i know i am going to start another forum controversy by asking this question
But what fin should i use and why/what the different ?
every one told me yesterday that i need a weed fin for my fantic viper while at the lake - 180L, 80cm wide, i am 100kg sailing with 5m or 6m
atm i have what moby call "38 powerbox" with a single bolt
was looking at
carbonsugar.com/design/everything-you-should-know-about-fw-fins/
Do i need to measure the hole to see if new one fit ? or is there 1 standard
i read that i need to get some thing with 45 rake , ( is more rake back better ?)
and that a eg: 45 weed fin is similar to a 38 power box so i got to get some thing bigger as a weed fin then normal fin
Now do i want longer , wider, thicker, flexible or stiff
I'm pretty sure your board has a powerbox finbox. I didn't look at it that closely though. However I saw the single bolt at the top, indicating its a powerbox.
Any powerbox fin should fit in that box. Its an industry standard. The bolt of another fin may be of a different length and have a different thread than your current fin. Thats really the only difference. Sometimes a bit of sanding is needed to get a better fit.
In regards to weed fins, see if anyone has a second hand powerbox weedfin. Someone may have gone to tuttle box boards and has fins they want to get rid of. At this point you just need any old fin.
More rake will mean the weed slides off the fin more easily. Weed fins are bigger than comparable upright fin.
If there is floating weed you'll need a weedfin so the weed slides off, otherwise you'll keep getting bogged down and wondering why you're not planing.
As a newbie don't go for an expensive pristine fin because you'll hit the bottom a lot and damage the fin anyway. Ask around, check the buy and sell for a second hand fin of suitable size.
As a beginner you won't want a whole quiver of fins, just a weedie for the lake and maybe a pointer as well if you sail somewhere with no weed. Set your mast track a little further back when using the weedie.
billyboy402 your a redneck to this sport, just give you some advise, everyone is going to be your best friend 
be aware of best friends that want to help you out with old "almost new" there's a few key words you will need to know
you need a fin , i just happen to have one in the truck, slight crack at the base but still works it cost $150 its your for $50
Ive got this great sail , only a bit of delam i paid $500 you can have it for $300
you need to upgrade Ive got this board i cant kill it, the decks just a bit soft i paid $1,800 its your for $500
hey billy boy you need some new harness lines, you can have my old one for $10 you can get a new set for $40 and you'll have one spare
i guess the list can go on but remember there's there's a Heep of crap out there that's worth a lot of cash
just remember im not far away and heeps of great unused gear, ill send you a pm and you can let me know when you comeing so i can dig out of the shed 
^^^Keef, I've been trying to get billyboy to make the trip down south but it just hasn't lined up with work and weather. I've got some bits he can have as well, 4.7 sail, adjustable harness lines, Pryde X3 boom and a Dakine t3 harness. All used but comes at the right price $0.00, just can't help out with a weedy as I sold my spare.
William, weed fins will make life a bit easier if there is any sort of weed about, it doesn't take much to stop you planing. Like all fins the weedys come in all sorts, wave, freestyle, freeride, slalom and speed. You'll just want a Freeride weedy for your viper and generally if you have a 38cm normal fin you will want a 38cm weedy. They will look vastly different when you put them side by side and the weedy will be a lot longer but because of the rake it will be at the same depth as the standard fin. The viper is power box and all power box fins are the same, The PB is the most common fin box in freeride boards. from memory the viper had a much smaller fin as standard, about 20-25 was what I was thinking. Yours must have an aftermarket fin, It had a smaller fin as the board is fitted with a centre board that generated the lift and stability and the back fin was mainly for tracking. If the centre board is not used or missing then something like a 38cm fin would be needed for the board.
There are plenty of weed fins out there but they are not all equal.
.....I was wrong, just googled the viper specs and it ships with a 34cm fin. It's been a few years but I would have bet my left on the fact that it was smaller.
Anyways you'll want the same length and stiff, flexy weed fins are rubbish. The angle can vary but 45 is about the standard.
Sounds like I need a larger weedfin..I borrowed a 30cm one for my 125ltre with 6.6m sail. I usually use a 44cm with it..
It was quite gusty so I spent some time not planing but I wasn't getting upwind very well.I know its also my technique as well.When it did blow I'd spin out occasionally so I'd just back off the back foot pressure after & it would be ok.
Its frustrating when you can only have one or 2 fins to cover a wide range of boards & sails..
^^^^Sue it's just technique, You just have to sail a bit differently with a weedy. The stiffer they are the better. I've use a lot of different ones and unless they are made out of g10 they are just to flexy and feel rubbish. In my 112 I use a 23cm wave/weedy and it's got more then enough grip, For my 95 in a 20 and it feels to big. And if possible try and find a weedy that has been set forward in the box and It will feel so much more like a normal fin, These type seem less prone to spin out.
I come from a surfing background and things are a tad different. If someone lends you their wax, whilst they lean over our shoulder so you don't use too much, you've done well. Only been sailing for 5 months now and every sailboarder I've meet has been super helpful and generous.
be aware of best friends that want to help you out with old "almost new" there's a few key words you will need to know 
