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Old 25-03-2020, 05:19   #1
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how to rig an inner forestay which MUST be parallel to main forestay?

My mast in down.... difficult to figure out where to position a mast bail with the double block which will feed the inner forestay and the sail halyard.

The position of that bail and double block is crucial for the two stays to be parallel.


My drawing shows "X" for distance from the deck attachment points....
What should be the "Y" from mast top to bail for the two stays being parallel?

Cannot figure it out.

Thanks
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Old 25-03-2020, 06:23   #2
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

Measure the mast from the point is crosses the shearline (may be virtual if deck stepped) to the point the existing forestay attaches This is "I"

Measure the distance from where the forestay attaches to the bow back to the leading edge of the mast.

The angle between at the sheerline equivalent formed by the mast and deck can be assumed to be a right angle - ignore mast rake at this stage but it will be important when determining the length of the inner forestay - physical.
You now have two sides and an included angle. Basic trig will calculate the other dimensions .and angles. There are free triangle solver apps available for smart phones.

Measure your "X" and subtract it from J. this gives J2. J-X = J2
Enter J2 into the app to determine I2.
You may need to delete I, J and calculated existing forestay length but leave the calculated included angles.

Take note of how much below the tang the halyard sheave is on the top most fitting and allow sufficient hang for whatever headsail attachment/ furling system you are using.

Measure the physical forestay length and compare it to the calculated length.

This will give you enough information about mast rake to get in the ball park but depending on setup - masthead, fractional, # spreaders, raked?, runners, checkstays etc the final solution will vary.

You will at least have enough information to determine if the rigger you are talking to is onboard with what you are trying to achieve.
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Old 25-03-2020, 07:26   #3
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

Question - what do you do first, locate the place for the tang on the mast or the chainplate location?
I had a cutter for many years and its incdredible how much the mast bends when only the staysial is on, and I had runners.
In a retrofit, I beleive the inner stay need to be countered by either running backstays or the existing lower shrouds, no?
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Old 25-03-2020, 08:10   #4
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

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In a retrofit, I beleive the inner stay need to be countered by either running backstays or the existing lower shrouds, no?
Yes!

Seems an overwrought modification to a Catalina27, but have fun with it...
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Old 25-03-2020, 08:17   #5
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

A set of spreaders is best location for I2
Loads are lateral as well as fore & aft.
Runners, checkstays and shrouds tend to coincide with the spreaders.

Welding a tang mid span even with engineering would not be considered best practice. Catastrophic out of column collapse aside forestay tension needs to be controlled / maintained as does mast bend to control draft in the main.

Setting a location on deck is secondary. A location that can be effectively reinforced, clear of other deck hardware and in the zone for parallel forestays may be hard to find unless the boat was designed with cutter rig as an option.

Parallel stays look good but so can a slight taper. The objective for sail efficiency is to maintain a slot to generate lift. Diverging stays (inner steeper - gap at bottom greater as is the chord of the triangle) works as well if not better than parallel at the cost of increased compression loads.
Most of the slot shaping is in the relative position of the draft along the chord and the cut of the leeches.
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Old 25-03-2020, 08:33   #6
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

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Originally Posted by Rucksta View Post
A set of spreaders is best location for I2
Loads are lateral as well as fore & aft.
Runners, checkstays and shrouds tend to coincide with the spreaders.

Welding a tang mid span even with engineering would not be considered best practice. Catastrophic out of column collapse aside forestay tension needs to be controlled / maintained as does mast bend to control draft in the main.

Setting a location on deck is secondary. A location that can be effectively reinforced, clear of other deck hardware and in the zone for parallel forestays may be hard to find unless the boat was designed with cutter rig as an option.

Parallel stays look good but so can a slight taper. The objective for sail efficiency is to maintain a slot to generate lift. Diverging stays (inner steeper - gap at bottom greater as is the chord of the triangle) works as well if not better than parallel at the cost of increased compression loads.
Most of the slot shaping is in the relative position of the draft along the chord and the cut of the leeches.
Very good observations.
I was thinking of adding a Solent stay, to be used with a storm sail. In this case, parallelism is not important... I think.
The mast tang would need to either envelop the mast or use a spacer inside the mast if using a thru bolt, and the chainplate needs a good ground above a bulkhead.
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Old 25-03-2020, 09:00   #7
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

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Originally Posted by SVTatia View Post
The mast tang would need to either envelop the mast or use a spacer inside the mast if using a thru bolt,
Another option is a riveted shoe like those often used to attach boom vangs.
This arrangements tends not to create stress risers when interacting with other loads.
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Old 25-03-2020, 09:33   #8
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

If the stays are parallel then the triangles are similar (in the geometric sense).
So J1/J2 = I1/I2 and J1/I1 = J2/I2.

J2 = J1 - X

I don’t care to do the rest of the derivation, too much like being in high school math again.
What are I1, J1 & X?
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Old 25-03-2020, 10:34   #9
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

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Originally Posted by SVTatia View Post
Very good observations.
I was thinking of adding a Solent stay, to be used with a storm sail. In this case, parallelism is not important... I think.
The mast tang would need to either envelop the mast or use a spacer inside the mast if using a thru bolt, and the chainplate needs a good ground above a bulkhead.

Why a spacer inside the mast? I added a non-parallel inner stay about 2' below the mast top. Drilled a hole through both sides of mast and ran a bolt through. No spacer needed, just snug the nut on that bolt up against the mast but not real tight, and use either two nuts tightened together with a lockwasher or a nyloc nut. Bolt runs through ends of two SS straps that meet together in front of the mast to take the inner stay pin. I think I bought that SS mast tang part as an assembly somewhere.

Mast has a rather strong cross-section and with only 2' separation below the backstay I have not noticed any mast bend when closehauled with staysail in a stiff breeze.
Running backstays are a big expense and a pain to use.

We use a detachable inner forestay with lever tightener. Wonder if you want to leave that inner forestay in place permanently?
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Old 25-03-2020, 10:58   #10
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

It's not critical that the headstays are parallel. Without running backs it may even be better that the inner forestay is closer to the masthead...
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Old 25-03-2020, 11:42   #11
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

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Originally Posted by ferrailleur View Post
My mast in down....

Thanks
Hello ferrailleur, Why MUST your inner stay be parallel? With a single spreader, your mast will not have proper lateral support with a parallel inner stay. You will also have to add running backs, which is a PITA to sail with. Why not run the inner stay higher near the masthead where the mast is supported better??
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Old 25-03-2020, 11:49   #12
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

Quote:
Originally Posted by ferrailleur View Post
My mast in down.... difficult to figure out where to position a mast bail with the double block which will feed the inner forestay and the sail halyard.

The position of that bail and double block is crucial for the two stays to be parallel.

My drawing shows "X" for distance from the deck attachment points....
What should be the "Y" from mast top to bail for the two stays being parallel?

Cannot figure it out.

Thanks
What cruising goal do you have that would require inner stay capability?

How are you going to determine your "X"? I would think the process of planning an inner stay on a sloop would be to consult a sail maker to determine the appropriately sized sail for the sailboat and conditions you might expect to encounter. Those measurements should determine where it would be placed on deck ("X") and the height along the mast (Y").

Have you considered the necessary deck reinforcement needed at "X"?

Good Luck.

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Old 25-03-2020, 12:38   #13
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

I'd be a ratio of the J measurement to the height from sheer to headstay attachment. Maybe it's 2:1 for examples. If 3' back from the bow go down the mast 6'.
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Old 25-03-2020, 13:14   #14
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

Firsdt thing is to make sure you have a good bulkhead and the chainplate is secure to the hull or you will just pull your deck off.
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Yes!

Seems an overwrought modification to a Catalina27, but have fun with it...
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Old 25-03-2020, 13:45   #15
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Re: how to rig an inner forestay which MUST be parallel to main forestay?

As others have said, the stays do not need to be parallel. My Carina is a cutter and they are no where near parallel. Other considerations are more important.

Adding an inner forestay will not make your boat a cutter, unless she started life as a "slutter" (a boat with the mast aft as a cutter but with only a single headsail). Before proceeding you should probably talk to a sailmaker or naval architect about it - just putting a staysail up may not work well. OTOH if the inner stay is intended for smaller, heavy weather jibs in lieu of the headsail then that might work well if not too far back from the jibstay. In that case adequate support of the staysail at the mast with aft intermediate stays or running backstays will be needed - the loads can be the same as the jib.

So please share: what is it you are trying to accomplish?

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