Select to expand quoteI've been repairing boards for a long time pretty much got those sussed out but foils are fairly new to me.
Just putting this out there for some suggestions/opinions, I could get crucified here as its a tough crowd sometimes.
I tried to repair the base of the mast that fits into the fuselage, long story short the repair failed.
initially the end of the mast/fin base had snapped off.
After discussion with other board repairers, I went this method...
I ground down one side, I didn't want to grind too far as it would then go into the mast/fin laminate below, potentially weakening another area.
I built up the area with one layer of glass 6 layers of carbon and repeated this pattern, I then compressed this area as hard as I could with two clamps.
I re shaped this side.
Flipped it over and did the same regrinding, laying up, compressing reshaping. Redrilled holes for barrel nuts.
The repair lasted a little bit of sailing then failed.
You can see a layer of my glass has adhered to the mast base, so that's good that it stuck and held onto the original material, yet it still delaminated, both side were done on different days using their own mix of fresh resin, carbon and glass.
Any suggestion as to why it may have failed? any suggestions on a better way to do this repair?
The area would have quite a load on it, The barrel nuts reduce the carbon in the area and are fairly close to the bottom edge. I think the whole box should have been designed to be deeper.
Pics to follow
Initial damage
Half of base ground away
Carbon and resin being compressed
The repair failure!
I'm far from an expert at repairs, but what i can tell you is that foils are always orientated to deliver up lift, so the front of the fuselage is being continually shunted up by the main wing, and the rear of the fuselage is being shunted down by the stabilizer. Its not at all uncommon to find the mast/fuselage bolts which you tightened evenly and securely have "loosened" at the front and "tightened" at the back with all the leverage that occurs upon taking the foil apart again after foiling.
The cross bolts that starboard puts in as a bit of added security do always have a teensy weensy bit of play in them. Just enough to let movement happen. I'd hazard a guess thats what happened to the repair. With nothing there to provide strength top and bottom, the leverage twisted, or tore the repair loose again when the pressure came onto the rear bolt and barrel nut.
What to do about that as a repair, not too sure. The starboard carbon foils usually come with a plastic sheeve on them to shield the aluminium fuselage from the carbon. could you use the space the plastic would usually occupy to wrap the whole mounting section in fibreglass and lend the repair some added strength that way?
Good attempt but I think The loads on that area are too great for the contact area of the repair. Good excuse to upgrade I reckon
I wonder if you may have compressed too hard, so that too much epoxy was squeezed out. Reminds me a bit of a slalom board I had with a large delam on the underside, where there was almost no signs of epoxy.
^^^ No such thing as squishing it too hard (not with clamps anyway). Look at G10 which is formed under tonnes of force.
Your board had the glass not wet out properly in the factory.
Strong laminates have the fibre wet out and the pressure is designed to squeeze out resin for the strongest product.
maybe extend the reinforcement up to the foil mast.. if properly shaped should not add much drag
Before you re drill the hole's for the nuts, drill from the end past the break and insert and glue in some 4mm carbon rod. One above and one below the cross bolt.
Select to expand quotepowersloshin said..
maybe extend the reinforcement up to the foil mast.. if properly shaped should not add much drag
This is on the money, have a look at Fangy's fillets. Fins actually perform better with fillets at the base, so you won't loose any performance by building up above the box, a bit like old, "Glassed on" fins used to be
All really good suggestions, the filliting, the carbon rods, replacing the plastic cover with carbon, maybe a combo of the above.
I did think cut off the base, making the mast 2cm shorter and make a complete new base, but that's a major job, lots and lots of hours plus making sure it's all vertical and lined up.
You will be in a world of financial pain should you lose the fuse and wings after a home repair
I would get a repair quote from a composite specialist, then weight up his cost V a replacement
If this is a gen one cheese mast it might be an opportunity to get the stiffer one
the problem here is the actual design, there's nothing the best expert repairer can do to make the existing design work reliably. A deep tuttle would work better. But adding extra tension elements in a fillet will hold the front barrel nut in place. expecting the existing small area around that nut to hold all the forces involved is crazy. Even for a biggish slalom fin, the standard is deep tuttle. To expect a standard tuttle to handle those loads is very bad design.
Apart from that Rider is very experienced, and he's learnt a good lesson with this one. He won't make the same mistake again.
Thats the fuselage end Decrep 
But YES you have just made me realise yes its a crap design as the upthrust from the foil coincides with the barrel nut that lines up with the LE.
even worse the entire thing relies on the force applied to the two barrel nuts. A taper lock fitting like tuttle or PB would be far better. A foil should have two MT or a PB at the FUSE END as well as the board end.
Whichever company starts that can give decrepit and me free stuff now.
Oh yes Decrepit you are sooo right, a hard lesson learned! I am now even more experienced. I did initially think thats impressive all the force and the weight of the rider plus board plus sail is going through this small area. I thought there may have been more documentation on the internet re this kind of failure, or repair.
If I had have ground deeper or further forward I would have ground into the laminate of the mast, which would have weakened that area and it would snap of at the point where I ground into it, I think even if I grind further forward it still will break off. Hmm?
Yes a deeper box and repositioning the bolts would make it stronger, but maybe a deeper box and fuselage would cause more drag.
I once tried a similar repair to the rear barrel nut of the tuttle head and ended up with a similar result - delam through shearing force. Absolutely not reveling in your misfortune but I'm relieved a bit that what I thought was a simple repair that I effed up probably isn't so simple.
While the rear barrel nut is taking the worst of the wind upward torque forces, I'm surprised that the other nuts and cross bolt holes look unscathed. Wondering if either the way the drilling was or that the other screws were a touch loose that the rear barrel nut is taking all the load rather than sharing it across the entire structure.
Select to expand quotePaducah said..
I once tried a similar repair to the rear barrel nut of the tuttle head and ended up with a similar result - delam through shearing force. Absolutely not reveling in your misfortune but I'm relieved a bit that what I thought was a simple repair that I effed up probably isn't so simple.
While the rear barrel nut is taking the worst of the wind upward torque forces, I'm surprised that the other nuts and cross bolt holes look unscathed. Wondering if either the way the drilling was or that the other screws were a touch loose that the rear barrel nut is taking all the load rather than sharing it across the entire structure.
Good point, In the intial damage its strange that the failure was around one nut yet there are 4 other points that secure it and no damage to them, when they are all done up its pretty tight and I would think they would all take an even amount of load, but when it comes to foils I'm a newbie.
Select to expand quoteR1DER said..Paducah said..
I once tried a similar repair to the rear barrel nut of the tuttle head and ended up with a similar result - delam through shearing force. Absolutely not reveling in your misfortune but I'm relieved a bit that what I thought was a simple repair that I effed up probably isn't so simple.
While the rear barrel nut is taking the worst of the wind upward torque forces, I'm surprised that the other nuts and cross bolt holes look unscathed. Wondering if either the way the drilling was or that the other screws were a touch loose that the rear barrel nut is taking all the load rather than sharing it across the entire structure.
Good point, In the intial damage its strange that the failure was around one nut yet there are 4 other points that secure it and no damage to them, when they are all done up its pretty tight and I would think they would all take an even amount of load, but when it comes to foils I'm a newbie.
Yeah it looks like the rear barrel nut. May have been from an impact or from damage from excessive wear between the fuselage an mast connection? That rear wing puts the first barrel nut in tension, and the aft sleeved screw in shear.
A hard load shoving the front wing up should compress the front and put the rear side in tension.
Could have been from a loosely fit shim between the fuse and mast causing wear as well.
Still a more sturdy design than the slingshots 
Be sure to leash it when you foil with it. Since any repair of that kind cannot reproduce the fiber integrity along the whole structure, it will almost certainly be pretty weak. If that happened to me, I would bite the bullet and replace it.
Select to expand quotesegler said..
Be sure to leash it when you foil with it. Since any repair of that kind cannot reproduce the fiber integrity along the whole structure, it will almost certainly be pretty weak. If that happened to me, I would bite the bullet and replace it.
Unfortunately, it's the other end that's suspect. One would need a leash to the fuse. 
Your solution works well for shallow spots or old originally non-foil boards where there's the chance of box or head failure.
Did you check whether the sleeves are working when the back screw is tight? Ideally, those sleeves should be sharing the work, right? In my SB carbon mast, I cannot insert the sleeves unless the fuselage is fully seated.
I'd do a repair using the plastic isolator, then remove it and add glass around the repair until it's tight in the fuselage adding a fillet to the rear. Then make sure the cross bolts are making firm contact.
Select to expand quotedecrepit said..
Even for a biggish slalom fin, the standard is deep tuttle.
You would think so... but the Starboard Fox has powerbox and they are recommending a 9.5m sail can be used, which would need around 45cm fin
www.severnesails.com/boards/fox-2/
( Having rolled two powerbox fins out of boards over the years, I can only assume you cant put a big fin in them and it is hard to get good quality fins with powerbox heads... is a shame for a well-regarded board. )
Agree with others, just not enough surface area to work with for the repair. Can try extending repair full length along both faces and epoxy/glass in (carbon, or aluminum/steel plate) but would have to remove material to do that unless there is room without the plastic sleeve, or just use metal plate on both sides to span the break. But should just chuck it IMO.
I think I'm flogging a dead horse. But Im going to give it another go, Ill look at maybe filling the holes positioning in the fuselage then re drilling, I like the carbon rod idea, I think the fillet and wrapping to take place of the spacer is to much time and effort for something that may fail againand I've also just seen there are some cracks on the other end of the mast near where it goes into the boards fin box.
I had a thought about the barrel nuts. Instead of taking most of the strength away drilling a hole for them right through the tab. how about something like a helicoil moulded in? That's going to make the tab much stronger.
Ages ago there was a guy making slalom fins without barrel nuts. I've never had to repair one, so don't know exactly how he did it, but looks like some sort of insert.
Select to expand quotemathew said..decrepit said..
Even for a biggish slalom fin, the standard is deep tuttle.
You would think so... but the Starboard Fox has powerbox and they are recommending a 9.5m sail can be used, which would need around 45cm fin
www.severnesails.com/boards/fox-2/
( Having rolled two powerbox fins out of boards over the years, I can only assume you cant put a big fin in them and it is hard to get good quality fins with powerbox heads... is a shame for a well-regarded board. )
True Ames fins go up to 54 cm in a PB sweeper style fin, very durable G10, used a 54 cm PB Sweeper for years on a JP MagicRide 142 with no problems.
