So I am at the gold coast having a lovely sail when the fin ripped the fin box out sideways managed to grab fin but lost it on the way in 
. So now i will have to wait till im in sydeny to get repaired . Thats the end of that. So how much to get repaired professionaly & how hard is it to repair myself ? Im so sad
Not easy to replace yourself, a bit more advanced than a ding repair but certainly not impossible.
You will need a router first up
US boxes are easiest for sure
im guessing it's a power box and the post where the fin is screwed in has given away
Its an f2 xtc 103 ltr and its ripped the entire box out .Its split the ca rbon & divi cell up into a flap about 10 cm long . I cant do pictures till im back home so next wk. The fin box looks like it was surronded by expanding foam its still there around three sides. Were its missing is were the carobon damage is . It was a power box . The whole idea of comming up was so my wife could see her twin sister baby & i would go windsurfing every day to get me out of the motherinlaws house .
I use this site to check out whats needed
boardlady.com/
Have a look at what she says while looking at your job.
Thanks ka43 I did go see surf fx they gave me a board repairs no so i went & saw him he said it would take a few days to repair as the epoxy needed more time to cure.I thought about buying a new board but i calculated that i couldnt afford the divorce.Ill get it fixed when i get home . Winds droped off anyway . Im looking on the bright side it could have happened when i was out in the ocean . I hit a rope about a month ago but it seemed rock solid . So was it me just overpowering it with a big fin or did i damage it & not notice & then it let go when i overpowered it. I dont want this to happen again thats for sure .
It will always take a few days to do a finbox, lots of steps in that kinda repair.
It is very rare to rip out a box nowadays, In the polyester waveboard days it was very common to roll a box until they put 5 ply in next to the stringers. Even then it still happened.
I reckon it is maybe a minor manufacturing fault, combined with hitting something as you said, and then using big fins with lots of load and it has slowly slowly weakened
^^^ sounds like a stoopid idea. The bolt in powerboxes and tuttles is designed with too much clearance with the female threads in the fin so the bolt can rip out easily.
The box should be strong as blazes and there is no need to design a substandard box..
the chinook boxes are different and have there own housing thats glassed from the bottom to the top of the board, the tabou i did was like a us box but with the very light tubular post resined to the top of the box as an anchor point, as you said stupid idea, but its easyer to replace the finbox opposed to the back of your board
"edit"
Keef I was saying making fragile boxes is the stupid idea because of the fact the bolt is designed to let go
It looks to me as thought the box was only glassed were it joined the bottom deck & the rest of the cavity was just filled with expanding foam.
OK new power box has been ordered . I asked at wind surf & snow & they said could take a month to get fixed so I am going to do it myself .
The board is a f2 xtc 103Lt.
The last photos are of the fin box with the vacuum pump on to dry it out . I have the vent screw out & only about a teaspoons worth of water was sucked out. As I am going to do this myself all help will be very appreciated .
My understanding is that i have to grind out the cracks into a taper away from crack so a new piece of carbon can be used to fill then sanded back . Also as Barns sugested a layer of carbon could be laid under the crack as i have acess from the side . Then i resin divi cell to the finboxto fill up the cavity .Then i glass the top of the box to the deck . Then I use expanding foam to bond that to the eps . Then the box is glasssed onto the bottom
deck .Should i cut the eps back were its all torn out & glue a block in to bring back to size or just more divi cell on that side to build up that void its a good 50mm torn out . Thanks people I'm looking on the bright side now . It didnt happen out to sea & this is another step on the way for me to be able to make my own board. And I'm going to make it stronger so it never happens again (I hope).
Basically for strongest repair....
Router it all out about 30cm x 10cm.
You won't be able to router it out to full depth or you will take some deck laminate out ( remember it is curved) and watch out for rear footstrap insert!!!
Then scalpel cut and dig dig until you have made that nice rectangular hole go all the way thru to the divinycell layer on the deck side. You want the next couple of steps to bond to the sides AND the deck.
Insert a block of divinycell (30x10cm ish) in the hole. Make sure it is profiled on the bottom (well the board is upside down so really it is the top 
) so it matches the deck shape well. This is a long step with sanding the d'cell block to fit well.
Butter it with epoxy / q-cell mix and bung it in. Vibrate the jebus out of it to get rid of air bubbles and refill around edges as necessary (NO voids). Get the mix in anyway you can, popsticks, syringes, anything. Helps to have excess resin/q-cell mix in the bottom of the hole before you bung the block in.
Make sure it is a tiny bit proud of the board.
Weight it so it does not try to float.
After it is cured, sand back flush with board.
Router hole for new box.
Draw a line on wall (about 5m away) horizontally with a spirit level and then give it a right angle line with a t-square. Put board on saw horses and line up bottom of board with horizontal line. Tape board down to saw horses so it can't move at all
Cover fin and inside of new box with Kiwi shoe polish (good mould release) just in case you get messy.
Butter box with epoxy/q-cell mix
Layup with 2 layers wet glass, then butter again.
Insert into hole
Line up fin with the vertical line on the wall by eye , and then it is well and truly close enough to 90deg to board. Masking tape it in place (tip of fin to rail both sides. Lots of good gaffa helps)
After curing, set router to about 2mm depth and route out all the divinycell and the box.... that will account for thickness of the glass
Bevel edges of old glass
Glass over with 3 layers, when close to cured cut about box slot with scalpel (being sure not to lift the glass away from box). Let it cure, sand, fill, sand, paint.
sweet.
(it really helps with pics, but that is the gist of it)
unless you stop the box twisting, the same thing will happen again!
The foam isn't strong enough to do this.
That broken black peg is what was doing the job before, and it obviously wasn't adequate. (I've had several finboxes go exactly the same way, the peg snaps and box rolls)
If you can get a continuous couple of layers of glass around the box and tied to the bottom that will help. Or a divinycell/carbon sandwich alongside the box going all the way through to the deck, and tied to the deck and bottom. then the damaged bottom area around the fin needs to be structurally continuous from the rails up to the fin box.
You beat me Mark,
Only comment I have, is that the glass around the fin box, has to overlap onto the bottom for a couple of centimetres to stop box twisting, if there's no overlap onto the bottom you're still only relying on the d-cell strength, depending on the density, that may not be enough. and there's no point using fibreglass at all unless it does overlap onto the bottom.
Mark, looks like this is a race to see who can out type who, but seems basically we agree with each other.
this all sounds very complicated,double Dutch actually to me.
10 years ago i needed new footstrap positions in my board.
went to hardware shop,bought some red rubber screw in plugs,drilled holes in my board,put plugs in,then footstrap screws in.
sailed board but did not work 
,took it to Steve who put in new plug box.
the box i repaired i made a new post useing a sparkplug socket as a plug and glassed it in with carbon roveings top and bottom
I hope the fin box comes in today.
I havn't been in such a great rush as the ladder slipped off a verhander at work and was on the top of it result i was pretty lucky 4mtr fall sprained ankle lots of bruising ( could be a bit of practice falling off ladders helped ) Plaster cast came off yesterday so i can start to hobel around . I think i understand the process & once ive got the fin box in my hand I will be able to play around. What materials do i need ?
A 30 x 10 block divinicell is there a special denstiy?
Qcell /Epoxy how many ltr
Carbon fibre whats the best grade for this ?1/2 mtr2 enough?
resin ?
I have a router so holes sweet.
I'd go for 80kg/m3 d-cell.
I'm not sure carbon's essential, but it won't hurt to use it, 200gm/m2 is about right. 1/2m2 will probably do.
You shouldn't need much epoxy, 250mls should be stacks.
Qcell depends how good a job you do and how much bog you need, again 250mls should do you.
did your new box come with the post already attached to the box, if it did you mite want too reinforce where its connected, the way i see it is if its glassed to the bottom of the board properly and the post is glassed to the deck(with roving's) it shouldn't need heaps of packing just as long as you have it plush with the bottom of your board and glassed solid at the deck
when the box and post arrives take a pic id like to see it
I'm still waiting for the fin box. I will post pics Keef .Thanks decrepit I wanted a shopping list to take To the glass suppliers. I hope the box comes soon as I'm still off work with sprained ankle & it would be a great time to do it. I have done a bit of fiber glassing over the years so i feel confidant enough to use carbon fibre. I think that the fin box repair is a step towards making my own board. Thanks to Mark as well without your excellent advice fellow windsurfers I probably still would have done it myself but an inferior job . I'm hanging to get it fixed I'm so frustrated limping around the house . I want the board ready for when I'm ready should be soon .
