what wood for daggerboard?

[sigurd]

It is high aspect and I need it to be stiff.

What do you think would be best?

[alan white]

Quartersawn mahogany or spanish cedar or luan. Well sealed in epoxy and paint or varnish. A view of the end grain should show all growth rings to be nearly vertical with the board flat.
Many will recommend plywood, but it isn't as stiff as solid wood.
The side pieces at the top of the dagger should be oak or similar dense wood to contribute to the board's flatness.
You can also take a single wide board and rip it into several pieces (say, 3" wide) reverse every other one, and laminate them front to back into a solid piece again. This tends to ensure a board whose movement is self-compensating.

[PAR]

The most reliable wooden board construction is strip planked, with opposing grains, every other strip, then of course sheathed. If the sheathing is fairly heavy, then a light weight strip can be used, like white spruce. If the sheathing is modest then a stiffer and more dense species like Douglas fur may be used. Hardwoods are an option with the weight penalty and swelling potential they bring to the table. On a daggerboard you'd be well advised to reinforce the trailing edge. A technique I've used for years, is to saturate single braid nylon rope and tack it to a groove along the leading and trailing edges. On the trailing edge it needs to be a fairly small diameter line, but the leading edge could be thicker. Soaked with epoxy, bonded in place and faired, it makes a very durable edge that can take lots of abuse.

Plywood is an option, but I'd only recommend this on a very small boat with a lightly loaded board. Your high aspect board will be heavily loaded, so do your best to make a light, stiff board.

[sigurd]

thanks alot...

[mikereed100]

Here is a link to some info that I have found usefull:

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


Mike

[sigurd]

thanks mike, I'll read it.

Par, Hehe, I already broke two plywood boards, one in the Tornado and one in the Europe Dinghy... kiting.

I'm getting some carbon tow now to put in grooves in the board I think. And I have some thin glass fabric to sheath it, so it will be super strong.
Since I am so nerdy I am trying to find strength charts for wood.

[PAR]

You don't need carbon, what you need is to not use plywood. Plywood isn't as strong as solid lumber in this application, because half of it's fibers are running in the wrong direction, to resist the loads imposed on it underway.

[mikereed100]

Sigurd,

Here's some more reading

http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

Mike

[Petros]

The two stiffest woods are doug fir and even better is red wood (I do not know how hard that is to find in your location). Doug fir is more stiff than most hardwoods.

I have made them from pine by laminating several large pieces about 3" widex 3/4 thick, edge to edge, alternating the grain direction as Alan suggests. Fast and easy to build. If you are going to cover it with carbon fiber, just use the lightest cheap wood you can find, all the strength and stiffness will come from the carbon outer skin.

[sabahcat]

It is high aspect and I need it to be stiff.

What do you think would be best?


What size and sort of boat?

When we know that, and have an idea as to the loads, then we can give you a better idea as to what timber is best suited

[sigurd]

A kiteboat. Since the power is applied so low, the force on the board can get much higher with a mast and its overturning moment. Also the kites can generate huge force from apparent wind.
Narrow hull, total crewed weight from 200kg but it has many hundred kg cargo capacity.

[Howaya]

Just because no one likes plywood for boards, I have to ask if you'll accept the following construction method. All of the following wood parts are epoxy coated before assembly.

Essentially, I used a 2" x 3" x 8' length of douglas fir as a spine. On either side, positioned so the spine is about 40% from the leading edge, I epoxied two thin plywood skins. The 8' edges were pinched together and temporarily stapled to a 1" x 1" x 8' long leading edge of douglas fir and clampled to a 8' trailing edge of flat aluminum while the epoxy set up. Once the epoxy cured I removed the staples and clamps. At this point the plywod skins take on a natural foil shape with open ends at the top and bottom. I fit a thick spanish cedar end cap on one end, filled the forward cavity with pour foam, and then fit an end cap on the other end. Then, I encased the whole shebang in a layer of light biax cloth followed with fairing compound. The boards are 8' long with a chord of just under 2' and weigh around 30 pounds each; they are actually bouyant.

After 14 months of use aboard a 30' catamaran they seem to be holding up well. One of them took a hit in soft mud and debris at the bottom of the leading edge; apparently the foam not only adds extra stiffness but also acts as a waterproof barrier/cushion during events of abrasion/impact.

[PAR]

Howaya, it's not a matter of disliking a material. Apparently you didn't read the full thread or examine the attached links. Plywood is inherently weaker for the task then solid wood. The veneers and more importantly the wood fibers themselves fail (sheer) with cyclic loading, which is the repeated type of loading a rudder blade, centerboard or daggerboard must tolerate. Personally, I prefer inert materials for these types of appendages, which can eliminate many of the other issued associated with wooden materials.

[sigurd]

par - inert - what do you mean?

probably it should be reasonable to calculate minimum strength from max CL of the sections, max speed, and the vertical location of the pressure on the board - then give it a safety factor?

edit - there will only be glass cloth available but the cabon tow can go both crisscross around the board, and along? in little grooves in the core?

[PAR]

Carbon and plywood don't really mix well. Not that they can't be bonded, but if you're going to pay the weight to strength penalty of plywood, then the carbon isn't being used effectively or at least is just along for the ride. The same will be true of a strip planked wooden core board, unless you used a very light weight core stock and a fairly thick (relatively) sheathing.

Unless you're going to employ balsa, foam or honeycomb cores, skip the fancy sheathings.

HDPE is a great board material. It's neutrally buoyant for the most part, isn't effected by the marine environment, is self lubricating, so it's less likely to jam, it's doesn't swell with moisture content, easy to machine, durable, etc. It has it's draw backs to, but everything has trade offs. This is one example of an inert material board. Another would be a foam core, 'glassed board (there are others as well).

[sigurd]

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.

[sigurd]

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?

[sigurd]

PAR said:
HDPE is a great board material. It's neutrally buoyant for the most part, isn't effected by the marine environment, is self lubricating, so it's less likely to jam, it's doesn't swell with moisture content, easy to machine, durable, etc. It has it's draw backs to, but everything has trade offs. This is one example of an inert material board. Another would be a foam core, 'glassed board (there are others as well).

In another thread PAR said:
Look, with just the basic numbers in hand, it becomes clearly obvious that this isn't a structural material. It's fine if you need a cutting board to clean fish, or an overly heavy center console podium for your helm, but it's got so many issues working against it in structural applications, that it's not viable in anything, other then very light weight injection or roto molded applications, where the weight of the material can be some what accommodated for with form.

In other words, cut a piece of 1/2" thick 3.5" wide, 8' long HDPE and clamp it on edge in a vice. Under it's own weight it will sag grossly and flop around like a noodle. Now take the same dimension in plywood and note the differences. The plywood will sag, maybe a 1/16" and will not willingly flop around without some force. The same would be true of solid wood, a 'glass laminate, metals. The HDPE will flop around when the wind blows gently.

Right. good board material me ar5e

[sigurd]

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

[PAR]

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.

[FramerDan]

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 :)

[sigurd]

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

It is high aspect and I need it to be stiff.

What do you think would be best?

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. :P

[PAR]

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.

[sigurd]

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!

[sabahcat]

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

[sigurd]

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.

[wet feet]

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.

[mikereed100]

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

[sigurd]

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.

[wet feet]

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.

[sigurd]

wet feet, I was hoping to get less induced drag for an AR of 5 with a 10% thickness. I think I will put taper to reduce root bending. something like (in cm) root:20 tip 10 length: 75

[sigurd]

Here is the answer to what i was asking earlier, strain to failure, it is called.

Sounds ok so far, except there's something we've left out: strain to failure. For e-glass it's 14.3, but for balsa it's only 4.4 . This means that at the load where the balsa lets go, you've only used about 1/3 of the strength of the e-glass.
http://www.djaerotech.com/dj_askjd/dj_questions/fuse.html

So, to ensure that the wood does not break before the carbon, the wood should have a bit higher strain to failure than the carbon. Right?

[peter radclyffe]

Here is the answer to what i was asking earlier, strain to failure, it is called.


http://www.djaerotech.com/dj_askjd/dj_questions/fuse.html

So, to ensure that the wood does not break before the carbon, the wood should have a bit higher strain to failure than the carbon. Right?

use teak or lignum vitae or ekki, greenheart, jarrah

[peter radclyffe]

if spruce breaks, build a wider slot, spruce is supreme in so many applications, so if it breaks you must redesign the board

[Boston]

my old Grand dad used to do exactly what Al White said in one of the first posts but with White Oak
was heavy but the stuff doesnt swell and its strong as doug fir without the splitting problems or the rotting problems

although that last from Peter is great
Sitka spruce is the bomb for lightweight and strength
White oak is one of the denser of the hardwoods
Hickory isnt all that water resistant and although its a good strong wood it has the same problem doug fir does with splitting

best
B

[PAR]

The original poster on this threads wants a light, very strong daggerboard. He has carbon tow on hand and would like to incorporate it into the build.

For his application increasing the chord, on his high aspect foil is likely not wise and probably out of the application's effective performance envelop.

Sitka's only real attribute is it's compressive strength to weight ratio, which makes it well suited to spars, not foils.

White oak is twice as strong as Douglas fir and nearly twice as heavy, live oak is stronger and heavier, but is less prone to checking and splitting. Both of these oaks will swell like a pregnant woman compared to the typical softwoods used in appendage applications.

Hardwoods in a sandwich laminate isn't a wise idea and is self defeating. The idea of these laminates is light weight, which negates the use of oaks or other particularly dense woods.

Douglas fir is a reasonable choice, but is farther down the list then other species, such as white spruce or red western cedar.

It's very likely the builder will use a foam or honeycomb core, some of his carbon tow and directional fabrics, which will produce the lightest, strongest bang for his buck.

It's important we speak what we know, not what we think we know.

[peter radclyffe]

The original poster on this threads wants a light, very strong daggerboard. He has carbon tow on hand and would like to incorporate it into the build.

For his application increasing the chord, on his high aspect foil is likely not wise and probably out of the application's effective performance envelop.

Sitka's only real attribute is it's compressive strength to weight ratio, which makes it well suited to spars, not foils.

White oak is twice as strong as Douglas fir and nearly twice as heavy, live oak is stronger and heavier, but is less prone to checking and splitting. Both of these oaks will swell like a pregnant woman compared to the typical softwoods used in appendage applications.

Hardwoods in a sandwich laminate isn't a wise idea and is self defeating. The idea of these laminates is light weight, which negates the use of oaks or other particularly dense woods.

Douglas fir is a reasonable choice, but is farther down the list then other species, such as white spruce or red western cedar.

It's very likely the builder will use a foam or honeycomb core, some of his carbon tow and directional fabrics, which will produce the lightest, strongest bang for his buck.

It's important we speak what we know, not what we think we know.

how about 25 mm bruynzeel ply, with several guitar neck tie rods, vertically to tie plate strip washers,full length of board, top & bottom, would this counteract the lateral breakage