Anyone have ideas on How to reduce vibration from my 5hp kicker?

I need a side project to hide from another bigger project so I thought I might try to tackle the annoying problem of engine vibration from my 20-foot sailboat's 5hp kicker. I think the transom design may be a big reason though I know dampening vibration from a 2-stroke is often difficult...here's a pic of the transom slot...it's on the starboard side...the transom is hollow ..just heavy glass on either side...with the liner providing a "cap". I was thinking of drilling a hole and pouring foam into that whole hollow area of the "inner"transom to dampen the area..but not confident that will work...Anybody out there with ideas or experience with this type of predicament?

 

I think a wooden block bedded in some kind of caulk would work better. The foam would need a bottom built to hold it in place and a hole in the top to pour it.

 

Before I could suggest anything useful, I like to know more about the problem and how it is manifesting itself:

1) Is the vibration being perceived as sound or is it actual repetitive movement that is being transferred to the hull?
2) Is the vibration being noticed in the performance (i.e. shaking and affecting propulsion)?

Vibration can be due to many things
- operating the propeller in a turbulent wake;
- poor primary balance in the engine
- propeller is hydrodynamically unbalanced i.e. nicked or blade damage
- bad fuel mix / poor jetting
- bad propeller selection for task
- bad depth setting of the propeller / partial cavitation

Letting us know more may help get better answers.

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Bill

 

Well...let's see....Gonzo..that's certainly an idea..would seem to have alot of dampening potential...the vibration itself isn't so bad I suppose...no more than most one-banger 2-strokes I guess....it's the cockpit gear lockers that really rattle I just realized... and thats an easy enuff fix......but I guess my question is what can I do to get rid of as much vibration as I can in my boat given the transom design. I guess I'm wondering if the fact that it's a hollow transom makes an echo chamber and more importantly perhaps a "vibration chamber "(so to speak)....like the body of a guitar or upright bass,etc.
I have a brand new prop on there...I got the one that had the least pitch that was available from the Tohatsu dealer in my area....I am easily pushing two tons around in normal use and a few hundred pounds more when I'm geared up for a camping trip so I needed lowest pitch prop I could find...anyways...I probably should just start with the cockpit gear lockers and go from there but I think there's more I can do to make the motoring side of my outings a bit less "rattling"...thanks for the replies...thats about all I can tell you guys at this point...

 

Increasing the surface area of the motor to transom contact point may help - try to put a large plate(s) under both sides of the clamp points, perhaps with a rubber sheet in the mix as well. The rubber sheet may act as a vibration damper, lessening the transfer and the wider plate increasing contact surface. This involves no drilling or filling of the void in the transom.

Unless you are willing to do the job right opening the void may be more trouble than it is worth.

Just a thought. I've got an aluminum plate on my dad's fishing boat doing the same job for the 3hp trolling motor and it worked for me. This is in fresh water, and we don't have the same issues as salt water boats.

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Bill

 

Sounds like a good idea to begin with Bill...at least before more drastic measures...I have only a thin but solid brass flat plate like you see on doors on the inside of the transom and nothing on the outer side or back of the transom so this whole area needed upgrade anyways.
It has been well over 6 months since the boat has been in water and my memory eludes me concerning the vibration now...so it would be best to do any further upgrades only after I have fixed the vibration from the cockpit seat lockers and can isolate the problem accurately as Bistros said...then go from there...in the meantime..I think I will add a wider plate on the inside with some rubber but there really isn't room for any more transom thickness like a decent thickness of rubber would need.....it is certainly a challenge because the bolt clamps on the motor are barely wide enuff at full extension to accomodate the transom.This is not an easy replacement fix since the travel is limited by the motor frame obviously...
Actually if you look at the transom pic above there might be a way for me to remove the liner shell cap in that area only and fair it out flush with some decent cloth and epoxy...that should yield a few millimeters to play with and some extra rubber thickness between the plate and the inner transom where the clamp-bolts tighten...

...thanks for the replies...

 

I agree with bistros on all points. Regarding rubber, viscoelastic polymeres increase the damping. Be sure to place it in the shear-span so it gets activated.
Regards,
Booster

 

Hmm...I'm gonna wholeheartedly implement and agree with what you guys are saying as soon as I figure out what you guys are saying....sorry...I'm afraid I need a little translation...Are visco-eaestic polymers some of Vulcan's children,.i.e.kinds of rubber I take it? And the" shear span" reference may bear translation too if you wouldn't mind...Is that the main shear load area laterally across the top of the transom in this case?I am always trying to learn proper terms but may need some help here....thanks!

 

Visco-eaestic polymers

These polymers provide vibration dampening on the horizontal sheer, rather than in compression like a rubber sheet. They are very effective at dampening vibration, and given the suggested plate will be distributing the point load over a larger area, will probably work better than my rubber suggestion.

Then again, I had bicycle inner tubes around and didn't have the 3M product so I adapted.

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Bill

 

Hi again!

Well, bistros provided a good link regarding the polymers.

Shear Span - For a symmetrically loaded beam, it is twice the distance between a reaction and the nearest load/point:

composite.about.com/library/glossary/s/bldef-s4826.htm

In case of a transom (which more similar to a plate) its little more complicated, but I think bistros solution should work. After you apply the polymer-product do not use the full clamping force you have. Read the instructions carefully. If too much clamping force is used the polymer-product will flip-flap around your entire transom the next day or so!

Regards,
Booster

 

It may be cheaper and more effective to dump the single cylinder 5hp for something with 2 cylinders and a little more grunt. 4-stroke even quieter...

 

Yeah..I agree Raw...a four-stroke or a 2-banger would naturally be alot smoother...I love that little Tohatsu though...she pushes me along real well when she has to...and I don't think I'm ready to part with her yet. I have never had her up against more than a 3 knot counter-current though and honestly I might look into an an 8 hp someday...In the meantime I'll try a few things mentioned above to minimize the "bad vibes"and maybe even give some thought to simply using it less except when I absolutely need to like in and out of tight anchorages....or against particularly strong currents etc...The whole idea of sailboats for me and alot of sailors I think is to take things a bit slower...but still be moving...and maybe having that rattle when the motor is run will simply remind me that maybe it's time to slow down....:idea:

 

Back in the day when I (badly) amateur raced motorcycles, two strokes had about twice the power of a comparable displacement 4 stroke. Your 5HP two stroke banger probably provides as much power as a 8-10HP 4 stroke - at the expense of a lot more vibration (it is a single cylinder with no balanced firing cycle), noise and smoke.

Two strokes generally run at higher RPMs with lower bottom end torque to reach the same HP ratings, so they don't do well at low RPM settings. You've got to get them in their narrow power band, but when they are there they pull like Jack the Bear and are pretty smooth.

I loved running a Yamaha RZ350 two stroke against 4 stroke 600 production bikes. You could ruin someone's day without a lot of effort.

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Bill

 

What you said is very true Bistros...I have read several posts over the last year where people have discussed how there 6 hp four-stroke comes up short in hole-shot power against the 4hp 2-stroke they had before or scenarios like that...My motor has the same body as the 6 or 8 hp I believe I read somewhere ...just different carb mostly and it's also the same motor as all the Nissans and Mercs from those years as Tohatsu made all that stuff up to 15 hp under license...probably still do...BTW..thanks for reminding me about staying in that narrow window of power...and avoiding idling. I also like being able to lay the motor down or in other odd positions and she is easy to carry...I even carried her a couple hundred yards across a mud flat one time last year...not a fun exercise but I was able to do it without cursing my Maker or even any of the Sailing Gods so I guess it could have been worse....

 

Hi!
I still vote for the visco-elastics. Would even cure an old Swedish tändkulemotor (flywheel about 20kg), used on the fishing boats "fiskestånka" around year 1900. One benefit of the tändkulemotor was that it could run on almost anything. Just get it started with a "blåslampa" and it kept going for ever.
Regards,
Booster