From the Drawing Board
Occasional ramblings from
a Small Craft Designer

by John Welsford

Balance is not just a matter of standing on one leg!

I had a call last night from a customer who had not long before launched a small gaff yawl to one of my designs. While in comparison to the old deep keeled classics that one would normally associate with that rig the boat was a light open dinghy with underwater lines that allow the boat to plane freely in the right conditions. A rig large enough to give the boat a performance that would have been unthinkable, and high form stability instead of a big slug of lead hung underneath the boat did have an important characteristic in common with the old gaffers.

On the modern cruising yawl the rig is well spread out, its jib is way out the front on a bowsprit, and the mizzen mast is about 3inches forward of the transom with the sail tagging along behind the boat as she sails. In fact on a boat length of 17 and a bit feet we have a total rig length of 23 feet! But unlike the old cruiser with its full length keel the new boat has just enough skeg under her to promote straight tracking, and a centerboard that is about a foot and a half from the leading to the trailing edge of the foil, and the rudder is of course not hung on a skeg so there is very little lateral plane aft or forward of the centerboard.

So we have a large and long rig blowing the boat sideways, balanced on a short and concentrated lateral plane that provides the resistance to that force. It is necessary to keep this boat in balance and that can be an advantage if used with skill, and cause all sorts of problems if not. There is an opportunity for a savvy skipper to trim this kind of boat to keep her balanced through a wide range of conditions, and to do this much more effectively than many other and older boats can.

My customers concern was that when he went sailing in really rough weather for the first time his beautifully balanced new boat had developed enough weather helm to make control quite difficult. While the weather he was out in was bad enough to keep anyone without a really compelling reason and a large seaworthy boat snug ashore, and the area was notorious for fast tidal flows and forcing through narrow channels against the winds he did manage pretty well and by the sounds of it he was not at risk in spite of a real lack of experience in handling open boats in those conditions.

But how to handle that boat in those conditions?

First of all, what was happening there.

One, the boat was sailing with the jib rolled up leaving the center of thrust from the sails a lot further aft than optimum. Two, as a sailing boats speed increases the center of effort of the sails needs to lead the center of lateral plane ( the underwater point around which the boat will pivot if pushed sideways) by more and more.

My mans problems came when the boat was getting close to surfing, and with the jib furled and the centerboard right down in its most forward position the sails effort was too far aft, and the center of lateral plane too far forward relative to the sails so the rudder was carrying a lot of weight and the boat was relying on that rudder to stop the stern being forced sideways. Its not designed to do that and under those conditions is easily overpowered resulting in an uncontrolled and often violent turn into the
wind. A "broach". Dangerous!

It's a matter of balance. To adjust the weight of the helm or to correct a tendency to round up into the wind it helps to visualize a seesaw with a moveable pivot. The speed of the boat, the trim of the boat and the position of the centerboard (as an adjustable keel, if fitted) can alter the position of the pivot. To adjust the weight on the ends of the seesaw, one must alter the center of the force that the sails are generating in relation to the pivot point. Reefing from the wrong end moves the center of the sails effort aft putting more weight aft and overpowering the rudder causing excess weather helm, moving the effective center of lateral plane (the seesaws pivot) forward does the same thing and makes the seesaw unbalanced. An effort needs to be made to move the pivot aft, and the sails drive forward to get that balance back.

In future that skipper will be shortening sail from the other end, mizzen down first, then reef the main, and only then furl the jib. Other ways of helping to restore balance is to swing the centerboard aft by lifting the board a little, to trim the boat stern down with crew weight and to ease or trim sails to spill wind from the after part of the rig.

Remember that an increase in speed moves the ideal position of the pivot aft, that the drive of sails can do the same with an increase in wind strength and that the ideal balance point of the rig will move forward as boat speed and wind speed increase.

Adjust the sails and the boats trim to suit that change and your boat will be a lot easier to steer.

Keep that seesaw balanced.

John Welsford
Designer, and not often enough, sailor.