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  1. #1
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    Why does more wind need smaller fin?

    I understand why as the wind increases I need a smaller sail but not a smaller fin, on the SAME board.
    Imagine I am sailing a 62cm wide board well powered on 6.5m with a 36cm fin. The sail is generating enough power to overcome the drag of my gear and off I blast. Then the wind increases and I change down to 5.5m and the gear feels unbalanced with the windward rail riding up so I change to 28cm fin and back to comfortable blasting along at same speed as before. Surely forces are similar, and maybe a bit less with smaller sail having lessdrag so why do I need a smaller fin on same board.

  2. #2
    Senior Member Silicon Beach's Avatar
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    I get what you're saying: that you change to a smaller sail when there is too much wind power for the sail area, but that the fin is under the water, not affected by the wind, and so the lift / force on the fin remains the same. I think that is why with my multi fin boards I never need to change the fins, whatever sail I'm using.

    But the other limitation of fin size is control dependent on the state of the water. Obviously as the wind increases the water gets more choppy / wavy and so a smaller fin helps with control. Again, with small multi fins it's not so much of a factor.

    The only other question might be: we know that that the size (volume, width etc) of a board tends to suit a certain sail size / range ... and does the size of the (single) fin contribute something to the perceived size of the board, such that a given fin size feels right with one sail, but not with a much bigger / smaller one? This is presumably why slalom sailors use a smaller fin in a smaller slalom board.
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  3. #3
    If we use multi fin waveboards then there is little or no torque from short fins so the modern waveboard does not over-power from the fins, but the board can still seem too big in terms of float.
    And that's because as the wind increases the board goes faster, the water gets rougher and the wind can get under the hull. Our ability to put body weight on the board tail may also change if your stance changes as the wind increases and you change rig size.


    So, to answer the question, a long fin on a slalom or freeride board rails the board as it develops lift. If you go faster then fin torque rails the board more and the board can get out of control – and it may be that you are going faster on the smaller sail but did not realise it. If the water gets choppier, or the wind gets under the hull then the sailor may have more difficulty holding the rail down.

    The smaller sail does not need to be matched with such a big fin as sideways loads on the fin are less, so changing to a shorter fin is often a simple way to regain control in windier weather.

    if you don't want to change fins when you drop down a sail size, then control issues might be solved by shifting the mast foot forwards a touch as this takes rig load aways from the fin in terms of torque distance, and has the added effect of dropping the sailor under the rig more to load the tail better with weight.
    Main boards: Flare 101, NuEvo 86, UltraKode 80, Reactor 82, NuEvo 73. Powered by Severne Blades and S1s.

  4. #4
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    I recently started to use multifin boards as well and was pleasantly suprised to experience like you that a given set of fins has a big sail range.
    Going back to single fins I understand smaller sails = more wind = choppier sea, but i find some changes, most prominently 8.5m (44cm fin) changing down to 7.3m don't change the sea state much where I am but 44cm is uncomfortable with 7.3m so i change to 38cm. My GPS tells me I am cruising in both cases at around 25 knots. My logic says 7.2m has less drag than 8.5m so less sail force is needed to reach 25knots so less lift off the fin, but experience says the opposite. I am missing something here.

  5. #5
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    I believe it is because larger sails put more load on the rail and the fin. So to provide the correct amount of lift to counteract that we need a bigger find. If we then reduce the sail, and hence the force on the fin, it is still providing the same lift at a given speed. That lift translates into sore legs and a flat nose as we catapult through the sail. Instead we reduce the fin size to bring balance to the force. Either that or I have had too much chateauneuf this evening.

  6. #6
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    Quote Originally Posted by phill104 View Post
    I believe it is because larger sails put more load on the rail and the fin....................
    but if big and small sail drives board at same speed surely force derived from sail is the same, or maybe the big sail as well as providing the same forward drive also provides more sideways force?

  7. #7
    Senior Member Gorgesailor's Avatar
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    Quote Originally Posted by Maker View Post
    but if big and small sail drives board at same speed surely force derived from sail is the same, or maybe the big sail as well as providing the same forward drive also provides more sideways force?
    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. The loads from the larger sail actually provide some stability which is why it is generally easier to sail an overpowered 7m than a 5m - all things being equal(though they never are ....)

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