IMCA PROPOSALS FOR CENTRE MAINSHEET AND REVERSE THRUST KICKING STRAP This document contains comments, explanations of the thinking, experience, calculations or background that lies behind the proposed rules changes for the introduction of the centre mainsheet and a change to the kicking strap rules including allowing reverse thrust kicking strap. It is proposed that one centre mainsheet arrangement is permitted as an alternative to the existing stern mainsheet arrangement, an alternative stern mainsheet arrangement is introduced and the kicking strap rules are amended to allow more space for the crew. Why a centre main sheet ? There are several reasons why the option of a centre main has been proposed. 1. At present in the UK, the Mirror is the only junior (under 16) trainer with transom sheeting. All RYA methods and teaching is in dinghies with centre mains. The transom sheeting system causes problems for children and adult beginners who normally sail Optimists, Toppers, Laser Picos or other small dinghies that have centre mains. When they try a Mirror, they can’t tack in the manner they are used to, they have to learn to tack facing over the stern. This negative experience puts them off and they may discount the Mirror as a possible next boat. 2. I understand that the Dutch Sailing Authority is now reluctant to support the Mirror as a Junior Class. They have medical evidence that aft sheeting systems can damage backs and knees, because of the twisting posture. The RYA has similar concerns. The view of Dr Frank Newton, ex Chairman of the ISAF Medical Commission is "It is highly desirable that we don't subject 'young backs' to unnatural postures whilst participating in our sport." He specifically concurred that sitting out, twisted, pulling on an aft mainsheet, fell into this category of an unnatural posture!.
The recent change to centre mainsheets in the Topper Class has led to circumstantial evidence that there is now much less back injury. 3. In the UK Mirrors are often used by parents teaching their children to sail and then to race. The same thing happens in Australia with the “Nipper” club training program for 7-11year olds and similar schemes in New Zealand. Centre mainsheets are an advantage in this situation as it enables the adult/trainer to manage the mainsail from the crewing position while the kids “steer”. 4. When Mirror sailors progress onto any Youth class (420s, 29ers, Lasers,…..) they will find it easier to change if they have been using a centre mainsheet. A single centre mainsheet option of a “laser” style mainsheet with fixed hawse is proposed.
Possible centre main systems There are three other possible centre main systems which we could adopt. In this section I will describe the proposed system and the alternatives so everybody is clear about the terms being used and then go on to look at the advantages and disadvantages of each. The word “hawse” is defined in the Oxford English Dictionary as a nautical term meaning “The arrangement of cables when a ship is moored with port and starboard forward anchors”. I’m going to adopt the Australian tradition and use this term to describe the arrangement of rigging (two strops of equal length or one rope with knot) used to hold a block centrally above the transom and tiller using the existing mainsheet attachment points on the transom. This being the case, the strops or ropes are called hawsers. Laser style with fixed hawse (the proposed system). The mainsheet starts on the boom or (more likely) the becket of a single block mounted near the end of the boom. It then goes down through a single block mounted on the hawse, back up to the block near the end of the boom. It then goes along the boom about 1300mm to another single block then either; i. Down to a single block mounted on the thwart or a mounting block just aft of the thwart. ii. Directly to the helms hand (29er style) Any of these blocks could be a ratchet block, but it makes most sense to mount a ratchet block as the final block in the system. Laser style with adjustable hawsers. As above, but the ends of the hawsers can be adjusted so the height of the block can be changed or moved to windward etc. Split tail. About 600mm from its end, the mainsheet divides in a smooth manner into two thinner tails of equal length. One of these tails is fixed to each of the existing mainsheet attachment points on the transom. The mainsheet then goes to a single block near the end of the boom. It then goes along the boom about 1300mm to another single block then down to a single block mounted adjacent to the thwart. When the main is fully sheeted in, the join between the tails and the single part of the mainsheet passes through the block on the end of the boom. 420 style. The mainsheet starts on the becket on one of two blocks (or a double block) mounted adjacent to the thwart, up to one of two blocks (or a double block) on the boom above the thwart. It then goes back down and around one of the blocks mounted adjacent to the thwart and back up to the other block on the boom. It then goes back down to the other block adjacent to the thwart. This gives the same purchase as the existing system (2:1). The reasons for proposing the Laser style mainsheet with fixed hawse over the other systems is as follows: 1. By retaining the block fixed to the transom, the load imposed on the thwart or top of the centreboard case is reduced. I calculate that ½ of the load will be on the transom and ½ will be on the thwart or centreboard case (and none if the lead is taken directly off the boom). This is because the block attached to the transom has 2 parts of the mainsheet pulling on it and the block adjacent to the thwart also has two parts of the mainsheet pulling on it. The tension in the mainsheet is constant (ignoring the effect of ratchet blocks). This is an important issue. Many experienced sailors in the class (for example Dave Gebhard) have real doubts
about the ability of the boats (particularly the older ones) to take the loads that would be imposed by a 420 style mainsheet system where all the blocks are adjacent to the thwart. The split tail system will have 1/3 of the load pulling on the transom and 2/3 on the thwart. 2. It puts less bending stress on the boom than a 420 style mainsheet. With the 420 arrangement, you have all the downward force concentrated in the middle of the boom from the main sheet and kicking strap, countered by upward force at the gooseneck and clew attachment points. This may not be a problem for an alloy boom, but is likely to cause failures with a wooden boom. With the proposed arrangement, the downward force is spread between the aft and centre mainsheet attachment points and the kicking strap or gnav 3. It’s a low cost option. The existing mainsheet blocks can be re-used. Only one or two additional single blocks (one with a becket) and a longer mainsheet are required. With a 420 style mainsheet, given the limited space available, either double blocks or single blocks with beckets would be required. A non adjustable hawse is cheaper than one where we allow the length to be changed. Australian experience was that changing the hawse height was ineffective, but that won’t stop sailors spending money on blocks, cleats and trying it for themselves unless the rules stop them. Split tail mainsheets are not easy to make, most would have to purchase them, so a little more expensive than just buying rope. 4. The purchase offered by this system is about 2.5:1, slightly higher than the stern sheeting system which is 2:1. This means that there will be slightly less mainsheet tension, which will make it a bit easier for younger sailors to pull the mainsheet in properly. The split tail mainsheet system is 1.5:1 so harder for young sailors to pull in when it’s windy. 5. Experience in the UK and Australia points to this system giving a race advantage since it is possible to get the boom very close to the centreline, without creating a lot of tension in the mainsail leech (particularly important in light winds). The hawse height plays a role here. The minimum mainsail leech tension is achieved when the block on the hawse is almost touching the one on the boom. However, since the boom height varies with the wind strength so does the ideal hawse height. In strong winds when more leech tension is required, the boom and hence, the ideal hawse height, will be lower. Since we are not going to allow the hawsers to be adjustable, crews will have to compromise with a hawse height suitable for strong winds. In order to compensate for the race advantage described above, this proposal includes a new stern mainsheet with fixed hawse option so that those who want to stay with stern sheeting can do so, and enjoy the same performance gain. This is a fairly inexpensive change for existing stern sheeted boats, only requiring the purchase of a single block with becket and a small amount of line for the hawsers. The disadvantage is that it creates yet another option (some people feel that the Mirror offers too much choice – gaff, one piece mast, two piece mast – wood hull, GRP hull, wood boom – alloy boom, ….). In other respects the proposed system gives no race advantage, if anything it gives a slight disadvantage. As there is more purchase, you have to pull in more mainsheet than with stern sheeting to move the boom the same amount, so there is no advantage regarding pumping the mainsheet as allowed by RRS 42 or when boat handling at marks or when doing penalty turns. The proposed system will have a bit more drag and a bit more weight (2 blocks, longer mainsheet, hawsers) but the effect will be very small.
The 420 style mainsheet would offer a race advantage as it would allow you to pump by pulling all parts of mainsheet or taking a lead from a different point. It would also allow longer tiller extensions to be used. On the down side it requires more leech tension to get the boom centred than do the hawse or split tail systems. The split tail mainsheet has less purchase so gives a small pumping and boat handling advantage. It also has an advantage over all the hawse based systems as it is possible to get the boom close to the centreline with even less leech tension. This is because there is no hawse height to worry about and this is why these systems are popular on other classes such as 505s, Fireballs and GP14s. Alternative stern sheeting system As described above, the proposed centre main sheet gives a race advantage as it is possible to centre the boom with less leech tension than the existing arrangement. In order to allow sailors who are used to or prefer a stern sheeting arrangement to enjoy the same advantage, the following alternative stern sheeting arrangement is proposed; Stern mainsheet with fixed hawse. This is similar the existing mainsheet arrangement, but with using a hawse to locate the block on the transom centrally. The mainsheet starts on the hawse or a becket on the single block mounted on the hawse, goes through a block mounted near the end of the boom, back down to the block on the hawse and to the helm. Increasing space for the crew Nobody who has sailed a Mirror can have any doubt that the space for the crew is very limited. We need more space for the crew for the following reasons; 1. Mirrors are often used in training programs with the adult/trainer crewing. However the amount of space available for the crew is limited, a major cause is the kicking strap. This lack of space problem also applies to older, bigger children and adult beginners. Having more space will widen the range of people who can sail the boat comfortably. It also makes the boat more attractive for those who want to take a family for a sail. 2. The lack of space issue will be compounded if a centre main is fitted (which has advantages for teaching and beginners who have just finished a course with a sailing school as described in the centre main section above). There are two methods of increasing space for the crew and the proposal is to allow both: A reverse thrust kicking strap. This is sometimes called a “gnav” which is derived from the word “vang” spelt backwards and is pronounced “nav” A gnav comprises a hinged strut fixed onto the mast above the boom with the other end bearing down on the boom via a slider in a track or a roller. The position of the strut end on the boom is adjusted with a control line. This system is used on 29ers, 49ers, International 14s as well as some modern cruising dinghies. As the whole mechanism is above the boom, it clears the cockpit completely. There are a number of points to consider regarding a gnav. 1. A gnav places loads on the mast and gooseneck that are very different to those imposed by a traditional kicking strap. Many classes with gnavs have lower shrouds or strengthened masts. Seldén voice some concerns that it may not be possible to retrospectively fit a gnav to a gunter rig or even a normal Bermuda mast. However trials in the UK using a normal gunter rig and a Selden Bermuda
mast did not encounter any such problems. I believe the angle and length of the strut plays a large part in determining the loads imposed. If the strut is more upright, the stresses will be more up and down the mast rather than trying to bend it. However, with a more upright strut, it will bear on the boom at a point that is closer to the gooseneck. This means it has less leverage. So to get the same amount of leech tension the forces from the strut bearing on the boom are higher. Using a longer strut would give more leverage for the same strut angle, but will disturb the sail more. 2. A gnav appears, if anything, to give a performance disadvantage for the following reasons: It disturbs the airflow over the mainsail. This effect is likely to be small as the top of the gnav strut is disturbing the flow near the mast that is turbulent anyway. Near the bottom of the gnav strut, the efficiency of the sail is low because of the tip vortex at the bottom of the sail. Between these two regions only a small part, if any, of the airflow is disturbed. It’s likely to give less leverage, meaning the crew will have to pull harder than on a normal kicker to get the same leech tension. It will weigh more. 3. You will need a well engineered track or roller allow the kicker to be adjusted without a lot of friction. This will increase the cost of these systems. One advantage of a system using a track rather than a roller is that it will distribute the loads over the length of the track rather than impose them at one point. Roller systems commonly employ a large “shoe” or “collar” to distribute the force on the boom over a larger area. 4. Calculating the mechanical advantage generated by a gnav is more complicated than with a traditional kicking strap. According to my calculations, the purchase of a gnav is a function of the interior angle, say of the strut to the boom and is given by the formula; Tan():1
gnav strut
S Y
boom
X
S’
This is calculated as follows; Consider a gnav strut at an angle to the boom as shown above. As the end of the gnav strut moves a small distance on a circular arc from the point
S to S’ it moves along the boom a distance X. At the same time the point on the boom S moves a distance Y. Since the strut is long compared to S - S’ then the angle of the strut to the line S – S’ is close to a right angle. Hence we can say the angle in the triangle whose sides are of length X and Y is also Tan = X/Y so X = YTan So to move the point S on the boom down by 1 unit, it is necessary to move the end of the strut along the boom by a distance 1 X Tan So if the angle is 45º, Tan 45º = 1 and the purchase is 1:1 If the angle is 63.4º, Tan 63.4º = 2 and the purchase is 2:1 If the angle is 75.9º, Tan 75.9º = 4 and the purchase is 4:1 If the angle is more than 75.9º then the strut is delivering more than 4:1 and would require a “reverse purchase” to keep it legal. Tan can be calculated for measurement purposes by; 1. Measuring the distance from the intersection of the centreline of the gnav strut with the aft edge of the mast to the intersection of the top of the boom (extended as necessary) to the aft edge of the mast. Call this distance A. 2. Measuring by the shortest distance from the point where the centreline of the strut intersects the top surface of the boom to the aft edge of the mast. Call this distance B. 3. Dividing A by B Another way of doing this calculation is to consider the forces acting on the car or shoe at the end of the strut. There are three forces acting on the car as shown below
Car Boom
S - force from strut. T - tension in line from car to any purchase. Forces on car R - reaction from boom
The car is in equlibrium, so the horizontal and vertical forces must be equal. This means; Horizontal Vertical T = S cos R = S sin
Mechanical advantage is T/R = S cos / S sin
T/R = cos / sin = tan Move the point of attachment. This method retains the existing kicking strap, but allows a point of attachment on the boom that is closer to the mast. This does not give as much space as a gnav, but is much cheaper, almost zero cost and easier to implement. There are a number of points to consider regarding allowing the attachment point to be closer to the mast. 1. The amount of leverage is reduced, meaning the crew will have to pull harder than on a normal kicker to get the same leech tension. So the loads on the boom when the kicker is and on the gooseneck will be higher. This probably won’t be a problem with an alloy boom, but could be for a wooden boom. I therefore propose to allow wooden booms to be reinforced at the gooseneck. 2. As far as I can see there is no race advantage in having the point of attachment closer to the mast. This is a classical example of a “self limiting” rule. The closer the kicker attachment is to the mast, the less leverage you have but the more space for the crew. The further it is away from the mast, the more leverage you get, but the less space for the crew. There is no “ideal” position, it’s a question of finding the right balance for your crew. I can’t see any problem with having two or more attachment points on a boom so it’s easy to switch from a big crew position to a small crew position and so on. Commentary on the actual rule change proposals Rule 1.4.14 With a centre mainsheet, new boats will look odd if the hawse arrangement is not symmetrical, as if it had been “cobbled together”. If we make the hole in the transom for the mainsheet optional, then new boats can use two eyes rather than a hole and an eye. I did consider allowing a second mainsheet hole so owners of old boats can drill another hole to get a symmetrical look, however, a second hole will be near the number engraved on the inside of the transom. I don’t want that obscured in any way, so old boat owners will only have the option of plugging their existing mainsheet hole if they want a symmetric look. Given that the drain holes are already optional on GRP boats, the logical way to proceed is to make all holes through the transom optional, hence the change of “Holes shall be through…..” to “Holes may be made through…..” and removal of “Drain holes may be omitted for GRP boats.”. Rule 1.7.6 I’ve moved the restriction on the distance between main sheet attachment points from 1.6.16 to a new rule 1.7.6 as it makes more sense in this section. I’ve renumbered the existing 1.7.6 to 1.7.7. Note that that this rule only states a minimum distance. I considered putting an upper limit on this, but, with fixed hawsers I can’t see that the distance between the attachment point has any impact on the leech tension as the main is centred. As the main is brought towards the centreline, there comes a point when both hawsers come under tension. At this point the block is on the centreline. As discussed, it’s the height of the block on the hawse that plays a crucial role in determining the amount of leech tension generated when centring the boom. However, if other systems were to be allowed (say the split tail), there would be an advantage having the attachment points further outboard and I think we
would need to impose a restriction. Rule 5.2.2 Rule 5.2.4 Allow the forward end of the wooden boom to be strengthened. This rule is deleted thus removing any restriction on the kicking strap attachment point.
Rule 5.5.20 The Bermudan mast rules use the ISAF approach of listing permitted fittings. We need to add the gnav strut fitting into the optional fitting list. Rule 6.2 We go into detail as to how the mainsheet is to be rigged to avoid anyone starting the centre mainsheet on the hawse and just going directly to the block on the end of the boom. This would give a purchase of 1.5:1 that would result in a race advantage. The limitation on cleating the mainsheet has been removed. I have been advised that cleating the mainsheet is no longer considered an unsafe practice and that it would help both some younger and older sailors. We state that the kicking strap can be of the reverse thrust type.