Prop change - new noise

C

C-Val

Member
Hi everyone
I have not able to get more than 4800 rpm on my original 70 hp Johnson pushing my 22' classic. So I changed my prop from 15" down to 13" of the same diameter (13 3/4"). I went with a Michigan Wheel 3 blade aluminum prop.

It works a lot better. Now I can get 5200 rpm and I think I only lost 1 knot. The only issue is in a narrow band of rpm (1800 to 2200) I am getting a vibration noise from the motor. Then is disappears and doesn't come back until I slow down and drop past that range again.

Does this make any sense to anyone ?
Does it hurt anything?

Thanks for your input
David and Val
 
All of my boats and motors for that last 50 years have had odd sounds at various RPM settings. If it is in a narrow range of RPM, then you are most likely looking at some resonance around the motor or even on the transom and other than annoyance, it is probably not harmful. That is similar to my airplane flying experience and automobile experience. I just move above or below those odd spots and don't worry about it. If it is something that is over a wide range of RPM's, then taking a closer look might be advisable. Cessna 175's had a dangerous (prop can fly apart) operating RPM range that was never fixed, so Cessna simply posted a warning on the instrument panel that said "don't operate in such and so RPM range for more than X minutes." Resonances and vibrations are part of the normal world of mechanical widgets. However, Cessna 175's didn't sell well.
 
C-Val

You might check the clearance between the prop and the trim tab on the bottom of the anti-ventilation plate that corrects for the rotational torque of the prop and makes the motor more neutral and steer straight.

Check (measure) it with the old prop and then with the new one. When the clearance gets down under a certain amount, which seems to vary, the high speed wash off the prop is felt as a thump or vibration to the motor. This may also vary with the trim angle.

You might think this would occur at all speeds, or all speeds above a certain threshold, but sometimes fluids can behave in unanticipated ways, or perhaps a certain resonance frequency is reached.

I don't remember what amount of clearance was generally required, but IIRC, once it gets down below 1/2" or so, the possibility exists.

Maybe you could call up a prop shop or a technical rep at Michigan Wheel and ask someone who deals with it routinely.

You can grind down the trim tab, if necessary, or some manufacturers make an offset trim tab for this situation:
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Try Googling "prop to trim tab clearance", which is also:
http://www.google.com/search?client=saf ... 8&oe=UTF-8

Hope this helps. Good Luck!

Joe. :teeth :thup
 
Agree with both above that it is a harmonic which is setting up resonance.

I assume that it is an aluminum prop. Few props are fully balanced or trued. The more expensive SS props included. You can take the prop to a shop which has "prop scan"--and they can true and balance the prop--but that will cost more than your new aluminum prop. Some times a little work with a file, or even just time/a little sand on the bottom, will eliminate this vibration. But not to worry--and 1800 to 2200 is probably not a band where you use the outboard much. If it is, and it bothers you or is a large vibration, then try what Joe suggest (the diameter is the same, but with the larger pitch the dynamics may change. If that does not take care of it, then true up the edges of the prop with a mill file. (something which needs to be in the tool kit, just in case anyway. (We ran the Grand Canyon a number of years ago, and the guides spent an hour os so every night pounding and filing the props on the 25 hp which they used to steer the "J" boat rafts.
 
Potter Water, Sea Wolf, Thataway,

You have not idea how much your input means to me. To be able to tap into the depths of your experience is worth more to me than than you can imagine. Your collective years is a resource that becomes a guiding light for those of us who are new to the C-Dory world and are only guessing at what to do.

I guess its why I love this club so much and why buying a C-Dory is one of the best things I ever did!

C-Val
 
Agree that we have to know the speed to know what is happening. When you get to the top speeds in a C dory there are lots of variables. Trim is #1. Then motor height and drag. Tom is correct that you would expect to pickup a little speed. You are developing more HP (but not more torque), turning the prop faster--and should have less slip. Putting the numbers in a slip calculator would help to be sure. Generally the slip will decrease as you lower the pitch. But a mile per hour is so variable--sea state, current, wind, bottom condition, temp, elevation etc.

We all appreciate the kind words, but we have all been helped along the way as we acquire knowledge and then appreciate a forum to pass it on for others to enjoy the sport!
 
Thank you starcrafttom also for your input

Speed has been a little harder for me to monitor because I have been boating with different loads lately.

My first observations were with 4 people aboard, full of fuel and gear. My best speed was 18 knots at 4800 rpm wot I cruised that time at about 4000 I dont remember my speed exactly - it may have been around 15

Since that time I have added a permatrim and raised the motor to the highest setting. (The permatrim is still under water a bit but once in rough waves the prop came out of water so I was thinking of leaving there for now)

Lastly I added the new prop down 2" to the 13". Now my max rpm is 5200 at about 17 knots and I cruise at 4500 rpm and doing 12 to 14 knots

I may have to take the boat out by myself sometime to do more standardized runs to compare. Thats all I have for now.
 
First I may not have some of the items correct, but your speed is very low.
I made the assumption that a 1990 70 hp has the same lower gear ratio as your motor: 2.414--that was a constant in most 70's. Using your speeds as "accurate" you prop slip didn't change much--it is 26% now and was 27%
before.

Those are high slip numbers--you would like to be down to 11% or so!

Factors: Adding the permatrim will decrease top speed about 1 mph by drag. The others have to do with the motor. Is this an original 1982 motor? Can we verify the gear ratio--from your manual? When was the motor tuned up? What about the compression and leak down? How many hours? etc... Also altitude--3.5% loss per 1000 foot elevation.

Ideally at 11% slip you would be getting 24 mph. That is about what I would expect with a 22 light, good engine tune up etc. With the 90 we see close to 30--again boats light.

If the boat is light, the bottom perfect, engine tuned and good compression etc--then there are specialized props which would improve your speed--but they are going to be SS and a bit on the expensive side.
 
It is the original 70 hp motor (1982). There is no way for me to know how many hours are on it though. The model number is J70ELCNB
The serial number is J5677667 I am getting between 2.25 and 2.5 nm per gal. All speeds are read off my gps. (My water wheel reads higher but I was assuming the gps is more accurate.)

It starts, runs, and idles perfectly as far as operating it is concerned.
I will do a compression check on it and get back to you. That could be the issue, but I would never know it by running it

I didn't buy the boat for the motor so I am just going to keep running it until it actually dies before I re-power. (Every mile is a bonus for me I have a 2007 4 stroke 8hp high thrust yamie for back up just in case.)

I have some time off later this week so I will take it out by myself with minimum load and get some standardized runs for you to compare. I have a big family and it seems I always have a heavy load of people or gear!

This is C-Val standing by
 
The weight in the boat most likely has a lot to do with it. The early 22's were designed for that 2 stroke 70 hp. You would get about twice the mileage with a modern 4 stroke,and go faster--but probably never break even because of the cost of the new motor. Hang in there and enjoy the boat! When I sold C Pelican about 6 years ago, I discounted the boat because of the older 2 stroke--and it lasted that owner for his entire time he used the boat--and then gave out on the current owner the first or second time he used it--so you never know. There are some 30's motors out there still running....you never know!
 
I think you are doing pretty well for a 30 year old 70 hp motor. It's a miracle it even still runs. Somebody took care of it for sure! For what it is, I don't think the boat is going slow at all considering.

Something to research and consider is that starting in the late 70s to mid 80s, outboards were rated for horsepower at the powerhead and not the prop like motors are now. If this is the case, you don't really have a 70 hp motor at all - more like a 63 (or less) hp motor.......It is entirely possible your 70 was rated at the powerhead. If it is starting easy, runs well, and is getting your boat with 4 guys up to 20 mph that is pretty good. I wouldn't bother even doing a compression check as it is probably just fine.

Also, like Bob said, weight is always a factor. The more folks and gear you put into it, the more power it is going to take to go a certain speed. I bet if you leave three guys at the dock that boat goes considerably faster. You might be surprised that find you are smoking along at 25-30 mph.

As for 30 year old 2 stroke motors OMC built immediately failing with new owners, I suspect somebody doesn't know how to mix 2 stroke gas......LOL......But then again, it doesn't surprise me at all if it had a good mixture and still failed. After all, those things are ancient and stuff does eventually wear out. In general though, those Johnson and Evinrude 3 and 4 cylinder 2 stroke motors were incredible motors that went and went for just about ever. It sounds like you got one of those.
 
Good points about the 70 hp motor. A few of us used to Slalom water ski behind 25 to 35 hp outboards, when there wasn't anything bigger! Those old motors could pull a lot! Maybe I was being too hard on the 70....

However the early C Dory brochure on our web site gives the 22 with a 70 hp Johnson, 12 gallons of gas and two people aboard, in Lake Washington, gives a top speed of 24.3 knots @5000 RPM and 4.6 nautical miles per gallon.
 
being the boat in question is in Canada lest make sure we are taking about MPH and KPH??

I had a 70hp rude on my 16 starcraft and then my 19 starcraft. That was a great motor and is most likely still running some where. Loved that thing. I bought if from the chief MWR mechanic for a air force base in Sac. Just before they closed the base he rebuilt all the motors and then bought them at auction for pennys. It was really a old /new motor and ran very well.
 
Just to make sure we are on the same page, all speeds that I have given are in knots and all gallons US.

Thanks for your thoughts.

(BTW the boat is in Blaine stored with the good people of Blaine Marine Services. I do nexus to use the boat)
 
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