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Double Spreader Mod

A forum for discussing topics relating to MacGregor Powersailor Sailboats

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Re: Double Spreader Mod

Postby BOAT » Mon Feb 12, 2018 9:39 am

NiceAft wrote:John said:
I think it would b sufficient if u r just using it for a jib or storm jib for heading up-wind

If that is the case, why install a backstay at all? :macm: ‘s sailing configuration works fine.

I’m having a difficult time understanding why anyone believes the :macm: mast needs a backstay for conventional sails (Main, Jib, Genoa, etc.). I am not aware of any reports of a mast coming down for other than a forestay failure.

Ray


The M does not need a backstay in any normally rigged situation - the numbers on the M rig on paper claimed the boat would break before the rig would but even Roger himself did not believe it so he had the first M boats built with backstays and after numerous design changes over time the numbers have been proven by the factory and users that the rig is strong enough to pitch pole the boat. That's almost TOO strong - that means if you held the boat in one place in the water and pulled on the mast forward it would actually bury the bow of the boat into the water and flip the boat before it would break the rig.

Mastreb proved it to himself by trying to take a marker buoy into port with him by hooking it with a mast spreader. The boat stopped dead like as if it hit a concrete wall and nearly pulled the boat under water. He bent the mast and a spreader.

The boat is just too light to break the wires - there is enough wire there to lift the entire boat right out of the water. You don't need any more wires.

Highlander is stacking headsails on his mast so he is attaching a lot of horsepower up higher than the stock jib hound. He has stacked two more working sails up even higher on the mast so the top of his mast is really doing a lot of work that a normal M boat mast does not do. In his case the backstay is to keep him from bending the top of the mast and messing up the rearward bend that the mast is supposed to have for a good mainsail angle. In his case the backstay makes sense.
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Re: Double Spreader Mod

Postby Ixneigh » Mon Feb 12, 2018 4:44 pm

The main reason I want the extra stays is the same reason I sail with the rudders up. It says right in the manual not to do It. But it suits my purpose. Ill be putting a lot more sailing miles on the rig, and in conditions no one recommends going out in, than Roger probably had in mind. The last few days were a wakeup call. The M doesn't come with mast winches, but I sure would like some. Tired of struggling to get that perfect luff that's not really important on a nice day sail, but is when you're looking at 70 miles upwind.
She doesn't come with 2 or 3 reefpoints. Fine for the way many owners sail. But if you want to maximize the sailing part of the boat, Its an important upgrade. The running back stays may certainly be overkill.
Re the angle. The angle is marginal. But using the cabintop winches I am certain I can set that up quite snugly. Even if it discourages mast pump in cruddy conditions it will be worth it.

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Re: Double Spreader Mod

Postby Highlander » Mon Feb 12, 2018 9:16 pm

Ixneigh wrote:The main reason I want the extra stays is the same reason I sail with the rudders up. It says right in the manual not to do It. But it suits my purpose. Ill be putting a lot more sailing miles on the rig, and in conditions no one recommends going out in, than Roger probably had in mind. The last few days were a wakeup call. The M doesn't come with mast winches, but I sure would like some. Tired of struggling to get that perfect luff that's not really important on a nice day sail, but is when you're looking at 70 miles upwind.
She doesn't come with 2 or 3 reefpoints. Fine for the way many owners sail. But if you want to maximize the sailing part of the boat, Its an important upgrade. The running back stays may certainly be overkill.
Re the angle. The angle is marginal. But using the cabintop winches I am certain I can set that up quite snugly. Even if it discourages mast pump in cruddy conditions it will be worth it.

Ix


I totally agree with ur safety mentality , tho most Mac sailors may never go through ur sea bearing experiences . If u feel that running back stays could b a life saver in an over extended situation then go for it , better safe than sorry !
But remember with all the safety aspects put into place it must be matched with equal skilled seamanship !

J 8)
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Re: Double Spreader Mod

Postby Signaleer » Mon Feb 12, 2018 10:57 pm

First, the :macm: doesn't need backstays, that's not going to help tighten a headsail on a fractional rig.

Now... back on topic.

The problem with the X Mast design is that because the shroud attaches too low relative to the length of the mast, it induces mast bend port & starboard.

Image

Yes I know those are two M's but using it for demo purposes
It essentially acts like a fraction rig.... to port and starboard. A fractional rig and mast are designed to induced bend to create optimal sail shape fore & aft. However, to port and starboard the mast is designed to, and should, stay (basically) rigid and straight. With the X this is not the case (from my eyeball).... there is too much force allowed to be introduced at the top of the mast which acts like a backstay tensioner (only to leeward). Where the shroud attaches remains fixed and the entire mast will flex, I've seen it.

My mast (if anything) was rigged very tight and after nearly two wrestled the pin in the forestay, it would come to weather very nicely. The headsail still sagged by my standards, but nothing like what I have seen on Youtube videos.

There are few things I critique about the X design (and M design for that matter [Although the X is way better, why don't you M owners keep crapping where you sleep ... literally)

But the mast and the attachment of the shrouds is too low and induces bend port and starboard. This is what I am aiming for fix.
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Re: Double Spreader Mod

Postby Tomfoolery » Tue Feb 13, 2018 9:51 am

Here's a photo of my :macx: sitting in a herd of :macm: boats at the 2013 MMOR near Toronto. Mast envy at it's worst. The :macm: has a 6 ft cantilever above the uppers and forestay (4/5 fractional), while the :macx: only has 4 ft above (~7/8 fractional).

For easier direct comparison. That's Highlander's boat on the left. The 'extra' white line shooting off to port is my topping lift, as the boom is off to port when this was taken.

Image
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Re: Double Spreader Mod

Postby BOAT » Tue Feb 13, 2018 10:43 am

Yeah, I always thought that was strange that the M had MORE unsupported mast above the spreaders than the X - yet, the X has the backstay and the M does not! It's counter intuitive, but that's just the way it is.

There must be some sound engineering there because the X masts are the only ones breaking. The M mast tends to bend before it breaks.

It might be the main sheet connection? does the traveler on the M take up more of the sail pressure on the rig than the pedestal mounted X mainsheet? Is that why it requires a backstay?

Very strange.
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Re: Double Spreader Mod

Postby Tomfoolery » Tue Feb 13, 2018 12:05 pm

I did this a couple of days ago and saved it as a draft instead of posting it because I thought nobody would be interested. But given the direction this discussion has taken, maybe that's not the case. So here it is.

_____________________________________________________________________________________

Since I'm in the middle of working on some solid models anyway, I thought I'd build the :macm: mast section and report here, like I did with the :macx: mast. For anyone that wants to know for design purposes. I carefully reconstructed the MacGregor extrusion drawing, and analyzed it's area properties.

Image

The moments of inertia and weight per foot from the model are extremely close to the sketch from MacGregor's suppliers' corner. The model calculates the centroid locations, which aren't on the Mac sketch, but are needed for calculating bending stresses.

And here's the :macx: mast, reoriented to be the same as the :macm: mast. For convenience.

Image

For the :macx: , with its 4 ft cantilever above the top of the upper shrouds and forestay, and staying at or below 14,000 psi stress which should give a fatigue life much longer than the boat could do during its lifetime (6061 T6), you could push/pull with 280 lb force at the top (horizontal load) in the fore/aft direction, and with 218 lb side-to-side.

Deflection at the top of the mast with the max forces shown would be 0.55 in fore/aft, and 0.72 in side-to-side.

For the :macm: , which has a 6 ft cantilevered mast above the shrouds, it would be 230 lb fore/aft, and 171 lb side-to-side. The mast is stronger, but it's also longer (above the last support).

Deflections at those forces would be 1.1 in fore/aft, and 1.6 in side-to-side.

In actuality, all the deflections would be even greater, as I assumed the bottom of the cantilevered part is mounted to an infinitely stiff surface, but the reality is that the mast below the start of the cantilever will actually also flex with the application of a point load at the top, so the plane at the bottom of the cantilever would not be horizontal.

And those deflections are based on the point load that causes a max bending stress of 14,000 psi. Of course, the loading on the mast is more complicated than just a horizontal force at the top, as it's distributed (unequally) along its length, acts in several directions at once, experiences some torsion, and is also in compression to some degree. But it's interesting to see the trends. Like the length of the cantilever is much more important than the moment of inertia, as the deflection varies with the cube of the unsupported length. So making a mast just a little longer above the last stay can make for a gross increase in deflection. But only a linear increase in the stress (twice as long = twice the stress).
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Re: Double Spreader Mod

Postby BOAT » Tue Feb 13, 2018 12:37 pm

Tom - your report says the M mast bends twice as far as the X mast.
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Re: Double Spreader Mod

Postby Tomfoolery » Tue Feb 13, 2018 12:57 pm

For the load that induces 14 ksi bending stress, which is different in each direction, and for each mast section, and given the different length of mast above the upper stays. Yes.

But that doesn't mean in practice that it will bend twice as much, as I have no idea what the actual loading is. But I can say with conviction that the loading is at least somewhat complicated, and certainly not as simple as a horizontal point load at the top of the mast. And I strongly suspect that the bending load is very much smaller, since only a small part of the sail is 'pulling' on it.

Deflections for a 100 lb horizontal point load at the top would be:

:macx: 0.20 in fore/aft, and 0.33 in side-to-side

:macm: 0.46 in fore/aft, and 0.94 in side-to-side

You can scale those deflections up and down from the 100 lb reference.

If you load the :macm: mast with 100 lb at the same 4 ft height as the :macx: mast, you get 0.14 in fore/aft, and 0.28 in side-to-side. You can see that cantilevered length makes a huge difference in deflection. Only a linear difference in stress (2/3 the length gives 2/3 the stress for the same loading profile).

It's all just academic anyway, as boat masts aren't really loaded like that, except when tensioning the backstay. And even then, it also curves the mast in the opposite direction below the point where the upper stays connect, and much of the load is downward, with only a portion acting to bend the mast.
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Re: Double Spreader Mod

Postby Signaleer » Tue Feb 13, 2018 1:08 pm

@TomFoolery

... uh...wowsers. Only one thing to add is that the lateral forces on the M Mast are handled (to an extent) by the rotation, which means the M Mast never really stresses directly laterally. But wholly cow - awesome analysis.

Now another piece of data: empirical data...

1. X masts break at the spreaders (does everyone agree to this?) I believe my rig was adequately tight, perhaps over tight (and may have caused a pinch at the spreaders) and this is the impending failure.
2. I also believe I have visually seen more mast flex than I am comfortable with vs. my 105 lb Pearson 26 extrusion on that boat (which was also a masthead sloop). This flex makes me think this is a weak link.
3. When we calculate stresses are we accounting for headsail? mainsail? perhaps mainsail & Spinnaker? (I tend to be a conservative sailor, and respect the environment, but still enjoy sailing in good conditions). Because the flex in the forestay, this often creates a non-flat sail that is definitely stressing that mast. (again, my was as tight as I've seen on Youtube and tighter... but still our foresails have a ton of flex)

I could be wrong but I don't think double spreaders will add additional windage worth worrying about and may help the mast stay rigid port and starboard. but I don't want to do that if it is dumb.

Thoughts on my analysis?

Ed.
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Re: Double Spreader Mod

Postby BOAT » Tue Feb 13, 2018 1:23 pm

I always prefer the flexible masts - there is a deflection limit from the manufacture - I like the masts with the highest limits. The first time you pitch pole your boat in a bad storm and watch your mast wiggle like a wet noodle you too will appreciate the advantage of a flexible mast. To me a good rig is one that can turn turtle under water and recover without breaking. People with stiff masts always carry bolt cutters so they can cut away the mast in the case of a turtle because they know their mast can't take it. I want a mast that will bend and flex and take the abuse.

I have been in bad situations on other boats where the stress on the mast is great. In particular heavy boats with short masts - in rough sea the mast is stiff with little deflection.

In a really bad slam the M mast is sort of like a wet noodle - not sure on the X. I can tell you that if the mast on a CAL30 bent like the mast on the M it would break - the mast on the CAL can't take that kind of deflection - it will just fail.

I always assumed the X mast had the same properties - I did not know there was any difference until people started talking about breaking masts on this site. I have read two kinds of failures on the site here for the MAC boats - bending, or cracking. The predominant failure attributed to the M is bending (mastreb and others that have hooked a tree branch or light pole at the launch ramp). There is also the similar stories about the M mast falling down in "slow motion" in stories on this site from forestay failure. That's what I want - if I hook a light pole I want it to either yank the boat right off the trailer or bend the mast - I do not want my shrouds to fail - EVER.

The only mast break stories I hear are attributed to the X mast but I'm not so sure that's because of the mast deflection limit?? Can that be calculated by Tom's computers? It appears to be more related to drilling holes in the mast?? That's an easy fix - why not just clamp or weld the hounds instead of bolting them?
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Re: Double Spreader Mod

Postby Highlander » Tue Feb 13, 2018 3:26 pm

FRI The :macx: mast doe,s not have the same standing rigging as an :macm: Mast
:macx: Mast is smaller , lighter & has shorter spreaders 36" long

:macm: early mod with the bigger mast 5" I believe & had 42" long spreaders, the 2008 :macm: & later had a lighter mast 4 1/2" I believe same shape mast the later just a little smaller Dia. & lighter but had 44" long spreaders as such the :macm: shroud chain plates are further aft of the mast thus giving a lot more support thus eliminating the need for a back stay also the mainsheet being on a traveler on the :macm: also acts as a back stay to some sort of extent . so the X & M r two different peas in a pod
If for say on a M u want to fly ur spinnaker from the masthead as I do , if u r not using ur main just attach ur main halyard to the end of ur boom & crank down ur mainsheet
or run a set of running back stays

On an :macx: or :mac19: or :macm: if u want to add more strength & support to ur upper top mast section another very simple easy method is to install forward facing Jumper Strutts as I did on my :mac19:
http://i78.photobucket.com/albums/j95/m ... 010037.jpg

http://i78.photobucket.com/albums/j95/m ... 010069.jpg

http://i78.photobucket.com/albums/j95/m ... ics/a2.jpg

http://i78.photobucket.com/albums/j95/m ... 0002-1.jpg

http://i78.photobucket.com/albums/j95/m ... CN1406.jpg

J 8)
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Re: Double Spreader Mod

Postby Highlander » Tue Feb 13, 2018 3:37 pm

y Signaleer » Mon Feb 12, 2018 8:57 pm

First, the :macm: doesn't need backstays, that's not going to help tighten a headsail on a fractional rig.

It will if u r flying a masthead Spinnaker :P

or u could just surf along :)
Image

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Re: Double Spreader Mod

Postby Signaleer » Tue Feb 13, 2018 11:56 pm

Highlander...

Dude... Jesus you are cool.

Sell me the 19?

Ed.

PS. Agree if you are running Masthead spinny ... my comment was without. :)

P.S.S. I think your mod may be better and easier. I'm going to consider that!
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Re: Double Spreader Mod

Postby sailboatmike » Wed Feb 14, 2018 12:38 am

Signaleer wrote:Sell me the 19?


Highlander has a 26M
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