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Thought for the Day - Hull finish, rough or smooth

I had a very interesting discussion with one of our members about hull finishes. Without going into detail the discussion went as follows.

We started by discussing Reynolds number and what this meant for laminar flow on a hull and flow separation points. I noted that in the early noughties, fine strips of material were put on fins of 1 metres and also we had Dennis Conner on stars and stripes used a stick on fabric of fine riblets on his Americas cup 12 metre and this featured on a NASA article which also highlighted a 5% reduction in fuel on a Lear jet which had the same riblet material applied. Sorry that's a long sentence. We discussed matt sanding a hull with different grades, use of Go Fast White and Grafspeed ( I think that was the name) paint from a bygone era.

The problem with our discussion is we could not validate any of the ideas because we would need a test tank and very sensitive equipment. The other problem was to achieve laminar flow on a one metre, you need flat water and how often does that happen. As soon as you get waves you get a breakdown of laminar flow. So all the fancy tech goes for a bowl of chalk if you get my drift.

In the end we decided to ask an expert and who better than my favourite designer, Brad Gibson. His reply, on his boats, he sands down to 1-2000 wet and dry and then polishes the surface to a smooth shiny finish. Exactly what I did on my home builds and all my previous dinghys and it has the added advantage that the hull is easy to keep clean.

So this might be an example of practicality over science or maybe just not understanding the science.

I for one will keep using the T-cut motor car polish on the hull and it is brilliant at buffing out scratches In gel coat and gives the boat a brilliant shin.

Fair winds to all.

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