Trimaran alignment of hulls with different rocker

When we have different rocker on the main hull and the floats, what is the best compromise? To align the sheerline of the bow in front of the main beam or to follow the theoretical waterlines? In the upper example the float is rotated nose down 1 degree.
Or should one equalize the sheerlines around the main beam line, ie with the floats rotated 0.5 degree nose down?

 

One option would be to alter the angle by making the rearbeam housing on the centre hull adjustable [with U shaped packers maybe?] , this was done successfully on a previous trimaran?[may have been a cat] shown here a few years ago whose name escapes me. He drilled a set of vertical holes in support brackets for the adjustment. regards

 

I would think the shear line is not very important except to aesthetics (looks), what matters is the water line or where the underwater parts of the float are in relation to the main hull.

Tb

 

The idea was to lift the rear beam over the center hull deck. As the forward beam will be attached at the beam pocket of the center hull and there is no beam pocket at the rear beam, I can build up whatever height I want to. I have checked Google for known designs and most of the floats seem to be parallell forward and somewhat lifted at the stern.

The drawn waterlines are when used as beachcats, question is what happens to them when loaded more to the rear due to the build up of a cockpit and the crew a little more to the rear? My idea was also to delay pitchpoling with a slight nose down.

Anway, as I am using existing beachcat hulls the design will be suboptimal.

My plan is to add an external beam pocket to rear. It can be bolted to the hull sides to allow vertical adjustment. See detail from rear beam plan.

 

Im just shooting from the lip here but I would be inclined to calculate the centre of buoyancy of the float and place it just in front of the sailplans centre of effort.
It should be around about where the daggerboard is. But my advice is worth what you paid for it.
Check out other beach cat tris and see what they have done, some of Richard Woods tris use beach cat hulls.

 

Hi redreuben,
Thanks for the tip, but I am to lazy to adjust the floats lengthwise, ie the original beam pockets will be used on all three hulls, Nacra 5.5 floats and Tornado center. As I mentioned earlier: "I have checked Google for known designs and most of the floats seem to be parallell forward and somewhat lifted at the stern."

This drawing of the latest version of F-22 shows the rocker being aligned like that.

 

I would go bow down if anything. As the leeward hull presses, I think you would want the volume in the hull to counteract earlier not later, then as it becomes over-pressed and submerges, the boat should slow and tilt to allow the sail to spill to avoid pitchpoling. This assuming the hulls will behave like semi-submersible hulls.

Farriers earlier designs behaved in this way as a safety precaution, and it saved my butt a few times, and the boat reacted exactly as he described in his sailing manuals.

 

Hi SolGato,
After reading oldmulti´s and your comments about F-22 in the "Multihull Structure Thoughts" tread, I checked and found it verified that somewhat nose down and fuller floats forward must be the way to go. Will lift the float 4cm at the stern, like in my first sketch, if it looks right when doing a mockup on land. Unfortunately I can´t move the floats further forward in this suboptimal design, though.

 

Found three modern designs that speaks in favor of nose down, the first one designed in Sweden. The two others show what Grainger advocates. They all seem to be deepest 1/3rd from the floats nose.

By rotating my float 1.2 degrees compared to its waterline it should be on par.

 

A little bow down on the floats does seem to help windward performance, but if you get them too deep, they lift too much and cause the main hull to start to drag its transom- which is slow. A little goes a long way.
B

 

Hi Bruce,
The 5.5 hulls aren’t that fat in the bows. Bow down just above 1 degree, ie the rear beam lifted ca 5cm, will probably solve it. Using the previous images as guides. As you say, you shouldn’t overdo it.