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  1. #8
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    It relates to the amount of leeward force exerted by the sail/wind strength hence the need to achieve a balance between sail size and fin size/type/construction. I say size/type and construction because fin length or even fin area is not the full story as far as fins are concerned. A well constructed carbon fin can have a far greater range of use than a G10 fin for example simply because it is easier to build the fin with controlled flex. On my boards I can use a 35cm carbon fin with 5.8/6.2 and 6.6 with little noticeable change in terms of control or drive..........I may change to a 37cm with the 6.6 if the wind is on the light side for that sail. Fin choice is far more critical on more performance oriented boards...e.g slalom........and much less so on more user friendly hulls.

  2. #9
    Senior Member PK1111's Avatar
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    It's because your large fin is for your light wind sailing. 36cm is pretty big!
    You generally need that depth to provide the lift to counteract the force of the larger sail but it will overpower quickly.

    as soon as you can change down sail size, you'll find the larger fin is actually providing too much lift so have to reduce the fin size.
    To put this into perspective, I use a 23cm swept back wave fin for my fsw 90 from 5.9 to 5m sailing. I plane in a solid 15 knots just like most fellow sailors.

    You should try using your 28cm fin with your 5.6. You have to use a lot more technique to get planing, but I think you might be surprised.
    There are limits though. I use a 44cm fin on my 120 litre slalom board with 8m sail for summer cruising in 10 to 15 knots!

  3. #10
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    Quote Originally Posted by PK1111 View Post
    It's because your large fin is for your light wind sailing. 36cm is pretty big!
    You generally need that depth to provide the lift to counteract the force of the larger sail but it will overpower quickly.

    as soon as you can change down sail size, you'll find the larger fin is actually providing too much lift so have to reduce the fin size.
    To put this into perspective, I use a 23cm swept back wave fin for my fsw 90 from 5.9 to 5m sailing. I plane in a solid 15 knots just like most fellow sailors.

    You should try using your 28cm fin with your 5.6. You have to use a lot more technique to get planing, but I think you might be surprised.
    There are limits though. I use a 44cm fin on my 120 litre slalom board with 8m sail for summer cruising in 10 to 15 knots!
    You are describing the experience, which I agree with, but I am stull unclear why it works that way.

  4. #11
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    Quote Originally Posted by Gorgesailor View Post
    I think you are on to the answer here. Larger sails usually have COE further back, more weight etc... hence put more load on the board & fin for a given amount of lift.
    Indeed, coupled with addition weight and balance the increased load through the legs to the rear of the board and hence the fin would increase. I suppose the easiest way to imagine it in the old grey matter would be to massively exaggerate everything. Compare for instance a 60cm sail with a 60m sail, which would load the rear of the board more?

  5. #12
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    Quote Originally Posted by Maker View Post
    You are describing the experience, which I agree with, but I am stull unclear why it works that way.
    You are asking for an explanation of the forces applied to and generated by a windsurfing rig and board and that is complex! A sail has a relatively small working range in terms of angle to the wind. So if you are aiming to sail on a beam reach the sail will create no drive if it is at 90 degrees to the wind, but will create drive if you sheet in up to about 20 degrees. Some of the force generated will however be trying to push the board to leeward rather than forward. The leeward rail of the board plus the fin provide resistance to that force converting it to a force lifting the windward rail. You resist that lifting force by keeping the board level. The foil shape of the sail meanwhile produces lift ( a force in a direction between straight ahead and leeward). As the board accelerates the rig creates it own wind in effect ( as you would if you ran on a windless day, you would feel wind on your face). That combined with the actual wind direction becomes the apparent wind acting on the rig which is forward of the actual wind, hence you can now progressively sheet in to match that wind angle. Meanwhile the leeward force continues to increase. The bigger the sail the bigger that leeward force so the fin needs to be big enough to counteract it and convert it to a lifting moment on the windward rail. There comes a point where you can no longer keep the windward rail down although your sail may still be able to remain stable in the windstrength. That tells you the fin is either too big, to stiff, or both. The symptoms will be difficulty getting the board off the wind and the board becoming unstable. Slalom sailors use this point close to loss of control to get the board riding up on the fin but it takes strength and courage to keep it there! If you are able to push the board downwind or upwind the problem will ease because the leeward force on the fin is then reduced. If you are going fast the apparent wind will however already be pretty fine so heading upwind can be dangerous. If you are at the point where the fin is causing loss of control, sheeting out will only compound the problem because you reduce the forward forces. The likely result is a crash as the board takes off! Even staying with the same board and sail, changing now to a smaller or more flex fin can bring everything back in balance.

    So assuming an optimum wind strength for any given sail size, there is a matching optimum fin size/construction/type.

  6. #13
    Senior Member PK1111's Avatar
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    Quote Originally Posted by Maker View Post
    You are describing the experience, which I agree with, but I am stull unclear why it works that way.
    I know, sorry!
    My point is that there is a turning point when the wind suddenly provides substantially more power, and generally that is 15 knots of wind.
    Below that you need quiet specialist equipment and or good technique to get going. Your 36 cm fin is certainly that.
    Wind speed provides power in a logarithmic way, so as you go below 15 knots, you have to increase size exponentially, so why big kit is big (formula).
    Im not sure but I think fin lift is similar.

    Once we get more than 15 knots of wind, the dynamics quickly change and frankly equipment size becomes more focussed on control than power.

    There is a magic zone of 15 to 20 knots which seems to be free game for everyone. Slalom sailors still hanging on to big kit, free riders either going faster but more likely changing down to 6m, wave sailors and freestylers on 5.3s.
    All generally flying around and having fun.
    That is the point where you are changing down sail and fin size, for more control.

    If your power and lift are not matched, then you will experience control issues.
    Like I said, to really improve your technique, try your smaller fin with your larger sail.

    My fsw 90 arrived with a 27cm upright fin and double back straps, but I've never found they got me planing earlier, or kept me planing through lulls, so prefer the smaller 23cm fin.

    The ultimate irony?
    I go fastest on my biggest gear.
    25 to 30 knots with an 8m, 120 litre slalom board with 44cm fin. Typical speeds on fsw are c20 to 25 knots and 15 to 20 knots on the wave board.
    Certainly something to think about!
    Pk
    Last edited by PK1111; 13th November 2016 at 04:14 PM.

  7. #14
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    I wonder if another factor is on a large sail in lighter winds the apparent wind is less across the beam than smaller sails in stronger winds to achieve the same forward speed.

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