Yanmar V-twin diesel conversion of OMC Saildrive

[sgancarz]

Originally Posted by a64pilot

A 90 degree V twin is an exceptionally smooth running engine, it is I believe in perfect primary balance.
A Harley is 45 degrees, was picked as a nice round number and for packaging, but is a horrible design and has done more for giving V twins a bad name than anything. (in my opinion)

The Yanmar 2V78 is an 80 degree v-twin with 749cc displacement. The diesels hit harder than a gas engine, so might be a little rough at idle. I'll just have to see once I get it fired up.

[a64pilot]

OK I had assumed a 90.
I let the Harley comment go as I'm bad about thread drift, and didn't want to drift this excellent thread

[sgancarz]

Originally Posted by Sailor Doug

Specs on Sail drive 2 stroke.

http://www.leeroysramblings.com/OMC_9.9_1993.htm


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Originally Posted by a64pilot

You know I think 5 to 6 thousand RPM is and has been pretty much the common RPM for large and small 2 and 4 stroke outboards for almost forever?
I think my 175 HP Verado was propped for 6,000 RPM if I remember right.

The OMC Saildrive of the 1970/80's cannot be compared to a modern outboard engine.

The Saildrive utilized a 35HP powerhead that was detuned to 15HP. With that the max RPM on the engine was only 3750 rpm. This lower rpm also allowed the engine to run with a fuel/oil mixture of 100:1 instead of the 50:1 recommended for the standard outboard.

[sgancarz]

Originally Posted by sgancarz

The OMC Saildrive of the 1970/80's cannot be compared to a modern outboard engine.

The Saildrive utilized a 35HP powerhead that was detuned to 15HP. With that the max RPM on the engine was only 3750 rpm. This lower rpm also allowed the engine to run with a fuel/oil mixture of 100:1 instead of the 50:1 recommended for the standard outboard.

Also, remember this is a saildrive for a sailboat. No matter how fast I turn that prop this thing will never get up on plane. I think that is the reason OMC detuned the engine in the first place.

[Terra Nova]

Originally Posted by sgancarz

Also, remember this is a saildrive for a sailboat. No matter how fast I turn that prop this thing will never get up on plane. I think that is the reason OMC detuned the engine in the first place.

Why would you not think they de-tuned it to more closely match the capacity of the sail drive?

[sgancarz]

Originally Posted by Terra Nova

Why would you not think they de-tuned it to more closely match the capacity of the sail drive?

The drive is nothing more than an outboard leg. The insides are the same parts as most of their other outboards. Bearings, seals, shift dog, etc. I can't say whether the gears are the same or not, but the design is outboard all the way, so I'm sure it could handle the higher RPM's. The housing is the only visual difference due to the fact of the way it mounts through the hull.

[Terra Nova]

Originally Posted by sgancarz

...I'm sure it could handle the higher RPM's...

You're just not getting it. It isn't the RPM that it can't handle, it is the HP produced @ that RPM.

[sgancarz]

Originally Posted by Terra Nova

You're just not getting it. It isn't the RPM that it can't handle, it is the HP produced @ that RPM.

Ok, original saildrive was 15hp @ 3750 rpm, the Yanmar engine is 18hp @ 3600 rpm. However I figure I will never run the diesel at 3600 rpm, so if you look at the power curve from the manual at 2000-2400 rpm (which is where I think it will be running most of the time) the engine is making approximately 14hp.

Now torque is another story as the original engine only had about 18ft/lbs of torque, where the diesel puts out 30ft/lbs of torque.

[Sailor Doug]

Originally Posted by sgancarz

Ok, original saildrive was 15hp @ 3750 rpm, the Yanmar engine is 18hp @ 3600 rpm. However I figure I will never run the diesel at 3600 rpm, so if you look at the power curve from the manual at 2000-2400 rpm (which is where I think it will be running most of the time) the engine is making approximately 14hp.

Now torque is another story as the original engine only had about 18ft/lbs of torque, where the diesel puts out 30ft/lbs of torque.

Horse power is force through distance through time. 18 ft/lbs can not have the same horse power as 30 ft/lbs at the same speed.


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[Sailmonkey]

Enough about the horsepower lessons, speed, torque, ratings of old power heads etc..,,,

If someone wants to take a Diesel engine and adapt it to an OMC sail drive applaud their enginuity.

Most of the vocal naysayers probably don't have the skill or drive to embark on what I see as a fun project.




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[sgancarz]

Originally Posted by Sailor Doug

Horse power is force through distance through time. 18 ft/lbs can not have the same horse power as 30 ft/lbs at the same speed.


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The power curves are very different between the engines. I did not say they created the same power at the same RPM. I merely stated the published ratings. Unfortunately I cannot provide the graphs for the OMC engine, but I do have them for the Yanmar.



As you can see, the Yanmar engine maintains 30ft/lbs of torque through most of the rpm range. Where as horsepower maxes out at 3600rpm.

[sgancarz]

Originally Posted by Sailmonkey

Enough about the horsepower lessons, speed, torque, ratings of old power heads etc..,,,

If someone wants to take a Diesel engine and adapt it to an OMC sail drive applaud their enginuity.

Most of the vocal naysayers probably don't have the skill or drive to embark on what I see as a fun project.




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Sailmonkey,

Thanks for your support on my endeavor!

[Sailor Doug]

Originally Posted by Sailmonkey

Enough about the horsepower lessons, speed, torque, ratings of old power heads etc..,,,

If someone wants to take a Diesel engine and adapt it to an OMC sail drive applaud their enginuity.

Most of the vocal naysayers probably don't have the skill or drive to embark on what I see as a fun project.




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A little engineering goes a long ways. Solutions don't have to cost more. I have a couple of friends with near end of life OMC sail drives that are looking a solution I think this is a great one. I was jut offering my 45 years in mechanical design engineering, no one has to take it. Force x distance x time = power

Never offer a criticism without a solution.



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[darylat8750]

I must say that I started reading this thread with large reservations as to your success. You have won me over with your attitude and willingness to consider alternatives. I think you are above 50% chance of success now! The new coupling is magnificent and addresses two problems that I thought would destroy your efforts. (IE misalignment during vibrations and torsional vibrations.)
Good luck and do keep up informed as you progress.

[sgancarz]

Originally Posted by Sailor Doug

A little engineering goes a long ways. Solutions don't have to cost more. I have a couple of friends with near end of life OMC sail drives that are looking a solution I think this is a great one. I was jut offering my 45 years in mechanical design engineering, no one has to take it. Force x distance x time = power

Never offer a criticism without a solution.



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I know I saw the torque rating on the OMC somewhere, however I can't find it now. But using the formula TORQUE = (HP x 5252) ÷ RPM. ((15 x 5252) ÷ 3750 = 21), the OMC calculates to 21ft/lbs of torque.

Here is a good article that explains this formula and how it is used to calculate HP, as Horsepower is dependent on time. However when trying to calculate TORQUE it does not work quite as well, as torque is independent of time.

Horsepower vs Torque

Great discussion by the way. Made me do some more research and taught me a few things.