Study Hall: Correcting Weather Helm

[cal40john]

Originally Posted by Chrisgo

I know this is an old thread. Really, I know, but I have to agree with Jedi.

I am looking at a modern 32' production cruiser that some say has weather helm issues, so this topic interests me.

Also I think people misplace there center of effort points. Is it not so much where the clew or end of the sail is, but where the mainsheet or traveler is attached to the boat?

I have heard of tightening the backstay to bring the c of e aft, but again, the pressure, (attachment points if you will), are still in the same place, on not sure why that would work.

If there were no air flow creating lift the center of effort on a sail would be the center of the area of the sail. It has nothing to do with attachment points.

Take a rectangular block of wood (hull). Nail a stick on one end (mast mounted on bow). Nail a sheet of wood to the stick that extends twice the length of the block of wood. ( Hard wing sail attached to the boat only by the mast rigidly held amidships. Half the sail overhangs the stern). Push sideways on the sail approximately at it's middle (CE) which is over the stern of the hull. Assume CLR is the middle of the hull. Which way does it rotate? If it depends on attachment point it will rotate downwind at the bow since it is solely and entirely attached to the boat at the bow. You should find the bow actually rotates into the wind.

John

[HappySeagull]

Chrisgo, not sure if I have you right, but it makes no difference where attachment points are....ie: attaching a sheet block further forward will not decrease weather helm. Weather helm is a measure of boat hull +sails as a whole. Changing positions of attachments is just changing local stress-loading on the boat and rig UNLESS it changes the position or force of sail ON the hull.
Tightening the backstay may satisfy that "condition"by changing the drive of sail...it's force and maybe the centre of effort too, relative to the hull and other sails but it's not near so obvious a change to make as slacking a sheet, reducing the mainsail size,moving the mast, moving ballast, and so on.

[Chrisgo]

I come from a windsurf backround, where the legs and feet perform a lot of the attachment points, so I'm still trying to figure this all out. I think my definition of ce is wrong maybe.

[Bash]

Originally Posted by Chrisgo

I come from a windsurf backround, where the legs and feet perform a lot of the attachment points, so I'm still trying to figure this all out. I think my definition of ce is wrong maybe.

Nope. It's the same on a windsurfer as a keelboat. The biggest difference is the size of the sails, which translates into more stretch, especially if the sails are made of dacron.

(Of course, when's the last time you saw a windsurfer with dacron sails?)

Weather helm is something any good sailmaker can help you solve with a new suit of sails. The CE can be designed into a sail, and sails can be cut flatter for boats with weather helm issues.

(BTW: With dacron sails, the center of effort can almost always be moved forward by increasing luff tension. This is an adjustment that isn't really effective on sailboards, largely because there's so little stretch in their sails.)

[cal40john]

Originally Posted by Chrisgo

I come from a windsurf backround, where the legs and feet perform a lot of the attachment points, so I'm still trying to figure this all out. I think my definition of ce is wrong maybe.

OK, let's put you on a Mistral Superlight 260 liter board. We'll make you a lightweight sailor so if you move a couple of feet fore or aft you don't significantly sink the bow or stern. Hold the sail is in the correct fore/aft position to go straight, now don't move the sail relative to the board. Moving your feet from one foot in front of the mast step to one foot behind the mast step will make no difference, you will continue to go straight. In reality it will change, because you will sink the end of the board you're standing on changing the CLR. It is the CE CLR couple that determines helm.

John

[malbert73]

Originally Posted by s/v Jedi

I agree fully with Barnakiel. The changing shape of the under water part of the hull when heeling is the factor for weather helm when it increases in gusts (= more heel).

Think about it: to move the bow off the wind you would want more pressure on the jib. But if you do the opposite, furl it a bit, weather helm decreases. This is because you heel less.

Same for the main. When you put in a reef, or flatten the sail, you de-power it, resulting in less heel.

So I really do not agree with Gord's point to "increase the pressure on the head sail". All effort should be to reduce heeling so reefing, flattening sails (if the main is baggy you can still flatten it by reefing), bending mast more if you have the luxury of backstay adjuster, tighten vang and sheets and let traveler out a bit.

"Narrow" boats with balanced lines don't have the problem up to some point of heel.

cheers,
Nick.

Correct. Sell the big assed aft stateroom boat and buy one with a normal sized transom and length/beam ratio well less than 3.


No substitute for a boat that won't lift the rudder out of the water when it heels. I have had the inclinometer pegged at 45 degrees and still had control of the boat in a puff... Even with roosters tails off of the winches. No rounding up unless I want it to by putting the tiller down to leeward...

Full disclosure, though, boats that don't have excessive weather helm with squirrely rounding up tendency also don't have aft berths with hot tubs underneath.

[stevensuf]

Correct me if i am wrong, but is a small amount of weather helm not beneficial, it aids in pointing by having the rudder out a few degrees creating more lift towards the wind.

[malbert73]

Originally Posted by stevensuf

Correct me if i am wrong, but is a small amount of weather helm not beneficial, it aids in pointing by having the rudder out a few degrees creating more lift towards the wind.

Correct. Just not the amount folks are complaining about.

[KiwiIC]

I believe that the effect quoted of ROLLER furling a headsail has had two outcomes.
1 is that the CE of that sail has now moved forward (and a side effect that if the fairlead was not moved the leech will have opened up some)
2 the boat is now not so powered up, has less heel on so hull distortions are reduced.
Overall outcome is that the weather helm would be reduced, even by default.

[datmbn]

Couples and Forces @ sailtheory.com

[GordMay]

Greetings and welcome aboard the CF, datmbn.

[Kestrahl]

Originally Posted by Bash

Nope. It's the same on a windsurfer as a keelboat. The biggest difference is the size of the sails, which translates into more stretch, especially if the sails are made of dacron.

(Of course, when's the last time you saw a windsurfer with dacron sails?)

Weather helm is something any good sailmaker can help you solve with a new suit of sails. The CE can be designed into a sail, and sails can be cut flatter for boats with weather helm issues.

(BTW: With dacron sails, the center of effort can almost always be moved forward by increasing luff tension. This is an adjustment that isn't really effective on sailboards, largely because there's so little stretch in their sails.)

Dacron varies greatly, some Dacron sails will still look fine after 10 years while others won't. It is worth taking a photo of the camber and showing a sail maker before assuming new sails or recutting will solve anything. Recutting is much cheaper and will make the sail maker less money. Dacron does respond to luff tension provided it is cross cut and not radial but it won't get rid of the camber out of the leech if it has stretched.

Sent from my GT-I9500 using Cruisers Sailing Forum mobile app

[Glenn.Brooks]

Old thread, but always a current topic! Seems like quite a few comments related to racing with luxury of a crew for every sheet. E.G. Tightening backstay. Who does that in a blow! And what do you get for the effort? Not very much improvement compared to trimming or reefing the main.

In my old CCA rule full keeler the optimum way to reduce weather helm is slack the main, then reef at around 18 kts. Second reef goes in around 25 kts. Third above 30kts. This reduces heel and improves weather helm to next to nothing. Rolling in the genoa helps point the boat, but the power comes from the main, hence the most improvement in the original posting about sailing in 20kts of breeze would ideally have been to put in the first reef in the mainsail and maybe slack the reefed main until weather helm disappeared. Anyway, very interesting discussion. Glad it's a sticky at the top of the page.

[ic_boy]

I see this is a very old thread, but can't resist adding my 2 cents. I cruised for 2 years on board my Morgan 382, it's a design that generates a pronounced weather helm in 15+ kts of wind. We had a monitor windvane (came with the boat) & did almost all of our passage making steering with the windvane, and it absolutely would _not_ work in the presence of even minor weather helm so we were fairly sensitive to tuning out all weather helm on that boat.

3 main sources of weather helm in my experience on the Morgan 382, in order of how much each contributed to weather helm:

1. CE to leeward of the fore-aft centerline (only seemed to be an issue from beam reach to dead downwind course, when the sails are way out to leeward)
2. Anything beyond very moderate angle of heel (only an issue from close hauled to beam reach, other points of sail we didn't heel enough to have an issue)
3. CE aft of CLR (only seemed to be an issue from close hauled to a beam reach)

I can say 1. was definitely the most pronounced weather helm source, it was a constant heavy weather helm & you could feel the sails pushing on only one side of the boat & twisting it to windward. 2 & 3 were more subtle and I was never sure whether it was a CE vs CLR imbalance or just increased heel ... if I had to guess I would say they both contributed in roughly equal proportions.

What worked for us to solve the problem on that boat

1. For broad Reach to Dead Downwind Courses we would either drop the main entirely (broad reaching) and that would balance the boat, OR we would run dead downwind with the Genoa poled out opposite the main and that would also balance the boat. Setting up the pole was enough of a pain that we often just drank our beer in the cockpit & sailed downwind 'tacks' broad reaching alternating courses (chicken jibe every 3 or 4 hours) under Genoa alone. The monitor steered surprisingly well broad reaching under just the Genoa even in very blustery conditions with a quartering sea pushing the boat all over the place.

2. For Beam reach to close reach / close hauled we would de-power the main by tensioning the vang hard and easing the traveler a little to leeward or easing the main sheet a bit till the telltales showed we were under-trimmed by a few degrees (you have to have a vang to do this without the mainsail starting to shudder a bit). This would immediately reduce our angle of heel & I felt it moved the CE forward a bit (no way to measure CE I guess, but we had an inclinometer in the cockpit & just flattening the main and undertrimming slightly would instantly take 5 degrees or more of heel off and you could feel the weatherhelm immediately settle down). We typically lost 1 kt of way with this tactic, but it had to be done if we wanted to steer with the monitor.

Reefing - we always reefed in this order no matter the point of sail (and this was neccesary for weatherhelm reasons): 1. first reef in the main 2. roll in Genoa to 25% mark 3. 2nd reef in main 4. (almost never did this, only needed in 30kts+) roll in Genoa to 50% making it more like a barn door than a sail & use it purely to balance (not propel) the boat.

Things that did NOT work for our boat:

1. New main sail. I've heard many say that an old stretched out mainsail will cause weatherhelm. We replaced just such an old baggy main halfway through our cruise, and I can definitely say that our spanky new main was a much faster more enjoyable sail, but it behaved EXACTLY the same with respect to weather helm as our old baggy one, no discernable difference whatsoever on any point of sail or any wind condition.

2. Tuning mast rake to move CE forward. We spent quite a bit of time playing with this, moving & changing chocks where our keel stepped mast passed the deck & tuning back & fore stay tension. Even radically changing the rake (we went so far as to remove all rake to straight vertical) had no effect at all on weather helm, we tried it on a number of different over-nighters. Changing mast rake made no difference at all no matter what we did, total waste of time.

My Final 2 cents:

If you're trying to mitigate a weather helm issue on your own boat, the best bang for your buck & time spent is to have an effective boom vang. Our vang was just line & blocks, I made it myself, very cheap & hugely effective at keeping the main flat while its eased when reaching, IMO its the single most effective thing you can do to mitigate weather helm. Also, having a traveler that's easy to adjust under load was very helpful (I replaced ours with a fancy $800 one from Garhauer with a ball bearing car, it was money well spent) ... not as easy/cheap to rig up as a vang, but still I feel the traveller was a worthy addition to the boat & it did help a lot when tuning out weather helm.