what wood for daggerboard?

Never mentioned I was going to use plywood. I said I broke two ply boards. one was 24mm by 500 by 500 and one was 20mm by 400 by 600 deep - and I hadn't really got started, either, the wind was light. I am hoping for a decent AR, at least 5, with a 10% section, so plywood is several orders too weak.

For a given thickness I think that wood/carbon will be difficult to beat in stiffness or strength.
I want it super duper strong, but I hope I don't have to use the whole roll of carbon tow. I thought I had heard wood and c had similar stiffness, or was that failure point? so they would go well together?
But if what you say is true then maybe I should make a mold anyway (but distruting the tow to make a strong, even skin sounds a bit difficult). I can't see how sheathing a core with tow can be done nicely and evenly?

Please consider the depicted construction. What do you think? will it be as strong with a foam core? I guess I'm worried that the tows will just rip through the foam, and besides the glass would have to be much thicker.

 

I did a little check in the "mechanical properties of wood" I got linked from one of you fellows. seems the modulus of elasticity or youngs modulus for the best woods are 12000 to 15000 MPa. I'm not sure what carbon tows are, probably ten times stiffer? I guess that means the wood will do nothing except being a shear web and shape, like PAR said. Or what?

EDIT: Hm. Youngs modulus says how much resistance to bending there is. But if two materials are to work optimally together in the same direction, I suppose they should reach plastic deformation at the same deflection, or something?

 

PAR said:

In another thread PAR said:

Right. good board material me ar5e

 

What failure mode to expect, given the above pictured construction? buckling of lee side near the trunk exit?

 

The second quote, you list my reply to a poster's request of a suggestion to use HDPE as a plywood planking replacement. In this capacity it isn't well suited, as stated. In this issue the previous post is out of context.

As a blade (rudder, centerboard, etc.) it's better suited, though there are better plastics to consider as well, then there's composite structures, all of which don't suffer from the ills wooden appendages can. Currently there are a number of manufactures marketing direct replacement rudders made of these high density plastics, for boats like Catalina 22's, Lasers, J-Boats, etc. IdaSailor Marine use a proprietary HDPE mixture for their rudders (wisely so). I also know of several composite blade manufactures, if interested.

 

HDPE is a great idea, I've used fiberglass and HDPE mortar tubes for pyrotechnics for quite a while, and after several "flowerpots" (in tube detonations) the HDPE actually held up to the shock better than fiberglass. Not sure it would behave the same way as a hollow fin, but for the price, it's worth a shot. Not too hard to mold either, if you can build a mold to withstand 500*

As far as the HDPE being too flexible...well that's it's strength...depending on how long a fin you're talking about, you shouldn't have a problem with flexing it too much. The beauty is, it can smash against rocks all day long and never fracture, that's what made it so safe with explosives.

I admit though, I did lose track of the original thread, just think hdpe would work well for me

 

PAR, I thought you were pulling my leg.
I did ask for your meaning of inert. However

To be more clear: I am getting tow, can get timber, glass cloth, probably some foam from somewhere far away, and I am going to build the stiffest and strongest and most accurate board I am able to, with the help of you most honored and generous gentlemen, thank you.

 

If I was to make a board like the one you want, I'd use an aluminum or stainless steel armature, which included the shaft or pintals, if it's a rudder. Over the this I would mold a fairly stiff density foam (say 8 to 10 pounds) or prefit manufactured closed cell foam (carved to fit around the armature and the foil sections desired). Over this I would use a number layers (depending on how big the board was) of 3K, 2x2 twill carbon fabric, set in epoxy, all bagged down tight. This would produce a very light, strong and stiff board, but it wouldn't be cheap nor especially easy for the novice builder to do. 3K 2x2 is running around $60 a yard, unless you can get a deal or have some "purchasing power".

You could substitute a honeycomb core, which would require a slightly different layup.

The key to all of these types of "high tech" layups is the laminate schedule. Too light and it will go boom in a spectacular kind of way. Too heavy and you've wasted costly materials.

This is one reason wood is often used and also why some of the high density plastics are also in use.

An inert material is one that isn't awfully affected by the environment it will live in. Plastics do well in marine applications, if you choose the right plastic. 'Glass set in epoxy is relatively inert, so is HDPE.

Using tow is the best way to get a good fiber to resin ratio and fiber orientation, but sweet Lord, it a tedious task to do by hand. If you want the lightest without having to physically wrap the boat strand by strand, then use a unidirectional 9 ounce fabric (properly oriented and layered for the expected loads). Next on the list would be a 1K plain weave (usually about 3 to 4 ounces, $250 per yard), but again you'll have a number of layers. You can ease this a bit by using a 12K triax braided fabric (about 16 ounces, $250 per yard) which is like applying 4 layers of 3K all in one shot. If this fabric is used, get the good stuff, which is flattened so it's only as bulky as 6 to 8 ounce. You also could employ a 6K 5HS weave (about 11 ounces, $80 - $90 per yard). You also could consider a 3K 2x2 prepreg (6 ounce, $160 per yard).

That strip planked, 'glass sheathed board is looking pretty good ain't it.

 

But the tow I will use since I got cheap, and not fabric.

I agree that trying to make a sheath with the tow sounds difficult. That is why I asked for response on the channels in the wood board. Only cheap materials needed then. Maybe the diagonal channels are overkill and a thin glass fabric sheath can take the torsion. The profile is bidirectional and center of pressure is ahead of max thickness (which is at 50% chord), so any twist would load the tip more, don't know the effect of that. I have the hunch that reducing lift at the ends is a good idea.
I plan to make use of a refrigerator pump I have.

If I build too strong board, I can use a bigger kite!

 

I built Western Red Cedar boards for the last boat with a bit of black down the thickest part, then all wrapped up with 1 layer of 400gsm DB and epoxy. The white stripe was where the cedar was routed out and some carbon put in and then bogged over before Glass.

The pics are of the board after we hit a reef at around 14 knots, hard enough that the board jammed and the hull was almost entirely clear of the water after collision and was left pivoting on that one board.

Powering back and forth eventually got the boat off the reef and as you can see the damage was minimal.

Strong stuff.

Dave

 

Cool! Now, if I could choose the woods, I'd use lignum vitae for the edges, and hickory for the middle, since it is tough and impact resistant, with channels of tow in it. Growth ring lines perpendicular to chord to increase shear strength between the tows.
But I don't know if the hickory or tow would fail first, when bending sideways. The data I have for woods says how much load must be applied for plastic deformation (Young's modulus), but not at how much deflection this happens.
Anyway we don't grow those here so probably not.

 

The problem I would foresee with the use of the carbon tow is that you would be creating a terrific stress raiser within the structure by creating the grooves to permit it to be below the surface.I would quite happily use a lamination of mostly spruce,with either mahogany or ash for the leading and trailing edges and for ten percent each side of the thickest section.I would use a sheathing of 300 gsm cloth and go sailing.

 

I like this recipe, but I imagine there is some point where the length:thickness ratio becomes such that reinforcement is necesary? This article from the Gougeon Brothers suggests that if the thickness of the cord is less than 4% of the length of the underwater portion of the foil (they are referring to daggers as well as rudders) then reinforcement is necesary.

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

Mike

 

I see, wet feet, but as I understand it, it wouldn't be a problem unless the wood is so unbendable that it breaks/deforms before the carbon does? Not sure if there are proper engineering terms for this?

Mike, thanks for the link.

 

I omitted any mention of carbon in my reply because I would suspect that having to create a shallow groove to set the carbon in would not provide much additional stiffness for the work involved.The glass on the surface ought to be adequate.The two examples of board dimensions earlier in the thread are a long way from high aspect ratio,why change the dimensions?
I have to admit not having encountered a daggerboard or centreboard with a thickness/chord ratio of less than 6% and if I had a good reason to use such a thing,I would definitely sheathe it with carbon cloth.I would still expect such a thin foil to be exceptionally fragile and the references on http://www.mothboat.com/CMBA/Building/foils.htm use 12% and upwards for examples.