twin keel design

Currently I have someone in England who will be testing on a 42 inch model my concept which is to attach a naca 00xx foil to the bottom extending outward like two spades welded together. I wanted a more fin shape however I was told that the anchor rope or chain could get caught-Design compromises. This should put the weight low and out wider than just a bulb and provide a cupping action(like a scheel keel).
I would appreciate any input to this design. They are to be attached flat parallel to the surface of the water. They will also act as stabilizers.
The theory here is that the depression formed amidships formed due to the bow wave and stern wake will be reduced and like a bulbous bow the theoretical maximum hull speed should be exceeded.
For comparable shallow draft single keels of any design this twin keel design should be superior.
I would appreciate any comments?

 

Twin Keel Development

I've had a great interest in this subject for a long time (reference the monohull section of my website, since~ 1973).

I assume that you have been to all of the various sites on the web, and that you've read Lord Riverdale's presentation to the Royal Institution of Naval Architects, "Twin Keel Yachts --Development Over 45 Years"?? I assume this as you are going forward with some testing of your own??

I would be interested in the results of your testing. And I may be able to supply you with some additional material documentation, BUT at the moment I am too busy getting ready for some comprehensive structural analysis of my sailing rig

 

Hi, had I not been notified by e-mail I would have forgotten my posting completely as I am extremely busy.
By the way since you are mentioning mast test have you ever seen a mast that is not circular but shaped like a symetrical airfoil and rotates thereby eliminating any turbulence created by the mast.
I even have a better idea than that that keeps your sail always perpendicular even when you are keel all the way over.
No, I have not read Lord Riverdales complete research however to fine tune my bublous bow and the location of my the bulb will be adjustable and at top speed(motorized) I will see the results immeadiately. Similarly with the twin keels I will do the same-bolt it on with a plate from inside and keep drilling holes and relocating it until I find the ideal spot. It may sound crazy but I will cut out the centre part anyway and if I reuse the holes I may end up with only one extra hole. I have been doing some further thinking and will drag my bulb keel through the water and measure where the trough occurs and similary with the keels find where the crest occurs and then match these to the hull's bow wave-the trough of the bulb matching the crest and the crest of the keel matching the trough of the wave created by the hull. Then I will know where pretty closely they should go.
I understand that Lord Riverdale wanted a heel angle of only 20 degrees and I would like at least 30 degrees. Also I beleive that the hull shape has a lot to do with where to locate your keels. Sometimes you have to adjust your design for convenience so that anchors don't get caught or removing the battens from your sails so that you have fewer repairs.
I believe many designers do not put the bulb bow close enough to the surface so that no wave is created. I have a book here written by an expert who shows two pictures of yachts one with and one without and no difference in the wave. I would hang my head with shame if I published anything like that.
The twin keels are supposed to create a crest right in the trough but I have never seen any pictures like that but I have seen an excellent picture of the bulbous bow on the Japanese ship Yamashiro Maru- if you do a search for " Ripples in Time Bulbous bow " you will see it and then say -Hey that's how it works!- the trough of the bulb coincides with the normal bow crest. The bulb bow is supposed to be only effective over 5 knots with increased drag and resistance below that. I question if designers have a proper handle on it so I was considering one that I could remove for slow speed and move it deeper to reduce slapping in higher wave conditions.
Anyway still investigating twin keel design.
A picture is worth a thousand words sometimes
Take care
Tom Triglav

 

mast design

I am currently working on a mast that is rotatable, electricaly and manualy , shaped like an airfoil that also tilts from side to side so that the mast is always vertical. Does anyone have any idea what precentage increase I will achieve? I would also like it to fold down however I haven't reached that point yet.
Thanks for any replies.
Tom Triglav

 

I would expect the horizontal component of the lift to vary as cosine(heel)^2, and the induced drag to also vary as cosine^2. There would also be an increase in dispalcement that goes as sine(heel).

So it depends on what your basis of comparison is. A catamaran wouldn't benefit very much, but if you normally heel at 30 degrees it could be significant.

 

twin keel purpose

Well, I had a dream last night and my completed yacht was sliding through the water and I could float outside, in the water, etc. and could see the way the water was flowing. The bulbous bow worked perfectly creating a cancelling wave for the bow wave but when I was floating underneath the twin keels it wasn't what I expected. Instead of creating a wave as I had expected the twin keels kept the flow of water underneath the boat and redirected it out at where the trough was supposed to begin reducing the wake. It worked perfectly but it was not what I expected. Any ideas on this? Also the shape of my bulbous bow and the bottom of my keels was extraordinary. The bulb shape was round on the horizontal plane but verically it was shaped like a foil with the chord being half of the length with it about centered. The end part of the foil was protruding above the surface. The wave generated was terrific as the size of the bulb was extremely small. I assume with those shapes there should be no pounding whatsoever. Oh and the bottom of the keel-so beautifull. I can't wait for testing. I am going to carve up a model today. Maybe I should just go ahead an build it as dreamed. The mast was terrific too. There was a pin on the port side and a rotating pin with a bolt through another pin on the starboard side. One side would pivot up and down on the pin and it sat on a big gear with teeth and this rotated the whole mast. If you wanted to fold it down all you had to do was rotate it 90 degrees and screw out the bolt completely and it would pivot back and be horizontal with the deck. I guess this dream yacht will soon be a reality!!!
I have now selected a NACA 63(2)-015 foil which has a low drag bucket at -2 to 2 degrees of .005 for a section coefficient of drag.
Theoretically if you design for speed the widest part of your yacht should be at 55-56% of water line. Co-incidently the location of the keel would be centered here(45 to 65% of water line length) and the trough forms just behind this location. Any ideas?

 

Thought I would add a little fuel to the fire with regard to twin keels.
As one who was associated with Lord Riverdales "Bluebird of Thorne" from the original table napkin idea to replace the old "Bluebird, via visits to boatyards all over the country to pick brains and ideas, through to the shake down cruise and first trip to the West Indies. To anyone who knows the boat it will come as no surprise that I would'nt change a thing. The twin keels were the outcome of testing two sets of aerofoil section keels and aerofoil section twin rudders. The final selection was almost a toss up they were so close. We had keels "A" + rudders "B" and Rudders "A" + keels "B" until we were seeing them in our sleep. The testing was done at the Saunders-Roe testing tank on the Isle of Wight. When the decision was made I doubt we would have worried much if we had got them mixed in error they were so close. She was a lovely handling boat, sea kindly and by no means sluggish as some people think twin keelers have to be. I think they are mixing true twin keelers with so-called bilge keelers which are usually a compromise on boats that were originally designed to be fin keeled.
She would not point as close as a good fin keel yacht. But she got pretty close to it. I have been in some fin keels that would have come second to her in the pointing stakes. Downwind and with the wind abeam she was a very comfortable boat and could be sailed hands free for quite long periods.
I said earlier that I would'nt change her. I must qualify that. Robin Riverdale was a great believer in his wishboom for its perfect mainsail shape when filled

 

Bluebird of Thorne (part two)

I agree that in practice the sail took up the theoretically perfect shape, which a standard boom is unable to do, due to the straight rather than curved foot. But on reflection I think I would have preferred roller reefing from a working standpoint. As point of interest, the handrails from wheelhouse to saloon 7ft of twisting ivory were Narwhal tusks, purloined from Lord Riverdales sister in Ambleside in what was then Westmorland. The two ships wheels in teak and "Birmabrite" sandwich construction were made by myself in our workshop in Lord Riverdale's cellar at his house "Ropes" in Grindleford Derbyshire. I read that some people are worried that they cant "rock" a twin keeler out of mud if it settles down. It brought a smile to my face. We have been "muddied" many times, she just eases herself up as you wait for the tide to rise. Never got anchor chains or lines caught either.
As for Lord Riverdale himself, he left a void in many peoples lives when he died. A peer of the realm with a lineage going back to some of the distant kings of England with names like "Beowulf" He was a tough,kind resolute man with a wonderful sense of humour. Godfather to my kids and he's missed

 

Twin Keel Experience

Hello John. Thanks for your participation, and first hand accounts.

I wanted to make you aware of a more detailed discussion of this subject on another portion of this forum, "Twin Keel Research" http://www.boatdesign.net/forums/showthread.php?t=5315&page=1&pp=15&highlight=twin keels You might even consider adding your comments to this other subject thread as well as any additional ones you may have.

I took notice that you just recently joined the forum. Just to make you aware of it, the "search" function button on the header is a pretty useful tool if you want to find all the subject threads that might mention this subject and/or others.

 

I would agree with you 200% having built a replica of Bluebird from the plans, but not overlaped plating, a fast comfortable yacht. It was interesting how many people knew of Bluebird and how many "replicas" had been built from the drawings in The Proper Yacht. One I was shown over had virtually 3 levels looked like a block of flats. Owner proudly informed it was the same as Bluebird.
Incidently Skips visited me to check on my workmanship, a great day, he could remember every detail a great help.
I built Kallista11 as a composite, steel hull 6mm alloy deck and deckhouse worked well.

Doug Probert

 

This is my first post to this forum. Hi all!

Many years ago...(about 40) I "amateur built" an "amateur designed" 19ft 6in bilge keel sloop "Alfresco" that appeared in "Sea Spray" magazine in NZ. It was quite a primitive hard chine plywood hull with a block of lead, and two steel plates bolted to the bottom. It wasn't fast but the ability to dry out, relatively shallow draft, and clear cabin sole, were big advantages. About 3 years ago I found a Robert Tucker "Ballerina II". It was a very similar, but prettier boat, of similar age. It was as if I had found my old boat so I just had to buy her. It took a couple of years work to realise that she was beyond restoration. I managed to track down a copy of the plans from Tucker Designs now operated by Robert Tucker's son Tony. We may rebuild her from scratch.

My wife and I would like a boat the we can use to explore Pumicestone Passage, the channels between Moreton Bay and The Broadwater, and trail to spots like Lake Wivenhoe, Lake Awoonga and Tin Can Bay. A bilge keeler seems ideal for some of these spots and I would quite like to design her myself.

Apart from the practical advantages of a bilge keel design mentioned above, there does seem to be a pretty strong argument that acceptable sailing performance can also be achieved mainly due to the ability to use assymetrical foils so that the shape of the fully immersed leeward keel provides optimum lift on each tack.

The major problem to address to my way for thinking is when running on a more-or-less even keel. Surely then space between the keels would act like channel forcing the water to flow for-and-aft only and preventing any flow to the side. Ideally I suppose the lateral angle of the keels could be aligned to the natural flow that would occur if there were no keels. It seems unlikely that this angle would be the same at all speeds, and quite possible that such natural flow would follow a curve.

In summary, is it not likely that the optimum keel design for builge keels would be significantly different for different angles of heel (from zero)?

If so, would it be worth considering trim tabs, or even rudders, on the keels to allow more flexibility?

I am just thinking "out loud" but would be interested to know what others think who may have progressed further done this path.

John Tennock

 

--------------------------------------------------------------------------------

To Whom It May Concern,

My husband and I are in the process of purchasing a sailboat for the purpose of offshore cruising. We are both 24 with limited sailing experience and we are throwing everything we have into this endeavor.

We have found a sailboat that we are in love with. It is a Marimba 35, a one-off, designed and refit by Michael McGrath from Victoria, British Columbia. Michael has succesfully sailed it for five years offshore and it is completely outfit for our intended destination.

The boat seems to be completly ahead of its time, all the boats we have found in our area subscribe to traditional design features. Our reservations about the boat are primarily to do with its design features that we do not understand. Namely the hull (Monocoque construction, strip planked Western Red Cedar with fibreglass overlay) and the keel (twin-keel "medium-aspect foils with an extended 'torpedo' foot"). We have done some reasearch into the construction of each and it seems that "if it's done right" it is perfect for our purposes. However, we are not in a position of experience to know if it has been "done right". The only person we know who understands the design of the boat seems to be the owner himself and we obviously cannot rely entirely on his opinion.

I have an extensive list of some of the design particulars. If you, or somebody you know has a better understanding of effective twin keel design can you refer us? We will soon have the plans for the boat as well as a photographic record of its construction. We are looking for somebody who can independantly evaluate this material as to the soundness of the boats construction and educate us as to its design principals, and theoretical handling. We are trying to get a sense of what the potential drawbacks are for this boat so that we can evaluate it better when we take it out for sea trial and have it surveyed.

Thankyou for your help.

Drea Pinnegar
604.845.6666
todreamalife@yahoo.com

 

Other Twin Keel Discussions

Drea, did you take a look at these other discussions ?? http://www.boatdesign.net/forums/showthread.php?t=5315&page=1&pp=15&highlight=twin keels