Downeaster outboard

I am new to this website and I have a question. I am considering building a 31ft downeaster. Is it unreasonable to add an outboard (like a Yamaha High Torque series) on a transom bracket to use for trolling and more so for a get-home engine in case the main engine fails? The boat has a 29.5 ft waterline length and weighs about 10000 pounds. How much horsepower would I need to go at 6-7 knots? Any help is greatly appreciated.

 

Hi vjb,
Adding a small outboard for trolling is not uncommon. For a boat this heavy you'd definitely want a high-thrust version with a big gearcase and big prop. Even so, I wouldn't expect 7 knots with it; 5 would be a more reasonable target for the kicker alone. Someone will surely give you the full calculation later, but my instinct is that you'd be looking at a 20-30hp outboard for what you want to do.
If get-home emergency power is your concern, your kicker has to have separate electrics and a separate fuel system from your main engines. (And you should never fill the kicker and main tanks at the same fuel station! Crappy fuel is one of the leading causes of at-sea engine trouble.) Since your main will most likely be diesel and outboards run on gas, the separate-fuel side is taken care of. The kicker should have its own battery (or better yet, have a rope start as well); if you decide to tie this battery into the boat's electrics, do it with a manual battery switch.

 

Marshmat:

Thanks. Really my main reason is to use is as a get home engine.The boat is going to be powered by a Cummins diesel. The outboard would have to be gas since I don't think you can buy a diesel outboard in the US at least not yet. If anyone has done something similar, let me know.