Shape of keelfin at the trunk

Hi guys,

What shape do you make the top of your keelfins?
Basically the foil section that is inside the hull/trunk.

I know the two basic plan forms are:

[li]You cut the rear or forward section of the fin at an angle which allows the fin to be drawn up tightly into the trunk.
[/li][li]You build a section slightly narrower than the width of the keel with angled sides and this allows you to pull the fin up and adust it forwards and backward for ballance.

But what is the plan view of that section? if you want to be able to adjust it forwards and backwards then surely the trunk and the appropriate bit of the fin needs to have straight, parallel sides?
How on earth do you build that? If you do get that correct, then what happens to the hole in the hull to accommodate the keel fin? It would need to be long and then you have this hole when it’s in the water to mes up the flow? I figured you could just use some play-doh or something to fill it?


bad drawings…

Plan forms.

On the right planform (it’s art I tell you) you would be able to move the fin/bulb but you need to flatten the sides to be parallel so it can move in the trunk.
Like this…

How do you do that elegantly?
Will the left planform be stronger? Or if you make the fin with a carbon rod through it from the very top - does that define the strength of it more?


Much depends on the model you are referring to.
In theory you can do as many arrangements as you can think about. IOM is a particular case since the total weight and dimensions are given by the Rules.
In one of the pictures is highlighted the one I will probably use for my Class M “1290”.
In general I use a Fin thickness of 7-7.5%, some modelers can manage to go down to 6-6.5%, important to get a rigid construction.
The Fin Box will be trapezoidal simply to gain weight.

It can be also shorter for the same reasons so far is tall enough to avoid water entrance when tilted.
Another argument consist on the Fin possibility to slide inside the Box, often used for the first prototype, but also for racing so far the Fin is positioned and fixed for the full time of the race event. Shims are needed to bloc the shifting.

Once, I wrote a paper about the Fin construction, is in Italian but plenty of pictures : Starting from a rectangular balsa assembly sheets and shaping the Fin profile while keeping the rectangular shape for the Top side.

This will allow better control when sliding into a rectangular fin Box, if so required. One could instead think to keep the fin profile also for the Top Side and use this “Top” as a mold shape to build a Fin Box by wrapping around Fiberglass lamination. No sliding.

Before lamination the Top shall be covered by a “non sticking’ material like several layers of “packing tape”.
Some will prefer to avoid de-molding troubles by building separate two halves.
Often this late construction integrates also the Mast trunk.
The 'Amon Fin” picture will indicate the PDF files about Fin construction once open the above link, just scroll down…

Added PDF File about a Fin Trunk Construction

Thanks Claudio :slight_smile:

Apologies for the very late response.

Don’t mind, hope was useful