Problem with lee helm

Shanti, a Bristol Channel Cutter has some serious lee helm. On a trip down from Florida I had the typical weather helm of a BCC which really isn't all that much worse than many other boats I have sailed on. But still a little more than usual.

So what is different? I had a new jib made. I changed the dimensions so the fair lead came down by the chain plates instead of back by the gate (about 8 feet difference in position). I did this to give more overlap with the staysail to generate more power in the forward part of the sail plan...looks like it worked to well.

This is not a trim problem! I am sure through a number of methods to verify that I am trimming all sails properly. Tell tales, trim stripes etc.

When the wind increases and I heel over more, the lee helm increases also.

I'm puzzled.

You can read another discussion over at http://www.samlmorse.com/forum/read.php?5,9644
You can see my boat at http://garyfelton.com/shanti/

 

Lee or weather helm are the response to an inbalance betweeh the center of lateral resistance of the hull and the center of effort of the sail plan.

Windsurfers all know this very well, its the way they steer.

When you had your new sail made you moved the center of effort of the sailplan aft increasing the lee helm. It doesn't matter where the sheet leads to, what matters is the center of effort. When you heel the center of effort on the sailplan moves aft as the forward portions of the sailplan "lose" wind and the main doesn't. if you were to ease your main the helm would balance.

This is the simplistic answer.

Steve

 

Thanks for the reply Steve. I am aware of the center of effort and the center of lateral resistance and the corresponding theory. I sailed my previous boat for 16 years without an engine.

The new sails center of effort moved down and forward maybe by about 2 feet down and 6 inches forward.

I tried easing the main, but it didn't seem to make much of a difference. what I normally do to trim the main sail is haul her in until the leech tell tails stall then ease back out a bit until they fly again.

Thanks,
Gary

 

Moving the center of effort of the sail plan aft would increase weather helm or decrease lee helm.

As you, Gary, say in your later post you estimate it has moved forward 6 inches.

Easing the main is likely to to increase lee helm as the reduction in force in the main means there is less force balancing the foresails.

Lee helm is dangerous, especially in your case where you say it gets worse as the boat heels. The increase in wind force will naturally increase lee helm as well as increasing the heel. The shape of the hull as it heels may or may not contribute to lee or weather helm. You can steer a dinghy in light weather just by adjusting the heel. Generally when racing a dinghy you heel it to an angle where the helm is neutral to reduce the drag of the rudder to a minimum.

If a gust hits you, lee helm increases, the boat heels, tries to bear away, you push harder on the tiller to counteract it, the rudder may stall and you lose control, the natural reaction is to let out the mainsail making matters worse.

Some weather helm is considered safer, as a gust causes the boat to luff up, spilling the wind from the sails.

Adjusting the trim of the sails to achieve balance is inefficient.

 

Have the new jib cut down to a smaller size. You may lose some potential power but you may not lose much distance made good. Lee helm is bad news. Do what you must to remedy that problem.

 

Try some rearward mast rake.

 

I was wondering if it might be possible to rig a temporary stay off your sampson post and hank your jib onto that. See what that does for the helm balance. Or will that shift the clew too high?

 

Was anything else changed to affect the waterline? If she is down by the stern that might introduce lee helm. In some boats raising the centerboard a little can counter lee helm to some extent.

 

The reason I posted here was the realization that lee helm is not a good thing. But wouldn't making the sail smaller move the COE even further forward? although you would be decreasing the power. I Will probably shorten the leech so the fairlead is placed further aft.

If this keeps up I will! I love rake in a mast, makes um look fast!

That would be too radical a change I think. What I am going to do is get the old yankee (fairly new sail actually) and take it for a sail again.

Nothing has changed in that regard. In fact, nothing has changed except putting on a new sail with a radically different fairlead position.

I do find it extremely interesting that just changing the geometry of the sail so it sheets at a much different position has apparently caused this. But maybe that 6 inches difference forward in the COE does make a huge difference. In fact it would have to make a huge difference to make the boat go from heavy weather helm to heavy lee helm.

In all the discussions I have read about sails and COE I see the COE placed in the middle of the sail. But what would happen if you drew a triangle that included all the attachment points, head, tack and fairlead. That is where the boat is being pulled from.

Lets say you wanted to tow the boat. But for some insane reason you were going to do it by tying off on the bulwarks (I know it would probably bust the bulwarks). Now if you tie off forward of the lateral resistance of the keel, the boat would tow at an angle but with the bow facing somewhat forward. If you tied off on the bulwarks aft of the center of lateral resistance of the keel, the boat would turn around and tow stern first, but at an angle.

So the standard BCC yankee sheets aft of the center of lateral resistance of the BCC. My new jib sheets forward of the CLR.

Now as I fall more and more off the wind, the weather helm returns until it is about average for a BCC on a beam reach. Shes fairly neutral at about 60 degrees off the wind.

Here is the sail plan with the new sail drawn in.



PLEASE NOTE: The fairlead position I am using is confirmed by 3 different methods.
First, the angle of the sheet to the fairlead point is determined by drawing a line from the midpoint of the luff through the clew. This method is confirmed by Doyle. Doyle places a trim stripe at the clew so one can line up the angle of the sheet for proper fairlead placement.
Second, in practice this works correctly as the telltales I have along the luff of the sail break at the same time when the sail starts to stall or luff.
Third, the sail shape looks correct. Draft is where it should be from top to bottom.

So this is not a trim problem. The fairlead is where it is designed to be. In the drawing the fairlead is not entirely accurate as it sheets just aft of the forward shroud.

 

I don't think the attachment points are as significant as the center of area for predicting the CoE. CoA is only an approximation in any case, as CoA and CoE are not necessarily at the same place even for a single sail and with more than one sail the CoE location will move as the sails interact.

The sheet attachment point of the STD yankee is presumably the as-built location, and is well aft of the location predicted using the Doyle method. It is likely that this has been established by experience and may not have been the initial as-designed point.

The new jib sheets at the theoretical spot, perhaps a little aft of that according to your note, but not nearly to the same extent as the STD yankee.

So it seems logical to try moving the new jib's sheeting point aft. This may not necessarily result in the optimum sail shape, but balancing the boat is more important. Adjusting the trim of the sails to achieve balance is inefficient as Latestarter wrote, but moving the sheeting point is a lot simpler than changing the mast rake. You can always experiment with that later.

 

The sheet lead is such a steep angle, that moving it enough would render the sail useless in my opinion. I know that the CoE and CoA are just estimates because of interaction (slot effect etc.) of the sails. But it does serve best guess.

I see two things in my immediate future.
1) I still have access to the standard yankee. I will bend that on and take a sail. See what happens.
2) I will probably shorten the leech of the new jib. which will move the optimum sheeting point aft to just aft of the shrouds. That position will be an average of the two locations discussed. This will only be done after the sail with the previous yankee to eliminate the possibility that something else is causing the lee helm that I am not aware of. Because at this point, that is the only thing I am aware of that has changed.

 

I'm very surprised that the new jib would have such an effect. The boat ought to be more tolerant of small changes such as would happen when replacing a jib. The boat might have already been close to neutral helm. Still, lee helm seems an extreme result.
Mast rake appears to be the solution if sheet lead relocation doesn't work. The amount you move the masthead aft won't even be noticable so don't expect a jaunty skipjack look. Try lengthening the forestay about 1/4" for starters and take up the shrouds/backstay from there. It won't take much.
Obviously, you've already tried easing the jib to see how the helm reacts, which should bring back the normal weather helm. Does the jib luff before the lee helm disappears? See if the new jib likes to be cracked a bit compared to the old one. Maybe you're sheeting too close. Telltales would help in determining the correct position.

 

The CoE/CLR relationship with regards to weather helm is in my opinion grossly overstated. Obviously the couple between these two centroids will produce a turning moment, but counter intuitive as it may seem, this doesn't always translate to alterations in the feel of the helm.

I have seen some hulls continue to have weather helm despite everything being done, including the adding of bowsprits and the addition of skegs. I have also raced boats where there is absolutely no change in either the feel of the helm or the yacht's manoeuvrability when either the mainsail or alternatively the genoa, is dropped. Ridding a boat of all the 'effort' from one side of the CLR, in theory should lead to an immediate and significant change to the helm. But you stand on the Green at Cowes Week and watch how many classes jig around right up to the 5 minute gun with just their mainsails raised. They all happily tack, bear away, and go up and down wind without a loss of control, but their 'centroids' must be showing enormous theoretical imbalance.

About 15 years ago we were paid to investigate the 'balance' of a proposed new super yacht with the construction of a large free sailing model. All the ballasting was done internally so the CoG remained constant while the keel position was varied over nearly 20% of it LWL. No changes in the helm were detected.

The oft quoted relationship between the centroids allows designers to produce boats of normal form. There is therefore every expectation that these boats will steer and balance within acceptable 'norms'. But it doesn't follow that such a relationship incorporates all the variables.

My first advice is to make sure the bottom of your boat and rudder are absolutely clean.

 

I'll tell you all that this has been a mind bender. When I first purchased the boat she had weather helm. I put a new jib on her and now I have lee helm. And this new jib was not THAT much of a change except the fairlead position. But maybe the fairlead position is just a head fake for something else.

Time and few experiments will tell I guess.

Thanks Crag, I was hoping a professional designer would jump in. Your comments are very well received.

Thanks to everyone who contributed, it all helps.

Gary

 

It has taken a while to compose this post and I now find it has to some extent been overtaken in the meantime by expert comments.
So for what its worth my thoughts are as follows.


I take ancient kayaker's point that COA and COE differ however for simplicity I will use COA.

Reducing the sail area of a foresail moves its COA forward.

However it moves the COA of the whole sail plan backwards.

The exception to this is a large overlapping genoa whose leech is astern of the combined COA.

It is similar to balancing a strip of cardboard on a knife edge. If you cut out a piece of the cardboard to the left of the knife edge, you need to move the knife edge to the right to restore the balance. The bigger the piece and the further it is from the knife edge the more you have to move the knife edge to the right.

I thought a little analysis might throw some light on the situation.

Comparing the standard yankee to the new jib and measuring off my monitor screen I found that the area lost between the red and blue leeches in mm was 132 height x 6 maximum width divided by 2 = 396.

The area gained between the foots of the sails was 68 length x 15 maximum height divided by 2 = 510.

As important is the distance of these areas from the combined center of area of the sail plan.

If we assume that the combined COA of the sail plan is roughly on the mast, the COA of the area being lost is about 8mm in front of the mast, whilst the COA of the area gained is about 35mm in front of the mast.

So that the moment lost in front of the mast is 396 x 8 = 3168 but the moment gained is 510 x 35 = 17850 about 5.6 times more.

Whether this on its own is enough to explain the lee helm I do not know.

When I first got interested in the technical aspects of sailing they used to talk about the slot effect of having 2 sails close together which produced more lift than if they were separated.
You said you wanted to achieve more overlap with the staysail, which you have. It might be that there is more interaction with the new jib than the yankee.

I am trying to work out the effect of moving the fairlead.
My initial thought is that you can not treat the force at the fairlead in isolation.
As you said:-

Moving the fairlead will alter the forces at head and tack, it might just cancel out. Also when close hauled the jib sheets tend to be pointing roughly fore and aft, so lateral force would be small.