- Status
- Not open for further replies.
P
Peter Albright
Mast length?
I have a 77H30. If you can tell me the length of your mast, I can compair it to my forestay.
I have a 77H30. If you can tell me the length of your mast, I can compair it to my forestay.
B
Bob Howie
Length of the forestay is basic trig
The solution to your question, if you are good in math and I'm not necessarily, is basic trig. A sailboat has four basic measurements in the sail/mast/stays area, P, I, E, J. In this case, J is the distance from the mast to the point where the forestay attaches to the deck. I is the distance from the deck to the forestay attachment point on the mast. According to specs I have on my 78h30, J = 12'10" and I = 40'6". Overall height of the mast is 45'6". I and J basically for the base and the 90 degree side of a right triangle with the forestay forming the slanted side of the triangle, or the hypotenuese. Here's the part I don't know; what's the formula for finding the hypotenuese (distance) of a right triangle? It's trigonometry at its best, but I never took trig! And, I be doggone if I can find it in any of my immediately handy reference texts. I'll keep looking.
The solution to your question, if you are good in math and I'm not necessarily, is basic trig. A sailboat has four basic measurements in the sail/mast/stays area, P, I, E, J. In this case, J is the distance from the mast to the point where the forestay attaches to the deck. I is the distance from the deck to the forestay attachment point on the mast. According to specs I have on my 78h30, J = 12'10" and I = 40'6". Overall height of the mast is 45'6". I and J basically for the base and the 90 degree side of a right triangle with the forestay forming the slanted side of the triangle, or the hypotenuese. Here's the part I don't know; what's the formula for finding the hypotenuese (distance) of a right triangle? It's trigonometry at its best, but I never took trig! And, I be doggone if I can find it in any of my immediately handy reference texts. I'll keep looking.
B
Bob Howie
Trig Formula Found
A smart cousin came to the rescue, so here’s the formula for determining the hypotenuse of a right triangle, aka, the length of your/our forestay. A2 x B2 = C2 where C2 is the length of the hypotenuse I’ve rounded off some of the numbers to give you a little fudge factor and as an amateur rigger, you want a little extra length which can be cinched up by turnbuckles or adjusted by hacksaws! 412 x 132 = C2 = 1,681 169 = 1,8502 = 1,850 ¸ its square root = 43.01, or the length in feet of the forestay based on the measures of I
A smart cousin came to the rescue, so here’s the formula for determining the hypotenuse of a right triangle, aka, the length of your/our forestay. A2 x B2 = C2 where C2 is the length of the hypotenuse I’ve rounded off some of the numbers to give you a little fudge factor and as an amateur rigger, you want a little extra length which can be cinched up by turnbuckles or adjusted by hacksaws! 412 x 132 = C2 = 1,681 169 = 1,8502 = 1,850 ¸ its square root = 43.01, or the length in feet of the forestay based on the measures of I
D
Doug T.
Careful: it is not a 90 degree angle!
Yes, "I" is the distance from deck to forestay fitting at masthead
Yes, "I" is the distance from deck to forestay fitting at masthead
D
Doug T.
Careful, continued
Hmmm. Looking back on my previous post, I see we never actually used the value of C except to compare it against our final answer. Oh well, it was instructional. A couple asides: Remember, the forestay length is the total length of the wire and all the fittings. When you buy/cut the wire, be sure to deduct the lengths of your fittings. Also, don't forget that when you tension the rig, the wire will stretch!!! I'm no expert in that regard -- I'm better at math than I am at rigging. I'd guess the stretch will be 1%-2%, depending on how tightly you tension the rig, but I'm not sure. With a wire that's 40 feet long, 1% is 0.4 feet -- almost 5 inches. That'll use up your turnbuckle real quick. Does anyone have data on this?? Now, what to do about mast rake??? I've made some drawings, but I'm still puzzling it out. This ain't gonna be simple folks...... PHIL, are you listening? If I get this worked out, can I submit the spreadsheet for posting here and let people download it?
Hmmm. Looking back on my previous post, I see we never actually used the value of C except to compare it against our final answer. Oh well, it was instructional. A couple asides: Remember, the forestay length is the total length of the wire and all the fittings. When you buy/cut the wire, be sure to deduct the lengths of your fittings. Also, don't forget that when you tension the rig, the wire will stretch!!! I'm no expert in that regard -- I'm better at math than I am at rigging. I'd guess the stretch will be 1%-2%, depending on how tightly you tension the rig, but I'm not sure. With a wire that's 40 feet long, 1% is 0.4 feet -- almost 5 inches. That'll use up your turnbuckle real quick. Does anyone have data on this?? Now, what to do about mast rake??? I've made some drawings, but I'm still puzzling it out. This ain't gonna be simple folks...... PHIL, are you listening? If I get this worked out, can I submit the spreadsheet for posting here and let people download it?
Why not have a rigger do this once!
Debra: Why not hire a rigger to be responsible for redoing your rigging. If you make ONE mistake and have to redo it you will have made up the cost for their labor. These guys do this for a living and can make suggestions on material too. PS: I do not want to undermine the astuteness of our math scolars!
Debra: Why not hire a rigger to be responsible for redoing your rigging. If you make ONE mistake and have to redo it you will have made up the cost for their labor. These guys do this for a living and can make suggestions on material too. PS: I do not want to undermine the astuteness of our math scolars!
H
Henry Weber
H30 forestay
Debra I've got a H30 1977. I do not think that the mast lengths are all the same. My measured lenghts are I = 40.2ft; J = 12.75. The max. hoist is 42. I replaced my rigging last year. Bought each length slightly longer (6") and then cut to lenght on the boatand used STA-LOCK fittings. Another variable is that almost all H30's have an area of rot and delamination either under the mast step or at the keel where the I-beam rots out. The result is that the mast and cabin roof have dropped 1 to 2 inches. If you need more info e-mail me. Henry
Debra I've got a H30 1977. I do not think that the mast lengths are all the same. My measured lenghts are I = 40.2ft; J = 12.75. The max. hoist is 42. I replaced my rigging last year. Bought each length slightly longer (6") and then cut to lenght on the boatand used STA-LOCK fittings. Another variable is that almost all H30's have an area of rot and delamination either under the mast step or at the keel where the I-beam rots out. The result is that the mast and cabin roof have dropped 1 to 2 inches. If you need more info e-mail me. Henry
D
Doug T.
Careful: continued w/mast rake calcs
OK (whew). Continuing with the example dimensions that I posted earlier... Pick the mast rake angle that you want and call it "A". I'm going to use 2 degrees for this example. (Warning: Make sure your calculator is set to work in degrees, not radians!!! Or convert, if you must: radians = degrees * Pi / 180) Calculate the distance that the forestay mast fitting will move aft because of the rake (R): R = sin(A)*I = sin(2) * 40.5 = 1.41343 feet Calculate the straight-line distance from the position of the unraked forestay mast fitting to its raked position(M): M = 2 * sin(A/2)*I = 2 * sin(2/2) * 40.5 = 1.413645 feet Calculate the distance that the mast forestay fitting moves DOWN because of the rake (L): L^2 = (M^2 - R^2) = 0.02467 feet Now calculate the length of the forestay in its raked position as the hypotenuse of a right triangle whose sides are I - L D (the vertical side) and R J (the horizontal side). Thus, (I-L D)^2 (R J)^2 = forestay^2 (40.5-.02467-.75)^2 (1.41343 12.833)^2 = forestay^2 Calculate that all out, take the square root and you have 43.6175 as the forestay length. Remember, the unraked length was 43.2, so yes, raking makes a difference. (To further illustrate, if you plug in an angle of 4 degrees instead of 2 degrees, you get a forestay length of 44.03 feet.) OK -- now subtract the lengths of all your fitting and that'll be the length of your STRETCHED wire. Deduct for stretching: I've seen one reference that says if you tension a wire to 10% of it's breaking strength (typical for standing rigging), it'll stretch by 1%. If you tension it to 20% of breaking strength, it'll stretch by 2%. I don't know if that's exactly true, but it seems reasonable. Anyone have any better data on that? I have all this in an Excel spreadsheet (including illustrations). E-mail me directly if you want it (dougtinva@hotmail.com)
OK (whew). Continuing with the example dimensions that I posted earlier... Pick the mast rake angle that you want and call it "A". I'm going to use 2 degrees for this example. (Warning: Make sure your calculator is set to work in degrees, not radians!!! Or convert, if you must: radians = degrees * Pi / 180) Calculate the distance that the forestay mast fitting will move aft because of the rake (R): R = sin(A)*I = sin(2) * 40.5 = 1.41343 feet Calculate the straight-line distance from the position of the unraked forestay mast fitting to its raked position(M): M = 2 * sin(A/2)*I = 2 * sin(2/2) * 40.5 = 1.413645 feet Calculate the distance that the mast forestay fitting moves DOWN because of the rake (L): L^2 = (M^2 - R^2) = 0.02467 feet Now calculate the length of the forestay in its raked position as the hypotenuse of a right triangle whose sides are I - L D (the vertical side) and R J (the horizontal side). Thus, (I-L D)^2 (R J)^2 = forestay^2 (40.5-.02467-.75)^2 (1.41343 12.833)^2 = forestay^2 Calculate that all out, take the square root and you have 43.6175 as the forestay length. Remember, the unraked length was 43.2, so yes, raking makes a difference. (To further illustrate, if you plug in an angle of 4 degrees instead of 2 degrees, you get a forestay length of 44.03 feet.) OK -- now subtract the lengths of all your fitting and that'll be the length of your STRETCHED wire. Deduct for stretching: I've seen one reference that says if you tension a wire to 10% of it's breaking strength (typical for standing rigging), it'll stretch by 1%. If you tension it to 20% of breaking strength, it'll stretch by 2%. I don't know if that's exactly true, but it seems reasonable. Anyone have any better data on that? I have all this in an Excel spreadsheet (including illustrations). E-mail me directly if you want it (dougtinva@hotmail.com)
B
Bob Howie
Tech Data on forestay strength and stretch
Y'know, guys, I think poor ol' Debra is starting to wish she never asked this question! And, all I can say is, well, I was pretty darn close to the length granting that maybe I wasn't dealing with a perfect right triangle. Great advice, too, on trimming to fit and using Sta-loks. Here's some tech data, Debra, on turnbuckles, wire size strengths and stretch. On turnbuckles, for standard, open-body bronze or ordinary stainless steel turnbuckles, the screw diamter should be twice the diameter of the rigging wire. The opening between the turnbuckle jaws should be twice the diameter of the rigging wire. The clevis pints should be twice the diameter of the rigging wire and always insert toggles between turnbuckle and chainplate and between mast tang and shroud eye. Type 302/304 commercial grade wirte wire should be used since Type 316 stainless steel wire, recommended for tropical use because of its increased resistance to corrosion, is approximately 15 percent weaker. This also uses 1x19 stainless steel wire as as the reference. Breaking strength of 5/16" wire is 12,500 pounds and 3/8" is 17,600 pounds. About stretch; two types, construction stretch in wire is permanent and results from the strands settling into place when the first load is applied. Elastic stretch is temporary, allowing the wire to return to its original length when the load is removed. The greater the number of strands in wire, the more elasticity it will have. The elastic stretch of a stainless steel wire increases in reough linear proportion to the load, up to about half the wire's breaking strength. Thus, stretch is a good indication of load. The following convenient metric formula will give acceptable results with 1x19 stainless steel wire: Load in Kilograms = (stretch in centimeters x breaking strength in kilograms) divided by total length of stretched wire in meters. (Hey! I'm sorry; they figured this in metrics and requires some conversion; I didn't invent this!!) Therefore, when a 10-meter (33 foot) wire of any thickness is loaded to half its breaking strength, it will stretch 5cm, or about 2 inches. Typically, a load of 25 percent of the breaking strength reprsents a moderate rig strain on average. So, in practical terms, a 50' length of 5/16" 1x19 stainless steel wire will stretch approximately 1.5 total inches. A 25 percent load in this case would be 3,125 pounds of load on the forestay. Henry was also very right about the deteriorating steel I-beam beneath your compression post and located in the bilge area. If you have this problem, let me know; I think I have a pretty good solution for it that I used on my boat. Steve also had a good idea; call a professional rigger and let him worry about the math!!
Y'know, guys, I think poor ol' Debra is starting to wish she never asked this question! And, all I can say is, well, I was pretty darn close to the length granting that maybe I wasn't dealing with a perfect right triangle. Great advice, too, on trimming to fit and using Sta-loks. Here's some tech data, Debra, on turnbuckles, wire size strengths and stretch. On turnbuckles, for standard, open-body bronze or ordinary stainless steel turnbuckles, the screw diamter should be twice the diameter of the rigging wire. The opening between the turnbuckle jaws should be twice the diameter of the rigging wire. The clevis pints should be twice the diameter of the rigging wire and always insert toggles between turnbuckle and chainplate and between mast tang and shroud eye. Type 302/304 commercial grade wirte wire should be used since Type 316 stainless steel wire, recommended for tropical use because of its increased resistance to corrosion, is approximately 15 percent weaker. This also uses 1x19 stainless steel wire as as the reference. Breaking strength of 5/16" wire is 12,500 pounds and 3/8" is 17,600 pounds. About stretch; two types, construction stretch in wire is permanent and results from the strands settling into place when the first load is applied. Elastic stretch is temporary, allowing the wire to return to its original length when the load is removed. The greater the number of strands in wire, the more elasticity it will have. The elastic stretch of a stainless steel wire increases in reough linear proportion to the load, up to about half the wire's breaking strength. Thus, stretch is a good indication of load. The following convenient metric formula will give acceptable results with 1x19 stainless steel wire: Load in Kilograms = (stretch in centimeters x breaking strength in kilograms) divided by total length of stretched wire in meters. (Hey! I'm sorry; they figured this in metrics and requires some conversion; I didn't invent this!!) Therefore, when a 10-meter (33 foot) wire of any thickness is loaded to half its breaking strength, it will stretch 5cm, or about 2 inches. Typically, a load of 25 percent of the breaking strength reprsents a moderate rig strain on average. So, in practical terms, a 50' length of 5/16" 1x19 stainless steel wire will stretch approximately 1.5 total inches. A 25 percent load in this case would be 3,125 pounds of load on the forestay. Henry was also very right about the deteriorating steel I-beam beneath your compression post and located in the bilge area. If you have this problem, let me know; I think I have a pretty good solution for it that I used on my boat. Steve also had a good idea; call a professional rigger and let him worry about the math!!
P
Peter Albright
DFebra, are you out there?
I have a 1977 H30. I have just replaced all the standing rigging. I upgraded one size from 1/4" to 9/32". My mast is still off the boat. If you can measure your mast, I can tell you the length you will need. I just need to compare your mast to mine. Any compression problems need to be fixed before you put the mast back on the boat. What about the rest of the rigging? Do you only need the forestay?
I have a 1977 H30. I have just replaced all the standing rigging. I upgraded one size from 1/4" to 9/32". My mast is still off the boat. If you can measure your mast, I can tell you the length you will need. I just need to compare your mast to mine. Any compression problems need to be fixed before you put the mast back on the boat. What about the rest of the rigging? Do you only need the forestay?
D
Doug T.
Thanks Bob
Thanks for the stretch info. By the way, the right triangle works fine if your cabin's about the same height as the forestay fitting at the bow. It goes awry when the cabin top is significantly higher.
Thanks for the stretch info. By the way, the right triangle works fine if your cabin's about the same height as the forestay fitting at the bow. It goes awry when the cabin top is significantly higher.
- Status
- Not open for further replies.