Diesel to 4 Stroke outboard conversion in displacement hull.

[Razorcanfly]

The plan would be to basically have a hole cut through the hull forward of the transom with a we’ll to mount the outboard and have it boxed. Reshaping of the hull aft of the prop would need to happen to reduce drag. Possibly a tunnel through the transom.
I wouldn’t consider a transom mount due to issues with keeping the prop at depth in high swell.

[a64pilot]

You can do it, exhaust will be an issue, yes most is under water, but if you look at idle you will see the exhaust occurs above water thru a separate hole, the motor wouldn’t like the backpressure, underway the prop actually forms a slight vacuum and the exhaust exits thru the hub, quiet brilliant and I think a Kiekhaefer design back in the 60’s? Maybe earlier. Brilliant man, really.
Unless the existing engine is completely knackered I wouldn’t consider it. But yes a 9.9 and definitely a high thrust

On edit Kiekhaefer marine and 1957
https://patents.google.com/patent/US4511339A/en

[rbk]

If you’re going that in depth with those requirements I would seriously consider a Yanmar 50 diesel outboard $$$ but might fit your needs perfectly while maintaining decent fuel economy. You can expect 1.5-2 gph Or about 1L/km at cruising speed with gas and severely reduced range.

[Razorcanfly]

Would the fuel consumption figures be improved with a fuel injected outboard rather than carbureted?

[malbert73]

Yes, a 4 stroke EFI will burn the cleanest and I believe be the most efficient. I have a 15HP EFI Tohatsu for my dinghy and so far love it.

[Q Xopa]

Originally Posted by Stu Jackson

That sounds incredibly high for fuel consumption.


1.5 litres per hour = 5.68 gallons per hour


I have a 21 hp in our 34 foot boat and get 0.5 gallons per hour


It appears you may have missed a decimal place.


You could compare this lower and, most likely more correct figure, to the fuel consumption of whatever you plan to use.

I think your conversion is the wrong way around.

To make matters more confusing there are Imperial and US Gallons.

The story goes the US Gallons came about after a mess up with the dimensions of an oil magnates large order for 44 Gallon drums. Instead the US Gallons was born, as a smaller volume to fit the barrels instead of redoing the barrels.

1 Imp Gal is around 4.5 Litres, or

1 US Gal is around 3.9 Litres.

[rbk]

Originally Posted by Razorcanfly

Would the fuel consumption figures be improved with a fuel injected outboard rather than carbureted?

Those number are for 4s efi

[slug]

Originally Posted by Razorcanfly

The plan would be to basically have a hole cut through the hull forward of the transom with a we’ll to mount the outboard and have it boxed. Reshaping of the hull aft of the prop would need to happen to reduce drag. Possibly a tunnel through the transom.
I wouldn’t consider a transom mount due to issues with keeping the prop at depth in high swell.

Sounds good

You should do some googling for other well mounted outboards . Perhaps find tricks for air supply and exhaust issues
Also what shaft length to choose needs some thought .
Longest shaft sounds logical ,but you never know

Others have used well mounts , should be some info on the internet
Boats like Florida commercial mullet skiffs have long used forward well mount outboards

[smj]

Yamaha has a new high thrust, extra long shaft fuel injected 25hp that may do the trick. I believe it swings a large 12” prop and weighs less than 140 lbs. Would work great on a displacement hull.

[Stewie12]

I had a boat with a well. Worked great as I could lift the engine out to reduce drag. Problem was fumes, so a long shaft on the stern would be a better idea. And when not in use pull it up. Getting rid of the current diesel engine weight will improve the yachts performance. I would only use an outboard hp that would give me the current cruising speed. Probably 15hp max to keep the weight to a minimum.

[AndyEss]

The original post doesn’t mention if the present diesel engine is running or not.

Seems like an awful lot of expensive work including an outboard well, ballast tank, etc. Do you have a buyer for the diesel, tranny, prop shaft and prop? I suspect none of this will add value to the hull, and in fact probably decrease the value.

What is the boat market like in Oz? Maybe consider selling this boat and buy another more to your liking?

[tkeithlu]

I would think carefully about taking on new problems as a result of avoiding old ones. You want to avoid noise, intrusion, and limited access. You take on roughly 1/2 the per wine bottle mileage, gasoline in the boat, added drag from the foot/three blade prop, a hole in the hull shape, and maybe a few none of us have thought of. Oh. Real fun when a wave comes up the well into the cabin. Where does investing in some sound deadening panels for the inside of your engine box fit?

[Cadence]

Originally Posted by Stu Jackson

That sounds incredibly high for fuel consumption.


1.5 litres per hour = 5.68 gallons per hour


I have a 21 hp in our 34 foot boat and get 0.5 gallons per hour


It appears you may have missed a decimal place.


You could compare this lower and, most likely more correct figure, to the fuel consumption of whatever you plan to use.

Stu, I think you have the decimal point in the wrong place?

[fourlyons]

We have two Yamaha 9.9 High Thrust outboards in a well pushing a displacement hull. At least with these engines all the exhaust goes out through the hubs, no exhaust in the cockpit. About 3/4 of a gallon/hour at 3/4 throttle.

They are lighter, cheaper, less smelly than the diesels of my experience.

They are a pain in the butt if there's a lot of floating grass/kelp, you end up having to clear them regularly. Mine pivot up into the well allowing me to clear them. You may want to make this possible in your application.

[fourlyons]

Forgot to mention, we had a Honda 7.5 on a previous sailboat, 21 foot waterline, 7000#. We could do 5 knots in ideal conditions.