Twin-Keels for Cruising?

[mitiempo]

It was probably an RM. They were designed as twin keelers and only recently was the single keel option added. Probably the highest performance for a production twin keel boat. RM YACHTS | Accueil

[Pete7]

Originally Posted by Dockhead

I saw a French bilge keeler dried out in Normandy -- afraid I didn't catch the name -- with extremely high aspect bulb keels -- two of them, very long and very thin. Now that must be something. I bet it goes upwind like a bat out of h*ll. The tidal range is in Normandy and Northern Brittany is the greatest in all of Europe, so bilge keels will be particularly useful there.

Almost certainly an RM from France, made of plywood but you wouldn't know even standing on board then finish is that good.

Love this photo, who needs a Rocna when you can just park a twin keeled yacht

RM YACHTS | Accueil

[atoll]

one of my puddle jumpers,at 2 1/2 foot draft we can pretty much go anywhere

[Dockhead]

Originally Posted by Pete7

Almost certainly an RM from France, made of plywood but you wouldn't know even standing on board then finish is that good.

Love this photo, who needs a Rocna when you can just park a twin keeled yacht

RM YACHTS | Accueil

That looks like it! Way cool boat!

And that photo is most definitely N Brittany! A scary place in a deep-draft fin keel boat!

[Billy Higgins]

There is a further advantage to twin keels I haven't seen mentioned - 'thwartships distributed ballast. By having the ballast keel in two separated locations beneath the boat, the rolling period will be longer.

For example, if you imagine a fin keel boat roatating around its rolling axis, the concentrated mass of the single keel almost has the boat whipping around over the keel, which itself is practically stationary. Whereas on a twin-keeler, the keel mass is not concentrated, and the boat has to move half the ballast up and half the ballast down in order to develop half a roll period. The need to accelerate the masses causes the roll period frequency to lengthen. The amplitude should be about the same, or a little deeper as the CG of both keels is probably slightly higher.

I would certainly consider a bilge keeler if one's cruising style and area of operation warranted it.

[delmarrey]

Originally Posted by Billy Higgins

There is a further advantage to twin keels I haven't seen mentioned - 'thwartships distributed ballast. By having the ballast keel in two separated locations beneath the boat, the rolling period will be longer.

For example, if you imagine a fin keel boat roatating around its rolling axis, the concentrated mass of the single keel almost has the boat whipping around over the keel, which itself is practically stationary. Whereas on a twin-keeler, the keel mass is not concentrated, and the boat has to move half the ballast up and half the ballast down in order to develop half a roll period. The need to accelerate the masses causes the roll period frequency to lengthen. The amplitude should be about the same, or a little deeper as the CG of both keels is probably slightly higher.

I would certainly consider a bilge keeler if one's cruising style and area of operation warranted it.

For some of the old farts that's just common knowledge. http://www.aivela.org/Aloisio_paper16.pdf

[Dockhead]

Originally Posted by Billy Higgins

There is a further advantage to twin keels I haven't seen mentioned - 'thwartships distributed ballast. By having the ballast keel in two separated locations beneath the boat, the rolling period will be longer.

For example, if you imagine a fin keel boat roatating around its rolling axis, the concentrated mass of the single keel almost has the boat whipping around over the keel, which itself is practically stationary. Whereas on a twin-keeler, the keel mass is not concentrated, and the boat has to move half the ballast up and half the ballast down in order to develop half a roll period. The need to accelerate the masses causes the roll period frequency to lengthen. The amplitude should be about the same, or a little deeper as the CG of both keels is probably slightly higher.

I would certainly consider a bilge keeler if one's cruising style and area of operation warranted it.

Well, wait a minute. Unless I am deeply mistaken, the inertial resistance to rolling created by ballast will not change depending on how many keels the ballast is distributed over, all other things being equal. One ton of lead one meter from the roll axis of the boat will produce exactly the same inertia whether it is in one keel, two keels, or a continuous ring arranged around the roll axis, assuming same distance from the roll axis.

Furthermore, if there is some resistance to rolling created by gravity on a single-keel boat, since 100% of the ballast will be raised by any rolling motion, this beneficial effect will be reduced in a bilge keel boat, since half of the ballast will be moving down at any given time when the boat rolls (but obviously only until the roll goes so far that that keel pass all the way under the centerline of the boat and starts rising on the other side), which will fight the weight of the other keel to some extent (depending on the angle between the keels) when the other keel tries to pull the boat upright.

So all other things being equal -- and not considering any hydrodynamic effects, about which I understand exactly zero -- a bilge keel boat should have less, not more rolling resistance than a single keel boat.

[Billy Higgins]

Dockhead - Thanks for the quick reply, but I am afraid you are mistaken in your main assumption as stated in the first paragraph, which is that distributing mass around a roll axis does not affect the roll period. It is basically an instance of Newton's Second Law. For a real-world example, imagine a racing cyclist preferring heavy wheels in a criterium where acceleration is important, or ask any racing sailor if placing half the crew on opposing decks in sloppy weather doesn't dampen rolling and keep the sails slightly less disturbed.

[Billy Higgins]

BTW, those RM 1260 sloops look really cool. I'd really like to sail one of those! (and see how they do in the roll-damping department). With that fat transom and twin keels, I'll bet it'd the most stable monohull around for its size.

[captainbri]

Originally Posted by delmarrey

Had a twin keeler once. Was great for inland waters and anchoring close to shore! BUT it didn't go to windward well in that it drifted to weather, especially with one keel out of the water. So getting up wind was time consuming in having to tack more often then others. I preferred motoring up wind.

If they made the keels longer, fore to aft, it might help a lot. And sitting on the rudder is not good for some of them.

If they made the keels longer, That would make it a catamaran.

[Eleven]

Mines an odd ball, lattice bilge keels and a swing dropping ballast keel. The bilge keels are a solid base on firmer ground/sand/gravel and the drop keel adds that extra stability by getting the weight low or reduces drag (lifted) when downwind.
I'm sure drop bilge keels (bulbous tips?) would be a worthy solution to grounding on all bottoms, running downwind smoothly etc. Has anyone tried that yet?

[highseas]

Most of Brent Swain's boats are twin keelers,a 36' on route to Cape Horn right now.A fellow sailer has a 42' junk rig Swain twin keeler,and he is convinced the motion is less.He also has a raw water keel cooler and big engine for going to windward.Brent's boats sit soley on the keels.

[Pete7]

Originally Posted by Billy Higgins

For example, if you imagine a fin keel boat roatating around its rolling axis, the concentrated mass of the single keel almost has the boat whipping around over the keel, which itself is practically stationary. Whereas on a twin-keeler, the keel mass is not concentrated, and the boat has to move half the ballast up and half the ballast down in order to develop half a roll period. The need to accelerate the masses causes the roll period frequency to lengthen. The amplitude should be about the same, or a little deeper as the CG of both keels is probably slightly higher..

I hear what your saying, but in practise I don't think there is much difference. We certainly seem to roll about the same as a fin keeled boat either at sea or in a marina when the wake of the ferry passes through.

You might also hope the larger surface area would have a dampening effect as a larger volume of water has to be moved (think of stabilisers). However, a yacht at sea sails a cork screw path so the keel cuts through the water like a knife through butter rather than a static object being forced sideways.

No, the main advantages have already been mentioned. Shallow draft and ease of drying or security ashore as they don't fall over. Strangely our yacht club still insist masts come down even for yachts with twin keels if we lift out over the winter, one day we will persuade the committee to change their minds . One member even suggested we would need to have a steel cradle I guess there are experts in bars the world over.

Pete

[ProtectTheOcean]

Originally Posted by atoll

one of my puddle jumpers,at 2 1/2 foot draft we can pretty much go anywhere

Atoll, just ONE of them? Don't be such a greed bastard, share one with me!

I'm mindful of places in west Africa that I've been, where the tide going out drops SIGNIFICANT amounts of depth, leaving anchored boats sitting on the floor, and the locals go out and play football (soccer) on the silty sand. Guinea Conakry, yet another place where one would be very glad to have those twin keels.

Seems to me the longer keels would be the way to go, rather than deeper ones. She'd be more like a full-keel vessel, and rest easier when the tide goes out, right?

[atoll]

Originally Posted by ProtectTheOcean

Atoll, just ONE of them? Don't be such a greed bastard, share one with me!

I'm mindful of places in west Africa that I've been, where the tide going out drops SIGNIFICANT amounts of depth, leaving anchored boats sitting on the floor, and the locals go out and play football (soccer) on the silty sand. Guinea Conakry, yet another place where one would be very glad to have those twin keels.

Seems to me the longer keels would be the way to go, rather than deeper ones. She'd be more like a full-keel vessel, and rest easier when the tide goes out, right?

yr welcome,i have a signet 20 bilge keeler you can take away tommorow for $800 dollars,ill even throw in some cruising kit!!