Twin Keels

[Boston]

these guys seem to have quantified there research a little better than most so I thought Ild post a link to em and see if anyone else finds it as interesting as I

http://encyclopedia.jrank.org/RHY_RON/ROLLING_OF.html

would be nice if they included a key though

[White Wolf]

I have been sailing a 56ft twin keeler for the last 6 years and there are definately good and bad points.

The good:

shallow draft of less than 5ft allows access to much calmer anchorages and less fear of grounding . I have seen much smaller yachts run aground near us assuming that there must be adequate water for a larger yacht!

allows some underwater maintenance whithout a haulout and no props when it is hauled

reduced roll at anchor and sailing downwind

excellent manouverability under power at low speed as the prop sits out in the open

once heeled the boat sails much better as the upwind keel sits more vertical and the other keel provides more lift

The Bad:

in light winds it tends to wander and directional stability is poor so sail trim becomes more important ( a new mainsail may help)

leeway is much more noticable in light winds as the keels do not bite as well when the boat is not heeled

whenever people see it out of the water you get the inevitable question " how does she sail with the bilge keels?"

I think that we have all heard the horror stories of poorly designed twin or bilge keelers having poor sailing characteristics but there seem to be very few articles on designs where they got it right especially on larger yachts.

I have sailed the boat in the Southern Ocean from Melbourne to Perth including some severe weather and the boat behaved very well however I doubt that It would be much good around the cans on a Saturday afternoon in light weather!

I have had to make several changes to the original design to make the boat sail better including a change in the steering,rudder profile and balance and have now changed the fixed 3 blade prop for a 4 blade feathering one. I am hoping that will help with drag and light wind performance especially tacking.

[Brent Swain]

Boats which have " Excess directional stability" which is exactly what you want for offshore cruising, do poorly around the cans, because the same directional stability that is such a huge advantage offshore , slows the boat down when she tacks around the cans.
Brent

[White Wolf]

Brent, I think the problem is that the boat wanders a bit in light airs which may be due to the high bow being blown off and an undersized main not providing enough drive. Directional stability improves as the boat heels and this may be helped by the hard chine. Under power we have excellent manouverability even at very low speed.

Obviously being 30 tonne we do not expect good light wind performance. As a cruiser we focussed more on heavy weather survival. In under 10 knts wind we tend to motor sail .

The new prop has now been fitted and the new main goes on next week so I am hoping that light air performance improves a little. The reduction in drag from the feathering prop may improve tacking.I am hoping the new prop will also reduce the strong prop walk we had previously but we may have to play with the reverse pitch to optimise this.

[Brent Swain]

Maybe cockpit dodgers aft will offset the windage of the high bows.
Brent