Importance of foil shape for LB and rudder

Howdy!

First off, I just put my very first boat in the water this weekend thanks in many parts to the great advice I've been getting here.

Here she is!


Anywho, my current question is - how important (really) is it that I make my rudder and centerboard into a foil (or some crude semblance of a foil). I'm sure foils can make a difference but in a small boat, sailed gently (and inexpertly) for pleasure on inland michigan lakes and reservoirs with a home-made sail rig, would I actually notice the difference between foiled boards and flat boards (with the edges router-rounded)?

Just for reference, I plan on making pretty simple boards out of 3/4 ply, probably with some lead weight poured in there. The boat is just a hair under 12' at the 'deck', it will probably carry about 65-70 sq. feet of sail.

I'm going to use the tied-on-with-a-rope-trow-it-over-the-side style of leeboard suggested by bataan in this thread: http://www.boatdesign.net/forums/sailboats/leeboard-small-skiff-38943.html

 

Considering your experience level and boat, you wouldn't notice the difference if you had NACA sectional shapes or a flat sided foil.

Make straight sided (slab sided) foils, by rounding the leading edge and tapering the trailing edge. This is all you need and it's easy to do, especially compared to making real foil shapes.

Don't weight your boards, just make a simple "U" shaped top section that will wedge over the rail at the location you desire. If you hit something it'll pop right off. Also consider at least multiple layers of plywood instead of one single piece. Plywood doesn't like to work as appendages for very long, but the loads on your boat will be fairly modest. 2 layers of 3/8" plywood glued together will last longer then a single piece of 3/4" for very little extra effort.

 

Thanks!

 

What PAR said.....But what ever you decide on for the total board/rudder thickness, make the cord length at least ~15-20 times that. So a 3/4" inch thickness board should have a minimum cord of 11-15 inches. And when tapering the trailing edge (4:1 or better), don't take it to a fine point, leave about a 1/16 to 1/8 inch ( ~5-10% total thickness and this applies to airfoil board also) flat on the trailing edge. Do those things and you'll see no real difference between a slab board and a true airfoil shape.

 

Thats a shame, I think he wanted an improvement on the plain slab board.

 

We have two boats with sheet metal centerboards. Both boats don't like to go to windward in very light air. My theory is that the boards at the very low speeds (less than a knot and a half or so) stall and don't produce much lift until the speed through the water is high enough. Once the speed picks up enough the boards are fine. Similarly when tacking in light air I need to bear off a little to make sure the speed through the water is sufficient for the board to be working, and then head up to the new course.

I expect a thicker board with a rounded leading edge and tapered trailing edge would work better at the low speeds.

 

Rwatson, I'm talking about straight sided foils, not boiler plates. A straight or slab sided foils is fairly close in preformance to a 00 series NACA section. It's very easy to make and will be much better then a boiler plate type of board.



I've had the opportunity to see a couple of Hartley cruisers, one with the hunk of plate steel centerboard and one with a weighted, foam core, slab sided foil. In all wind strengths the slab sided foil killed the steel plate equipped boat. In light air, it was especially advantageous. In modest wind strengths they both were fairly equal, though the slab sided foil was able to foot slightly better. On the only experience with heavier winds (a thunderstorm rolled through) the two boards preformed about the same, though I think the slab sided foil made better VMG. Both boats were essentially identical except for the boards.

 

Sounds consistent with my experience.

 

Nifty looking little flattie there Hospadar. Have fun.

For the slab sided board conversation I add this. The Windmill class rules specify a 3/4 inch thick board with a maximum taper of one and one half inches on the leading and trailing edges. That means Y = less than 3/8 and X is 1.5 inches or less. Pretty crude by some standards but the Mill, in the hands of a competant sailor will go to windward with some very good boats of other classes.

This builder intends to use lee boards. Lee boards might need to be located at a point along the waterline that creates the least divergence or convergence of flow with respect to the sides of the boat. That requirement, if it is a requirement, eventually dictates the location of the sail CE. I'll bet we could get into a fun converation about Bernoulli effect and such. Never mind that lee boards are surface piercing and may ventilate.

Hey Hospadar, I did not mean to introduce a lot of complications to such a straight foreward question. Just build the rig and go sailing. Never mind all that technical jazz.

 

Messabout is quite correct in the location of the leeboard for optimization of the boat's balance. In fact, most purpose designed leeboard craft, have their beam carried a bit further forward then usual for this reason. This is also why barges used leeboards, as the slab sides on this type of craft lended itself well to this arrangement.

Just round over the leading edge of the board, maybe a little more then a pure radius (slightly elliptical), then begin your taper from about half the length of the cord, even though 2/3's the length is shown in the above sample.

 

longest reg. no. I've ever seen.

 

That looks like the HIN, not the registration number. MCZ is the preface for a Michigan issued HIN for a homebuilt boat.

 

I'd agree, in that he's placed his HIN in the location of the registration number. Hospadar, you'd do well to remove the HIN from your bows or you'll just get a ticket every time the local patrol comes upon you. The registration number and likely the current registration sticker belongs there.

The HIN belongs on the transom of your boat; the upper starboard corner in fact. The numbers need only be about a 1/4" tall and they should be permanently placed (carved or stamped) in the transom, not stuck on numbers. The way I do this is to make a small 1/2" tall by 4" or 5" long box, say about a 1/2" deep. I pour straight epoxy into this, letting it self level for a couple of hours. When it's gelled up good, I use regular stamping dies, available from anyplace that sells tools. I lightly tap each number or letter into the green epoxy, then let it finish curing. When it's hard, I break it out of the mold, then carve a recess to receive it on the transom corner. It's glued in place flush, with more epoxy then painted with care not to obscure the numbers with paint.

 

PAR, thanks for the great tip on making the HIN.

C.O.
http://whatsintheshop.blogspot.com/

 

He has the current Michigan registration sticker in the correct location. Probably one trip to the Secretary of State's office to get both the HIN and registration. The registration number will be on the registration form which the sticker was attached to.