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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 04-24-2008
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Quote:
Originally Posted by KeelHaulin View Post
Again; nice writup Giulietta. I was wondering about the difference between rake and pre-bend. When you measure pre-bend is that in addition to the rake? So first you measure the rake and then you put additional pre-bend on the mast?

I also have a question regarding the wedging at the cabin roof for a keel stepped mast. "Spartite" was installed at the partners so I can't remove and replace wedges. Can I add rake and bend with the Spartite in place or would trying to rake the mast at this point only result in bending? The mast already has a factory taper and pre-bend IIRC; it's a tall rig, the I is 52'. Visually I don't see much rake; but there is bend above the upper spreaders.
Look here:



KH..the bend is mesasured by bringing the halyard you used to set the rake to near the mast foot, so you can measure it, by measuring the distance between the halyard and the mast, and this is obviously independent of the rake, since the halyard starts at the mast head and comes to the mast foot, and that happens even if the mast is lying down on the floor.

technically, the bend should be there independent of the rake, that is why you set the rake first and the bend after. Now the bend can be increased at a later stage by the backstay, if you have one.

As far as the mast thru deck seal..obviously it would be better if it was not there, as the thru deck is creating a "pivoting point" in the mast, thus affecting the ability to properly set rake and bend.

But if you can't remove it, "he that doesn't have a dog, can hunt with a cat", as we say in my country.

The rake can to some extent still be set but I doubt you can get better than 3 deg rake, as it will start bending, yes.. Give it a try..

If you knew how many people I know that complain about boats with excessive or lack of weatherhelm that are sailing boats with wrong rake, you would be surprized....

The tapper is to reduce weight aloft, make the top more flexible and make the mast loom good, and less "tree stump" on the top... the bend is done once on the boat, not at the factory..they ship them straight as a whistle..
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  #12  
Old 04-24-2008
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Quote:
Originally Posted by guilietta
The rule is simple each 1mm of stretch means 5% of the breaking load, and that is valid for ANY CABLE IN A SHROUD, no matter what the diameter is!!!


Careful... I did some basic calcs on this and yes; for an equivalent diameter and equivalent length the linear stretch is roughly proportional to tensile stress regardless of diameter. BUT if the smaller diameter shroud is smaller in length (which it should be) then the amount of pre-load you apply per mm of stretch is increased. That's because strain = change in length / original length.

Example:

Let's say you have a 50' mast with 7/16 upper shrouds. The calculations I did suggest that you would need 5.98 mm of stretch applied to them to get ~1600 lbs of pre-load.

In the same example; if the shrouds were 1/4" you would also need to stretch them 5.98mm to achieve 540 lbs of pre-load.

But; if you have a shorter mast (as you should) with 1/4" wire the stretch required to get 540lbs of pre-load will be less. If the mast is 30' tall the stretch required will be 3.6mm to get the same 540 lbs of pre-load. If it were tightened to 5.98mm it would have roughly 900 lbs of pre-load or 25% of break load.

So; while the rule-of-thumb is probably OK for a rough tune I would say that if you apply it to taller rigs it will result in shroud tensions that are on the loose side; and on shorter rigs it will result in shroud tensions that are a bit too tight.


Thanks for the additional info on rake VS bend. I understand it now; and now I'm not sure if the mast actually has rake or not. I'll do some measuring and adjusting according to your excellent procedures.
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Old 04-24-2008
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Ok but when and how do you adjust the baby stay?
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Old 04-24-2008
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Giu,

Sorry for getting your name wrong. This thread simply has to much info for me to appreciate all of it at one go.

I guess I was not clear enough when I asked for clarifications on the various terms. I have tried to upload a picture of the rig to clarify but for some reason Sailnet is not accepting the file.

So I will try with words.

The rig I am interested in has a twin spreader rig consisting of a wire A starting from the deck to the tip of the first spreader then to the tip of the second spreader and then on to the mast.

It also has a wire B starting from the same place as A going up at an angle to the mast at the lower spreader height.

There is a second wire C going up at an angle from the outer tip of the lower spreader to the mast at the height of the upper spreader.

I guess my question is: what are the names of wires A,B,C?

regards
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Old 04-24-2008
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Giulietta

Thank you for your hard work and effort. You saved me from starting a thread about how to check and adjust my rig's tension. The various stays are all over the place tension wise and I planned to re-tension around launch.

All these good deeds you do for us will hopefully earn you a nice bunch of beam reaches this season.

Thank you very much.

LH
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Old 04-24-2008
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Thanks Alex,


Great post.
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Old 04-25-2008
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xuraax,

Using Alex's examples under rig tension, 1) shroud tension,

A would be upper shroud,
B lower shroud
C intermediate shroud.

Alex,

Thank you for info, this is tomorrows list to do before saturdays race, or sunday if not time if found tomorrow friday. Got a loose guage the other day too!

Marty
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Old 04-25-2008
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thanks a lot Marty.

Rereading Alex's mail several times I was deducing the same thing but being a pessimist it feels good to get a confirmation from others.
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Old 04-25-2008
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Quote:
Originally Posted by KeelHaulin View Post
[/FONT]

Careful... I did some basic calcs on this and yes; for an equivalent diameter and equivalent length the linear stretch is roughly proportional to tensile stress regardless of diameter. BUT if the smaller diameter shroud is smaller in length (which it should be) then the amount of pre-load you apply per mm of stretch is increased. That's because strain = change in length / original length.

Example:

Let's say you have a 50' mast with 7/16 upper shrouds. The calculations I did suggest that you would need 5.98 mm of stretch applied to them to get ~1600 lbs of pre-load.

In the same example; if the shrouds were 1/4" you would also need to stretch them 5.98mm to achieve 540 lbs of pre-load.

But; if you have a shorter mast (as you should) with 1/4" wire the stretch required to get 540lbs of pre-load will be less. If the mast is 30' tall the stretch required will be 3.6mm to get the same 540 lbs of pre-load. If it were tightened to 5.98mm it would have roughly 900 lbs of pre-load or 25% of break load.

So; while the rule-of-thumb is probably OK for a rough tune I would say that if you apply it to taller rigs it will result in shroud tensions that are on the loose side; and on shorter rigs it will result in shroud tensions that are a bit too tight.


Thanks for the additional info on rake VS bend. I understand it now; and now I'm not sure if the mast actually has rake or not. I'll do some measuring and adjusting according to your excellent procedures.
KH..I don't know how you did your calculations, but I confess you are puzzling and confusing me a lot...

where did you get the numbers and are you sure about the values you are using for breaking loads?

Normally 7/16 cable which is around 11mm has a BL of 27.815 lbs, roughly (as my tables are metric)..and a stretch of 5.98mm as you suggest is 30% not 6%, as that is the stretch for the 1600lbs you are refering to.

a 1/4 cable that has a BL of 7054 lbs roughly, at 900lbs it is at 13%...
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Old 04-26-2008
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From the web, Young's Modulus for 1x19 wire rope is 107.5 kN/mm^2.

Using the table of breaking loads for different diameters of AISI-316 wire rope as given in the book "Principles of Yacht Design" I got the following table for wire stretch for a 2000mm wire loaded to 5% of breaking load:

diameter(mm)____breaking strength(kN)___delta L(mm)

3_______________7.7__________________1.01
4______________13.8__________________1.02
5______________21.6__________________1.02
6______________30.o__________________0.99
7______________40.9__________________0.99
8______________53.5__________________0.99
10_____________69.1__________________0.82
11_____________83.5__________________0.82
12____________120.2__________________0.99
14____________160.1__________________0.97

This practically confirms what Alex is saying.

regards
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