Twin keel research

Twin keels have been tried for a long time (maybe 100 years). In the seventies it has been a fashion about them in Europe because of the high tides (more than 7 meters in some places of Brittany in France). And have almost disappeared because this solution is not truly efficient. Also the race rules conduct to the same kind of boat.

The "Deriveur integral" (inner ballast with daggerboard(s)) is in my opinion a better solution. You separate the problems of center of gravity and center of lateral resistance. You simplifly the study of the the foil(s).
That give sailboats with very gentle movements and very stable as you can adjust the surface of the center or daggerboard while sailing.

I've sailed on both kind of sailboats ( of similar size ana weigth) and there is no match the deriveur is superior. And the pleasure of beaching close to the cocos of a tropical island is incomparable...

Against the common feeling the deriveur is very safe in rough weather and breaking seas. There is a proven record (on more than 30 years) of safety as these boats are used for long range travels and many have been in Artic and Antartic waters.

There are inconvenients; the ballast takes interior espace as does the case of the foil. It needs also more maintenance.

Finally a catamaran with a cutter rig (two "small" rotating wing masts and fully battened sails with wishbones -no expensive hardware, no floppy headsails, no fragile rollers- and an asymetric spinnaker, no ballast to carry) is even a better solution. Safe, good speed, easy to handle (you have just to move the tiller for tacking) with plenty of deck space and beachable. Maybe the best tropical sailboat.

 

Other Discussions of Twin Keel Vessels

Just recently received this note from a gentleman who I had past discussions with. A visit to their forum highlights the fact that their are other folks scattered about out there interested in this same topic.
________________________________________________

name: Kurt - CP23D - Winyah Bay

message: Hello Brian -

I spoke with you a year or two ago on the phone one day, I was/am enthused by your mast-aft designs. I've always appreciated the time you took to speak with me that day.

Today I'm writing to ask if you'd be willing to help some sailors clarify a
point of discussion on a web board I frequent.

One fellow who sails a bilge-keeled Weatherly contends that, due to the
split-keel design, his type of vessel becomes more stable when heeled than a
comparable single-keeled vessel would. In effect, his windward keel "hikes out", like moveable ballast almost.

The primary other participant, who seems to me to be more right in his
arguments, contends that a vessels CG is fixed irregardless of keel
configuration, and therefore righting moment does not increase due to heeling
more than it would in a single-keeled vessel.

I understand completely if you don't have the time or desire to get involved. I
also know that I saw you on a boat design website forum that was quite active, but do not recall its URL. If you don't feel like adding to our conversation, would you pass this message along to that design forum, so that we can get the perspective of an actual boat designer? I'm sorry for bothering you, but you are the only designer I "know". Thanks for taking the time to read this, also.

The discussion started with this comment from the split-keel fellow:

http://bbs.trailersailor.com/forums/tsbbcomp/trailersailor/index.cgi/read/381614

No registration is required to participate on the board.

 

A scow's bilge boards are also retractable, and only sails with one down, except when tacking. On a scow, the preparatory command is not, "Ready about," it's, "Board down."

But with twin keels, the keels have to be down for both tacks so there's not as much motivation for toe-in as there is on the scows. What toe-in will do on a twin keel is shift the load to the deeper keel - which is a good thing, as it reduces induced drag. The "lift to windward" will be exactly the same as for a symmetrical single keel setup, of course, because that is determined by the sails, not the keel.

Even symmetrical twin keels get a certain amount of this automatically as the boat heels because the windward keel sees a lower angle of attack than the leeward keel. You can visualize this by considering a case in which the windward keel rotated all the way to the hoizontal (but was still submerged) - its angle of attack would be zero and it would provide no lift. So the outward cant of the twin keel is like a scow's outward cant of the bilge boards so the leeward board is vertical and has its greatest depth when the hull is at its design heel angle.

Toe-in would provide a bias to the lift of each keel so that each keel was lifting in opposition to the other when the boat had zero leeway. The net lift would be zero, but there would be a drag associated with the lift produced by each keel. However, when the leeway angle corresponded to the zero lift angle of the windward keel, then the leeward keel would be carrying all the load. Camber in the twin keels would do the same thing, since camber shifts the zero lift angle of attack. So toe-in might be a way to trade off small gains in windward performance against small losses downwind.

I doubt toe-in would have any effect on steering. The change in lift per change in angle of attack is the same anywhere in the linear lift range of a hydrofoil. The fact that the two keels are operating about different points on the linear lift curve line might have a small influence on the trim steering angle - mostly due to differences in drag. But I don't see there'd be any effect dynamically on the steering from toe-in.

 

I fully agree on that - lifting the board up will have 2 major effects on security:
1) the CLR moves aft, increasing directional stability downwind, easier to control and to keep the stern to the waves
2) if broached, there is less lateral surface in the water, unlike a deep keel that will dig into solid water, the hull stays atop and tends to bob sideways.

Stability is not an issue if designed & ballasted properly. The only disadvantage that I see is that for the same type of boat, higher ballast means more of it and thus overall more displacement.

The other problem can be the space the board takes up inside - on Yago (see my gallery) I opted for twin lateral daggerboards, with the cases relatively close to the hull. They are also more effective than a central keel or board, as the are inclined and have a higher aspect ratio for the same draft and because they come out just below the waterline they are not covered by the hull when heeled.

 

Those are quite high-aspect foils you have there; will they be operating at the required speed on a boat like that?

Even in something as quick as a Farr 52 or Open 66, you need to crack off a fraction to get the boatspeed up before you can point, because otherwise the high-aspect foils suffer from too much drag. Given that this hull doesn't seem to have the basic pace of a raceboat, will you be going fast enough, particularly when coming out of a tack in chop? I'm just curious.

 

I hope so, I hope so...
Actually we built a 10 m some years ago, with just a single daggerboard like that (less effective when heeled) and it tacked just fine. Also, when you see some of the production lifting (pivoting) keel boats, they are quite narrow because they have to get them into the bilge so as not to take to much room. There were quite a lot of them in France, benteau among others. I sailed on those as well. Can't really see a problem there.

 

Area for area, the most efficient foil is one which is long and narrow - in other words a high aspect ratio foil.
This feature is taken to an extreme by some gliders, whose wingspan is only limited by the structural difficulties of attaching the wings to fuselage.

Because a keel / foil on a yacht operates at a low angle of attack, a high aspect ratio generally is the most efficient.

Fair winds

Wynand Nortje

 

Yeah, I know that Wynand, but at low speeds when sideforce is high, the high-aspect foil can stall or at least move to the wrong part of the drag "bucket", can't it???

I'm not saying there's a problem in actually making it through the tack, but I was wondering about coming out of a tack slow and just stalling, or stalling when moving slowly through heavy chop at low speeds. In those conditions, a larger fin can actually be more efficient, of course. It's a real problem on something like a Farr 52 when you get someone in a boat with a larger surface area foil jamming you up hard off the starting line; he's holding you up high and slow and you need to crack off a few degrees to build up speed to get the high-aspect foil operating.

You won't get that problem with this boat, but there are some horrific cats around that have ultra high-aspect foils which work beautifully on paper, and don't work at all in the real world where you are operating at lower speeds at many times.

For that sort of reason, guys like Farr design their cruiser-racers like the Bendytoy 40.7 with a lower-aspect larger-surface keel, as we all know. I was just wondering about Yago's foils but he's obviously happy with them.

The old rigs are fun to work with, Stephen Rogers (I think) designed a gaff ketch with high-aspect fin keel and bulb for a family that love having lots of strings to pull but wanted to go faster. It sounded like a fun boat.

 

Lord Riverdale's paper "Twin Keel Yachts, Development Over 45 Years"

Please excuse the posting of a less than new copy. I thought I had a better copy, but could not find it. And excuse the mark-ups.

 

Here's an Australian magazine article from a designder/builder of twin kelers.

If anyone knows the whereabouts of the author please let me know.

Any clues 249?

 

Rudder Configuration??

Any idea of the rudder arrangement on this fellow's vessels (single, double, etc?)

 

Brian
I think he had twin rudders and used Riverdales paper as a starting point for his designs.

The member "CT 249" sailed against him.

I am still hoping to find him alive and kicking, but so far no leads.

 

I met Warlock and its buider Mr Wright some years ago now. I read the article posted by Mr Johns and I concurr that there was turbulance behind (as in all boats) but little of the normal wave pattern when moving very fast.
Her performance was not too sluggish either, although she was no winner she did finish well. I felt that her hull could have had a gentler run aft and that she could have been a real winner with a bit of hull tweaking here and there.

She was vice free and a real lady to her crew. As to Mr Wrights whereabouts, I do not know, if I find out I shall let Mr Johns Know.

Very intersting to see a sister ship of Bluebird was built. I hope we here more of her.

Keep up the work

Aaron

 

Twin Keel research, Southampton Univ reference

I've excerpted a portion from the paper by Lord Riverdale....might be interesting to see if we could get a copy of this research?
_____________________________________

GENERAL MODEL RESEARCH

In recent years the interest in full-scale twin keeled craft has been paralleled by model experiments of varying degrees of technical sophistication. These have been aimed more at a comparison of the absolute performance characteristics than at the evaluation of a particular yacht design and they are particularly interesting because of this.

Tank tests

Southampton University have sponsored tests on a hull fitted with a range of alternative side force producing appendages and this work, which was carried out at the tank at the British Hovercraft Corporation, Cowes, was completed in 1966. Early in 1967, the author and Mr. Paul Spens of Southampton University discussed the findings of these tests and their possible relationship to those obtained during the ‘Bluebird of Thorne program mutually agreed that it would be a pity if either set were treated in isolation as there was much to be learned by comparison and cross comment. A full report by Southampton University will be available in due course and the author therefore confines himself to brief remarks intended only to draw attention to points of interest in the work which relates solely to twin keels.

The object of the Southampton University series of tests was to examine the relative merits of a centerboard, bilge keels, and leeboards when fitted as alternatives on a shallow draught hull. The hull form chosen was that of a large yacht of 50 ft. L.W.L. designed by Philip Rhodes, an example of the beamy, shallow-draught, centerboard type which has proved most successful and popular, particularly in the USA, as a combined cruiser and racer. In order to avoid the complications which would arise from simultaneous changes of tne hull form and appendages, the small central ballast keel with single rudder was retained for all the tests, and consequently the V.C.G. remained constant. On account of possible structural and other complications it was decided that the bilge keels should not extend below the central keel. The draught was therefore proportionately less than that of ‘Bluebird of Thorne’ and the bilge keels were of lower aspect ratio and correspondingly less efficient. They were symmetrical foils of NACA 66.006 section set at 25 degrees to the vertical and parallel to the centerline. The experiment program did not permit an attempt to obtain the optimum in the design and location of the bilge keels.

 

Riverdale's dicussions with Arthur Robb

Here are some discussions between Riverdale and Arthur Robb as contained in some estate papers of Robb's I obtained from the museum library at Mystic Seaport, CT.

These discussions are dated as 1961, so I must assume they are speaking of the 'older' Bluebird vessel in preparation for designing the newer vessel (testing in 1963, commissioning in 1967).
___________________________________________

RECORD OF DISCUSSION WITH ROBB ON FRIDAY,
1st SEPTEMBER 1961


1. There is no steering power or maneuverability at low speeds under the engine. This is a definite fault and criticism. It makes extra work, often necessitating warping the ship round and is a possible danger factor when maneuvering at low speed and in restricted areas in a strong wind.

It is due mainly to the fact that the wash aft from the propellers does not impinge at all on the rudder blades, even when the helm is hard over, and therefore there is negligible steering effect in a strong wind until the vessel is moving at 3/4 knots. This is one of the points where marked improvement is required in the new design.


2. Dead and insensitive feel with the helm. To most people a criticism as it takes fun out of sailing. Some find it difficult to get used to it. There is another view point to which I subscribe to some extent, that for cruising purposes this is offset and to some extent advantageous under the three headings:

(a) Self-steering is so good that little tiller movement is required anyway.

(b) The helm can be left without locking to do jobs around the deck or in the chart room.

The worst condition for heavy and dead helm is at about 5.5 knots under power. The cause is presumably inward pressure or suction on both blades due to the propeller effect.

Note: The whole of this problem is due mainly to the combined weight and friction of two massive rudders with very heavy cone bearings. The weight alone can give a lee helm feeling at large angles of heel. Realizing this and having played safe on the areas I had two new rudders made - much smaller and narrower blades. This was a tremendous success, and though the same conditions are still marked there was an improvement under almost all conditions.


3. The tiller comes alive and is extremely pleasant at about 8 knots in a seaway. This presumably is because the forces are then sufficient to overcome the weight and friction effects.


4. The position of the propeller and the free flow of water to and from the propeller unimpeded by dead wood results in remarkable efficiency in performance and fuel economy for the power produced.


5. ‘Bluebird’ creeps slowly to starboard when unattended due to the normal propeller effect.


6. 'Bluebird’ will come about in any condition of wind and sea with any reasonable sail combination. The turn is slow, rather ponderous but certain. I should assess it as half way between the old pilot cutter type and a modern reasonably smart cruiser. It has of course some advantages when cruising with a weak crew when one has sea room. It would be desirable to smarten this up slightly but not too much. The new design should do this.


7. ‘Bluebird’ “goes where she points” to a marked degree. This can be a possible danger when the sea is so great as to pick her up bodily and alter course. She will then continue into a gybe without any self-righting tendency


8. ‘Bluebird’ will heave to almost perfectly at a slightly broad angle but unless over canvassed with little tendency to range to and from the wind and little or no forward speed. The drift to leeward is considerable and she leaves a good slick to windward. She has hove to in a force 9. Under the storm jib set on the inner stay and reefed or storm mizzen.


9. She can be made virtually self-steering on almost any course with any reasonable sail combination, and the performance with the wind on the quarter is positively sensational in this respect.


10. She will run dead or with the wind on the quarter with the main sheet pinned in, and it is consequently possible to run safely up a channel in a gale gybing under control. When she gybes under these conditions she can be held from broaching to.


11. She will bear away under almost any condition e.g. without sufficient headsail or from the hove to position.


12. She can be sailed in a circle with sheets slightly eased when entering harbour under e. g. mainsail and staysail.


13. The hull design is so tolerant that an unusually wide variety of sail combinations and reductions of sail are possible.


14. When hove down by a squall she remains under full command of the helm (and retains self- steering characteristics) to far greater angles than a normal design.


15. She is slow when tight on the wind in light winds and then makes too much leeway. The reason is no doubt a relatively large wetted surface, the keels are not at an efficient angle when upright and the draught is small.


16. Rather similarly she is slow when running in light airs. Again due to the wetted surface and the rig shows little are set.

17. She is not slow with sheets started in a breeze and she can be driven to the theoretical maximum 8 knots is frequently recorded and under exceptional conditions she has done 9.3 knots for two hours.


18. Like I suppose all vessels, there is a length and amplitude of sea she does not like but in general she carries her way well due perhaps to modest beam and fairly heavy displacement.


19. She thumps the keel particularly in a short sea when heeled slightly too far. For cruising it is well to reduce sail at this point but it is a criticism. She would be better if the keel did not emerge or thump so soon. No harm is done. It is possible to force her and sail with the weather keel almost airborne. This is noisy, uncomfortable, splashy but fast. No detrimental effect of sea breaking on round or under the weather keel has ever been recorded in this or my other two (twin) keel boats.


20. As regards power to carry sail I should assess this at just, but only just enough for cruising purposes. Nevertheless she does not in general sail at undesirable angles of keel, and the unexpected characteristic of being slightly stiffer than one would expect from the beam, the hull form, the ballast, weight ratio and the draught has been remarked and appears to be a feature of these twin keel designs.

21. The buoyant rather blunt bow form is probably associated with a lower relative resistance co-efficiency at higher speeds than at lower ones. It does not slam. It will climb any sea but it does push some water about, and I think it tends to throw her head up when pitching. As the stern is relatively fine it may be, though the ends are so well balanced, that she pitches rather more than would a boat with slightly more bearing aft. The new design will probably correct this somewhat at both bow and stern with benefit to resistance and reduction of pitching tendency.


22. At six knots under power there is little or no squatting aft. At 6.5 knots (rarely achieved as this is maximum trial speed) she begins to sit down aft 3”or 4”. With this form of stern she does not bring much added bearing into play by doing so and the hull form would not therefore react well to a speed under power of e.g. 8 knots. Under sail of course there is little or no squatting effect
rather.
__________________________________
NOTE: The museum has indicated that they could find no 'tank test data' in Robb's files, so we must assume that data is in Riverdale's papers somewhere.