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· Telstar 28
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A couple of problems you may run into with using an outboard, even a long-shaft version on an outboard bracket, is that most boats will have a problem with keeping the prop of an outboard, mounted at the stern in the water. You might be better off using an outboard well, a bit further forward. Not only would this prevent having the 100 lbs of outboard hanging off the very stern of the boat, but it would probably allow it to grip the water much more efficiently. A marine surveyor or architect can give you a better idea of how well the boat would work with a stern-mounted outboard. The stern would probably also need some structural reinforcement to handle the weight and power of the outboard.

I would strongly recommend that you go for a four-stroke design, as they are quieter, cleaner, and more fuel-efficient than the two-stroker varieties.

You don't mention the length, displacement or size of the vessel in question, or how large the Yanmar diesel that you are looking to replace was. The horsepower to boat size ratio of diesel engines to gasoline engines isn't quite a linear one, as the diesel engines generally have a lower rpm and higher torque curve than the gasoline engines.

Most outboards have a very limited selection of props they can be used with, and finding one that has the appropriate pitch and power rating for your boat may be an issue.
 

· Telstar 28
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While the electrics can be a problem, especially if you previously had a high-output alternator attached to the previous engine...I don't think that is the case here. Some of the four-stroke motors have a decent alternator on them. The Honda 20HP I use has a 15 amp alternator, and that's generally all I need, since most of my power is suppied via two solar panels.
 

· Telstar 28
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The other problem with having an inboard that is non-functional is the additional weight it adds to the boat. Same with having an outboard attached to a boat with an inboard. A good four-stroke outboard is several thousand dollars, and that may go a long way to fixing the inboard engine.

My boat was designed to use an outboard, and as such, allows the engine to sit low enough that the prop coming out of the water is almost non-existent as a problem.

The other problem with an outboard on a sailboat is how to control the throttle and steering on the outboard. That can be a huge problem if the outboard is mounted low enough to keep the prop in the water...
 
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