Sail drives, how bad?

[getyourbone]

Not to steer the conversation away from the original question but I think it could affect someone’s thinking on the subject...


Are there substantial efficiency differences between the two? I would think that the extra complexity/“bends” of a sail drive would result in lower efficiency?

Really curious and have no idea...

[a64pilot]

If you find shaft alignment a problem, this eliminates the issue.
https://www.bruntonspropellers.com/sigmadrive

[MJH]

Originally Posted by stevensuf

I am considering a boat with saildrives, though having only ever had shafts I would really prefer shafts, but just how bad are sail drives?

I hear stories of constant problems, mostly expensive, I never had any issues with shaft drive and boxes, some say sail drives are not much of a problem, others report they are hell and considering i am talking probably 500 hours or more a year, just how much down time and costs are we looking at?

I know to avoid the yanmar sd40/50, any others that are particularly bad?

I once thought of buying an older boat and replacing the engine/shaft with a saildrive. After researching saildrives it I scrapped the idea.

I wouldn't buy a boat with a saildrive or V-drive, there are too many other options out there. The former was created for easy designer engine placement and to reduce manufacturing costs not to increase reliability or reduce owners maintenance cost; the latter for achieving a specific interior design, again at the cost of greater complexity.

KISS...Keep It Simple Stupid!

~ ~ _/) ~ ~ MJH

[stnick]

A Sail Drive - Are the parts in the water Stainless Steel ? Are they Bronze ? No to both they are Alu ! DAH ! its Just cheeper kick the can down the road to the customer to replace it in 10 years !!

[Jammer]

Originally Posted by getyourbone

Are there substantial efficiency differences between the two? I would think that the extra complexity/“bends” of a sail drive would result in lower efficiency?

Really curious and have no idea...

Saildrives are slightly more efficient. While the gearing losses are slightly greater, the thrust from the propeller is pointed directly aft. With an inboard, the downward slant of the propeller causes some of the thrust to be directed downward where provides no propulsion.


Inboards also have greater drag from the driveshaft and strut, than does the smaller and somewhat foil-shaped saildrive. This is a big deal on planing boats, less so on sailboats.

[alansmith]

A charter captain in San Francisco on a large cat had to replace his saildrives due to galvanic action. Cost him over 20 K to do it. Ouch! I think my Chinese wife would stroke if I told her that.

Obviously they work and many platforms use them. I’m just a coastal sailor and outboard engine on a trimaran works for me. Can’t get cheaper.

[a64pilot]

Originally Posted by Jammer

Saildrives are slightly more efficient. While the gearing losses are slightly greater, the thrust from the propeller is pointed directly aft. With an inboard, the downward slant of the propeller causes some of the thrust to be directed downward where provides no propulsion.


Inboards also have greater drag from the driveshaft and strut, than does the smaller and somewhat foil-shaped saildrive. This is a big deal on planing boats, less so on sailboats.

I’m not so sure I buy either of those theories, and they are of course dependant on boat design. I measured my shaft angle and I believe it’s three degrees, which is negligible of course, and look at the pic to see how much drag my shaft has.

[petermjdavies]

I have had a sail drive on a vessel that I have owned for 30 years. Never had a leak. Replaced the seal twice. Both times it looked like new when it came off. I did have a professional install the new sail drive.

If you are venturing to the ends of the earth it might make a difference. If you are cruising in normal waters, no difference at all.

Buythe boat you want. It either comes with a sail drive or it doesn't. I wouldn't spend a minute thinking about it.

[JOHNMARDALL]

The advantages and disadvantages of sail drives have been well covered already, and I would simply confirm that there is a vital need to maintain anodic protection of the aluminum drive leg of a sail drive and be sure that there's never a short circuit to any component of the saildrive through worn insulation of a nearby electrical wire or cable. Keep those things in good order and you'll be fine, don't and you'll have expensive problems. Of course, the same applies, perhaps to a lesser extent, to a shaft drive system.

Good luck
John Mardall
Vetus Maxwell Boat Equipment

[Captain Bill]

I will not argue that some sail drives designs are poorly executed and I have owned two, both the Yanmar SD40 and SD60. Water intrusion is not one of the problems they have. You must pay attention to galvanic corrosion, but that is easily accomplished. You must also not use copper based bottom paint on the drives. This is pretty easily accomplished and adds very little to the maintenance overhead. If you get an older sail drive like the Yanmar SD20 or SD30/31 changing the oil can be a real pain because the only way to get all the old oil out is to drain them from the bottom. In trade offs with shaft drives you give up alignment issues, cutlass bearing, and shaft seals, all of which every shaft drive boat owner I've ever known have experienced trouble. The only time I have ever hit something and had water intrusion in my boat was with a shaft drive. I hit a crab pot and wrapped it up pretty good. It sheered the shaft coupling, pulled the shaft aft destroying the shaft seal, and jamming the rudder. When did this happen, 0200 of course. I had to go into the water and push the prop off of the rudder with my feet as the prop and shaft were so locked on the rudder I could not pull it forward from the inside. I wrapped towels around the shaft to stem the water flow and lashed the shaft forward with some line. This slowed the water flow and the pumps keeping up and I limped home on the other engine. I was doing about 6 knots when I hit the pot line. With the sail drives I have hit many pot lines and never had a drop of water from any of them. This includes one time I hit the line of a 10 pot string that had been drug under by the current while I was sailing at 8.5 knots. It dragged my boat to a halt in less that 100 feet while under full sail in 20 knots of wind. It didn't do anything to the sail drive or the seal. If you can search the internet you van find examples of sail drive seals failing because somebody hit something, but they are quite rare.


One thing to consider is if you plan on cruising is that pretty much no sailboat transmissions are designed for cruising. They are pretty much designed for a service life of 1500 hrs. On most sailboats which are used for weekends and day sailing this is long life in years as 50 to 100 hrs per year is a common usage. This is the sailboat transmissions target market, not cruisers who might put on 500 to 1000 hrs per year. To get longer life you need a transmission that is designed for continuous duty or commercial use. I personally don't know of any shaft drive sailboat transmissions that meet this standard, but I certainly don't know all of the ones out there. The Technodrive SP60 by Twin Disc is advertised as designed for the charter/commercial market and has much longer service intervals than Yanmar, Volvo, or ZF drives. I don't have any personal experience with them, but I know a guy who sells and services Beta Marine which now uses this drive. He claims he now only keeps one example of each part in his inventory because he has virtually no call for them. He says most of the parts in his inventory are the original ones he purchased years ago. Beta used to supply ZF SD10 drives and I have seen his parts inventory. There are lots more parts on the shelf for those drives and in shiny new boxes. When I re-engine I'll be looking for moving to a beta engine with the SP60 drives.

[sand6000]

Saildrive legs are aluminium and you cannot use copper based aftifouling so they have growth issues and must be hauled out at no more than 12 months and sometimes a lot sooner. With shaft drive and a good self polishing antifouling you can get 2-3 seasons.

[Captain Bill]

Originally Posted by sand6000

Saildrive legs are aluminium and you cannot use copper based aftifouling so they have growth issues and must be hauled out at no more than 12 months and sometimes a lot sooner. With shaft drive and a good self polishing antifouling you can get 2-3 seasons.

I use use Ultima Eco by Pettit and get 2 years out of it, but I put on lots of coats and until recently I've been a full time cruiser. Prior to that I put epoxy primer on the sail drives and used Pettit Vivid bottom paint. I've never had corrosion issues with either of them. The Ultima Eco seems to be a bit better from a fouling standpoint, but since it's ablative you might expect it to stay cleaner. Any growth on the Vivid came off with a light scrubbing. 1 Qt was way more than enough to do both drives.

[MAJICDAN]

Most catamarans between fifty and thirty-five feet have sail drives.
There are thousands of sail drives currently in use.
They are fine for small horsepower, light-duty applications.
Saildrives are much less expensive, very compact, leaving more interior room available, and are easier to install for the builder.
They are much lighter and easier to replace than an inboard.
They typically have a shorter life and need more maintenance than straight inboards.
Straight shaft inboards are only seen in larger or commercial duty boats.

[admiralslater]

I have saildrives on our 03 Outremer which work well . When we purchased I had a complete rebuild done simply because the boat had been an estate sale for years in the making.
I read with interest ( somewhere) about the reduced drag over shafts . I also noticed that the Volvo boats and others like them use saildrives

[Compass790]

I hear a lot more complaints on the slipway about saildrives than shaft drives.They chew anodes & the servicing prices are eye -watering. I'm a budget sailor so throwing 2k$ for a service is a no-no. Guess if you have a $500k cat it ain't an issue. Never owned one though I must confess. (Both saildrive & 500k cat lol )
I'd walk away from a possible purchase boat if it had one. I'd call bulldust on the drag claims in most setups. Wouldnt worry me too much from the safety point if it sat behind a keel except for line tangling ruining the seal.