Vibration in specific location in hull

[SuspectDevice]

Hello.
Might have a chance of buying an Antares 710 with Volvo Penta TAMD31P. I need some help with it. The current owner has pointed out that it has a vibration in the hull that is in a very specific location - above, water, starboard side, just behind the cabin, right at the moulded step. If pressure is applied to the place where it happens, it goes away. It happens between 2700 rpm and 3000rpm, and is not present at lower or higher rpms. The engine is not vibrating. The propeller, drive shaft etc have been checked and all are ok. Engine running smooth. Hull, prop and rudder are clean.

Is there any reason why the vibration would be so specific, and what could be the cause?

Any help greatly appreciated.

Best....

SD

 

[DavidJ]

Your engine will be transmitting vibrations of increasing frequencies as the revolutions increase.
I suggest you have a loose something which will have a natural frequency of between 2700 and 3000 (or some multiple of that) as you said.
If your totally sure that the engine and transmission isn’t excessively vibrating, then its just a case of finding the loose bit (which may not be easy or the owner would have fixed it)
Its always recommended to have a hull and engine survey before buying anyway and this will put your mind at rest that it’s not something serious.
Credit to the owner for pointing out the problem.

 

[westernman]

Big ships generally have a red band on the rev counter which they avoid operating in because of undesirable resonances.
See 8 minutes 8 seconds into this video

 

[penfold]

If there's access apply a pad of sound deadening material to the inside of the surface; it vibrates noticeably at a certain rev range as that matches the harmonics of the surface, to get rid of it change the harmonic by dampening(add a pad) or stiffening(add some foam ribs and glass in) or softening(grind/sand away some of the grp(not a good plan here obviously)).

 

[Portofino]

It wouldn’t have harmonically resonated when new .
Its one of the issues I keep highlighting with boats built with liners or adhesives fixing grids or what ever .= failure of the adhesive system used .
Its this what happens when something comes literally unstuck ? You are left with unsupported areas and stuff like this happens .
As mentioned ^^^ larger areas of flappy plastic emerge .

 

[SuspectDevice]

Won't quote all the replies. Thank you all for the replies.

The owner took it out this morning with a marine engineer/boat surveyor and a mechanic. Everything engine. drive and prop is perfect. It appears the issue is down to failure of adhesive between two panels on the starboard side causing it to resonate at a particular RPM. Likely it can be solved by cutting a small panel out and applying some packing or fixing some wedges to it as the problem goes away when you press against the area where the noise comes from, or when one pulls slightly on the upright piece that supports gaff storage.

Now to see if ther other interested party are deterred enough for me to step in.

Thanks again everyone.

Take care.

SD

 

[DavidJ]

Sounds a bit of a codge but you could drill a small hole and squirt in some expanding foam, the stuff they use for fitting double glaze windows (and sometimes they use only this!!!!!)

 

[penfold]

Won't quote all the replies. Thank you all for the replies.

The owner took it out this morning with a marine engineer/boat surveyor and a mechanic. Everything engine. drive and prop is perfect. It appears the issue is down to failure of adhesive between two panels on the starboard side causing it to resonate at a particular RPM. Likely it can be solved by cutting a small panel out and applying some packing or fixing some wedges to it as the problem goes away when you press against the area where the noise comes from, or when one pulls slightly on the upright piece that supports gaff storage.

Now to see if ther other interested party are deterred enough for me to step in.

Thanks again everyone.

Take care.

SD

If it as you describe a hole or two drilled(in an inconspicuous place if possible) to allow PU adhesive sealant(sikaflex or similar) to be injected into the unbonded area would solve the issue, then the holes epoxied and gelcoat to hide the evidence.

 

[simonfraser]

have done similar with sika
vibration between a floor and bulkead, jammed in some thin wooden wedges
sika in the gaps
took the wedges out
more sika
end of prob