V-drive gear ratio

[steve34cat]

I am currently building an aluminum power catamaran. It was originally designed for outboard motors, but I'm thinking about changing to V-drives. I have Yanmar 6LP diesels to put in it. I have no prior experience with V-drives. I need to know how to engineer gear ratio. I have a friend that says he has a matching set of Borg Warners at 1 1/2-to-1. Also, I am considering V-drive vs. stern drive. I hear the Bravo II are good. Is this a good option, or am I getting into a mess I don't need? Any input would be appreciated.

[FAST FRED]

I need to know how to engineer gear ratio.

You need to know the overall ratio the vessel needs , and simply accomidate whatever the V drives are in the engine transmission ratio choice.

Conventional drives are usually cheaper to create and operate than drive leg deals.

Your ability/need to take the ground should be part of the plan.

FF

[steve34cat]

I have always had conv shaft driven 10 knot boats and have heard so many bad stories of out drives but, on this cat I'm building I do'nt have the room so it is either v drives or out drives and I'm trying to make an inteligent choice and this boat with yanmar 6lp will srew up 3800 rpm's what are your thoughts

[Aliboy]

The Bravo 2's (X series for diesels) are pretty good drives, but like all boats with out drives you will need to pull the boat every 6 months to inspect, clean, and also do oil changes if you use the boat regularly. Out drives are generally much more expensive to maintain than shafts. If you have the room I would personally go for the V drives and conventional shafts unless you are going to be beaching the boat or running very shallow waters.
For the gear ratio you probably want to start with getting some prop design work done. Once you have some idea of the best probable prop diameter and pitch you can iterate the process to fit with the available shaft diameters, shaft angles, prop to hull clearances, and gear ratios. At a guess, you would probably get a decent solution working around a pair of 1.5:1 ratio gearboxes, but it should all start with getting an idea of what a good prop for your rig would look like.