July 8, 2021

Blue Peter Dinghy - Making a daggerboard

This evening I started routing out the daggerboard. In fact, to be clear, this is an oversized board to emulate what the finished board will be like once it's coated with fibreglass.

I've written a spreadsheet using the NACA 4 digit formula to calculate the aerofoil section, and then from that extrapolated cutter position settings from a fixed point and vertically so I can emulate a CNC machine.

Here's a few pictures of the first side. My plan was (is) to sand the bits between the routed channels, but if that proves tricky, I may route those remaining sections out to reduce the sanding.

Once I've finished and I'm happy with the section, I'll upload the spreadsheet here too.

Getting things positioned ready for routing

The leading edge is starting to take shape

I did the other side today before cutting out the shape and finishing with a plane, rasp, and rough file.

Here are a few images showing the completed board. I'm not very happy with how flexible the trailing edge is. I suspect that even with a sheet of 180gsm woven matting on each side, this is not going to be especially strong. Current thoughts are to ensure the foil has a minimum thickness at the trailing edge of perhaps 2mm.

Looking nice from the bottom. With some sanding, this could work.

Looking nice from the side. Note the part that is in the hull has been left thicker

Trailing edge is very sharp - perhaps too much so even with fibreglass

The level of flexibility in the trailing edge is concerning!

Here you can see the section difference from the existing original but warped daggerboard

To make the daggerboard dummy (because it's full thickness to emulate the real one that will be coated in fibreglass to this thickness), I used the following spreadsheet.

Daggerboard Aerofoil Shaping Calculations.ods

Having received a mahogany (or perhaps sapele) board, I'm going to try using this as a the leading and trailing edges for the board to improve stiffness and strength.

The board is quarter sawn, but the grain does move about, and is at roughly 45 degrees to across the board. I'm currently trying to decide whether to cut the board into thin strips at 45 degrees to the face, then join these together again. A key advantage of this will be that the rings all go straight across the width of the board, but also, the board is effectively stretched by 30mm like this as I only need a thin section. Rather sadly, even if I do stretch the board like this, I'll still be about 120mm short, so I'll have to use a piece of pine to fill the gap.