Hi, I'm modifying an older waveboard to make it a bit wider, and I want to use a powerbox for a central fin, and two US-boxes for two side fins.

Where do I place the fins, and do I need to cant them in or make them parallel?

Are there any production boards like this? Or at least a three-fin setup?

I want to use a powerbox fin for the centre fin as I want to be able to handle bigger powerbox fins if I need to, and I seem to have a surplus of 'wave' powerbox fins that I never use!

FN,
Most production board use no cant and have no toe in. This makes it a much simpler install, if you choose to go down this path.
It will keep the board fast too, without the toe generating drag, which is impostant in a bump and jump style board or light wind wave.
For more pure wave a little toe wouldnt hurt but i mea little, like 0.5 degree.

Keep it simple and parallel, then all you need to decideis how close to the rail and how far forward you want the boxes!

Thanks Bertie. With my level of accuracy, 0.5 degree is parallel! As is 3 to 10 degrees

I've been having a bit of a search around, and it looks like people put the side fins ahead of the centre fins. Is this normal?

Does it work either way, or is there a benefit to one over the other?

Agree with Bertie, go with fins straight ahead and then you can play with toe by getting toed in fins like K4 ones.
But a slight amount of toe-in is always preferable.
So I'd measure the side fins as 1 deg toe in and draw the lines about 1m long on the board so you can check and recheck.
Then at least if you are off a bit you know it won't be toe-d OUT which would be bad.

As to positioning you will have so much fore / aft movement in a US box that I'd not worry too much about it. Just look at pics of similar boards.... but my guess is side boxes should have their REAR maybe 1" ahead of the FRONT of the centre box....?

Cant is helpful but too hard to achieve withough a jig.... when getting depth, toe and level all at the same time you will be stressed enough :)

But Bertie, toe-in does not make drag, it reduces it. :)

Select to expand quote

Mark _australia said..

But Bertie, toe-in does not make drag, it reduces it. :)

Ummm..... any time you increase the angle of attack ( below stall point) you will increase drag. It might not increase much but it will increase drag while traveling in a straight line.

Oh and FN, how long are the front US boxes you bought? i'd put the front boxes about 30-35mm infront of the rear but it all depends upon your fin choice( and how long the fin base is) and the box length.
I'd guestimate my leading to trailing edge fin gaps would be 70-80mm? but i'm not near my board right now, so get somebody to measure it to be sure.

Water does not flow straight along the underside of your board, it also escapes sideways. A couple of deg toe in improves most boards as it reduces drag.

My front boxes were slot so not much wiggle room, but yeah my LE to TE measurement would be similar to what you've suggested

Select to expand quote

Bertie said..

Mark _australia said..

But Bertie, toe-in does not make drag, it reduces it. :)

Ummm..... any time you increase the angle of attack ( below stall point) you will increase drag. It might not increase much but it will increase drag while traveling in a straight line.

Oh and FN, how long are the front US boxes you bought? i'd put the front boxes about 30-35mm infront of the rear but it all depends upon your fin choice( and how long the fin base is) and the box length.
I'd guestimate my leading to trailing edge fin gaps would be 70-80mm? but i'm not near my board right now, so get somebody to measure it to be sure.

I bought 8"/200mm boxes. Should the finbox be 35mm in front, or the leading edges of the fins?

I've added front boxes to a few boards and I would suggest the rear of the front box is in line with the front of the centre box, especially if you are using 8" boxes.

Draw a line across the width of the board at the front of the centre box, and put the rear of the new front boxes at this line.

Once you mount the fins you then have the ability to move them through a decent and useful range.

With toe-in I would put the boxes in with a very small amount of toe-in. Maximum of 3-4mm along a 200mm box. This gives around 1 to 1.5 degrees. The main point for me though is to ensure you don't have toe-out when the resin has set, rather than to achieve toe-in.

Same with cant. Aim for slightly out, not to achieve cant, but to ensure you don't end up with them pointing inwards.

And then distance from the centre line I always go with the furthest distance I think I can get away with without the box sticking through the deck (or getting too close)