Adding an Outboard to the Swim Platform on a 37 Footer

[sdowney717]

The swim platform is very robust made of 2 inch teak and well attached and supported to the hull.
the boat weighs 17,000 lbs. It is a semi displacement hull 37 footer. waterline length is more like 35 feet.
I was thinking a couple thoughts.

A little gas powered outboard OR

some type of electric AC powered outboard which could get power from my 6500 watt generator.
I think the electric drive is more reliable idea.

Could you get an old outboard leg and put an AC 120 volt electric motor on and how would you control the speed?
How much wattage would it take to push the boat at 3 knots? What HP for the motor? Could the electric motor push the boat at all? 2 knots?, 1 knot?

How much HP gas outboard to push the boat at 3 or 4 or 5 knots? etc...

currently, the boat has twin engines. Running one engine at 1500 rpm pushes the boat to 6 knots. Prop size is around 20 by 21

[Tellie]

You have twin engines, what would be the point of a third hanging off a swim platform that would seem to get a lot of dunkings underway which would rule out an electric motor in my book.?

[sdowney717]

An experiment in fuel savings. Say run the gen set and putter around slowly on the outboard. Also adds a small amount of reliability to get home if the main motors had issues.
I just had 2 break downs involving both motors. On one, the fuel filter failed and let dirt into the carb which flooded the motor.

The other, we ran home on one engine. Motored into a shallow area and went swimming and scraped off some barnacles.
It was shallow enough to stand and scrape. Hull draws 3.5 feet.
When I pulled out the intake might have gotten some mud and it blocked the water intake. I had to anchor and replace the impeller. Or it was just it's time to fail. Took it apart and every vane was catastrophically destroyed like no water flow.

Both motor problems on the same trip. Otherwise it was a great 4th and 5th on the water.

[jaxfishgyd]

Never seen/heard of anyone wanting to put an outboard on a decent size boat..
You would need a big outboard if you were to be serious, a small one could never push a boat like yours with any current or winds.
Plus like already mentioned, it would be dunked if there was any seas...
Your best bet is to maintain the two engines.. Proper maintanance is your best insurance..
I change my raw water impellers yearly.....Main and secondary fuel filters twice a year.....

[micah719]

What is the shaft output of your engines at 1, 2 and 3 knots?

An old outboard leg and prop of a size for that much power would likely be geared and pitched for a high revving petrol engine driving a planing hull, not a displacement hull at sustained low speed.

Wouldn't it be more realistic to look at putting a decent electric motor (or two) on the existing driveline/s? Perhaps even convert to a diesel-electric drive with the option of battery charging from the props, and the genset as a third backup.

Just musings....

p.s. and a redneck-tech yuloh off the stern in case nothing else works. At least it keeps you busy as the quarter million ton tanker is bearing down on you at 20kts, and you even have something floating to hang onto when it has passed......

[zooter]

if you need something more reliable, improve what you got. Better fuel filter system, protection from alcohol, scheduled maint. (impellers, zincs, etc.). Go back and fix the poor fixes you did when you bought the boat. Many boats were built with bad design and hardware, your job is to keep making it a little better. My rule is that after fixing what broke, always make one improvement, each time you use the boat. It would take a 50hp motor to do any good. Bad idea