Curved foil design

We have curved foils on our boat similar to a orma 60 type foil.
They generate around 4 tons of lift each.
We made them in solid carbon uni/db layup using good epoxy and good vacuum bagging techniques.
We have had one fail and need to make the new one stronger. (It failed on the tension side)
The mould will not cope with prepreg temperatures.
Any suggestions?

 

Generally a less abrupt sryle for asking a question is apreciated. A bit of politeness helps to get answers. So the answer will be as abrupt as the question.
A orma 60 foil is a piece that cost several thousand bucks for a boat largely over the million bucks. Even if your boat is under this size, such a piece is far from cheap. Why not to hire the services of a composite engineer? Maybe you'll need a good mold and prepeg, you're in very high tech pieces so you have to assume high tech costs. It's a big drawback that your mold can't cope with prepeg temps because high fiber ration is a primordial requisite for fabricating such high stressed pieces. So you have to go to prepeg so to make a new mold or to make a bigger foil so to make a new mold...

 

Thank you for the reply.
I take on board your politeness comment.
The original foil was designed by a structural Engineer, however it is all a bit of a black art.
I can build the next foil out of Pre preg low temperature cure. (Polyster mold should take a few parts at 70 degrees)
I will probably only build the Tension side in Pre preg though, the compression side would have little benefit I believe.
To get one of the high tech builders to design and make a foil would cost around $150,000 each which is completely out of the question so I am building them myself.
Someone suggested building the new one in Glass instead of carbon. Although flex would be significant breakage may in fact be less likely. Also flex would be a good thing as the foil would become more horizontal and work more efficiently.
I am just trying to see if there are any other ideas out there.
Cheers

 

Thanks for your answer. Yes composites are some times a bit black art, as the evil is the minimal details.
The pic is highly appreciated, it's a rather basic foil.
You are right is the tension side the most important.

The causes of breakage are extremely numerous;
1- Engineering error. Generally the underestimation of the stresses. Nowadays it's not so common with an experimented engineer in composites, able to define the quality and brand of the resin and fibers as composites are only good when you know what are you using, and the methodology of fabrication. The quality of a composite depends of the quality of the materials, the methodology and the quality of the workmanship.
2- Fabrication mistake. There are many. One very common mistake is the use of different resins ans fibers of those specified. There are hundreds of resins, and different qualities of carbon and other fibers. The capacity of elongation of the resin is a crucial factor. Another very common mistake is the non respect of the methodology defined by the engineer.
After we go in the evil, the details. I'll mention a few.
1-Presence of micro bubbles in the composite, a very common factor of structural fail as we saw after studies made in 1982 by the CETIM in France. A micro bubble means a zone of concentration of stresses. The fact is very well known in high tech gliders.
So the degassing of the resins if using the wet method, and in case of prepegs a special care in the application of the fibers with sometimes when the composite is thick, a vacuum used every few layers to be sure that no micro bubbles air are trapped in the piece that will be finally cooked.
2- The respect of the specifications of the method of final curing of the resin.
3-The quality of the material used and precision of workmanship used in the guides of the foil in the hull, specially at the zone of maximal stress of the foil. It's a common cause of breakage.

Better not to mix different materials using different techniques. It's all wet, or it's all prepeg. The resins used in these two methods are pretty different, and are not cooked in the same way, nor by the final temperature and most important the curve of rise of the temperature and the subsequent cooling.

All glass...you may end with a resonating spring where the lift is close to zero because of poor hydrodynamics as the the foil is vibrating and worst has too much torsion. If the piece was calculated for carbon, the same piece in glass will ridiculously bend and twist.
A possibility is a mix of glass and carbon as the two materials have different stiffness and "natural" resonances under stress. That needs new calculations.

As you can see, there are a lot of paths. The simplest one is to remake the foils as specified by the engineer, same materials, same methodology. After it's adventure with the inherent risks.

The domain of high tech composites in highly stressed pieces is truly high tech, ie not easy. That explains the cost.

 

in tension wouldn't Kevlar be good?

I am total amateur so take it with a block of salt.

 

The foils works in the 2 ways as it has stresses by vibration and torsion above the flexion stress. The problem with kevlar is no resin adheres correctly on its surface, it works simply mechanically embedded in the matrix. So it's very easy to delaminate.
That explains why kevlar is no more used in competition boats since about 30 years, and in composite planes since ages except as armour in military planes. Kevlar has many uses but no more in serious big structure high tech composites. It's a bit used mixed to glass as marketing argument for some boats and a lot of sport goods.
Furthermore Dupont de Nemors was too greedy, and the kevlar became far to expensive compared to the competition of the twanron, the dyneema (which is cheaper and better than kevlar in a lot of applications as sails and ropes) and the carbone which is far superior as it's twice better in traction, 10 times stronger in compression, and 6 times better in modulus of Young. Cherry on the cake the resin adheres perfectly so you obtain a true composite where the resin and the fiber work together. Kevlar is obsolete in a lot of applications.
There is no comparison.

 

You missed out the part about Kevlar being difficult to work with and needing special shears to cut.Not to mention going very fuzzy if you abrade it.The last time I saw any used it was a fairly cynical 200gsm laminate in the bottom of a 58 foot cruising boat to provide reassurance that the bottom would be less penetrable in the event of an impact.The polyester resin wasn't exactly the best companion for it.Ilan Voyager is giving good information.

 

Thanks for the heads up was not aware that it is so out of fashion (apparently for good reasons too).

 

Thank you for the replies.
Foil is a basic Clark Y design 12% chord chosen for its ease of build and its ability to work well even if the surface is not perfectly smooth.
I take on board the fiberglass comment. I agree with you Voyager.
We made a split mold for the build of the foils so that it is built in 2 parts and then glued together.
My plan is to build the tension half of the foil using low temperature cure prepreg 70 degrees. UNI 70% DB 30% Vacuum bagged and oven cooked. I believe the polyester mold will cope with this temperature, for a couple of pulls.
Tension laminate thickness will be 12mm.
The other side will be standard T700 carbon and a high quality epoxy UNI 65% DB 35% vacuum bagged and post cured.
The 2 halves are joined using epoxy glue and vacuum bagged together.
If this one breaks I will have to make it shorter next time.

 

Bushtailor, that seems good.
The clark Y 12 % is tolerant, rather draggy and not a high performer as it will cavitate rather soon but it will probably work at "normal speeds" for a cruising boat under 25 knots.
The true profiles for foils look closer to some wortmann or some Nacas for fast planes profiles as the big problem (only really solved around 10 years ago) is the sudden loss of lift by cavitation and the exponential drag. The "new" specially dedicated profiles for foils have a constant lift on a very wide range of speeds without stalling.
Good building.

 

Thank you for your reply.
The foils have not shown any tendency to cavitate yet but most of our speed is less than 25kn with the max around 30kn.(25kn is plenty fast enough)
Angle of attach is low at 2.5% as well which reduces the drag to less than .01
The foil is actually 11% cord
I am sure that new designs are better but I believe that flow detaches quicker on some of the newer designs especially if the surface is not perfectly smooth.

 

I've nothing to add on the construction of the foil but wanted to ask how the area of the hull at the foil exit is done. It looks like an insert from the picture you posted. Do you have any additional pictures of that? Many thanks in advance.

 

We built a box in the hull 30mm bigger than the foil on all sides. Thickness of the box laminate depends on the size of boat and foil but for the large cat it is about 10mm. Loads are distributed to the hull and preferably a bulkhead.
When I built the boat I designed the foil cases into the boat build so it was easy for me.
You then get a plastic block (make sure it can be used underwater(no nylon for eg and not too hard) and fit it into the box you have made. We fixed this to the box by heavy ss batten screws. You then cut the foil shape through the block which is the foils bearing. This is the hard part as you need to get the foil angle of attack correct etc.

 

Thanks for the explanation it's much appreciated.