Saturday, March 30, 2013

Shorter forestay

When we removed the mast at the end of the season in 2010 I stupidly released the forestay under tension and it finished up in a tangled mess but a least no one was injured.

I replaced the forestay last year but noticed that the mast rake was too great when compared to boats Little Grace moored along side. There was also more weather helm than I remembered from the 2010 season.

I contacted the supplier of the forestay (North Sails Turku - very helpful) and we agreed that they would reduce the forestay by the minimum possible, which was 5cm. Last year the bottle screw was closed right down (see photo below) and it has a combined thread of 8 cm, so if the replacement forestay proved to be too small I would be able to compensate by extending the bottle screw and I won’t be caught out with an unusable forestay at the beginning of the year.

Of course, I should put the mast up and do some proper measurements but that is simply not possible at the moment and I don’t want to waste time I haven’t got at the beginning of the season sorting it out.

I asked those smarter than me on the Alacrity/Vivacity forum; how much will a reduction of 5 cm of forestay change the angle of the mast (which is 7m from deck to masthead)?

This was the helpful reply I received from one of the forum members after I gave the additional measurement of 1.9 m from the bottom of the forestay to the bottom of the mast (the J measurement).

 "By a crude process of working the angles backwards I estimate a change in length of 0.0546 m would result in an angle change of 0.7º."

That got me thinking. After using grid lines in Photoshop to check the horizontal on the coach roof in the photograph below I used a Breton Plotter (I couldn’t find the protractor) to calculate that the mast rake is between 2 and 2.5 degrees.

The circumference of a 7 meter (the height of the mast) radius is 43.982297150257104 divided by 360 = .122 (12.2 cm). x 2 degrees (to be conservative) = .244 (24.4 cm - 9 ¾ inches) rake at the masthead. The ideal is on an Alacrity is about 15cm (6 inches), apparently.

Using the calculation of – 0.7 º should mean (if I have got this right) .122 ÷ 10 x 7 = 0.085 (8.5 cm - 3 1/4 inches) leaving a mast rake of just under 7 inches?

A 8 cm reduction in forestay length would be 1 º and 12 cm (4 ¾ inches) at the masthead leaving a rake of 12.4 cm (5.5 inches). I can safely reduce the diffrence down to 4 cm by extending the bottle screw, i.e. .5º = 6 cm (2 ¼ ish inches) at the masthead. So if the current rake is 2.5 º I am covered, I hope. So I sent of the old forestay as the measure for a new one of 8 cm shorter. They have both arrived back, leaving me with the option of going back to the one we used last year if my calculations are all wrong.


On inspecting the turnbuckle for the forestay that came with the boat I found that the threads had seen better days. In addition it had been wearing the bow fitting as the clevis pin is 6mm and I measured the hole in the bow fitting to be 8.5 - 9 mm. Concerns about the threads on the original turnbuckle now that I may have to extend it to accommodate the new forestay and an article that discussed the issue of the wrong sized clevis pins damaging bow fittings in Practical Boat Owner prompted me to buy a Selden 5/16 turnbuckle. The clevis pins are 8 mm and the overall quality is better.

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