Fin and Rudder Profiles?

I’m working on a Marblehead and I’m at the point of diving into making my fin and rudder. I’ve searched high and low for a good profile, but no one has really stated what they use.

I know I should dive in with XFLR but dang, it isn’t easy to figure out. And I really don’t want to reinvent something that someone else has already done.

So, anyone want to share what foils they have found to work really well for a Marblehead? Or any other class might be good too.

Standard Naca foils work well enough. If you want the really good foils, then you have to buy foils as Sails etc and Creed, do no share what they use. They spent the $$ to develop them and want to make some $$ back for the time spent to develop a model yacht specific foil. I personally use either a naca 0004 or 0006 for the fin and a 0009 for the rudder.

Well, I guess it’s time to dive into XFLR and download some foils for analysis.

Using Slotracer’s suggestions as a starting point NACA 0004,0006 and another persons suggestion of the SD series of foils. I’ve been filtering through the Airfoil Investigation Database (AID), so far I’m starting to like the GEO 445 or the S1010 HPV foils for the fin.

I thought I liked the BOEING-VERTOL VR-9 foil for it’s high stall angle, till I ran it though XFLR and noticed a weird “dip” at the front of the foil. That and when I ran the numbers for the Re range of these boats, it didn’t look very good.

What Re numbers are you using? I think for fins, the high stall is not as critical as for the rudder. My guess on the fin is that they dont see an angle much beyond 5 deg. I certainly could be wrong on that, but my thought is if you are in those realms of angle, the boat is not working very well or the current fins are way too thin. On the fin, I would think you would want to be able to support the required lift at 2 to 3 deg to be efficient.

Honestly, i don’t see the fin getting that much angle of attack either.

The Re numbers I’m using are: 329 - 329164

One thing that I am noticing is that the amount of lift generated by any foil at the Re, is that it is really insignificant. The same for drag. For how fast it moves through the water.

Generally in our RC models we use 7 to 8 % for the fin that will not see more then 5°due essentially to boat drifting, while for the rudder the risk of stalling is greater due to cotinuing manouvers over15°, therefore it is better to use 12 to 15 %

I looked at the GOE445, it looks pretty good. More of a bucket than the standard naca foils. I estimated that most of our boats are less than 100,000 Re most of the time. I think the RG65’s are less than 50,000. There really is not a lot of good symmetric airfoils designed for that range of Re. Going to plot the two against each other to see how much difference there is in actual shape. Might be less that I can reliably mfg.

I wanted a good range for the Re so I could look at the “meat” of the polar graphs. Here were my assumptions for calculating the Re:

Root chord dimension: 100mm (0.1m)
Tip chord dimension: 50mm (0.05m)
Span: 750mm
Kinematic viscosity of water @ 5℃ = 1.519×10^-6 m^2/s
Speed range: 0.01 m/s to 5 m/s (I know 5 m/s is huge, but again needed a a good range)
329 <Re< 329164

Oh yea,
Making the darn thing will be interesting.

Sounds reasonable for a M. Note that Bantock went with a longer root chord on his fins which leads to a raise in the Re. Note that the change in Re has way more effect than changing the foil shape on lift vs drag.

I was just verifying the fin dimensions on the plans. Yep, a bit bigger. Re changed to 460 to 427913. I’ll re-run the numbers. Shouldn’t much difference though.

Interesting thing about the extra width is it moves up your minimum Re for once the boat is moving which changes the lift/drag pretty substantially as the boat starts to move.