arithmetic for shorter spreaders...

[ALX357]

ok, this is not a completely new thread, but a new twist question on shortening spreaders - first i plan to keep my original 44" spreaders full length, as spares and for potential "restore", and order new ones for cutting to 36" . I DON'T want to cut and shorten my shrouds, and would like to raise the tangs on the mast a bit to take up the extra wire length produced by the shorter spreader. Have to factor a bit of increase in the length from chainplate up to spreader, which will offset a minimal amount of the height increase above the spreaders, after re-adjusting the spreader-to-stay intersection upwards. I would think it is safe to figure that the original setup has the outer shroud parallel to the mast, so shortening it would provide an imaginary right triangle to use in the calculation.
(note that my Genoa furler attachment "hounds" (?) are already raised up about a foot on the mast to gain visibility under the sail. I sail mainly with the Genoa alone anyway, strong topping lift and mainsheet pulled tight on the bare boom.
Furler drum housing is held in position by using the same OEM vernier adjusters that won't spin, at least not past the entry of the furler line. relocated the last block for the furler line to the bottom of the stanchion for a square-to-the-drum angle.) ... but i digress,
the QUESTION was ---- How much HIGHER should the attachment be for the outer stays if i shorten the spreaders from the original 44'" to 36" ?
I know all the parameters are there, can visualize the geometry - , attachment points, spreaders at a ? right angle to the mast, and it all has something to do with the Pythagorean (sp?) equation, and maybe T rigger-nometry but i'm kinda flunky with cypher'in, and cant remember how to make the numbers jump thru the hoops to come up with the right answer. ( Me Carpenter, you engineer ) Would like to get close enough to only need fine tuning a bit with the verniers at the chain plates. And hopefully they (the upper spreaders) will end up closer to, or even by chance in the same holes, where my heightened Genoa attaches.

[baldbaby2000]

Is this for an X? I think we need to know:

1. distance from the bottom of the mast to the upper tang attachment point
2. distance from the base (center) of the mast to the chainplate
3. distance along the shroud from the chainplate to the spreader
4. distance along shroud from the spreader to the upper tang

This is actually a little more info than is needed but it doesn't hurt to have redundancy.

It can be calculated or you could make a scale model with dowels and thread and just measure it and scale it back.

Of course this doesn't mean that what you end up with is structually sound or that the mast will bend in a good way when the wind comes up.

BB

[richandlori]

At the risk of sounding ignorant....why? What are you looking to gain? Is all this more truoble than it is worth?


Just a simple country boy inquary?

Rich

[dclark]

I was going to ask the same thing...why? I always thought it was a pain that you had to tuck them under the lifelines, but I finally got smart and just drop the life line first. I've heard it said that you can sheet the genny in further and consequentlly point higher, but the impact at the deck seems minimal and not enough to make a difference. As it is, if I pull my genny in as close as it will go, it's barely touching the shrouds and about as tight as it will go regardless. With that, Ive been able to point higher then people say this boat can. But that's not usually the problem. At that point, more time then not, it's excessive weather helm that wants to suck me which casues me to want to fall off some. I'll be raising the forestay tang about 1.5 inches sone to reduce some of the rake. I think that'll have a bigger impact then anything.

I also don't understand why you would go through all the trouble of shorter spreaders but try to keep the same shrouds. Why not just cut and reswag the ones you have?

The concern I would have (especially since you are the carpenter and not the engineer) is a structural one. But then again, I'm not an engineer either..where's Chip? The only thing I really know is that the whole purpose of the shrouds are to keep the mast from falling over. Common sense tells me that the wider the angle of the shrouds comming off the mast, the more stable it's going to be. So I would think that a shorter spreader (and smaller angle) would reduce stability. To compensate for it, wouldn't you have to increase the tension? If so, then that's the scary part. I don't know how much tension the Mac deck can handle. In any case if you'd like to try, go for it and let us know. But you might as well cut the spreaders and the shrouds. If you'r experiment fails, you'll probably have bigger problems then just putting back the old stuff.

[jmdefino]

The purpose of the spreaders are to decrease the compression force imposed by the shrouds on the mast by increasing the angle of the shrouds to the masthead. Decreasing the spreader length will decrease the angle of the shrouds at the masthead and will increase the compression load.

[ALX357]

The reasons for shortening spreaders : My Genoa hits the shrouds there long before it is sheeted all the way in, and can't point as high therefore. Also the shorter shrouds would also be easier to deal with on lowering the mast. Elsewhere on this site, the Sailing-Tuning section, it is mentioned numerous times by Mac owners who have shortened the spreaders several years ago with no ill effects, so i thought it would not be a problem structurally. Since i have already remounted my Genoa Furler higher by drilling the mast and relocating the bolt, (and i have no re-swaging tools nor experience, and that would be unreversable for those wires,) i know it is easier to raise the upper attach point of the shrouds than cutting and re-swaging them. Good point about the theoretical and possibly actual mast-to-deck compression difference: Note that the compression post inside the cabin, and next the "keel" bears most of the compression forces. Would it be a problem, though ?

[Moe]

Besides increasing heeling, I see two problems with raising the headsail that appears to me to be causing the sail/spreader interference problem. One is that it puts a wider part of the sail at the spreader level, the other is that, without raising the genoa cars an equal amount, it's pulling down more on the leech, which will pull it in at the head, again putting it closer to the spreaders. Increasing this sheet angle also reduces your ability to tighten the foot of the sail, and possibly could impact your ability to adjust it to spill air at the top. Finally, if you do shorten the spreaders and haul the genoa in tighter, you may find that with the tighter leech, it just backwinds the mainsail. At least that's what I see, in addition to the increase in mast compression.

--
Moe

[ALX357]

Moe:
i agree with the observation that a wider part of the Genoa is now higher and on the spreaders, part of the reason i want to shorten them.
I will be mostly sailing without the mainsail at all, as i said.
the sheeting angle doesn't yet seem a problem to me, but not being able to see forward on the leeward side of the boat was a bigger problem until i raised the Genoa. Elsewhere in the Performance and Tuning section, other Mac X owners that have cut their spreaders to 36" have not has any structural problems, and have been advised by their professional riggers that there was no problem with shortening the spreaders that much. Compression did not seem to be a problem there. Just going where others have gone before. I actually do value all advice i get on here, and weigh pro's and con's, but the question i was asking was "how much higher would the attachment point be if the shrouds remained the same? " Living here in Middle Tennessee, numerous lakes and rivers with dams and areas to sail, the wind isn't like coastal areas. Nor is the problem of boat pitching and swells as severe a strain on the rig. What i want to do would be easily and quickly reversible if i wanted to use the boat in maximum stress conditions, or with the mainsail most of the time. But i really enjoyed the Hunter 22 i had using almost always just the Genoa, and want to sail the Mac X the same way. It really makes for a lot more managable cruise especially with guests, and is lots safer for night sailing.
While the stock rig is fine or preferable for conventional Main and Fore setup, i want to maximize the convenience and ease of using the Genoa alone most of the time. Note that i have been sailing since i was 10 years old, now 53, learned on Lightning and Scows wooden boats, and have had plenty of white-knuckle sailing time. If i want to cross water quickly i will use the motor, and if sailing fast was primarily my goal, i would not have bought the Mac. Lakes here being relatively small compared to the Coastal areas etc. there is no advantage to sailing fast.
AND remember also my "mod" would be quickly reversable.

[Moe]

With the caveat I'm doing this before my first cup of coffee...

To find the new cable length from the spreader tip to the chainplate, measure that distance now (in inches), then square it, add it to 64 (the square of the change in spreader length), then take the square root of that. There may be enough adjustment in the adjuster to take care of that.

If your cable length from spreader tip up to the mast is 12'0" (144") as the owner's manual specifies, the length up the mast should change from the square root of (144 squared minus 44 squared) = 137.1" to the square root of (144 squared minus 36" squared) = 139.4". Just move the tang up until the new spreader is just being pulled upward in the slack in its socket, and mark it there.

Don't loosen the spreader tip and move the cable in it. Leave the existing bend in the cable where it is now.

--
Moe

[ALX357]

Moe,
hey thanks for lining me up on the math, ... i see the advantages in not moving the cable bend point at the spreader tip;
avoid having a permanent kink in the wire & not have any complications on the math, and apparently without much difference in length below the spreader. Not much difference in the rig geometry overall, 2 + something inches up, for the gain of 8 inches less spreader length, might even be able to get between the lifelines without dropping them. thanks again.

[dclark]

Raising the forestay tang an inch or two is just an easy way to reduce mast rake without replacing the forestay itself. I don't think it's nearly enough to add any additional heel.

Swagging only requires a swagging tool (which aren't cheap, but not terible expensive), a wrench, and no real experience (practice twice and you are anexpert). They are nice to have for the occassional use, but if you don't want to buy one, try the rigging section of west Marine and you'll most likely find a few you can use right there. Last I check, if you are a member (no charge) and leave a drivers license, you can take it with you for a few hours.

There are a few things on the Mac that make me a little nervous to mess with. The load on the deck is one of them. If I remember right, the rule of thumb for tuning is to reach a tension of 10-15% of the break strength of the cable. Last time I tuned mine using a loos gauge I think I stopped at about 8%. At that point it was tighter then any MacGregor rig I have checked and I wasn't feeling real confident in adding any more. I seriously doubt the deck could handle anywhere close to 15%. With a shorter spreader and an increased load, I don't think I would have gone that far.

Perhaps I'm completely wrong, but it seems this mod is an awful lot of work and ris to accomplish very little.of work and risk. I mean how much is it really going to move the shrouds in down where the sheets are? It doesn't seem like enough to make a noticible differance.

[ALX357]

well i guess my perspective on "alot of work " is different .... after having fabricated a mast cap, stuffed my mast with a conduit, wire and foam, re-wired my steaming light with diodes, installed hangars in the portside rear berth to stow the boom and poles, chased down leaks in the anchor locker drain, (had to reglass up there from the V-berth, a real PITA,) and changed the backstay from the starboard side to the portside, so i could re-hinge the helm seat on the portside to get it out of the way for boarding thru the transom, the process of merely cutting off the ends of the spreaders and re-attaching the tips, and drillinjg two holes for moving a bolt to a slightly higher location on the mast is really minimal work.
The sail is hitting the spreaders before the sheets run into the shroud, and if i can pull the sail in 8 inches more there, its a lot to gain.
Moreover, if the spreaders are 8" shorter over the cabin top when the mast is being lowered or raised, thats alot less interference to watch out for. Risk is minimal if the other sailors who have done it saw no ill effects on structural strength, (its only 2 inches higher on the mast) and the rig can be returned to stock configuration as easily as replacing the two spreaders and moving a bolt back to the original hole. Permanently altering the length of shrouds and messing with a swaging process i have no experience with seems alot more risky.
Do check the Performance and Tuning section on this very same Discussion Board to view the shorten-the-spreaders thread. And at the same time, thank you for the safety concerns, it's always wise to pause and listen to experienced advice. The same sort of care is taken by most of the Mac owners who have bigger than 50hp motors, 'though the margin of design strength for a motor 20% more powerful, (60hp) doesn't seem to alarm.
I was amazed at the owner who pulled his temporary step pin out, to put in the oem bolt when the mast was up, but some even thought that was not too risky. !!

[Chip Hindes]

Despite the nearly universal agrrement that what you're proposing is not a good idea, you seem bound and determined to go through with it.

Let me submit one more negative vote.

You have raised the genny one foot. That increases bending loads on the mast by some amount.

You want to shorten the spreaders by 8". Given equal spreader tension (after versus before) you will have decreased bending stiffness by some amount. Or, as John DeFino said, for equal rig stiffness you'll have to increase rig tension by a significant amount.

Your first mod causes higher loads. The second, proposed mod combines either less stiffness, or still higher loads. Do you see where this is going?

Others have shortened their spreaders without incident so far. IMO, it's a bad idea, but even if not, it could never be construed as a good idea. To my knowledge you're the first who has ever compounded the problem by raising the genny.

TINSTAAFL. You don't get something for nothing. By raising the genny and shortening the spreaders, you have compromised the strength of your rig and made it more likely to fail. Twice. About this point there is no maybe.

I don't pretend to know how to design boat rigging. Though as some are fond of pointing out, he's not a marine engineer, Roger has some significant experience. He's got well over 5,000 X boats out there with the rig he designed. Despite comments from many who think it's marginal, surprisingly few of them have come down. It's adequate.

Now for the sake of argument, let's try to find some contrarian who thinks the X rig is overdesigned. Anybody? Anybody? We might wait a long time to find such.

Yet by doing what you're doing, that's exactly what you're saying.

[ALX357]

will think about it some more... haven't shortened spreaders yet, ...i have raised the Genny with no apparent problems but there are load differences as you say.
btw, the bending loads of the higher Genny would be alleviated somewhat by moving the shrouds' attachment point closer up to the Genny, at least by some amount. And all engineered machines have margins of safety built into them; if everything was only just strong enough to be servicable, then those 5000 Macs would be falling apart all the time, instead of rarely. Now i am out of my league when talking to engineers about specifics of structural strength, but apparentlty there is no exact way to measure and evaluate the exact necessary strength that will apply for every rig. Good point about the compounding effect of the two mods however.

[Moe]

...let's try to find some contrarian who thinks the X rig is overdesigned. Anybody? Anybody?

I believe if over the years, Roger ever had even the slightest inkling the X rig was, in any way, overdesigned, he'd have downsized anything he could to save a penny or two per boat... and then marketed it as a dramatic reduction in weight aloft.

You gotta believe Roger considers it just adequate, as it is, if he didn't do that.

--
Moe