Mount Honda BF90D in highest position?

D

Darkwater

New member
After I bought our 2008 22 cruiser with Honda BF90D a couple years ago, in running the boat it felt like it was dragging a parachute. Adding a Permatrim helped, but not much. Honda 90 was mounted in the next to lowest position, so I raised it one hole to be next to highest position. That was better but boat still feels stern-heavy, even when lightly loaded with little fuel and only 1 battery. So presently, if the motor is trimmed to have the permatrim parallel to the bottom of the hull, the permatrim is on the same plane (even) with the bottom of the hull. However water rushing from under the stern rises rapidly, and the permatrim is still running about an inch under water (see picture below)

Permatrim_UnderWater.jpg


I am thinking of raising the motor to the highest position which should raise the Permatrim about 3/4 inch, but first I would much appreciate the comments of any of you who have experienced this situation.
 
It does look like yours is low. There is more discussion in a previous thread (2014 or so), but essentially the Permatrim should have just the thinnest film of water going over it when you're on plane.

Mine was maybe not as low as yours, but it was too low. I couldn't see the Permatrim really. I took the engine off and raised it to the top hole (and while I was at it overdrilled/filled the mounting holes, plus did a few other things).

I'll see if I can dig up the previous thread...

Okay, well the thread I was thinking of was a bit of a slog to read, but the same info is in this thread (with photos showing how things should and shouldn't look). They don't have Permatrims but are using the anti-ventilation plate so you can see how things should be.

http://www.c-brats.com/viewtopic.php?t=21941

Good thing the photos are here, as the original link (Verado forum) no longer works (404).
 
I tried that with my boat (measuring to the hull when on the trailer), but I wonder if that is the best way. Because different boats "sit" at different heights in the water once you are loaded and underway. (e.g. compare aluminum skiff to C-Dory 22).

Mine measured out about like they show in your link (on the trailer), but then -- as in the Verado Club's "bad" photo, the Permatrim (and thus the anti-ventilation plate) were deeply buried when on plane. The club says (and shows) that you are supposed to see splashy water over the plate, but not "green water." In other words, it should almost skim the surface.

Not that I'm an expert - I'm just going by what I have read here on the forum and at the Verado Club link plus some other forum threads.

I did raise mine all the way up (which I think was a couple of inches of height gained). I think it's probably better but still not perfect (but it could not be raised more without resorting to special measures). I can't say for sure as I have only been out solo since I raised it, so I can't just run back and look at the plate when I'm on plane. And I couldn't go by speeds/RPM either, because I was comparing heavily loaded with two people and a heavy kicker, to more lightly loaded with one person and a light kicker. It will be interesting to see once I have someone to look at it and take photos. I took photos (posted in linked thread) of how it was before when it was too low. Now I just need the "after" photos.
 
The link Sunbeam gave, was "Darkwaters" original post about this same subject in 2014.

Did what effect (boat speed, trim attitude, performance in chop) did adding the Permatrim give? (it may slightly decrease top speed).

Yes, I would move the motor up on more set of holes. Record numbers, trim if possible before, and after the move up.

Not sure I understand what "feels like dragging a parachute" means. Please give ups some RPM and speed numbers, which will help. Also if you have a protractor/Inclinometer (such as in i phone) to compare the boat's attitude (trim) with when it is setting "level" in the water. At some times, one gets best performance with slightly bow down trim--but not too much. The fore and aft trim of the C Dory is somewhat different than the average boat its size.
 
Brent,

That's the set of photos I got a 404 on, so linked to in another C-Dory discussion where I had posted them over to this forum. Glad you found them on the Verado club again.
 
yep. That ain't right, but I'm not sure about your trim angle on your motor.

Do you have trim tabs?

Sometimes if your trim tabs and engine are working at odds you can get your sterm in a funny position. What I mean is, if your trim tabs are down (driving your stern up and bow down, and your engine is trimmed way up, these alternate inputs can cause your prop to run in an awkward position. Also waste fuel and add stress to your hull and running gear.

I'm sure you've gone through the simple things, but this would be a very simple thing to make sure is right first.

Otherwise, yes, raise it up one spot.

However, I, too, would like to know more objective info to provide advice I'm certain is good.

There are a lot of things that could impact performance. Even an engine that isn't running properly, or the wrong prop could impact performance. Poor performance can be the cause of your anti-cavitation plate being underwater. It's not necessarily the other way around. What I'm saying is that if your boat is not getting to its normal speeds then your hull will not lift and you will not have your anti-cavitation plate just at the surface while on step.

So, engine RPMs and speeds are needed.

Other things to consider if the depth of the prop is the symptom and not the cause:

-bottom growth
-damaged or wrong size prop
-engine needs tuning or maintenance
-prop hub fouled (can result in more damage if not fixed)
-lower unit losing oil and failing (if you've run a lot like this then this is certainly not the case. it's not much time between things getting slow and things failing entirely)
-saturated core (added weight)

Any of these could cause your boat to feel like you describe, and perform poorly. If your anti-cavitation plate is indeed in-line with the bottom it is likely that you are not seeing it when you think you should because other problems are impacting performance and your hull isn't getting as far out of the water as normal.

I'm not saying this is the case, just that the one thing you have clearly identified (prop too low when on step) could be a symptom and not a cause. If your hull is just staying too wet you can raise your engine until it's out of the water and it won't fix anything.
 
Thank you all for the responses, and especially link to Verado site with the photos of proper engine height.

"Dragging a parachute" meant the boat would only do about 12 kt at 4000 rpm and trimming the engine and trim tabs did not help much. Felt like the prop was whipping, rather than gripping, the water. Boat seemed to want more downward trim than the engine and trim tabs could give it. This was with very light load of 1 person, and and water and fuel tanks about half full. This was before getting the permatrim and raising engine to second highest position.

Info regarding conditions when I took the picture of the Permatrim in the wake:
15 gallons fuel
10 gallons water
350 lbs people
100 lbs fishing gear (downriggers, weights, etc.)
1 group 24 battery
Honda BF90D 370 lb
Yamaha F6 kicker 65 lb
"Standard" Honda aluminum prop 3 blade 13-1/2 x 15. Prop in good shape except some cavitation burns from original owner (a clue).
Engine trimmed so ventilation plate and Permatrim parallel to bottom of hull, and mounted at second to highest position (of 4 positions) so Permatrim and ventilation plate are also at same level (even) with bottom of transom.
4500 rpm, about 18-20 knots
Engine has about 120 hours, seems to run great, and no codes showing.

I have few opportunities to run the boat under consistent conditions due to rarity of calm water out of Noyo Harbor. However I think the most rpms I can get right now is about 5500 (5300- 6300 is spec at WOT). Probably 25 knots top speed.

I was hesitant to raise the motor to the highest position because I thought the boat is probably already rigged by the factory/dealer for best performance. Now I know otherwise. I plan to raise engine to top position this month. If I am still not happy maybe try a prop perhaps with 4 blades and 13 pitch. Some 4 blade props are claimed to increase "stern lift". Also I want to install a second battery, for house loads, and I like the idea of putting AGM in port side forward area of cabin.
 
I understand what you mean, but 12 knots ~14 mph, 26 knots = 30 mph. We get 30mph at 6000 RPM, with standard 15" prop, and light boat (which is close to the weight you describe) at sea level. Very unlikely in your boat, but water in the core, or in the forward foam, could cause an increase of weight, and make the boat substantially slower.

I find that some folks do trim more bow down than is necessary--at least in calm water. Is the 5500 RPM the max speed, no matter what trim.

Yes, a 4 blade will give more stern lift, we use these on the cats. But it may also lower the RPM at top speed. You are changing two variables--3 to 4 blade, and 13 1/2 to 13" pitch.

Your current prop slip is 43%! Very high. Mine is 13% at 30 knots. Yours would be 14%. So at the upper range, you slip is fine. You have excess drag--maybe a "parachute" in the lower unit or boat trim. You should also be able to get that engine up to 6,000 RPM. That much drag, usually means that the boat is not planing properly--for the C Dory--would be expected in a deep V trying to get on a plane. (our boat has a slight disadvantage, and that is the DoelFin, which came with the boat, and is notes "good" as the Permatrim--subjective opinion.

Cavitation "burn" also suggests that something is causing drag, or air injection in front of the prop. Is there any fitting on the bottom, rough bottom, damage to the keel, etc?

First bring the engine up.

On edit here our our numbers: (actually Boat Test. Com numbers, but ours were so close that the difference is not significant) lite boat, Honda 90, stock 15" prop, sea level calm

RPM MPH Knots G/h MpG Nm/G
670 2.3 2.0 0.5 4.60 4.00
1000 3.9 3.3 0.5 7.70 6.70
1500 5.2 4.5 0.8 6.93 6.03
2000 6.6 5.7 1.1 6.00 5.22
2500 7.5 6.5 1.9 4.05 3.53
3000 9.4 8.1 2.2 4.35 3.78
3500 14.2 12.3 3.2 4.51 3.92
4000 19.1 16.6 5.0 3.81 3.31
4500 24.7 21.5 7.0 3.55 3.09
5000 27.4 23.8 8.1 3.38 2.94
5500 30.9 26.9 9.1 3.41 2.97
6100 32.9 28.6 9.6 3.45 3.00
 
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