Measurement of chainplate location

[lenfly]

As the full implications of the recent rule changes emerge, this post aims to generate discussion on the chainplate measurement proposal. In my initial response to the proposals I questioned the chainplate measurement but this appears to have gone un-noticed.

As I understand the intent of the rule changes was to provide a new way of measuring and not intended for the rule changes to “change” the sabre in any way. The proposal to change the way the chainplate location is measured will allow chainplates to be moved further forward and will disadvantage existing boats.

It is generally accepted that reducing the distance between the mast step and the chainplates allows the boom to be squared further on a run and therefore increases performance. As masts have moved back, there is also a benefit in keeping the centreboard case as far back as possible. Therefore, under the current rules, there is a trade-off between the aft place centreboard case and forward placed chainplates as they are linked by measurements to the centreboard case bulkhead (CCB).

The proposal to measure the centreboard case, CCB and chainplates from the transom will unlink this trade-off allowing the optimum centreboard case (further aft), chainplate (further forward) and mast step locations to be achieved.

I have three boats and would be able to move the chainplates on all three by up to 9mm. Under the current rules they are as far forward as possible.

The chainplates should be 28mm in front of the CCB. This is half the width of the chainplate block plus 4mm ply. I have confirmed this with the previous national measurer.

There is no other possible interpretation of the location of the chainplates in the Building Instructions. Diagram 2E shows this and the top-right photo on page 29 also shows the chainplate block. There is no other way of orientating the nominated fittings on the chain plate block so that the fastening holes actually align with the timber. The RF488 is 127mm long and the block is only 48mm wide. If the chainplate was located along the gunwale, there would be flat section in the gunwale unless it was also shaped to fit. The building instructions do not mention shaping. The instructions also say that the chainplate is to be fixed to the centre of the chainplate block. If they were intended to be located along the gunwale, the chainplate should be 79mm in front of the CCB.

The rules say at Section B 3.2 that the FRP Sabre must emulate, as nearly as possible…. a fully complying plywood boat. There is no reason that an FRP boat can’t emulate a ply boat in regard to the chainplates.
Has a boat ever been built with the chainplate block running along the gunwale?

I understand that many measurements have been taken to set the proposed measurement. I hope that as well as a measurement from the transom, a measurement from the CCB to chainplate was also taken. It would be useful to report these measurements to the membership so that everyone can understand how the rule as derived. For example, is it set to measure in all boats checked? What are the maximum, minimum and average observations? And, dare I say, has the standard deviation been calculated to identify any outliers for investigation?

A small sample of 3 boats with chainplates as far forward as previously allowed could be move by up to 9mm. This benefit is magnified at the end of the boom.

I object to the possibility of having to drill more holes and weaken my gunwale to move my chainplates forward to achieve the same benefit of new boats or existing boats that may have been allowed to get away with forward chainplates. This is the effect that the change in the rule will have. Any boat that has intentionally moved their chainplates forward should be made to correct them.

Therefore, I recommend that this part of the rule change be reversed so that the location and measurement of the centreboard case, CCB and chainplates remain linked. This is due to the new rule allowing a change to where chainplates can be located. An assessment should be made of how many boats have been built with chainplates too far forward and then a decision made how to deal with them.

What do others think of this?

John Campbell
1935
1842
1668

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