# Foil Question

Discussion in 'Hydrodynamics and Aerodynamics' started by JustinT, Nov 25, 2013.

1. Joined: Jul 2013
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### JustinTJunior Member

I have a small sailboat from 1968 or 69 with LOA 4.67 m, LWL 4.34 m beam 1.80 m, and sail area 13.66 sq m sail area. Think Wayfarer with a Mariner cubby cabin. It was originally build with a 34 kg steel plate centerboard at a displacement of 213 kg. It came to me with a 9 mm aluminum plate approximately 1/3 the weight of the original. I have since ripped out the old centerboard trunk because of delamination at the base, wet 9 mm low quality plywood sides and mat for all tabbing through out the whole boat. I am amazed it mostly stayed together after over forty years. I can not imaging hanging 34 kg of steel in that box.

My thoughts are since I am having to rebuild the whole case (and much much more) I should put some actual foils on this sucker. I was thinking something like a NACA 0008 or 0009 for the centerboard and 0012 for the rudder. The current centerboard has a 39.5 cm chord and a 95.25 cm length underwater approximately 0.37 square meters but with curves etc it is really about .30. I have read that Marchaj said 4% of sail area which seems a little excessive at 0.55 square meters. I also read that John F Sutton wrote that 1 sq ft to every 40 square feet of sail area equaling 0.34 square meters on this craft. All of the NACA profiles are based on Chord length. Is Chord length determination based on area required in a centerboard and weather I want a long skinny board or a wide short board? For example it could have a 20 cm chord by 168 cm length, or a 50 cm wide board with a 67 cm length both with approximately the same area. I am leaning towards something along the lines of 33 cm Chord and length of 102 cm with an area a little less then 0.34 meters with the curved and narrower tip. What factor does the lift play into all of this? If I have a NACA 0006 compared to a NACA 0012 I should be able to have a smaller board area in the thinner board. Is this correct? What say all of you on area of my foils and Chord length?

My current plan is to install the new centerboard so the leading edge is the same as the precious board. Do I have any room to move this around? Say 5 -10 cm back or to one side. I half thought about sliding the trunk off to the side or even asymmetrical bilge boards but then I realized I was stepping out way to far from the previous design and my knowledge. Though I feel crew and gear weight will play more affect then board location. If I could slide the centerboard back a little it would free up some space. Do I have any leeway to move this trunk location with out major changes in performance, or mast location?

Cheers Justin

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### JustinTJunior Member

The more I think about all of this. Is length and chord choice mostly determining the trade off between light air (drag) and heavy air (lift) performance? Justin

3. Joined: Nov 2003
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### PARYacht Designer/Builder

Yes, you can move the board in a boat of that size, though balance issue should be addressed. Much of this can be accommodated with crew placement and a few inches one way or the other isn't going to hurt much. You'll likely notice the differences in light air, if the board is moved aft and she goes by the lee, but once winds freshen a bit, you'll probably be fine. As to moving it sideways, well again the small amount you're talking about should hurt anything.

About the foil sections, you should consider the general speeds the boat will do, then select the section that best represents the attributes you'd like at those speeds. For the most part, unless you're racing similar boats, going nuts with NACA foil shapes isn't necessary. A slab sided foil will do nearly as well (fractions of a knot difference), which are a lot easier to make that NACA sections.

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### JustinTJunior Member

Par when you refer to slab sided foil are you talking about something like Mik Storer often employs with his designs?

How do I estimate the best length and chord of the foil? If the wind is light I plan to row or not care.

Thanks Justin

5. Joined: Nov 2003
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Location: Eustis, FL

### PARYacht Designer/Builder

Yes, Mik uses these slab sided foils as do I, on some designs. Developing a foil section can get quite involved. A look at the last America's Cup would show quite a few different approaches.

There are many things to consider, so you'd be best off sticking with a similar plan form of the original and selecting sections that fit, in a performance level the boat is capable of. Since we can assume this isn't a speed demon (cuddy burdened Wayfarer), the 00 NACA series will likely do well, but you have to carve these up, which can be tedious, hence the slab sided remark.

A higher aspect (within reason) appendage will perform better then a short, fat chord "slice 'o pie" style of board. In most small craft like this a compromise has to be made to offer sufficient aspect ratio, yet not impose itself too much on the interior of the boat. This usually means a less then desirable aspect ratio, so the case isn't so long. The length of the chord is increased, to make up for the lost area the shortened span, as another concession to the accommodations intrusion. Tapering towards the tip can ease this intrusion even more and permit better sections at the tip. Again, there are a lot of ways to approach the "issues", but I've fond on this type of boat that simpler is usually better and easier to make, such as parallel leading and trailing edges, so the sections are continuous, using a good lift section (like the 0012), so you get better stall characteristics and better performance over a wider range, in spite of it's drag and concentrating on a symmetrical foil, even if slab sided, will usually get you as much as you can expect, without driving you nuts with templates and sectional transitions.

Simply put, you can drive yourself nuts with these decisions, just to gain an additional 1/10th of a knot or you can accept the boat's performance envelop for what it is and enjoy building a new appendage, with it's abilities underway.

6. Joined: Jul 2013
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### JustinTJunior Member

I think I finally get the trade offs between chord length, foil length, foil location, and foil profile in boats of this type. Thanks for your input. Cheers Justin

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