vibration problem with new motor

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storm63

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I've searched this forum and haven't found anything quite like the problem I'm having. I'd be grateful for any help or pointers to relevant information I missed.

I have a 1994 Lund Alaskan that worked great with the original 50 hp Yamaha 2-stroke. Sadly after 24 years, I had to replace the outboard last month. I bought a 50 hp Tohatsu 4-stroke. The boat runs great at planning speeds, but has a lot of vibration at lower speeds. I primarily troll, so this is a major problem. It vibrates the floor and pedestal seats so much it is unpleasant to be in after 1/2 hour of trolling.

The installer said the transom was hollow when he drilled the holes to mount the motor, so added cross bars for support. Clearly I wish he'd given me a chance to rebuilt the transom first, but the stern does appear to be sturdy.

Also, many of the rivets in the floor have come come loose and probably should be fixed.

I know the boat needs some work, but I don't want to put a lot of effort in and still end up with the vibration problem. So, I am seeking advice on what I should do for the vibration problem:
- take the outboard off and rebuild the transom with something like CarbonBond that I've seen mentioned elsewhere on this forum
- replace the worn rivets in the floor
- buy a new boat that is built better for the new 4 strokes
- other ideas?

Thanks!
 
All motors will have a harmonic range that can build up vibrations. I would have though Tohatsu would designed them to occur in a range above typical idle/trolling speeds and lower than planing speeds.

You could try adding rubber sheet over the transom anywhere where the motor contacts it.

I used CarbonCore for my transom rebuild, but the area to filled must be made nearly water-tight, or it would leak out. You may not need as much structural, so a std wood transom could be easy. Otherwise Seacast is a less expensive ‘pourable’ transom filler.
 
How's the prop setup? Could you go down a pitch or two?

Less pitch may let you increase the RPMs a little bit while trolling and lessen the vibration.
 
Prop could be damaged too ...

But I'd verify you have the correct pitch for the RPMs achieved, before I'd swap it out. Run your boat, motor, and amount of gear always carried through this calculator before you do anything: https://www.forum.tinboats.net/viewtopic.php?f=4&t=37433
 
Which 50hp did you get? MFS50 or MD50? 2 totally different motors...both 50hp.

The MFS50 is a 3 cylinder 4 stroke. 3 cylinder engines all have an inherent vibration. Some of them are balanced better than others, and yet some incorporate a counterbalance shaft. I don't know Tohatsu's 4 stroke 50. I know Yamaha's 50. It is silky smooth but it's an inline 4, and not an inline 3 cylinder. The only 3 cyl Yamaha now is the F30 and F40, and neither one of them are rough running either but not quite as smooth as the F50. The F50, F60, F70 are so smooth and quiet that the loudest thing you hear at idle speed is the pee water hitting the lake you're sitting in. And no you can't hear it running at idle, nor can you really feel it.

Perhaps Tohatsu made theirs a 3 cylinder and omitted some features that would otherwise smooth the engine in favor of lighter weight? I do not know. Again, I haven't looked up the specs on them to compare. Yamaha left the counterbalancer out of the '17 and newer F25's to get rid of about 10-12 lbs of weight...but it came at a cost of a little more vibration. That's an example only.

Prop ok? Hub not spun? Blades not bent? Prop shaft not bent? Being new, I wouldn't think so, but always worth checking.

Just watched some youtube videos...yeah...that MFS50 is a little rougher running than I'd like too, based on some of those videos.

Also some hulls will exhibit resonant vibrations at certain vibration frequencies, and perhaps yours is like that. Dad's old flat bottom was like that and it was nearly unbearable at a certain RPM. Even mine does it at about 1100rpm and it's a good hull. I just had to learn to "drive around" the shake (keep it below or above 1100). For trolling sometimes it would unavoidable but I don't troll much.

It may be just inherent in the design of the Tohatsu.
 
Thanks to everybody for the quick responses.

The prop is new and there is no sign of damage. I assume the suggestions about changing the pitch is so I could troll at a slightly higher rpm and minimize the vibration. If so my problem is the vibration occurs pretty much at any RPM below planing speed. It doesn't seem to matter if I troll at 800 RPM or 1200 RPM. I am trying the prop selection tool DaleH pointed out. That's pretty cool, but it said my configuration was outside their products. They are supposed to email me a recommendation.

I think my first course of action will be installing a rubber pad under the motor to see if that cuts the vibration to a tolerable level. I'm reluctant to go to the effort of rebuilding the transom if that may not resolve the problem.

I am concerned that TurboTodd might be right and this motor/boat combo is a problem. It is a MFS 50. I never expected a new 3 cylinder 4 stroke would vibrate more than a 24 yr old 3 cylinder 2 stroke, but here I am. The boat is also very light for it's size and seems to be magnifying the vibration.

Thanks again for the input
 
FWIW I did a google search on 2-stroke vs 4-stroke OB motor vibrations and 4Ss definitely vibrate more!

Some report switching to a cushioned hubbed prop works, where others play with or push through these RPMs, etc. and then OB makers like Suzuki claim how they use 2 different durometer runners in their isolation mounts to cancel out the vibration from the 4S drive train.
 
NVH. Noise/vibration/harshness.

Some manufacturers go a little further to reduce NVH, others compromise NVH for a more competitive price point, or perhaps compromise NVH for lighter overall weight.

Why does a 4 stroke typically have a little more vibration at idle? Power pulses are further apart. A twin cylinder 2 stroke will fire a cylinder every 180° of crankshaft rotation. A 4 stroke twin will fire a cylinder once every 360° of crankshaft rotation and because of this, there's a little bit of a shake. A 4 cylinder 4 stroke will fire a cylinder every 180° of crank rotation-exactly the same as a twin cyl 2 stroke-and they feel real similar in the vibration department. That's one reason a 90° cross plane V8 is the engine of choice for luxury pickup trucks.

A 3 cylinder 4 stroke engine has one power pulse every 240 degrees of crank rotation. (figured by 720 degrees-which is a full cycle on a 4 stroke-divided by the number of cylinders). A power stroke only lasts about 90 degrees of crank rotation. There is 180 deg of crank rotation from TDC to BDC. So, if you think about it, an inline 3 will fire a cylinder, the gases push the piston down the hole for about 90-120 deg of crank rotation which accelerates the crankshaft rotational speed, then the exhaust valve begins to open somewhere around 110-120 deg, and then it's got to "free" rotate (and rotational speed slows) another 120 deg of crank rotation until another cylinder does some more work. In that "free" rotation, the crankshaft wants to slow down a little. A heavier flywheel aids, but weight is not a good thing to have on an outboard. Manufacturers a lot of times will cut flywheel weight down but that in turn increases vibration at idle speeds. Tractors...using all sorts of inline 3's, usually have a flywheel that weighs between 70 and 200 lbs....not only to smooth the engine, but also to aid in engine "torque" as some people call it...which is simply adding to the rotational inertia by reducing the acceleration and deceleration rates of the crankshaft under a load.
 

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