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Old 08-10-2015, 18:57   #1
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Yanmar V-twin diesel conversion of OMC Saildrive

Looking for some expert advice on my planned diesel conversion using an OMC saildrive. I recently purchased a Yanmar vertical shaft v-twin 18hp water cooled diesel engine and am planning to replace an old OMC saildrive powerhead with this engine.

My first question has to do with the Iso-Ring used on the saildrive.

Does anyone see any problems with the durability/strength of the Iso-Ring when used with the new engine? The new engine weighs almost double the original power head (60lbs vs 112lbs) and probably has twice the torque.

Second.

I will need to mount the new engine approximately 5 1/2" above the mounting surface of the original powerhead due to the mating of the two shafts. My plan is to use 1" round bar stock with 7/16" bolts securing them to the two adapter plates. Anyone think this wouldn't be strong enough?
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Old 09-10-2015, 07:04   #2
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Greetings and welcome aboard the CF, sgancarz.
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Old 09-10-2015, 07:53   #3
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

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Originally Posted by sgancarz View Post
Looking for some expert advice on my planned diesel conversion using an OMC saildrive. I recently purchased a Yanmar vertical shaft v-twin 18hp water cooled diesel engine and am planning to replace an old OMC saildrive powerhead with this engine.



My first question has to do with the Iso-Ring used on the saildrive.



Does anyone see any problems with the durability/strength of the Iso-Ring when used with the new engine? The new engine weighs almost double the original power head (60lbs vs 112lbs) and probably has twice the torque.



Second.



I will need to mount the new engine approximately 5 1/2" above the mounting surface of the original powerhead due to the mating of the two shafts. My plan is to use 1" round bar stock with 7/16" bolts securing them to the two adapter plates. Anyone think this wouldn't be strong enough?

I personally would mount the engine on flex mounts above the sail drive and couple it with a flex coupler rather than hard pipe. That engine is going to move a lot at various speeds and a rigid couple will likely break things you can't replace.


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Old 09-10-2015, 08:50   #4
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

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I personally would mount the engine on flex mounts above the sail drive and couple it with a flex coupler rather than hard pipe. That engine is going to move a lot at various speeds and a rigid couple will likely break things you can't replace.
I will be using a Lovejoy L-type coupling for the shafts. My concern with mounting the engine on it's own separate mounts would be that the engine would move independently of the drive unit and cause problems with the shaft alignment. Thus my though to mount solidly to the drive housing and rely on the Iso-Ring for isolation from the boat.
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Old 09-10-2015, 09:09   #5
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Stiff mounts won't allow for much engine movement and will keep the extra weight and vibration from damaging the lightweight drive leg o the OMC unit. I think the love joy couple will take enough of the movement encountered mostly at idle in stride.


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Old 09-10-2015, 10:27   #6
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Buy a different boat...
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Old 13-10-2015, 06:48   #7
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

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Buy a different boat...

Nice to see that you have so much to add to the conversation.
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Old 13-10-2015, 07:36   #8
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

It's a forum, some will pontificate endlessly and say nothing, some will be short and sweet, given the price of used sailboats which much more desirable drive-trains this makes zero sense.
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Old 13-10-2015, 12:24   #9
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

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...given the price of used sailboats which much more desirable drive-trains this makes zero sense.
I'm inclined to go along with this.

The sail drive is bad enough without bastardizing an engine to fit it.
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Old 13-10-2015, 22:28   #10
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Well sometimes you just gotta try something others have not. There's a lot of boats out there that still have the old OMC saildrives in them. Face it, sail drives are here to stay now, so why not try to take the original and make it better.

And actually believe it or not, there are people out there that love their old OMC saildrives. They've been around for nearly 40 years now and many are still going strong.

Unfortunately OMC came out with the saildrive just as the diesel started taking over. If they would have put a diesel on it like I am trying to, they just might have conquered the diesel revolution of sailboats.
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Old 13-10-2015, 22:46   #11
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

I think you will be fixing cracked things a lot if you try to mount the engine solidly to the drive unit.

I will be interested to hear if this works out financially . I think Phil Boldger said once "Just because something can be done doesn't mean it should be."
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Old 13-10-2015, 23:13   #12
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Well, you will surely give the old Sail-Drive the acid test by doubling the engine's weight and hp.
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Old 16-10-2015, 12:32   #13
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

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I personally would mount the engine on flex mounts above the sail drive and couple it with a flex coupler rather than hard pipe. That engine is going to move a lot at various speeds and a rigid couple will likely break things you can't replace.
Sailmonkey,

Thanks for the input. I think I will take your advice and mount the engine on it's own mounts. I also will be using a Lovejoy S-type coupling instead of the L-type. The S-type provides more torsional dampening than the L-type jaw coupling.

I plan to use the internal raw water pump in the lower unit to feed raw water to a heat exchanger. I am hoping this will be enough, however I am installing a pulley on the output shaft of the engine just in case. This will allow me to add an external raw water pump should I need to in the future.

Below is a drawing of my current design. Of course this all could change as the project moves forward. Everything from the blue intermediate housing down is the original OMC drive leg and mount. Everything above that will be part of the new install.

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Old 09-11-2015, 07:00   #14
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Moving forward on the conversion. So far I have obtained the parts needed to rebuild the lower unit and have been working on the drawings for the mount and coupling.

I have decided that I will use an external raw water pump instead of the internal one in the leg. After doing all of my measurements and drawings, I discovered that I would not have enough room for the outlet of the internal pump. So, I have obtained a used Yanmar raw water pump that I will drive from a pulley attached to the upper portion of the Lovejoy coupling.

I have also completed the design of the exhaust header and mixing elbow. The exhaust will be made from 304 Stainless tubing. I will end up needing to wrap the primary tubes to help keep down temperatures in the lazarette, as it would be too expensive to try and water cool the entire system. So the wet exhaust portion will not start until the mixing elbow.
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Old 21-02-2016, 07:14   #15
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Re: Yanmar V-twin diesel conversion of OMC Saildrive

Been a while since I posted on this, so thought I would provide an update.

I have completed the rebuild of the lower unit with all new bearings and seals. I filled in the original water intake and have purchased a new external raw water pump as the original I purchased has sever scoring that I just didn't trust a rebuild. I have also obtained all of the components and completed the machine work on the original input shaft for coupling of the engine to the lower unit.

Yesterday I test fit the base for the engine mount and began reopening the hole in the bottom of the original saildrive mounting. I discovered that the previous owner filled in the mount with a 1/2" of fiberglass and 3" of wood. Now I have to figure out how to get all the wood out of there without damaging the original housing.

Here are a couple pics of the progress.







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