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If you watch the contestants in the Fizz Cup struggle for a place on the stage, you will no doubt have seen some of them capsize or nose dive. At first a nose dive looks quite funny and you think he or she must have made a mistake. But there are not many who know what mistake the sailor actually made and what lies behind this malheur. I will try to explain and illustrate. My apologies for the poor quality of some of the images.
There is a big difference in the forces on a boat when it sails upwind or downwind. Just look at the 2 images of laser sailors. The first is sailing upwind in a gentle wind and the second is reaching in a strong breeze. Notice how the sailors in the first image are sitting towards the front of the boat while in the second image the sailor is hiking backwards.
If you sail upwind, the forward pressure in the sail is relatively small and there is a wind that blows against the rig, trying to tilt the boat backwards. The speed is relatively slow and therefor the resistance of the hull in the water is not big either. The sailor sits at the front to make the water leave the transom in a smooth way.
If you sail on a reach, the forward pressure in the sail is big, there is almost no wind drag and the hull is literally dragged through the water with great force. The sailor needs to sit at the back of the boat to prevent the boat from pushing the nose down.
Unfortunately you cannot sit at the front or back of the boat in SL. And therefor a Flying Fizz doesn’t nose dive if you sail it normally on any course. But there are more forces in play….
If a boat heels over to leeward, the sail will try to turn the boat towards the wind. This effect is called weatherhelm.
In order to sail in a straight line, the sailor needs to steer away from the wind. But the rudder is located at the back of the boat and is not only steering, but also lifting the transom. And if you lift the transom, you automatically push down the bow. A normal boat will still not nose dive if you do this gently though.
But now add the effect of waves to this all. Imagine you sail on a reach or downwind and you approach the gybe mark. Big pressure in the sail pushing the boat forward, tremendous force on the hull that is dragged through the water, the boat heels over to leeward, you hike full, but it doesnt help much, you are fighting to keep it under control, you steer to compensate, the bow is pushed down further, the boat surfs down a wave going really fast.
The mark is coming towards you at great speed! It’s frightening! You have to gybe now!
But right at that point in time, you have fully surfed down your wave and the bow hits the next one. But still you steer away, because if you dont some one else will sneak between you and the buoy. You compensate the weatherhelm and try to bare away to make the gybe.
BANG! In a blink of a second, the bow pierces the wave, buries itself in it, the boat suddenly stops, the sail pushes further and there is nothing you can do anymore except swim.
Look at the diving cat in the picture. Do you see their course with respect to the wind, the wave they hit and the sail pushing further?
That’s what makes a Fizz Cup contestant pitch pole. If only we could show the waves a little better, improve the terrifying sounds of sailing at high speed, feel the pressure on the rudder and experience the fear of loosing the control over it.
thanks for your colourful explanations Moth . I very appreaciated that you builded in this nose dive feature
On the other hand, I have the feeling, that sometimes there is something going wrong with the calculation of the forces in your script; so I nose dived several times in moderate wind going to windward. You hardly can reproduce that in any RL boat.
Yes, guilty
In order to squeeze the script size of the sailing engine below 64K I made some huge simplifications. One of which is this line of code:
if(absRelWindDir>60 && (actualHeelXWhenSteering*steeringDir)<0) blah blah.
In normal terms it means: If you sail on a course more than 60 degrees to the wind and if you steer in the opposit direction of your heel, the nose dive script will add some points and after a few cycles will actually make your bow go down.
This means that is doesn’t matter if you sail 61 or 180 degrees to the wind, which is silly.
And it doesnt matter if you heel to leeward and bare away or heel to windward and steer up, which is also silly. I’ll change that in the next minor update after the Cup to make it a bit more realistic ok?
I have my hopes set on the annoucement that someday the total script size will count and not the size of the individual scripts. If that happens I can get rid of laggy interscript communications and use the experience with Fizz 3 to do a proper job with the nose dives and many other things.
Love it! Stuart, Moth and I were discussing the effect of the Center of Effort (CoE) of the sail and the Center of Lateral Resistance (CLR) on the nose-dive tendency of a foiling Moth. At about 1:00 into this video, you can see the centerboard coming almost completely out of the water — loosing both lift and lateral resistance. In your view, what is the main factor in producing the nose-dive?
http://bit.ly/2008MothWorldsNoseDive
Easy… the answer is somewhere on this page:
http://culnane.navidat.com/dc/moth/frame.jsp?nocache=981142472&url=/dc/sailing/moth/wshpaper/longitud.htm?nocache=981142472
… don’t ask me where exactly
In the 60th mothies added “flares” to their hulls: whilst keeping a narrow underwater hull you added some bouyancy a feet above water. That also added weight – what wasn’t appreciated. So some guys added horizontal foils to the bow – next to the waterline, wich were working like plane foils. Unfortunately they increased the resitance, and sometimes worked vice versa…
So we actually sailed since around 1990 with T-Foil-rudders – means, a foil with around 500 to 600m lenght was added to the rudder tip. The foil worked with downforce and it helped to avoid nosediving in certain situations. In light air it was painfull – becuase of the added resistance – some guys used it simply upside down
On Doug’s website you can see the more static relationships between CoE and CLR – sort of easy calculations. But the problem are the dynamic forces, which can be much stronger and overrule the statics.
With some experiences you could avoid nosediving in the dangerous situation – gybes for example. Or you simply turn hard to windward – and capsize comfortably to leeward
With the Foiler moth nosediving has become less violate – although the geometry looks very odd. The two foils produce a more stable static situation, because they produce more “definited” forces that a hull, that can occure odd forces when heeled.
There is just one solution that helpes in any cases: sail downwind as fast as possible and your rigg forces stay small
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