22 - 24 trimaran

Dagger board strength

Dagger boards need to be thought through just a tad more than putting enough carbon in to not break. In fact you do need them to break if you hit an obstacle with them and that means sufficient strength in the case to hull areas just behind the bottom edge and the front edge at the top. Too much strength in the board will seriously compromise the hull and you are far better to break the board and sail on without, than punching a hole in the hull.

Also nothing is worse to repair than the case ( from experiance of breaking a board or two ) and around it as it inevitably means replacing the whole case ( surprising how a screwdriver in the appropriate place can delaminate the resin sufficient to get a whole case out without damaging the hull ) or cutting big holes in the boat to get to the case.

 

damage control?

Wayne, we are talking about lifting foils here, not conventional daggerboards, loads are much higher - and anything is going to break slamming into sunken rock at 20 knots, don't care how heavily you have constructed boards or foils.

 

Things that go bang in the night.

I have navigated by "touch" all to many times, and have rebuilt several trunks because of it. The forward upper part of the trunk usually splits, builders seem to re-enforce the trunk to hull joint but forget the upper part. The glass/foam production boats are often not put together as well as the wooden ones I have seen. I build them strong, add crush blocks, and hope for the best. It is after all a very thin boat that goes really fast, sometimes it is going to get damaged. That is another good reason to sail a tri- a pontoon boat with a spare hull.

 

That is why a pivoting centreboard is the best of all worlds,--for a Trimaran.

It can be fitted in the main hull and concealed in the cabin furniture. The control lines can be fed up a PVC tube immediatly behind he mast foot and led back to the cocpit, to nylon jam cleats. Upon going aground whilst under way the "Down" line simply pulls out of the jam cleat and the board pivots back without damage. Control from the cockpit is safe and convenient, and when the board is fully up the centreboards slot is closed with a neoprene seal. It does it's job of resisting leeway without the extra induced drag of having to produce a vertical lift component, as in a canted board.

In lifting foils, this induced drag is vastly more than compensated for by the reduction in form and skin friction drag of the hull(s) being lifted out of the water. (Ask Doug Lord).

With stabilising foils this doesn't happen. The hulls still stay in the water and so the extra induced drag of the canted foils actually SLOWS the boat unless they are very carefully, and mathematically designed.

My first experience with pivoting centreboards was on Dave Greens Kraken 40, "Ringo". The board in my Buccaneer 28 was an improvement of this feature and was a joy to operate. The board could be adjusted to any angle to balance out the helm on any point of sail and it could be set forward over centre to put the boat in a hove to mode for lunch, or on the starting line before a race.

Way to Go.

 

Lots of boards

I agree a centerboard is the safest and most practical in most boats, but the trunk does take up some space. A good gasket does help with the slot drag. I am just finishing a long thin dagger that fits the stock trunk with filler blocks fore and aft. It can be "raked" fore and aft about 15 degrees so it gives me something to tune with. If I hit something hard it will do lots of damage somewhere, but I did not build it too heavy. I still think I am going to put the angled trunks in the floats, I need to do some rot repair/inspection in the beam saddles and the trunks are not that heavy. I can keep foam filler plugs in which ever trunks I am not using. They will give me something to play with. What thickness ply were your boats built of? Mine and most surviving 24s here are 1/4 inch fir- a little heavy but durable. Some were glassed also, mine is not, does not seem to have been a problem. All the water/rot damage on any that I have seen came from rain water. We have a tough climate for boats here, very hot summers and freezing winters. It opens up foam/glass decks as well as wooden boats. The wood is easier to repair. Bruce

 

Very interesting Bruce.

My first four Trimarans were built and sailed in Canada where the water is not salty.
The first two were made of 1/4" Douglas Fir/Oregon). They rotted out quickly in the fresh water.

My next two were made from Samba Mahogany 3 ply (Buc 24), and Brunzeel mahogany 5 ply (Buc 28) and I had no rot problems with these.

Douglas Fir/Oregon ply is OK in salt water, except --as you pointed out--for rainwater lying in the bilges. It is also heavier.

If I was building any of these types of multihull again I would use Gaboon ply. Lovely stuff.

 

B24 changes

Miranda's original main hull board and case was a real pain - there was a mickey mouse canvas drawstring arrangement to keep the water from spraying out at speed - never worked, soaked the interior. Also by removing the assembly much more room was produced in main hull. The foil/boards in the floats would give less drag than the original setup (because they are better designed, asymmetric and have the ability of raising or lowering to alter wetted surface area without changing the balance of the boat).
I'm not bagging Lock (thought his Kraken boats were very aesthetic plus being good performers - K40 Krisis was top boat here in the old days) BUT ..... there were some dumb design solutions on the B24. The worst was the savage turn in the after sections rocker - which made the boat sail like it was a heavy displacement keelboat, dragging a very ugly stern wave. If you really want to increase performance, that is what I would do first (extend and fair the after underwater sections). Miranda just skimmed along effortlessly after modifications.
A point about centreboards: they drag (because of their long slot and difficulty of sealing same .... and they can jamb .... and to clear mud, sand or marine growth buildup, you have to disassemble case top, pivot pin, lift out board etc. Not a problem with daggers, easily cleaned. The big plus is the dagger is a far superior foil shape compared to a swing board.
jpeg gives you a better view of foil/board angles.

 

Gary, I agree with a lot of what you said.

I was not keen on theB24's original board box and location. I built mine with a vertical daggerboard with a rectangular planform. It had a fibreglass closure on the bottom which closly followed the foil shape of the board which eliminated turbulence in the box. The box, being vertical, removed the top of the box away from the neat little galley and its athwartships supports formed a nice seat for two---or the galley occupant to sit on. It was upholstered and when the board was fully down the board cap sealed the top of the box. I admit the rectangular planform was not as efficient as an elliptical one would have been, but the passing Dolphins never noticed the difference.

As for the "Savage turn in the after sections rocker", I measured the angle of the tangent to the curve of the buttocks, and it came to 10 deg. Twice the recommended maximum of 5 degs. Locks reason for that was to get enough buoyancy in the after section to support the weight of crew in the cockpit. To reduce the negative effect of this he made the aft sections more Veed and raised the transom so that (for normal use) it didn't drag in the water. this was a compromise he made to achieve his goal for the overall design of the B24. You couldn't do that with todays wide flat Trimaran tail sections.
As I said earlier:-
"The Buccaneer 24 was Locks most successful design in terms of numbers of plans sold. He set out to design the simplest, yet best performing small CRUISING trimaran, based on the idea of three sheets of plywood butted end to end. The result was a pleasing looking boat which could be amateur built very quickly, in a small space, at minimum cost."

I understand the main reason for extending Mirandas stern sections was because the two big heavy guys sitting right at the back caused the transom to drag excessively.

Nevertheless I agree that if anyone is willing to make the changes, the stretched stern is a good idea and is also very pleasing to the eye.

Regarding the pivoting centreboard, you said:-
"The big plus is the dagger is a far superior foil shape compared to a swing board."

I don't agree with that.
A pivoting centreboard is (should be) made with the optimum section when it is in the fully down mode. IE:- Sailing to windward. As the board is progressively tilted back, the effective section becoms a finer thickness to chord ratio, reducing the form drag, and, since the angle of leeway will be less on a more downwind heading, the induced drag will be reduced too. Also since a large part of the board is now retracted the skin friction drag is reduced as well. A win-win situation all round.
All the other disadvantages you state are eliminated by a properly installed neoprene seal over the bottom slot.

As for leaving the boards fully down all the time.

I remember on an aforementioned "Filthy black night " in the Summerville race on Lake Ontario, two monohulls lost their masts, three lost their rudders, but a home designed home built plywood 40' Catamaran--which had both of its daggerboads fully down, broke up and SANK when one of the boards tore the Starboard hull apart.

Nuff Said.

 

foils and steerage

Right oldsailor, the pivoting board on the Tornado works pretty well (understatement) ... BUT I'm sure if Rodney March was asked to re-update that excellent design (which is still competitive, even better, with many of the latest cats) he would put highly refined daggers into the platform. Yer can't convince me otherwise, mate, they're just better. The only advantage a pivoting board has is when you touch bottom ..... and everywhere else where it is important, the dagger is superior.
My point of leaving both foils down on a tri with dihedral, is that only one foil/board is deeply immersed - I agree in crappy conditions on a cat, both boards fully down in heavy seas is going to load them up. In a perfect world you would lift them halfway or so - but I can understand people too frightened to move from the cockpit in inclement conditions.
A little point about crew position on Miranda, we try to sail, even with the extended stern, with the crew sitting as far forward in the cockpit as possible.
Just remembered an incident which happened before we altered Miranda - returning from Te Kouma beam reaching across the Firth of Thames in a 20 knot cold southerly, we crossed the 15 nautical miles in an hour, but many times I could feel steerage disappear - and had to keep the tiller absolutely centred, with no movement until the foam and aeration disappeared and steering returned. That convinced Malcolm to begin the B24bis.

 

Got to give it to you Gary.

Miranda is a stunning looking boat.

Multis rule.

 

details

Thanks for the Miranda pic, Gary, there is much to learn from that boat. You and Oldsailor have a lot more experience in larger multis than I do, but I think we are all going in the same direction. My boat was built in Canada in 74-76, but it spent most of its life so far on the Chesapeake in salt water and had very little rot. From the remains of the daggerboard it came with, it had numerous encounters with the bottom. Good light marine ply is readily available now, and I would not use anything else for changes/repairs. It is not even very much more expensive than fir. I will have to be careful on all counts as long as I am sailing mostly in thin fresh or saltwater. One of my friends was in a long distance race in a light beach cat, (coastal) going fast, and sailed over a large manta ray- he had his boards up, the boat behind him did not, and it basicly exploded. We don't know what happened to the ray. I do have lots of airbags, I don't want to have to use them, but at least I should be able to salvage the pieces if I hit something I will keep in mind the aft extension, I know I have noticed a large wave following me around- much more than I would have expected. I have usually moved out on the tramp in those conditions, so it isn't so much my rearend dragging, and my crew is light and forward. (and she would be greatly offended if we said her weight was causing the wave- I might be single-handing) Bruce

 

Small world?

Oldsailor, I am not sure of the builder, the boat was owned in 1984 by an H William Snyders of Mississauga,Ontario, who sold it at that time. It is hull #151 and was built from plans purchased from Canadian Multihull Services, Mr Snyders may have been the builder, but my records do not show that. All of the construction was very well done, almost production quality, and followed the plans exactly. It was epoxy coated in and out, but not glassed. The fittings/crossbars seem to be as supplied by CMS. The mast has been replaced by a rotating one from a Stiletto 27 cat. I would be very interested in learning more of the boat's history. I think it is a great boat. Bruce

 

Below is a photo of the exit opening for the banana lifting foil on Coville's ORMA60 while sitting in the shop. I got to crawl all over this boat while I was in France in September and found this component of the design to be very interesting. Obviously, this section is derived for max lift at the operating speeds of the O60 potential, so your needs may vary considerably.

I just love this whole thread, you guys, there is so much collected knowledge on this topic, that it is truly fun to read each time one of you guys posts a response.

 

Great photo

Chris, thanks for the photo, I think I am convinced- Mine will be 12%. It looks like it is set at a relatively high angle of attack also. I will be lucky to go half as fast as an ORMA60 , but the dynamics should be the same. Coville's have to work in light air also. Bruce