| Vol 2 - Issue 8 | July/August 1998 |
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What's All This About Weather Helm? Exploring One of Life's Great Mysteries. By Kim Miller
Here is the answer to these questions and more. More than you ever wanted
to know about lee helm, weather helm, mast rake, rudder rake, tiller tug,
rudder moment, and ruddertrack, and how they control your enjoyment and
safety while sailing your Hobie Cat.
Go and get your bathtub boat and sit it on the table in front of you. You don't have a boat in the bathtub? Better use your computer mouse then, even something with wheels will do. Push the boat forward across the table with your right hand. The wind is in your left index finger. As you sail the boat across the table, push your “wind finger” against the bow of the boat. It will turn away from the wind. Now push the stern of the boat with the wind finger. The boat will turn into the wind. You have demonstrated the effect of the sideways wind balance of the boat. When your Hobie is on the water the wind will tend to push it one way or the other in exactly the same manner. “What about the rudder?” you say. Yes, the rudder does control steering, but this balance effect is measured when you let go of the tiller. When the boat is free to turn as the wind pushes it, then you see its natural balance. Your boat is set up to turn away from the wind, or set up to turn into the wind, or it might just continue on the same course. This balance effect is called “helm”. You have either lee helm, or weather helm, or neutral helm. Neutral helm is when you are sailing along, let go of the tiller and just keep going in exactly the same direction. This form of balance of your boat is controlled by the sail size,
sail shape and fore/aft placement, and the shape of the boat under the
waterline.
Imagine that you are in a heavy wind, you get into some trouble and fall overboard. If you have lee helm the boat's balance will take the boat away with the wind. You will be left in the water and the boat will be sailing away by itself. Lee helm can also make the boat's steering “twitchy” and unpredictable. In a gust the balance of the boat will force it slightly to leeward and tends to push the bow into the water. If you are already running well off the wind this is when you can have a sudden jibe or pitchpole. If you have weather helm and fall overboard the boat will turn up into the wind and stop and wait for you. This is the next best thing to having it circle around and pick you up. A sudden gust with weather helm will turn the boat slightly towards the wind and tend to lift the bow (safer than nose-diving) with little effect on steering. You can continue without worry of the boat becoming upset. Balancing Your Boat Balance is achieved by getting the right relationship between centre
of effort and centre of lateral resistance.
Imagine surfing along with the wind coming from your port beam. Move
the sail aft to increase weather helm and you turn into the wind to port.
Move the sail forward to increase lee helm and you turn to the lee, to
starboard.
Now Let's Get Back to Your Hobie 14/16 You have no centerboard around which to pivot, and you have a lot of control over mast rake. So what does this mean for normal sailing? The centre of lateral resistance will not be located in a single place as with a centreboard boat. The CLR moves back and forth with the loading of the boat. Push the stern into the water so the bow comes out of the water, and the CLR moves aft. Just sitting over the rear corner casting will move the CLR aft. As you move forward, so will the CLR. There is a natural aft limit to which you can push the CLR like this. Because the hulls have less depth at the rear than in the front portion, the CLR will not move all the way aft. The deeper profile of the hulls in the front portion keeps the CLR further forward than we might sometimes like for good helm balance. The centre of effort is controlled by sail configuration. If you have
a jib it will be forward. If you rake your mast it will move aft. Balancing
the CLR and CE is not difficult, as long as you know what you want to do
with the boat.
If you are a social sailor who does not need to get every ounce of speed out of the boat, you will not need extreme mast rake. If you race your Hobie, increasing mast rake will give you extra speed and more stability. However, you pay a price for this as the boat with extra mast rake has extra weather helm. To compensate for this extra weather helm and keep the boat running straight, the skipper has to supply extra tiller effort against the tendency to turn up into the wind. This effort comes from tugging on the tiller. It is easy for your arm to tire if your boat has excessive weather helm. The load that the skipper is supplying is what I call "tiller tug". Another disadvantage with excessive weather helm is that the extra drag
from turning the rudders to achieve a straight course slows your boat.
Getting the maximum speed is a matter of balancing the extra power against
the extra drag.
Reducing Weather Helm When You Have Extreme Mast Rake.
At this point it is easy to confuse weather helm with the effort required to keep your boat running straight. Many Hobie sailors confuse weather helm with tiller tug. These are different things and they are reduced by different adjustments. The only way to reduce weather helm is to more equally balance the CE with the CLR. If you have excess weather helm, you can reduce it by moving the CE forward, or moving the CLR aft. There is no other way to balance your boat. If you move the CE forward you lose the benefits of extreme mast rake. To move the CLR sufficiently aft both skipper and crew have to sit so far aft that they will fall off the boat. This is because the hull profile is not deep enough in the rear portion to balance the extra depth of the hull in the front portion. The CLR is a function of the profile of the underwater hull shape. Your Hobie 14/16 does not have enough aft hull profile to move the CLR far enough aft to keep it under the CE with a raked mast. You are stuck with high weather helm. Let me repeat, "the only way to reduce weather helm is to more equally
balance the CE with the CLR."
Rudder rake is where we adjust the rudders so that they point forward underneath the boat. We can adjust rudder rake to reduce the tiller tug, but this does not change the weather/lee helm balance of the boat. Welcome to confusion city. Consider a normal Hobie 14/16 just out there sailing around. It has slight weather helm, the skipper does not have to apply much effort (tiller tug) to keep on track. It's just a good fun way to laze away a sunny afternoon. Now we will really mess up that fun loving skipper. Lift the tiller castings to unlock the rudders. The rudders are now pointing slightly backwards. The skipper continues, tiller in hand, but the tiller tug on his arm has increased to a point where he thinks he'd better start working out at the gym again. What has changed with the balance of the boat? Nothing at all. The CE is still in the same place. The CLR is still in the same place. What we have done is to mess up the rudder configuration. The weather helm is still the same gentle balance, but the efficiency of the rudders in correcting it has been greatly reduced. With the reduced correction efficiency, the arm of the skipper is about to be pulled from his shoulder. We have increased his tiller tug without doing anything to his weather helm at all. Push the rudders into locked position again, and their efficiency is restored. If we were to push the rudders further forward the skipper would have to put even less effort into his correction for the slight weather helm. But if we did too much of that his steering could become very erratic and dangerous. And here is the irony. Moving the rudders like this is not balancing out weather helm. You can't balance excess weather helm by raking the rudders forward. Here's why. When you unlock your rudders they point further aft. Consider the underwater shape of the boat. Moving the rudder profile further aft moves the CLR further aft. And if you move the CLR further aft, it comes closer to being directly underneath the CE, which should give neutral helm, which should make the steering easier. But it doesn't make it easier, it makes it harder. In the same way, raking the rudders forward moves the CLR forward, which should increase the weather helm, which should increase tiller tug. But it doesn't do that, it reduces tiller tug. Remember these two rules:
When you move your rudders forward or aft, the adjustment you are working with is not about weather helm. In fact, this rake effect works directly opposite to the weather/lee helm effect of moving your rudders. There are two effects working here. One is helm balance, the other is what I call rudder moment. The helm balance has only slight effect, the rudder moment has far greater effect in the opposite direction.
When you rake the rudders forward does it mean that you have changed the balance of the boat? No. You have not moved the CLR of the underwater profile of the hulls, nor have you moved the CE of the sail.. You have just made the rudders easier to turn by reducing the working leverage.
When you rake the rudders forward you are moving the WC closer to the PC. If you were to increase this movement you would get to a place where the WC was directly in line with the PC, and as you passed this point the WC would be in front of the PC. Soon, the rudder blade would be totally in front of the rudder pin. The rudder would be hinged at the rear edge. Why does the rudder hinge at the front? Why not hinge it at the rear? This is why not. Imagine that your rudders were hinged at the rear edge. You are about to turn. As the rudder blade starts to swing the water pushes against it and swings it all the way to the side. And there it stays. There is no real control of the boat like this. So we can't have the working centre too far in front of the pivot centre. In fact, we can't have it in front at all. Further, we can't even have it in line with the pivot centre. Its handling would become like a supermarket trolley and totally uncontrollable. So we need to have the rudder blade mostly behind the rudder pin. But if it is too far behind the pin, the rudder moment is too high and the tiller is hard to manage, such as when the rudders are unlocked. Somewhere there is a balance point where the rudder is easy enough to
handle, yet not twitchy or unstable. This is the point we need to find
to reduce tiller tug without destroying boat handling. People who have
made this modification tend to say that to have the leading edge of the
rudder about one inch ahead of the pivot centre is sufficient.
Ruddertrack Rick White adds another element to this equation, what he calls ruddertrack. This is the angle at which the rudder must be held to compensate for weather helm. You are sailing along with slight weather helm and the rudder is out of line by about 2 degrees. Increase the weather helm and the rudder has to be held at 4 degrees. Increase weather helm further and this angle increases. The angle is called ruddertrack. You have 2 degrees of ruddertrack, or 4 degrees etc. As ruddertrack increases, so does drag. But in the early stages of weather helm ruddertrack is actually good for your boat's steering as it decreases drag. It is when you move into higher levels of weather helm that ruddertrack has to be balanced off against any power advantage of extreme mast rake. The drag free benefit of low ruddertrack is a byproduct of having asymmetrical hulls. Thinking time again, folks. Start your engines. You are on a beam reach with the wind from port. You are on the port rear corner casting with your right hand on the tiller. The port hull is just kissing the water. Consider what is happening underneath the waterline of the starboard hull. This is what the water flow looks like from above.
The water flows past the hull like air over a wing. The flat outer side of the hull cuts the water cleanly, the curved inner side of the hull pushes the water outwards along its foil shape. When the water joins up again at the rear of the hull, the tendency of the two streams is to flow outward from the boat. This foil shape also gives the Hobie lift to windward, so the boat is moving forward as well as slightly to windward. This slight tendency adds to the tendency of the water streams to flow outward from the boat. Now we will add the rudder.
The water is flowing off the hull and outwards. The rudder wants to fit that water flow and it turns slightly outwards. This means that the natural tendency of the rudder will be to steer slightly to lee as if to compensate for weather helm. The rudder has a couple of degrees of ruddertrack but there is no force on the tiller as the ruddertrack is merely matching the water flow. This is why there is no drag at early values of ruddertrack, it is the way the boat's hull works the water. The flow of water off the hull has its own tendency to steer slightly to leeward, even when the hull is being pulled slightly to windward by the foil shape. This outward flow of the water from the hull is one reason why Hobie sailors often set their rudders up for slight toe-in. Each rudder then more easily matches the natural water flow. When we have two hulls in the water, the windward hull is working in
the opposite direction from the lee hull. If you are sailing flat with
both hulls fairly equal in the water, then any effect of the foil shaped
hulls is neutralised. You can see from this that if you are to get the
benefit of drag free ruddertrack, you must have the windward hull out of
the water where ever possible. When you are just starting to fly that windward
hull, the underwater design of the Hobie is giving you its maximum efficiency.
The angle of the rudder is smoothing the water flow, reducing drag, increasing
speed, and increasing lift to windward. And it is doing it all for free.
Try and get all that from KMart.
As the drag of having excess weather ruddertrack builds up, speed drops off. But how much does the drag reduce the added power of extreme mast rake? Where does the tradeoff between extreme mast rake and resultant drag become detrimental? This is a question that Hobie racers will debate over many kegs of beer. However, the boats which win the races have lots of mast rake, even to the point of the mast head overlapping the rear crossbeam.. If you want to join the debate, invite me to your party. (An airline ticket and a few days in a beach side hotel with a Hobie parked outside will also be appreciated.) Conclusions There must be some conclusions out of all this. Here's a short list. 1. Mast rake gives more speed, more power, more efficiency at certain points of wind, and more stability under power. 2. Mast rake increases weather helm which increases tiller tug and resultant drag. 3. Weather helm can't be adjusted out by rudder rake, or by anything except decreasing the mast rake again. 4. Tiller tug can be adjusted out by rudder rake but ruddertrack remains. 5. Ruddertrack, and associated drag, can't be adjusted out with anything. 6. The power advantage of mast rake far exceeds the increased
drag induced by extra ruddertrack. For this reason racing skippers set
up for extreme mast rake and take the drag as part of the package.
Note about Rick White. Rick White is one of the catamaran world's highly respected sailing trainers. He runs sailing seminars all over the US and has written, with his wife Mary Wells, "Catamaran Racing for the 90s". This book is considered to be an essential resource for anyone who races their catamaran. You can find Rick on the internet at http://www.catsailor.com/ Back to your place in the text.
Coming up in part 2 of this series. The How-To Guide. How to adjust for mast rake. How to adjust rudders for toe-in. How to adjust for rudder rake. Kim Miller stalbans@mail.wts.com.au Back to Features |
