Here is my design sketch for a trailer improvement that will fit at the back of a standard trailer, slightly behind the rear bow roller. It is designed for a typical home built plywood dinghy which might have 6mm curved/twisted panels, a V section bottom and at least one chine. Clearly the loads must be spread over a large area - so more rollers is good. It is also designed for single handed use where it may be impossible to keep the boat perfectly aligned with the trailer due to tide, wind and waves.
The assembly matches the midship shape, as this must pass through when winched in 50%. The theory is that a combination of gravity and the rigid shape of wide rollers and wood side arms will ensure the keel eventually rolls on the rear bow roller, even if winched at an angle due to tidal flow. The wide rollers also keep the boat upright from about 30% to 60% winching.
Beyond 50% winching these rear rollers will have less and less effect, eventually they will not touch the boat which will eventually sit on the rear bow roller and be pulled/twisted to sit on the 2nd and 1st bow rollers.
Heavier plywood boats designed for coastal cruising often have a central hog and bow rollers conveniently support the entire boat weight, as the hog is the strongest part of the hull. MilliBee's hog for example is two lengths of 4in x 1in running the entire length of the hull. They are laminated with epoxy and screwed - the keel then becomes incredibly strong.
Logically a similar assembly could fit midships. This has 2 uses: first to force alignment of the keel on the 2nd bow roller, second to provide excellent support of the boat whilst towing.
Its mostly inspired by recovering MilliBee, when I realised any supports must allow the low angle V section at midships through and not place point loadings on the 6mm hull panels (although its not impossible to strengthen the panels internally)
The LED lights are legally required, because the short wooden arms will be wider than the hull. Hence they must be white at the front and red at the rear. They also make inspection of the wheels at night much easier!
What do you all think of the Mark 1 design?
Paul
PS: metals4u.co.uk are very reasonable for aluminium and stainless strip or tubing. In fact a 2 1/4in wide aluminium tube, thickness 1.6mm or 16 guage is £29 for a 4 metre length.
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