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Converting to a rotating mast

A forum for discussing topics relating to MacGregor Powersailor Sailboats

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Converting to a rotating mast

Postby walt » Mon Sep 17, 2007 11:46 am

I am seriously thinking about converting a 26S to rotating mast and am guessing this is probably the best place to find someone who has done this. I turned up little with a search here but maybe used the wrong key words.

So here is what Im trying to find out

* can you purchase the rotating base or did you have to roll your own? Do you need to limit the mast rotation? Some old Hobies limited the rotation to about +/- 45 degrees, but I dont believe you really need this..

* what about the spreaders and shrouds? Does all this need to be changed to some different configuration which allows the rotation?

* What about the back stay? Can you leave the (for example) 26X back stay as is? I dont believe the 26M comes with a back stay but some probably have put one on - any special considerations?

Any other considerations?

Why do I want a rotating mast? Im a pretty firm believer in their usefulness on a Hobie cat and also land and ice boats. However, these boats (my Hobie is a 14 non turbo) all have no jibs and the use of a jib probably makes the angle of the main mast less sensitive.

I can fairly comfortably sail my 26S with the main only in some good breeze with the traveler way out but it just doesnt feel like the sail is working well and Im fairly certain its the really rank leading edge shape you get with a non rotating mast (Ive seen the arguments about the full sail shape which is probably also significant - but I think the mast rotation is more significant). I beleive Im losing a lot of lift and probably also have a lot of drag especially when the sail is angled out such as on a reach in windy conditions. In the high winds and with the traveler, the main sail lift vector is more pointed in the direction the boat is traveling and with a rotating mast, I think the boat would just get up and go. The boat is in control now with the non rotating mast but Im guessing things would be a lot more "fun" with the sail actually creating a lot more lift (which is what I beleive would happen)...
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Postby Frank C » Mon Sep 17, 2007 12:15 pm

Mike Inmon, the MacDealer in SoCal, has converted his Mac 26D to the rotating mast. He has also written here about upgrading several X-boats to the new mast. You could search on rotating mast with his member ID (mikelinmon) in that second search field.

It might be easier to look up his number on the Dealer Info page, then make a phone call.
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Postby kmclemore » Mon Sep 17, 2007 1:07 pm

Remember to click the "Search for all terms" field!
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Postby walt » Mon Sep 17, 2007 1:32 pm

A couple things I found - cost is about 2K to 2.5K to have a dealer do it (may Inmon is the only one who does this - or has ever done this?) and you need a flatter sail to complement the better leading edge the rotating mast is going to provide.

However, I also saw no reason given but that if your in a windy area, you may not want the rotating mast. Running on the main only in higher winds is important to me and the place I sail at is definatly on the windier side. Im wondering if the problem is the mast itself going into oscillations. On a big landsailer with a wing mast I sailed, it had a seperate "sheet" on the mast alone and in a tack, you had to pull the mast tight to center to prevent it from oscillating and destroying itself. But the Hobie 14 doesnt seem to have this issue. You can also "mass balance" by hanging a weight connected to the rotating mast out in front of the mast but Id rather not have complications like this.

Is there any drawback to the rotating mast in windier conditions?
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Postby baldbaby2000 » Mon Sep 17, 2007 2:03 pm

Walt,

I assume you want to use the stock 26M mast and not just convert your existing mast to rotate? The 26M mast is probably a heavier stock than your mast now and I suspect that's one reason the rotation helps more on the M than it would on a smaller mast. I used to race Hobie 18's and the rotation could be controlled. On the 26M I think it's pretty much limited by the spreader assembly. I have not problems with oscillations. When I have the sail down I tighten the main sheet to keep everything tight through the topping lift and that seems to work fine.

I'm not sure the change would be worth it. The 26S is a pretty fast boat from what I've seen and putting a heavier rig on it with the added weight aloft may not do you any good.

I think you are right in not getting the best airflow around the mast when there is no headsail; however, the presence of a headsail does tend to "smooth" out that turbulent region so it's not as bad.

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Postby walt » Mon Sep 17, 2007 3:03 pm

Good to hear of no oscillation problems - looks like the M setup probably has figured out.

Since this is the power sailer forum, I think the case to use the M rig would be of interest. Right now, Im just exploring the options but am thinking my current "S" mast would work just fine and I need a new main sail regardless of what I do with the mast. There could be some mast shaping which made the mast better for either rotating or non rotating applications but Im guessing I wont find out if this is the case and likely everyone buys about the same extrusion shapes. Im thinking I just need "a" rotating base (with the ability to lower the mast such as the M base does) and "a" rotating spreader assembly and maybe something at the top of the mast which wears better with the rotation.

If I always sailed with the jib, I dont think I would need the rotating mast and also if I always sailed more upwind/downwind only. But there is the (really fun) high wind case where I can keep my setup simple by just droping the jib and running on a full main. There are other options such as a storm jib, reefed roller jib, reefed main, etc but I would bet the rotating mast full main only is just going to significantly outperform the other options and my experience is that with the traveler, I can handle this config in some good whitecapping conditions - and it would be more fun to really get the boat moving.

How late in the season do you guys sail the M's at Chatsfield res?
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Postby bscott » Mon Sep 17, 2007 10:17 pm

I am in the process of converting an X mast to a M. With some advice from 4M members and others, the following is rather important:
1) The X and M masts are the same, M being 2' longer.
2) A new main is required and must be cut the same as a rotating cat main which is flatter than a typical non rotating mast main
3) New chain plates installed 8" aft of the stock location
4) Halyards led aft via Harken 140 pivoting mast exit blocks/cam cleats
5) Shroud lengths need modicication--we are going to add shroud adjusters to lengthen the shrouds to facilitate the pinning of the furler.
6) Back stay--I am installing a port side tang to provide 4:1 block and tackle tuning from each stern tang. Each stay will attach to a single wire yoke which will attach to a pulley running in a mast bail.
7) Experiment cutting the spreaders shorter by 6" to get better down wind main performance .
Other X improvements required to get max sail shape are:
1) A 4:1 traveler mounted on top of the cockpit seats at the cabin hatch
2) 6:1 boom vang and
3) Replace the CDI with a Schaeffer Snapfurl to be able to adjust the jib halyard for better close haul headings

This has become a winter project as the new mast has yet to be delivered.
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Postby Frank C » Tue Sep 18, 2007 3:06 am

Are you sure the mast sections are identical, X vs. M?
I thought the M-section was larger & heavier.
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Postby Highlander » Tue Sep 18, 2007 7:36 am

All the X mast's I've seen are the same size as my mac19 the M mast is away bigger & heavier

Cheers John
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Postby baldbaby2000 » Tue Sep 18, 2007 10:50 am

From what I've seen the M mast is heavier duty, not just longer.
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Postby bscott » Tue Sep 18, 2007 12:01 pm

I rec'd a PM from Mike Inman stating the mast sections were identical, that all replacement masts were Ms cut to length and holes drilled by the user. I was also told by a dealer that the newer M masts were the same cross sections as the X but lighter.

I guess we'll have to compare them side by side when the new mast arrives.
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Postby Frank C » Tue Sep 18, 2007 12:32 pm

Hmmm ... sounds as if the earlier M-boats had a heavier section.
Guess they've decided they didn't need the extra strength, preferring less weight-aloft.
  • You mention adding new chainplates, aft by 8 inches ... have a specific recipe for that? I'm wondering about backup for the hull-lam. Guess it will require some slash'n gash to the interior liner?
  • I like the specs of the SnapFurl. Anything specific you like on the Schaefer? You might also want to look over the Hood SeaFlex.
  • And, sounds as if you're planning to use a backstay, attached to an aft-wise boom-bail? Good idea~! Someone posted something similar in the MacMods. I'll link to it later, if I can find it.
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Postby walt » Tue Sep 18, 2007 12:44 pm

For anyone who cant sleep at night and needs some aid..

Read this:
http://www.tspeer.com under: Airfoil - wingmast - pdf version

This article really applies to land sailers but its certianly applicable to a mono slug sail when used without a jib in higher winds.

The thing I wanted to point out is something very roughtly derived from figure 1 but it shows that about 22 percent of the sail lift is derived in the first 10 % (leading edge) of the sail area. If you screw that first 10% up (such as with a non rotating mast and with the sail sheeted out), your guess is as good as anyone elses about what happens but I think you lose a lot of what a sail is supposed to do.

Some more of the quick and dirty conclusion to get the 22% is given below:
------------------
On the x scale on the figure 1, there are tick marks that divide the sail in to 10 zones starting at the front of the mast and ending at the leach. On each zone, I made a rectange which approximated the total area of the positive and negitive pressures which I beleive is a direct correlation to the lift generated in that section. I then compared the area of each rectange to the total area and assume this gives the percent of lift that area or zone produces. This was a rough and not too accurate measurement but here are the results:
Zone % of lift
------------------
0-1 22.6
1-2 15.9
2-3 12.7
3-4 11.53
4-5 9.9
5-6 8.6
6-7 7.2
7-8 5.4
8-9 3.6
9-10 2.1
(ie, zone 0-1 is from the leading edge of the mast to 10% back - first 10% can generate 22.6% of lift "if" done correctly.
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Postby johnnyonspot » Tue Sep 18, 2007 12:54 pm

I recall them saying on the dvd for the M that the masts are the same except for the added length on the M.

I guess I question the necessity for a rotating mast. How much speed can one expect to add by changing to a rotating mast anyway? Does the trouble and expense justify it? Unless you are racing, I can't see it being worth it. And if one adds an M mast to an X or a classic, what implications will there be from the extra length versus the amount of ballast and form stability being designed for a shorter mast? Anyway, just my 2 cents.
Last edited by johnnyonspot on Tue Sep 18, 2007 12:55 pm, edited 1 time in total.
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Postby tangentair » Tue Sep 18, 2007 12:55 pm

I would say I am impressed with these details as an engineer I can analyize the curve, consider the first 40% or 50% s contribution and determine ways to tweek my sail shape to get the best foil, but I just want to sit back and sail people and admire the beauty in and outside the boat.
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