Multihull Structure Thoughts

Skyking1. I have sailed on 37 and 42 foot cats that have destroyed 6 plus daggerboards. Each board was WRC with 100 mm of uni down each side of the thickest part and 400 biax glass wrapped around. The boards were had a 500 mm chord and were 65 mm thick. All of those cats just sailed on with no hull damage. The real trick is to ensure the board breaks before the dagger case does, in each of the above cases the cats were designed to do that. Speak to Richard Woods, he will advise about what happens when one of his cats hit something. I am pretty sure the board will break first. In the 8 meter tri jpeg 37 shows the boards and rudder. The boards have white patches in them low down. I am sure that is foam inserts to reduce weight but also weaken the board low so it will break at that point if it hits something hard. PS I have also wiped out a few rudders, again they were designed to bend the shaft before the spade rudder damaged the hull. Forward looking depth sounders are required in my world.

 

Thanks, so many people will provide disinformation regarding these things. It is difficult to understand the motivations, but what you say makes sense.
I see a Woods cat in my future.

 

Mr Woods has gone off swing centerboards due to noise underway from water churning in the case. I recall him telling a story of hitting something with a daggerboard. Apparently it got stuck but didn't leak. His website is a treasure trove of articles.

Can't find the FAQ atm but this is a good start:

Sailing Catamarans - Useful Articles http://sailingcatamarans.com/index.php/articles

If you are concerned about dagger boards or dagger rudders modifying to centerboards and swing rudders is straight forward. Many designs also have lar keels as an option. RW's designs are not the only good option but they are a good option. Ray Kendrick's cats are another possibility. Swapping to foam from ply is straightforward.

 

Hey Oldmulti. have we covered ideas for sealing centreboard slots ?
Might be a plan ?
There are lots of positives, I particularly like the centreboard bunk front idea as it does open up the interior in small boats. Servicing the board could be problematic.
There have been some interesting ideas in sealing the case over the years involving all manner of levers and flaps. might be interesting to put them all in one place.

 

I'd be interested in this topic too. I did my best with the G-32 centerboard flaps and they are better than the stock flaps, but still not great. The tri that I want to build will have a centerboard.

 

Maybe put up a sketch of the thing for me. I am not a boat designer or builder, but I do my own design/fab lightbulb moment things. Some of my best stuff was on the back of a napkin to begin with
One nagging question is how do we deal with fouling in those spaces? Just keep the board in, and then it gets fouled in if you don't keep the paint at tip-top? It seems that a slot would be very difficult to fight with mother nature over.

 

I have sometimes wondered if there could be an inflatable solution to this - something like lengths of inner tube either side at the bottom of the case which can just be pumped up a bit to give a nice, conforming seal. Release the pressure when you need to move the board then just a few strokes on a pump to make everything snug and sealed again...

 

Redreuben, you ask interesting questions. Centreboard slot “covering” leads to a few ideas. First the reason why it could be useful and a few generalisations about centreboards.

The choice between a dagger board, centre board, low aspect keel and Wharram deep V no keel is about performance especially upwind versus draft, convenience of handling, internal space and ability to run into something with minimal or no damage. Please understand a kickup centre board can still be broken if it hits something when the board is heavily loaded and does not hit something head on.

The efficiency of boards varies. The daggerboard is best with minimal slot drag, the centre board is nearly as good but has higher slot drag, the low aspect ratio keel is about 50 to 66% as efficient as a daggerboard with minimal drag bar increased wetted surface. Wharram deep V has have minimal efficiency. There is a difference between pointing high and the leeway a hull produces. Daggerboards point high with minimum leeway, deep V hulls may point high but with a lot of leeway.

Next a kickup centre board case needs to be well designed. Great Britain 3 the 80 foot Kelsall racing trimaran in 1977 blew the top of its centre board case off when sailing at high speed due to water pressure build up in the case. Flooded mainhull. Second you carry a lot of water around in a daggerboard case when sailing. Even a Buccaneer 33 case can carry 100 plus lbs of water when sailing. Thirdly the shape of the case edges can cause drag. There is a jpeg showing slots and the drag coefficient of edge treatments to a slot. A well shaped edge can minimise drag by a large margin.

Now to the real question. Centre board slot coverings. The function of the slot covering is to minimise drag of the hull slot. Racing dinghies EG Finn etc virtually require a slot covering to be competitive. The Finn hulls have a I mm indent around the centreboard slot to put in either a Mylar or stiff sail cloth covering over the slot with a cut down the middle for the varying angles of the board. Jpegs give the idea. Steve Clark C Class cat designer builder even uses slot coverings on his daggerboard cases to minimise slot drag. Quote “The best I ever came up with was .0015" Mylar, stuck to the hull with seam stick or equivalent double sides tape. 1" either side of the trunk. Don't slit the Mylar until you are done. Clean the gel coat with acetone to make sure it is sqeaky clean first. Squeegee Mylar into the tape really hard to make sure the air bubbles are all pushed out. Double sided tape is much cleaner and easier than contact cement, sticks better too. Then put a layer of insignia cloth or sticky back over the top, extend it about 1/2" beyond the Mylar. This stops the Mylar from getting caught and peeling too easily. Then slice down the middle. With care it lasts about a year, but is quick and clean enough to do before big regattas.”

Next is a variety of ideas ranging from Clarkey suggestion of inflatable slot filler to 1 guy who had solid flaps that opened for board up or down movement and closed once the board was EG down. All these ideas have 1 problem, if the board is down and it hits something you can do damage to you slot covering device. Therefore a stiff flexible solution is better.

One big warning. On larger boats that live in the water barnacles etc can grow on your slot covering scratching of locking up your board as you try and raise or lower it. Also, if you need to clean inside your centre board case you will have to do some type of removal of the slot covering or case top.

Materials for slot covering can vary. It ranges from stiff rubber, mylar, stiff sail cloth etc. The real trick is have a product that is stiff enough to hold shape but can flex to allow movement of the board. The problem is how you attach this to the hull to allow maintenance and minimal drag of the slot covering unit itself. The Finn dinghy approach of having a 1 mm indent around the centre board case for the slot covering material and some fairing compound over provides a reasonable solution for minimal drag.

Finally, please analyse the type of sailing you are doing. If you are truly a cruiser then a low aspect ratio keel may be good enough. If you trail a performance cruising tri then a centreboard may be required with a slot covering. If it can be done a daggerboard is the best but often at the cost of some accommodation.

Attached are some jpegs to give the idea. Also, the full plans for the Buccaneer 28 are at MediaFire https://www.mediafire.com/folder/aaltgc3owsb1u//Buccaneer%2028 or the Command 10 are at MediaFire https://www.mediafire.com/folder/56wr7egnhypqc/Command_10 In each case they have kick up centre board plans that are included in the PDF’s below. A jpeg of Buccaneer 33 centre board plan is also below.

Hope this helps. Further information on approaches or materials will be appreciated. Opps later add on, there is another way to fill a centre board slot, that is using Prindle 19 or Tornado 20 foot semicircular boards that always fill the slot. Not the most efficient board shape but it works well and would match a kickup board and uncovered slot.

 

My build uses dagger boards in the ama's and indicates you can cover the board with a release agent and slot it. Epoxy with microballons is packed around the board at the hull exit filling the gap. Has anyone used this method? It seems fairly easy and as it wears you could freshen it on haulouts. It would make cleaning the case from the bottom challenging I think.

 

Clarkey, my ideas revolve have revolved around using the water pressure. And perhaps fabricated in copper.

 

We have discussed the Outremer 45 before but I have found some additional jpegs showing the build. The 45 is a high performance cruising catamaran designed by Barreau-Neuman and built by Outremer to last “50 years”. The cat is 48 x 23.33 foot with a light displacement of 17,400 lbs. The cat started at 45 foot but had an extension out the back early in the build series and ended up 48 foot although it kept the “45” designation. The carbon mast is 59 foot high with a 731 square foot mainsail, a 420 square foot self tacking solent, a 473 square foot genoa and a 785 square foot gennaker. The length to beam on the hulls is 9.7 to 1. The draft is 3.33 foot over the rudders and 6.66 foot over the dagger boards. The underwing clearance is 3.15 foot. It has two 30 HP Volvo’s for manovering etc.

The accommodation is 2 double berth and toilet in one hull with the master cabin and large bathroom in the other hull. The bridgedeck cabin has the galley, dinette and navigation area. The bridgedeck is kept short to minimise weight and keep the main accommodation aft of centre of the boat. It’s a good layout for a real sailing catamaran.

The performance of these cats is very good. One tester said its well “We bore away onto a close reach in around 15 knots of breeze and were soon storming along at a very satisfying 9-10 knots. Equally impressive was the boat’s motion in the 4-5ft seas, which was easy and predictable, in stark contrast to all too many cruising cats out there. Marketing hype aside, a half-knot of boat speed more or less isn’t going to make much of a difference, even on passage. However, a nasty, whippy motion in a seaway is. The Outremer 45 is a boat that takes good care of its crew.” This is a 300 mile per day boat with peaks of 20 knots plus.

The structure is described by Outremer as “The hulls and deck are built in vinylester with a divinycell core. However, the hull layup beneath the waterline is solid glass in the interest of security should you ever experience a grounding. Outremer is not afraid to use a little more material, including carbon, in high-load areas to ensure rigidity and durability over time. The hull/deck joint is not just glued on an inward-turning flange, but securely glassed in all around, and the bows include multiple “crash boxes” to prevent water ingress in the event of a collision. The daggerboard trunks are also not only rock-solid, but the boards themselves designed to break away first, acting as “fuses,” as it were.”

The jpegs are the real story. A very good cat.

 

Another thought on daggers and centerboards.

Rather than a quarter circle what about an L shaped board. When swung down the horizontal aerofoil shaped leg goes vertical while the other previously vertical leg swings down to teh horizontal plugging the slot.

You get a pretty efficient and quiet fin but still with the ability to kick up on contact.

The case needs to be big but otherwise I can't see much downside ?

 

Guzzis3. Good idea that I have seen done elsewhere. The only down side I see is the increased size of the centreboard case. The centreboard case fills with water to above the waterline when sailing fast which then is added weight for the cat or tri to drag around. I have sailed on a 40 foot plus cat that had 6 foot water spout coming out of its daggerboard case tops at speed due to the water pressure pushing pass the very close fitting daggerboard case bottoms.

 

It also spoils the bunk front centrecase ergonomic. Which to me is a big plus in under 30’ ers
I’m sure I’ve seen somewhere before a centreboard that does a similar thing but the closing flap was on some sort of hinge and lever arrangement, anyone ?

 

Isn't there also another version of the centreboard case where the dagger board has a semi eliptic shape at the nose and a 45 degree rear vertical on the first part of the case, allowing the daggerboard to slide foward and rotate down through the slot from the confines of a sealed case ?