Monday, 7 May 2007

Trimaran float (ama) buoyancy

From time to time we get questions like: “Can you sail on one hull only” or “On one of the videos you can see a float almost submerged in the water, does it not indicate to little buoyancy”. (In one of our old videos a sequence was shown where the float was partially submerged. This we thought would show how kind the Seaon behaves, however it seems to have caused more questions than answers.)

Basically these questions boil down to: Why don’t you have more buoyancy in the floats? This is a relevant question. I will try to elaborate on this.

Already at the requirement specification phase the float displacement is to be defined by the intended use of the boat: Racing without interior, racing/performance cruising etc.

For racing purposes only, the float buoyancy would be generous enough to be able to lift the main hull out of the water and the leeward float will carry the whole weight of the boat. So why are not all trimarans designed this way? Sailing on one hull will obviously improve performance due to reduced wetted area, however you will also sail very close to the stability limit of the boat, meaning you will be sailing close to capsizing. A small increase in wind force could turn the boat upside down.
Obviously this needs very active and proper sailing by an experienced racing crew. There is little indication whether being on the right side of stability (and yes….there is occasional capsizing with racing trimarans and in rare cases also with cruising trimarans).

Another important consideration is the volume in the float compared to the length. If you design a light weight racing boat to a certain length and want it to fly on a hull it is possible to achieve this with a slender float design that cuts the waves nice and even in lights winds. On the other hand, if you design a heavy cruiser with the same objectives, the float will be blunt and will have to much reserve buoyancy in light winds, witch will cause the boat to respond to excessively to wave action and slow the boat down in choppy light wind conditions when sailing upwind.

How much displacement and its distribution through out the float is part of the sailing characteristics of the boat. Amongst other it influences performance for upwind sailing, power reaching etc. Furthermore it defines the “predictability” of the boat. It is very important to design a predictive and smooth behaviour of the boat. One does not want a boat that reacts with surprises. To achieve this it is important to have the longitudinal buoyancy distributed in a suitable way. For power reaching you need a lot of volume in the bow, but not too much to affect upwind performance. When pressing the boat hard upwind, it is also very important to have enough reserve buoyancy in the aft part of the float to avoid that the boat trims excessively backwards in longitudinal direction, when the boat is hit by a hard wind gust (due to sail center of effort moving excessively backwards when e.g. trying to luff to depower).

Summing up: There is no one single design that is right or wrong – it is the intended use that decides the design. A boat intended for top speed power reaching will look different than a boat optimized for upwind sailing and as mentioned, total weight of boat (building technology i.e. weight of laminate, interior, cruising amenities etc) also is part of the design strategy.

The Seaon 96crb design objective was to create a high overall performance (…if I remember correctly the marketing spec was: “The damn fastest boat with interior that a family can handle”) and to be as versatile as possible in various winds and courses. Also the objective was to minimize the power needed (mast length and sail area) for easy and safe handling, which is why we put some real efforts into the building technology (carbon, pre-preg etc. – for more information check our web site under “Production”). A light boat is fun to sail in light winds and the reduced need for power will make it easier to handle in tougher conditions.

We spent a lot of time and energy in designing the floats (and the rest of the boat of course!) and are extremely pleased with how well the performance, kind and predictable behaviour of the boat came out.