Comments Thread for Giu's Sailing Videos

[Rockter]

Good video there Guil, and well explained.

I would question ony the comments at about 8.45 concerning the traveller angle influencing heel. If the sail trim is the same in both cases, that traveller force vector... the blue line.... does not influence the boat heel. That force vector is within the free body of the boat, and it's an action-reaction pair within the structure. Although the tension of the blue line changes with the angle of the blue line, for the same sail trim, the boat toppling moment does not change, and so the heel angle will not change. The toppling moment comes from the sail shape, and the angle of heel, and not from how the forces from the toppling moment are distributed within the structure.

It would be great to have a video on how to sail downwind without a rudder. That one makes me nervous.

I never move the traveller on my olde long keeler. It does not run freely anyway. I use a big kicking strap at mid boom to influence the sail shape. It make a big difference.

I particularly liked your sail trim diagrams and the use of the tell tales.

Rockter.

[tdw]

saudação

(hope that's correct)

[Giulietta]

Originally Posted by Rockter

Good video there Guil, and well explained.

I would question ony the comments at about 8.45 concerning the traveller angle influencing heel. If the sail trim is the same in both cases, that traveller force vector... the blue line.... does not influence the boat heel. That force vector is within the free body of the boat, and it's an action-reaction pair within the structure. Although the tension of the blue line changes with the angle of the blue line, for the same sail trim, the boat toppling moment does not change, and so the heel angle will not change. The toppling moment comes from the sail shape, and the angle of heel, and not from how the forces from the toppling moment are distributed within the structure.

It would be great to have a video on how to sail downwind without a rudder. That one makes me nervous.

I never move the traveller on my olde long keeler. It does not run freely anyway. I use a big kicking strap at mid boom to influence the sail shape. It make a big difference.

I particularly liked your sail trim diagrams and the use of the tell tales.

Rockter.

Rock, yes it does, sorry to contradict you but you're wrong. Let me show you why.

When the boat is heeling and the traveler is above the centerline of the boat, it's pulling the boat on the higher side. This is why it afects.

When the boat heels, the center of gravity is in the center, BUT, the center of floatability, and the center of effort are both under the centerline of the boat. If the traveler is above the centeline, the momentum is greater, because the arm pullling the boat, from the cneter of effort, in relation to the center of floatability is gretaer.

The boat, because it floats, rotates around the axis of the center of floatability, that is the pivot point..the traveller bellow it has a shorter arm than above, and so is the leverage effect or momentum.

I don't know if my English was good to explaind, so I made a drawing

See bellow please.

In black arrow is the center of gravity, the green arrow is center os effort, and red arrow is center of floatability.

[Rockter]

Not so Guil.

You could weld the boom goosneck in that same position, so locking the boom in that position, and the heel would not change if you took the yellow chord away completely. That yellow chord you show there is an internal force WITHIN the structure of the boat. To change the heel of the boat with the yellow chord, it must act outwith the boat. It must interact with some other body (or force) outwith the structure of the boat.

It would not matter what angle the yellow chord is at, or even if it were absent, as long as the boom angle is frozen to maintain trim.

It's the green force that is acting and heeling the boat. The yellow chord is part of the structure of the boat that gives the system reaction to that green force action. It does not matter if you lock the boom with the yellow chord, or weld the boom solid, or put a (weightless) post sticking up on the port side and tie the yellow chord horizontal to it... the heel angle will not change.

Best wishes.... good discussion.

[Giulietta]

I am not going to argue...if you weld the boom at gooseneck, you effectively attaching the effort to the boat's center line, and can't therefore compare with the pull being exerted above the center of flotation or bellow it.

This is something I learnt almost 26 years ago, when I started sailing boats with travelers..

[sailingdog]

Rockter—

Gui is correct. The sail is not rigid, and as such the outer edge of the sail and boom move as the wind pushes on them. If the outer edge of the boom is connected to the high side of the boat, it will have more leverage to pull the boat over with... than if it is connected to the low side of the boat. If the boom and mainsheet are on the leeward side, as the boom tries to rise, it is actually going to be pull up on the leeward side, helping counteract the heeling force caused by the wind a tiny bit.

You're making the mistake of treating the sail, mast and boom as a single rigid unit—which it clearly isn't—and because it isn't it won't act as one.

If you look at the forces exerted on the boom, they'll be roughly in line with the mainsheet...or the mainsheet would be pulled in a different direction.... so the cyan lines represent the force on the boom of the boat...and therefore the boat...



As you can see from my illustration, the forces on the right-hand boat are clearly less likely to cause it to heel because of the mainsheet's location relative to the boom. While the mainsheet on the right-hand boat does contribute a bit to the heeling force, it also contributes a greater segment to the righting force on the boat, as it has a greater vertical component than horizontal component and is to the leeward of the center of buoyancy... so will tend to right the boat more than heel it—that is simple physics...

The force on the left boat has a much greater tendency to heel the boat, as it has greater horizontal component above the center of buoyancy...and a much vertical component pulling upward on the hull to windward of the center of buoyancy... which means that the total of its components are working to heel the boat...

It ain't rockter science...

[Valiente]

I think SD has grasped the problem clearly.

I am only guessing, though, that Rockter's term "toppling moment" has something to do with the more familiar (to me) term "angle of vanishing stability" or AVS.

I believe what is being discussed is a three-dimensional lifting surface that has the ability to change its angle of attack at one end (the traveller) AND to impart twist to the lifting surface (the mainsheet in conjunction with the traveller) AND to tension that surface to move draft, and thus centre of effort (the mainsheet tension).

This is therefore a dynamic interaction that can indeed steer the boat if the centres of effort, buoyancy and gravity are known in relationship to each other, and if the sea state allows it.

This understanding also allows the sailor to know why his boat will self-steer on certain points of sail and not others.

[leliu5th]

Awsome and very informative video's GIU! I learned more sailing just watching those videos! Looking forward to more! Oh BTW how are the Elk!
LOL!!

[Rockter]

Look there guys, these are free body diagrams here. This is what I do for a living. Sailingdog, the blue vector you depict pointing skywards is real, but where does the reaction to it act? At the other end of it.
There is an equal and opposite reaction to it at the lower mount of the yellow chord. The net result of an equal and opposite reaction pair within the structure?... abosolutely nothing! THERE IS NO NET FORCE APPLIED ON THAT BOAT BY THE YELLOW TRAVELLER CHORD...ZERO. The yellow traveller chord is a tensioned action:reaction pair within the structure. The chord is not subject to an externallly applied force. The boat is, and it heels until the counter moments will equal it. It heels to gain moment balance, but what is heeling the boat is not the traveller chord, it is the wind.

I emphasised again and again that the sail trim is constant.

When the wind hits the sail, it does not give one damn how the boom is attached to the boat, and IF THE SAIL TRIM IS KEPT THE SAME, you will NEVER change the heel of the boat by altering the traveller angle.

So, for the same sail trim... weld the boom, lock the boom, use a traveller chord at any angle, or nail the boom to a wooden frame OF ANY GEOMETRY with 4x4 timber, and for the sail trim constant, the boat heel will be constant, no matter how long you have sailed, or will sail.


Rockter.

[sailingdog]

Rockter— Isn't moving the traveller, by definition, changing the trim of the sail. It is one of the sail controls.