Prop/WOT/engine height - would appreciate your thoughts.

[Sunbeam]

I hope you can all put up with one more thread on this topic. But although I get the general concept (and have read through the archives), I'm still not sure what to change (prop-wise) on my individual boat, and I would appreciate any suggestions as to where to go, prop-wise.

So, first what I have now, and then some background to fill in more details:

Yesterday I went out for the first time in my "refit" boat. This being a 22 Cruiser, with Yamaha F80 main and Honda 8D kicker. The main has a Permatrim, and I have Bennett M-120 trim tabs. We were fairly heavily loaded, with about 15 gallons of water and 40 gallons of fuel, plus enough food for a couple of weeks or more, most of the tools I would ever have aboard, other usual gear, and two people (combined weight of about 400#). We were in fresh water at around 400' above sea level. Conditions were calm and smooth. Prop is a new Solas aluminum 4-bladed, 13 pitch x 13.25" diameter. (skip down a couple of paragraphs for comparative info). The bottom is gelcoat, although I waxed it recently (which I realize will slow it down slightly, but I did it for easier scrubbing of growth/slime).

Running at WOT, I was able to make around 5400 RPM and 20mph (currently have things set on statute miles as that is how it was for initial trial at purchase). It didn't seem to change a lot with various adjustments, but this was with engine trimmed fairly neutrally and maybe 30% tabs or so. I tried for getting the "spray trail" as far aft as possible looking out the side window.

My engine manual recommends a range of 5,000 - 6,000 RPM at WOT, so I'm within that range, but I would rather be closer to the upper end. Also in my mind is that I'm about to head to Powell, and so things can only get "worse" there. I wonder if I should get another prop - could be a spare/Powell prop.

Some background:

When I sea-trialed the boat briefly at purchase, things were a bit different. The boat was very light (no fuel, 15 gallons water, no gear, two people combined around 280#). At that point I had no Permatrim, no tabs, and the prop that came on the boat, which was a 3-bladed aluminum Michigan Wheel, 15 pitch x 13.5" diameter. We made 5600 RPM and 28 mph at that time. This was on a freshwater lake at about sea level, a very light chop and wind.

So, in a nutshell:

Then
Light boat
Sea level
3-bladed 15 x 13.5" aluminum prop
No Permatrim or tabs
5600 RPM/28 mph

Now
Heavyish boat
400' altitude
4-bladed 13 x 13.25" aluminum prop
Permatrim and tabs
5400 RPM/20 mph

I consulted with someone more experienced than me to choose the prop, and I'm guessing that with the original prop things would have been much worse, but I still think I'd like it better for sea level running, which would make it "less worse" on Powell too (which I'm about to leave for). I'm fine with getting another prop, but just not sure what to get, either in number of blades or other dimensions. Can someone make a suggestion?

*****************

The other factor I wonder about is engine height. Roy & Dixie from C-Way have a 22 that's three years older than mine, but with the same engine and from the same dealer. They found that their engine was set too low on the transom, and moved it up around 1-1/2" as a result. I'm not positive, but I think mine is about 1" higher than theirs was. Ultimately it just matters how it is on my boat, but their findings did raise the question in my mind about that dealer maybe setting engines too low.

From what I have read in the archives, a first, "loose" check is to see if the bottom of the anti-ventilation plate is level with the bottom of the boat. Mine is an inch or so lower than that, I think (I wasn't sure exactly how to angle the engine vis-a-vis the bottom or the transom; I guess the gap would be smaller if I had raised the engine to make them more parallel). You can see it here:



I also read that a more precise way is to see that the Permatrim rides just at or under the surface of the water when up on plane. I tried to ascertain that yesterday, but there is a lot of turbulence back there when on plane (is this normal?) so it was hard to tell. Maybe it's easy to tell when it is at the proper height and can be seen? Or does one always sort of "extrapolate" through the spray?



If the engine is too low, I imagine that is the thing to correct first. On the other hand, I'm not sure I would do that before Powell, and so I still think maybe I would rather order a prop now that might help. If I later move the engine up, it seems it would still be useful to have a "gradient" of props for different loads/altitudes, and they can be spares as well. If I can get one ordered tomorrow, I should be able to receive it and put it on before Powell. I'm hoping to spend a few weeks there and don't like the thought of lugging my engine that whole time.

Thanks,
Sunbeam

 

[thataway]

I thought that I was the only nut who took a boat on Crescent Lake! Hope you had lunch at the Lodge. We go up there every year when we are in Sequim!. I have taken both the 22 and the 25 up to Crescent, but we always had more chop than you had! Thanks for the great photos!

I have always had 90 hp outboards on the 22, but the 15" pitch is been what I used for sea level and 13" pitch for Powell. I think you have the Powell prop on the boat now, even if it is a 4 blade.

First, remember that these boats are very weight sensitive. But when light and well balanced should get close to 30--and that is what your boat did with the 3 blade when you were at sea trial.

Lets look at things. If the engine is properly trimmed, the Permatrim should be just at the surface of the water--yours is well buried. So your motor needs to come up a little (was it the 300 lb person or the 100 lb person taking the photo?--that could make some difference.

I would have fiddled with the engine trim and the tab trim to find my fasted WOT--Generally with the permatrim on the engine, I have the tabs pretty well retracted. the 30% tabs sort of puzzles me. You look at boat speed, not where the wake is breaking. For WOT trim the tabs all of the way up, trim the engine about neutral--and then with the throttle full forward move the engine trim up and down slowly to see where you get the maximum speed of the boat--and that should also be the MAX RPM

Looking at the photo of measuring the level of the outboard--Have the cockpit floor or cabin floor level on the trailer, and then put a level on the Permatrim , and put the yard stick or what you are using on the bottom of the boat to establish a plane. The stick held against the bottom of the boat should show the Permatrim or top of cavitation/antiventillation plate should be at the same level as the bottom of the hull. Yours is not visible at all, showing that the motor is set a bit too low. I would take it up one hole and check again.

Who ever suggested the prop knew that you were going to load the boat--and suggested a lower pitch with 4 blades to give stern lift. Going from 3 blade to 4 blade may cost you from 100 to 300 RPM (depending of cupping etc.) So the 4 blade prop may be what you are going to get...but I am wondering about trim etc. and weight. Crescent Lake is about 500 feet so that should not make any different. My concern is that you have a lot of prop slip by calculation: 30% slip. The light boat was 18 slip--and that is a reasonable number. If your boat had gotten 30 mph light, the slip would have been about 12.5%, which is ideal. So maybe the motor being down a bit low, plus the drag of the permatrim in the water slowed the boat down.
(not saying the permatrim is bad, but it needs to be on the top of the water)

So you have some things to do--put the 300 lb gorilla on a diet,
See what tools you can off load. Raise the motor one hole. I don't think that you are going to hurt the motor at Lake Powell, but you are going to be slower--if you are going to stick with 4 blade then go to an 11" pitch, If staying with 3 blade, then I would stay with 13" pitch. I might also try the boat with the 15" pitch prop that came with the boat.

Again--all of this shows the sensitivity of the boats to weight...

 

[Sunbeam]

A bit more info, and maybe I could get some comparison thoughts from you-all.

I called the place from which I ordered my current prop, gave him the scoop, and asked for his thoughts. In somewhat random order:

- I did go the right direction in prop selection; with my original prop I would have made a lower RPM now.

- 5400 RPM isn't too bad, but that's a big drop in speed.

So, fellow Brats: Is that kind of a drop in speed something to be expected going from a lightly loaded boat to a heavily loaded one? (28 mph to 20 mph).

- There may not be a logical "slight improvement" size of prop, but he could potentially tweak my current prop; however that would give me more RPM at the expense of top speed (not that top speed is a priority for me at all)

- 4-bladed is usually good for a "heavier" boat (I asked if I should have stuck with a 3-bladed one).

- As far as engine height/looking at Permatrim it is also a "feel" sort of thing. (I certainly don't have that expertise yet.)

- I would lose RPM at altitude (Powell) with this prop, but there is no "easy" next prop to move to. It would need to be an 11 pitch; he is going to look around for options and call me back. This is the prop I have now:



My thought now (which may change as I learn more) is that since my "sea level" prop is not so bad that I'm really lugging the engine, I will wait to do anything to it until I get a bit more experience running the boat, and perhaps figure out engine height. But I will likely order a different prop for Powell now as this one is obviously not going to be optimum. I don't yet have a target prop to order though.

 

[thataway]

There are few 4 blade props for that outboard, because few folks use the 4 blade prop--most use the 3 blade. Ken at Prop Gods is the Go to guy about props.
http://www.propgods.com/Content2/Default.aspx
I consider him to be the best in the country. If you were near his shop he would give you several props to try. He told me to go to 11" pitch on the c Dory 25 for powell--and it did the job. Picked up 5 knots on top speed. Barely on a plane to full on good plane.

The reality is that most of the C Dorys are running 3 blade props. We get in the low 20's cruise speed with a loaded 22 with a 15" stock prop and Honda 90. You noted our speed--about the same as Les's boat at Powell, with a 3 blade 13". C Cory's will run fine any where from 14 to 20's Look at fuel efficiency.

 

[Sunbeam]

I see I replied earlier but it came in after your post - which I had not read at that time. So maybe it looked like I was ignoring your (valid) request for info. Let me respond that now.

I thought that I was the only nut who took a boat on Crescent Lake!

I guess I'm now a confirmed nut then, thanks :wink At least I'm in good company though :thup

Thanks for the great photos!

You are welcome! I thought maybe I was "over posting" photos so glad to hear they were enjoyed.

...remember that these boats are very weight sensitive. But when light and well balanced should get close to 30--and that is what your boat did with the 3 blade when you were at sea trial.

Yes, and it was also just above the middle of the WOT range, which I was happy with. Although I know I'll never run the boat that light (shoot, even Rana Verde can't be *that* light!). But it does provide some context for my current measurements.

Lets look at things. If the engine is properly trimmed, the Permatrim should be just at the surface of the water--yours is well buried. So your motor needs to come up a little (was it the 300 lb person or the 100 lb person taking the photo?--that could make some difference.

Okay, good to get your input and help reasoning it out. With all the spray and etc. at the transom, I wasn't sure what I was seeing as far as Permatrim height (or what I should be seeing). I was taking the photo and the "gorilla" :smilep was at the helm.

I would have fiddled with the engine trim and the tab trim to find my fasted WOT--Generally with the permatrim on the engine, I have the tabs pretty well retracted. the 30% tabs sort of puzzles me. You look at boat speed, not where the wake is breaking.

I won't say that someone more experienced couldn't have got more out of it, but I did play around quite a bit and I never got over 5400 (occasional "glimpses" of 5500) RPM. The reason for the tabs was twofold (although maybe not correct). First, because I can't see over the bow otherwise. Second, because of the "side wake." I'm not sure if you think I was looking at the wake aft, nor what the real name of the "side wake" is, but what I was looking at was the spray that comes out the side of the boat right around the helm. Reason I was doing that is that last fall on Powell (on the Marinaut, not my boat), that boat had a fuel efficiency meter. So it was easy to see what adjustments were the most efficient. The further that "side wake" went aft as compared to the vertical bar in the starboard side window, the better the fuel consumption, so that's what I would shoot for. I was figuring that meant there was more "magic carpet" with the bow up and out of the water (in a planing, flat way, not in an "I can't see where I'm going over the bow" way). Maybe I'm on the wrong track? (I don't have a fuel consumption meter on this boat; so just speed/RPM.)

That said, I probably need to try it all again, because there is every possibility that I could come up with a better combination of Permatrim/tabs. I only feel a bit like "I want to figure something out NOW," because I'm about to go spend 3-4 weeks on Powell, and know it will only be "worse" at that altitude. So if I could help things along with a different prop I would like to try. Otherwise I would be content to experiment more (and of course I will still do anyway).

For WOT trim the tabs all of the way up, trim the engine about neutral--and then with the throttle full forward move the engine trim up and down slowly to see where you get the maximum speed of the boat--and that should also be the MAX RPM

I will try that specifically next time.

If I'm remembering correctly, I did try taking off the tabs and it just put the bow up more but didn't give me more RPM, but I could be wrong. I do know that at a certain point, adding more tabs gave me "bow drag" so I put them up enough for that to go away. I had 2-3 of the little lights showing on the Bennett indicators. Partially to balance port/starboard (it was perfect with me on port side; with me at the helm I need to balance with one tab). I'll have to take (more) notes in future, too.

The stick held against the bottom of the boat should show the Permatrim or top of cavitation/antiventillation plate should be at the same level as the bottom of the hull. Yours is not visible at all, showing that the motor is set a bit too low. I would take it up one hole and check again.

That confirms my thoughts. It looks like I have room to easily take it up "two holes" in total (at about 3/4" per hole) just by using the sliding slot and without changing the holes in the boat. So there is some room for adjustment. I'm not sure if I will do this before Powell though.

Who ever suggested the prop knew that you were going to load the boat--and suggested a lower pitch with 4 blades to give stern lift. Going from 3 blade to 4 blade may cost you from 100 to 300 RPM (depending of cupping etc.)

Yes, I did say that as long as I was buying a prop, I'd like to make it more suitable for a loaded boat on a cruise as opposed to the original one (the boat had just one prop, so I figured why not make original the spare). But are you saying that a 3-bladed one of the same size as my 4-bladed one would give a bit more top-end RPM? So in other words, I "gave up" some RPM improvement by going with the 4-bladed one for a same given size? I wonder if I need the lift with the Permatrim and tabs now or if a 3-bladed would have been a better idea.

So you have some things to do--put the 300 lb gorilla on a diet,

That shouldn't be hard because it's not me :wink

See what tools you can off load.

I had a feeling you might say that. Ha ha. The thing is that I really kept weight in mind when choosing which tools to bring along. My tool box is a mere shadow of its usual self! Seriously! And I didn't even have a cooler aboard yesterday, nor clothing and bedding, nor.... other stuff. So I can't see it getting too much better, weight-wise (darnit), and can see it getting heavier (at least at the beginning of a trip). I have already started thinking of changing kickers. My current one is a fine engine, but it weighs 108# (talk about a gorilla). I see my use of it as getting "over to the shoulder" so I can anchor, and for that I would be comfortable with a smaller engine. The only reason I have this one is that it's what came on the boat. If I were going to be trolling with it I think it would be worth its weight, but I'm not.

Raise the motor one hole.

That I can do. Roy from C-Way gave a nice description of doing his (same engine) by using the trailer tongue jack and some timbers.

I don't think that you are going to hurt the motor at Lake Powell, but you are going to be slower--if you are going to stick with 4 blade then go to an 11" pitch, If staying with 3 blade, then I would stay with 13" pitch. I might also try the boat with the 15" pitch prop that came with the boat.

Let me make sure I get this:

1) I may be okay with my 4-bladed prop for for sea level use; try raising engine and improving technique (I can't see boat getting any lighter, and can see it getting heavier).

2) For a Powell prop: If I stick with a 4-bladed prop (which... should I?), I look for an 11" pitch.

3) If I move to a 3-bladed prop for Powell, then the same pitch I have now might make a difference (in other words, only the number of blades changes?) Maybe I should not even have gone 4-bladed to begin with (not that I had any way to know, but now I do have data).

3) I don't totally understand trying the original prop. Reason I say that is that I thought my new prop (even with its 4 blades) would be "better" already in terms of giving me more RPM at WOT. Is there any way the original prop could do that? I mean, I read you saying that but don't understand it. Props confuse me a bit - maybe it's that blend of art and science.

Thanks so much for your input.
Sunbeam

 

[Sunbeam]

So here is one reason I'm confused. I searched "Powell" and "prop" and was happy to see a post by Kerri-On. Reason I say that is that there are two people living on the boat, so it must not be super light, and he has a Yamaha 90, which at least bears some resemblance to my 80 (although mine is really the equivalent of the 75 in current parlance). Here is what he had to say (and George, if you are reading, I'd be very interested to hear your thoughts):

Just picked up my Powell prop 13 1/4 by 15. Should get me the 500 rpm I lost with the 17 pitch. We shall see. George

So apparently Kerri-On had a 13.25 x 17 (we'll assume 3-bladed as I guess that's more usual). And then went to a 13.25 x 15 for Powell. But I now already have a 13.25 x 13 for "non Powell" which still has me in the lower RPM range at WOT. So is the difference that great with a 4-bladed vs. a 3-bladed? I'm not stuck on 4 blades, so I can switch to a 3-bladed if that's better. Or is the 10 hp that big a difference? Or am I hopelessly confused.... ?

The idea of spending a month on Powell either lugging or running along at displacement speeds (or higher speeds sucking excessive amounts of fuel) is a bit of a bummer. So my first priority is to figure out and order (tomorrow) whatever prop is most likely to work there. I'm not too crestfallen about "previous investment trap," so if it turns out the 4-bladed was just a mistake, I have no problem going back to a 3-bladed.

I think I maybe should have just started a Powell prop thread separately (?), but now I don't want to clutter things up on the board. And of course it's all related anyway.

 

[thataway]

I am trying to figure out why your boat is behaving the way it is. I carry a Waeco freezer, extra group 31 battery, , Full ice chest, 5 gallons extra water, 3.5 hp outboard, plastic box (contains propane stove, grill, Honda Generator, extra anchor) in the cockpit and inflatable on the top, plus clothing for a month and food for a month, Marie, myself and the dog are about 330 lbs. and seem to be running better than you are (and I also compare the first 22 I owned which had no Permatrim or tabs, as well as the current boat with a Doel Fin and Lenco tabs). In that I don't seem to have a problem with bow up so high, I don't have problem with side spray....How did the boat run on sea trials ref the spray and bow attitude? Maybe you are running with the bow down too much? But you say that you have tried different combinations....

The reason I suggested to go back with the first prop to compare the amount of slip. The slip is way high currently @ 30% [assuming that I have the correct lower unit ratio (2.31)] With the 4 blade 13 pitch prop.

With the 25 and Tom Cat, you could see the Permatrim on top of the water. The current boat has a Doelfin--which, if I get well, I will change out for a Permatrim.

I probably would go with the prop you have to Powell and move the motor up. Try one hole at a time. Run, than go back--unless it is obvious when you measure that you need to go up two holes. If you want to buy another prop, either a 3 blade 13"pitch or a 4 blade 11" pitch. I cannot find a 4 blade in 11" for your engine--but it may exist. But I would give Ken at Prop Gods an e-mail.

This is from Boat Test. com:

Take two propellers, identical in design (blade shape, diameter, rake, cup, etc.) that is appropriate for a given application, and simply add a propeller blade, we get a truer representation of just where the difference lies. The addition of the extra blade causes increased drag, which, in turn, requires more horsepower, in order to achieve the same rpm. Since the horsepower is limited, the rpms drop, and the speed will tend to drop with it. This is why, when going from a 3-blade to a 4-blade, the pitch is dropped an inch, or more, in order to keep rpm parity. It is this difference in pitch that causes any potential speed differentials between the 3-blade and the 4.

 

[digger]

One other thing of note...when you have a slight chop, the boat is faster...takes some of the drag off the bottom. This is especially noticeable when you are at a near critical point of the power range. On my boat, I found the fuel consumption and speed were both improved by having the bow of the boat slightly downward. It made the engine feel like it was about to start laboring. Also, it would help to start the test by not having the tabs deployed -- just trim with the motor at around 4800 rpm, and find the trim that gives you the most speed. Then slightly bring the tabs down until you determine if the boat speed is improving or slowing. Find the best speed with that adjustment, and tweek the motor once again and see if you can improve the speed by adjusting the motor trim up slightly. Make all the adjustments without changing the throttle setting.

 

[Sunbeam]

I am trying to figure out why your boat is behaving the way it is. I carry a [bunch of gear] and seem to be running better than you are.

I don't seem to have a problem with bow up so high...

I don't have problem with side spray....

Hi Bob,
I appreciate your help in getting to the bottom of this - thanks. Let me try to add some info that will (I hope) clarify.

1) My boat is near what would be my top weight/loading, but I would guess it's lower weight than yours above, just because no generator, freezer (yet) etc. I don't plan to lighten it much though, as the reason I got it is for cruising. Not that I'm not going to pay any attention to weight, but just that realistically, it's probably even lighter than it will be at the beginning of a cruise (although when I'm solo it will probably be lighter).

2) I don't know that my bow is extra high - it might be that I notice it more because I'm not that tall and plus am relatively short torsoed -- so any amount of bow up feels like it blocks my view. I will probably raise the helm seat.

3) What I did was just put the throttle at WOT and let it run, and then I played with the tabs and trim, until I was going fastest and the spray from the bow/side moved as far aft as possible (those two coincided anyway). I took all tabs off, then added them to get the bow down (which actually seemed more efficient, unless I'm remembering wrong).

4) I don't think I had a problem with side spray, but then I guess I assumed that it was normal to have some slight line of spray off the boat near where the windows are (but down at water level), since the Marinaut had it too (and that boat ran sweetly!). Maybe I made it sound like it was a true wake, or up by my window, but that's not what I mean to say. It's down at the waterline and just a slight white line of spray. I probably didn't describe it well though.

The reason I suggested to go back with the first prop to compare the amount of slip. The slip is way high currently @ 30% [assuming that I have the correct lower unit ratio (2.31)] With the 4 blade 13 pitch prop.

My F80 does have a 2.31 gear ratio, so you have that right on. Okay, I see what you mean about comparing. Another data point with the same load would be good (vs. my super light sea trial load).

With the 25 and Tom Cat, you could see the Permatrim on top of the water. The current boat has a Doelfin--which, if I get well, I will change out for a Permatrim.

Okay, mine is low for sure then. Maybe I'll try to raise the engine tomorrow (I'm slightly loathe to do it without epoxying the holes as I want to, but then it's already not epoxied anyway (was figuring I'd find someplace to hoist my engine up so that I could fix the holes properly). I'm beginning to think it's just the ol' thousand paper cuts: Engine a bit low, 4-bladed prop, healthy load, operators who could use more experience.

I probably would go with the prop you have to Powell and move the motor up.... If you want to buy another prop, either a 3 blade 13"pitch or a 4 blade 11" pitch.

Thanks for the specific suggestions. That gives me something to go on and is much appreciated. I will try Ken at Prop Gods (great name!) too. Appreciate the recommendation.

Also, it sounds like that 4th blade might be part of the problem. Unless I find some reason that is a huge advantage, I think I will just go to a 3-bladed one. I don't have any underlying reason to "have to have" a 4-bladed one - it was just suggested as a good alternative (before testing it).[/quote]

Thanks again for your time,
Sunbeam

PS: Digger, thanks to you for your info as well. I will keep all this in mind as I test things out. Just feeling a bit of a rush since Powell is coming up - other than that plenty of time to test/go back/test/etc.

 

[KickerDRB]

Glad you got on the water and enjoyed the fruits of your summers labor. Sounds like you are getting to the bottom of your prop dillema.
As far as motor height, I thought the best time to tune or adjust was when the boat was near the lightest state. It would be a real bummer if you tuned the system for a heavy load, then ran it one day by yourself (without full up water and fuel) and the prop was cavitating away.....

regards,
Dave

 

[Sunbeam]

As far as motor height, I thought the best time to tune or adjust was when the boat was near the lightest state. It would be a real bummer if you tuned the system for a heavy load, then ran it one day by yourself (without full up water and fuel) and the prop was cavitating away.....

Oh, good point. I hadn't thought of that. Too bad I never checked that on the sea trial after having the engine tuned up (was very quick and last year!). That would be a bummer! On the other hand, it is lower than the boat bottom now, which seems like a semi-universal rule of thumb (from what I can tell?). i.e. the anti-ventilation plate should be level with the bottom of the boat.

Regarding what I am describing as the "side wake" (which probably has a real term I just don't know yet ), here is a photo of what I meant, as shown on a Marinaut 21. I marked the forward point of the "side wake" (SW) with a red arrow. So in the first photo, the SW is further forward, and in the second photo, it's further aft. That boat had a fuel flow (consumption) meter, so along with to RPM and speed, it was pretty easy to see what was most efficient (and it felt best too). I found that it seemed to coincide with the SW moving aft (as shown in second photo), and it gave the best "magic carpet" feel, so I tried to use that as a guide yesterday on my boat. Would be interesting to have fuel flow meter.

I could get the "SW' on my boat yesterday partway back on the window (but still down by water surface), so quite a bit further back than the first photo; but not as far back as the second photo.

 

[Sunbeam]

Okay, I'm posting back to myself now , but I found this link in another thread here (while searching archives) and just now remembered that I had it open in another tab. Okay, so the "buried/too low" AV plate photo sure seems to look a lot like mine.

Their photo of "too low" AV plate:

My engine (I think we were at 4300 RPM and 16 mph here; they say to "take it up to cruise," so I think this should qualify:



Kinda looks like the "too low" one to me - do you think?

And the "proper" one? I never saw anything like that - it would have been pretty obvious with that big Permatrim up that high! I would have assumed that was too high before having read/learned more (here and at the link).

I'm a bit uncertain about the recommended loading for testing. They say to test engine height at 50% fuel, and I had one full tank and one just over half full at that point, so 75% fuel. On the other hand, I didn't have a cooler/ice/food in the cockpit, and I wouldn't be surprised if that weighed 100# (or about 16 gallons of fuel worth). Also I didn't have the kicker tank along (3 gallons, for another 20#).

But then for testing WOT RPM, they say 50% fuel and "a light load for fishing. cruising, etc." So I'm not sure if they meant to say that for the height test and just left the second part off, or what. So I don't know if the engine height should be tested at fullish load or at 50% load...

If they really meant 50% fuel, but normal load (which for me would be heavier as I figure I'll mostly cruise) I was probably at that equivalent. If they meant 50% load overall (because not talking about boats typically loaded for cruising), then I'd have to take stuff off the boat, as I was probably closer to 80% overall load for cruising.

Anyway, seems like a really good link (and also refers to the Prop God).

http://www.veradoclub.com/smf/index.php?topic=137.0

Sunbeam

 

[tpbrady]

On both C-Dory's I have owned, the engines were mounted too low by the dealer by 1 1/2 to 2 inches. In both cases, I wish I had more upward adjustment but ran out of transom. I used Permatrims on both. On the 22 the Permatrim didn't seem to make a whole lot of difference other than cutting the top end speed by about 2 mph. Yes it helped get the bow down (or stern up), but its main impact was allowing a pretty efficient cruise speed at 12-14 mph. With the Permatrim off that cruise point move up a couple mph which is a lot rougher in a 22. On my 25 the Permatrim was a real asset as that boat is just plain stern heavy and it made a lot of difference in trim. In both cases I could never get the motor high enough to get the Permatrim up to the surface as far as it should be. In your case, with the Permatrim buried, it represents more drag than anything else. Moving it up will reduce the drag both from the lower unit of the motor and the Permatrim.

Tom

 

[Sunbeam]

On both C-Dory's I have owned, the engines were mounted too low by the dealer by 1 1/2 to 2 inches.

Interesting -- seems like there might be a lot of that going around :wink:

I remembered that Roy/C-Way (who has my same engine rigged by same dealer) had raised his engine, and we had some discussion, but I didn't remember ever conclusively digesting the info as it compared to my engine. Well, I just now looked back through my PM's to where he had sent me back a specific measurement. From that I can tell that my engine now is at the same place as his was originally (which was too low). He moved his up to the limit (that is, without changing the holes in the transom), which raised the engine around 1-1/2" higher than it was, and with a heavy cruising load it sounded like his Permatrim was still not "floating" on the surface. That makes me think that even more lightly loaded there would still not be a problem of it being too high (and since no holes are being changed it could always be lowered). Since it sure sounds like it's a lot less fruitful to be playing with props while dragging the AV plate (with large Permatrim) through the water, I think I will try to raise my engine. Not sure if I will get it done before Powell, but I will take at look at it (and at my tools/resources) tomorrow. I may still order another prop for Powell just to have it as an option - not sure yet.

 

[Chris]

I run a 13 1/2 X 15 three blade all the time. It is the sweet spot where I get good performance at all elevations and loadings. I loose about 200 rpm at Powell but it really doesn't hurt because I'm still turning 5400-5600. It's important to let these motors turn up. I think the goal should be to prop so that you are turning the high side of your rpm range at sea level while light. Then when you go to altitude or load you will loose a little rpm but still be in an acceptable range...until you go extreme load or altitude. The dealer propped my boat with a 17 which I knew would not work once I loaded the boat. Dealers do that because they are dealing with boats that are new and light.
Your motor mount looks OK to me...looks real close to where mine is. Being a little low is definetly preferable to too high.
You absolutely should have a spare prop at Powell. File and a hammer a good idea also. Those rocks can be sneaky!

 

[Sunbeam]

Thanks for the input, Chris. I think your Yamaha 75 is about as close as one could get to the F80 without actually being one, so it's especially helpful. I'm going to assume that your prop is a 3-bladed one (seems like I kind of got an oddball by going with the 4-blade, which is making it hard to compare. Maybe that was an "oops" but the reasoning of the person who suggested it sounded good at the time).

Of course I kind of wrecked my data set by changing so many variables between my first and second try. And next thing you know I (hope to) be at Powell, which is yet another variable! I don't know that I'll get out again before heading there. Maybe if I get there in time, I'll try out both of my props before the cruise. At least then I will have "equal data" and be able to really properly compare the original 13.5 x 15 (which is the size you have good luck with) with the new prop I got (13.25 x 13).

I may decide to get either a 13.5 x 13 (3-bladed) or a 13.x x 11 (4-bladed) to bring along, but I'm not sure about that yet. I'm going to try to understand the 4-bladed thing better first.

Sunbeam

PS: If you see this - do you get a sort of "sing" at certain RPM's? I noticed one between slow and about 1900 RPM (8mph) - it seemed like it was coming from the prop area. I'll have to see if it stays the same with another prop.

PPS: I'll be very interested to see Rana Verde again now that I have specific things to look for (like that special boat-lightener you carry along )

 

[thataway]

In the middle of the night I was thinking about the motor level at fully loaded and very light boat. The boat certainly sets a couple of inches deeper (I have never calculated the amount of weight to bring the C Dory waterline down one inch--a factor I usually figured on my cruising sail boats).

There will be some difference in the prop and water flowing off the transom, but it will not be much. In the Tom Cat it did not seem to vary as far as the visibility of the Permatrim. Recently I saw a photo of a cavitation plate or foil which was correct--I'll see if I can find that.

Sunbeam, with your intended use, I suspect that the boats waterline will remain fairly constant, so I would adjust it for your current loading. I had more data for the C Dory 25, since it did not have the foil when I first got it, and my experience was very similar to Tom's.

I have been lucky to not have hit any rocks at Powell--but I agree that a hammer and course file are always in my tool kit, as well as at least one spare prop, trust washer, at least one extra crown nut and several cotter pins. (I once had a cotter pin break in a dinghy, and both the prop and crown nut spun off a dinghy when I put it in reverse. Fortunately it was in very clear water and I found them both--put a piece of wire in there to get back to the big boat to replace the cotter pin!.

What you clarified about the spray is normal. I had thought about the visual lines for you and raising the seat also. If one has a vessel with a clear windshield and sloping bow, the visibility in the C Dory seems a bit restricted, with the pilers and fore deck, but you get used to it

 

[Sunbeam]

I was just putting my hammer into the "goes to storage" pile, but I hadn't thought about possible prop work (uh-oh, there's another pound in the boat ). I'll put that aboard now (already have a file in the toolbox). I think I have spares of the others because I got all new with the new prop, and then packed away the full set of previous parts, but I will double check - glad you mentioned it.

... with your intended use, I suspect that the boats waterline will remain fairly constant, so I would adjust it for your current loading.

That sounds logical to me. If I somehow manage get the boat lighter at some point in time (ha), I'd be happy to adjust (not a bad problem to have, after all). Right now I just don't see myself as a day-tripper; if it changes then okay, I'll figure it out then.

What you clarified about the spray is normal.

Okay, cool. I guess "side wake" is probably not the real term

I had thought about the visual lines for you and raising the seat also. If one has a vessel with a clear windshield and sloping bow, the visibility in the C Dory seems a bit restricted... but you get used to it

Good to know. I really did feel a bit "hemmed in," plus had that neck crick that you get from driving a too-big car where you are straining to see. That did take away some from the enjoyment, but I think (hope) that will abate as I get used to it, and if I get the seat adjusted for me (hope to find a Cabela's on the way to Powell as I have heard they have quite a few helm seats on display that one can try - catalogs/online just doesn't cut it sometimes). I've saved a couple of photos wherein C-Brats added a nifty little drawer below the helm seat in order to raise it -- although I'll probably start by just using scrap wood spacers to try things out.

Sunbeam

 

[Sunbeam]

Edit: See bottom of post.

A specific comparison question:

(I know I could probably get this info from a prop shop, but maybe it will prove helpful to others too if it's here.)

I'm looking at Solas aluminum 3-bladed props that say they will specifically fit my engine. In 13 pitch, there are two diameters, one is 13.5" and the other is 13.75" (in other words, everything else about them is the same).

Given that all else is the same, what difference(s) would the change in diameters cause?

Sunbeam

****
Edit: I got in touch with the prop shop where I bought my current prop (note that when I ordered it I basically "knew" what I wanted and just ordered it, so they didn't really help me select the one I have - so they can't be faulted for it). In talking to him he did say that going with the 4-bladed when I did wasn't totally illogical, as they can be better for a "heavy" boat. However now, he felt that going down to an 11 pitch 4-bladed would probably be "too much," and would really cost in top speed (not that that's a priority, but I don't want to totally lose it all).

Upshot is that he suggested I try the 13.75" x 13 pitch 3-bladed, and he said why didn't he ship it to me at Powell, I try it, and if it worked well I send him some money, and if not I send him the prop back. I can't exactly argue with that kind of service :thup I think I will do that as then I will have all three props, and can try them all with the rest of the variables remaining constant.

I did also ask about the 13.5" vs. the 13.75" in 13 pitch, although I had already found that in the "real world" online, the 13.5" was pretty much impossible to find, or at least far, far less "normal" to order. He said that would be more for a smaller/lighter boat, and the 13.75" would be the logical one to try.

So, once I find someplace to receive a UPS package around Powell, I'll get it on the way, and I should be positively bristling with props (I haven't ruled out raising the engine at all, but am not sure I'm going to do that before Powell).

 

[thataway]

Did you use the storage facility in Big Water? They can get a UPS package for you. They did it for me.

You would have slightly less blade area, with the 13.5" diameter, and it would spin slightly faster--not quite as much water moved per revolution. Usually as you go down in prop pitch you go up in diameter (slightly). The 13.5 inch diameter Solas prop has a bit more rake to it than the 13.75" and is definitely more of a lighter boat prop, with a little less blade surface area and different cup.