The world of ideas works two ways. You give and you get back. As an example, in a recent copy of PBO following Dave Sumner's Mirror article, a letter appeared telling of a modification the author had made to his racing dinghies to make them safer for cruising. A check with our membership secretary shows that this man has never been 'one of us'. A further check through the back numbers of the Bulletin shows that no one has ever written up a similar modification. I traced the author and wrote to him for more details, and he kindly replied. He turns out to be a man of over 70 now who used to sail Merlin Rockets, Enterprise and Wineglass boats.

And what is this idea? Simply to temporarily convert a dinghy into a trimaran!

He felt that his idea would appeal to two types of dinghy sailors which will include many of our members, I think. The first being the man who enjoys, to a greater or lesser extent, the 'round the cans' racing at the local lake or river club, especially during the winter when cruising is not generally undertaken. And those looking for an inexpensive dinghy. An old performance dinghy is often sold cheaply. Many development class boats become redundant as newer designs prove more successful.

John Jermain – for that's his name – used three sheets of 8' x 4' exterior ply to stitch 'n glue together two triangular shaped outriggers each 10 feet long and 16" beam. They were double ended with the shaping beginning from a point about 2' from the stem and stern. He attached them to cross beams some 10 feet in length which in turn were secured to the hull by lashing them with nylon cord, to two extra sets of chain plates - one set forward of the mast and the other abaft the helmsman's position.

Suitably sized and placed blocks of wood, fixed to the spars, took up the space between them and the deck camber, and also lifted the bows of the outriggers slightly, thus avoiding the risk of 'digging in'. And for trailing, the floats and spars came apart and were carried inside the dinghy. Rigging them added only a few minutes to the normal launching time.

Take good look at the three accompanying photographs. They're old, in black and white and of only snapshot quality. Yet they clearly illustrate how the floats were used. Two photos show a well loaded Enterprise and the other a Merlin Rocket.

I called to see John at his home recently and he kindly made drawings and the photos available to enable others to follow should they wish. Starting with 3 sheets of 8'x 4' exterior ply of 3 or 4 mm thickness (the lightest you can get, says John), cut one piece in half to give two 4'x 4'pieces, and then divide each lengthways into 3 x 16" wide planks. The other two sheets are cut into a 6'and a 2' piece. Each of these is then cut into 16" wide strips. Join the 4' long pieces to the 6 footers, to give you six 10' long planks. The joins are backed using part of the 2' long pieces left over from the original cutting.

The side pieces are stitched together after the ends have been shaped as shown below. The actual shape of the curves is found by drawing a straight line between points. Using this as a base, bisect it and draw a right angle from this point. Form an isosceles triangle upon it. The side of the triangle then becomes the radius of a circle which will join both the point of the bow and the start of the curve.

Next shape the deck, bringing it to a point following a gentle curve from a position 60 cms from the bow and stern. Again you can use the isosceles triangle method above to find the curve. (Diagram C). Glue a few small blocks of wood to the inside of the float, and then temporarily screw the top down to ensure that everything goes together comfortably. Once this is done, the exterior tape and resin can be applied to the bottom joint of the sides, and when this is set, the top can be removed and the inside tape, resin and paint applied to finish off the interior. The top is now re-screwed and glued into position and finally taped and glassed around the gunwale edges.

John attached his floats onto the 10' long cross beams by simple nylon lashings and two extra pairs of shroud plates on the sides of each float. Some reinforcing is obviously needed at the points where the shroud plates attach and could either be put inside before the top is fixed in place, or glued and screwed on the outside. The latter might not look quite as good but unless you know exactly where the float and the cross beam are to be fixed; the outside reinforcements more easily permit some experimentation. As already mentioned, another two sets of chain plates are attached to the dinghy itself and the cross beams will be lashed to them.

Diagram C These floats were used successfully on a 13', a 14' and a 15' boat. But is should not be too difficult to take the principle and apply it to a boat smaller or larger than these. However, using them will enable you to sail a lighter racing dinghy, which will be more easily launched and recovered than a heavier cruising boat. There will be no need to add ballast or think of replacing the wooden centreboard with steel. Nor will you need to cut down the sail area. Indeed, the larger sails and consequent speed of a racing dinghy becomes an asset rather than a liability when you're trying to reach safety should the weather deteriorate.

As to performance – John says that with masses of buoyancy outboard – equivalent to a couple of men on trapezes – it behaved as a very stable and unstressed trimaran. In light airs it ghosted along like a monohull or with one float just kissing the water. He tells me that his boats would tack readily in any conditions except of course that you couldn't use the racer's 'roll-tack'. However, they would plane – without panic - and on board you could sit to windward or leeward, move about without anxiety, walk up onto the foredeck to pick up a mooring (try doing that on a knife edged bow of a Merlin Rocket without floats!). Reefing or having a brew up in a F5 becomes a doddle. John told me the only modification he would now make if he were starting again, would be to add some form of spray deflectors to the floats since waves tend to be forced upwards between the dinghy hull and the float, with the risk of a wetting when moving fast.

It seems such an elementary idea and I am surprised that no one appears to have done this before. Indeed, past Bulletins have been full of ideas to de-tune performance boats – GP14's, Enterprises, Mirror 16's, (even Finns!) and the like in order to adapt them for cruising – all with the consequent loss of performance, the loss of resale value, and the loss of an alternative use such as racing.

I contacted Paul Constantine, who is the only DCA member I know who has studied multihulls in any detail. He actually has a Tri which he designed and built himself and in which he has sailed extensively. I asked him for his comments. Paul's first thought was that these are not trimarans in the true sense of the word. "You can add stabilisers or trainer wheels to your child's bike without turning it into a tricycle after all", he says. "The idea is more like adding extra buoyancy to the perimeter of a RIB."

On construction, Paul says, - "You can see why they were equilateral triangles as this will give three panels 16ins wide to one (standard) 4ft wide board. Stitch and tape is a good method." But Paul feels there should be internal bulkheads placed at the beam fixing points for strength and to make watertight compartments.

On 'true Tri's', Paul says that those early ones, (about 1960s) "had wide floats which were similar to the equilateral triangle in cross section. Such a shape picks up buoyancy very quickly and will have the effect of lifting the bow rapidly. If the triangle was more steeply pointed towards its (upside down) apex it would have a softer action as it immersed more gradually and picked up the buoyant lift of the water in a smoother way. Experiments showed that if the float is too wide, too far forward, the boat will act like a raft trying to conform to the profile of each and every wave, which can be very uncomfortable. One has to get buoyant lift well forward, for that is where the sail pressure is applied, but one can have a more slender shape which pierces through the top of the wave rather than having to go right over it.

"It is a similar story with the back of the float. A straight float (like a tube, pointed at each end) with a constant cross section, contacts a wave. The wave tries to lift the front of it, but then as it passes along the float it will eventually reach the back. The wave's action is then trying to lift the back of the float and consequently depressing the bow at the same time. This wave action gave trimarans of a certain era a very pronounced pitching action. A partial solution was to make the float curve upwards from the water towards the stern when viewed from the side. The wave then gradually hits the pointed bow and moves back to give lift to stabilise the boat at the point of maximum volume. It travels onwards but its lifting action is diminished as the rear of the float curves up and away to be carried clear of the water. This float shape also gives a safer ride when running with waves coming in on the quarter, for the slender, lifted rear tip of the float can be partially immersed without tipping the boat into a nose-diving attitude.

"It can be seen from this description that the positioning of the buoyancy along the length of the float is a very important factor. The cross sectional shape at any given point is also crucial. Triangular shapes initially pick up buoyancy slowly so the boat will heel quickly to any puff of wind, but then stiffen up and sail with a steepish (for a multihull) angle of heel. Circular (cross section) shapes carry the maximum volume for the minimum surface area but they are noisy as they slap the sea, and quite stiff as they immediately deliver lots of righting power. In addition to all this there is the question of whether to place the floats in the water when the boat is at rest. Early boats had the floats in the water, but when it comes time to tack, all three hulls have to steered around, so in general it's not a good idea, especially as there will be greater structural stress due to each hull attempting to conform to the wave shape that it is experiencing. Lifting the floats clear of the water gives a number of advantages. The windward one acts like it is on a trapeze and adds to stability. It goes over incoming waves on the beat and it allows crew weight to lift the leeward float out of the water in the very lightest of airs.

"There's other stuff about floats, such as the percentage of the total boat's buoyancy that they should carry, but I think I have probably conveyed the most important points to consider when looking at a sheet of plywood and wondering where to start cutting. The magic of floats can be seen when watching them working on a real boat. They are in constant balance with the force being applied to the sails. A light puff puts the float onto the water, a heavy gust depresses it just the precise amount to counteract the force. Up and down they go operating in constant adjustment, like ballast on a sliding scale; when you want it you've got it, when you don't want to drag it with you, it isn't there any more. Wonderful system.

"I've saved the best until last. Because they make the boat so stiff with a predictable angle of heel, the weather helm can be controlled. No more stretched arms hauling on the tiller, just point the boat where you want it to go and relax."

So there you have it! An idea which no one seems to have tried – or have they? Do tell me if you have. And if you haven't and fancy having a go, I'm sure there are other members keen to learn from your experience. After all, it's been said that a clever man learns from his mistakes, whilst a wise man learns from someone else's!

My thanks go to Paul for his considerable contribution and to John Jermain for the details of his original idea.