Just for everyone's clarity, mainesail (12 posts) is not the same as Maine Sail (moderator with 9,943 posts).
Why design a boat with prebend?
- Thread starter Daveinet
- Start date
We have an imposter in the house and he's not me... Don't know how that screen name got throguh the cracks.
Oops! Thanks for that correction, folks, and sorry I was fooled. But so far, I'm still with Jackdaw on this one! ;-)
I just went thru the process of re standing and tuning my B&R rigging after having dismasted the H28.5 for truck transport.
I read up on the process and found the details in the owner manual and what I found online .....vague.
I was interested in comments in the beginning of this thread -that prebend is for fractional rigs. Note- The H28.5 is not fractional, the forestay goes to the mast top plate.
I obtained a Loos tensioner from a friend. It had concave detents in the heel of the tool to gauge your wire size and which you match up the numbers on the stress gauge to the tool's stress chart/ matrix for each wire size.
Before stepping and before side shroud tensioning, I had to lay the mast on its front, run a tight string top plate to boom bracket to mark and guide tensioning while keep the mast straight down the mast center line. I also use the distance from line to mast to reference the prebend distance as I tension/tune the masts upper and lower diamond wires. Per my reasearch on this boat the bend calculated to 3.3 inches as I evenly tension the 3/16ths diamond wires. I let it set at about 2.75- 3" as I assumed some sag in the line.
Now to dealing with the side shrouds -
I assume the prebend I'd put in was done when I stepped the mast. I did not intend for the side shroud tension to compound prebend in the diamond wires tension, only to match it and snug the fore/aft and side to side of the entire mast. Am I correct???
Using the mainsail halyard as a guide, I made sure the mast was roughly centered with equal lengths of main halyard to the outboard gunnal.
I started with almost hand tight side shroud turnbuckles and used my Loos tensioner in increments of 2-3 turns to alternately tighten each side starting with the top shroud line, then the lower, then the thicker midish mast cable. The latter I tightened to approx same stress "on the matrix" which was a different number on the tension gauge than the other thinner cables. I assume I'm not add loading my initial prebend created by the mast's diamond wires, as I'm noting wire gauge (diamond/ side shroud and thicker mid mast ) and tightening to same "chart tension" load all around
I assume I may need to tweek a turn buckle once I'm asail and observe how much, if any , slack I see on a port and starboard tack.
My back stays are no more turnbuckle tight than they were before I dismasted and I believe they really should NOT be especially loaded as they seem to be mostly dedicated to aiding the stepping process and ( per my readings online) are not the primary means of pinning/securing the mast.
I could be wrong on some of this prebend and tensioning but I'd appreciate comments.
I read up on the process and found the details in the owner manual and what I found online .....vague.
I was interested in comments in the beginning of this thread -that prebend is for fractional rigs. Note- The H28.5 is not fractional, the forestay goes to the mast top plate.
I obtained a Loos tensioner from a friend. It had concave detents in the heel of the tool to gauge your wire size and which you match up the numbers on the stress gauge to the tool's stress chart/ matrix for each wire size.
Before stepping and before side shroud tensioning, I had to lay the mast on its front, run a tight string top plate to boom bracket to mark and guide tensioning while keep the mast straight down the mast center line. I also use the distance from line to mast to reference the prebend distance as I tension/tune the masts upper and lower diamond wires. Per my reasearch on this boat the bend calculated to 3.3 inches as I evenly tension the 3/16ths diamond wires. I let it set at about 2.75- 3" as I assumed some sag in the line.
Now to dealing with the side shrouds -
I assume the prebend I'd put in was done when I stepped the mast. I did not intend for the side shroud tension to compound prebend in the diamond wires tension, only to match it and snug the fore/aft and side to side of the entire mast. Am I correct???
Using the mainsail halyard as a guide, I made sure the mast was roughly centered with equal lengths of main halyard to the outboard gunnal.
I started with almost hand tight side shroud turnbuckles and used my Loos tensioner in increments of 2-3 turns to alternately tighten each side starting with the top shroud line, then the lower, then the thicker midish mast cable. The latter I tightened to approx same stress "on the matrix" which was a different number on the tension gauge than the other thinner cables. I assume I'm not add loading my initial prebend created by the mast's diamond wires, as I'm noting wire gauge (diamond/ side shroud and thicker mid mast ) and tightening to same "chart tension" load all around
I assume I may need to tweek a turn buckle once I'm asail and observe how much, if any , slack I see on a port and starboard tack.
My back stays are no more turnbuckle tight than they were before I dismasted and I believe they really should NOT be especially loaded as they seem to be mostly dedicated to aiding the stepping process and ( per my readings online) are not the primary means of pinning/securing the mast.
I could be wrong on some of this prebend and tensioning but I'd appreciate comments.
jtm, I may be off base here, but... On a B&R rig with a (adjustable) backstsay like yours on the H28.5, I've always assumed that the ONLY function of the backstay was to add prebend! Mind you, with a masttop rig like yours, the pull of the backstay is directly opposed by the pull of the forestay (& jib luff/halyard), so it shouldn't add much. First, it should tighten the forestay and the jib luff, eliminating any sag there.
Us, we've got the newer-fangled version of your boat, and they made it fractional rig and eliminated the backstay completely. And ours also has in-mast roller furling, so significant prebend would interfere with that.
But I do plan to check our mast tune in the Spring, so any light the gang here can shine on those vague instructions would be appreciated!
Us, we've got the newer-fangled version of your boat, and they made it fractional rig and eliminated the backstay completely. And ours also has in-mast roller furling, so significant prebend would interfere with that.
But I do plan to check our mast tune in the Spring, so any light the gang here can shine on those vague instructions would be appreciated!
Thanks normofthenorth,
If the prebend is accomplished as you seem to indicate by the back stay and perhaps the (B&R ) swept back and inboard side shrouds, Why is the prebend calculated and initially accomplished/induced by the laying down of the mast on its face and diamond wires tightened to achieve the needed bend? How would/could one properly tension the diamond wires post facto of masting
I should add that I found better information (vs the owner manual) on the Selden website ( thanks to a pointer from SBO/HOW member - warren Milberg. Also, somewhere there or in another forum, it was mentioned that the backstays on these boats (besides aiding masting process) will be/seem looser because the mast top angle of the backstay is much wider than that of the forestay.
If the prebend is accomplished as you seem to indicate by the back stay and perhaps the (B&R ) swept back and inboard side shrouds, Why is the prebend calculated and initially accomplished/induced by the laying down of the mast on its face and diamond wires tightened to achieve the needed bend? How would/could one properly tension the diamond wires post facto of masting
I should add that I found better information (vs the owner manual) on the Selden website ( thanks to a pointer from SBO/HOW member - warren Milberg. Also, somewhere there or in another forum, it was mentioned that the backstays on these boats (besides aiding masting process) will be/seem looser because the mast top angle of the backstay is much wider than that of the forestay.
The Hunter 28.5 is a bit of a weird duck; a masthead B&R rigged boat with a backstay! Not really sure what Hunter intended there; I bet no one alive really knows; but they did do this on several boats.
In general, pre-bend is added to a B&R rig statically with the reverse diagonals; indeed you often see B&R rigs before set on the boat with pre-bend already set into them. With the reverse diagonal going out the to the spreader tip, the spreaders+diagionals can act like 'diamond spreaders' to stabilize/bend the rig without a deck attachment.
There is nothing particular about the B&R rig that makes them 'backstay-less' per se; that is really a function of spreader sweep. With the close to the regular 20 degree sweep on the 28.5 I assume the backstay is there (like normal) to stabilize the rig. Normally a rig will have 28-30 degrees of sweep if the rig is supposed to be stable without a backstay, B&R or not.
In general, pre-bend is added to a B&R rig statically with the reverse diagonals; indeed you often see B&R rigs before set on the boat with pre-bend already set into them. With the reverse diagonal going out the to the spreader tip, the spreaders+diagionals can act like 'diamond spreaders' to stabilize/bend the rig without a deck attachment.
There is nothing particular about the B&R rig that makes them 'backstay-less' per se; that is really a function of spreader sweep. With the close to the regular 20 degree sweep on the 28.5 I assume the backstay is there (like normal) to stabilize the rig. Normally a rig will have 28-30 degrees of sweep if the rig is supposed to be stable without a backstay, B&R or not.
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Thanks for the input jack.
It is sort of a "belt and suspenders" approach to prebend by Hunter.
So you seem to feel my prebend approach ( first do the diamonds laying down, then do the similar stress tuning to the shrouds standing) was reasonable.. the Seldin PDF has tuning info for the several variations fractional, full, swept and B&R- albeit it's doesn't detail stress levels other than indicating 20% ish of max tension.
It is sort of a "belt and suspenders" approach to prebend by Hunter.
So you seem to feel my prebend approach ( first do the diamonds laying down, then do the similar stress tuning to the shrouds standing) was reasonable.. the Seldin PDF has tuning info for the several variations fractional, full, swept and B&R- albeit it's doesn't detail stress levels other than indicating 20% ish of max tension.
Jim,
Your approach is sound. I think B&R tuning is a bit of adjusting everything a little in the end... For sure your V1s should be near the 20% as well and not just hand tight. That might mean leaving some off the Ds and letting the Vs pull that on. At least thats what seems to me would happen! ;^)
Your approach is sound. I think B&R tuning is a bit of adjusting everything a little in the end... For sure your V1s should be near the 20% as well and not just hand tight. That might mean leaving some off the Ds and letting the Vs pull that on. At least thats what seems to me would happen! ;^)
Hi Jack
I have a Hunter 35.5 and purchased new sails last year from North sails. The boat was new to me and I just found out they built in 2.7 inches of luff curve into my main. Luff curve was not something I was thinking about when I placed the order. This mast is very flexible and I understand the purpose of pre bend to avoid an inversion. My question is, is 2.7 inches enough? I have read some owners of this model are using the depth of the mast which I think is around 6 inches, fore to aft.
I have questioned North about it but they haven't given me an answer as of yet.
From your experience do you think I have an issue?
Thanks Floyd
I have a Hunter 35.5 and purchased new sails last year from North sails. The boat was new to me and I just found out they built in 2.7 inches of luff curve into my main. Luff curve was not something I was thinking about when I placed the order. This mast is very flexible and I understand the purpose of pre bend to avoid an inversion. My question is, is 2.7 inches enough? I have read some owners of this model are using the depth of the mast which I think is around 6 inches, fore to aft.
I have questioned North about it but they haven't given me an answer as of yet.
From your experience do you think I have an issue?
Thanks Floyd
Floyd,
I'm sure you'll be fine. ANY curve in the mast will prevent inversion and add rigidity. 3 inches of static pre-bend in your mast will allow the new sail to sit as the designer intended. Because your boat has a backstay, adding backstay tension will bend the mast even farther, flattening the sail more and making it better on breezy days.
I'm sure you'll be fine. ANY curve in the mast will prevent inversion and add rigidity. 3 inches of static pre-bend in your mast will allow the new sail to sit as the designer intended. Because your boat has a backstay, adding backstay tension will bend the mast even farther, flattening the sail more and making it better on breezy days.
I was worried I was reading some old posts some by Alan He has the same boat and I believe he said he has 6" prebend and 14" rake
Anyway I guess I'll stick with it and set it up for the 3"
Thanks appreciate it
Anyway I guess I'll stick with it and set it up for the 3"
Thanks appreciate it
You're good. Rake, pre-bend, and bend are three separate things.
Rake efffects rig BALANCE, and helm feel. One-time setting for most boats.
Pre-bend creates tension in the rig to stabilize it; your sail is cut to match.Or vice-versa. Racers will adjust this daily, cruisers set and forget.
Bend is added dynamically to flatten the sail. racers again play this (via backstay) as much as their traveler, cruisers maybe once a day depending on conditions, or never.
Rake efffects rig BALANCE, and helm feel. One-time setting for most boats.
Pre-bend creates tension in the rig to stabilize it; your sail is cut to match.Or vice-versa. Racers will adjust this daily, cruisers set and forget.
Bend is added dynamically to flatten the sail. racers again play this (via backstay) as much as their traveler, cruisers maybe once a day depending on conditions, or never.
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Floyd- go to the selden (spell) website where they have a PDF detailing the approach for different rigs. There's a formula computed with the overall length of mast and top plate to boom attachment.
Iirc- in my situation
measure mast from boom attachment to top plate = x
X times .01 = amt of bow
Measure bow to be at "center" of mast
I got the bend induced by eaqually tensioning my diamond wires "alone" on a full aspect B&R rig for the h28.5
Iirc- in my situation
measure mast from boom attachment to top plate = x
X times .01 = amt of bow
Measure bow to be at "center" of mast
I got the bend induced by eaqually tensioning my diamond wires "alone" on a full aspect B&R rig for the h28.5
I have a Beneteau Oceanis 461 with a pre-bend mast. Even thought it is not a fractural rig ?!
This causes some problems with the in-mast furling of the main. I wish Beneteau would offer a straight mast for in-mast furling.
This causes some problems with the in-mast furling of the main. I wish Beneteau would offer a straight mast for in-mast furling.
Yep. Like a fract, it's done to stabilize the mast and prevent pumping.
Its also the reason you have a babystay.