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Old 06-01-2019, 18:50   #1
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Useful life of inboards vs. saildrives

The useful life of traditional, diesel, inboard propulsion systems -- engine, transmission, shafting, propeller -- is often given as around 10,000 hours, as a rule of thumb. Many installations make it longer than that, a few fail early. After these systems reach end of life, usually the boat is either repowered or scrapped.



In another thread, the useful life of one particular saildrive was quoted as being 2,000 hours. Anecdotal evidence suggests that this is not unusual. Saildrives are derived from outboard motors and stern drive systems, and neither of these ordinarily last much longer than 2,000 hours; saildrives themselves have been part of the boating landscape for many years and have never enjoyed a reputation for longevity.


The trend, of course, is towards saildrives. No one buys a new sailboat with the intention of putting more than 2,000 hours on the auxiliary. The powerboat market accepted the short life of stern drives and outboards long ago in exchange for the benefits they offer in speed, fuel economy, upfront cost, interior layout, and weight distribution.


In the stern drive market, there are at most a dozen or so major outdrives and volumes are high enough that margins have thinned out and costs have come down. The savvy owner of a stern drive boat can purchase a lower unit over the counter for under $1000 or an entire outdrive for around twice that in most cases. In many cases there are local stocking dealers, and once the boat is out of the water, installation is straightforward, taking only a few hours.



Is there similar hope for saildrives? Can we expect to see a future with a handful of standard sizes from two or three manufacturers, with ease of replacement of the entire assembly as a design goal? Can the long-term reliability and lifespan of these systems be improved? Does anyone think that direct-drive inboards will remain an important part of the new boat market over the long term?
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Old 06-01-2019, 19:06   #2
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pirate Re: Useful life of inboards vs. saildrives

But of course.. built in obsolescence is the rule in todays world.. keep costs down and profits up..
But then many of us have an aversion to that big hole in our hulls waiting to happen..
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Old 06-01-2019, 21:44   #3
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Re: Useful life of inboards vs. saildrives

Yeah, I do think direct drive will remain important unless the saildrive folks get their longevity quite a bit. As a group their reputation is declining and people are more and more realizing that they involve much bitter holes in the bottom of the boat.

Personally I have chosen to go the outboard route which doesn’t involve any holes in the boat.
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Old 07-01-2019, 03:50   #4
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Re: Useful life of inboards vs. saildrives

Our first pair of SD20 Yanmar Saildrives were still going well when we changed both Saildrives and engines at 6500 hours. Regular oil, seal and zinc changes and keeping free from electrolysis should last as long as engines. Current SD20s going well at 2000 hours. Apart from the output shaft which can score, not much else to go wrong unlike the bigger units with clutches etc.
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Old 07-01-2019, 09:48   #5
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Re: Useful life of inboards vs. saildrives

I agree that a shaft is easier to service, but I can not see that the life of saildrives should be less than for a standard transmission.

Saildrives need regular maintenance to live a long life. If you can keep water from intruding and protect the housing from electrolysis, they should live as long as a the engine. Many of the original Volvo sail drives from the 70s are still in service. Yanmar had problems with the clutch assembly on their SD50s, strange enough, as the same cone clutch is used in their standard transmission. There is a fix to this, though.

The same goes for sterndrives. They are used on many commercial boats, and if serviced and of the correct rating, will live forever.
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Old 07-01-2019, 11:29   #6
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Re: Useful life of inboards vs. saildrives

I have had both Volvo and Yanmar saildrives in catamarans and owned a monohull with a Yanmar conventional shaft drive that I circumnavigated the world on.
There is not much difference in the maintenance of either. The saildrives require lower shaft seals, the conventional drives require shaft log and P-bracket maintenance.
Having owned both and having done both long distance and local sailing, the convenience of the saildrive works best for me.
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Old 07-01-2019, 13:38   #7
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Re: Useful life of inboards vs. saildrives

I have an Isuzu c-240 diesel with 27,000 hours and it’s runing great. I change oil every 150 hours. In all that time I’ve changes two starters, 3 or 4 alternators, two sets injectors, two water pumps and rebuilt injector pump at 23,000 hours. No oil leaks ever and no smoking exhaust. Buy a good engine and twin disk transmission, change fluids often, don’t let it idle a lot and keep rpms under 2000 and it will last decades.
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Old 07-01-2019, 18:08   #8
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Re: Useful life of inboards vs. saildrives

I lived in Darwin for nearly thirty years and met many international cruisers. Issues with sail drives were just about a standard event. Overheating was a common complaint and this may have been due to the local high water temperatures.(32 C) Also blocking of the cooling water inlets.
A Perth company installed saildrives on a fleet of pearling work boats around Broome which also has high water temperatures (about 1995 I think).
Due to boiling of the sail drive lubricating oil they had to remove all of the sail drives and replace them with inboards/shaft drives. An expensive lesson.
I recently spent time in some South African catamaran yards where sail drives are the norm. The builders generally claim that the newer drives are better engineered but none would discuss warranties on the drives. During design and construction it is really clear why sail drives are used. They take up less space!
I asked about relative fuel consumption considering that energy must be consumed in the two right angle changes in drive direction. No readily available information.
If I was in the market, I would be doing considerable homework. If Isambard Kingdom Brunel didn't endorse it I would stay clear!
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Old 08-01-2019, 00:38   #9
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Re: Useful life of inboards vs. saildrives

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Originally Posted by billgewater View Post
During design and construction it is really clear why sail drives are used. They take up less space!
Well that, and having the engine aft means you’re berth is not on top of the engine.
But— I have to agree with boatman61 here. That’s a big hole in the hull. Where was the last one to rip out, coast of Thailand?
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Old 08-01-2019, 07:32   #10
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Re: Useful life of inboards vs. saildrives

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Originally Posted by Rapanui View Post
Our first pair of SD20 Yanmar Saildrives were still going well when we changed both Saildrives and engines at 6500 hours. Regular oil, seal and zinc changes and keeping free from electrolysis should last as long as engines. Current SD20s going well at 2000 hours. Apart from the output shaft which can score, not much else to go wrong unlike the bigger units with clutches etc.
I believe this to be of utmost importance. We've recently sold a tug that carried two generators driven by the same Perkins diesels I have in my boat. One had 23k hours and the other older one was closer to 35 or 36k hours. But the skipper who happens to be my father in law is obsessive about engine maintenance. These were inspected by Transport Canada every year and found to be perfectly fine for their intended service.

The care given any engine pays off.

I would also argue that the numbers quoted for outboards is mighty low given the fishing community I know.
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Old 08-01-2019, 10:51   #11
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Re: Useful life of inboards vs. saildrives

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Originally Posted by billgewater View Post
I asked about relative fuel consumption considering that energy must be consumed in the two right angle changes in drive direction. No readily available information.

I do know that at higher (planing) speeds used in power boats, stern drives are more efficient than inboards. I understand this to be a combination of the drag created by the shaft and cutlass bearing of the inboard, and the non-horizontal angle of the propeller centerline.


Since the vertical component of the thrust is wasted even at displacement speeds, I would imagine that this is still true at sailboat speeds.
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Old 09-01-2019, 12:48   #12
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Re: Useful life of inboards vs. saildrives

I think the comparison of saildrives to stern drives (outdrives) is a fair one. The technology is similar as is maintenance and longevity. There are many more stern drives out there than sail drives, so there is a lot of information. They definitely fail much sooner than in-board installations. no argument about it in my experience.
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Old 12-01-2019, 08:03   #13
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Re: Useful life of inboards vs. saildrives

[QUOTE=billgewater;2796964]
I asked about relative fuel consumption considering that energy must be consumed in the two right angle changes in drive direction. No readily available information. [QUOTE]


In the marine propulsion field it's generally accepted that inboard propulsion systems have about 7% power loss between the engine flywheel and the propeller. This consists of transmission losses of 3 to 4% (gear friction, oil pumping losses, windage losses) and the balance is lost in the stuffing box friction and shaft bearing friction.

The typical power losses in a saildrive run about 3% between the engine and the propeller, so they are significantly better in that respect. Add to that the gains in propeller thrust from 100% horizontal thrust as opposed to an angular thrust component in most inboards. In addition, the drag of a saildrive thru the water is less than most exposed shaft/ strut/ propeller arrangements, giving better effeciency both under power and under sail.

Seems to me that when it comes to overall efficiency a saildrive is the clear winner.

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Old 12-01-2019, 10:48   #14
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Re: Useful life of inboards vs. saildrives

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Originally Posted by Dooglas View Post
I think the comparison of saildrives to stern drives (outdrives) is a fair one. The technology is similar as is maintenance and longevity. There are many more stern drives out there than sail drives, so there is a lot of information. They definitely fail much sooner than in-board installations. no argument about it in my experience.
If sterndrives fail it is normally due to poor or no maintenance. They need to be pressure tested yearly and shaft seals need to be changed when needed.

I have seen many 40 year old Volvo sterndrives and sail drives still going strong. 40 years is in my eyes a good life span.

But, yes I think the failure rate is higher, because people tend to ignore what they can not see.
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