Centerboard Slot, Gybing board, Forward Rudder and Canard

The nature of  the Murrelet centerboard is not well understood. MacGregor Yachts states in the cruiser’s brochure that

A long, thin airfoil is far more efficient than a short, wide one. This is why racing sailboat keels are deep, and why sailplane wings are long and thin. The relationship between the fore and aft width of the board and its length is called its aspect ratio. Most boats have keels with aspect ratios of 2 to 1 (meaning that the keel or centerboard is two times as deep as it is wide). Our centerboard has a ratio of five to one (it is 13″ wide and 5′ 6″ deep). The high aspect ratio increases lift as the boat sails into the wind and reduces drag. This is one of the major reasons that the X will point closer into the wind and sail faster than other trailerables.

It is important to note that the trunk (often incorrectly referred to as the centerboard slot) is wider than it need be for the foil so that it can be hung in a way that enhances pointing. The foil is better, but not perfectly, described as a jibing board.

Jibing boards (also spelled gybing boards), when cocked to windward slightly (see animation), provide much better pointing. Our centerboard is self jibing, just like multi hulled boat masts are self rotating. This jibing feature has been portrayed as a defect because if the board is left down while the vessel is stationary it can clunk about with wave action. I would much rather lift the board and put out a second anchor, to prevent swinging, than eliminate the jibing feature. Gybing Centerboards have been well studied on 505s.

 The 505 is unique in that it has outstanding performance in all conditions. In light air it is quick and responsive, and in breeze it just goes faster. Planing begins in 10 knots of wind.

The Class Rules are One-Design, with the emphasis on controlling aspects that most directly affect boat speed. The sailplan and hull shape are tightly controlled, while the rigging layout, spars, and the foils are open.

505s do not have the hanger bracket of the Mac26x but like the Mac26x, the foil is shaped in a way that allows it to cock to windward. Findings from the 505 racing dinghys appear to apply to Mac26x vessels. The 505 self gybing board gives better pointing in light to medium air and flattish water. However, in windy conditions you do not want the centerboard to gybe. The 505 gybing boards are designed so that by bringing them up very slightly, part of the center foil blade goes inside the trunk and jams at full width, stopping the centerboard from gybing. The same behavior is observed on our vessel.

tasarstartIn 2005 a new term was invented that avoids controversy over the terms gybing board and centerboard. The new term is ROTATING KEEL. I very much like this term which appears to have been derived from a  mini transat swinging rotating keelfoil.

My conclusion, from the animation below and the width of the centerboard trunk, is that under sail there is very little “centerboard slot” drag to be concerned about with Murrelet. At any heel over 11 degrees the slot is out of the water.

thullTasars, loose speed noticeably when they tack or jibe. This is likely owing to the hard chine and cods head structure of the hull which when on a heel track the vessel in a straight line that resists a turning action. A 26X with a standard jib shares that behavior. It is eliminated by using a Genoa. The Tasar crews (which are not allowed the use of a Genoa) work the behavior by making as few tacks as possible in moderate and heavy wind and viewing each tack as a loss of 10 boat lengths (a huge penalty)

Future 60 foot and single handed ocean racers are to have up to two centerboards, the second being called a canard. The second centerboard will be used just as the centerboards on the Mac26x cruisers are which is for steerage by introducing weather or lee helm and like the Mac26x off centerline water ballast likely plays a part in its desirability.

11 degree heel puts one water tank<br /> keel on lineMacGregor Yachts’ change in the water ballast system so that the ballast was no longer on centerline represented a huge advancement in the firm’s monohull design. It created the equivalent of a twin-keeled catamaran-like-structure in the X boat. (See animation showing 4 slices through the X hull at 11 degree lean.)

Because the rudders of the X are on the keel lines, the windward one is not very rudder like when the boat is on heel. In winds over 17 MPH you see behavior involving lateral drift supporting the conclusion that the second rudder, the one on the windward, functions as a centerboard. What is currently called a centerboard, or jibing board, may (at least at planing speeds) be better described as a canard.

The canard is a controlling, rather than a weight bearing, foil. The main foil on sailing surf boards and other craft that use canards are mounted almost directly under the rider, supporting almost all the weight of the craft. The weight bearing foil is designed to remain submerged at all times, and it is guided in doing so by the canard foil, which pops to the surface before planing and generally remains there during subsequent operation. The importance of this foil in sustaining a plane is not known. The X downwind “hang glider configuration” does not use the centerboard to break plane and sustained planing involves little effort. But for other points of sail the centerboard is likely important in maintaining the planing mode of sailing. In displacement mode sailing the centerboard acts more as a rudder on upwind points of sail by jibing slightly. Johnathan McKee’s minitransat boat incorporated two concepts not unsimilar to the Mac26x. First, a swing keel with both lateral and fore and aft movement allowed McKee to balance the boat properly on all points of sail. Second a canard, or forward centerboard (sometimes called a forward rudder) also swings laterally to keep the foil vertical allowing more efficient trim while heeled. Most of the Trans Atlantic ocean racers have water ballast.

The behavior of the X described as “she is hard to knockdown, ballasted or unballasted” likely supports the above. With all three foils extended in normal operation, the aft lee one serves as the weight bearing foil and primary lifting force for planing. When not planing, the forward foil serves both as a rudder on upwind points of sail by jibing slightly to the wind and to compensate for the weight of water ballast by lifting the hull. The windward aft foil provides additional lateral force to the lee side keel to prevent crabbing when winds are over 17 MPH. When the operator is inattentive or inexperienced, all three foils likely provide counter balancing forces that prevent knockdown. Like three legs on a stool, any one of the foils might function as the weight bearing one during an unplanned action, propping the hull up and righting it. The point is that the behavior with foils extended means X owners can expect fixed keel boat behavior. Behavior like capsize preventing stability even when surprised by unexpected wind while unballasted.

 

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