Fins problem

[Oceanbreather]

I have FCS g5 fins on my board. I cut my ankles on the side fins sometimes. I'm considering either buying two center fins to replace the sharper outer fins or just sanding down the edges on the outerfins so they aren't so sharp. Are there any reasons why I shouldn't do either of those? I don't know why outer fins are flat on the inside side anyway. My only guess is price.

[surferdude_scarborough]

well the reason for the flat side on the inside is to do with the foil and stuff. you'd be better off listening to someone elses explanation of this because i dont understand it. therefore buying two centre fins for the outside wouldnt be such a good idea.

no reason not to sand down the sharp edges a little so they are not cutting you so much. in fact its the safest thing to do. if they can cut you they can cut someone else. just make sure you dont change the shape of the fins foil much if at all.

[the.ronin]

ProTeck fins

[Oceanbreather]

They don't use the same design for attaching to the board . Cool idea though.

[the.ronin]

They make them for FCS and Future ... is it the size they don't have for you?

[drowningbitbybit]

If you put two centre fins where you're outer fins are, you'll have a barge that will have the turning ability of an oil tanker. You dont want to do this.

The flat inside of the fins is part of their design, not to save money (although fins with a shaped foil on the inner surface are now coming onto the market).

So just sand them down ever so slightly.
Any more than that and you'll suddenly realise how important fin shape is.

[kitesurfer]

I don't know why outer fins are flat on the inside side anyway.

Its for exactly the same reason that aeroplane wings are curved on one side and flat on the other. Lift. The way a thruster fin set-up is arranged is what allows it to generate lift and thrust as both you and the wave apply pressure/force to it.
As DBBB says if you have centre fins for outer fins the board will have much less ummph (and yes that is a technical term that i picked up off sar) to it!

KS

[the.ronin]

I don’t mean to veer on a tangent here but this is all pretty interesting stuff.

So how does this “lift” translate to, technically speaking, “umph”? I never doubted that there was a reason outer fins are foiled like that but I also never fully understood why. Using the principals of flight, the lift would be created since there would be lower pressure on the inside if the fin than on the outside. That would create a sort of “pull” force on the outer face of the fin, right? So we’ve got these foiled fins on both sides of the board creating pull forces on opposite directions.

Is it that fact that the outer fins are (correct me if I’m wrong) pointed towards the nose of the board thus the forces are actually not perpindicular to the axis of the board but slightly facing forward? Does that create forward thrust as water flows along the outer fins? (Oh snap, is that why they are calle thrusters??)

If I’m not mistaken, the outer fins are also slightly leaning outwards, right? So not only do we have forward thrust, but there is also upward lift being created?

[eastcoastsurfshop]

Yeah, sand them down a little, but only a little! Most new FCS fins like the M series have a slight inside foil now, but its nothing like a centre fins foil so you'd deffo notice the difference with 3 centre fins! Proteck fins are a good option, thay have a rubbery edge witch...

1. doesn't hurt you or others
2. gives a bit of flex & twang on the edge of the fins whilst keeping stiffness throughout the fin so not losing to much on the turns and still gives a bit of twang when your do turn

The other option is to keep ur ankles away from ur fins, shouldn't be there anyways

[Roy Stewart]

Does that create forward thrust as water flows along the outer fins? (Oh snap, is that why they are calle thrusters??)

If I’m not mistaken, the outer fins are also slightly leaning outwards, right? So not only do we have forward thrust, but there is also upward lift being created?

The leaning outwards is called cant, it does create horizontal fin area, but this does not necessarily lift upwards in relation to the bottom, due to the fact that toe in ( fins pointing in towards the nose) lowers the lift angle of the fins. The trend has been to use less toe in to minimise drag from the sidefins working against each other, this also stops the side fins from pulling downwards ( although I have never seen anyone actually mention this fact)

.

[the.ronin]

Thanks for the response Roy ... I never actually put much thought into the engineering behind the surfboard but I imagine there's quite a bit of rhyme and reason to even the smallest detail.