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Old 26-06-2014, 20:01   #1
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Changing saildrive to shaft drive

I am researching the possibility of chopping out a saildrive bed, closing the hole in the hull, and fitting engine beds and a shaft drive.

The boat is a conventional fin keeled 36 footer, late 80's vintage, fairly flat run.
Current engine is a Volvo 2003 and a 120 saildrive.

Nothing urgent, both working fine at the moment, but if something goes wrong I don't waste good money on another Volvo.

Has anyone done this? What are the issues?
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Old 27-06-2014, 08:50   #2
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Re: Changing saildrive to shaft drive

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Originally Posted by olaf hart View Post
I am researching the possibility of chopping out a saildrive bed, closing the hole in the hull, and fitting engine beds and a shaft drive.

The boat is a conventional fin keeled 36 footer, late 80's vintage, fairly flat run.
Current engine is a Volvo 2003 and a 120 saildrive.

Nothing urgent, both working fine at the moment, but if something goes wrong I don't waste good money on another Volvo.

Has anyone done this? What are the issues?
I read a blog once about doing just this. It was very detailed with pics and descriptions. I don't know how to access it again.

I don't think there are any issues with doing so other than the obvious ones of needing to glass up the saildrive hole, add a stuffing box and possibly a strut and getting the alignment correct. If your engine is mounted backwards or too close for a shaft, that could be a bigger issue.

Mark
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Old 27-06-2014, 08:57   #3
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Re: Changing saildrive to shaft drive

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Originally Posted by olaf hart View Post
I am researching the possibility of chopping out a saildrive bed, closing the hole in the hull, and fitting engine beds and a shaft drive.

The boat is a conventional fin keeled 36 footer, late 80's vintage, fairly flat run.
Current engine is a Volvo 2003 and a 120 saildrive.

Nothing urgent, both working fine at the moment, but if something goes wrong I don't waste good money on another Volvo.

Has anyone done this? What are the issues?
The biggest problem will be getting the prop deep enough in the water, so it would not cavitate.
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Old 27-06-2014, 12:45   #4
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Re: Changing saildrive to shaft drive

As Cotemar noted, your problem will be that boats with saildrives often have very wide and shallow sections aft with little rocker (slope up to the rransom). They use saildrives to get the prop down deep and the thrust horizontal. I suspect to get it deep enough with a conventional shaft, you are going to have a steep and inefficient shaft angle.

Also the engine may have to go forward to have enough room for a conventional transmission, coupler and shaft log.

It won't be easy.

David
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Old 27-06-2014, 14:18   #5
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Re: Changing saildrive to shaft drive

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Originally Posted by djmarchand View Post
Also the engine may have to go forward to have enough room for a conventional transmission, coupler and shaft log.
Look at V-drive options on transmissions. Does not take care of all issues, but might be only way to get engine and prop in decent locations.
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Old 27-06-2014, 14:31   #6
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Re: Changing saildrive to shaft drive

I talked to a guy that did this. It cost him $35,000 on a cat (2 engines). If I remember correctly he was able to use the original Yanmar engines, with some changes to the bell housing. He said he should not have gone with the lowest bidder, but should have paid the guy who wanted $50K. So on a mono you might get it done for Half of what he paid or should have paid. Since you are buying a new engine anyway I guess that is a wash. A new sail drive might cost you 5 boat bucks or so versus 1.5-2 for a conventional transmission. I doubt you'll save any money by going to a conventional drive. It will probably cost you more to make the modifications than you'll save. Also, why buy a Volvo. There are plenty of other suppliers of sail drives these days. Even Beta Marine has one now.
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Old 27-06-2014, 14:32   #7
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Re: Changing saildrive to shaft drive

I don't see it as brain surgery, but a lot of work: remove all, glass the hull with 12:1 taper from both sides. Clean and finish. The rest is pretty much standard engine install.
Install new engine and determine location of thru hull tube. Attach shaft to engine with shaft thru cutlass bearing and glass tube in place. add strut and etc. (maybe strut first?) a lot of work really.
There are a lot of fairly aft section boats with standard shaft drive, I wouldn't worry about cavitation. If your prop is under the hull, flat or not it's going to be in the water.
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Old 27-06-2014, 16:36   #8
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Re: Changing saildrive to shaft drive

I think you should keep the sail drive. As others have pointed out, fitting a conventional shaft is going to bring the prop up close to the hull and not very far below the surface. My boat was configured this way and the thrust from the screw follows the hull and creates a small but embarassing rooster tail, wasting a lot of the energy that would otherwise be pushing your boat. With anything but a small prop (12") I had this problem, so I built a new strut and extended the shaft 8" aft. This gave me 3" of clearance to the hull with a 14" screw and lessened the roostertail effect but did not squelch it entirely. Just my experience.
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Old 27-06-2014, 17:19   #9
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Re: Changing saildrive to shaft drive

I think you guys are smoking funny cigarettes worrying about hull shape and depth. Here's a couple bene's with shaft and sail drive.
Now the work involved.. that's another whole thing!
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Old 27-06-2014, 17:42   #10
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Re: Changing saildrive to shaft drive

IMO, the reason that Benies etc use saildrives is not because of the flat hull shape at all. Rather, it is because it is far less costly to build than with a shaft drive. It is a great deal for the builder... dunno about the buyer. We've had endless arguments here on CF about this, and I don't want to reopen that subject!

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Old 28-06-2014, 22:00   #11
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Re: Changing saildrive to shaft drive

Olaf, in the last 2 months I have bought 2 Volvo Diesels. First an MD7A that came from a saildrive, and then an MD6B to get a conventional transmission. The saildrive engine had front motor mounts that were at 45 degrees, where as the conventional shaft engine had 90 degree mounts. For what ever reason, the engineers at Volvo thought that the saildrive required 45 degree mounts when the shaft drive engine didnt. You might want to consider the chance/cost of remounting your engine for a conventional transmission. Just a thought!!!! ______Grant.
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