Weather helm

[Touchwood]

I have long been an admirer of John Vigor, and I'm sure I'm not the only forumite who regularly reads his blog. However, I was a little surprised today to come across the following: -

"NEWCOMERS TO SAILING are often surprised (and perhaps a little dismayed) when they discover that their boats suffer from weather helm. In their innocence, they never expected to have to pull their tillers up under their chins every time their boats heeled over in a puff of wind. Why, they ask, should a boat try to head into the wind just because the hull tilts one way or another? Well, it’s not an easy question to answer, but here are some of the factors that cause weather helm on the average deep-keeled hull:

Ø Excessive beam, especially beam carried far aft
Ø A mast raked too far aft, or positioned too far aft
Ø The hull assuming an asymmetrical shape it heels Ø Sails that are cut too full, or have stretched over the years
Ø The fact that when a boat heels, the center of effort of the sails moves farther out to leeward, away from the centerline of the boat, and thus has more leverage to shove the boat into the wind. That’s why it’s usually advantageous to sail the boat as upright as possible."

I am a little puzzled by the last of the above factors. Far be it from me to question Mr. Vigor's undoubted knowledge and experience, but I just don't see how moving the centre of effort of the sails to leeward gives it "more leverage to shove the boat into the wind" The leverage that does this is surely that along a fore and aft axis - that's why raking the mast too far aft does have this effect. I would have posted a comment on John's site, but every time I try to do so the internet eats it. Am I missing something?

 

[yachtorion]

I have long been an admirer of John Vigor, and I'm sure I'm not the only forumite who regularly reads his blog. However, I was a little surprised today to come across the following: -

"NEWCOMERS TO SAILING are often surprised (and perhaps a little dismayed) when they discover that their boats suffer from weather helm. In their innocence, they never expected to have to pull their tillers up under their chins every time their boats heeled over in a puff of wind. Why, they ask, should a boat try to head into the wind just because the hull tilts one way or another? Well, it’s not an easy question to answer, but here are some of the factors that cause weather helm on the average deep-keeled hull:

Ø Excessive beam, especially beam carried far aft
Ø A mast raked too far aft, or positioned too far aft
Ø The hull assuming an asymmetrical shape it heels Ø Sails that are cut too full, or have stretched over the years
Ø The fact that when a boat heels, the center of effort of the sails moves farther out to leeward, away from the centerline of the boat, and thus has more leverage to shove the boat into the wind. That’s why it’s usually advantageous to sail the boat as upright as possible."

I am a little puzzled by the last of the above factors. Far be it from me to question Mr. Vigor's undoubted knowledge and experience, but I just don't see how moving the centre of effort of the sails to leeward gives it "more leverage to shove the boat into the wind" The leverage that does this is surely that along a fore and aft axis - that's why raking the mast too far aft does have this effect. I would have posted a comment on John's site, but every time I try to do so the internet eats it. Am I missing something?

Unless I'm missing something... Yes you are... the effect is much the same whichever way the mast leans - if it moves further away from the keel it will have more leverage to swivel the boat around it.

 

[mjcoon]

Unless I'm missing something... Yes you are... the effect is much the same whichever way the mast leans - if it moves further away from the keel it will have more leverage to swivel the boat around it.

That, combined with:

The leverage that does this is surely that along a fore and aft axis - that's why raking the mast too far aft does have this effect.

is rather confusing.

The crucial effect, it seems to me, is that the sail is generating a forward force to propel the boat (as well as a lateral one that we would like to avoid). If this forward force is applied near the centre-line, by the mast being vertical, then there is no turning force causing the boat to bear away or round up.

If the mast tilts to leeward then the forward driving force will be applied through a point away from the centre-line which will therefore generate a turning force causing the boat to round up. Compensating for this requires lee helm.

"Leverage along an axis" is a strange phrase since an axis is for turning around. But lateral force that is applied aft of the centre of lateral resistance (of the keel and hull) will also generate a turning moment causing rounding up. Same symptom but a different cause.

I hope I am clarifying without contradicting...

Mike.

 

[Seajet]

Unless I'm missing something... Yes you are... the effect is much the same whichever way the mast leans - if it moves further away from the keel it will have more leverage to swivel the boat around it.

Maybe, but I'd suggest the effect whereby the rig produces more weather helm when heeled is also that the top of the main is a lot larger in proportion to the top of the foresail, compared to presenting the full sailplan to the wind.

It's a reasonably complex set of factors, but I'd suggest weather helm of the 'tiller under the chin' type described is largely a function of hull shape, and in the extremes described, rudder stall.

 

[sailor211]

Sketch it out as a plan view.
Centre of force, Centre of resistance , centre of effort
The more the heel the greater the turning force rather than pushing force.

 

[yachtorion]

After reading the other replies I think perhaps I see where you are coming from more than I did.

Yes - fore and aft misalignment will generate a torque, but unless the rig is perfectly balanced there will always some level of torque, the boat healing and the mast being further outboard will increase the leverage and amplify that force.

Or perhaps I'm just more confused than I was to start with

Edit - yes I was more confused. mjcoon clarified the crucial thing.

 

[Touchwood]

Thanks for the replies - mjcoon has clarified it for me, I was thinking only of the turning moment generated by the lateral force, there is of course the driving force to consider, and moving this to leeward does generate a longer lever and therefore a greater turning force.