# Twin Keel is it an advantage?



## APP Mode (Dec 20, 2011)

Most of us think about twin keel as an advantage to moor in low tides scenarios. In the market there very interesting twin keel available, and my question is how these boats perform in severe weather, comparing with fin keels?

When the boat heels there is one of the keels that is further down than the other. Does the opposite one which is upwards helps to trim the boat to help the boat navigate more comfortable?


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## SloopJonB (Jun 6, 2011)

APP Mode said:


> Most of us think about twin keel as an advantage to moor in low tides scenarios. In the market there very interesting twin keel available, and my question is how these boats perform in severe weather, comparing with fin keels?
> 
> When the boat heels there is one of the keels that is further down than the other. Does the opposite one which is upwards helps to trim the boat to help the boat navigate more comfortable?


My first boat was a British bilge keeler called a Vivacity 20. It was far from fast but it was as stiff as a church. I had no basis for comparison until I went out on a Cal 20 - I couldn't believe how tender it felt compared to my twin fin.

I guess they act a little bit like the modern canting keels - as the boat heels, it is trying to lift the weather keel (and 1/2 the ballast) up and out of the water. Also, if the twin keels are canted somewhat, say 15 degrees, as the boat heels, the leeward keel becomes more vertical and theoretically should resist leeway better than a single centerline keel.

I've long wished a good designer like Mr. P would do his magic on this concept to see where it could go re: performance. There are a number of potential advantages but the increased wetted surface always seems to put people off the idea. They are popular in Europe because they can dry out without legs but the concept is usually seen on pretty tubby boats, not on anything with any performance illusions.


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## Familycruisers (Dec 15, 2011)

I have a Westerly Centaur bilge keel. Will lay over quick to a point and try to round up with some weather helm. If the wind stays constant she'll balance out and start humming but that 20+


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## zz4gta (Aug 15, 2007)

SloopJonB said:


> ...The concept is usually seen on pretty tubby boats, not on anything with any performance illusions...


Just answered your own question. 
If you want to see what's fast and what works, look into race boats.


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## Faster (Sep 13, 2005)

Here's a link discussing twin/bilge keels..

Twin-Keeler Newsletter: The ultimate Twin-Keel advantage list

It seems there are some still being made in France - Wrighton Yachts | Builder of the Bi-loups, customizable twin-keel boats.

Agree with JSB.. it would be nice to see someone (designer) tackle this from a performance perspective. To some extent the twin daggerboard/canting keel designs in ocean racing are working on this principle as the canted keel has little leeway resistance on it's own - but the righting moment is not provided/nor required by the boards so it's not quite the same thing.


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## SloopJonB (Jun 6, 2011)

zz4gta said:


> Just answered your own question.
> If you want to see what's fast and what works, look into race boats.


Actually, it's more complicated than that. It has primarily been used for it's grounding ability which is why it's generally seen on tubby little cruisers. As far as I know, no-one has ever spent any time designing one from a performance perspective. There are some potential advantages that have never, to my knowledge, been investigated or experimented with.

I'm not talking about top level race winning performance here but I think their performance could very possibly be improved a bunch while retaining their existing virtues. For example, it might be possible to improve their performance along the lines of the twin bilge-board, canting keel boats, albeit to a lower level, but without the expense, incredible complexity and need for big, highly skilled crews. I think something better than the current crop of shoal, winged keels seems within the realm of possibility if a good designer pursued it.


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## zz4gta (Aug 15, 2007)

Sloop, the reason top level race programs don't pursue a daul keel shape is b/c it's not fast. Programs don't spend millions of dollars trying to put lipstick on a pig. 

There's nothing wrong with two keels, different way to skin a cat, but it's hardly the most efficient. It's a fine design if you're looking for something you can beach and let the tide run out, but there are better ways to do everything else. Like sail.


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## mitiempo (Sep 19, 2008)

SloopJonB said:


> Actually, it's more complicated than that. It has primarily been used for it's grounding ability which is why it's generally seen on tubby little cruisers. As far as I know, no-one has ever spent any time designing one from a performance perspective. There are some potential advantages that have never, to my knowledge, been investigated or experimented with.


Actually they have RM YACHTS | Accueil


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## SloopJonB (Jun 6, 2011)

zz4gta said:


> Sloop, the reason top level race programs don't pursue a daul keel shape is b/c it's not fast. Programs don't spend millions of dollars trying to put lipstick on a pig.
> 
> There's nothing wrong with two keels, different way to skin a cat, but it's hardly the most efficient. It's a fine design if you're looking for something you can beach and let the tide run out, but there are better ways to do everything else. Like sail.


I'm actually quite aware of that - I've owned both a twin fin and a world championship level 1/4 tonner. My point was as follows (condensed);



> I'm not talking about top level race winning performance here but I think their performance could very possibly be improved a bunch while retaining their existing virtues. I think something better than the current crop of shoal, winged keels seems within the realm of possibility if a good designer pursued it.


My thinking was more along the lines of improving boats like the current crop of Hunter's, Catalina's and the like with their short winged keels. Their windward performance is pretty compromised, their stability is only O/K and they can't dry out. The RM that Miti posted is more what I was thinking of.

Hey, it works for rudders on extreme race boats.  (I know, I know. )


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## ccher (Jun 24, 2011)

A neighbor has a Bene 323 with twin keels-didnt even know they built them until I saw his. Never been sailing on her but he is an avid proponent of the design. However I think one of his primary reasons for his liking the boat is that he's able to put it on a lift at his dock-very little bottom maintenance.


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## -OvO- (Dec 31, 2011)

And that's a damn good reason. Any boat is faster than one that's out of the water being scrubbed.


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## APP Mode (Dec 20, 2011)

Faster said:


> Here's a link discussing twin/bilge keels..
> 
> Twin-Keeler Newsletter: The ultimate Twin-Keel advantage list
> 
> ...


This bloog is very interesting. Helps to understand the logic of the twin keels performance.


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## Rockter (Sep 11, 2006)

I once owned a Magyar 7 a while back.

It was a twin-keeler.

Its performance to weather was poor.

I am very much into long keels.
.


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## Brent Swain (Jan 16, 2012)

In the 80's when I first began building boats for other people, 80% wanted single keels. Now 80% want twin keels,and those who have single keels wish they had gone for twins. All who have had twin keels, including circumnavigators, would never consider going back to a single keel. I certainly wouldn't. In my current boat, nearly 28 years old,, I have never paid to tie to a dock. That gives me a huge mileage advantage over someone who worked to pay the moorage all that time. They would never sail far enough to make up for the time they wasted.
On my last two trips home from Tonga, I sailed from around Hawaii to BC in 23 days , the first half hard on the wind, not super fast, but I don't think I would have done any better in any single keeled, heavily loaded 31 footer. Heading south from Vancouver Island, I was south of Hawaii in 14 days.
One performance advantage of twin keels is you are far more likely to be sailing with a clean bottom, given it's so much easier to keep clean. 
When a boat rolls, the keel is the centre of the roll. With two centres counteracting one another, the rolling of a twin keeler is far less than that of a single keeler. Cruise ships use bilge keels for that reason..


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## PCP (Dec 1, 2004)

zz4gta said:


> Sloop, the reason top level race programs don't pursue a daul keel shape is b/c it's not fast. Programs don't spend millions of dollars trying to put lipstick on a pig.
> 
> There's nothing wrong with two keels, different way to skin a cat, but it's hardly the most efficient. It's a fine design if you're looking for something you can beach and let the tide run out, but there are better ways to do everything else. Like sail.





mitiempo said:


> Actually they have RM YACHTS | Accueil


I guess both are right. ZZ is right stating that nothing beats a long fin with a streamlined bulb on the end and mitiempo is right saying that the French (and not only RM) make performance boats with twin keels.

Actually they use them both on RM, that can have a streamlined twin or a mono keel. The difference is small but the performance is slightly better with a mono and that's why the RM that race use a mono keel (performance version).

But the guys that buy RM like to go fast and almost all chose the twin keel because that difference is really small and under a cruising perspective, even for the ones that like to cruise fast, the twin keel offers more advantages, namely a smaller draft for the same RM and the possibility to put the boat on the hard (at the low tide) on its twin keels.

I have posted about that on the interesting sailboat thread:

http://www.sailnet.com/forums/boat-...62341-interesting-sailboats-5.html#post591460

Regards

Paulo


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## PCP (Dec 1, 2004)

Brent Swain said:


> ..
> When a boat rolls, the keel is the centre of the roll. With two centres counteracting one another, the rolling of a twin keeler is far less than that of a single keeler. Cruise ships use bilge keels for that reason..


And I guess you are right too about this.

Regards

Paulo


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## APP Mode (Dec 20, 2011)

Paulo,

Thanks very much for your participation in this forum.

On the other one some coments have been very interesting, like the Sirius yachts, owners can opt for twin keels, and i want to do this forum a usefull tool !

Regards


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## SloopJonB (Jun 6, 2011)

PCP said:


> But the guys that buy RM like to go fast and almost all chose the twin keel because *that difference is really small and* under a cruising perspective, even for the ones that like to cruise fast, the twin keel offers more advantages, namely a smaller draft for the same RM and the possibility to put the boat on the hard (at the low tide) on its twin keels. Paulo


Paulo, regarding the performance difference - is it primarily seen in light air, where skin friction is such a drag factor? Do you know of any performance minded advantages to the twins? Are they stiffer for example? How about their leeway angle? Tracking ability? (is the helm more docile)


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## Brent Swain (Jan 16, 2012)

One should be wary of the keels shown in post#8, a dangerous design. If the anchor rode gets caught on the trailing edges of those keels on a lee shore, you are in deep trouble. The only way to get it unfouled is by putting a kellet on your rode, and even that wont necessarily pull it off.
When I designed my twin keeler, I copied the general keel shape of Bluebird of Thorne, and the Westerly Centaur, both tank tested designs. I found getting the anchor rode caught between the keels, and hooked on the trailing edge of one, a very commonly occurring problem. That is something neither their tank tests nor their computer calculations told them. I resolved it by welding a vertical piece of flat bar up the trailing edge of the keel, to let the rode slide off. On subsequent boats, I made the trailing edge of the keels vertical, so the rode would slide off the bottom when the boat came broadside to the wind. Experimenting with a model makes clear what works and what doesn't.
Another problem with the boat in post#8 is the extremely short and narrow attachment of the keels to the hull, which makes getting adequate structural support for the keel difficult. It takes a lot of structural support for twin keels to survive a bouncy grounding, or hull speed impact with a rock. Deep, high aspect ratio keels make this even more difficult, and more important. That is why I prefer longer, wider , lower aspect ratio twin keels.
By angling my twin keels out 25 degrees from vertical , each is 100% more effective than an equal sized single keel in reducing leeway, when heeled 25 degrees. For this reason, my twin keels have the same area combined as my single keel , giving them the same total wetted surface. Too many twin keel designs have large vertical keels, totally failing to take advantage of this factor.
Twin keels have a fraction the surface area of a full length keel.


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## PCP (Dec 1, 2004)

SloopJonB said:


> Paulo, regarding the performance difference - is it primarily seen in light air, where skin friction is such a drag factor? Do you know of any performance minded advantages to the twins? Are they stiffer for example? How about their leeway angle? Tracking ability? (is the helm more docile)


I have seen a comparison test made by the guys from Fora Marin on a RM 1200, I mean two, one with twin keels single ruder and another one with two rudders and a mono keel, but I can't remember properly. I do remember however that the mono keel tracked slightly better upwind (more close to the wind) and was slightly faster downwind. In what regards sailing the twin keel had not any advantage, however it could do almost as well with less draft and had the big advantage of standing on its "feet" on the low tide.

Regarding a bigger RM, that depends on the ballast, but for similar ballast I don't think so. However it is possible that the righting moment produced by a twin keel at small angles of heel is comparatively superior to the one produced by a traditional fin bulbed keel and that may explain your feeling regarding a stiffer boat.

Fora Marin is the builder of RM and they continue to think that the best option is the twin keel, as well as the French clients. Most of the boats that are made with a single keel are for foreign clients were the twin keel is associated with slow boats.

RM are really fast cruisers to the point of winning a comparative promoted by one of the major Italian magazine "Vela", to elect the best fast cruiser between 13 an 14m. The boat was compared with some of the fastest performance cruisers on the market, including some great and fast Italian boats. The comparison included the Salona 44, Dufour 45, Elan 450, Vismara 46, Fisrt 45, Grand soleil 46, Hanse 47, Comet 45 and Mylius 15.50.










http://www.rm-yachts.com/images/fich4b90d6b65641f-Top_Ten_Fast_Cruiser_GdV.pdf


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## PCP (Dec 1, 2004)

Brent Swain said:


> One should be wary of the keels shown in post#8, a dangerous design. If the anchor rode gets caught on the trailing edges of those keels on a lee shore, you are in deep trouble. The only way to get it unfouled is by putting a kellet on your rode, and even that wont necessarily pull it off.
> ...
> Another problem with the boat in post#8 is the extremely short and narrow attachment of the keels to the hull, which makes getting adequate structural support for the keel difficult. It takes a lot of structural support for twin keels to survive a bouncy grounding, or hull speed impact with a rock. Deep, high aspect ratio keels make this even more difficult, and more important. That is why I prefer longer, wider , lower aspect ratio twin keels.
> ...


The boat you are saying that is dangerous is a RM 1060 that is the new model from For a Marine that has substituted the RM 1050 that had similar twin keel.



















The RM are fast long range offshore cruisers and the 1050 were sailed extensively everywhere. It is one of the more popular voyage French boat and about 120 boats were made. Never heard nothing about any problem with the keels and the anchor rode and believe me, if there was a problem, with so many boats built they would have changed the design.

Regarding the problem that you mention about "the extremely short and narrow attachment of the keels to the hull", I guess that you don't know the boat. Those twin keels are a part of a steel structure that molds the hull on the interior of the boat. The support is therefore huge.


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## Brent Swain (Jan 16, 2012)

I've driven the trailing edges of keels up into the boat thru much heavier transverse plate, with much shallower, longer (less leverage ) keels . 
When the anchor rode goes between the keels, and then the boat turns sideways , it hooks on the trailing edges of those keels, in a way that is extremely difficult to get off, especially in any wind. A trendy brand name on the boat doesn't prevent this from happening. On a lee shore, it is extremely dangerous. I and others have often had that problem, so no point in claiming it doesn't happen , even if you do have a trendy brand name on your boat, to prevent it from ever daring to happen.
A couple of my 36 footers , one with a single keel and one with twin keels, had a race. The single keeler was about 5% faster upwind, but they were perfectly matched on all other points of sail. The twin keeler wanted a rematch, but he is now in the Straits of Magellan ,enjoying the huge advantage of being able to hang out in drying anchorages during the storms.


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## PCP (Dec 1, 2004)

Brent Swain said:


> I've driven the trailing edges of keels up into the boat thru much heavier transverse plate, with much shallower, longer (less leverage ) keels . ...


I believe you but a good design is not necessarily heavier and this steel structure seems strong enough for me, especially if you consider that it is over a very thick plaque of marine plywood.

I believe you but a good design is not necessarily heavier and this steel structure seems strong enough for me, especially if you consider that it is over a very thick plaque of marine plywood.










RM are designed by Marc Lombard, a trendy NA with an huge experience. There are about 200 RM built according his projects.

Marc Lombard

They are meant to voyage and several have circumnavigated. I never heard of any structural problem with any RM. The resale value of these boats is huge, probably among the best if not the best on the market and that's because they have a well deserved excellent reputation of reliability, seaworthiness and durability.

Regards

Paulo


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