Worst Nightmare

[Tomfoolery]

Well, that stinks.

If you're talking to a mast supplier, then I'm sure he/she has looked at the properties of the original, or at least the latest version, of the custom extrusion MacGregor used. http://www.macgregor26.com/suppliers_co ... ty_rr.html

Do be careful about the strength of the new extrusion, which is easy enough to determine from the specs in the drawing. Make sure the Ix and Iy values (those are 'eyes', not 'ells') are met or exceeded. They're a measure of the distribution of the metal in the two primary directions of bending (fore/aft, and side-to-side), and relate in conjunction with the material strength, to the bending stiffness and strength of the mast. Outside dimensions alone aren't enough, though you can probably get close(ish) with outside dims and wall thickness. That's why all mast manufacturers give those specs. Same with booms and spreaders.

But I also agree that you should call or email BWY - they're great folks, and they may know of either a mast in your area, or how to get one to you for cheap(ish), or perhaps a reasonable replacement from a catalog that you can source locally. Worth an email at least.

Oh, and if it were me, I'd look into splicing the broken mast. Maybe it's the junkyard mentality in me, or the New England attitude (where I grew up), but an otherwise perfectly good mast that's very expensive to replace would warrant serious consideration for splicing. At least to me.

[Neo]

The mast on the M model is too light in my opinion.

BWY said the 4.5" is not available but why do you think 4.5" is too light (weak?).... Having just see one crash to the ground, I'm thinking the lighter the better lol!

I do think any mast could withstand an impact with a tree.

[Neo]

Do be careful about the strength of the new extrusion, which is easy enough to determine from the specs in the drawing. Make sure the Ix and Iy values (those are 'eyes', not 'ells') are met or exceeded. They're a measure of the distribution of the metal in the two primary directions of bending (fore/aft, and side-to-side), and relate in conjunction with the material strength, to the bending stiffness and strength of the mast. Outside dimensions alone aren't enough, though you can probably get close(ish) with outside dims and wall thickness. That's why all mast manufacturers give those specs. Same with booms and spreaders.

This is great advice tom. Thanks for that.
I'd consider splicing as a very last resort but I can't see anyone wanting to buy my Mac after that.

[cptron]

If memory serves, Mastreb replaced his mast with a carbon fiber mast. You might PM him and see how his is holding up.

[Neo]

I've discovered that my previous mast is 3" X 5.2" so I am guessing it has been replaced before.... It seemed strong enough but was a bit heavy to handle.

My local Mast supplier has suggested the Mast Profile below ( 2.95" x 4.38") but I am concerned the front part is not pointed (aerodynamic) enough for a swing mast..... I'll get more information on it's construction but I also might talk to him about a 5" mast.... but I guess that goes against my desire to keep the Mast as lite as possible.

Have PM'd Mastreb ... Thanks for the tip Cpton

[Ixneigh]

It's too ligh because there's no margin for screw ups.
With that no back stay rig and only one set of lower shrouds the mast should have been a tapered, slightly thicker spar.
That being said there currently is no case that I am aware of it failing under sail. Failures on this board have been attributed to fitting failures only.
My keel boat had a spar it required two people to carry. She had two lowers, spreaders you couldn't bend with a vice and 3/16 wire. I hit trees putting her in the mangrove creeks and snagged the shrouds on branches frequently and never gave it a second thought. While that might have been overkill for a Mac sometimes the quest for saving weight is taken a bit too far.
Ix

[Neo]

Hmmm... some good points there Ix.
I'd love to go Carbon, even at double the price, but I'm told it would be more like 10 times the price!... but that's currently under investigation

[Tomfoolery]

I've discovered that my previous mast is 3" X 5.2" so I am guessing it has been replaced before.... It seemed strong enough but was a bit heavy to handle.

My local Mast supplier has suggested the Mast Profile below ( 2.95" x 4.38") but I am concerned the front part is not pointed (aerodynamic) enough for a swing mast..... I'll get more information on it's construction but I also might talk to him about a 5" mast.... but I guess that goes against my desire to keep the Mast as lite as possible.

Have PM'd Mastreb ... Thanks for the tip Cpton

Here's the MacGregor mast, from their Suppliers' Corner.



Dimensionally the one your supplier is proposing is very close at 111.4mm (Mac) vs 111.13mm, and 75.6mm (Mac) vs 75.0mm. No wall thickness or moments of inertia are shown (the Ix and Iy values), though. Or material specs, for that matter. Weight per foot (or other unit length, of course) isn't as good as actual section properties, but it's the next best thing I suppose. But the mast is a close match.

[Ixneigh]

Iirc mastreb did not go for the carbon mast. Only the boom. Carbon fiber fails in a more dangerous way then aluminum.
It is subject to minor damage that can render the whole structure unsafe.
Aluminum is much more foregiving and that's key to casual users.
Best option is a custom fabricated tapered aluminum spar with a sleeved insert about six feet long in the spreader area.
In my opinion. As always
Ix

[Neo]

Ok, The Mac diagram states 1.5729pd/ft (2.34073307Kg/m).
From information I've received the suggested replacement is 6061T6 alloy - 2kg per metre (1.34394 pd/ft)... So I'm guessing the walls are thinner... weaker?
Its available in 9.5m (31ft) lengths. The price is not so bad but need to be quoted on delivery yet.

Ix custom made spars are as rare as hens teeth in Australia.... unless you have deep pockets that is lol!

[Tomfoolery]

... So I'm guessing the walls are thinner... weaker?

Maybe. Or the metal is apportioned differently. That's why I said that unit weight (lb/ft) is only approximately representative of strength and stiffness. 15% less metal doesn't necessarily mean 15% less strength.

A 3" x 3" x 3/16" wall tube like the one used in the Mac trailer pole tongue weighs 6.87 lb/ft and has a moment of inertia (the Ix and Iy values, which are both the same for a square tube) of 2.6 in^4.

A 1.42" x 1.42" solid steel bar also weighs 6.87 lb/ft, but only has Ix and Iy values of 0.34 in^4. Same amount of material per unit length, but distributed differently.

It's why so-called "I"-beams even exist - to put material where it does the most (far away from the center), and remove it from where it does the least (the center, or web area), for a given amount of material, depending on how it's loaded of course.

So I would press them for a catalog cut sheet showing section properties. Or make and model, so you can look it up yourself. Then you have to make a judgement as to how close you need to come to the original mast properties.

[Neo]

Thank Tom... your making my head spin but I understand what you're saying.

My other concern is floatation foam in the new mast.
I'm thinking I might need a chemical mix to fill to void.... Any ideas on this?

[NiceAft]

I noticed on the drawing it shows “version 4”. Does that imply previous mast styles, or previous drawings?

Ray

[Neo]

BWY said there were 2 actual versions 4.5" and 5' ... that's all I know.

[mastreb]

Hi Neo,

I think you should go with the spar your local supplier has found. It's more than close enough, and it's wing-enough shaped to function well.

I explored going to a carbon fiber mast in detail, but ultimately decided that it wasn’t worth the expense. The problem is that there are no stock carbon spars made that are the right size to work from, and custom carbon is extremely expensive. The closest I could get cost-wise was $4000, and I also did not like the failure scenarios with carbon that Ix pointed out. Aluminum fails exactly the way you want it to for a mast, and that's valuable.

That said, I learned a lot about mast building, and could help you build a custom mast from either aluminum or carbon as you see fit. It’s not nearly as difficult as it seems.

You can turn any un-slotted aluminum tube (or carbon tube for that matter) into a mast with a bolt-on track. You'll lose a bit of aerodynamic performance, but most boats don't really try to rely on aerodynamic properties anyway--any non-rotating mast will perform basically the same as a round profile. At most you'll lose a half-knot in otherwise perfect upwind conditions. In all other conditions there won't be any performance loss. Using stock tubes dramatically lowers the price and makes it possible to acquire spars in town from industrial metal suppliers.

The simplest way to go is the Harken Batt-car AA system, which I did on my boat. The only two issues with it are protecting the track from the mast carrier when the track is down, so it doesn’t get damaged, and the tendency of the ball-bearing cars to lose bearings. The second problem is resolved by simply getting the cheaper slides without ball bearings—they work perfectly well for our boats. It's also a bit expensive, costing $1500 for my boat.

You just bolt the track to one side of any tube, and you’ve got a mast system. Then have the track slides mounted onto your mainsail by a sailmaker, and transfer the rest of the hardware from the old mast to the new mast at exactly the same distances from the foot.

When you source an AL-tube, use the same interior wall width as the stock MacGregor tube.

Matt