International Mirror Class Association of Tasmania.


Toll Tasmania: Supporting Tasmanian Mirror Sailors

 

 

 

 

COMMENTS ON UK MCA PROPOSALS FOR BERMUDA RIG AND ALLOY BOOM

This document contains comments that are explanations of the thinking or the experience that lies behind the proposed rules changes for the introduction of the Bermuda rig. Note that the Oxford English Dictionary defines “Bermuda rig” as a yachting rig with a high tapering mainsail. I have used the term “Bermuda”, rather than “Bermudan” throughout.

Part B Rule 5.2 Main Boom.

It is proposed to introduce aluminium alloy as alternative to wood to allow a low maintenance option and to overcome the existing problem of wooden booms splitting, usually at the gooseneck, in higher winds.

Rule 5.2.1 Overall length - no change. End-plugs on alloy booms are viewed in a similar way to fittings. As the length of the mainsail foot is limited, there is no speed advantage in using end plugs to make the boom longer.

Rule 5.2.2 Material - unchanged for wood. Allowing common marine grade aluminium alloy extrusion (90+% Aluminium). Integral tracks are permitted to allow manufacturers standard sections and kicking strap fittings to be used. Existing rule 7.6.4 stops the use of the integral tracks for the mainsail foot. The End-plugs could incorporate a hole for a standard gooseneck fitting or be cosmetic.

Rule 5.2.3 Section – unchanged for wood. Alloy booms will not need to be tapered for the gooseneck fitting. 53mm diameter limit is to allow for up to 50mm tubing (a standard size) to be used with a small manufacturing tolerance (corresponding to that on the gunter alloy mast).

Rule 5.2.4 Kicking strap attachment point – unchanged. Alloy booms may be equipped with manufacturers standard kicking strap fittings mounted on a track on the underside, hence need for wording “effective point of attachment”.

Rule 5.2.5 Rigging – no change to rule regarding fittings on the side of the boom. Need to define “side” for non-rectangular sections. Allowing internal rigging would look neater and cause less windage, but could give a performance increase and will increase cost.

Rule 5.2.6 Recessed track – only permitted on a wooden boom. However the alloy boom may have an i ntegral track (Rule 5.2.2) which could be used for the clew outhaul.

Part B Rule 5 Spars

It is proposed to introduce an aluminium alloy Bermuda rig as alternative to the existing gunter rig. This is to allow a low maintenance option (no wooden gaff). It should reduce the cost of new boats (wooden gaffs are difficult and hence expensive to manufacture). It should make the boat easier to rig (by eliminating the mainsail lacing & the need to pull the gaff tight up against the mast) and easier to land and recover in high winds (by allowing the mainsail and boom to be quickly dropped into the boat). The rules have been written to allow the use of standard spar manufacturer’s parts and thus to eliminate costs that would otherwise be incurred if custom fittings were required.

New paragraph after 5. SPARS Defines the rig configurations and only allows one rig configuration to be used in a regatta (similar to rule 7.6.2 applying to sails).

Rule 5.1 Change the name of this section to clarify that this is the mast to be used with the gunter rig option.

Rule 5.5.1 Since 5.5 is a whole new section, we can use ISAF Equipment Rules of Sailing (ERS) definitions throughout. The ERS is available from the ISAF web site http://www.sailing.org

Rule 5.5.3 The Trident “Weekender” mast is an aluminium alloy tubes plus plastic track.

Rule 5.5.4 Allows the option of a two piece mast for ease of transport, storage or shipping. The Trident “Weekender” mast is a two piece mast.

Rule 5.5.5 Tapering the spar or using a series of sleeved tube to change the cross section is allowed above the shroud rigging point (hounds). The Trident “Weekender” mast uses a smaller size tube above the hounds. Feedback from UK trials of untapered one-piece and “Weekender” masts has been that they lack gust response. Tapering the spar above the hounds increases cost, but should solve the gust response problem. It is important that the new rig is “well behaved” (like the existing gaff) in the upper wind range as the boat is often sailed by juniors and beginners.

Rule 5.5.6 Exact cross sections are optional within this overall limit. This should maximize the number of mast manufacturers who will be able to supply a suitable “off the shelf” section. It will also allow sailors of different weights to choose a section (stiffer, less stiff) to match their weight (in a similar way as can currently be done with the choice of timber and cross section of the gaff).

Rule 5.5.7 Corresponding to existing rules 5.1.8 (gunter mast) and 5.3.4 (gaff). Both existing rules allow a 15mm deviation from straight. As the Bermuda spar is longer, a deviation of 30mm over the length of the spar will give a similar overall limit on curvature to that on the gunter rig.

Rule 5.5.8 Given that tapered masts are allowed, in order to use manufacturers standard masthead fittings, it will be necessary to allow the main halyard to be internally rigged. Allowing other halyards and control lines to be internally rigged would look neater, but will increase costs.

Rule 5.5.9 To keep the rigging of the Bermuda mast similar to the gunter mast.

Rule 5.5.10 Movement in the mast step is limited as per existing rule 5.1.9 for gunter masts. The position of the middle of the Bermuda mast is controlled, rather than the position of the mast step and the position of the peg on the mast.

Rule 5.5.11 Most mast manufacturers heel fittings are not compatible with the standard Mirror mast step and many manufacturers mast steps will not meet the existing 12mm high requirement in rule 1.6.13. This definition of the mast datum point and taking all critical measurements from this point means the height of the mast step will not affect the height above the deck of the mainsail black band, shroud rigging points, forestay rigging points and spinnaker hoist height. Thus manufacturers will be able to use their standard heel fittings and mast steps. Note that the “top surface of the foredeck butt strap” already appears in rule 1.6.13.

Rule 5.5.12 Dimensions. Heights are measured to the mast datum point which is 100mm above the surface of the foredeck butt strap. Thus 100mm has to be added to the height dimensions to get the distance to the surface of the foredeck butt strap.

Mast limit mark width ISAF’s term for the width of the band on the spar. 16mm minimum is the same as the painted band on the gaff in exiting rule 5.3.5

Upper point height ISAF’s term for the distance from the mast datum point to the bottom edge of the painted band. This is 4757mm + 100mm = 4857mm above the foredeck butt strap. This is the same as the existing gunter rig distance of 3200mm from bottom end of gunter mast to bearing surface of sheeve plus (1733 –76)mm = 1657mm from gaff band or pin to the bottom edge of the painted band, giving a total of 4857mm.

Forestay height ISAF’s term for the distance from the mast datum point to the point where the line of the forestay (projected) meets the mast. The corresponding point is not precisely controlled on the gunter mast. It has to be less than the maximum overall length of the gunter mast which is 3296mm. The forestay has to be looped within 50mm of the top of the mast, but this does not limit how close to the top of the mast this can be. Typical gunter masts from Bell and Widebeam that I have looked at have the reduced section for the standard rigging loops extending about 40mm from the top. The figure of (3165mm + 100mm) = 3265mm above the foredeck butt strap comes from the UK proposal for a Bermuda rig from the mid 1990’s. It is 31mm below the top of a maximum length gunter mast. A typical gunter rigged boat will have a forestay height which is less than this so it seems a reasonable figure for a maximum height.

Shroud height See comments on Forestay height.

Spinnaker host height ISAF’s term for the distance from the mast datum point to the bearing surface of the spinnaker halyard pulley or lead. This is (3183mm + 100mm ) = 3283mm above the surface of the foredeck butt strap. This dimension is not precisely controlled on the gunter mast. A typical spinnaker crane (such as the Holt Allen-4184) has the bearing surface for the halyard 13mm below the top of the fitting which, in turn, would typically be level with the top of a gunter mast. On a maximum length gunter mast this would make the bearing surface 3283mm above the surface of the foredeck butt strap.

Spinnaker pole fitting projection Corresponding to existing rule 5.1.10 on the gunter mast.

Mast datum point to the top of the main boom spar This is (559mm + 100mm) = 659mm minimum and (579mm) + 100mm = 679mm maximum above the surface of the foredeck butt strap. This corresponds to rule 5.1.6 on the gunter mast. The word “edge” has been removed as the boom may not be rectangular in section.

There is no need to limit the overall length of the mast. Anything above the upper limit mark (measurement band) is unnecessary weight and windage.

Rule 6.1.1 makes a strop for the jib halyard mandatory. There are no limits on length of strop for gunter rigged boats so I’ve not introduced one here.

Part B other rules

Rule 1.6.13 Existing rule applying to gunter and Bermuda rig mast steps.

Rule 1.6.13.1 Existing rule now only applies to boats equipped with a gunter rig.

Rule 1.6.13.2 New rule for boats equipped with a Bermuda rig. No need to restrict height of the mast step since this is taken account of by the definition of the mast datum point. Optional design allows manufacturers standard parts or existing mast steps to be used (Trident Weekender uses a standard mast step). The centre of the mast spar at its lowest point is measured rather than the position of the mast step and the measurement corresponds to the position of the centre of the gunter mast step. The centre of the mast spar is found by measuring from the transom to the back surface, then measuring from the transom to the front surface and taking the average of the two measurements.

Rule 1.6.13.3 Existing rule applies to gunter and Bermuda rigged boats.

Rule 6.1.1 Minor change of wording “…protect the gunter mast…”.

Rule 6.1.2 Minor change of wording to make it clear the existing rule applies to gunter mast. Additional sentence covering the Bermuda rig allowing standard manufacturers fittings to be used.

Rule 7.2.1.9 No need for any luff lacing eyes if the mainsail is for a Bermuda rig so they are now optional. But, if fitted, there is still a maximum of 6.

Rule 7.6.3 Additional phrase covering position of top of the mainsail on the Bermuda rig (the upper point is ISAF’s term for bottom edge of the measurement band).