Discussion in 'Sailboats' started by jarcher, Mar 25, 2012.

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### jarcherJunior Member

Hi All...

I am trying to determine the best way to calculate the length of spreaders. If my basic understanding is correct, I need a spreader that:

Is strong enough to withstand compression.
Is long enough without being too long.

Strong enough I'll set aside for now.

Too short gets a mast to fall down, while too long could restrict sheeting an therefore reduce pointing ability.

A rigger told me that in a two spreader rig where the spreaders are inline, the lower spreaders are 85% the length of 1/2 the chain plate width while uppers are 65% of that width.

Well that's easy enough, but from reading, it seems that what I really need to achieve is that the shrouds are at an angle no less than 12 degrees from the mast.

So is spreader length primarily determined by what is needed to get the 12 degree (or whatever is desired) angle?

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### Perm StressSenior Member

Yes, spreader's main purpose is to increase angle between mast and respective shroud. Quoted percentages are good enough, if 10-12 degrees angle between mast and shroud is maintained.

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### michael pierzgaSenior Member

I assume you already have chainplates installed to the designers location.

The shorter you make the spreaders the more compression load will be transmitted to the spar. The more panels...spreaders....the shorter the spreaders may be.

Somewere on the internet is a handy rig calculator that allows you to observe the load changes by adding more spreaders or shorter, longer spreaders. Do some googling. perhaps with the words "mast compression"

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### jarcherJunior Member

Thanks for the replies! Yes the chain plates are where the designer and builder left them, each one 3.5 feet exactly from the center of the mast step. To calculate spreader length, I did:

3.5 * 12 *0.65 - (3.6 / 2) = 25.5

The 3.6 / 2 part takes away half the width of the mast itself.

I have not checked the shroud angle yet, I'll do that later tonight when I get home from my trip.

No luck finding that calculator yet, I'll keep at it.

Thanks!

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### michael pierzgaSenior Member

It sounds like you are trying to get a bit more upwind ability by shortening the spreaders and narrowing the headsail sheeting angle. . It works. Other modifications also let you sail higher.

Im just a sailor. On the boat my internet conn. does not allow me to surf, I have no access to books or software so I cant comment.

I suggest you post a sketch of your boat and its sail plan, with measurements, so that engineers like Perm Stress can run the numbers and make suggestions.

The " numbers" are simple , suggestions for sailplan optimisation are well known and many times easy to accomplish. .

6. ### Paul BPrevious Member

Any rigger who tells you this should be avoided.

Have a quick walk around any marina and look at the boats/rigs similar to yours. You will see that none of them have a rig with this sort of dimension. There is a reason for this.

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### jarcherJunior Member

Actually, I am in the process of replacing my mast and I am truing to figure out the length of the spreaders I should use. I could be ultra safe and make them very long at the expense of pointing ability, but as this is a racing boat, I am trying to avoid that.

Still, I agree a diagram is a good idea. I'll draw one and scan it.

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### jarcherJunior Member

Hi Paul, I actually have toured a few marinas just to do that, but as the boats are up on poppits, and even when they are in their slips, its hard to estimate the length of their spreaders vs the chain plate width.

So then is the proper way to get the spreader length to calculate the length that gives the desired angle between the shroud and the mast?

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### michael pierzgaSenior Member

Yes !!

But what is the desired angle ?

How much compression can your boat and mast section stand.

Post measurements and one of the Boat Design net engineers can rapidly run the numbers and make comment

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### jarcherJunior Member

Hm, which measurements are relevant? Each chain plate is 3.5 feet from the center of the mast step. The mast itself is deck stepped, and the section is a Kenyon 3656 and it will be just about 38 feet 3 inches long. The RM@30 deg is 13,500 foot pounds. I calculated the shroud loads at:

- upper and intermediate shrouds 4,629 pounds
- lowers 6,220 pounds.

Details of how I came up with that are here:

http://www.jims-scampi.com/drupal/node/353

The mast section details:

http://rigrite.com/Spars/Kenyon_Spars/3656-MORC.html

The section was broken and sleeved and riveted right about where I need to put the gooseneck. Paul B helped me with this project last fall with some numbers, and I read all about to get those in Brion Toss' book, which is fascinating.

I am thinking the needed measurements are the height of the spreader bar openings and the height of the T Ball openings where the shrouds attach. Yes?

11. ### Paul BPrevious Member

If you look at the boats from dead astern you might notice the old IOR boats and MORC types (masthead racing rigs) usually had a lower spreader a bit longer than the CPW/2. You might want to use a string plumb bob to see this. We're only talking a very small percent.

If it was my mast, using your previously given numbers of 38.8' "I" and a CPW/2 of 42" I might use a layout similar to the attached photo.

The design of rigs is not simple. It really helps to have learned the nuances from people who are really good at it. You can follow the suggestions in books, but often they are lacking in some important details.

The layout I did is not optimal, but given your CPW/2 it isn't bad. If you pull in the upper spreader an inch it would appear to be OK angle-wise, but the rig might be difficult to tune due to the imbalance of the poke on the spreaders.

Pulling in the lower spreader won't help the sheeting angle at all. If you sail with a 150% genoa you'll notice that when the sail is sheeted hard against the chainplates and it is touching the upper spreader you still have a lot of room between the sail and the lower spreader.

I suppose you could reduce both spreaders by a half inch or an inch and the poke would remain pretty balanced. However that might not help your sheeting ability, and given the rig you have is at the lower end of the moments it is best to have a bit of extra angle working for you.

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12. Joined: May 2011
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### jarcherJunior Member

Paul, this was VERY nice of you, thank you! I have a few questions, if you don't mind...

(wow, this is a long post - sorry!)

Very interesting, I never noticed or heard of that. Why did they do this?

That's good to know, I was wondering about that. The rig I have is a single spreader rig, and I don't know the length of the existing spreader. I know it is less then CPW/2, because the upper shroud is not vertical but rather leans in.

When the guys sheet the 155% (or the 140% for that matter) in against the chain plate, whether or not the top of the sail hits the spreader depends, of course, on the lead car setting. I can't say for sure that I can reduce twist enough with the exiting spreader as it is. Its very hard to look up and estimate twist other than by gauging distance from the spreader tip.

I'll measure Monday (I should be able to get down there) but its a good bet the existing spreaders are longer than the 28.5 inches you proposed here. So even if I can't sheet in more, I may gain from being able to reduce twist when conditions merit it.

But I thought, perhaps incorrectly, that the top of the head sail would be further inboard than its foot when heading upwind. I just have no idea how much further inboard.

Well I sure can agree with that premise. I know about software design and writing, and the same is true of that, in a big way. So I am very grateful for your help and the help of others!

Why? When you day "given your CWP/2" do you mean because it is so wide? Or is it due to something else, perhaps what I am forced into because the mast is at the edge of what is acceptable from an MoI standpoint?

What problems should I expect? Is it just a decrease in performance? That would suck, but I think it would still be better than what I have.

Okay, a few more thoughts/questions...

First, I am trying to avoid making more openings in the mast, and so I am trying to keep the slots for the two spreader bars where they are.

If I do that, the upper section would be 124 inches (reduced from the proposed 130 in your drawing). The middle section would be 142 (reduced from proposed 149) inches and the lower section would be 179 inches of spar plus another 7.5 inches of deck (the mast step is 7.5 inches above the chain plates) for a total of 186.5 inches (an additional 9.5 inches from proposed).

So the bottom section is much longer than you proposed, but that still leaves the shroud to mast angle at 12.69 degrees. In fact, I can go as long as another 11 inches before I get down to 12 degrees.

There is also one more potential difference, although i am not sure. The spreader bars I am planning to use are here:

All the way to the bottom of that page, model number K-11508. In the description it says 'Unit is 10" long x 2 1/4" wide, and has a 5-degree vertical angle.' I don't know if that means each spreader is 5 degrees elevated or each spreader is 2.5 degrees elevated. If 5 degrees, your numbers change noticeably. If not, then very little change.

Either way, if I use your proposed spreader lengths, then shortening the length of the middle and top section increases the angles a good bit. The top section would be 13.06 degrees (~.g degrees more). The middle section would be 17.04 degrees (~ .75 more).

Would that change your proposed lengths? I assume not, since you mentioned that the MORC masthead rigs have a spreader slightly longer than CPW/2, and that is what you had suggested.

At first I was disappointed when I saw how long the spreaders needed to be. I have one set, the lower ones, and they are only 35 inches long. But then I realized I could cut them down a few inches and use them as the upper spreaders, and order new lower spreaders. They are tapered, but the taper does not start until 17 inches, so I can shorten it by 6.5 inches.

RigRite is going to make the new set of spreaders from the Kenyon SP3 stock they have. The existing ones are SP3 as well. This is a very pricy solution, but the SP3 stock works with the spreader bars, which in turn work with the existing mast openings.

Another option is to use the lowers as lowers. The 35 inch length would create a shroud angle of 13.38 degrees, but would provide less support. And of course this would not be wider than the CPW/2, which you suggest. Then I would just order the new upper spreaders at 28.5. But now I have the imbalance you mentioned that will make the rig difficult to tune.

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### jarcherJunior Member

Oh Paul, one thing I forgot to ask. Are those lengths from the center of the mast, so that I should reduce them by 1/2 the width of the mast, or are those already adjusted for the width of the mast?

Thanks again...

14. ### Paul BPrevious Member

I don't think this is good practice. Of course we already know the existing rig is some sort of replacement that ws not optimal.

The leech profile is not a straight line. I'm attaching photo of a sail we were looking at a couple of years ago. You can see the curve of the leech. You can manipulate this curve with lead position and sheet, but it is always going to be there to some extent.

The spreaders of the single spreader rig should be a lot longer than 28.5 inches.

I have no idea what you are trying to say here. Yes, the head of the sail is further inboard, as the very top of the sail (at the halyard shackle) is on centerline.

Yes, because of the wider than optimal CPW. You can't use really narrow spreaders on a rig with a wide CPW.

I see what you are trying to do. The "I" of the E32 is 38.0. You are trying to use the rig as an "I" of 38.8. It appears you would like to add the whole 10" difference to the bottom panel.

I would not do that myself. I'm not the one writing the checks for this project.

If you move the upper spreader higher you effectively lengthen it, so it would have to be shorter.

I hope you realized you would not use a 35" spreader with a 42" CPW. Draw that up in a quick diagram.

I did make a small mistake on the layout I did yesterday. I didn't account for the cap shroud height being on the mast wall. I've fixed that, and now with a 29" upper spreader it is still under 12 degrees.

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15. ### Paul BPrevious Member

If you look at the bottom of my sketch you'll see the cpw/2 dimension is 42". That is to the CL.

I'm not sure where your leads are on this old production HT. I would expect the #1 to sheet at approx 10 degrees, maybe more. You can't really have tight sheeting angles with such a wide CPW. A full race boat of that era might sheet at 8 degrees, so require narrower CPW and spreaders.

I would also suggest you use MORC-style checkstays on this rig.

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