The Boat Whisperer Upwind DVD QUESTION on windward helm physics

T

tkjazzer

Member
Hello,

In The Boat Whisperer Upwind DVD
Location: Title 2, Chapter 4 @ 7:29

he is talking about weather helm, tipping the boat forward, moving the wet-to-surface area forward...

(first time I've heard anyone call it "wet-to-surface area" ??? - but I got the idea - british nomenclature?)

He makes a statement which I do not understand:

"as the wet-to-(?wetter?)-surface area goes forward... the sail plan is effectly 'FURTHER BEHIND' the wet-to-surface area... generating windward helm... which means you have to move the rudder harder over to keep you in a straight line"

I get what he is saying -- it's a viscious cycle.

But I would like to understand the physics so I can teach the principle in future lessons.

When he says the sail plan is "behind" to wet-to-surface area. Doesn't he mean further forward.

If you are tipping the boat forward, the mast would then go further forward (not backward).

So is he using the word "behind" to talk about the direction of the vector of the tipping motion and not a position in space?

I don't windsurf much and need to read more about adjusting masts in relationship to sailing...
I actually thought pushing the mast forward causes the boat to bear off.

If that logic is correct, then maybe he is talking about the sail plan going behind as in aft which would cause the boat to head up....

I really need to attend one of those North U lectures on trim - or bust open the book and start reading it.

In googling on this topic, I just skimmed:
http://sailing.about.com/u/sty/adva...Using-CE-and-CLR-to-Sail-without-a-Rudder.htm

It reminded me leeward heel - obviously makes the boat head up -- when he says "sail plan behind" is just referring to it causing more leeward heel?

Maybe I don't understand "sail plan" vs "mast rake" - which the link confirmed my thoughts of mast foward -- bear off.

Looking for some clarity.
 
What you said about a sailboard is correct - moving the mast toward the bow makes the board bear off and tipping it back towards the stern makes the board round up. This turning is accomplished because the center of effort of the wind on the sail changes relative to the center of lateral resistance of the hull/daggerboard/skeg combination. When the center of effort of the sail is moved forward (tipping the mast toward the bow) while the center of resistance of the hull in the water remains unchanged, the hull rotates to leeward. This effect is seen on any sailing craft and also on kayaks.
 
I haven't seen the DVD but this is what I think he means.

When you sit forward, the front of the boat goes lower in the water making more waterline length in front of the mast. However the rear lifts out of the water. Therefore the waterline has moved forwards in relation to the position of the mast.

As it does not take much of an angle to lift the rear of a laser out of the water, the sail will not move as far forwards as the waterline, therefore the waterline moves forwards in relation to the sail - or said the other way round, the sail moves behind the waterline (wet-to-surface area).
 
Can't comment on the physics, but (and I will need to check the video to be 100% sure), he is saying 'wetted' surface area, not wet-to-surface. In other words, the wet bit of the hull :)
 
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