On Aug 1, 4:44 am, "Marc" wrote:
....
My main objectives are :
- having a backup engine with which she should at least run 4 kn without
having the need for double propellors (too much drag it's a fast sailing
yacht nog an moto sailor ;-)
....

Well, I don't want to get into the my boat is faster than... ALL
sailboats are slow. Still, I do have a cruising boat with top speeds
right around twenty knots under sail and it has two engines and two
props. It also has two rudders, two dagger boards and two hulls...
An interesting side effect of having two propulsion engines is that
the props are sized to WOT with both engines running and are thus
"over propped" when only one motor is running. I think this is good
since it means that I can run one engine slowly and still have it
loaded up when motoring through a calm. I don't have a pyrometer on
the engines and they are too small for commercially available flow
meters so I can't really claim to know if the theory really works, but
I run on the assumption that it does... Anyway, it might be fun to
explore the differences in installed cost of twins vs a single and bow
thruster. I think it would be easier to get home on one twin than on
one bow thruster . Another possibility would be to put in a
continuous duty drop down style bow thruster on an "Z" drive. Your
genset could then run that as a "get me into the dock now that I've
sailed home" engine.

-- Tom.