Shortening Mast - Calculations for Stays

Jan 15, 2014
79
Morgan 323 Portland
PROBLEM: 1979 Paceship 23. Irreparably bent mast during haul out. Bought a new-to-me [used] mast. New mast is shorter. Have practically new standing rigging which I want to re-use. Side stays will transfer, but need to shorten fore and back stays.

Old Mast: 27' 6"
New Mast: 27' 1"

QUESTION: What "formula" do I use to calculate how many inches to remove from the fore- and back- stays?

Thank you in advance for any help!
 

DougM

.
Jul 24, 2005
2,242
Beneteau 323 Manistee, MI
sides of a right triangle: distance from mast base to chain plate = A, distance from base of mast to stay attachment =B. Length of stay approximately =C. A squared+ B squared= C squared.
 
Nov 6, 2006
9,885
Hunter 34 Mandeville Louisiana
New stay length (in inches) will be equal to the square root of the square of the (old stay length in inches minus 3275. ) or NewStay squared = Old Stay squared - 3275.. so take PREECISE old stay length from point to point, square that, subtract 3275.. that is New Stay squared.. take the square root of that number and it will be the new stay length.. Another Injun-Ear should check me on that because it has been a while since Pythagoras and I have collaborated.. Even that number will be off by a very little (maybe a half inch) because I don't know the difference in height between the mast step and the top of the stay tangs. But it should be plenty close for this exercise and taken up in the turnbuckles..
 
Jan 15, 2014
79
Morgan 323 Portland
Thank you, while I'm a virtual math illiterate, I knew there had to be some astonishing mathematical process to address this. I'm very grateful and most impressed with your wizardry. I'll plug in the numbers and see what pops out.
 
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Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
The mast is already off the boat and it will light enough to move around.

Get some mason's twine. Secure one end to the forward chainplate take the other end to the mast step and mark it. The twine will not be horizontal, measure the angle with either a protractor or one of those carpenter tools that measures angles.

Do the same with the back stay.

Now, with the mast on the ground take the line from the front of the boat and stretch it out and put a stake in the ground or mark the pavement. Be sure to replicate the angle. Measure the distance from this mark to the mast tang. That will be your forestay length.

Do the same with the back stay.

Give these dimensions to the rigger. He'll compensate for turnbuckle length and fittings.
 
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LloydB

.
Jan 15, 2006
806
Macgregor 22 Silverton
I don't think that you can do it with any kind of formula unless you know what the original mast rake a was.You'll also need to know the true relative position of that bow and stern connection points actually on your boat.Sailboat data makes it look like both those connection points on your boat are not level with each other.You could however construct a template using the new mast with the existing fore and back stays and adding a temporary 5 inch extension to the new mast.Measure from the tabernacle as others have suggested using a tape or masons twine to define the distance from the tabernacle to each station on the boat.Attach your existing stays to top of mast and bring them to meet the apparent positions of the existing attachment points while flat on the ground.Now remove your temporary extension and slide the mast to the tabernacle position on the ground and measure the distance of the stays to find your new lengths.Having said that if I were in your shoes I would be tempted to buy a new Johnson-lever for one and extension bar for the other stay, then put the mast up sloppy then figure out(measure) where to cut the stays. Just keep the mast up by using the halyards while cutting/measuring each stay.
 
Nov 6, 2006
9,885
Hunter 34 Mandeville Louisiana
My "formula" is not exact but because the triangles are similar (or very close) the difference in the stay lengths are going to be very close.. plenty enough for this exercise.. sooo for giggles, I calculated a typical backstay for this arrangement using mast height only (and a typical distance from mast to backstay @ 90deg), then calculated again using mast base 1 foot above the stay tang(s) .. The difference in the stay lengths between the two cases is about 1/8", which is close to how much hull flex ya get when ya tension the stays up.. The stay itself is going to be considerably different, but using the original, measured precisely, will give a good useful number.. I don,t know a couple of things but it turns out that they'll mostly factor out..
 
Last edited:
Jan 15, 2014
79
Morgan 323 Portland
Very intriguing idea about building up the Mast step.... I'm going to give that serious consideration as it would save having to redo the lengths of my (nearly brand new) standing rigging.

Otherwise, I'll spend some time sorting through the formulas and other ideas. Thanks all!
 
Aug 3, 2012
2,542
Performance Cruising Telstar 28 302 Watkins Glen
Could you repair the old with a section of the new welded in? Then you would have the full length mast.
Otherwise, won't your sails need to be changed too?
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
I'm going to give that serious consideration
If your mast is deck stepped, I would not. Your challenges are great to secure it to the boat deck. If you have a keel stepped mast then it can be done. You would be better of getting a piece of mast extrusion and fitting it to the bottom of your mast with an internal sleeve.
 
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Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
Could you repair the old with a section of the new welded in? Then you would have the full length mast.
Otherwise, won't your sails need to be changed too?
A couple of factors here. The main may not be effected if the gooseneck is at the appropriate distance from the masthead. The boom will be lower by the amount the mast was shortened.

The headsail is a maybe. If it is a full hoist 150% genoa, it may be a problem as the foot of the sail will be too low. If the headsail is not full hoist, then probably not. With the genoa, an issue will be angle at the top of the mast. The space between the forestay and the mast could be too small to let the sail tack through the foretriangle without chafing or catching on the mast. If the genoa is a 135 or smaller, this probably won't be an issue.
 
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PaulK

.
Dec 1, 2009
1,223
Sabre 402 Southport, CT
Since the mast is stepped on deck, adding a 6" sleeve to the new spar is probably the best idea. Probably cheaper than adjusting all the shrouds, stays, and sails to fit a shorter mast in any case. If the bottom of the new (short) mast fits into the existing mast step, the sleeve could be added a bit above it. Otherwise it could be added to the bottom and made to fit into the step. K.I.S.S.!
 
Nov 20, 2021
9
Beneteau OC 400 Jews bay
New stay length (in inches) will be equal to the square root of the square of the (old stay length in inches minus 3275. ) or NewStay squared = Old Stay squared - 3275.. so take PREECISE old stay length from point to point, square that, subtract 3275.. that is New Stay squared.. take the square root of that number and it will be the new stay length.. Another Injun-Ear should check me on that because it has been a while since Pythagoras and I have collaborated.. Even that number will be off by a very little (maybe a half inch) because I don't know the difference in height between the mast step and the top of the stay tangs. But it should be plenty close for this exercise and taken up in the turnbuckles..
Please! Is there a good reference guide for "math challenged" sailors who are attempting something similar?
Specifically taking forestay w/roller length to calculate new mast height?
 
Nov 20, 2021
9
Beneteau OC 400 Jews bay
The mast is already off the boat and it will light enough to move around.

Get some mason's twine. Secure one end to the forward chainplate take the other end to the mast step and mark it. The twine will not be horizontal, measure the angle with either a protractor or one of those carpenter tools that measures angles.

Do the same with the back stay.

Now, with the mast on the ground take the line from the front of the boat and stretch it out and put a stake in the ground or mark the pavement. Be sure to replicate the angle. Measure the distance from this mark to the mast tang. That will be your forestay length.

Do the same with the back stay.

Give these dimensions to the rigger. He'll compensate for turnbuckle length and fittings.
Hello,
So you're saying lay it out on the ground do your calculations then cut.
I'm thinking take our job laser at dusk at check heights of forestay attachment, mast step, and aft stay attachments.
 
Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
Hello,
So you're saying lay it out on the ground do your calculations then cut.
I'm thinking take our job laser at dusk at check heights of forestay attachment, mast step, and aft stay attachments.
It is all trigonometry, if you get the measurements correct and the angles correct and you remember how to do the calculations from high school trig. The last part is the hard part. :confused:

Yes, lay it out on the ground or use a bunch of saw horses.
 

PaulK

.
Dec 1, 2009
1,223
Sabre 402 Southport, CT
"If you get the measurements correct and the angles correct... "
And if either one is off, the results will be too. Trig is lovely. When we get the angles and measurements correct it allows us to use a sextant to find our where we are on the globe within perhaps a mile of where we actually are.
Marcus is coming in with a new question about a new mast height, but it is unclear what he is actually looking for. The calculated height - if the angles are measured correctly - will be the distance from the end of the forestay to a perpendicular point on the mast. This point on the mast does not necessarily correspond to the deck line, the partners, or the mast step. So what good does it do him to know this? How do you determine the angle without having the mast stepped? What about rake in the mast? It would seem that Marcus needs to explain his question better and might be better served by starting a new thread.