CE on Trimaran how to calculate?

Does anybody have the formula for calculating the the CE and CLE for a Trimaran? I know how to do it for a Monohull but its not the same for a multihull.

The boat in question is a 8.5m Tri, the centre of the mast is 4.25m from the bow. The boom will be 4.15 long and the mainsail has a 1.8m squarehead with a moderate roach and around 35m2
The forstay will be on the stem and will be overlapping with around 20m2.

Is it beneficial to have the cetreboard verticle not raked?

How bad is it to have it 100mm to one side of the centreline? the hull is quite flat on the bottom.

There are no foils in the floats, just one in the middle hull, however the rudders are in the floats like a seacart 30.

 

Sam, just draw your lines through the sails as per conventional monohull to get your CoE. The difference with a rotating mast, and more so with a
larger chord wing mast, is that the more efficient rig provides more "drive" - so the CE "position" is a little more forward than conventional fixed rig position. But everything is a compromise; monster headsails and reachers will drive the bows off and you'll have some lee helm, small flat headsail will shift CE aft and if you sail with just your large square topped, long footed main, that will give you weather helm (with dagger in conventional position a la by the book keelboat positioning) - really you need a set of balancing boards forward if you want your neutral/slight weather helm at all times ... but I guess you won't be bothered with that. Also you going to get sort of weather helm feel with your transom hung rudders - they are not like balanced rudders. Split Enz had four daggers ... which drove the crew nuts in the early days - but you can set the boat up to be perfectly balanced by lifting various boards up or down, on all points of sail. There is absolutely no difference to balance and helm feel if you shift the dagger to one side of the main hull. I mean you could put them in your floats if you wanted to, still no helm feel difference. Daggers are better vertical or even slightly forward raked. Raked aft boards are only that way because the mast and bulkheads etc. are in the road, its just a convenience, compromise thing. Also the board has to be much longer if heavily raked.

 

Any chance you can expand on that a little? Is it to do with the endplate effect?

 

If you rake them forward it slows or halts ventilation running down the leading edge of the foil, board, rudder when extreme angles of attack occur, like savage rudder movement or dagger ventilation after jumping out of a wave that has pushed you sideways. It is especially important on surface piercing lifting foils, because that's when the foil wipes out and you crash; you can easily see the mass of bubbles envelope almost the complete foil destroying lift. Big splash! It would be worthwhile doing that to your rudders ... around 3 to 5 degrees is a safe angle. Go too far and the helm will suddenly go nuts from being overbalanced. But picking up weed can be a problem - you'll have to stop and reverse. Raked aft daggers don't have this problem but they ventilate easily - because the air bubble races unhindered down the sloping leading edge. This is not quite so important with daggerboards because there is a top end plate (main hull) slowing air movement- but the other two foils work better raked slightly forward, just IMO. Underhung rudders are also better than transom setups because of again top end plate protection - also the rudder can be smaller to do the same job, plus having less drag and wetted surface. This all sound BS ... but it is what I have observed, learned empirically.
The point about Ron Given's Split Enz' four boards, she is a catamaran, hence four - and the after, larger daggers were set well aft of amidships near the cockpits and the smaller balancing boards were well forward. But a single small balancing board forward on a trimaran's main hull would allow perfect balance for all the differing sails carried.

 

Why not?

 

because the floats also act as foils and change the CLE as the boat powers up. also they dont heel like a keelboat?

but correct me if im mistaken?

 

Excerpted from "Boats To Go" by Thomas Firth Jones:

"For the first several years we sailed Vireo (a variant of James Wharram's Tanenui), the balance of her sail plan was similar to a monohull sloop. Close hauled in any wind above 12 knots, she developed a strong weather helm. Sometimes we reefed just to spare the helmsman's arm. Finally, Locke Crowther explained to me that the balance problem is different in multihulls. The mono heels and produces weather helm from her asymmetrical shape under water, and from the wind pressure on the rig being way out to one side. The solution is to shift the center of effort by reefing the main and continuing with the genoa. The multi produces weather helm from the sail area forward pushing the fine bows down into the water and moving the center of resistance forward. The solution is to move the whole rig further aft, and if need be, reef the headsail first.
I moved Vireo's mast aft a foot, and moved the forestay chainplates halfway down the bowposts. The weather helm diminished, and the boat became far easier to manage. In Hummingbird (a 28'-6" trimaran of Jones own design), I set the mast far aft to begin with, and set the forestay well in from the bow. There is almost no lead of effort over resistance. Balance is good in a variety of wind strengths."

Tom Jones website is here:
http://www.jonesboats.com/index.html

 

I have a very fast 31 foot tri I made the boards way to big. On race day in light air the boards never stall no matter how much I sheet in. First to the weather mark. Keep the boat light put a lot of sail on and keep the mast not to tall.I am 23 feet wide 43 foot rig 2100 # 650 square feet upwind,13 square feet of board and rudder . Get it close and rake the mast. Muti guys always change stuff. good luck

 

A rule of thumb for a tri is to have the rig center of effort at or just forward of the boat's center of lateral effort. Float shape, especially bows can affect the helm balance as well as fore and aft trim when pressed hard. Bows up is always better than bows down. The old fashion low tech way of figuring the clr is to cut out a silloette of the main hull underwater profile and find the balance point with a pin. Boats are so dynamic you are only finding the place to start tuning up with mast rake etc...

 

In his case would you ignore the floats? when the boat is fully powered up the main hull will be clear of the water and the floats 50% immersed so it should change things a lot right?

 

For initial balance ignore the floats. If you are sailing with the main hull clear of the water your amas are then functioning like catamaran hulls and their clr should be placed accordingly. The initial weather helm is to compensate for the lee helm that is caused by the ama immersing. Different float shapes have different effects . A 200% ama with full sections isn't going to immerse much at all, while a rounded v is. Most tri amas add area forward as they are immersed to compensate for the lee helm and give diagonal stability. There are a lot of variables in tri design, ama shape, section and profile all have different effects and should be selected for the use required. Those full amas can be uncomfortable cruising etc...

 

http://www.blairboats.com/demon-85.html

here is the finished sistership. I want to move the centrboard to modify the inside so thats why im trying to work out what can be done. also they get a fair bit of weaather helm how its setup.

 

With that big square head main sending the center of effort aft you might be able to move the board back. Of course the big screacher/code zero moves the area forward so it would be good to know how the sistership balances in that situation. With big rig center of effort changes a trim board forward starts to look good as Gary said. Having balance/trim boards in the amas might keep the mainhull clear. Offseting the board to one side won't affect the balance.

 

suck and see

With square tops and rotating masts it is pretty hard to find out where the CLR and CE will be. On top of this you actually want weather helm on a buoy racing boat. Maybe not on a cruiser but a racer sure.

Having a large rudder set at a low drag angle of attack will make the rudder produce lift to windward. This is something the monos have been doing for decades. What you want is for the boat to be well balanced with lift from the rudder. A balanced rudder will assist with helm.

If you are really worried about helm then consider making a square daggerboard case from ply and making it extra long fore and aft. Then you can put inserts into it and move the board fore and aft. (make the inserts foil shaped) Let's not think you can nail this straight off. Mono sailors often move keels fore and aft. Even Ben Lexcen moved the keel around on Australia 4 and he really knew 12 metres by then. My reckoning is that you won't nail it by calculating.

It is so much more difficult to move a daggercase than a fin keel so if racing is going to be your thing consider the tunable daggercase. You could incorporate a Newick style crash box if it ends up the front or middle.

I would talk to the designer with your concerns and sail on the sistership some more. Are they over trimming the main? Most cases of weather helm are caused by poor trim. The boat has to be able to talk to you somehow. Weather helm is one way your boat says - EASE THE SHEET.

Also check the rudder for rake. Any rake and the loads go up dramatically.

Be pretty careful about saying the weather helm is due to design. There are a few reasons why a well designed boat will have weather helm. Check these out first.

cheers

Phil

 

The sistership isnt wrong, but I just want to make sure there isnt a better way. Iv sailed on it a lot and steered it a lot already so pretty familer with how it handles.
Im also probably going to move the rig back 250mm as I think a mast on a multi shouldnt be forward of the centre of the boat so this will change things but its mainly my altered interior that requires a different board placement.
I was already planning to build an oversize case and put inserts in in case I change the board at a later stage.
A forward board is not an option, too heavy, too complicated and takes up too much room inside.

Im beginning to think there isnt really a way of calculating it as there are way to many variables?

A local multi designer apparantly lines up the CLR and CE and says it more or less works. I know a little weather helm is good but I like to be able to steer with one hand and not have to ease the sheets just to steer!