Lifting Spinnakers:does it lift the bow?

There is no force forward of CG depressing bow in my illustration.

Bow rises until lift of spin + lift of bouyancy = weight of wee boat.

 

Your illustration is depressing. It is wrong and incomplete. Find a moment to think about.

The lift of buoyancy must = weight of wee boat, else wee boat would sink.

 

If you lift the bow with a crane by 50kg the CB will be CG-50kg.
Use Newtons to make it fancy.

(Re illustrations: I am using a Japanese language computer and struggle along just using MS Paint)無化し　欄簿

 

Still haven't found that moment to think eh?

The CG is a location. Lifting the wee boat with a crane won't make the CG move relative to the boat. You can move mass to alter the CG's location. The CG does not have mass, so you cannot remove 50kg from it. You can figure it out in a matter of moments. You just have to find the moment to figure it out.

 

Together, spinnaker lift and bouyancy resolve against gravity. CG doesnt move in my illustration or explanation.
Losing lift from the spin would 'drop' the bow, until CB aligns vertically with CG.
The moment youve been waiting for starts when spin lift is lost and ends when CB and CG realign vertically.

 

If the line you have drawn between the CB and the CE of the sail is the total force vector, You can see that both the upward component (aft of the CG) and the Drive component (above the CG) act to pitch the bow down.

The is no uplifting bow force in the diagram, it is depressing.


You didn't happen to actually read the thread did you? :?:

 

I did not draw the line connecting CE and CB.

It is not the force vector.

But you can resolve my red lines from CE and CB on it if you like.

I read the thread with relish.

 

The small amount of spin lift is some meters ahead of the CG.
A long lever.

Opposing that is the greater amount of bouyancy lift only a little aft of the CG.
A short lever.

When spin lift is lost bouyancy moves forward as the bow comes down.

Remember its a planing skiff sort of boat thingy some one else drew, but it is good enough to use for this discussion.

 

You forget that the drive is acting on a lever that is nearly as long as the "lift" lever. Even if the drive lever was half the length of the "lift" lever, the Drive is much more than twice the lift. The large amount of drive is some large distance above the cg, it is a large force on a large lever acting to pitch the bow DOWN. It is countered to some small extent by the bow up pitch moment you describe. The net is bow down.

Granted, the spin has some lift so it drives the bow down less than other sail combinations, but it does not cause bow lift or pitch up.

The only way you can "prove" that the spinnaker lifts the bow is by ignoring the drive moment, as you have done.

 

I thought I'd managed to show that, with the right sail plan geometry, it IS possible for the bow to lift? If anyone still disagrees with my demonstration, can they explain why in a clear manner, rather than just "you've made a mistake". I fully agree, that for the majority of boats, the pitch moment depresses the bow, but this is not an 'always and forever' situation. There are designs where a bow up moment can exist - a long pole, high speed (hence planingon aft sections), perhaps a slightly bow up attitude etc, will all help to achieve this.
The Cherub analysis (referenced earlier) showed the lift force acting at 41 degrees. At this angle, the seperation of the CE of the kite longitudinally from the LCF only needs to be fractionally bigger than the vertical height of the CE of the kite above the waterline. In my opinion (and I haven't done any analysis to show this), a 12' Skiff is one class where a bow up moment is quite likely, especially if the bow is already slightly lifted due to other forces/moments (hence altering the angle of the kite's drive force).

 

Lift is acting about the CG. When lift forward and aft equal weight at CG all is well. Vertical forces in equilibrium.

Thrust and drag are in equilibrium, when the wee thing is planing along.

There is no moment.

 

PI, it don't matter how logical your analysis is. The "you are wrong" approach will stick if one keeps yelling it loud enough. The world is flat and you are practicing witchcraft. We should burn you at the stake.

 

Ah, I see! The 'world is flat' idea always struck me as rather daft - wouldn't the sea fall over the edge?

P.S. What time is it in Maryland - shouldn't you be asleep???

 

Sleep? 4 hours a night on this side of the pond- I've been at my desk since 6am.

 

The drawing that rayk is using shows a bow down pitch moment.

Since you have not done any analysis to show it, why not do some and share your thoughts? I think you will find that in the x - y plane the wind is flowing on the x axis, the force from the sail will be on the z axis (heel) and the x axis (drive). I would like to see where the y axis component comes from when the mast is upright. Every time I place a force vector at the CE of a spinnaker I can't prove a positive y axis force.

The one certainty is the drive acting on the x axis is above the CG. This creates a bow down moment. To lift the bow there must exist a force or moment that is greater than the bow down pitch moment. I've tried to find it off and on since the 1995 discussion and have never been able to prove it.