Rudder and Dagger Board Specs?

[Darrell H]

I have 12' sailing dinghy with no dagger board and no rudder and I need to construct both. Can anyone give me a clue on how deep the dagger board and the rudder should be below the water line?

Darrell

[Ken_Potts]

Hi Darrell,

Can you tell us more about the boat? Rudders and daggerboards can differ greatly from one boat to another.

[Darrell H]

Thanks for responding Ken

i'm sorry it has taken me so long to respond to your response,I've been so busy I'm going nuts.

The dinghy is of fibreglass construction and is 10' overall in lenth and has a 4' Beam, the mast is mounted forward about 3/4 up from the stern. It has 1 main sail and is not set-up for a jib. The shape of the hull is very radiused from the keel up to the top of the free board. I have never had this boat in the water but the shape of the hull looks like it will respond very rapidly. If you need more specific information I will provide it. I really appreciate your help.

Darrell

[wkisting]

First, a disclaimer: I'm no expert... I've just read about this subject...

From what I understand, a general rule is to try to make the submerged surface area of the center/daggerboard approximately 4 - 5% of the sail area. That seems a bit high, however, if your boat has a large sail on it. My Pocket Cruiser has 130 sq. ft. of sail, but there's no way I have 6.5 sq. ft. of daggerboard area... in fact, I only have about half that... so in my case, 2% appears to work fine.

I have read that it's best to make the daggerboard 2 to 3 times taller than it is wide. A tall, narrow daggerboard produces less drag, and if I remember correctly, it's less-than-ideal to make the board wider than about 12 inches, which shouldn't be a problem in your case. I'm going from memory here (haven't seen my boat all winter!), but I think I ended up building each of my daggerboards to be 12" wide by 3' tall when fully dropped... which puts (obviously) 3 sq. ft. of area underwater... I usually only drop the leeward board, and not quite all the way, but it appears to do fine preventing leeway and allowing the boat to sail.

It is said that the top 6" of water is too turbulent to provide good hold against a daggerboard or rudder, so ideally your rudder and daggerboard will get below the top 6" where the water will flow across the surface with less disruption, which translates to greater control. Having said that, at slow speeds, it might not matter much. As I recall, my rudder on my 14' Pocket Cruiser extends down into the water about 20 - 22 inches (the blade is 8" wide, foil shaped), and works very well for turning. I'd probably want to shoot for about 20" of depth on the rudder on your 12' boat, but I'm guessing. Somewhere, I dimly recall reading a recommended formula for rudder size in relation to boat length and weight, but my memory fails me on that one.

For a proper foil shape (a.k.a. NACA profile), the thickest part of the blade should be 1/3 of the distance back from the leading edge. The leading edge should be smoothly contoured, and the trailing edge should be cut off square (not tapered to a very thin edge) in order to minimize the likelihood of vibration in the board. There's a formula for calculating how thick the blade should be for a given width, but darn if I can remember it. On a boat as small as yours, I don't think you'll see any real benefit in "lift" from a foil-shaped daggerboard (though you might still want to shape the rudder). So, for the daggerboard, I would consider opting for 1/4" aluminum or 1/8" steel (maybe thinner if it's a small board), which would add ballast too... though it might add more weight than you desire... depends how big the board ends up being.

Again, I'm sure others will chime in with better expertise... that's my two cents.

[Howard]

To add to what the others have said, if you have a trunk for the daggerboard, you would build to match those dimensions. It will give you fore and aft distance of your DB and subtract at least 1/8" from the width to get your finished width of your DB. You want it to fit pretty snug so it doesn't vibrate or rattle, but not so tight that it won't easily slip up and down. If the trunk is swept back, your DB will be too, so let it guide all your angles.

As for the rudder, you can look at lots of designs and copy those...either a kickup rudder, which is harder to build, or just a fixed one. If you go fixed....you would need to consider the rake of the transom and put an angle in your rudder such that the leading edge of the blade hangs vertical to slightly swept back in the water. Dimensions are roughly 10 inches by 18 to 20 inches. For the rudder, go with a standard width so you can get standard pintles and gudgeons.

Both the rudder and DB should have some foil shape to them to work at their best.

[Barry Pyeatt]

Darrell,

You may want to read the following article. It is from some well respected people in the business and good information. Particularly good points to consider when approaching your project(s).

http://mothboat.tripod.com/CMBA/Building/foils.htm

[PAR]

In most modern small boat designs, both the rudder and the dagger/centerboard work to provide lift and lateral plane. This is why the 5% rule doesn't usually apply to small craft.

Quick educated guesses would be 80 sq. ft. of sail (lively but safe) or 70 sq. ft. if in a windy location. 14" wide board with 20" exposed under the boat, use a NACA 0010 sectional shape if desired, but a rounded leading edge is all that's needed on a boat like this. A 8" wide rudder blade that has 16" of immersion, will provide enough helm feel. This too can also have a foil shape (NACA 0012 or 0010 will be fine). These may seem like larger numbers then might seem reasonable, but it's common to have more board and rudder area in round bottom boats, particularly if they're especially slack bilged. In fact, I'd like to see them 4 to 6 inches deeper, for the best performance (foil shaped of course)

The location of the daggerboard will be determined by the position of the rig. If you'd like I can scale up a working sail and appendage plan, that will balance and work well for your needs, with minimum of fuss. Drop me an email (click on my icon) and we can discuss it.