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Old 11-04-2020, 10:44   #46
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Razorcanfly View Post
Yes, that is full achievable speed in calm seas and running around 2100rpm.
Most cruising is around 4.5 knots at 1700rpm.
One more thing:
If you do not want to damage your diesel, use it no less than 60% of load or 75% of rpm (it is not petroleum!). Propellers are designed to be most efficient at the max rpm of the engines.
Also, I did not understand what you mean about "4-stroke". Do you mean petroleum? Do you know, that 90% of diesels are 4 -troke? One exception I know - Detroit Diesels.
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Old 11-04-2020, 13:55   #47
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Increased speed is definitely not on the agenda. I want win back some of the excessive room dedicated to the Diesel engine and address the noise issue. Undoubtedly my fuel economy will suffer, but I have a lot of wiggle room there. I consider even 25hp is excessive, but Iím after fuel injected 4 stroke and havenít seen it in smaller outboards.
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Old 11-04-2020, 14:11   #48
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Razorcanfly View Post
Increased speed is definitely not on the agenda. I want win back some of the excessive room dedicated to the Diesel engine and address the noise issue. Undoubtedly my fuel economy will suffer, but I have a lot of wiggle room there. I consider even 25hp is excessive, but Iím after fuel injected 4 stroke and havenít seen it in smaller outboards.
My humble opinion is a 15HP four stroke would probably suit you well. It sounds like a lot of work. What about getting the lower unit out of the water?
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Old 11-04-2020, 15:03   #49
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

It is a trailer boat. So raising the lower unit is not important.
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Old 11-04-2020, 16:45   #50
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Diesel to 4 Stroke outboard conversion in displacement hull.

Tohatsu makes a nice 15 hp fuel injected 4 stroke- I have one. I think same block and engine is also detuned to a 9.9.
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Old 11-04-2020, 21:17   #51
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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You did the math wrong. 1.5l =0.4gal. Which strikes me as kind of low, thatís 15nm/gal for a power vessel.
15 mpg is certainly reasonable for a light displacement boat 21 feet wl at 1.15 square root is at 5 knots.

I have a 22"8" displacement New River Boat Company "Sea Gypsy " It has an M-30 perkins 3 cylinder 28 hp engine. At half rpms it does 6 knots at 0.6 gph and at 5.5 knots at 0.5gph so 11 mpg is no problem.

Last summer i had a governor spring brake and as a temporary propulsion unit put a 15 hp Yamaha 2 stroke on a bracket mounted stern. Poor steering, 2 gph no reverse response - terrible, but it did do the job. Glad to have the diesel engine running again. Outboard motor is good for a speed boat. Not so good for a displacement boat. Besides gasoline is a safety issue and doesn't store very well, soon to gum up.
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Old 12-04-2020, 01:50   #52
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Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by roverhi View Post
An outboard in a well will handle seas way better than one on a transom bracket. With seas kicking up much past a couple of feet immersing a bracket engine becomes a reality. Good thing is you probably wouldn't need the engine as there would be enough wind to sail. An engine in a well won't be subjected to the large gyrations of a pitching stern and be safely used in most conditions. Yes, if seas get really lumpy even an outboard in a well could be flooded. Had an outboard in a well on my first boat and never had a problem using it even in some pretty steep ocean waves.


All good points, but have a look at the drawings of the OPís boat and think youíll agree that itís pretty impractical.

Iíve sailed on a RL24 that has the outboard in a well, but that boat was designed with that setup in mind. Trying to squeeze something in between the skeg and the stern on the OPís boat would be beyond my skills. And I donít think heís going to be crossing oceans in a 21 foot motor boat, not by choice anyway.
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Old 12-04-2020, 08:19   #53
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Razorcanfly View Post
Iím after fuel injected 4 stroke and havenít seen it in smaller outboards.
There is this Mercury 15hp 4 stroke EFI model:

https://www.mercurymarine.com/en/us/...okicker-99-15/
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Old 12-04-2020, 20:27   #54
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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One more thing:
If you do not want to damage your diesel, use it no less than 60% of load or 75% of rpm (it is not petroleum!). Propellers are designed to be most efficient at the max rpm of the engines.
Also, I did not understand what you mean about "4-stroke". Do you mean petroleum? Do you know, that 90% of diesels are 4 -troke? One exception I know - Detroit Diesels.
Hi, I'm probably mis understanding you, but I'll say it anyway, the reason they call Detroit a 2 stroke is because they fire every stroke, the diesel is the same as all diesels use.
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Old 15-04-2020, 09:15   #55
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Hi, I'm probably mis understanding you, but I'll say it anyway, the reason they call Detroit a 2 stroke is because they fire every stroke, the diesel is the same as all diesels use.
4-stroke diesel is similar to 4-stroke petroleum: has suck valve and exhaust one. The piston changes position 4 times for one cycle. 2-stroke diesel has only exhaust valve and external supercharger to compress air. The piston changes position 2 times for one cycle.
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Old 15-04-2020, 09:41   #56
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Kolchac View Post
4-stroke diesel is similar to 4-stroke petroleum: has suck valve and exhaust one. The piston changes position 4 times for one cycle. 2-stroke diesel has only exhaust valve and external supercharger to compress air. The piston changes position 2 times for one cycle.
Just to clarify the above as it could be interpreted differently. The piston changes positions 4 times to complete the combustion and exhaust cycle, which is 2 full rotations of the crank in a four stroke. A two stroke completes the combustion and exhaust cycle in one rotation of the crank.
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