i’ve read that some are using heli blades as keels. would these work?
i don’t think they’re the right foils. i reckoned that naca 65’s would give the proper lift. am i wrong?

I’ve got some heli foils. Not got a micrometer so I don’t know how thick they are - but they’re pretty thick.

In an ideal world an NACA 6 or 6.5% section is probably not far wrong. However, compared with a full size boat, Footys are badly saild - purely because the helmsman does not have the same feedback. I am therefore going for 9% folis in order to make the boat more forgiving. It doesn’t just stop, stall the fin and go sideways.

Hope this helps.

To put it plain and simple :
NACA type 65 on a footy keel is not a wise choice.

( my own opinion, of course )

drag is almost the same of other profiles
lift is almost the same ( may be even a bit smaller )
stall is more abrupt

NACA 65 are laminar flow profiles, their max thickness is moved backward, and their nose radius is smaller than “usual” NACA

Their drag is a bit less than usual within a given range of angle of attack ( only few degrees ) and a bit more outside this range ( called “laminar bucket”)

They have been used for the very first time to design the wings of the well known P-51 Mustang american fighter during WWII

Their properties have been investigated using wind tunnel testing at 3, 6, and 9 millions of Reynods number.

A footy is working around 25 000 RN

In other words a footy foil is something like Luna Rossa sailing in a pond of honey

In this field of flow ( small and slow things ) almost everything is laminar, your credit card, my mobile phone, and my uncle’s sunglasses.


conclusions :

take it easy,
many very succesful planes have been designed using wing profiles drawn by hand

I feel that boat performances are much more sensitive to size and position of

By the way I would appreciate your opinion about : “rudder under” vs “rudder on transom”

Did you compare ( on water ) two of your sister boats same hull, same weight, same sail, but different rudders ?

Even if this point has been discussed from teoretical point of view, a real two boat testing would improve the truth about the matter.



The boat that won the Regional regatta in Laconia last year had a keel made from a helicopter blade as does my latest Micron 2. Both boats seem to perform very well in a variety of conditions. I remember Bill was surprised that a boat with such a small keel area behaved so well in the very choppy conditions. It also worked very well in the smooth water and light wind in Raleigh. The only possible exception would be in extremely light conditions when the keels suddenly just stop working. I suspect that this is a Reynold’s number problem rather than the foil section because I use a very narrow chord. As soon as you can feel the wind move, they ‘hook up’ and work fine.

As to the rudder question: I have built six boats with the rudders about 2/3 of the way under the hulls and one with a transom-hung rudder. In general, I prefer having the rudder under the boat for the smoothness of the steering response. I haven’t noticed any difference in speed.


Its been mentioned before, but is worth repeating, that Lester Gilbert’s web page discusses a bulb design tool by Marco Majic :-

Although set up for bulbs I also use it for fin sections simply because the default NACA 00XX sections just seem to look right.

I suspect Flavio is talking of a true NACA 65 section with a concave trailing edge profile and a max thickness at about mid chord. The basic NACA section with a 6.5% T/C has its max thickness at about 1/3 chord. Marco’s program shows this quite clearly.

I try not dotting "i"s and crossing “t"s w.r.t. these profiles since I’m still sanding my shapes fron 1/16” plywood.



I have had the file for a bit. Has anyone used it to calculate, then build a bulb?

I was thinking the other day about the rather “fat” sinker being used on my RG65 class boats. My thought is if one were to print, then cut a profile template, then turn a wooden plug to use as a male plug for a mold. Has anyone tried/done this from the program?

Just curious, as it sounds like with weather warming, it might be time to clear off the lathe and have a go.

Thanks for any thoughts, observations and opinions.

Yes. I have learned a lot since that post.

As for heli blades, the idea and post, came up after messing with the current blades I had on my heli. Not a very Technical observation, but the blade went through the water easily, compared to a hand-carved, more rhobust foil. This was done by mounting it on a spring / pivot contraption, and placed under running water. Then the pointer on the contraption made it’s mark, and was compared to the other carving. The heli blade had less resistance moving through the water.

Now on the boat, the heli blade didn’t hold as well as the fatter carved foil. The vessel slipped to lee more. I figured that the quickest way to a mark, was to have the boat point more towards it. The fatter foil held better.

Back to the heli blades. I found a more fatter blade on my buddies heli, and when he upgraded to carbon ones, I took possesion of his wooden ones.
These were half profile foils, and they were bonded to a center piece of wood, to give me a base on which to carve the foil I have now. It is some what between the original test heli blade, and my original fat carving. I figured to have it more in favour of creating more lift, and to have the boat track rather than slip (and, while thinking of the resistance through the water) make it a little thinner than the fatter original carving.

I will know how it performs very soon, as my current Footy is nearing completion. I think (without trials done yet) that it will work just fine.



Yes, I’ve cast three Footy bulbs using Marco’s program.

Although Marco’s program seems to be limited to a 1kg weight you can get round this limitation.

Go to File - Set Bulb Parameters - and input the TC ratio you want and just leave the weight at 1kg.

Then go to the - View Bulb Data - section. It will show a table of dimensions for a length which gives a weight of 1kg. You can then select a different length where highlighted and click O.K . It will then show weights for different lengths. Just iterate until you get what you want.

If you then go to the Print section and ensure that you select Print Preview you can print off a full size diagram. If you adjust the Number of Sections to a value one less than the length in cms you can get easily usable increment lines on the diagram.

I use the table data to set up a series of diameters at 10 mm intervals and then use the P/O tool in the lathe to turn those diameters. I finish off by merging these grooves with a wood turning chisel and glass paper.

The top photo shows an as cast bulb with only just enough lead, and the lower photo after initial dressing. I don’t go too fine at the trailing edge in order to reduce the chance of damage.

Turning the plug is relatively trivial - but casting the lead is another matter all together :scared:



Thank you for your quick reply.

Appreciate knowing it works. I am in the 500 gr (16 oz) range which should allow a direct calculation and print. It’s for the RG65 class boats currently being built, and I had some reservations about the huge frontal area of the currently available lead fishing sinkers. Was looking to reduce the massive front area to a longer, thinner bulb.

I have cast before, but it takes a bit to get me motivated each time I do. Basically I use a bucket of damp sand, covered with several layers of aluminum foil. I press the bulb plug half way in, and then carefully pour two halves. The aluminum foil is only good for a single pour, and I wind up with wrinkles from the foil, but it works to a point where I can shape each half, then epoxy together. Have gone as high as 2.7 kg (6 lbs) as a hot lead pour. I would much rather do a mold and use lead shot, but it is getting harder to find the small, fine lead shot locally with all of the hunting and pollution laws.

I am resigned to having to use lead alloy from automotive wheel weights. Freight to ship lead shot is a “killer”.

I’ll give the software a go, and see how well it works - warm weather dependant for sure. Thanks again for your post.



Here in the U.K. house builders will use 2 mm lead sheet about 150 mm wide to line the joins between angled roof sections, or where a vertical wall meets a sloping roof.

Buying new from a builder’s merchant is expensive as it comes in quite large rolls - but local scrap metal dealers will often separate lead from other metals and offcuts can be obtain there.

Your method of casting 16 oz bulbs is interesting. As you can see from my casting photos the odd wrinkle would seem inevitable - nothing that a big file and some glass paper can’t put right.

I cast my 8 oz Footy bulbs into Plaster of Paris moulds. I don’t think they’ve got a very long life though - I’ve noticed small cracks appearing after only 5 or 6 castings ( 6 casts but only 3 useable bulbs ;))



Dick, for my RG65, I used a polystyrene foam plug, plaster of Paris for the two half moulds (they’re best left for a week to dry out completely) and scuba belt weights for the lead. The mould was a bit oversize, but a rasp quickly removed the excess lead from the bulb and gave a fair finish.

I find that I can cast bulbs easily in a block of wood with a cavity dug out with a small gouge. The bulb must be poured in two halves, but mounting holes can be provided easily by adding small pieces of thin dowel at the appropriate positions. In the attached photo I show one bulb, a bit overfilled, with another that was made in a plaster cast and then turned on a small lathe. I no longer use lead, because of its biological harmfulness, but instead use the new solder which is intended for drinking water pipes. It is only slightly less dense than lead.