Interesting Duckworks article on foil shaping

[Thorne]

http://www.duckworksmagazine.com/09/howto/foilcuts/index.htm

http://www.duckworksmagazine.com/09/howto/foilcuts/progression2.gif

[John Meachen]

Not new,Mark Drela (sp?) had the same thing on a webpage devoted to aeromodelling some while ago.The process can work very well with a skilled practitioner but is very dependent on the ability to create a fair shape if ultimate performance is desired.

Now to be provocative,I will suggest that if you make by such a process a centreboard,it may be worth trying a piece of rubber hose screwed to the edge of the board and in contact with the sides of the case as a means of holding the board in its desired position.It will be safer than playing with a few pounds of molten lead and it will not necessitate fairing dissimilar materials or dealing with the rot that eventually occurs after several hundred thermal cycles.

[Thorne]

Phil's foils also has a great page that helped me a lot with several CB's and rudders.

John, not sure I'm following your "not to be provocative" point == are you talking of ways to keep the CB down, or centered / angled in the slot? And if the lead is glued in place rather than poured (I use sheet lead), wouldn't that meet your objections to the dissimilar materials and rot issues?
http://www.luckhardt.com/newcb4.jpg

[Bill Perkins]

That's the best presentation I've seen , thanks . I may try to make an improved dagger board for my boat . I'm limited in thickness by the existing case though so I'm not sure how much I can improve on the plywood plate I have now .

[The Bigfella]

On Monday, I'll be sheathing the board that I've got a thread running on somewhere. I'll do an update. I'm probably going to vacuum bag this one with carbon fibre.

[John Meachen]

Sorry for the delay in getting back to Thorne,the wbf has seemed a bit constipated this evening and it has taken an eternity to log on.The use of a piece of rubber hose for positioning the centreboard was popularised in the 1950's by Jack Holt to the best of my knowledge.It was certainly used on his GP14 which was designed in 1949 and Enterprise in 1953 and works well enough.The centreboard is not compromised structurally by having the fibres of the wood cut to allow for the insertion of a slab of ballast which behaves in a different way to the parent material when the temperature or humidity change.Instead the board is left intact and the short length of hose which is screwed to the top edge of the board can be compressed by the screws or bolts which hold it in place to provide sufficient friction to prevent the board moving while subjected to normal sailing loads.It also removes the elongating force generated when the boat is trailed and the lead slab bounces up and down.I would attempt to post a sketch but is is around midnight and I have never managed to post a sketch before and it seems like a lot of learning at a late hour.Maybe tomorrow.

[frank pedersen]

The "old rubber hose trick" also is used in the Wayfarer, now some 10,000 strong. It strikes me as sheer genius in its simplicity and effectiveness.

[Hwyl]

I cut slots using my skilsaw to the right (or a bit less) depth. Finish sand and plane down to the bottom of the slots and finish off with templates and mark 1 Eyeball.

Jack Holt was a genius and a gentleman. The hose idea works well and is adjustable (just tighten of loosen the screws) and if there's a bit of slop at the pin, the board naturally gybes to give you lift.

[Eric Hvalsoe]

I'll be the wet blanket. I don't see much payoff in shaping a 3/4 thick centerboard or rudder for non competitive sailing . . . whatever floats your boat. I buy lead 'bricks' from a company here in Seattle. They are roughly 3/4" thick. One of the sizes is about 8 lbs. Just right to sink a centerboard on an HV 13 or 16. Simply lanyard, gravity drop. I cut the hole (with draft) in the board and epoxy the brick in place. No glass, no pins, it stays put.

[Thorne]

Eric -

I'll agree that we'd be hard-pressed to determine the difference between a plain flat CB and a foil-shaped CB on most small boats, but the time and effort to shape a foil can be minimal for a small benefit -- kinda like tweaking the trim on your sails.

;0 )

I've never used the rubber hose system so can't comment on it, but for most small sail and oar boats under 18', the weight needed to sink the CB isn't enough to cause a problem, require a winch, or damage the board or case during trailering. And I've always loved weighted rudders and CBs -- you can sail right up on the beach without fiddling with anything other than the tiller and sheets.