A question for the mathmaticians

castaway

castaway

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My mast is raked back at the top by about 12 inches. Now I want to correct this to get it vertical.

Can anyone tell me how much I would have to take off the forestay (the bottle screw has run out of adjustment) to bring the masthead forward by 12ins.

The mast is 40ft and I will have to check this but the mast step is 13ft aft of the stem head fitting.

I am doing this as the mast is down at present and although it makes it easier for the job of shortening the forestay it doesnt make it easier for estimating how much to chop off.

Also what is the best way of cutting rigging wire? In the past I have done it by cutting a nut in half and placing it over the wire ( 1/2 either side) and clamping in the vice.... I found that this held the wire together whilst I cut with a hack saw, otherwise all the strands tend 'ping' off...Poss there are better ways ??

Thanks Nick
 
If your mast height and distance from stemhead are right your forestay needs to be 42.06 feet.
The square on the hypotenuse (forestay) is equal to the sum of the squares on the other two sides (mast and distance to stemhead). So the forestay length is equal to the square root of mast height squared plus stemhead distance squared.
 
The forestay needs to be sqrt(40 squared + 13 squared)

approx 42ft less the bottlescrew.

It's normal to have some mast rake however so I would get some advice from a rigger or designer.

Best way I found for holding and cutting rigging was a Black and Decker Workmate Tape over where you want to cut and cut through the tape.
 
[ QUOTE ]
how much I would have to take off the forestay

[/ QUOTE ] Based on the figures you have given approx 3.84 inches.
Plus presumably about half the total adjustment of the bottle screw
 
The previous posters seem to assume that the angle between the base of the mast, and the point of forestay attachment, is precisely a right angle. T'aint necessarily so. Unfortunately my maths is not up to the calculation, but I'm sure some member's will be.
 
We need a little more information to answer your question directly.

What height is the mast step above the forestay fitting? Are the 13 and 40 foot measurements exact?

But it raises a further question. Why do you want the mast upright? A previous owner may have found that the rake gives better lee/weather helm balance?
 
[ QUOTE ]
previous posters seem to assume that the angle between the base of the mast, and the point of forestay attachment, is precisely a right angle

[/ QUOTE ] Well not quite. I have assumed an I measurement of 40 and J measurement of 13.

If the I measurement is a bit more than 40 as it will be if the foot of the mast is higher than the stemhead the intermediate figures I calculated for the total forestay lengths will be different but the amount by which the forestay has to be shortened will not be vastly different.

For example if the foot of the mast is 2 ft above the stem head, making I= 42ft then the stay will require shortening by approx 3.67" plus half the bottle screw adjustment
 
I think it's the difference in the hypotenuse of a triangle 40' x 13' = 42.06'
& one of 40' x 14' [the extra foot of rake] = 42.38'
= difference 0.32' [aprrox 3.5"]
The error caused by height of step above fitting fairly negligable?
[Be easier to do in metres , I dont like doing decimals of feet]
 
I would make pretty sure that the mast is not meant to have that much rake before doing anything drastic. Have you contacted the owners association.
 
[ QUOTE ]
I think it's the difference in the hypotenuse of a triangle

[/ QUOTE ] Thats right. Thats how I worked it out.

No problem with decimals of feet. 0.32 x 12 = 3.84" or approx 1"27/32.
 
My mast is raked back at the top by about 12 inches. Now I want to correct this to get it vertical.

WHY? - is your boat a galleon?
 
You asked how much to cut off the forestay., presumably because it's difficult to measure.

First, you need to work out the angle between the deck & the mast. That's just simple right angle trig.

tan = opposite / adjacent. So the angle = arctan (1/40) +90 = 91.43 degrees.

Now you have 2 sides & and angle from a non-right angle triange so use the cosine rule to work out how long the third side is - the forestay.

That comes out at 42.37 feet.

Now you can work out how long it should be to get a right ange triangle using pythagorous theorum That comes out at 42.06 feet.

The difference is 0.31 feet or just under 4".

Still ask the question "why no rake" though.
 
Oh come on chaps, let's get some agreement here !

One of you is saying just under 4", another giving two different answers of 3.84" and 3.67", a third saying 3.5".

How is the poor chap supposed work out what to do ?!! /forums/images/graemlins/smile.gif
 
[ QUOTE ]
Oh come on chaps

[/ QUOTE ]

One saying 3.5". That's because he was unable to muliply 12" by 0.32 and arrive at the correct answer (Couldn't do decimals of feet! /forums/images/graemlins/confused.gif)

One saying just under 4" because he also could not do any better for 0.31 x 12"

Me giving two answers. One assuming I= 40 feet, the other estimating that it might be 42 feet.

All agree that it'll be between 3½" and 4", subject to the figures given being correct, except of course it will have to be a bit more than that to bring the bottle screw into the middle of its adjustment range.
 
Ah yes this was really what I wanted ... I'm happy with Pythagous, but wasn't sure if I should simply make the assumption that the angle between mast and deck was 90 deg ( which of course it isn't ) and allow a bit for 'luck' or whether more accurate calculations are needed.

For those who quite correctly raised the point about whether the mast should in fact carry that amout of rake... no I think not, It has just become that way over the last 40 years due to re rigging etc.... She also has a bit to much weather helm which is the reason for looking at this issue now.

Thanks to all for the replies.

Roll on Summer !

Nick
 
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