Bilge keels/fin downwindrolling

snowleopard said:
Downwind rolling is a direct result of a shortage of hulls. :D
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This is fred drift,But I have to admit that I have slept very peacefully in a 27 foot Wharram cat running downwind in a gale ,but there is no space to keep such a boat in the marina in Cangas,so the choice of boatis between a bilge or fin.
 
Sections 4,9 address the rolling issue
The main advantages are as follows:

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. Modern twin keels are of high aspect ratio and present less wetted area then a full keel or long fin keel while retaining the steady helm associated with full keels.

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. Also with this cant outward from the vertical, leeway forces water up to the root of the fin as opposed to spilling over the tip in a single keel. Hydrodynamic tests have shown that decreasing end tip loss can double the effectiveness of a fin (the sole purpose of keel winglets).

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; by reducing wave making resistance at hull speed where it constitutes 85 - 90% of total resistance. To ensure the desired effect is at cruising speed the correct fore and aft placement of the keels and proper proportions must be checked by model testing. This placement is critical, as the model data shows. Too far forward or too far aft and the resistance will dramatically increase.

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. The effect of this lift is to flatten the trim angle, i.e. reducing squatting, which flattens out the wake and lowers the resistance.

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. In the case of the motorsailer we were testing, 85 h.p. would produce 14 knots. Also the yacht can be controlled in reverse, which is seldom true of single keel yachts.

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 ability equal to that of an ordinary yacht is achieved on a fixed draft approximately comparable to that of a centerboarder without the problems associated with lifting foils. 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. Righting moment and range of stability are at least equal to those of a well designed centerboard yacht of relatively deep fixed draft, because ballast can be placed in each fin the ballast is as low as any keel-centerboarder.

11. The general advantages of twin keels include the ability to take groundings in a level position. This allows the bottom to be cleaned and painted (although the shorter and shorter keels are making this more precarious), without the cost and nuisance of a haul out, as well as being easily shipped without a cradle. 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.
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Read the rest at http://www.brayyachtdesign.bc.ca/article_twinkeels.html
 
I find that sea & wind conditions are more critical than keel configuration for 'orrible rolley sailing! But, given that all things are equal, I doubt that my bilge keels will provide a noticeably smoother platform than a fin keeler.
 
Many bilge-keelers in the past were designed with a hull shape that discouraged heeling or roll, and contrary to LS's data made them rather slow. It would be interesting to compare versions of boats like Sadlers with fin and keel options, and though I know that information about stability and leeway are known, I am not aware of anything about rolling.

As well as the pendulum effect, you need to take into account the damping effect of the area of keel, where a deep fin will score better by virtue of the length of its lever. I have compared my 1.5m shallow fin against the standard 1.8m and although speeds are mostly very similar I had an experience this summer sailing on a broad reach in light wind but in disturbed water (near the Alderney Race) with a deeper sister-ship when it appeared that the other boat was sailing more steadily and I was losing wind from my sails flapping more.

My guess is that for the same hull design bilge keels would roll more as the keels have less "bite".
 
It either is or isn't the case that bilge keelers are better or worse, and talking about it won't solve the question, and I'm not sure that tank testing will either. The trouble is that current craft with both options have been optimised usually for the fin keel and all, like the Sadlers are worse in most respects with bilge keels. If one were to compare a hull optimised for a fin against one designed for twin keels I suspect most of us would be sceptical about the validity of the results because of the confusion between hulls of different design. I don't doubt the theoretical advantages of bilge keels (as well as their appeal for practical reasons) but that doesn't always translate into real differences in practice.
 
My two ha'porth.
Having owned a bilgekeel that I did take offshore, once, I can attest to the kinder motion downwind and the definite dampening effect of having two generously sized keels-(though part of this may also be that twinkeelers tend to be shallower drafted)...

Mine was quite small ( Corribee at 21feet) and what made a huge difference ( improvement) to general behaviour was to grossly load up the boat with stores, gave it a much dampened motion without losing speed, at least downwind.

As an aside, Mogy, I reckon we met, Cangas, got to be 12 years ago now though? Some reference to Spanish hospitals treating their clients like a saussage-making enterprise, ring a bell at all?
 
Blueboatman said:
My two ha'porth.
Having owned a bilgekeel that I did take offshore, once, I can attest to the kinder motion downwind and the definite dampening effect of having two generously sized keels-(though part of this may also be that twinkeelers tend to be shallower drafted)...

Mine was quite small ( Corribee at 21feet) and what made a huge difference ( improvement) to general behaviour was to grossly load up the boat with stores, gave it a much dampened motion without losing speed, at least downwind.

As an aside, Mogy, I reckon we met, Cangas, got to be 12 years ago now though? Some reference to Spanish hospitals treating their clients like a saussage-making enterprise, ring a bell at all?
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I remember the phrase as I was fed up with the number of paitents my wife had to see daily,what boat did you have?
 
Wee blue junkrigger, but we all met ashore in the evening cafes-as you do. The beautiful Angels ring any bells and that weird big white mfv conversion with epoxied(!) box masts that later sunk...or my lovely young English neighbour who sailed out there with her 3 yr old son, all impressive.
Is Cangas still as laid back now ?

Regards Tim
 
I was impressed by the information supplied on the performance of bilge keelers. But then I started to think that if they offer superior performance downwind and upwind, why is it that our racing brethren haven't adopted them?

As for rolling downwind, as has already been said, I suspect this owes as much to hull shape as anything else. What has surprised me is the effect of speed as the faster you go, the less you roll, and because of that I have found that a cruising chute improves life tremendously.

I think the reason for the improvement is that as the speed through the water increases, the keel can provide more damping as it is always passing through clean water, and secondly the frequency that with which you are overtaken by waves and the speed at which the pass underneath the boat is greatly reduced.
 
The builder of Kingfisher Yachts, Rags Nierop, was an aircraft designer who apparently always insisted that they were twin keeled, not bilge keeled. He incorporated the principles of air/water flow to improve the way the boat went. There is no reason why they should be inferior to a fin keel, but its a bit difficult to compare because he never put a fin keel in production. He tried one, but there was no advantage.

It was good enough for Blondie Hasler who helped in the development and sailed Kingfishers, on which he developed his wind vane.
 
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