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Old 09-08-2010, 11:06   #16
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Originally Posted by barnakiel View Post
Negative. Consider where the forces are attached and their vectors. Does not seem like easing the tack and the clew will lift the bow.
I wouldn't be able to measure the force vectors, but as the tack is raised the center of pressure in the chute rises and rotates to lifting as the top third of the chute bellies upwards. Lifting the bow.
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Old 09-08-2010, 17:24   #17
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I wouldn't be able to measure the force vectors, but as the tack is raised the center of pressure in the chute rises and rotates to lifting as the top third of the chute bellies upwards. Lifting the bow.
If the kite pulls up it does not equal the moments are pulling the bow up. The sail attached by the sheets, halyard and the spinnaker pole, build yourself a paper model then pull the strings with the boat moored bow in the water. Then you will see what is pulled and where.

I bet any sail pulling by the halyard or any sheets attached aft will have its bow PUSHED deeper into the water. If she lifts it is only because she moves faster with the kite and the hull creates more lift fore.

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Old 09-08-2010, 17:25   #18
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Sure talking spinnaker not a jibe.

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Old 09-08-2010, 17:51   #19
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If the kite pulls up it does not equal the moments are pulling the bow up. The sail attached by the sheets, halyard and the spinnaker pole, build yourself a paper model then pull the strings with the boat moored bow in the water. Then you will see what is pulled and where.

b.
Don't need a paper model. I've some boats already. Okay, here's how it is: The sails and bow wave and hull-form conspire to force the bow down. Anything one can do to lessen the downward force is thought to be a good thing. Flying the spinnaker high can reduce the downward force on the bow.

Here's a non-paper model for ya. Next time you're flying a chute in a good breeze tie a line between the tack and clew of the chute. Tie the middle of it to your harness. Have the crew release the tack and sheet. Report back to us which way you went. Hint: You'll stay dry. We call that direction 'up'.
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Old 09-08-2010, 19:09   #20
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Another way to look at it is that sails work because there is less air pressure on the leeward side, which causes lift perpendicular to the surface of the sail.

If the surface of the sail is pointed towards the water (as is the case with a jib and a main on a beat) there is a significant downward force that has to be reacted by the hull and foils.

If the surface of the sail points up (as is the case with a well eased sym spin on a deep run) then the resultant force is up.

The only downward force on the bow on a deep run with a sym spin is caused by the main - It's not creating lift, but acting as a 'barn door' - which causes an overturning moment, which pushes the bow down
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Old 09-08-2010, 19:45   #21
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Don't need a paper model. I've some boats already. Okay, here's how it is: The sails and bow wave and hull-form conspire to force the bow down. Anything one can do to lessen the downward force is thought to be a good thing. Flying the spinnaker high can reduce the downward force on the bow.

Here's a non-paper model for ya. Next time you're flying a chute in a good breeze tie a line between the tack and clew of the chute. Tie the middle of it to your harness. Have the crew release the tack and sheet. Report back to us which way you went. Hint: You'll stay dry. We call that direction 'up'.
But the bow of the boat is NOT harnessed between the tack and the clew. The upward force of the spinnaker can only act through the points it is attached to the boat (top of the mast and where the sheet and guy are fixed to the boat). Pulling up and forward on the top of the mast may still push the bow down (admittedly less than just pulling forward alone)? And if the sheet and guy are fixed rear of the mast, will pulling up on those points push the bow up or down?

If the (assy) spinnaker is tacked to the bowsprit then sure, it will pull the bow up, of course...
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Old 09-08-2010, 20:37   #22
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The sheet and afterguy rise at an angle of about 30 degrees. The law of sines says the upward force is therefore about half the total force. That half is pulling up on the tack, or somewhere forward. That pulls up on the bow. If there's no foreguy it pulls up on the afterguy or head.

I used to sail an 11:Meter. Exceedingly light bow. She would try to launch skyward. The chute and main both raked back pointing at the sky. She'd leap over the bow wave.
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Old 12-08-2010, 18:59   #23
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But the bow of the boat is NOT harnessed between the tack and the clew. The upward force of the spinnaker can only act through the points it is attached to the boat (top of the mast and where the sheet and guy are fixed to the boat). Pulling up and forward on the top of the mast may still push the bow down (admittedly less than just pulling forward alone)? And if the sheet and guy are fixed rear of the mast, will pulling up on those points push the bow up or down?
I am completely with paradix here.

Pulling the boat by the top of the mast or by any point aft (and esp so if the sheets point up) will push the bow into the water.

Sure, if we attach the sheets fore enough, the situation will change.

But then, if we do lift the bow, how much driving force from the kite do we lose? Maybe it is better to use the force on the sail to go a bit faster so that there is more hydrodynamic lift. Water is thicker than airs - we go a bit faster and we get much more lift ...

Ideas?

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Old 12-08-2010, 23:56   #24
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Spinnaker lifts the bow?.... a tricky question for sure.... and I don't profess to having enough engineering prowess to calculate it all and give a definate yeah or nay, but here's my 2c.

There sure are a lot of forces running around a symetrical spinnaker rigged on pole - Forces are transferred to the boat via, the pole downhaul, the backstay, the lead block for the guy, the pole, the mast, the winches and also the tweakers.

Now, I just can't see any of these exerting a downward force on the bow - because nothing attached to the bow is in compression. What I can see, is that some of these force attachment points will cause the stern to lift (the sheet/ spin block and the head/backstay perhaps) and that this lifting of the stern causes rotation.

However, I don't believe that these forces will exceed those which are lifting either the centre of the boat or the bow - so the overall effect is not to lift the stern, but lift the bow. I say this not because I've calculated and resolved the forces, but because I've raced on many boats and have observed that usually the stern squats under spinnaker and usually the bow lifts (on lighter boats, it comes clear of the water).

Now what we are missing is the effect of the main. The main exerts a lifting force on the stern (via the backstay mostly). In lighter winds this force is not significant, as the boat will be moving only a few knots slower than the apparent wind (depending on the boat of course). However, as windspeed increases above hull speed, this force has an exponentially increasing effect on pushing the bow down (by lifting the stern).

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But then, if we do lift the bow, how much driving force from the kite do we lose? Maybe it is better to use the force on the sail to go a bit faster so that there is more hydrodynamic lift. Water is thicker than airs - we go a bit faster and we get much more lift ...

Ideas?

b.
Yes - we definately loose by lifting the bow - and so we should alter the trim of the boat accordingly - by moving crew weight. Look at the water comming out from the transom as a guide.

The only time we want the bow out of the water is when we are over powered and we don't want to trip the boat.
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Old 13-08-2010, 16:24   #25
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The only time we want the bow out of the water is when we are over powered and we don't want to trip the boat.
But this is much faster achieved by moving the crew aft than by adjusting the kite!

Unless there is no-one left to run aft ...

Also, if the boat squats under the kite it is (IMHO) because of the speed, not because the kite pulls the bow up.

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