Shape of floats for trimarans

Hi,

Looknig to get testing my iso dinghy proa again in feb when work calms down and looking to get some more insight on the shape of the flaots for trimarans.

The floats on my CLC Sport Tandem "kaymaran" trimaran were very simple smooth shapes that are pointy at both ends. Very low volume but work very well and leave no wake apart from where the water rides up and over and hits the simple vertical bracket for he crossbeams. I am going to fair that in to reduce the drag but the point of that is to be super light weight and not put too much stress on the kayak hull.

For my ISO trimaran project, the floats are hugely bigger but I am wondering about the influence of the float shape, particularly at the rear.

The shape is currently dictated largely by me liking the reverse bow look, wanting to try and reduce pitching and having a lot of volume up front.

However, what is the significance of the back of the float being squared off, pointy or other variations?

What if that was more vertical like the front and with a much thinner section?

Why do a lot of floats cut back in towards the front at the back eg the weta floats? Just looks or providing a cleaner break point with the water?

I am also changing the ride height of the test float for better balance and will try to get some video to see where it is sitting in the water and see what could be improved.

I will probably add some extra volume towards the rear as well.

I don't understand this yet and any insight would be helpful.

 

Give a lot of thought to the amount of total buoyancy you want the ama to have. First question: do you want to fly the main hull? Give some thought to how you might right the boat if it turtles.

 

Bouyancy amount

The amount of bouyancy is sufficient that statically it will easily support both our weights on one side for easy low speed clambering about.

A reasonable amount of dihedral will ensure that only one float is in the water at any given time but without the windward hull being too high. The main control of the boat is us scampering about shifting our weight - my wife is known as ballast - to keep things under control and flat.

Initially (and probably just not) not flying main hull, but will be going foil assist sort of like the c-fly as can be more easily engineered with my limited skills. Also would help if I am single handing it as my movement can be a bit suspect while enabling some decent speed.

If it ever goes completely over we are *****ed.

The mast bag would hopefully stop it completely turtling and if so righting lines might enable us to get it back.

If not, we can flood the sitting hull on the water, right it and then try to drain it as well as possible then get back to shore.

If totally over then it would mean removing the floats. They are held on by four bolts so quite feasible and it can then treated as dinghy with huge tramps. Getting the floats back on while in the water would be interesting....

At some stage this summer we will artificially put it on its side and see what we can do.

Having said that, the sort of incident to result in the boat turtling will almost certainly leave me not walking so I might not be that worried about righting the boat.

Main thing at the moment is the back of the float is sitting a bit deep in the water compared to the front when starting to load it up, hence looking to change the geometry to get the front a little deeper.

Now we have the trapeze ready we will able to balance it better and keep the float out more as we start to ramp the speed up a bit.

Just interested in theory of the shapes at the back, getting clean separation and reducing turbulence when the float is in the water.

Anything you can point me towards is greatly appreciated.

 

These are of Dr. Bradfields 18' Osprey trimaran. It is a foiler and the ama shape is somewhat different than most. I think I would want the fully submerged center of buoyancy of the ama to be slightly ahead of the CB of the main hull to help with pitch control. This ama shape is good for a foiler and for a non foiler that won't be flying the main hull.

 

osprey hull shape

So, the rear of the float is a thin vertical line looking like the front of most floats.

Why are more not like that?

What effect does that have when it is not foiling and travelling compared to it being square or rounded with a flat vertical face?

How does that affect the turbulence and drag?

 

You could go here to look at more ama shapes: http://www.boatdesign.net/forums/multihulls/small-trimarans-under-20-a-43650-57.html

It all depends on how you will use the ama. If you plan on the ama supporting a large percentage of the boats weight you probably don't want a double-ended ama. It works on the foiler because the ama is just for static buoyancy-under sail it is designed for incidental contact with the water and doesn't provide significant pitch control for the boat so Bradfield designed it as low drag as possible within the framework of his buoyancy requirements
Personally, I'd keep the ama as small as possible that also meets your "comfort" level. The smaller it is the lighter it is etc. The more dihedral you have the weirder the motion will be at rest-say at anchor. But under sail it will be better than having the windward hull constantly slamming into waves.
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I'm developing an entirely new kind of ama for use on a foiler that uses a very small ama hull combined with a "curved piece" of equal buoyancy. The idea is to keep the ama hull as light and low impact as possible while have a lot more total buoyancy if it is ever needed. The ama hull is like Bradfields except a flat planing surface is extended aft. This Two Stage Ama allows the crossarms to have high dihedral keeping them well clear of waves and avoids having too large an ama hull just for buoyancy. The high dihedral allows the total ama buoyancy to be fairly high which makes it extremely difficult to capsize or pitchpole. Highly experimental Test Model:

 

Ours will be pretty incidental through moving our weight - there as a safey factor or training wheels as much as anything.

We will see how the next round of testing goes and then I will weld up the frame and we'll start properly thrashing it and take things from there.

The current size is a jack of all trades, we will probably go smaller and lighter once we start looking at the foils.

These larger ones will be ueful where I am single handing though as a precautionary measure.