Curved foil design

Hi,
We have curved lifting foils on a catamaran.
They fail after about 2 years due to fatigue. They break at the hull due to the tension side failing.
They are made solid with prepreg carbon so we have exhausted the strength of carbon alone. I know that americas cup type foils they use metal in the high load areas.
I am thinking a high tensile steel piece through the hull exit high load area and fibreglass structure elsewhere.
Does anyone know what the Americas cup guys do?
Cheers

 

Not a huge amount of information in the first post,for instance is it a sailing catamaran or a power catamaran?What do the hulls look like at the point where they join the foils?Another thing to keep in mind is that the America's Cup guys might not have expectations of long foil life as the pace of development could see a steady trickle of revised and improved foils coming along.After the final race of a series the entire boat is a piece of history that really has no future at the same level,so different considerations apply.

 

Hulls have a hdpe soft bearing that is a tight fit on the foils.
Foils have no marks from the hull bearings.
Boat is a sailing cat.
Foils generate about 3 ton of lift.
There was another thread on here last time one broke with more info.

 

It is not 'just' the material. But the detailing.
Do you have an image or dwg showing how this connection is done.
Since the devil, is in the detail, for fatigue.

 

There's the inside of a MOD70 foil. I can't remember where I read it, it may have been an article by Paul Bieker, but it was talking about using different modulus carbon on the outer and inner skins to allow it to flex more instead of just breaking.

 

Thank you for the replies.
The cross section of the mod 70 foil pictured is the end in the boat which does not require alot of strength.
Mod 70 foils have a bad habit of breaking every couple of years as well and they cost around 100,000 euro to make each!!.
I have come up with a plan...... I am making a titanium box beam which in theory should be capable of taking the loads with an acceptable amount of flex. The laminate will be Fiberglass with epoxy. Still solid through the hull exit and tapering to foam core in the ends. A cross section of the ends looks similar to the picture above except I run more DB to the leading and trailing edges of the foil to take the twisting loads.

 

Is your plan via a series of calculations to demonstrate the changes versus the existing method, as being valid and verifying your 'new' design?
Or, is it just a guess and over engineering it to the eye?

 

We did get some new calcs done on the carbon version by a high end NZ engineer. The max strength (perfect) carbon layup was right on the max load generated by the foil which is why it was failing about every 2 years.
The new layup is as per your name...... except I did some engineering on the Titanium beam. Although complicated to make it should be strong enough. The titanium beam will be 1 m long and tapered to avoid point loading of the foil.

 

Do you have any dwgs you can share showing your - should be strong enough - new design?