In 1990 we introduced a new float shape for our trimarans that was deployed on the TR series and later as the shape evolved adopted on other trimarans including Trilogy and the Essential Eight.
The V90 float replaced the earlier designs that featured more rocker and a more rounded sections amidships. These shapes had been used on Born to Run, ATL/Oaks, The Spoon Bay, and other trimarans of that period.
The V90 floats were the same length as the main hull because waterline length is the primary determining factor in seakeeping, performance and safety, and the optimum sailing trim for a trimaran is to be sailing on the float with the main hull just clear of the water.
Over time the V90 shape evolved with gradually increased buoyancy relative to boat weight, a flatter rocker line to reduce pitching, and fuller sections aft that provided more power in reaching conditions.
Larry Ketten's TR40 was one of the first designs to use the V90 float shape. The sterns were still quite fine at this stage.
The evolution of the V90 shape was incremental and culminated in the design of the floats for the RAW30 and Airplay 30 Trimarans which shared a new float design penned in 2012/2013.
Trimaran float design really revolves around three fundamental parameters:
Total buoyancy, fine bows for good penetration, and the buoyancy as far forward as possible to keep the bows high and dampen pitching.
A number of other factors must also be considered like the decks being able to easily shed water, keeping the centre of buoyancy as low as possibly to minimise heeling, and optimising the buoyancy distribution in the fore and aft plane (the prismatic) for high speed performance and minimal pitching motion.
Keeping the buoyancy down low and well forward helps to resist diving and steadies the motion that-that in turn gives you more drive from the sails. Ideally the floats should be just clear of the water at rest to minimise drag in light air.
In 2016 while working on the lines for the new 63’ Mauritius design I began to think about two other factors. The first was shedding water (spray) that would otherwise go back to the cockpit in fresh conditions. The second was ease of construction.
It wasn’t exactly the first time these issues had crossed my mind but with a lot of development in making the catamarans easier to build it was an opportune time to address these two issues.
On the construction side I had already done quite a bit of modelling work to determine whether I could get the float shape I wanted with panel construction that didn't require compounding, maybe in conjunction with strip planking on the hull bottom.
I was happy with the shape I had developed, but aesthetically it was pretty much a box in the back end and I wasn’t convinced it was the best solution. When it doesn’t look right there’s a message somewhere in the cerebral network struggling to tell you something and I was not satisfied the job was done..
On shedding water, preferably, down rather than back, I was interested in the first photos I saw of the flared bow shape on the IMOCA 60 Hugo Boss when she came out of the shed. It looked as though the shape deployed in the design would serve two functions, shedding water downward and helping with some dynamic lift in the bow.
This seemed to be confirmed when I was able to observe Hugo Boss’s motion compared to other boats in the fleet at the start of the Vendee Globe in 2016.
It’s difficult to be quantitative about these things especially as it was rare to see a camera angle that captured two boats side by side, but Hugo Boss certainly appeared to have an easy motion through the water and appeared to be shedding water from the bow better than other boats in the fleet.
The Boss’s performance in the 2016/2017 Vendee Globe left no doubt there was no serious defect in the hull lines.
Hugo Boss (left) photographed prior to the 2016/17 Vendee Globe. She lead the race in the early stage and finished a very close and credible second place after breaking one of her foils in the Southern Ocean. The bow is broadly flared below the deck line
Having determined that floats needed some compounding in the panels, at least for the aesthetics, I set about determining the most cost and time effective solution for one off construction.
The advantages of using pre laminated composite panels are beyond question, regardless whether the panels are pressed, infused or simply vacuum laminated.
There are basically three options to achieve compound curvature in the panels..
The first is to kerf one side of the panel and then lay a new layer of glass on the kerfed side after the panel has been bent into place.
The second is laminating the panel with a thin layer of uni glass in the x axis on each face. This requires a second laminate on each face once the panel is bent into shape.
The advantage is that unlike the kerfed panel, the original uni laminate retains its full strength and contributes to the panel stiffness, not just the laminate thickness.
The third option is to simply leave the glass off one side of the panel until it has been bent into shape in the forming jigs. This kind of panel is rather floppy and difficult to manhandle until you fit it into place, and you might need more closely spaced formers than the other two methods.
Designed by Edoardo Anastasia Diamond Eagle features a flared bow below the chine.. Owner- builder Peter Sowman achieved the concave forms using ATL's pressed DuFlex panels with a light uni laminate each side of the core, and then over laminating with second laminate.
CNC parts for a trimaran
Which method is best?
I’ve seen all three in use. They all work and they all have their relative advantages and disadvantages.
For trimaran floats I would suggest the light uni laminate on each face.
One thing that complicates the process a little is that you can’t unroll or develop a panel with a compound surface in the design process.
There is no way to accurately determine the shape the panel needs to be in its flat state on the laminating table.
However there is a work around that gives us a pretty good approximation of the shape we need.
We can use a flat panel in our model as well as the compound panel. We unroll the flat panel to get a close approximation of the shape. Then we measure the circumference of the curves of the compound panels at a number of stations to determine how much wider the flat panel needs to be.
The difference is usually only a few mm depending on the size and the amount of curvature required. We can add a few mm for good measure and this is simple trimmed off after the panel is fitted.
The red hull is the shape used on the Airplay and RAW30 designs, The transparent gray is the C17 hull. Total buoyancy and centre or buoyancy location are almost identical for the two hulls, but the chine in the lower bow sections of the C17 float allow for more buoyancy low down while retaining a fine entry.
It is also reasonable to expect that new shape will better deflect spray and create some dynamic lift at speed.
In the image below the bow of the Airplay hull can be seen in light gray; slightly taller and with less reverse rake on the stem profile.
The new C17 float shapes have already been integrated into the Mauritius design and will be deployed on the Rocket Factory Trimarans and the TR and R Series trimarans as well.
The combined Newsletter for Grainger Designs and Rocket Factory Trimarans
Our DuFLEX kit systems streamline the construction process for amateurs and professional yards alike. More details here…