Saildrive vs. Conventional Shaft Drive

kloudie1 · Nov 22, 2008

For those of us who don’t pull the boat every year, the maintenance issue with the hull penetration seal (bellows/diaphragm) is an issue as is the big chunk of aluminum (typically) in the water sitting there waiting for corrosion. (price of a leg with gear change-out versus price of a strut) The other issue is having expensive gear and bearing parts separated from seawater by a lip seal with a .007” wide contact patch.. although this can be controlled if the oil reservoir feeding the leg is about 3 feet above the waterline so that oil pressure is always more than water side pressure.
On the plus side for saildrives is the straight line drive which is more efficient (horizontal thrust line). The normal folding prop is a low drag benefit over the normal fixed prop on conventional drive. (minimally more drag than a conventional with a good folding prop). As noted, alignment, once correctly set, is not an issue for the owner.. The engine placement by the builder can be optimized for access or interior considerations. Typically, saildrives are more quiet because of better mechanical isolation from the hull.

sailingdog · Nov 22, 2008

One advantage of a saildrive is that they usually don't suffer from prop walk, since the prop is normally vertical, not angled.

Don12364 · Nov 22, 2008

Negs:-
* Aluminium Prop corrosion problems
* Aluminium Drive Leg corrosion problems - a friends leg was scrap after two seasons
* Cannot change lube oil in leg unless boat is lifted or dried out
* Huge hole in hull filled only by a rubber diaphragm
* Need to change diaphragm every seven years - Volvo
* Need to move or remove engine to replace diaphragm
* Yanmar has double diaphragm with water sensor in between. I know of three cases where the outer diaphragm tore away and fouled the prop
* Leg projecting vertically downward is more prone to picking up lines and debris etc.
* Some legs have the cooling water inlet in the leg which can be covered by plastic bags and the like
* Doubtful if gearbox double bevel drive gears could stand abuse of rope, plastic bag or chain entanglements like a straight drive can.
* Centres of Volvo props are rubber mounted and known to fail after only three years
* To keep weight out of the ends of the boat builders want to mount the engine as far forward as possible. This leads to prop being well in front of rudder thus making the desirable features of prop wash over the rudder and its response time much less effective
* Am aware of a recent case of leg becoming completely detached from engine due to bolts vibrating loose - nearly sank the boat. Engine installer needs to remove the leg to mount the engine!

Positives:-
* Much cheaper for the builder (though not for the customer due to higher price of unit).
* No shaft alignment problems - these are eliminated by fitting a $75 flex coupling anyway see (http://www.boatsandoutboards.co.uk/view/LCM016)
* No cutless to change (every ten years or so)
* No stern gland to leak - I overcame this with a PSS gland

They are an abomination! Just ask a powerboater with two in/out drives.
I sleep more soundly with a shaft drive for all the above reasons.

All U Get · Nov 22, 2008

My first advantage that I would see with the saildrive would be the elimination of the need for shaft to engine/transmission alignment. When it would come to major work, can you split the "outdrive" portion with the boat still in the water to access internal components?

Could I ask the question that really bugs me about the way boats are built? If the water comes in through the seal around, why doesn't the shaft terminate above the waterline inside the boat. There would be less water pressure around the seal, so drippless or no seal needed eh. If the engine resides in the lowest belly of the boat for ballest, then a transfer case with gears could be added like my 4 wheel PU. Now I'm going to suggest another gear to shift into once at speed to allow for a smaller, fuel efficient engine maintaining hull speed. I already have a dripless rudderpost, Why not the shaft.
All U Get

Ross · Nov 22, 2008

All U Get said:
My first advantage that I would see with the saildrive would be the elimination of the need for shaft to engine/transmission alignment. When it would come to major work, can you split the "outdrive" portion with the boat still in the water to access internal components?

Could I ask the question that really bugs me about the way boats are built? If the water comes in through the seal around, why doesn't the shaft terminate above the waterline inside the boat. There would be less water pressure around the seal, so drippless or no seal needed eh. If the engine resides in the lowest belly of the boat for ballest, then a transfer case with gears could be added like my 4 wheel PU. Now I'm going to suggest another gear to shift into once at speed to allow for a smaller, fuel efficient engine maintaining hull speed. I already have a dripless rudderpost, Why not the shaft.
All U Get

My propellor is about 18 inches below the waterline. The shaft enters the boat a 15 degrees above the horizontal. that is about 3 inches to the foot. I could bring the inboard end of the shaft above the waterline if it were six feet long but I want to allow for people and cargo near the stern so I would like about 6 inches of margin. That will add 2 feet to the length of the shaft before it emerges from the stern tube. Now I need a bearing to support the end of the shaft, so add 3 inches.My cabin settees are at the same elevation as the waterline. At this point I have a propellor shaft dividing my cabin down the center and I still haven't installed the engine in head. Nancy is raising cain with me cause she wants to sleep in the vee berth and can't get there because I put the Damn engine in the middle of the boat.

All U Get · Nov 22, 2008

So could you get away with a smaller engine? Could you slide it aft? On my Catalina 30 it might be under the quarterberth and the shaft might be a lot shorter, or maybe two small, offset props like those big yachts.

All U Get

Ross · Nov 22, 2008

All U Get said:
So could you get away with a smaller engine? Could you slide it aft? On my Catalina 30 it might be under the quarterberth and the shaft might be a lot shorter, or maybe two small, offset props like those big yachts.

All U Get

You want to get rid of the packing gland by bringing the shaft up above the water line. The other way to do that would be to install a pod external to the hull and install a hydraulic or electric motor in that. But a hundred and fifty years of packing glands seem to indicate that they are pretty good.

All U Get · Nov 22, 2008

Ross,
If I could get a jet-ski engine with no external prop, higher horsepower, and better fuel economy I would be changing my keel to have a tube down the long axis like Red October squirting water out the back. Do you think we could get it to work?
All U Get

Ross · Nov 22, 2008

It would work but I get almost 20MPG with my 10 hp volvo and a fixed prop. I doubt that you could better that with a jet drive.

Hermit Scott · Nov 22, 2008

All you get, you sound like me, trying to think of better ways to do all these things. If you are going to be an inovator or inventor, just be ready for everyone to tell you that 'that's the way it's always been done and it must be better'. You know why they say that? Cause it's true! And trying new things is expensive, wrought with unforseen problems, and unreliable because it's not been done before.
The way I see it now, boats are like aircraft depending on how far from the shore you are. It's best to go with what's proven to be reliable.
With a jet ski you still have seals though. You will always have to have a seal. The packing gland is just the easiest to change the seal. You want the prop under the boat also. That is one of the greatest benefits of having an inboard engine. Besides I know how easy your packing gland is to replace in your C30. I am very glad I don't have an engine in the back of the boat. I was trying to buy a catalina 27 because they were more affordable, but I never could get a good look at the engine because of it's location much less the packing gland.

Jenni · Nov 22, 2008

Just outa curiosity. Is the Sail drive style when the boat has the internally mounted Outboard motor that drops down through a hatch in the boat or is it something else entirely? And really as far as ease of maintnace im really fond of the outboard on my boat.

LANCE ON CLOUD NINE · Nov 22, 2008

Jenni-

The saildrive is different from an outboard in a well. The best way to describe is that it looks similar to an out boards lower shaft - except is bolted to bottom of hull- usually right behind keel. An inboard engine passes power through a hole in hull bottom to drive unit. Usually unit does not steer and boat still uses a conventional rudder. Most use diesel engine but some were made with gas engines. You can find a picture at beta marine .com

Sandy Stone · Nov 22, 2008

saildrive

Yeah, I really want a huge hole in the bottom of my boat, protected only by a rubber seal. I completely agree with Donalex - they are an abomination, their only reason for existence to make things easier & cheaper for boatbuilders, owners be damned. Sort of like front-wheel drive for cars. Don't get me started.

Jim Seamans · Nov 23, 2008

Sail drive pictures

Sail drives are an interesting setup for boats and are an outgrowth of inboard/outboards. But I think the future is electric with diesel generator. We don't need to go fast just aux power available when needed.

Link is to Yanmar saildrive pdf.

http://www.yanmar.com.au/express/Dealer/Prodspec/ENGINE PDF/3YM30-SD20.pdf

Barnacle Bill · Nov 24, 2008

There are lots of advantages of saildrives. They take up less space in a boat so it can be used for other things. There is no shaft alignment problems to deal with, no shaft leaking problems, no need to replace a cutlass bearing, they are quieter, they have more thrust, they have larger props usually for more torque, they back up in a straight line, they require less maintenance in the shaft portion of the engine, etc. I have experienced their use for over 25 years now and they are awesome. I am delivering a Wauquiez 40 right now from Annapolis, Md to Charleston SC and the boat is equipped with a Yanmar saildrive. It is a very nice engine, we have tons of power and life is good (but cold).

All U Get · Nov 24, 2008

Hi Barnacle Bill,
How much access do you need for that lower unit? Do you have to haul the boat? I'm looking at Jim Seamons" thumbnail and would also like to know if there are different lengths to that lower unit?
All U Get

firehoser75 · Nov 27, 2008

Saildrive Owner

I am not going to debate the virtues of shaft vs saildrive, I will just tell you a little bit about my saildrive. It came with the boat when we decided that this was what we thought would be the best boat for us. I didn't give a whole lot of weight to whether the boat had a shaft or a drive in this decision.
I haulout my boat annually. At that time I change the lubricant by taking out a drain plug in the bottom of the drive. It takes about 5 minutes (total) until I am ready to refill. I have never done it, but I have been told that you could use the same oil removal pump that you drain engine oil with to do a change with the saildrive through the fill hole in the top (although you would not get complete removal of the old fluid). Manufacturer recommends fluid change every 200 hours (if memory serves me) and I don't put on that many hours in one year.
My boat has a completely dry bilge because of the saildrive. This eliminates any odours that some boats have due to wet bilges. It backs very well (little prop walk, not none), and it performs very well in forward. I do worry about corrosion on the leg, but so far (after 4 years) there is none (well a very tiny almost invisible amount along the edge where the zinc attaches). I recently changed the saildrive hull seal mainly because the engine was out due to a failure I suffered (another thread on this one) and I thought it would be a good idea while access was so good. The old seal looked brand new! The engine can be removed with the saildrive left in place. With the engine out, it cost me about 6 hours labour and the cost of the parts (expensive parts, after all it is Volvo) to replace this seal. I have never suffered from the "afflictions" that some have talked about in earlier replies (like lines, plastic bags plugging the water intake, etc.) but I guess it could happen. The drive seems relatively protected behind the keel, so I don't worry much about it getting hit by floatsum etc. either. Other sailboat owners who have gone out with me comment on the lack of vibration and how quiet my engine is. Well, that's about it, just that my overall impression of saildrives is pretty good.

Tom

Barnacle Bill · Nov 30, 2008

Just returned and only read your reply today. To the best of my knowledge, if I understand your question, there are not various lengths available for sail drive units on any given manufacturer's engine. Do we have to haul the boat for what? I guess you need to give me more info.

Saildrive vs. Conventional Shaft Drive

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