Leeward Heel and Weather Helm

P

phosphor

Member
Hey.

I just have a quick question from an engineering standpoint. I understand from experience and some reading I've done that leeward heel upwind creates weather helm, while windward heel creates lee helm. I can't seem to get my mind wrapped around exactly why.

In order for the weather helm to be created, I understand that the center of effort of the sail or underwater area must shift, but I don't understand how they shift with the heel angle. It seems more likely to me that the center of effort of the sail is the one that's shifting but that's just a premonition. Can anyone set me straight on what exactly is happening in the physics of this?

I did search the forums a bit before posting this but didn't see a technical explanation.

Thanks! :)
 
I'm no engineer but I think it's more to do with the shape of the hull than with the center of effort of the sail. When the boat is heeled to leeward the shape of the hull makes the boat turn to windward and causes the weather helm. And vice versa,
 
I didn't think of that one. Thanks very much. I'll look into that a bit more. If I do end up finding a definitive reason I'll post it here.
 
The way I picure it is that if the boat is standing straight up in the water, then the under water part will be symetrical. If the boat heels to leeward, there is more of a straight line along the windward side and a rounded line on the leeward side making it asymetrical and wants to turn. Thus, a shorter, chubbier boat will have more weather helm with heel than a long slender one. There is something going on with the sails too, but Im not sure exactly what it is.
 
I consulted one of my professors today, I'm currently a student at Webb Institute in NY, and he pretty much told me that exact thing. :p The thing with the sails is that upwind, even though the turning moment is much less pronounced close hauled than when on a downwind run, there is still a turning moment caused. The sails and the waterplane shape both want the boat to head up.
 
I see that you've already answered this question, but thought I'd pass along my experiences with this issue. As a sailing instructor, I have to answer this question all the time. If you look in a sailing manual from about twenty to fifty years ago, you will see an explanation which addresses hull shape-- the thinking was that heeling the boat to leeward presents an asymmetrical hull profile, deflecting the bow to weather. This makes some intuitive sense, especially to skiers who have learned the snowplow. But in fact the most important factor when the boat is heeled is the position of the daggerboard relative to the position of the sail, i.e. the center of lateral resistance relative to the center of effort. An asymmetry or imbalance in these forces produces the turning moment which we describe as weather helm or lee helm. I ask my students to think of it this way. The sail creates lift, which, for the purposes of small-boat sailing, is forward drive. (In fact, of course, forward propulsion is created by the difference between two lifting surfaces, the daggerboard and the sail.) It is, I say, the engine of your boat. It provides thrust. If you put that engine to leeward of the hull, it will want to spin the boat to weather, causing it to head up. If you put the engine on the windward side of the boat, it will spin the boat to leeward, causing it to fall off. If you put the engine in the center of the boat, it will produce a (nearly) neutral helm.

So the principle while beating to weather is similar to the principle at work while on a dead run, with the exception, I'm told, that the turning moment on the run isn't the difference between the lift provided by the sail and the lift provided by the daggerboard, but the drag provided by the sail and the drag provided by the hull.
 
Great explanation dog. I stand corrected. It's amazing how many myths and misconceptions we all have about how a sailboat works from the old "the wind going round both sides of the sail has to arrive at the trailing edge at the same time" to the famous (but non-existent lee-bow) effect.
 
yell0wd0g said:
I see that you've already answered this question, but thought I'd pass along my experiences with this issue. As a sailing instructor, I have to answer this question all the time. If you look in a sailing manual from about twenty to fifty years ago, you will see an explanation which addresses hull shape-- the thinking was that heeling the boat to leeward presents an asymmetrical hull profile, deflecting the bow to weather. This makes some intuitive sense, especially to skiers who have learned the snowplow. But in fact the most important factor when the boat is heeled is the position of the daggerboard relative to the position of the sail, i.e. the center of lateral resistance relative to the center of effort. An asymmetry or imbalance in these forces produces the turning moment which we describe as weather helm or lee helm. I ask my students to think of it this way. The sail creates lift, which, for the purposes of small-boat sailing, is forward drive. (In fact, of course, forward propulsion is created by the difference between two lifting surfaces, the daggerboard and the sail.) It is, I say, the engine of your boat. It provides thrust. If you put that engine to leeward of the hull, it will want to spin the boat to weather, causing it to head up. If you put the engine on the windward side of the boat, it will spin the boat to leeward, causing it to fall off. If you put the engine in the center of the boat, it will produce a (nearly) neutral helm.

So the principle while beating to weather is similar to the principle at work while on a dead run, with the exception, I'm told, that the turning moment on the run isn't the difference between the lift provided by the sail and the lift provided by the daggerboard, but the drag provided by the sail and the drag provided by the hull.
Click to expand...

word...its all about the CofE. I did a few tests last year where I towed a Laser and then cut the tow rope, I heeled the boat to one side to see if it would cause the turn, there was a small amount of turning, but not nearly enough to cause weather helm that occurs when heeling. So, this proves yet again that its all about sail trim. d0g have you watched Higher and Faster?

c'mon, what are they teaching you over at Webb? Consider yourself lucky, I'm hoping I can get into Michigan, if not Webb for school.

TC
 
There really are a lot of different ways to think of it. I'm thinkin' of playin' with a model in the tow tank a bit to mess around with it too. Nothing official, just heeling a model and giving it a bit of a forward motion to see what it does. Both CofE and the waterplane theories seem to make sense (I'm not saying that to doubt anyone at all. I love hearing others' view points because I don't want to be victim to tunnel vision). If you guys are interested I'll tell ya what goes on.

As far as what they're teaching us at Webb, it's mostly commercial ships. :p We barely get any small-boat design. (Small in this case meaning under 100 feet, and not much performance dinghies.) There are just two professors that are insane sailing buffs that do a lot of big boat racing. Talk to ya lata.
 
I just read both your posts-- yours and Tim Clark's. I often wondered what percentage of the weather-helm felt from heeling the boat was due to the center of effort, but I couldn't think of an easy way to test it. It never occured to me to try the question backwards-- what percentage is due to the hull shape-- by towing a hull. It sounds like an experiment in the old-school empirical tradition-- Benjamin Franklin with his kite. Are there any other forces in play?

I'd be interested to hear what you come up with in the tow tank.

Sean
167260

phosphor said:
There really are a lot of different ways to think of it. I'm thinkin' of playin' with a model in the tow tank a bit to mess around with it too. Nothing official, just heeling a model and giving it a bit of a forward motion to see what it does. Both CofE and the waterplane theories seem to make sense (I'm not saying that to doubt anyone at all. I love hearing others' view points because I don't want to be victim to tunnel vision). If you guys are interested I'll tell ya what goes on.
Click to expand...
 
Unfortunately things piled up fast and doing that test fell to the way-side. I had to do it only when the actual professor who runs the tank isn't around but he just sails and works so he's there a lot. So I'll see what I can do when I get back to the school in August. I'm back home now. Sorry for the disappointment.
 
From a long-time pro sailing instructor (since 1962):
Imagine a toy boat in the bathtub with a siick for a mast. Push on the top of the mast and the boat goes forward. Heel the toy boat and push on the top of the mast and the boat pivots. A "keel" added makes the effect more pronounced. The heel causes the helm. That's why a boat with the mainsail flogging and a spinnaker way up forward of the boat making the boat heel will produce an uncontrollable windward broach, despite all that force in the front of the boat seemingly trying to turn it down.. It is the heel.
As for the assymetrical hull when heeled effect, it is minimal, as shown by Peter Barrett in an old One Design Yachtsman' magazine. Think Hobie Cat. On one hull, which is build assymetrical, the boat has some lift, but does not turn because of the shape of the hull.
181281 GM
 
All I really think about is how I want the boat flat all the time. I know that if I have weather helm i have to stear against it to correct for the problem. The steering motion acts as a break and pushes the bow down into the water causing me to go slow. Sometimes it is just better to keep it simple and go with the flow...
 
I know this is bringing back something from a long time ago, but I finally did the test I wanted to. I went into our tow tank, and picked out the closest hull to a Laser that I could find. It was closer to a J24 shape with a rounder bilge and a fin keel, but I felt it would do the trick. So by putting some brass weights on one side of the hull I made it heel a good 10-15 degrees to port. I just gave it a light shove in a direction along its centerline as best as I could and it wanted to turn to starboard all on its own. I tried it a few more times just to make sure I wasn't pushing on it wrong and giving it some amount of spin and the result was the same. Even if I induce a minor amount of spin on the hull it wants to head up. Whether the aerodynamic forces or the hydrodynamic forces are greater when it comes to the boat wanting to round up I have no clue nor know how to measure at this time, but I thought I'd share my findings.
 
One aspect to the heeling boat without sail might be friction on moving surfaces. As an approximation, think of the hull section being circular i.e. the wetted surface area changes little when the boat heels within reasonable limits (e.g. gunwale out of water). The Centre board (or keel on a J24) also has a significant surface area. As drag is probably the same 'ish per sq inch of surface, heel the boat to port and the centre board moves out to starboard. Thus, whilst the hull drag stays close to the same, the centre board drag has been moved out to starboard, tending to cause the boat to turn to starboard (i.e. "away" from the direction of heel).

Such an effect would have nothing to do with hull shape but only due to moving a significant surface are to one side of the boat (a bit like putting your foot in the water - put it in on the starboard side and the boat will want to turn to starboard).

I'm not saying that hull shape has no effect, just that another effect on the "no sails" case might be (in part) friction.

Note that I am no expert and did little physics after leaving school many years ago.

Ian
 
Ow! Now my brain is starting to hurt. Thanks for doing the tank testing. Also, good thought on the "keel" offset perhaps being the turning effect. Seems the more I think about it the slower I go.
 
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