Help weigh issues re:Centerboard redesign

[Billy Bones]

I've stumbled on an older design which meets my needs very well. It is a 20' x 7' speedy daysailer with 170sf sa

Its only drawback, after much study, is the fact that it has a 3/4" steel (galv) centerboard. If I build this one, for my own use, I'll not use a steel centerboard.

I'm leaning away from a daggerboard, despite it's simplicity, as it is less beach-friendly, esp with a gaggle of people on board. In this design it would be 6+ feet long.

So I'm thinking about redrawing the existing centerboard plan for a wood board, many of which I've built in the past, and incorporating the corresponding ballast (about 160# plus board displacement) either in a slim shoe outboard or some pigs beside the trunk inboard.

Here's the question: I could easily make a nice foil-shaped centerboard within the designed outline, but it would be much wider than the steel, necessitating a wider slot. Do the merits of a foil shaped board outweigh the drag associated with a wider centerboard slot?

Thanks for your thoughts

[Raggi_Thor]

I think it does. The slot will be in the wake anyway.

[tom28571]

The most recent issue of Professional Boatbuilder addresses this issue. The conclusion is that a NACA foil is superior, but only marginally better than a flat sided foil with parabolic leading edge and smoothly tapered trailing edge. The flat board would require a narrower trunk. The flat sided foil is much easier to shape also. Since the steel board must have been used for ballast, you should maintain a similar righting moment which would require a lot more weight unless it is built into the board. Unless you are a recer, I would not be concerned about turbulence caused by the slot.

[h_zwakenberg]

Upwind sailing will improve with your proposal. You pay for it downwind, where the higher drag will be somewhat detrimental.

bye
Hans

You could consider a glassed wood board properly shaped with lead encased in the lower section with a wider slot sealed using a simple gasket that would virtualy eliminate turbulence.
Lots of dinghy's use this kind of gasket...
The lead could likely be quite a bit less than the total weight of the steel board for the same righting moment because it would be down low even considering the extra buoyancy of the shaped board. The negative ,though probably not much of one, is that the cg might move further aft than with the steel board when retracted.

[Billy Bones]

Thank you all for wonderful ideas and suggestions! I think the wood/glass/lead board with gasketed slot should answer perfectly. Is the gasket typically just standard soft rubber? Something I should be able to find in McMaster Carr or similar?

APS ; http://www.APSLTD.com supplier of small parts. I believe the ones I've seen are just sewn out of dacron but several different materials would work.
-------
page 89 2005 catalog; part no's SS50-150 ; SLG001 & 002 ; HA302 &303 ; HA30 ;HA5393