US Finbox fin for iSUP - 6.5"

I recently bought a Nixy Mahattan iSUP. It comes with this 9" fin. Note thumbscrew that also is captive.

Since I also go on rivers I wanted to buy a shorter fin. I have another iSUP that has 5" side fins and even those get scratched up. So a 9" fin is a no-go. Our rivers are very low this year (a few boat landings I saw being a few hundred feet away from any actual water)

Best I could find is this 6.5" fin. Note that it has a screw without the plastic knob and a threaded square washer (I know this is the wrong term). I tried installing and it only worked without that square thingy and the screw is not captive. The fin itself has a wide base and doesn’t slide well in the fin box. This no-sliding makes it hard to align the screw. I guess I could sand a bit off. On the upside, it is a bit flexy, so it should survive hits?

But I looked for alternative fins since I also like a more captive screw system (because if it isn’t captive, it will be a hassle). but it seems all shorter fins have that weird system. In one review I read this is a system for regular (solid) SUP, not inflatable. I didn’t realize they have different box systems.

Can someone school me:

  1. is there a better way to install that fin i bought? I didn’t see a way to get that square fin inside the box.
  2. Do iSUP indeed have a different system and what I bought is not for iSUP? Is there a specific name or keyword for the correct one?
  3. is there a reasonable source for shorter fins that are more like the OEM fin (but quite a bit shorter than 9")?

A totally wild alternate idea would be to just not install a fin at all when I go into rivers. But I suspect this won’t track well despite the sleek shape.

Have you tried any surfboard sites?

Most kayak and board builders buy their auxiliary parts from someone else and just install them.

Measure the box and see if you can find a name and model #. If there is a surf shop near you, go there.

The skeg depth and width are usually tested so the paddler can get 8-10 strokes on one side before the tracking goes off. A shorter skeg will make that interval less, no skeg will make it a pool float.

I have an iSUP (inflatable), not a surfboard. And I’m inland, so no physical surf shop nearby.

Some more research shows the fin I bought (or the screw system with the metal plate) seems to be common for hard long boards etc. I’m able to insert the square plate in my fin box. But that is redundant since my fin box already has a permanent threaded whole to thread in. This system also seems to be designed to keep the fin installed permanently. I have an iSUP and roll it up after use, so the fin gets removed every time. This system of screw with metal plate is not practical for frequent installation.

What I need is a fin with the same type of screw like my original 9"fin. that screw is captive and stays with the fin without needing that plate. that fin also relies on some threaded hole already being in the fin-box (like mine does). If I can’t find anything, i always could butcher the 9"fin and shape it to 7"or so. But ideally I just buy one that already is shorter.

My original fin is 8.33 mm wide and fits well. the new fin is 9.02 mm, so it is the standard 9mm. The inner width of my fin box is about 9.3 mm (apparently 0.3mm gap still makes the 9mm fin be sticky). Obviously with manufacturing tolerances and changing temperatures this all is a trial & error situation.

I’m perplexed that there are so many standards that create very similar fins that have just tiny differences. In addition the manufacturers still deviate a bit.

I see that my original fin screw was made captive by adding a small U-shaped metal washer it threads in. Maybe i can find something like that to make my 6.5" fin captive. then i can sand it to make it thinner so it fits into the fin box. But again, if I could find one that already works, that would be preferred.

I realize a shorter fin will track less… but this isn’t really a choice unless yo put some more water into our rivers.

Edit: I took the iSUP out this morning on the lake with the 9" fin. This was the first time out. I kayaked on a stadium seat with kayak paddle since it was a bit choppy. I hit ground once :slight_smile:
At the end i took the fin out entirely and kayaked with just the fin box being my “fin”. It was a bit less stable, and tracked a bit worse, but was totally acceptable. it wasn’t worse than an IK. So in a pinch it is do-able without fin. But I think a 5"fin (many seem to be 4.4") still would be better.

Then try google. I did and found this

Trust me, I did research before buying the one I bought. And I researched again before my OP.

I had found that (and similar ones), but it doesn’t have a captive screw. And it has the screw port on the wrong end. My fin box (and the fin it came with) has the screw on the bow side. Nixy actually sells a similar fin, but also with he square piece that will make frequent installation cumbersome.

Just to make it clear:

  • I have an iSUP (inflatable) and take the fin off after every ride. any fins or fastening methods for hard boards or boards that keep the fin on all the time won’t work.
  • the fin should have a captive screw on the bow side since that is there the finbox has the hole to screw in. This is the fin I have (the nose towards nose option) and any new fin should be similar. Basically just slide in and screw tight. No fiddling with metal plates etc.
  • I don’t want to fasten it with the square plate since that is a hassle and things get lost
  • the new fin should be as easy to install and take out as the OEM fin and have no lose parts i need to keep track of.

I’ll try to go to a hardware store to see if I can get a captive screw of the size. If I find something, I could buy the above but based on what I learned so far I’ll return the 6.5"fin I bought. This is how the Nixy fin captive screw seems to work (top right picture is close to it). So what all the advice above is missing is the captive screw.

Looks like Starbaord has screws labelled as M4-0.7 and 22mm long. Is that the correct standard thread? where would one get 22mm long versions of that as thumb screw?

i found some screws (that are too short), but they don’t have the E-ring. I also don’t know if they have enough space between screw head and the thread for the fin height to fit in. So I really would need to find the exact screw that Nixy uses. Maybe just cutting a Nixy 9"is the best.

I found this on the Nixy website.
It seems like it would work.

No, it doesn’t. It requires to insert that square plate into the box every time I install the fin. I take the fin out (and deflate my iSUP) after every paddling.

And it wouldn’t work without that square plate since the already installed threaded insert is in the front (bow side) of the fin. So it would be the wrong way.

I really want to have a screw similar to the original 9" fin with captive screw (on nose side) since that doesn’t require me to keep track of parts, and just screws in without fiddling with the square plate.

I don’t know if the captive thumb screw is common among iSUP. I start to suspect it may not. But both my Nixy iSUP use them and it really simplifies things. I just cheked Red and their spare fins also use the square thingy. So that captive design may be a (great) Nixy special.

The site says about the fin, “Compatible with all NIXY center fin box”

Yes it may work. But not conveniently. It still requires to install and take out that plate every time.

Did you look at their 9"fin? That design is 1000x better and doesn’t require fiddling around every time you install or take the fin out.

The design you don’t like because you have to insert the plate and align the screw is actually a better system for being secure. One thought I have is since the fins you like are so inexpensive just buy a couple and cut them down with a hack saw to the height you want, and shape it with a rasp and and sandpaper or an orbital sander. Board surfers used to do this all the time to make their own custom fins from cheap biggerfins. You can use the extra fin as a template for the curve, and outline it on the bigger fin with a marker before shaping.

Yeah, the square plate is less proprietary since the Nixy system depends on a threaded hole already in the fin box. I guess one could glue in that square plate or so…

Unless a better fin comes up I’ll either suck it up and buy one of those 4.4" fins (ebay for $22?) , or sacrifice one of the Nixy 9" fins. This weekend i plan to go out on a river and will just test how fin-less works out on a real river. That obviously would be the cheapest and would not hit the ground at all. So i try that first, then decide what compromises I’ll accept.

I also could just use my other iSUP that has two 5" side fins.