Always ready to try (smile). The first issue on dimensions is to estimate the planform area. More area, the less the fin loading, ie the fin operates at a lower coefficient of lift for a given side force, hence at a lower leeway angle, and is less prone to stalling. Downside – more wetted surface, more drag, more noticeable in light airs. Having said that, Graham Bantock routinely times out the A fleet in very light airs sailing with his higher area fin, so the laws of physics apparently suspend for him…
Then, you need to know if the cross-section is designed to have a drag bucket. If it is, then the lift coefficient (and hence angle of attack, ie leeway) needed under ‘usual’ conditions should be located in the middle of the bucket. To do that, you play with the planform area (as above).
Now to the shape.
I’m thinking that they are trying to move the max lift towards the top of the keel. Assuming that the center of gravity of the boat is going to be somewhere along the length of the keel this would try to put most of the lift above the COG. This may help pointing and righting???
Well, the idea of a tapered fin is to approximate to an elliptical planform. This planform has the advantage of minimising induced drag overall. Along the way, it turns out that the actual lift (NOT the lift coefficient) generated by the foil is evenly distributed along the span. As you say, a parallel-sided fin of the same planform area will have its centre of effort fractionally further away from the root that will a tapered fin, but I’m reasonably sure that this is not the design objective. (If you wanted to pursue this line of thinking, I’d suggest considering the centre of effort in relation to the centre of buoyancy, rather than to the centre of gravity. Heck, if you really wanted to pursue this, you need to reference it to the metacentric height – http://www.onemetre.net/Design/FinArea/FinArea.htm – (grin))
Hi Lester
I had forgotten about the elliptical shape, you’re right, that is probably his intent.
The quote above brings up something that has confused me since I started this hobby. I have always thought that when a boat heeled it was rotating around the waterline (more correctly the centre of buoyancy-I think). Referring to Fig 3 of Larry’s article there are two forces, the sail side force(SSF) and the fin/hull side force (FSF). If the boat rotates around the COB then these forces are assisting each other and if it wasn’t for the bulb the boat would lay over(in fact if it wasn’t for the two mediums, water and air, it would spin like a propeller) but the bulb IS there so does this mean that the boat now rotates around the bulb or that the whole thing is just stalled and drifts sideways. If I’m following Larry right(he says the SSF and the FSF are in opposition) then it must be rotating around the bulb. This is what’s confusing me. Is the FSF making the boat heel more or less. Please enlighten me.
I thought of an experiment that may help. Next time I go sailing I am going to tie a line to the backstay anchor so I can hold the boat back. The boat should stop but still be heeled(SSF) but no FSF cause it ain’t moving. When I let go of the line the boat should stand up or heel more as the FSF develops. Does this sound logical?
Thanks for your time, I really appreciate it
Don
We went RV’ing(sailing) for a week so I tried my experiment. I tied a 50’ line to the backstay and let the the wind take the boat on a beat. When the boat came to the end of the line and stopped the heel did not change. The boat did fall off the wind (lee helm) which seems to indicate that the fin is helping to lift the boat to windward, an increase in the canoe hulls tendancy to screw up into the wind when heeled. This made me wonder if any lift from the keel is necessary. The hull will already point upwind when heeled and the rig position will control how much. Is the fins primary reason for existance just as a strut to get the bulb away from the hull for righting and the airfoil shape just happens when you try to reduce the drag. My next step will be to make a strut from a piece of round aluminum and see what it does to the boat balance. The drag will go sky-high but it may show something and it’s easy to do. Is all this of any consequence? I doubt it but any knowledge is useful in one way or another.
Thanks for reading my drivel
Don
I have had sucess making CF foils for an RG65, and an IOM rudder by shaping a core, in the case of the fin i think I used 4 layers of 0.8mm modelling ply, anfd balsa for the rudder. The ply was carefully shaped with reference to the glue lines, to ensure it was symetrical, and using a template, which ws understandably difficult given the scale. All i nall I came somewhat close.
I sheathed all of them in 200gsm twill CF cloth one layer a side as it was what was available. I didnt try to wrap around the leading edges but layed them up per a trailing edge. Once finished it worked fine. No here is the key to all this, any idiot, even barbera can shealth a random peice of ply with black stuff.
I made a compress using an 18mm ply boards each side, a layer of hard foamboard forom an art supply place, a layer of scotchbrite pads as i was worried the foam wouldnt conform to the shape properly, a layer of peelply, then the carbon cloth. This sandwich was then but in my bench vice and clamped up as much as possible. I added G clamps to ensure even pressure to the ends.
Result was a very high quality consolidated carbon laminate much mroe highly compressed than any vacuum bag will manage. The shape isnt perfect, for perfection try a cnc mold but who can DIY that. The press did ensure the laminate reflectedthe shape of the underlying core. I had a slight bend in the end of the keel, 20mm which i will cut off. Apart from that light foils shaped as well as I can imagine any homebrew can be done, at this small scale.
The foils are in the process of being set up to clearcoat by brushing on resin, letting it sure and sanding same old same old. This has given some scope to fine tune the shape especially around the trailing edges. They will look fantastic in clear carbon on my Goth RG.
God after typing all that I need another sherry, maybe a Bex and a good lie down as well. Especially after the effort of my posts about being ripped off by Brighton Boatworks this morning.
‘Hope you enjoyed Barbera’s CF fin making method employing her press’
How about the two articles/method by Eric Rosenbaum for rg65 and USOM (IOMs?) fins published in the AMYA magazine (#165 for the RG65 and current one - #172 - for USOM)?