I want to add extra fins to a shop bought twintip kiteboard to give extra tracking. Beyond drilling the holes, is there anything else to be careful of? e.g. do I need to seal the hole edges with something to avoid water penetration, or reinforce the new fin area, etc?

Just use bigger fins

Bramber, Hi there is a quick and not so durable or a more involved effort.. Depends on the value of the board. If you have a look at djdojo's profile you will see a few piks of modifying a TT for foiling he does not hold back. Do a google on him with Site set to seabreeze (kite / foiling forum guestimate was mid 2019 +/- few months) should pickup an excellent post he did on how he did his mods. My efforts are based on that. I have only done TT once, not a g8 result cosmetically, did job. used big hole carbon fiber tube and epoxy with microballs and chop. so suggest djdojo's if you have trouble finding it let us know. Have fun you can always patch a hole with epoxy and chop easily .
Cheers
AP

Don't bother. Fins only aid in stability

Thanks for the tips.

I was thinking of adding extra fins to an old twintip to make it more mutant like... If I add two more fins to the existing rear fins, is there much science in terms of where to put them (beyond making it "look" like a quad arrangement)? Also being a twintip is there any point in angling the fins at all, or just make them parallel like the existing two?

Plummet said..
There is more science to it. You also need to offset the foot pads and get the rear foot pad and quad find as close as practical.
The more rearward the footstance and the closer to the fins the better the pivot turning. However the more offset the worse it is backwards.
You have to find that magic point. Check fin/pad location and the supernatural. Very similar indeed......

I love the idea of not having a toeside front fin. I expect it gives a very natural surf feeling for slashing waves.