View Full Version : Twin Engine Conversion

07-24-2001, 10:05 PM
Can anyone provide insight on converting a single-screw to a twin-screw for a 42 foot wooden powerboat? Primary reason is better manuevering, but a second engine would be nice backup on long coastal voyages. Can it even be done with a single engine? What are the issues with adding a second engine? I am interested in all issues - mechanical, aesthetic, historical, ethical(!) etc. Any good links in this regard? Thanks!

07-24-2001, 10:10 PM
A bow thruster or stern thrusters may be a heckuvva lot simpler and cheaper...and as for reliability...most problems with modern diesels are contaminated fuel...put three RACO filters in series and filter your fuel.
Enjoy.(I am assuming diesel power of course...)

Phil Young
07-25-2001, 01:16 AM
If you had a rear wheel drive pickup, would it make sense to convert to 4wd? It could be done, but better to trade it. Start looking for a boat you like with twin screws. Or install a genset, with a belt drive going to your shaft, a bit of emergency auxilliary power. Fit a bow thruster if you need it.

07-25-2001, 01:37 AM
Single screw versus twin .....
well, on the down side the twin screw/twin engine setup won't be twice as fast but it will have more drag. Of course, for all of that reliability you are going to get a much much faster fuel gage! There would be structural modifications as well as mods to your control system ect. Basically it all sounds very very expensive. On the plus side, you would have that second engine for better reliability but maneuvering a big twin screw boat with only one engine can get very tricky -enough to make you wonder why you didn't opt for a cheaper alternative.

I am with the others in wondering why you would want to do this. As an academic exercise I suppose it has some interest but you'd be far better off supplementing your main power with a 20 HP outboard as an emergency auxillary (get one that uses the same type of fuel; yes you can get diesel power outboards). As to manueverability . . . get a bow thruster or just go out and practice walking your boat between some buoys you set up for the purpose. Nothing is quite so impressive to another power boat skipper as to watch an expert walk a single screw boat into a tight slip.

[This message has been edited by PugetSound (edited 07-25-2001).]

07-25-2001, 07:29 AM
A BIG outboard off a swim platform will get you home. I see you live in Cambridge, Ma---I might be your neighbor as I do too---do you own this 42 footer or thinking of buying her? I am a member of the Watertown "Yacht" Club(really boat club--I just can't think of the 40 foot glas boats we have here as yachts).Call me if you wish---617 491 6140. http://media5.hypernet.com/~dick/ubb/smile.gif

[This message has been edited by norske (edited 07-25-2001).]

Dale Harvey
07-25-2001, 08:44 AM
You will have to be a lot more specific to get any worthwhile information. There are boats on wich this can easily be done. Most of them would be production boats that were offered in both configureations. Moveing props out from behind a massive deadwood on some Lobster/Trawler types can gain actually efficency. Wether or not the hull can stand the weights being higher up and outboard will require more than amature speculation. Moveing weight in a finely tuned displacement/semi-displacement hull can make an unholy roller in a seaway.

07-25-2001, 10:21 AM
I second the bow thruster idea.Most single screw designs are as manuverable as twins with a proper thruster.
Since I dont know the boats design the rest is speculation.
Prop and rudder combinations make a big difference in handling.If your boat is designed as a disp. or semi disp.,the rudder/prop combo will more than likely be correct.If the boat is originaly intended to go alot faster than hull speed,chances are it will have a small rudder and low speed agility will be poor.
You would be surprised at what small well engineered rudder size increases will do for manuvering.
It is very easy to permantly ruin a good boat with single to twin conversations.

Adam C
07-25-2001, 01:09 PM
A single screw boat is a good thing....Your rudder sits protected behind a keel, and similarly the prop spins in a protected aperture. This translates to a robust platform which can withstand damage and groundings. The engine can also sit lower in the boat.

Twins (double trouble) have twin shafts set further outboard, and are usually unprotected unless skegs are added forward of the props. The rudders are similarly fragile.

Engines are placed necessarily higher in the hull and therefore are a detriment...more weight added higher above the waterline is a bad thing.

Get a bow and stern thruster.