Daggerboard thickness

[tealsmith1]

The boat I'm building calls for a 1/2" thick leeboard. I've decided to change that to a daggerboard. Would 1/2" be thick enough for a daggerboard or should I make it 3/4"? Thanks for the help.

[Tim B]

Don't know your boat size, etc. but a number of smaller boats (<12 ft) use 1/2" thick daggerboards. I have one on an 8' pram that is fine. Just for fun I've capsized it. The 1/2 thick board seemed plenty strong enough to handle me pulling on it to right the boat. The board is only about 2' long below the boat. 1/2 thick plywood stock was harder to find than 3/4". I would have switched to 3/4" but it would have complicated other things about the design too much. The outer keel was only 1" wide, so a 3/4" slot wouldn't leave much.

[tealsmith1]

Oops. It's 15 1/2 ft. I figured that if it would work for a lee board it would work for a daggerboard. But I got to second guessing myself. I haven't put the trunk together yet so I wanted to settle it before its too late.

[Tim B]

I agree that a 1/2" thick dagger board would be stronger than a 1/2" lee board of the same size. The lee board would have all the stresses concentrated on a smaller point. However, a 3/4board would be even stronger. Your already modifying plans to add a dagger board anyway, if it isn't too much trouble why not make it 3/4"?

[John E Hardiman]

Board thickness is not the question here, case strength is. In switching from a leeboard (which is outside the hull) to a dagger board (inside the hull) means that the structure which holds the board (the case) has to be able to support the full load of grounding at maximum speed. Now reshaping the leading edge can help thrust the board up rather than back or to the side, but the case still has to be strong enough. I have seen small boat hulls split open by grounding only the board.

Modern state-of-the-art racing sled designs deliberatly make the board weaker so that they break off rather than take the bottom out of her when they hit something at 20+ knots. Even a small boat can develop considerable loads (500 lbs @ 5 knots and a 2' lever on the board is ~3000 ft-lbs trying to wrench the case out of the boat :eek: ) Rather than make the board thicker, find a set of rules for building the case or calculate it out so the board breaks first.

[Tim B]

Great point John. I need to learn to think about what's going to happen after something has already gone wrong!

[Buddy Sharpton]

I've seen daggerboards with the unsubmersed part cutaway to what looks like the open truss work on a bridge so that one of those "struts" will breal in compression before the hull, the case, or the nice pretty foil end is hurt. You just glue back the strut for the next time.

[john welsford]

John is dead right on the strength of centrecases thing, but in small boats I dont go to too much trouble. Fitting crush boxes as I do in multihulls or high end racers is an option but a pain to build.
Try making the end posts about four times the centreboard thickness in the fore and aft dimension, glueing and screwing at 2in intervals ( go in from alternate sides with screws that get well into the ply on the "other" side) and put a bolt through each corner.
Then of course comes the issue of how to hold the case in the boat.
Ultimately though, if you hit something hard enough something will break.
So, dont run agound going straight downwind on a full plane with 25 knots of wind up your tail.
JohnW

[Todd Bradshaw]

As long as you're redesigning things, why not make the board thick enough to get some lateral stiffness and a decent airfoil shape out of it. 1/2" is pretty minimal and unless it's plate steel, I can't see much reason to go that thin.

[John E Hardiman]

Originally posted by Todd Bradshaw:
As long as you're redesigning things, why not make the board thick enough to get some lateral stiffness and a decent airfoil shape out of it. Sorry Todd; but you just pushed one on my pet peeve buttons. A study of lift/drag curves in the Renyolds region of small sailing craft will show that there is NO advantage to airfoil shapes once the cord/thickness ratio passes ~8:1. Indeed, given the non-homogenity of near surface flow patterns, it is counter productive to thin the edges as all you do is make the blade stall more often in the seaway.

It is FAR better to put a layer of glass on each side if stiffness is what you are after than to thicken the blade. The leading edge should only be a 2:1 ellipse (i.e. only the first 1/2" should be rounded) and the trailing edge should be no more than a 3:1 taper with a MINIMUM 1/8" square edge thickness at seperation. Everything else is just a waste of time and adds problems.

There...I'm done ranting....

[Todd Bradshaw]

John,
How do you adress flex in the board? I may be uninformed and am certainly not a highly skilled boat designer, but I generally believe that parts of my sailboat that bend and flex of their own accord don't seem to contribute much to it's efficiency. If something bends, I'd like it to be because I pulled the gizmo that's there to bend it. I have canoe paddles that are stiffer than 1/2" of wood with a layer of glass on each side and it would seem awfully weak and flexible for a daggerboard. Granted, we don't know much about the boat here. I guess I'm picturing something along the lines of a Laser when it may be more like a Peapod with a 45 sq. ft. spritsail, but what is the current feeling on flexing foils?

[John E Hardiman]

Todd;
If your worried about transverse flex in the board, that's what the glass skin is for. Fiberglass is not an isotropic material, it has different properties of strength and stretch in the direction of the warp and fill fibers. Lay the glass with the warp fibers (the straight ones in the direction of the roll) vertical on the board. As the board attempts to flex to port, the starboard outer fibers of the board must stretch and the inner port fibers must compress. As the glass fibers effectively do not stretch, the board is prevented from bending as much as it would have normally (insert long discussion on sectional moduli and material properties interdependance here, I can point you to sources if you want). I'll have to do the numbers (or ask Bainbridge Island ), but I'll be willing to bet that a single layer of glass provides as much or more stiffness than increasing the the thickness 1/4".

(stupid spelling error corrected)

[ 01-14-2003, 01:34 AM: Message edited by: John E Hardiman ]

[john welsford]

I use a "knitted" glass fabric with a much higher proportion of the material running in one direction, and in that direction it is straight rather than wriggled around the cross ( warp? or Woof? whichever, the ones that go across) threads. This stuff adds enormously to the stiffness of the panel I get about 30% of the deflection on a given load with a layer of it on each side.
On foils, For centreboards I use the 00 series NACA foils, but never go above 12% which is consistent with your 8/1 John H . On really quick boats I go thinner but in general purpose boats use the thicker one with a very rounded leading edge so they dont stall when you are trying to tack in marginal conditions. But with 1/2 plywood plate what JH is saying is right , dont get complicated, you can get 90% of the result with 10% of the effort.
John W

[John E Hardiman]

Todd;

Just a short explaination on my little tirade

Though JohnW only goes to a NACA 0012, for a "foil" rather than a "faired keel" a reasonable maximum shape is a NACA 0020 (i.e. the thickness is 20% of the cord or a 5:1 cord/thickness ratio) with an aspect ratio (AR, span/cord) of ~2.0+. As the AR decreases, the thickness must also decrease (to reduce root losses), but once you get to a 7-10% thick foil or so I've seen no shapes that provide any noticable increase in performace. Indeed, the most noticable differences are poor L/D ratios at high angles of attack (i.e. large leeway angles) caused by poor trailing edge management.

For this dagger board however, I expect the AR to be in the 2.5-3 area (say a 250mm root cord by 650-750mm span with some taper for a 15' boat). At this AR everything I read points to the AR so dominating the lift that for the foils geometry in a boat, the pitch, roll, and wave orbital velocities have a greater effect than foil section shape for "reasonable" cord/thickness ratios. As I expect this foil to have a cord/thickness ratio of ~20:1 or a 5% thickness, a quick look into the lift/drag curves shows that even a flat plate compares favorably with the similar thickness foil sections. At high angles of attack, loss of lift force seems to be a loss of lift area due to TE vortex rollover. Thicker trailing edges seem to stablize this effect and keep their L/D ratios up. So thining the TE, i.e. having an "airfoil" shape actualy makes the problem worse.

Of course as the boat grows in size and speed increases, then the importance of L/D ratios at high angles of attack changes. Then there is still the "faired keel" which is another animal altogether. ;)

[Todd Bradshaw]

I think I got most of that, but hope there isn't a quiz later.... I had one boat (14' planing dinghy called a Speedball) that I swear the tip of the daggerboard swam back and forth (to windward, then to leeward even after shimming the well to a very close fit). Drove me nuts and got worse at high speeds. I messed with the board, rounding the leading edge and cutting the trailing edge to about 1/8" square, but never did completely stop the problem. Any ideas?

[tealsmith1]

Thanks for the opinions. John, thanks for bringing up the importance of beefing up the case. Now, please allow me a few more questions.
BYW, this is a Bolger featherwind.

How much clearance should be allowed between the board and the sides of the trunk? Any tips on securing the board? Does it have to be weighted?

Again Thanks.

[John E Hardiman]

Originally posted by Todd Bradshaw:
the tip of the daggerboard swam back and forth ... I messed with the board, rounding the leading edge and cutting the trailing edge to about 1/8" square, but never did completely stop the problem. Any ideas?Sounds like a tip vortex problem if squareing off the trailing edge failed to stop it. Was the tip rounded or rounded back by any chance? A square tip is better for controling/reducing/taming tip vortices.

Tealsmith1,

My choice is to have a board that is a "slip" fit by having thin "lands" (either on the board or in the case) on the front sides, back sides and bottom of section buried in the case. In this way the board does not rock, but also does not heavily bind as it only rides on the lands and seals the case bottom when fully extended. You can weight the board to help hold it down, but do not weight it enough to sink it. I will gurantee that at some time, you will lose it over the side, it's a law of nature.... ;) .

[John of Phoenix]

Regarding constructing the daggerboard trunk, Glen-L has a neat trick for a custom fit at this link. http://www.glen-l.com/weblettr/webletters-2/wl22-shop1.html

It's a little light on instructions so I'll fill in what I can.

First, what you're making is the trunk case to keep the water out. It's lightweight and strong but not "structural". The strength is derived from the support the trunk attaches to at its upper end and where it comes through the hull at the lower end.

Form the trunk around the blank of the daggerboard. By "blank", I mean that the board is at its final dimensions and has been rounded at the leading and trailing edges, but any shaping of the foil, if any, has not been done. The reason is that the trunk is initially made 2" or so oversize and trimmed to final size after it's installed.

You asked how tight should the trunk be. As tight as you can get it or the board will wobble, but loose enough for the board to slide through. :rolleyes: Now to the first picture. The board is wrapped with enough paper to provide a snug fit after the board is sheathed with a layer of glass, filled with resin and several coats of varnish or paint. How many wraps? Dunno. Are you using 4oz or 6oz or what oz glass? I mocked up on a sheet of plastic, peeled it off and measured. Pretty anal for being so unscientific, but it worked. I used 6oz glass and mine worked out to be six wraps of the local news rag and one wrap of six-mil plastic. Figure out the depth of the trunk and add a couple of inches. Figure the length required to get six full wraps around the board and cut the glass to width and length. Clamp the board to the bench to get the orientation shown in the picture and spread paper under the board as this is about to get messy. Wrap the board with paper and plastic and tape it down nice and neat. Next, tape the glass fabric to the plastic and take a full wrap with the glass. In the pictures, they do six wraps with three pieces of glass. If you work steadily and use a slow hardner, it can be done quite neatly with one long piece of fabric. Ok, mix the epoxy and slather away.

See picture two. Take another wrap and slather. Wrap, slather, repeat. Wrap, slather, repeat to the end of the fabric.

See picture three. Squeegee off the excess. Miller Time. :D

Next day, separate the trunk from the board and dress one end of the trunk to get it square. A pass through the table saw did the trick for me.

This is how it comes together.

http://bluemax44.home.pipeline.com/Finished2.jpg

Attaching it to the boat. The upper support is made in two pieces. The first is the mahogany piece with the cleat attached to it. Trim really. The second part (hidden under the first part) is a 1/2" piece of plywood that is the structural component. It surrounds the trunk under the first part and extends under the thwart where it is securely screwed to it. Rock solid. Now, slip the trunk, neat end first, down through the upper support into position on the keel. Mark its outline and cut the lower slot. Reinsert the trunk neat side up with the top slightly below the top of the upper support. The raggedy end should be protruding from the hull by an inch or so. Apply thickened epoxy to make a fillet around the top support and bottom juncture with glass tape to strengthen the lower joint. Miller Time. :D

Next day, trim the raggedy end flush to the hull and work on the daggerboard. The board, when finished, should slip snuggly through the trunk. Sounds more difficult that it is.