A couple of questions on yacht design....

beancounter

beancounter

Well-Known Member
Joined
28 Feb 2003
Messages
1,334
Location
Cambridge
Visit site
...that occurred to me whilst looking at boat ads.

1) Why have saildrives become nearly universal? To my simple mind, bending the drive around a couple of corners introduces complexity, & therefore more opportunities for Murphy's Law to be invoked. My guess is that it allows the engine to be located to maximise the accomodation space.

2) Likewise, what caused the decline in skeg (or semi-skeg) hung rudders, and the dominance of spade rudders? Performance? Cost?

Just idle thoughts during a tedious day.....
 
Interesting thoughts. I'd guess the saildrives are just simpler than a shaft type installation both for building and for maintenance. You just neet to drop in the whole unit. Whereas a shaft needs to be fitted to the boat and aligned etc. Plus the shape of the boat has to take the shaft. You can't get a flat bottom so easily. Then there's always the cutlass bearing, stern gland etc that needs maintaining. The saildrive does have its gasket to be replaced from time to time though.

Not sure about skeg vs spade though. There are certainly performance differences but what generally drives things these days is cost.
 
Saildrives are cheaper for the boatbuilder. Glass in the engine mount, saw hole in the bottom of boat, fit engine, jib done.

No tiresome faffing about with piano wire, P brackets etc etc.

Result for the owner. Saildrive a rather high maintainence system when compared to shaft drive. Possibly less vibration too.

In my current search for a new boat, sail drive is a minus point.
Blue water boats can do without the anode regime that saildrives need.

Spade rudders cheaper to build too. Narrow skegs are difficult to laminate properly If strong enough, a spade rudder is much nicer to steer than a skeg hung one.
 
Saildrives have become universal for reasons of cost, builder convenience and a certain user friendliness - win, win, win.

The installation is much easier and more flexible. Less accommodation space is lost. Annual and routine maintenance is less because no glands to grease, stuffing boxes to restuff etc. Because there is no need to align engine and drive, there is less risk of vibration due to poor alignment and less chance of the system becoming out of alignment due to movement in the beds, minor collisions etc.

The main drawbacks are: the need to replace the seal at approximately seven-year intervals and the risk of corrosion to the aluminium drive leg, which needs non-copper antifouling.

Skegs are much more difficult to mould and therefore more expensive and more likely to fail due to poor workmanship. Modern rudder design and materials technology mean that spade rudders are pretty much as strong as old skegs but there are, of course, other issues.

Spade rudders tend to give optimum performance - less wetted surface, quicker response, more turning effort per square foot and lighter tiller loads.

Skegs, or half-skegs, usually, but not always, give better handling with greater straight line stability, better control down wind and much better handling under power in a straight line.

Spades are more vulnerable to damage from underwater obstructions and more likely to get jammed by stray ropes, nets etc, though partial skegs come a close second here.

For general family sailing I can see no reason why you should not want a saildrive and a spade rudder. If I was doing a lot of deep sea stuff I would probably prefer a full depth skeg for strength provided I knew it had been moulded properly and it did not lead to too heavy steering. I might also opt for conventional stern gear because I wouldn't want to have to change a saildrive gasket on a remote south sea island.
 
Both also suit the modern, saucer shaped boat hull.
One big drawback for me is their vulnerability to floating ropes, nets etc. and this motivated me to go back to the long keel/shaft and propellor within rudder aperture for sailing around Ireland and west coast eEngland.
Ken
 
James,

Wow - thanks very much - a most comprehensive answer!

Just to put my niggle to bed tho', are saildrives significantly "higher maintenance", as michael_w suggested?

I'm happy with my shaft drive, semi-skeg Moody, but one has to think ahead to the next boat, doesn't one? /forums/images/graemlins/smile.gif
 
I built my boat with saildrives and spade rudders for the following reasons:

Saildrives: No propshaft tunnel, stern gland, thrust bearing or alignment problems so MUCH easier to fit. Engine rubber mounted and able to move freely so reducing vibration. No leaks.

Spade rudders: semi-balanced so less force required, simpler to build. Strong enough (2" solid shafts). Protected by keels & saildrive leg from ropes etc.
 
To change a saildrive seal, you need to break the engine-gearbox joint. As the modern builder has buried the engine within the boat this is usually unnecessarily difficult to do.

To change the anodes, you have to take the prop off. Also there seems to be quite a problem with tube worms taking up residence in the waterways of the leg. Plus a rather large hole in the bottom of your boat.

I'll stick to a conventional shaft, stuffing box et al. When it leaks, it's usually managable. Now think about that saildrive/ clever shaft seal giving way.
 
You must log in or register to reply here.

Share this page

Top