The engine sitting on spring elements has as many as 6 possible resonances
(3 parallel, 3 rocking around three axis) and the reason for noise and
vibes in the hull structure is all too often that the designer has
failed to choose elements that keep these resonances away from the engines
excitation frequencies, which is normally 0.5x, 1x, 1.5x, 2x and up of the
speed or the hull structure is not stiff enough where the engine sits..
Most engines are fairly silent on lower harmonics but for instance x3 can be
a high level. The shaft in turn has bending resonances, just like a
string of a guitar.
Excitation is here a bit of unbalance and blade passage pulsation from
the prop.
The shaft system, prop to crank shaft has torsional resonances as well.
If you suffer from high vibes, check if it is a broad speed range issue or
a rather sharp span of the speed. For the Vega and the Combi system,
most problems are wrong or aged even loose or cracked rubber elements.
The bed below can also be cracked loose.
Using a soft spacer disk (like most old cars had as a Hardy disk) or
a Centaflex can be nice on a bit bigger ships but focus below 20 Hp
should first to ensure the engine sits correctly. A disk is cheap but a
Centa is not. Make sure you get a correct one adapted to your
drive line components. The designing is normally a free service.
If you like some DIY engineering, and increase your ability to
understand tech sales talk, you could get a program called
ISOMAG (google for it, english or german) and do the
calculation on any ship, small or big and survey all products of
spring elements and shafts/couplings. The soft is very competent
and easy to handle.
I do this as a work on machinery and on a hobby basis, and would
be glad to help anyone in need.
Arne, S/Y Charlotta Vega 239Steve Birch wrote:
