CE on Trimaran how to calculate?

[catsketcher]

Gday

As an ex Laser sailer I am really into the idea that you should steer with the sails AND the helm. This will make the boat go faster. It is probably one reason why dinghy sailors are put on big race boats. They have had all the bad habits thrashed out of them.

If in doubt reverse engineer. Get as many shots of good tris and see where there boards are relative to the mast. Usually racers are 3/4 rigged and have their mast stepped on the forward beam. Some have the board behind the mast - most Graingers, under the mast Farrier or in front, ORMA 60s often have this with rake. My Twiggy and the Newick I built had the board just aft of the mast.

cheers

Phil

[catsketcher]

As to twin boards - they can help with trip but they make the boat harder to maneuver. You don't have a central pivot point but have to drag both boards through the water around some point in between. Good for offshore but not for bay racing.

Why move the mast so far back? The Twiggy did this and it was not a good idea. The boat would have been better with a more normal rig. Most tris have the mast 40% from the bow. That seems normal.

[Samnz]

Originally Posted by catsketcher

As to twin boards - they can help with trip but they make the boat harder to maneuver. You don't have a central pivot point but have to drag both boards through the water around some point in between. Good for offshore but not for bay racing.

Why move the mast so far back? The Twiggy did this and it was not a good idea. The boat would have been better with a more normal rig. Most tris have the mast 40% from the bow. That seems normal.

twiggys mast was well aft of 50% wasnt it? more like 60%?

My theory on why it didnt work is as it burys the bow the top of the rig goes higher... not good to prevent pitchpoling!

I cant remember seeing a racing multi with the rig foward of the centre, thinking of orma 60s, and the 100+footers.

Normally when I helm well balanced mulihulls I find dropping the traveller 100mm if the hull flys to high then immediatly raising it back to where it was as the hull starts dropping is fast, correct me if im wrong? dont see any reason why you should have to drop the traveller to prevent massive weather helm?

[catsketcher]

It wasn't really the mast raising that was the Twiggy's problem. She was very stable sideways and so Lock put a big rig on her. He knew that she would be deficient in stability forwards so moved the rig back. It sort of worked but it wasn't enough - the boat did not have enough buoyancy up front.

I get the weather helm problem. You shouldn't have too much but I was talking about manouvring like bearing away. In this case you should need to ease sheet. Here is where the Farriers are so good. As they have the rig up front they have a large distance between the daggerboard and rudder. So you get a long lever arm. That has to be one of the reasons Farriers are so sweet to sail and steer. Pulling the rig and board back will reduce the lever arm and may make the boat crankier. The Twiggy could get pretty stroppy. You may find you get the same rudder loading as the rudder needs to be at higher angles of attack than a boat with a mast and board forward.

The Orma boats are a box boat so if yours is too then that may be a good way to go. Like the Twiggy it seems as though there is a need to get the CG aft and taking the rig and beam back does this.

cheers

Phil

[terhohalme]

CE and CLR are normally realized as geometrical (static) numbers. In multihulls the lead shoud be 0 (+/- 2%). Dynamic steering balance is much more compilated than just CE and CLR.

[Samnz]

Originally Posted by terhohalme

Dynamic steering balance is much more compilated than just CE and CLR.

please expand on this?

[terhohalme]

When a multihull heels (few degrees) CE AND CLR moves sideways to leeward and turning arm stays small, so in general a multihull is less sensitive to turn upwind than monohull and lead can be smaller.

For other elemets of dynamics of steering, see:
Ross Garrett: The Symmetry of Sailing

[Micronot]

I know this is a little off topic. If this is out of bounds I apologize in advance. I'm interested in the CE problem. I've actually been subscribed to this thread for a while now. I think we all need a little more.

I've seen any number of books on mono hull design & actually own a couple. Does anyone know of more comprehensive design info regarding multihulls??? I have "Building Outrigger Sailing Canoes" but I'm interested more in design elements what we are discussing than overall designs.

Any references

[cavalier mk2]

Most multihull books will give you some design information but for its size Derek Harvey's "Multihulls for Cruising and Racing" gives more than most. If you like math anything by C A Marchaj is helpful, try "Sail Performance". If you like history the late Rob James "Multihulls Offshore" covered his time period well and the first hand racing accounts are fascinating as they illustrate the development in multihull design. Chris White's book "The Cruising Multihull" is well known and helpful. Of course studying the designs themselves and sailing them teaches the most but hitting the books can save you a lot of time and help you evaluate what you are experiencing.

[bushsailor]

I have just been through this. Upwind you need the board behind the mast to deal with the powerful main. Downwind it is desirable to have the board further forward.(that is why the orma boats have their board on a large angle so when the board is half up its centre of resistence is moved forward.)
I installed the board behind the mast vertical, but still had weather helm so am now moving forestay forward and going to a bigger jib. Downwind the boat still feels good, probably because the speed tends to balance the boat.

[Samnz]

Would 10 degrees be a good amount of rake? if the top of the board stuck out just in front of the cabin (like the orma's) or need more?

[bushsailor]

I would suggest closer to 20%. However why not consider lifting foils in the floats. That is what I am doing after sailing on a foil assisted boat. Over 12knots they lift the hull and let the boat accelerate. They also solve the problem of any lee helm with a big screacher. They need to go forward of the mast. Then you can put the board behind the mast.

[Gary Baigent]

Put daggers, or even slightly angled foils, in the floats Sam, mounted forward of CoE, as bushsailor so sensibly suggests, then you can forget about all this compromising with a dopey angled, very long, daggerboard cluttering up your beloved accommodation area - plus you will also get rid of weather/lee helm problems when carrying differing area sails.

[Samnz]

Thanks Bush Sailor and Gary for the suggestion, however I will stick to my centre hull board with no foil floats as designed. Main reason is that the seacart 30 is way faster than the catri 28 also the vplp 50 tri has no float boards and hes quite a successfull designer wouldnt you agree? Foil floats add weight and complication on an already very difficult boat to sail with three crew. Keep it simple I think.