Weather helm CE & CLR

[Silkie]

Does anyone (not just on here) really understand weather helm? Despite extensive research I've never come across a fully convincing explanation.

My current understanding is that it's caused by a combination of the relationship between the centre of effort of the sails and the centre of lateral resistance of the hull and the changing underwater profile as the boat heels. Sail trim has got to be in the mix too I would imagine.

Hurley 22s are prone to it but my experience has been markedly different in each of my 4 seasons so far and I'm determined to sort it out once and for all for next year.

In my first year it was frankly scary and I put it down to the mast compression problem. Having fixed this over the first winter the transformation was astonishing. From requiring both hands hauling the tiller up under my chin the helm, while still heavy, now only needed the tiller a few degrees to weather even in quite brisk conditions. I thought I had cracked it but 'twas not to be so simple. In my third and fourth years I again had significant weather helm.

Rig set-up has got to play a big part if the conventional view about the CE/CLR relationship is correct.* During the first season the rig couldn't be fixed since the mast dropped significantly as the wind increased and during the second I had almost no rake IIRC. For the third the forestay was too short (mast raked for'ard and weather helm) and it was too long for the fourth year (mast raked aft and weather helm again.) If the CE thing is as simple as it sounds I surely shouldn't have had weather helm in year three.

There is presumably some other factor at work which I've failed to notice.

Whaddyathink?

* [I've always been a bit dubious about the notion that the CE can be considered as a single point on a line between the centres of the two sails whose position on the line is weighted by the relative sizes of the sails. After all the forces are transmitted through the luffs of the sails and through their sheets. Also the shape of the sails and the angle of the wind make a big difference to how much of the wind force is translated into drive and how much into heel.]

[Nick]

All we do to fix it is reef the main . . .

Why not invite that PBO sails expert to come up next season and do an article?

[ash]

I've also read about the importance of mast rake, but IIRC it's more that too little rake creates lee helm in light conditions.

Silkie has a good underwater shape so shouldn't create excessive weather helm when well heeled.

I believe that when well heeled, the driving effort of the sails is outwith the beam and therefore creates a turning force to windward.

In lighter conditions with the boat more upright, I believe that the cause of weather helm is having the boom too close to the centreline. If using an overlapping headsail, the traveller can be let down to leeward until the ( mainsail ) luff is backwinding slightly as long as the main part of the sail is drawing well.

Nick and I sail the same boat, but as the wind speed picks up Nick probably drops a reef into the main and hangs onto the full genoa whilst I tend to fit the jib and use full main. The Vega is very much headsail driven, and Nick has the advantage that he can reef his genoa whilst my headsails are hank on but I find that she goes well with the jib. Raising a headsail to windward doubles the speed achieved under main alone, I believe because of the increased wind speed across the lee side of the main - the slot effect even though the headsail isn't overlapping the main.

Ash

[Silkie]

You are both quite correct in all you say, I've no doubt, but the fact that Nick reefs the main first (it's not unknown for him two have 2 reefs in the main with full genoa) while Ash changes down headsails would indicate to me that some boats are more tolerant than others to differing sail balance. Silkie carries a fair spread of canvas for a wee boat (which gives her surprising light air performance in relation to bigger boats sometimes ) and it may be that this makes her more sensitive to sail balance.

However in some years she seems to need only a couple of inches of helm regardless of wind speed (within reason) while in other years the end of the tiller is a foot off the centreline nearly all the time when on the wind.

Against this, even in "off years" there are occasions when she balances beautifully (when reefed) like the Titanic Moment in year three. Apart from the fact that I'm not happy that I'm doing the best I can for her, the only real problem is that the tillerpilot can't cope with excessive weather helm. Certainly she loses a fraction of a knot but this is insignificant in a wind over-supply situation.

I just don't understand all the issues involved.

[ash]

and it may be that this makes her more sensitive to sail balance.

Well - you would go buying a highly strung thoroughbred !

I was interested to read the translation of the German Review of the 22 on the Hurley website. They didn't mention weatherhelm but thought that the mast should be moved further back which would reduce the main, increase the fore triangle, and thereby reduce any tendency to excessive weather helm.

You also provide some clues yourself

she balances beautifully (when reefed)

Fitting a bigger genoa doesn't help as the extra area is behind the mast. You're stuck with the fore triangle given by the designer - but you're not using it fully as the foot of your sail is well off the deck. As an experiment you should try pulling the bottom section of the sail out of the furling groove and fit the tack to the stemhead and see what difference this makes to the balance.

On the other hand ( or at the other end ) you say that she has a generous spread of canvas, so maybe follow Nick's advice and drop a reef into the main as soon as the wind picks up a bit. I think that we all may have a tendency to hang on to too much sail - you may be tempted more because of the excellent ballast ratio of Silkie. The boat may be fine in terms of heel, but are the sails able to maintain a good shape ?

Ash

[Silkie]

she balances beautifully (when reefed)

You missed the crucial "there are occasions when" from your quote.

There are also occasions when we can go 20 miles with the wind direction varying 20-30 degrees and speed from 10 to 20 knots, first reef in and out, variable genoa and sails drawing beautifully the whole time and still always need loads of helm to keep her in a straight line. Yet in the golden year 2 when I was surely even less skilled in sailing her than I am now she was nearly always much easier on the helm.

Perhaps you're right and I just reacted much more quickly to increasing wind back then but I don't think so since I'm still a big fearty.

[Nick Vass]

Weatherhelm is the tendency for a yacht to round up into the wind.
If the helmsman has to pull the tiller towards him or her to make the boat go in a straight line then the boat is said to ‘carry weatherhelm’.
The hull of a Hurley 22 turns on a point called the Centre of Lateral Resistance CLR.
The CLR is the central area of the underwater profile of the yacht.
The wind force on a sail creates a point called the Centre of Effort CE.
Both the main and the jib/genoa have their own CE. Both CE points combine and create the boats Centre of Effort.
If the boats CE is aligned with the CLR then the boat will be balanced.
On the Hurley 22 the CE is often aft of the CLR. The pressure of the wind turns the stern away and the bow towards the wind creating excessive weatherhelm.
At this point the main should be reefed.
The hull shape of the Hurley 22 makes a big difference too.
The Hurley 22 has overhangs at the bow and stern so that the wetted area of the hull is kept low when the vessel is upright but the waterline length is increased when the boat is healed over when sailing. The Hurley 22 is narrow compared to modern yachts and carries its beam just forward of the CE. This means that when the boat is healed over the buoyant beamy part of the boat kicks in and tries to float up pushing the bows around to windward thus creating forces exasperating the weatherhelm.
The Hurley 22 widens dramatically as you look at the profile above the waterline.
Modern boats carry their beam much further aft making the boats a lot lighter on the helm.
The maximum speed of a hull is around 1.4 times the square root of the waterline length in feet or x 3.28 in metres.
Therefore the hull speed is greater when the hull is heeled over and therefore the faster the Hurley will go but conversely the more rudder is needed, putting on the brakes and slowing the boat down.
It was considered that a slim boat was a fast boat, as seen on Bloodhound for example. Modern boats are very beamy and blisteringly fast but need to be sailed upright as their waterline length does not increase much when heeled as they have stubby bow stems and flat transoms.
Hurley Marine altered the shape of the Hurley 22 rudder in 1972. Increasing its size by 1.3 square foot. The newer shape rudder is shown on the HOA site on the Technical page. It is the same as the Ravensail/Hurlwind/Blaxton mould. This new rudder is available from Blaxton Boats.
Increasing the size of the rudder is not a total solution to the problem as a larger rudder can in itself be a disadvantage.
A small amount of weatherhelm is desirable as it gives you a feel for the helm but if weatherhelm is excessive you have to put the helm hard over and effectively putting the brakes on as the rudder would sit at a right angle and act as a flap.
A ‘balanced boat’ is referred to when the forces put on the hull by both sails when it turns on its central line of lateral resistance are equal. That is when both the genoa and mainsail are equally balancing the boat out laterally.
My Hurley 24 can often be described as well balanced. The high aspect rig with narrow main and large genoa balance the boat well but give a little weatherhelm which feels reassuring as the boat would round up into the wind should I fall overboard. More weatherhelm is felt during strong winds when the genoa is furled in and the mainsail takes over as the main source of propulation.
Although the masts are nearly the same height the Hurley 22 sail plan is very different. The mainsail is comparatively larger. In fact the sail area of the 22 is more despite the displacement being very different!
H22 disp 1,769 kg Sail Area 24m
H24 disp 2,285kg Sail Area 20m
Therefore I would conclude that reefing in and balancing the boat by effective reefing and good quality sails is of paramount importance and a more effective way of reducing weatherhelm on a Hurley 22 than the size of the rudder alone.
Weatherhelm on a Hurley 22 can be managed by reducing the size of the mainsail or increasing the size of the genoa/jib.
This adjustment in sail area and relocation of the main driving force can be described as moving the centre of effort.
The opposite to weatherhelm is leehelm.

[little boy blue]

no excuses now silkie