Engine mounts

[SmileyGiley]

Recently looking at some potential replacement tubs...

One vessel I saw had a largish Thornycroft mounted pretty much directly onto the fibreglass engine stringers. I'm used to seeing hefty rubber mounts, this boat didn't seem to have any visible (though there was a suggestion that there was a thin rubber insert). I was also told this is quite a usual installation for small commercial craft.

Any comments, people???

 

[TheBoatman]

Giles
It's not unusual to have engines in commercial vessels bolted directly to the engine bed or sole plate especially if vibration is not a consideration.

In engineering terms it's much better for alignment and FWIW I think that the vibration question doesn't really arise, I've skippered many boats with single & twin engines upwards of 500Hp each and you couldn't feel any real vibration.
With the engine bolted down "hard" you could get everything lined up so that when you give her a small burst astern the engine didn't rock backwards & forwards on the mounts cocking up the alignment.

Peter.

 

[boatmike]

Yes that's absolutely right, plus the fact that larger engines with 4 cylinders or more can often run rather more smoothly anyway. It is usual to have a hard rubber block, not very thick, to absorb some vibration and take up any unevenness in the bearers, but the more power you have the harder the mounts need to be anyway to stop the increased torque twisting it out of line. You do still need a flexible drive coupling to the propshaft of course but otherwise check if it runs OK and doesn't shake the fillings out of your teeth. If so probably OK!

 

[mbay]

my engine is rigidly mounted,(sabb 18hp) yes there is vibration, I've just learned to live with it.

 

[SmileyGiley]

Cheers for the responses. I guessed it was something to to with smaller engines have fewer cyls and therefore more vibration.

I was recently in Singapore, Pan united shipyard. I had to ordered some pneumatic mounts for a rack of computer gear I was fitting on the survey ship I work on. Local purchase order from the RX catalog... no need for manager approval. Two days later I am handed shipment papers for rubber mounts priced at USD 50 THOUSAND each. Holy CR&P!

Oh - seems the main engines were being re-mounted. Mine came in a couple of days later at about a hundred bucks apiece.

Cheers

SmileyG