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Old 28-08-2009, 08:40   #1
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Sail-Drives or Shaft Drives?

Can anyone explain the advantages / disadvantages of: sail-drive engine as oppose to shaft drive engines.
. . . from what I can see, the main trade-offs are – price and that sail-drive the engines often need to be located under the aft bunks. The question I dont understand is why.
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Old 28-08-2009, 09:46   #2
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Here is a good discussion on the subject...
http://www.cruisersforum.com/forums/f48/pros-and-cons-of-sail-drive-29727.html
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Old 28-08-2009, 13:56   #3
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The engine is aft and it sits almost on the saildrive. Where else could we place it? Anyway, probably the best set-up in a cat.

Also - cat's hulls tend to be flat aft and the shafted prop would be facing too much downward.
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Old 28-08-2009, 17:15   #4
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I suspect saildrives have more wetted area.

Steve B.
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Old 28-08-2009, 18:45   #5
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Ruby, I was very apprehensive about switching to saildrives when we bought our cat. I anticipated more maintenance and probably more haulouts to change zincs, fluid and seals.

After 4 years and several cruises, I'm more comfortable with the sail drives and they do have several advantages: prop thrust vector doesn't angle down so more efficient power; no stuffing box with water in the bilge; don't have to align engine and prop shaft. I changed the seals/o-rings in the saildrive during first haulout myself and it was an easy job. I can only drain about 60% of the transmission oil from the top without hauling out. I change it about twice a year and the oil looks clean and no sign of water so I don't think an annual haulout is necessary just to change saildrive oil.

I think the biggest negative for saildrives is the large rubber grommet that seals the saildrive to the hull. I worry about springing leaks in the rubber even though the condition looks good. I haven't changed grommets yet and I understand it is a big job and engine must be shifted forward. The other big negative is changing the zincs on saildrive. The props must be removed to change the zincs. That isn't a big deal on fixed props, but my folding props must be disassembled in the water... all 30 plus parts. (The newer saildrives may have an easier zinc change without prop removal)

If I had a choice, I would still prefer conventional shafts over saildrives, but I wouldn't consider saildrives a "deal breaker" if the boat met my other criteria.

I suspect that many cats with conventional shaft drives have engines located under aft bunks also. Saildrives probably give designers a little more flexibility in locating engines since they don't have to allow for the length of the conventional shaft. The saildrive bolts directly to the back of engine and the outdrive leg goes straight down so the engines can be moved further aft.
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Old 29-08-2009, 06:50   #6
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Actually after I did this post I found heaps of info, even in this forum. The main differences others have reported is in expected maintenance and maintenance cost of a sail-drive. Also the point Greg brings up concerning possible leaks. It seems the only real advantage of the sail-drives is in the boat design / manufacturer.
Even on that score of having the engines further aft raised concerns of maintenance at sea, and the potential of making the boat hobby-horse
. . . I haven’t heard of one boat owner that would choose sail-drive over shaft-drive. Having said that, I think Greg summed it up by saying this choice should not be a deal breaker
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Old 29-08-2009, 19:13   #7
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. . . I haven’t heard of one boat owner that would choose sail-drive over shaft-drive. Having said that, I think Greg summed it up by saying this choice should not be a deal breaker
I agree 1000%. Wish my cat didn't have sail drives but was not a deal breaker. Have changed the rubber seal boots and was not that big of a deal. Not real impressed with the design however, nor with anything painted green in the engine room.

If only the Leopard 37 (shaft drives) came in a 44-ish model....

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Old 03-09-2009, 15:53   #8
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I agree 1000%. Wish my cat didn't have sail drives but was not a deal breaker. Have changed the rubber seal boots and was not that big of a deal. Not real impressed with the design however, nor with anything painted green in the engine room.

If only the Leopard 37 (shaft drives) came in a 44-ish model....

Mark, I'm hauling out this winter and may try to replace the rubber seal boots on saildrives. Can you give me a brief description of what's involved in changing them? Do you need to take engine off mounts and move it forward or can you just unbolt saildrive and remove it? I assume the saildrive must be lifted up inside the boat rather than dropping out thru the hole? Do you have Volvo 120 S drives also?
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Old 03-09-2009, 17:03   #9
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I'm not sure I could bring myself to buy a boat with a saildrive. Ask any powerboater you see with an outdrive.... they all have to be worked on. The saildrive isnt probably as bad, but I just dislike having a huge hole in the bottom of my boat with a rubber gasket! and I really dislike aluminum mixed with other metals under water. Definitely wouldnt go offshore with it. Everybody anal-izes over thruhull/seacocks, hoses etc etc.... why have that huge hole! My catamaran had V drives which worked perfectly and kept the engines in the rear of the boat. So did my Pasport 47. It may just be me, there are quite a few out there.
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Old 03-09-2009, 17:07   #10
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Mark, I'm hauling out this winter and may try to replace the rubber seal boots on saildrives. Can you give me a brief description of what's involved in changing them? Do you need to take engine off mounts and move it forward or can you just unbolt saildrive and remove it? I assume the saildrive must be lifted up inside the boat rather than dropping out thru the hole? Do you have Volvo 120 S drives also?
Hi Mark, just to add another question - approximately : how long to would take to replace one grommit ? Thinking of beaching and replacing during slack and low tide. Same engine as in SVCattales : Volvo Penta MD 2040B with 120 s.
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Old 03-09-2009, 18:49   #11
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Mark, I'm hauling out this winter and may try to replace the rubber seal boots on saildrives. Can you give me a brief description of what's involved in changing them? Do you need to take engine off mounts and move it forward or can you just unbolt saildrive and remove it? I assume the saildrive must be lifted up inside the boat rather than dropping out thru the hole? Do you have Volvo 120 S drives also?
I have the 120's yes. You are correct that the saildrive is just unbolted from the rear of the engine and lifted up through the hole. In my case I wanted to preserve the boot clamping ring - which is just mild steel of all things - before the new boots went in. So I wire-brushed and painted, let dry overnight and then had them ready for the reinstallation. (I hired the boot replacement done as it was a first-time experience for me...) You will also want to preserve the thrust bracket at the rear of the sail drive - it too is mild steel. Mine were unpainted and starting to corrode when I bought the boat.

There are 2 pieces of rubber in the boot replacement kit - the boot itself and the rubber hull fairing. Rreplace them both of course. And if your engines have started to weep oil from the rear crankshaft seal it's a great time to replace that while the sail drive is off (mine were leaking). $50 seal that can only be changed with the drive removed. The engine mounts were unbolted from the bed rails and the whole engine slid forward to allow sail drive removal. I don't think there's enough play to remove the drive if you don't move the engine fwd. Was not a big deal, and you don't have to worry about shaft alignment afterwards anyway - that's about the only nice thing about the sail drive arrangement

My brilliant mechanic managed to pinch the boot beneath the clamping ring during reassembly, so when we launched I had lots of water coming into the people tank.... not good! Was an easy fix but we hung in the slings for a while as he sheepishly made things right. So, don't pinch the boot as you tighten the clamping ring unless you want to flush the bilges a bit

I would not hesitate to do this job myself the next time after having seen it done. Those clamping rings are pricey - did I mention that you should preserve them during this job?

Good luck with it!
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Old 03-09-2009, 18:53   #12
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Hi Mark, just to add another question - approximately : how long to would take to replace one grommit ? Thinking of beaching and replacing during slack and low tide. Same engine as in SVCattales : Volvo Penta MD 2040B with 120 s.
I would say 3 - 5 hours assuming all goes well. Problem is that big hole in the boat should you run in to troubles. You'll also want to replace the lower drive seals while you can since that can only be done out of the water (well - at least without performing some unnatural acts as others have described...)

And did I mention that you should preserve / paint the #$%@& clamping ring while you're doing this?
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Old 03-09-2009, 20:41   #13
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I have the 120's yes. You are correct that the saildrive is just unbolted from the rear of the engine and lifted up through the hole. In my case I wanted to preserve the boot clamping ring - which is just mild steel of all things - before the new boots went in. So I wire-brushed and painted, let dry overnight and then had them ready for the reinstallation. (I hired the boot replacement done as it was a first-time experience for me...) You will also want to preserve the thrust bracket at the rear of the sail drive - it too is mild steel. Mine were unpainted and starting to corrode when I bought the boat.

There are 2 pieces of rubber in the boot replacement kit - the boot itself and the rubber hull fairing. Rreplace them both of course. And if your engines have started to weep oil from the rear crankshaft seal it's a great time to replace that while the sail drive is off (mine were leaking). $50 seal that can only be changed with the drive removed. The engine mounts were unbolted from the bed rails and the whole engine slid forward to allow sail drive removal. I don't think there's enough play to remove the drive if you don't move the engine fwd. Was not a big deal, and you don't have to worry about shaft alignment afterwards anyway - that's about the only nice thing about the sail drive arrangement

My brilliant mechanic managed to pinch the boot beneath the clamping ring during reassembly, so when we launched I had lots of water coming into the people tank.... not good! Was an easy fix but we hung in the slings for a while as he sheepishly made things right. So, don't pinch the boot as you tighten the clamping ring unless you want to flush the bilges a bit

I would not hesitate to do this job myself the next time after having seen it done. Those clamping rings are pricey - did I mention that you should preserve them during this job?

Good luck with it!
Mark, Thank you very much for the detailed description of the job. It sure increases my confidence in trying that job during haulout.

I already have the parts to reseal the lower drive, so I may as well do the whole thing. During last haul I replaced the rubber fairings using glue and method recommended by the yard. They lasted about a month before they fell off. Now I have a nice crop of barnacles in the hole that are very difficult to remove with the drive installed. When I remove the drive, it will be much easier to remove the growth without fear of poking a hole in the rubber boots. I'm also going to fill the hole with some type of foam before putting on the rubber fairing. That should keep the critters out and increase my buoyancy aft so we can take lots more crap on our next cruise (at least 2 more sixpacks anyway).

Thanks again.
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Old 04-09-2009, 11:28   #14
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..... I'm also going to fill the hole with some type of foam before putting on the rubber fairing. That should keep the critters out and increase my buoyancy aft so we can take lots more crap on our next cruise (at least 2 more sixpacks anyway).
Interesting idea Greg - what type of arrangement do you have in mind? It will have to allow removing the drive again, be sturdy enough to stay together in a pounding sea and not absorb water. Would be interested to know how you plan to do this.

As you've found out the hard way, gluing the fairing rubber is not very long-lived. I have thin ss frames that screw to the hull and hold the rubber fairing in place.

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Old 04-09-2009, 14:15   #15
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[QUOTE=markpj23;327650]Interesting idea Greg - what type of arrangement do you have in mind? It will have to allow removing the drive again, be sturdy enough to stay together in a pounding sea and not absorb water. Would be interested to know how you plan to do this.

As you've found out the hard way, gluing the fairing rubber is not very long-lived. I have thin ss frames that screw to the hull and hold the rubber fairing in place.

Mark, I got the idea from an earlier thread (Saildrive Boot) from Fastcat. He stated: "we also fill up the hole around the leg with a high density polyurethane foam to keep the water out of there for a bit of gained lift."

The easiest solution may be to cover the hole with cardboard and squirt Great Stuff thru a hole in the cardboard and fill it up. Great Stuff is easy to carve with a knife and the fairing rubber could be attached later. This might also avoid a rubber boot failure if i filled the entire cavity. The downside is the great adhesion of Great Stuff to everything it touches. Later service of saildrive would require sanding or wire brushing the stuff off.

Another possibility would be to cut two pieces of closed cell foam (like cockpit cushion foam) and wedge them in the hole before putting on the rubber fairing. Foam is easy to glue together so I could make a custom size plug to fit the hole pretty closely with a center cutout for the saildrive leg.

I would certainly like to hear an other ideas on how to fill the cavity (any multihull dentists around?)

Mark, I like you idea of the stainless steel strips to hold up the fairing rubber. I need to use that idea too.
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