Tom Cat 24 resonance at low RPMs

C

Coastal crew

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May 26, 2015
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C Dory Year
2001
C Dory Model
24 Tomcat
Vessel Name
Salty Roads
Hello all,
I've been looking forward to this day for sometime, becoming a C-Brat that is. Well I'm hoping to. I sea trailed a 24 Tom Cat over the weekend and found that at lower RPMs a loud resonating sound was coming from the transom area. In fact there were two. One sounding like the boat had a diesel motor thumping away and another at higher RPMs that sounded like a high pitched droning sound, like the props of a small plane, for reference. There was definitely a sweet spot at about 4000 RPM and she quotes right down.

Can anyone with a Tom Cat or experience on one comment on this.

Thank you

A soon to be budding brat
 
I have ridden in several Tom Cat 24's, and didn't remember any sound/vibration as you describe.

Was the cabin door open. We usually ran with the door closed.

I would try running the boat at an idle on one engine, then the other, so see if it is isolated to the engine.

Is there any possibility that there was some hull slap from chip?

I would check the transoms for any deflection when you put your full body weight onto the lower unit, with the motor trimmed all of the way up to the trailering position.

Welcome aboard!
 
Hi Bob,

The sounds only occurred while under power and not at idle. Closing the door definitely helped but the cause is my concern. I will be having the hull and motors surveyed next week and will hopefully get some professional options, though the surveyor will likely not have any experience with the a Tom Cat.

Unfortunelty the water was flat calm the day we were out. I had hoped to go out in some chop to see if there was tunnelling or slapping. I have a 18 Nautico and am experienced with hull slapping in rough water.

If I understand your last comment correctly I will have to wait to pull the boat to check for deflection in the transom.

If you have any other tips or comments they would be highly appreciated.
 
I would check the props (type, size, condition, connections, installation, etc) and check lower unit fluid levels. It is nice to have a second person driving and you can listen then trade positions,

Let us know the outcome
 
When the RPMs are closely matched on twin engines, one can often hear the "beat frequency" of the two engines. It's usually not that loud but one can hear it. The frequency is equal to the difference in RPM's between the two engines and will vary if you independently adjust the RPM's of one engine.
 
Motors are 2000 vintage 90 hp Honda's. One looked to be running a little rougher then the other. The other odd time was that the throttle controls were wrong, the left starboard control controlled the port motor and vise versa.
 
Glad to hear you are looking at a 24' Tomcat. We love our boat and I would be happy to answer any questions you might have. We purchased ours about 18 months ago and we now have most of the kinks worked out.

We have relatively new (400hrs) Honda 135's. They don't really make any noises at low RPMs. I am guessing that you need a tune up or something.

Good Luck!
 
I have Suzuki 140's on my TC. The only time I get those kind of sounds is when the RPMs are out of sync.

There is a difference between a marine surveyor and marine mechanic unless you are getting one that takes on both resposibilities. If there is a question about the engines, get a marine mechanic to run through it. Will be money well spent if even just for the piece of mind.

Feel free to ask any questions you may have.
 
A marine mechanic from Honda checked the motors out and gave them a decent bill of health. Aka there is nothing mechanically wrong with the motors that would cause the vibrations and resonance. He did suggest that it could be a result of not having counter rotating motors.

Should these be counter rotating?

If I cannot figure this out I will likely walk away from this boat. Thanks for all your help.
 
We have a 2003 with the Honda 90's and our engines are NOT counter rotating.
 
I'm not as familiar with this engine size range as many others here, but I'm not sure there are any "smaller" engines that are available with counter-rotating props. I thought that started around 150hp or so? I imagine there may be some Tomcats with the larger/counter-rotating engines, but this would mean that all 22's (for example) with twins do not have counter-rotating props. So it must be possible to have them and not experience issues.

I have been out on a 22 with twins a few times, and I "have" to get the engines synchronized or it bugs me; I would describe that as an RPM hunting type sound (when they are not quite synchronized). I also find that the prop on my (single main engine) 22 will "sing" a little at certain low RPM levels. That sound changes with different props.
 
I have 135's that ARE counter rotating?

Do you have an audio recording you can share? If so, you can PM me and email me.

While I am sure that my engines to make a "bad" sound, they do make a different noise when you put them into gear at low RPM, than they do at a higher RPM.

If you have something I can listen to, I can tell you if it is similar or not to my boat.
 
Aha, so it goes down to at least 135hp.

Still probably no 22's with twins have counter-rotating props, so it must be possible to have a properly functioning twin setup without them. 'course the TomCat ones would be further apart.
 
Well it looks like I'm going for it!!! Buying the boat that is. I spoke with a very knowledgeable person (my apologies I have forgotten his name) at E.Q. Harbor Services. He provided much information on the boat and based on the information I provided him he figures it's likely a prop issue. There are new SS props and the old aluminum props which I'm going try. Figures crossed! The motors are over due for a tune up which will be completed in the near future. Hopefully this helps as well.

Looking forward to my future cruising of the PNW.
 
Congratulations! I'm going to guess you spoke with Les at EQ. It's always so nice to have good information, isn't it? Much better than just wondering.

Yay new boat! :cake
 
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