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I have this boat with a 1998 150hp Evinrude 2 stroke on a metal jack plate. It was purchased this way three years ago. I recently went from a four blade 16" prop to a 3 blade 17" Stiletto. Perfect prop but it caused this weird vibration/ hum harmonics from 3000 rpm to 4500 rpm in THe fine folks sent me 4 blade 17" and the problem mostly went away but now it is back. I wonder if it is caused by the jack plate, lower unit, something shaking in the hull or the prop. I can't find anything loose but it is hard to check while underway. Anyone have an answer? Thanks!

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I've had more vibration with four blade props than three blade props. This is because prop with even number of blades will have pairs of blades pass through the "shadow" of the gearcase at the same time while props with odd numbers of blades send only one blade at a time behind the gearcase.

The jackplate could be making things worse if there is any slop in it. Likewise, the outboards motor mounts could be getting worn and soft.

But it may just be a characteristic of the boat/motor/jackplate/prop combination.

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I should add that any time you have a prop/vibration problem, it always a good idea to check the runout on the propeller shaft to see if it bent in any way.

What type of propeller hubs are you using in your props?

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Tom:

The runout looks good. The hub is the one that came with the Stiletto prop- a hard rubber insert. Everything went on correctly-I spoke with the prop tech as I was doing it. Could it be the large diameter of the prop? The boat came with a 4 blade, 16" prop that was a smaller diameter. I tried a Stiletto 3 blase but the vibration was unbearable-especially when turning left. I haven't tried any other props and I hope I can find the cause because I love how this prop works with this boat. The jack plate seems fine but you never know just from looking-it doesn't look like it's shaking while under way.

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Any motor needs to run within the manufacturers suggested range.

What are the RPM's of the engine at WOT?
You should be within the range of engine specs.
Many engines have a max RPM at WOT of around 5k to 6k as a example.

A prop is selected by the above along with a few other things like a bow lifting prop, stern lifting, etc.... However, the max operating RPM's must still fall within the manufacturers suggested range.
Otherwise, you will be under-propping or over-propping your motor and that isn't good and could cause damage, excessive wear, poor performance, and possibly vibrations.

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Max operating RPM's are 5500 and that is what this current prop runs @WOT. The 16" overrevved. This is the correct prop for the engine spec wise, I just wonder if this is a Stiletto made unique problem?

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Well, your Stiletto is the only Stiletto prop I have ever heard of vibrating so I doubt it's a Stiletto unique problem. Furthermore you report the vibration occurs with the three blade as well, so a freak defect seems extremely unlikely as it would have to have coincidentally happened to you twice.

I am further perplexed by comments you made in November about this prop elsewhere when you noted that:

I talked to the super folks at Stiletto and they are awesome! They sent me a Bay Pro III, four blade, 17" prop to try. I did that today and the results were perfect. No vibration, stellar hole shot with almost no bow lift, tight in turns and fuel economy very close to the three blade. Low rpm's keep it on plane too. This is the perfect prop for this boat. I believe all the suggestions of trim tabs and doel fins, etc. can be eliminated with this prop. The folks at Stiletto are incredible and so is this prop!

So, what has changed between then and now?

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I don't know. That is what I am trying to find out. It certainly has reared its ugly head again except the vibration/hum is less pronounced than the 3 blade. Good point on the freak defect. I am just trying to find out if anyone has had this happen to them and how they cured the problem. What would be the best way to trouble shoot this?

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Man, this is one of the more interesting threads I've come accross. Thanks Tom for providing the insight as to the Harmonics or "shadow" from the engine when using an even bladed prop.

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Would the motor trim tab help in creating this problem? The way it is angled now it steers the motor so the boat turns slightly right and it takes two hands to turn hard left. Also-the vibration is worse when turning left and goes away when turning right. Yes-trimming it up softens the herd to turn left problem but not the harmonics problem. Just looking for thoughts. Thanks!

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The trim tab is generally adjusted so that turning in each direction requires the same amount of force when you are at your normal cruising speed.

The trim unfortunately will always be different at lower or higher speeds than your normal cruise speed making it easier or harder to steer in one direction or the other. This seems to be the way it is.

So, set it up so it takes the same force in each direction to turn the wheel when you are at your normal cruise speed whatever that is.

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Joe is exactly correct; the trim tab should be adjusted so there is equal steering effort in both directions. That is what it is for.

In your case, you need to move the trailing edge of the trim tab to starboard to make the steering more balanced.

But to answer your question, no, the trim tab will not be affecting the vibration.

I think that since you have the motor on a jackplate, the next thing to try is lowering the motor 1/2 inch.

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Check the flywheel magnets.....you may have an issue with loose or broken magnet on the outer rim of your flywheel.

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Thanks! I will start checking all these things.

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Well, I adjusted the trim tab this weekend; to the right, halfway between center and maximum allowed angle. It reduced the harmonics some. I keep the trim down and the vibration is less but the steering is hard to turn to the left. When I trim it up the steering loosens up but the vibration increases and the boat tends to porpoise. I do fiddle with the jackplate height also.I guess the next step is putting the trim tab maximum right and live with it. I just can't figure out the cause of the harmonic vibration short of taking the jackplate off. I don't know how to begin checking the fly wheel magnet. Kinda weird.

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Well I took the boat out this past weekend and it seems the harmonic vibration is most noticiable from the hull at the port side of the reversible seat. It seems to come from that area. The tone changes when bouncing over waves so I'm pretty sure it's the hull making the vibration. I also moved the engine trim tab over halfway to the right but unless I trim it up so the boat porpoises, it still is very hard to turn left. Don't know what my next step is.

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Sounds like what you have is a resonance - the prop rotation is hitting a frequency that's causing something else to ring like a bell. To fix it, you'll need to either change the freqency of the prop rotation, or find out what's "ringing" and damp it.

When my wife complains about something rattling in the car, I first tell her to clean all the crap out of it, so that's my first advice - remove all loose items from the boat and see if it helps. If not, then try some damping - if you have it localized to an area of the hull, try stuffing some cushions or life jackets up against it and see if that helps. Or duct-taping a couple of old magazines may help if it seems to be a hull panel that is vibrating. Once you have found what's ringing, you could try a permanent fix some sound dampening mat like that used by car stereo installers.

Good luck! Sounds frustrating - you'd hate to have to mess up the performance with trim tab or other adjustments just to get rid of the noise. Our Aquasport has had a resonance since new, it's only just above idle so it's easy to avoid.

Tim

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That is interesting. I'll try it! Thanks!

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It is not the prop. I ran the original prop that came with the boat when I bought it three years ago and, though less noticeable, the harmonic vibration was still there. By reason of deduction-I have come to the conclusion that it is the jack plate, even though it seems tight and strong. It did not do this when I first bought the boat. I wonder what happened?

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Next thing I'd check is the engine mounts. If they're going bad, could be letting the vibrations from the engine pass through to the hull....

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I know this is a dumb question but by engine mounts do you mean the ones that bolt to the transom or under the cowling and how do you know if they have gone bad?

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No dumb questions, glad you asked. Click on the link below to take you to the Evinrude parts site. In the left column, click on the year, horsepower, and specific model of your engine, then select Midsection. You'll see an exploded parts diagram and I believe it's parts # 3 and 6 you want to look at.

http://shop2.evinrude.com/Index.aspx?...p;siteid=1

I've never had a bad mount, but understand that sometimes they can be checked by watching the swivel bracket/steering arm area for excessive movement as you power up and then power off the throttle. You can also try pulling back on the lower unit to try and move the engine and check for loose mounts, but I think they'd have to be quite worn to see any play using this method.

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Thanks so much. I will check and see what I can find.