Twin-Keels for Cruising?

[s/v Jedi]

Brits? They watched and learned this from the Dutch of course







cheers,
Nick.

[atoll]

untill the dutch got too big for their boots,and we took over the east and the cape with our supreiour sailing vessels lol

[s/v Jedi]

Originally Posted by atoll

untill the dutch got too big for their boots,and we took over the east and the cape with our supreiour sailing vessels lol

Lol... we both tried to get French ships because they were so much faster how we could be so daft is beyond me heheh.

Took over the east? You stole a fertile nutmeg from the Dutch Indies, brought it to Grenada in the Caribbean where we didn't look and so broke our nutmeg monopoly... and the opium trade was slipping from our hands too, what horror. But we ended up with a Dutchmen King of England, Wales and Scotland anyway

The cape... well... we found it a bit harsh for you having to sail so far to the cold south just to stay far enough from the cape.



cheers,
Nick.

[atoll]

Originally Posted by s/v Jedi

Lol... we both tried to get French ships because they were so much faster how we could be so daft is beyond me heheh.

Took over the east? You stole a fertile nutmeg from the Dutch Indies, brought it to Grenada in the Caribbean where we didn't look and so broke our nutmeg monopoly... and the opium trade was slipping from our hands too, what horror. But we ended up with a Dutchmen King of England, Wales and Scotland anyway

The cape... well... we found it a bit harsh for you having to sail so far to the cold south just to stay far enough from the cape.



cheers,
Nick.

aggh man,ons wil laat bygons bly as bygons
gooie niuew jaar! an alles van die besste van hongerie

jou vrinde atoll!!!

(my new year resolution inflict my foreign languages on innocents)

[boatman61]

Jedi... do my eyes decieve me... or is that bottom picture a Maurice Griffith Waterwitch ketch....the black with red bottom... still got the plans for that..
another fantasy...
And we all learnt it from the Vikings who kept kicking our ass....

[Vic_222]

I have a Brent Swain 36 bilge keeler and the experienced Sailors who have Sailed with me are impressed with the way she Sails. I am very happy with the bilge keels.

The Swain 36 on its way around the Horn could be the s/v Silas Crosbie from Comox?

Cheers

Vic

[Capn Jimbo]

Another old thread that comes alive I'm sure when one of us considers a Westerly, et al. Unfortunately the consensus of the forum - from mostly single keelers - is to rather damn with faint praise, ie "Bilge keels are good for very limited circumstances, like big tide differences. They are not good to windward, don't handle as well, etc.".

In other words, just fine for the Brits and the North Sea, but here? Not so much. The basis for these feelings are understood, but how much of this attitude is spot on, and how much is just repetition and possibly mythological?

One poster took the time to post quite a nice link which if read, would have gone a long way to correcting things, or at the least to present solid evidence to the contrary:

Bray Yacht Design and Research Ltd. - The Advantages of Twin Keels

For those who retain an open mind, this one is a must read. To offer a summary:

"1. Higher sailing speeds than an ordinary cruising yacht of similar dimensions. Surprisingly, part of the reason for this is the relatively small wetted surface, which yields improved light air performance."


2. The twin keels become more effective with increased angle of heel, while a single keel becomes less effective. Because twin keels cant outward at the tip, the leeward keel becomes more vertical and deeper in the water as the boat heels. The windward keel is working more horizontally creating downward lift that increases righting moment giving more power to carry sail.

3. The wave pattern reshapes to reduce the fore and aft crests. At hull speed a hollow forms amidship, but the bilge keels cause a wave to form in this hollow, canceling out the stern wave and giving a flatter wake. This increases the maximum speed of the hull, as much as 15 - 20%, in the same way that a bulbous bow or stern bustle works...

4. The deep plunging of an ordinary hull is avoided by the stabilizing action of the fins which are also very effective in dampening out rolling motions. The fins also provide a certain amount of lift to the stern at speed when the hull is upright.

5. Directional stability is markedly enhanced by the fins. This is demonstrated both by tank tests and full size yacht performance.

6. Speed and fuel consumption under power are better then usual. The prop can work in clear water without being shrouded by the keel and rudder.

7. The rudder areas are smaller for the same reason as the keels. Each rudder is more effective as it works upright, deep in the water.

8. Both keels and rudders can be asymmetrical (more curve on one side than the other) like a wing, and tailored to work on their one specific tack. This again makes them more efficient allowing smaller appendages. Generally it is felt that both the rudders and keels can be made 25-30% smaller because of the greater efficiency.

9. Windward performance in rough water is superior because of the roll and pitch dampening abilities of the keels.

10. Stability is equal to that of an ordinary yacht without recourse to extreme beam.

11. The general advantages of twin keels include the ability to take groundings in a level position. When sailing in shallow water, if one should touch bottom, the boat rights and clears itself. This is possible because twin keels draw more water when heeled than upright, unlike single keel boats which when righted dig themselves in deeper.

Indications are, from all work that has been done, that twin keels will perform as well or better in a shallow draft then a centerboarder, and definitely better then a single keeler. The key is in the understanding of the complex hydrodynamics involving the interplay of keels with the hull..."

It is well to note that these conclusions were the result of extensive test tank and real yacht testing, not just "common consensus".

My favorite myth is the alleged "running aground" advantage of a single keel. This never made sense to me, as when running aground, dropping or easing sail only brings the boat upright, and the single keel then digs in. Compare to a bilge keel which draws up when the sails are eased. I recall reading one bilge keel skippers experience which allowed him to sail to windward up a narrow channel. As each keel began to drag, he'd tack, the outside keel would lift, the tack would be completed, and he would continue until he began to touch the other side of the channel, and repeat.

This is not to take a firm position, but it IS take a much more open minded, science based source as worth our strongest and fairest consideration.

I for one, am sold, particularly in the our nearby Bahamas, Florida Keys and Caribbean.

[delmarrey]

As a previous twin keeler, I can agree with most of the above copy. But I really wouldn't wait to tack when the lower keel hits the ground. But they are more like sailing dinghies. Way too much leeway. If I had to get up wind or against the tide I ended up motoring straight into it, unless I was just out for a day sail.

What really got me was when my GPS showed me sailing backwards. IAW tacking against the wind/tide and loosing ground.

[Capn Jimbo]

Just like monokeels - some are effective, others aren't - there are twins and there are twins. For example the earlier Westerlys - pre-Centaur - did not do anywhere nearly as well as the Centaurs and later Westerlys'

Then after tank tests and for the Centaur, Dubois decided to slightly angle in the keels, which were also made aerodynamic for better lift. The result was the kind of superior performance the above cited design group found in their own studies.

The point: this extensive tank and real world tests and development showed that properly designed twins are superior to monokeels in most ways.

We would all do well to read the article in its entirety with an open mind.

Bray Yacht Design and Research Ltd. - The Advantages of Twin Keels

[GTom]

The shallow draft and the ability to dry sounds very appealing to me but apparently taking care of two keels is a rather complicated business in case of the Westerly's (centaur, fulmar, discus): https://www.westerly-owners.co.uk/we..._bedding_Keels .

I almost decided for a bilge-keel Discus but this seems to be a rather costy enterprise (hull strengthening/thorough check for fatigue + resealing).

Any of these lost keel(s) underway? That's an even bigger problem than sinking on a mooring...

[boatman61]

Originally Posted by Capn Jimbo

Just like monokeels - some are effective, others aren't - there are twins and there are twins. For example the earlier Westerlys - pre-Centaur - did not do anywhere nearly as well as the Centaurs and later Westerlys'

Then after tank tests and for the Centaur, Dubois decided to slightly angle in the keels, which were also made aerodynamic for better lift. The result was the kind of superior performance the above cited design group found in their own studies.

The point: this extensive tank and real world tests and development showed that properly designed twins are superior to monokeels in most ways.

We would all do well to read the article in its entirety with an open mind.

Bray Yacht Design and Research Ltd. - The Advantages of Twin Keels

To the best of my knowledge the Centaur, Longbow etc were all Laurent Giles designs..
The give away is the distinctive 'knuckle at the bow..
Centaur Statistics
Designer:
Laurent Giles

Keel:
Twin
Cockpit:
Aft
LOA (feet):
26' 0"
LWL (feet):
21' 4"
Beam (feet):
8' 5"
Draft (feet):
3' 0"
Centaur Keel Width:
Width at leading edge 78",
Width at trailing edge 83",
Length 64"
Displacement (lb):
6,700
Ballast (lb):
2,800
Berths:
layout A: 6; layout B: 5; layout C: 4
Heads:
1
Rig:
Sloop/Ketch (probably < 20 Ketch)
Fuel (ltr):
45
Main (sq. feet):
161
No. 1 Genoa (sq. feet):
223
No 1 Jib (sq. feet):
133
Sail ID:
CR
Water (ltr):
67
Built:
1969 - 1980
Number Built:
2444

[Capn Jimbo]

Originally Posted by boatman61

To the best of my knowledge the Centaur, Longbow etc were all Laurent Giles designs..
The give away is the distinctive 'knuckle at the bow..
Centaur Statistics
Designer:
Laurent Giles

Righto about the designer, but the point remains in re the by then improved bilge keel designs. The following is a quote by Giles regarding the Centaur:

At this stage I can do no better than to produce the words of Jack Giles from his press release of March 1969:
Westerly Centaur
L.0.A.: 26ft.
L.W.L.: 21ft. 4ins.
Beam: 8ft. 5ins.
Draft: 3ft.
Working sail area: 341sq ft.
T.M.: 6 tons
Designers: Laurent Giles & Partners Ltd.
Builders: Westerly Marine Constructions Ltd.

To be commissioned to design for Westerly Marine was indeed a challenge. With the resources of their own Research Department allied to a highly developed production line and a keen Buying Office there seemed to be a danger of too many outside pressures working on the Designers.

In practice we found that this was not so, providing trouble was taken to appreciate and allot priorities to the requirements of the various departments concerned and provided we were prepared to work within a reasonable, but tight, time table.

The first result of this co-operation, the "Centaur", has a very low waterline over beam coefficient, a clean canoe body and twin bilge keels of aero-foil section.

The power installation has its propeller working in clear water and the balanced skegiess spade rudder provides ample control under sail or power.

Under sail the boat performs remarkably well to windward at a very modest angle of heel and under power did all that was expected of her and, in addition, showed unusually good handling characteristics when going astern.

The layout below decks is spacious for a boat of only 26 ft, overall having four comfortable 6ft. 6in. berths, a separate toilet compartment and a dinette arrangement which can be converted into an additional double bunk. Despite the space below decks, there is still a 6ft. 6in. self-draining cockpit, a feature of the boat which has appealed to the American market to which an appreciable number of "Centaurs" have already been exported. 20.3.69

There is a world of difference between the earliest bilge keels and those that followed as the twin keels were refined. BTW the earlier link to rebedding the keels makes clear the work is quite amenable to the amateur/owner. Good link.

[GTom]

Originally Posted by Capn Jimbo

There is a world of difference between the earliest bilge keels and those that followed as the twin keels were refined. BTW the earlier link to rebedding the keels makes clear the work is quite amenable to the amateur/owner. Good link.

What is considered early and late? AFAIK ~early '80s Fulmars were still recalled.

[boatman61]

Originally Posted by GTom

What is considered early and late? AFAIK ~early '80s Fulmars were still recalled.

A few of the early new designs were recalled for strengthening however the later models were built to the uprated spec.. however it was not a mass problem and I think.. to be honest.. you are overly picky on boats getting on 35yrs and more old.
There's as much chance of you buying a 5yr old boat and having its fin keel drop off..
If you want to go cheap you pays yer money and take your chances.

[GTom]

Originally Posted by boatman61

to be honest.. you are overly picky on boats getting on 35yrs and more old.

Absolutely right. But this year I'll definitely decide, now the family is pushing too. Browsing infinite walls of advertisements (just e.g on boatshed I see 200 boats between 9-11m, 20+year old) makes the mind picky.

There's as much chance of you buying a 5yr old boat and having its fin keel drop off...

Yepp, that's one of the reasons I am looking around the 30yr+ boats. With a bit of exaggeration: they miss everything the new boats have and I don't need and have everything the new boats miss and I need