In Part 1, we looked at how naval architects position their sail plan and underwater surfaces, in order to have a generally balanced helm for their designs. In this part, we'll look at what might be done to correct excessive helm, should the final boat helm work out different from the design.
In order to retain the shoal draft advantage of a multihull, good performing boats use a center-board or dagger-board to provide resistance to side-slipping when sailing upwind. With this mind, we'll primarily consider that arrangement in this article.
One of the predicaments in designing a well balanced boat with a dagger board, is that it's position is often required just aft of the mast. With an underdeck pillar required to take mast compression, this space is not readily available for a trunk and some compromise often has to be taken. Many boats end up with the trunk surfacing just forward of the mast and unless the trunk can be well inclined aft, this forward placement often ends up giving the boat a fairly strong weather helm. Designers might try to offset this with a skeg or larger rudder aft, or perhaps incline the dagger-board case rearwards, so that the board itself is close to the ideal position as it emerges below the keel.
Even designers with a series of similar designs, but of different lengths, have to reconsider this arrangement for every design and some may have boards forward of the mast and some aft. Often the former will result in a strong weather helm, while the aft placement will make it close to neutral.
The Dragonfly 25's typically have a fairly low weather helm and this was achieved by giving their forward-of-the-mast daggerboard a fairly good rearward slope and spreading the mast compression down each side of the case. The photo on the right of a F25A, shows a dagger board directly UNDER the mast—though this means it cannot be fully removed for any repairs, unless the mast is lowered first. But some boats have gone away from the theoretically more efficient dagger board, and now sport a pivoting centerboad.
Such a centerboard no longer requires a case up to the deck, but as it pivots up in the event of running aground, it also pivots aft and that moves the center of lateral resistance aft as well, so reducing weather helm. Unfortunately, a pivoting centerboard also requires a long slot in the keel, not only weakening that critical part but also causing considerable water disturbance that results in a slower boat. (Although outside the present subject, it's worth mentioning that small stones can jamb in a centerboard case and become an issue too. Small dinghies with such a case, commonly have a rubber or plastic flap over the slot, but as they can be more easily flipped over for maintenance and cleaning, this solution can work well. Without the same ability to clean the slot, larger boats typically have no closing flap and with more than half the slot open with the board down, there lies their problem of added resistance. Daggerboards are also better supported over their length and less likely to 'flap around', while a centerboard requires a certain slack to function easily.) But once designed and built in, daggerboards have little ability to be adjusted fore and aft, so the designer had best get their location right!
One means to correct a strong weather helm, as already mentioned, is the possible use of a small fixed aft skeg. This increases the underwater wetted surface though and serves little other purpose than protecting the rudder a little. Increasing the rudder area can help weather helm too and if the area is added forward of the line of pivot, this will reduce the rotational force on the rudder with a resulting lower force on the tiller.
The other place to make changes to reduce weather helm, is to adjust the location and distribution of sails. If you've done all you can with the underwater surfaces, perhaps you need to move the CE of the sailplan, forward a couple of inches. Major changes in balance will generally require that the tack of the jib be moved forward and even the mainsail modified with area cut off the leech (trailing edge). This could more logically be done if you ever plan to re-rig with a taller mast and shorter boom.
A sail with more fullness up forward will move the CP forward, while a flatter sail will move it aft. While adding area to a foresail may seem a good idea, it seldom helps much as the added area is just about where the old CE was—just aft of the mast. If the mast has a good rake, then you may be able to reduce that rake by shortening the forestay turnbuckle a bit. Shortening the forestay by ½" will normally bring the mast forward from 1-2" depending on the original forestay rake. (The more vertical the stay, the greater the mast rake change will be for each ½" shortened). What can help with the foresail, is moving the tack (fwd lower corner) forward—something perhaps possible if you have a sprit fitted.
In very general terms for the size of boats covered by this site, moving the jib tack forward about 6" will give about the same correction as inclining the mast forward about 2" at the hounds; either change will move the overall CE forward about 1". You might discover an increase in weather helm if you've recently changed your mainsail to one with a square top, as this adds area at the mainsail leech. Moving your mast forward an inch or so, could compensate for that or you might even go for moving the foot even 2" and then 'fine tune' with more mast rake, as rigs with more rake often go better upwind. But relocating a mast step is not always possible and one must be careful to keep any mast-lowering pivot at the mast base, in close alignment with the supporting lower shroud plates, so that these shrouds do not become too loose while mast lowering, or you risk to lose the mast over the side!
It goes without saying that if you do not have weather helm but the contrary (lee-helm), that the suggestions for correction will be exactly the opposite of what is given here.
Reducing excessive weather helm will certainly make your boat much less tiring to sail, but you always need just a touch, so that the boat will round up into the wind when you let go the tiller—even if that motion now starts more slowly. By contrast, eliminating lee-helm on a boat that had this malady (by bringing the CLR forward and/or bringing the CE aft) will give your boat a totally different feel. It will become more lively and responsive and you'll start to wonder why you struggled so long with the simple task of tacking ;-)
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