Saildrives?

[amphibious1]

Originally Posted by DotDun

Please enlighten us as to the quantity of moving parts in a saildrive vs a conventional transmission...?

Hope this helps
this is a yanmar sd30 saildrive 20 to 30 hp

[geoleo]

Concerning Yanmar Saildrives------Ive heard that the smaller SD20 is fairly reliable but that the larger SD50 has design related "cone slippage" and that this Cone slipping can happen even on very new ones..................

[amphibious1]

Originally Posted by DotDun

Please enlighten us as to the quantity of moving parts in a saildrive vs a conventional transmission...?

Hope this helps
This is a yanmar km2p 20 to 30 hp conventional transmission

[Jim Cate]

Originally Posted by colemj

Jim, you are making a lot of noise about "seal replacement" and "efficiency losses" on saildrives without any actual personal experience or information about them.


Efficiency losses through a saildrive is a red herring. The gears are helical and matched sets and any efficiency losses are engineered out through gear ratios and prop sizing. Exactly the same way it is done in a normal gearbox, except prop sizing can be more exact than many full keel and skeg boats with apertures and shaft drives. It isn't like these things are slipping and hardly turning while the engine screams at redline. I will bet that the practical mechanical efficiency loss (as measured by prop output as a function of engine torque) compared to a rather long shaft drive going through a stuffing box and cutlass bearing is immeasurable. Heck, I may even take the bet that the saildrive is more efficient.

I have owned boats with both saildrives and regular gearboxes. The only tranny that has given me problems was the Hurth gearbox, but both types have been equally reliable.

Mark

G'Day Mark,

I'm happy that your experiences with saildrives have been so good. But not everyone else's have been so wonderful, and so I still think that for a long range cruising boat they have drawbacks... which is, I think, what the OP was asking about.

As to efficiency, well, ALL saildrives include two pairs of gears, one to translate the horizontal output shaft of the engine to a vertical shaft down the leg, and another pair to again make the prop shaft horizontal. These are not present in a conventional shaft drive tranny. And yes, these gears are surely "engineered" as you state. BUT, the losses can not be "engineered out". A look at an engineering text will show you that helical gearsets have a best case efficiency of around 98%, and the the losses are multiplicative. Thus the two sets have a loss of .98 X .98= 0.96... an inescapable 4 % loss. In addition, there are small losses associated with the bearings that support those gears. Whether this inefficiency is important is a personal evaluation, but it is there as stated.

Maintenance issues... you have had good seal life, but if you refer to amarinesurveyor's post, he often finds water present in the gear oil when he surveys saildrive equipped vessels. This suggests that not everyone has had such good service from their seals, and that maintenance can be a problem for a cruiser, especially if not near a slipway or travel lift. Some folks with multihulls report changing their seals whilst beached, but not everyone is comfortable with this approach, and it is not always a useful solution.

Cheers,

Jim

[Savanna]

Amarinesurveyor stated on 5th January (Post 11 in this thread) that "I like the valves on the Volvo drive water intake valve better than the Yanmar, the Volvo is a ball valve and the Yanmar is a gate valve, which seems to freeze up very easily if it's ignored for very long.".

My personal experience would support this. The inlet valve on one of my two Yanmars completely froze up recently. The valve looks like it was an original (10 years old), and perhaps a little more regular use by myself would have made it last somewhat longer. However, I had both replaced with half inch SS ball valves which have the added advantage of opening and closing with a quarter turn of a robust lever instead of a number of full turns of a much smaller "T" on the end of the Yanmar gate valve. The job job took about 10 minutes for each drive. The SS ball joints were $NZ25 each. An easy and relatively cheap fix.

[DotDun]

Originally Posted by Savanna

I had both replaced with half inch SS ball valves which have the added advantage of opening and closing with a quarter turn of a robust lever instead of a number of full turns of a much smaller "T" on the end of the Yanmar gate valve. The job job took about 10 minutes for each drive. The SS ball joints were $NZ25 each. An easy and relatively cheap fix.

What Yanmar drive do you have?

Can you specify the ball valve you used? I'm interested in the thread size in the drive housing.

Thanks!

[DotDun]

Originally Posted by Jim Cate

G'Day Mark,

I'm happy that your experiences with saildrives have been so good. But not everyone else's have been so wonderful, and so I still think that for a long range cruising boat they have drawbacks... which is, I think, what the OP was asking about.

As to efficiency, well, ALL saildrives include two pairs of gears, one to translate the horizontal output shaft of the engine to a vertical shaft down the leg, and another pair to again make the prop shaft horizontal. These are not present in a conventional shaft drive tranny. And yes, these gears are surely "engineered" as you state. BUT, the losses can not be "engineered out". A look at an engineering text will show you that helical gearsets have a best case efficiency of around 98%, and the the losses are multiplicative. Thus the two sets have a loss of .98 X .98= 0.96... an inescapable 4 % loss. In addition, there are small losses associated with the bearings that support those gears. Whether this inefficiency is important is a personal evaluation, but it is there as stated.

Maintenance issues... you have had good seal life, but if you refer to amarinesurveyor's post, he often finds water present in the gear oil when he surveys saildrive equipped vessels. This suggests that not everyone has had such good service from their seals, and that maintenance can be a problem for a cruiser, especially if not near a slipway or travel lift. Some folks with multihulls report changing their seals whilst beached, but not everyone is comfortable with this approach, and it is not always a useful solution.

Cheers,

Jim

Please explain the efficiency gained by the prop moving water horizontal to the movement of the hull vs. downward at x% angle. Does that comparatively overcome the 4% loss?

[DotDun]

Bottom Line: Saildrives are transmissions that live submersed in saltwater. Yes, saildrives require maintenance. Yes, some people have had problems with saildrives.

I'm going to make a wild guess - there are 10's of 1,000's of saildrives in service. Single digit percentage of them have issues. Of the ones with issues >50% of those are due to lack of proper maintenance.

1) The most important issue with saildrives are the two back to back lip seals on the prop shaft. They keep oil in and water out. Lip seals as a technology are very good. In an environment where temperature is not an issue, they have a very very good service history, but they do not last forever.

2) An owner of a saildrive must realize they have aluminum hanging in the water and take appropriate precautions, i.e. proper bottom paint, regularly change zincs, consider galvanic corrosion possibilities and take appropriate steps.

3) An owner of a saildrive must be cognizant of thru hull seal technology and agree to check/change at periodic intervals, normal use is >7 years.

Those that jump on their boat and turn the key and only think about sundown and cocktails and not addressing routine maintenance will have problems with their saildrive eventually, i.e. the greater than 50% of those with issues.

In contrast, a properly maintained saildrive will last many years and thousands of hours.

Saildrives enable features such as engine rooms outside the living area of a boat. If that is a priority for you then you probably will live with a saildrive especially if you are buying a production catamaran.

Decide your priorities and make your choices. There is no perfect boat.

[Savanna]

Originally Posted by DotDun

What Yanmar drive do you have?

Can you specify the ball valve you used? I'm interested in the thread size in the drive housing.

Thanks!

I have Yanmar SD20 saildrives. I will try and find out what the thread size for the ball valves is when our suppliers/marine engineers start work again after their summer break in a week or so. The ball valves were listed on my invoice as half inch, but I guess that that does not tell you the thread size. However it was the same as the thread size for the piston valves which were replaced.

[Jim Cate]

Originally Posted by DotDun

Please explain the efficiency gained by the prop moving water horizontal to the movement of the hull vs. downward at x% angle. Does that comparatively overcome the 4% loss?

Well, DD, that beats the hell out of me! You will have to ask a Naval Architect that question... one who is involved with small boat design as opposed to ship design. (I think that this is important since smaller vessels often have their WL plane pitching by larger angles than the down angle of the shaft.)

Once again, my comments have been for the OP who was curious about the use of saildrives in cruising boats. If you enjoy the benefits of saildrives, that's cool with me. My personal observations of saildrive problems over the years has lead to some distrust, and I have chosen to not use one. In fact whilst searching for what turned out to be our current yacht, I turned down an otherwise very nice Sayer 42 simply because it had one. That's my view, and if it disagrees with yours, that's fine too.

Cheers,

Jim

[goboatingnow]

Having owned both. I went out of my way to avoid sail drives on the next boat . Which is why at the time I changed to a Benny 393. In my case removing the drive nearly needed a complete dismantling of the engine cabinetry.

Shafts can cope with poor maintenance and generally degrade slowly. Sail drives if they give trouble, typically require removal.

I've also experienced similar in outdrives , shudder.....

Dave

[Jimbo485]

Originally Posted by smj

I don't think I've heard of an outboard falling out leaving a size able hole in a boat. Maybe the outboard is a viable option to both shafts and saildrives. I know for me it is.

Yes, smj, it is for us also. Outboards were on our shortlist when looking for a cat and they have proved themselves so far to have more advantages than disadvantages.

This discussion, only for us and YMMV, is probably like listening to two groups of people in the early 1910s and 1920s talk about the pros and cons of horses and camels for transport when Henry Ford had already come up with a much better solution.

Outboards these days are not what they were last century, especially if you can put them in a well rather than hang them off the transom.

[DotDun]

Originally Posted by Savanna

I have Yanmar SD20 saildrives. I will try and find out what the thread size for the ball valves is when our suppliers/marine engineers start work again after their summer break in a week or so. The ball valves were listed on my invoice as half inch, but I guess that that does not tell you the thread size. However it was the same as the thread size for the piston valves which were replaced.

I appreciate the help, I would like to get rid of the stupid gate valves.

[DotDun]

Originally Posted by Jim Cate

Well, DD, that beats the hell out of me! You will have to ask a Naval Architect that question... one who is involved with small boat design as opposed to ship design. (I think that this is important since smaller vessels often have their WL plane pitching by larger angles than the down angle of the shaft.)

Once again, my comments have been for the OP who was curious about the use of saildrives in cruising boats. If you enjoy the benefits of saildrives, that's cool with me. My personal observations of saildrive problems over the years has lead to some distrust, and I have chosen to not use one. In fact whilst searching for what turned out to be our current yacht, I turned down an otherwise very nice Sayer 42 simply because it had one. That's my view, and if it disagrees with yours, that's fine too.

Cheers,

Jim

So, we really don't know the difference in overall efficiency between a saildrive and shaft drive?

I'm simply trying to understand the claimed efficiency difference.

[Savanna]

Originally Posted by DotDun

What Yanmar drive do you have?

Can you specify the ball valve you used? I'm interested in the thread size in the drive housing.

Thanks!

Hi DotDun,

My marine engineer tells me that he used a 1/2 inch nipple which screwed into the SD20 on one end, and into the 1/2 inch ball valve on the other, and then a 1/2 inch "90 5/8th inch" hose tail which screws into the other end of the ball valve. He tells me that the threads on all three components (and thus the thread size in the drive housing), is 1/2 inch BSP (British Standard Pipe).