The below is a follow-up from sailor Andy White. He shares more information about the 420 Dinghy he has converted into a trimaran. Andy answers a some questions that resulted from the first post.
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Follow Up…
by Andy White
My response to Thom Davis’ query and comment is based on my present experience in sailing my small trimaran. I don’t think extra buoyancy in the floats is needed because I am happy with they way they have performed. I know they look small when compared to floats on other small small trimarans, but looks can be a bit deceptive.
Perhaps the following description will clarify what I mean. When you read it, I would like you to bear in mind that I have yet to test the boat in stronger winds and rougher water. I will limit my sailing to enclosed waters, not venture out on the open ocean in Western Australia where condition get pretty wild, especially in the Southern Ocean, the home of Great White sharks, Humpback Wales and ferral kitesurfers near where I do much of my summer sailing on the Hardy inlet in the town of Augusta near Cape Leeuwin.
The size of the floats on the 420 Dinghy converted to a trimaran
Firstly, I will give you some numbers: The floats are each 3.5 metres (11 feet 6 inches) long. They are made from 165mm (6 inch) diameter poly pipe with a 4mm (3/16 inch) wall thickness. They each weigh 9kg (20 pounds) which includes the weight of the aluminium fittings used to attach them to the cross beams. Each float has a volume of approximately 0.566 cubic metres (2 cubic feet), giving a buoyancy at rest of about 55kg (120 pounds).This seems adequate for the trimaran’s overall beam of 3.2 metres (10 feet 6 inches), bearing in mind that the lift from a float increases with speed through the water. Because the floats have no fore and aft spring or curvature, they use all of their length at all times for buoyancy and lift.
The size of the floats has, so far, proved adequate when combined with the hull’s stability. They have been designed to play a secondary or supplementary role in giving the trimaran sufficient stability, whereas a small trimaran with a narrower, less stable hull, relies on the floats to play a greater role in keeping it upright. On my trimaran they are intended to provide enough additional stability to help prevent a capsize, but if driven too hard without easing the sails, the boat will probably capsize – all small trimarans are the same and depend on the crew having sufficient sailing skill to prevent them capsizing in strong winds. However if you examine the image in the original post that gives a stern view of the 420, you will see the boat has wide rolled side decks, a fairly deep cockpit and hiking straps for the skipper (the forward hand normally uses a trapeze, but this is not used on my trimaran). The comfortable hiking configuration, with the wide beam, really helps the crew in keeping the boat upright in strong strong winds. This is an important factor in the stability of this trimaran. Even so, I was surprised how effectively the floats contribute to the overall stability, and smooth out the ride upwind and downwind.
My trimaran was designed as a simple, low cost conversion, using readily available materials. In this respect it has been successful. It points to similar success using boats like the International 470, the current 2-handed Olympic design. The drawback is the weight of the fibreglass hulls of the 420 and 470 – they are fairly heavy and this makes their launching and retrieval a bit more difficult than with a lighter hull. But this does not seem to affect their speed and boat handling qualities very much. The total sailing weight of my trimaran (without crew) is 129 kg (284 pounds) and the hull alone weighs 80kg (176 pounds). I wish it were lighter.
The floats do look quite small. This is partly due to their appearance next to the much larger 420 hull, which has a maximum beam of 1.63 metres (5 feet 4 inches).
When it comes to the amount of lift generated by the floats, I can change that quite easily by the way in which they are attached to the cross beams. I can raise their bows to increase their lift, or to help prevent them nose-diving should that be necessary. This is done by changing the bolt holes which are used to attach the floats to the 6 mm (1/14 inch) thick aluminium angle brackets bolted to the ends of the cross beams. This system of attachment also makes it easy to assemble the floats on the trimaran, because each float is attached to an angle bracket on the end of a cross beam by a single 6mm stainless steel bolt. I bolt one end of a float to an angle bracket while the other end rests on the ground. When the first end is bolted in place, the second end is swivelled up for attachment. I think the bolts are strong enough because they mainly rely on shear strength to cope with the upward thrust of the floats, and while the angle brackets alone may be strong enough to cope with sudden loads, lateral support is also provided on the forward end of the floats by an angled strut connecting the top of the float to the bottom of the cross beam. There is no angled strut for the aft end of each float because they are much closer to the point of attachment. The 4mm thick marine grade aluminium plate used for the attachments is very strong. This consists of a vertical plate welded to a 150mm (6 inch) long curved plate that wraps around the top of the float. The floats are held in place by large industrial stainless steel hose clamps,T-bolted through a slot at the top of each curved plate. This system allows the floats to be moved forward or back if necessary for trim and makes it easy to align the two vertical plates used for the attachment of each float. The attachments were quite easy to make, but I won’t go into that now. The upward thrust of the floats is countered by water stays of 4mm single braid, low stretch dyneema fibre rope with a nominal breaking strain of 1250 kg (2700 pounds). These help reduce the load on the hull flanges to which the cross beams are attached and eliminate bend in cross beams. Quite a lot of thought went into designing this system because I wanted it to be relatively simple to make and apply, and to keep water resistance to a minimum by using single rather than twin parallel struts for attaching the poly pipe floats. Besides, I had little faith in using materials like plywood or fibreglass to make the attachments because of the difficulty of bonding them strongly enough to poly pipe and being able to vary the angle of attack of the floats, as described above. I hope this description has been useful.
The following image shows how the floats are connected…
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As I pointed out in my introduction, the trimaran has not been tested in the strong winds (18-20 knots or more) we often experience in Western Australia. If the wind is too strong to carry full sail, I will sail with mainsail and no headsail. If I need to re-balance the boat to alter weather helm for windward work, I will simply rake the centreboard to do this. I doubt that shortening sail will have much effect on the trimarans abillity to tack or gybe.
In 1961, a GP14 sailing dinghy in England was fitted with floats. The results, which are similar to what I have experienced with my trimaran, make interesting reading and help explain how such a conversion can work. For more information, see the artice titled ‘Trimaran Conversion’, in Floats, Foils and Fluid Flows, published by the Amateur Yacht Research Society, publication no.36, pages 15-17, available on the Internet. You may also find another article in the same publication of interest on pages 6-7 It describes Trifold, a 15 foot long trimaran designed by Eric Manners in the 1950s that also uses small floats with strut attachments, but they are v-section and asymetrical to generate a lot of lift. Sadly, the plans are no longer available.
I have attached 2 images. The first gives a different view of the size of the floats compared to images in the original post. The second shows the result of a static heeling test with a combined crew weight of about 165kg (364 pounds). The float did not fully submerge. I would expect a narrower, less stable hull to heel more.
I have had experience with small multihulls going back to the 1960s, but most of my sailing has been in racing dinghies and keelboats. I think the combination of experience in mono and multihulls has been useful in designing my small trimaran.
I hope this message helps with your query about the size and adequacy of the floats.
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Along the rack track
I’m not sure I understand Thom’s comment, so I will answer as best I can.
I presume a rack is what we call a trampoline, as used on off the beach cats, like Hobies, or it could mean a frame like those used on Australian 18 foot skiffs, which have a trampoline within the frame:
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I have used both. In 1963 I designed and built a 23 foot trimaran out of 3/16 inch thick plywood. It was light, fast, and had a what might be descibed as a rack – a plank spanning the cross beams that could be used to sit out on. Very basic! I sailed the tri from a trapeze when necessary:
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The 420 conversion is a lot safer, more comfortable and stable. I’m past high performance dinghy and multihull sailing and don’t think a rack would improve the 420. There are much better platforms out there for speed. Besides we are now into the next generation – foiling multis and monos, and the foiling Moth class, which gave a boost to this, originated where I live and sail.
Regards,
Andy White