Effect of weight on boat speed

Does anyone know how boat weight effects speed on a planing dinghy for example a Comet, Solo, Streaker etc?

Newtons Laws apply and as Uffa Fox said “the only use for weight is in a steam roller”.  That being said, any difference will be most apparent when “like for like” racing, and will manifest itself as a lack of acceleration compared with similar but lighter boats, and possibly lateness to get on the plane.  Obviously it is the all up weight that also counts ... so the combined weight of helm and boat has to be taken into account.  If I were looking to buy a boat, I would avoid one that is overweight.

A 5% difference in hull weight won't make 5% difference in speed over the wind range, in fact I'd suggest it would be completely unnoticeable in most conditions. But if I was buying a new boat I would expect it to be bang on minimum weight. A used boat OTOH there are so many other factors that 5kg would not put me off buying an otherwise good boat.

Related to this is how heavier boats deal with waves on a light wind day. Try catching a K1 upwind on the sea in a light breeze if you need convincing! 

I know the post is about boat with but bare with me, I bought a Streaker because with its ideal weight is 73kg (based on class recommendations and ties in with the average to ten helms weight for a number of years at the nationals) so I would keep my weight down and healthy. My boat is 20 years old, all wood, been repaired a few times and I suspect heavily overweight. For most of my time I was at the top end of the weight range but after a few years of getting to know the boat frequently I would be at the windward mark with the lighter, more experienced helms in much new boats and better rags. When we went down wind however despite being right alongside hot shots and in clean air they would get on the plane and just get away from me. I would copy exactly what they where doing but still couldn’t get it on the plane as quickly as they did and it made a big difference.