About the only way a homebuilder could effectively make a long dagger without a mold would be to get a machined wood core and lay up carbon on the outside. Even this keeping a fair and constant section is the work of only the most skilled pattern makers and finishers.

I went out with Matt when they were testing the long daggers for use on the F16. They purposely used the same chord length on the boards so it was possible to interchange them and better test varying lengths and foil sections. The one thing about the longer boards was that it made it obvious that board height was a critical setting for performance. Too much board in the water and your vmg went to crap. The board height position is near as important as mast rotation. (very bad deal IMO for beginner and or even moderate skill level crews as now there is even more to do to sail) The long ones did help getting the boat to power up in the transitional speed winds. Once you were moving, you were quickly pulling up some as the wind built. Once double trapping it took nothing to have them up at the same depth as the normal high aspect foils, but now you have the dagger sticking up on your hull all the time, in the way for going in and out and moving crew weight. Never mind the danger of the sharp trailing edge meat slicer way up in the air just waiting to catch the crew sliding forward if you happen to stuff it.

A couple of comments on the above.

1. F18 boards are really heavy due to their length, the rules and the boat's huge righting moment compared to an F16.

2. I have to dissagree with Rolf, for projects like this wood is not good it is great. Built properly a carbon skinned timber cored foil will be about the same weight as the foam carbon alternative and if you're not paying for the labour they're much cheaper (this was demonstrated to me by a very good i14 builder).

3. I agree with Macca with regards to a bit of extra thickness. 2mm (19->21mm) will add over 20% more strength

My grip is with plywood. Plywood have nothing to do inside high performance foils.

Details on "proper" build of a wood core carbon covered foil is most interesting!

What wood (species, fast grown/slow grown, weight pr cm3 etc).
Cutting and staggering of timer staves to avoid warp.
Laminate schedule.

I like wood! As in "I love wood!"
The material is cheap and strong, easy to work with, excellent durability, relatively lightweight, very environmentally friendly and so on.

With the Gougeon book available as a PDF for download I hope more people try their hand with wood.

So, if not plywood then what? As regards plywood, I was thinking Brunzyeel for the finished product but anything from the local box store for a model.

Rolf,

the way I we've built "timber foils" before is.

1. Select straight grained timber with good strength to weight ratio (ie cedar) of say 1" (25mm thickness).

2. Cut into narrow strips, reverse alternate layers glue and clamp back together.

3. Place whole thing in thicknesser and dress until "flat"

4. Laminate solid glass strip up to same thickness (de-bulk and bag)

5. glue glass to front and back edges.

6. Machine to shape (you want solid glass lead and trailing edges)

7. Create rebate for uni strips at maximum thickness point. Note unis can be tapered out between 50mm below hull exit and lower end of board.

8. Fair over unis.

9. Apply laminate.

10. Fair and paint.

Cedar? Here in the U.S. I'm guessing the most suitable species would be western red cedar. Anyone hazard a quess?

Not western red, it has too much oil in it so will delaminate too fast. Go for Spruce, Hughes didn't pick it because he liked the colour.

Hi Scarecrow,


Questions:
7. Uni strips with carbon? How much would you guesstimate is enough for a 1995x21x180mm board?

8. Fairing. How much work is this and how precise is the result? How critical is the trueness to the designed profile?

9. Is this laminate only to make the foil durable so a 120 to 220gsm glass cloth is enough?


In your opinion:

How critical is accuracy to the designed profile. Is a tolerance of 1mm, 0,1mm or even 0,01mm the target?

What about geometry of the foil. Any tapers complicate work. Same with rounding off the tip etc. Yet this is often not discussed even if it is "standard" to aim for an elliptical pressure distribution and to avoid drag from tips.


Thanks for your insights, much appreciated!

Why not a hardwood? It'd be twice the weight, but there isn't much mass to begin with.

FRUIT NINJA!

petrified wood? Talk about sailing a "classic". How about one thousands of years old?

Sitka spruce is pretty much the standard in aviation for its characteristics in weight, strength and flexibility. Marine applications such as this should be much the same.

We are operating at reasonable Reynolds numbers, close to 1 million when sailing at 10 kts. Doing some first order calculations, assuming the flow is laminar (i.e smooth), looking 0.25in (6.4mm) aft of the LE, the boundary layer is 0.098in (2.5mm). Effectively this means any scratches or bumps within that distance would have no effect on the performance of the foil as the water around the foil can't even see them. However, there are A LOT of assumptions made in that first order analysis and you always want to build the best foil you possible can. My #1 tip if selecting a foil for a home build is to go with something whereby the performance doesn't degrade drastically due to imperfections. For example, a laminar flow section like the NACA 63-209 discussed here (http://www.foils.org/hysecdes.pdf) is much more sensitive to surface imperfections and things like kelp than a more traditional foil. The NACA0010 that was designed to operate in a more turbulent flow environment where the boundary layer is even thicker and hence you can get away with even more, besides it is easier to build accurately.

I am not an expert at wood construction by any means. I guess you have to laminate strips together to avoid warping along a grain line?

The smaller the strips, the less warping you'd have. So long as you flipped the pieces, assuming they came from the same stock.

Rolf,

too many variables, to little time to give you a back of the envelope estimate. Design path goes something like this:

1. Determine maximum righting moment.
2. Determine worst case lift distribution from rig.
3. Using above info calculate max side force on foil
4. Calculate lift distribution on foil.
5. Calculate moments and forces on foil.
6. Determine young's modulus and allowable stresses on selected timber.
7. Determine young's modulus and allowable stresses on proposed laminate.
8. decide on allowable flex on fully loaded foil.
9. determine section modulus of proposed foil.
10. adjust number of carbon layers until deflection goal is achieved.
11. confirm maximum stress due to bending in various components.
12. adjust laminate to suit.
13. check shear strength of core (particularly in way of hull exit).
14. Repeat all of the above because I've almost certainly lost a decimal place somewhere.

The exterior laminate makes it more durable but also must take into account hoop stength and twisting, yes you can use glass but it will be heavier.

Profile (meaning the shape of the board when view from the side) is in theory very important, however there is a trade off between efficiency with the foil down and up. If you taper the lower end you can reduce drag, however, it is at the expense of drag when you pull the foils up as you end up with slop in the case or a small amount of foil left down.

Section shape is important but don't dwell on it too much the shapes people use (with the exception of Richard Roake, Martin Fischer and their peers), be they NACA or any other profile are merely part of systematic analysis programs where one variable has been changed at a time, they are not "ideal solutions", so pick one which has the characteristics you want and do your best to match it but your priority should be a fair (consistand curvature without waves) shape.

Rudder and daggerboard section selection is not only important in theory. Some years ago we replaced the original rudders on a Tornado. We changed from the classic thin profile to a thicker foil with higher cavitation resistance but still the same drag level which made a huge noticabale difference to boat handling and speed. It removed all weatherhelm we had a high speed, which was significant and drag from the partially cavitating rudders.

Don't select laminar sections as they are inferior when operated in turbulent conditions and surface water is always turbulent if there are waves. That's why most people are happy with NACA 4 digits sections.

Concave sections are difficult to build, hence better stay with simple convex sectuion. Again a good reason for NACA four digits.

Other than stated here in this forum, skin smoothness or quality is very important, 1 mm roughness would have an catastrophic impact, and 0.1 would be still significant. A foil should be as smooth as possible and smoothness is more important than the section.

Cheers,

Klaus

Thanks Smiths.

Hi Klaus,

as smooth as possible is an "unclear" target In your opinion, how smooth should a homebuilder aim for? The combination of surface roughness and a true section is a challenge if working by eye and hand accuracy.
Is there a crossover point between the importance of sectional trueness and surface smoothness?

A CNC millcan work down to 0.001mm accuracy and some light sanding will give smoothness (or the machine can produce that as well if it is good enough).

I think this bears repeating.

"Other than stated here in this forum, skin smoothness or quality is very important, 1 mm roughness would have an catastrophic impact, and 0.1 would be still significant. A foil should be as smooth as possible and smoothness is more important than the section."

So what degree of smoothness can a home builder hope to achieve using hand tools?