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  1. #36
    Senior Member Radialhead's Avatar
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    Quote Originally Posted by Wing 11 View Post
    You think on that?

    It is much more easy to hold falling tree(Sail) whit rope 1 then rope 2.
    Rope 1 represent = higher boom instead low boom,seat harness instead waist harness,lean far more out instead upright stance,tall surfer instead short surfer

    Attachment 15732
    I already said nobody's disputing that. Now imagine your tree is leaning towards you rather than away from you, & that the person is firmly attached to the ground. A strong wind builds up from behind you. The part of the tree where rope 2 is attached can't move because the person is holding it firmly. The tree above rope 2 bends away from you, & the tree below rope 2 is therefore pushed towards you. It has to try to pivot around the point where rope 2 is attached - there is no other option.

    There will of course also be some wind trying to push the bottom part of the tree away from you, but the lower you move rope 2, the less affect that has, simply because there's less area under the rope for it to act on, & more area above it for the opposing force to act on.

    In short, yes the harness line angle is important, but it's not the only force involved, & any talk of internal versus external forces is a complete red herring & totally irrelevant.

  2. #37
    I was pushing a shopping trolley around Lidl this evening. They are free-wheeling trolleys, fttted with castor wheels.

    So you can push the trolley forwards, straight down the aisles as long as you can get traction with your feet on the supermarket floor. Once moving, it's pretty easy to keep going in a straight line. You might get hit by another trolley along the way, perhaps from the side, but unless the collision is extreme you just take the impact and keep going.

    While my trolley is heading forwards, I can also spin it from side to side and a second motion can be set up within the forward motion. My feet keep the trolley moving forwards but my body can move the trolley from side to side in a sweeping movement, which the trolley castors adapting happily.

    I also have my son in the trolley, and he can jump up and down and misbehave and yet that doesn't take the trolley off course as long as I keep using my legs to push it.


    The point of this silly story (true by the way) is that once you have a forward motion with continual power, it takes a lot to dislodge it. And the internal forces have very little affect on the main mechanical relationship, which in this case is leg power driving a trolley forwards.


    Boom height and mast foot position and footstrap positions are all settings which allow us to drive the board forwards efficiently – and the idea that boom height has a special relationship in levering the mast foot doesn't hold any relevance for me. At best, it's about whether my son rocks the trolley from a sitting down position or if standing up.
    The amount we move the boom height is so small that any change to this internal force has little effect on the bigger picture.
    Thinking of changing boom height to alter some leverage (by that changing the ratio of sail area above and below the boom) is a red herring for me.

    I change my boom height for sailing comfort and for control over the board and sail. Most people here seem to agree that you might raise the boom for early planing and then lower it to gain more control. Once in control, you can adjust the trim of the board mostly with body weight.
    And in a way, that's all you need to know.
    Last edited by basher; 16th November 2017 at 11:40 AM.
    Now back in the UK.

  3. #38
    Senior Member Radialhead's Avatar
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    Quote Originally Posted by Wing 11 View Post
    This is example of fulcrum in outboard engine on boat..
    Upper engine holder(red) want pull from stern of boat,and lower holder(blue) is pushing on stern on boat.This two forces are internal forces and they can NOT move this boat in any direction (roll,pitch,yaw,forward,backward.....)
    You can move this holders where ever you want,but nothing will change with boat motion
    If this holders are for example made of springs,upper spring will stretch and lower spring will get together..


    Thrust force is external force and only this force can move boat,in this case in forward motion...
    So I dont understand this example with boom as fulcrum, and sail area below/above.

    Attachment 15735
    Do you think the pitch of the boat would be the same regardless of how long the engine shaft is?

  4. #39
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    A lot of theory here whilst I suspect the underlying issue for the OP is that very small changes to the various tuning elements on a slalom board /slalom rig can fundamentally change the controllability and overall performance. Just to add to the challenge, different wind strengths, sail sizes, and water state, require some resetting of some or all of those elements. What is important therefore is not necessarily an understanding of all the physics at play, but how to return the board to the "sweet spot" ( as TWS refer to it) for every session.
    I do not move my footstraps once I am happy with their positions on any given board. I have already identified the best mast foot position for different sails on each board so rarely change that during a session. So the quickest and easiest settings I can change are downhaul ( North XT), boom height, and harness line length (the latter only on my big boom with big sails). For my money the TWS "how to make your board fly" video has the best advice. Incidentally their video on gybing using the step gybe ( minus the Charlie Chan foot positioning!) is also good.

  5. #40
    Senior Member Gorgesailor's Avatar
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    Quote Originally Posted by basher View Post
    I change my boom height for sailing comfort and for control over the board and sail. Most people here seem to agree that you might raise the boom for early planing and then lower it to gain more control. Once in control, you can adjust the trim of the board mostly with body weight.
    And in a way, that's all you need to know.
    About this I agree. The whole explanation is just to rationalize what many - including the Pro sailor mentioned have found. Usually I adjust boom height very little. I find there is an ideal balanced stance for a given board sail combination & once you have found it, any deviation is a compromise in some way. Usually you can adjust your sailing some using technique or else re-rig.

  6. #41
    Senior Member Graemef's Avatar
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    Quote Originally Posted by Wing 11 View Post
    This is example of fulcrum in outboard engine on boat..
    Upper engine holder(red) want pull from stern of boat,and lower holder(blue) is pushing on stern on boat.This two forces are internal forces and they can NOT move this boat in any direction (roll,pitch,yaw,forward,backward.....)
    You can move this holders where ever you want,but nothing will change with boat motion
    If this holders are for example made of springs,upper spring will stretch and lower spring will get together..


    Thrust force is external force and only this force can move boat,in this case in forward motion...
    So I dont understand this example with boom as fulcrum, and sail area below/above.

    Attachment 15735
    I used to race powerboats and there are similies between the forces an out board exerts on a boat and a sailboard rig on a board, we had to take great care to get the height and offset correct or it could end up with one side of the bow being cracked as the boat constantly landed on one side due to the sideways torque of the propellor as well as the forward motion.

    A sailboard rig also exerts a degree of torque, take for example the uj suddenly snapping, the bottom of the rig will come toward you with quite a force as you are acting as a fulcrum, contrary to what a lot believe, most of the power passes through you then your feet, the forces at the mast foot act very differently.

    Take for example the act we have to employ on long boards of dragging the rig right from the front of a 70cm mast track to the back in order to sail offwind, the technique to do this isn't dragging the rig back, it is achieved by lifting the boom up, as you lift the boom, the forces above it pivot the mast foot back down the board there being more force above the pivot point or fulcrum than below it.

    So, the effect of raising the boom, raises the pivot point at which the power is transmitted and decreases the ratio of power above to below the boom, therefore decreasing the 'lift' at the mast foot at certain points of sailing. Now short boards have very little variation, they are by and large small low power rigs on a small light board with very little movement to speak of compared to the massive movements of big rigs on longer more powerful and weighty long boards or even fast slalom boards where the loads and power transmitted is greater.

    Applying mast foot 'pressure' is something you do, or you get told to do as part of your basic training, but the reality is the forces on the mast foot naturally tend to be negative and the higher the boom the less the leverage the top half of the sail has upon them.

    As to getting the nose up the further up the boom the further back you can stand and this can lift the nose, if you have a board prone to being nosey, look to the fin, it's probably too vertical and a more swept back fin will enable the nose to lift, especially if you raise the boom which rakes the rig back a bit you stand back a bit further and the nose lifting effect works. Given you ride a board long enough to permit fore and aft movement.

    These little stubby multi fin things y'all ride these days have little option for trim variation so i wouldn't worry your head about it.
    Last edited by Graemef; 15th November 2017 at 10:31 PM.

  7. #42
    Good to see there are a lot of differing views on this.
    And try this for another thought:

    We talk about internal and external forces, but the key thing here is to remember that several things can be going on at once.
    You can be driving up the motorway with your family in the car, and you as the driver control the speed of the car via the engine power which you control with your foot on the throttle. But the car is heading up the motorway, getting from a to b in a fixed time, and that's the key reason for the journey.
    At the same time, your kid might be being sick on the back seat. The disaster in the back of the car is a separate event and only affects the car moving forwards if you as the driver lose concentration and perhaps crash the car.

    And mast foot pressure is like that.
    (We've had threads on this before – with greater or lesser success. Rigs have also changed a bit since then.)

    You might be sailing along on a big slalom board or on a longboard, and if the U/J fails then the bottom of the mast might well swing to windward as the mast tip falls to leeward. That in turn suggests a leverage on the mast foot to do with the boom height.
    That catastrophic event is also seen by some as showing the mast foot is in tension, and not compression – and so, the argument goes, surely 'mast foot pressure' cannot exist?

    The reality is that several things are going on at once, with internal and external forces at play. It's only when the U/J fails that the internal leverage force (normally neutralised) comes into play. I can assure you that for most of our sailing the mast foot is in compression – and indeed the U/J over time shows a swelling that confirms that. We are not kite-surfing here, and in most normal circumstances there is a downward component from our rig drive acting through the mast foot.



    Modern rigs are also more compact, and once you are on wave sails the sail area above the boom is relatively small compared to gigantic Formula rigs. The common ground is that our booms remain the same height, due to our physical height as human beings. Big sails have longer booms but they tend to be higher aspect, with the drive point well above the boom. That drive point (the Centre of effort) is even higher with a tight leech lightwind sail.

    Then again, we windsurfers simply observe different things with different gear. If my U/J fails on a wave board or on my freestyle board then the mast extension might scratch the board deck or even puncture a hole in my deck as the rig comes downwards. Is that mast foot pressure or is it the weight of the rig coming downwards? Probably a bit of both.

    To test this properly I guess what we need is an electric pressure pad under the mast foot which tells us when mast foot pressure is happening – not when the rig fails, but when we sail along.
    I'm pretty sure that mast foot pressure exists in my short boards. I'm hanging off my rig when sailing along and my mast foot is usually and mostly under compression – and not often under tension. So the load is mostly downwards, but with some sideways component too.

    Ironically, none of this affects my boom height choice.
    And we can still say that the rig power connects to the board via the three points – our two feet and the mast foot.
    Last edited by basher; 16th November 2017 at 10:57 AM.
    Now back in the UK.

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