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Old 21-12-2009, 12:21   #1
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What Do You Think of this Mast for a Cat?

My neighbor built this "truss" mast, (as well as the pictured 49' steel monohull). He cut the mast pieces from 1.5" aluminum tubing then he and a welder put it together in a day. Its about a 100 pounds. His thinking is that there is much less turbulence on the sail's luff verses a standard elliptical mast. (easy to climb too). The boat has circumnavigated Vancouver Island, Canada a few times so its had some miles.

I'm thinking of building a ~45' cat and wondered what the reader ship thinks of this mast as a way to improve laminar flow along the luff. He says he put telltales 6" behind the luff up and down the mast and they showed no turbulence at all.

My questions are:

1. Are forces on a mast comparable on a cat vs a mono, (it works for him, would it work for me?).

2. Do you think this will work on a cat? (is it wise?)

3. Would it be comparable in performance to a standard rotating mast, or a rotating wing mast as far as sailing efficiency? Maybe not but it would be a lot simpler.

Please chime in.

Thank you. Eric

PS. This is the first time I've attempted to post pictures. My son walked me thru it so if they don't appear I'll have to try again.
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Old 21-12-2009, 14:16   #2
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Eric,
The pictures appeared on the post.
1 Since cats don't heel much, the aerodynamic force isn't reduced as it would be on a mono.

2 In a seaway, the motions of a cat might be more bumpy than a mono, resulting in stronger accelerations at the masthead. If your cat is intended for ocean cruising, some computations would be necessary for safety purposes.

3 This mast design will certainly not improve laminar flow on the mainsail. IMO, any smooth mast would be more efficient, rotating or not. But your neighbor's monohull doesn't seem to be intended for high speed under sail.

Alain
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Old 21-12-2009, 14:17   #3
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As an old junior engineer:
It looks right, maybe a bit flimsy.
The option to widen the base hasn't been taken, in fact it might be better to be wider in the middle, you'd need a structures man with sail experience to advise on that.
Look up the Euler formulae for reasons why this is stronger and lighter.
Remember that gale force winds will have a lot more to work on.
And no racers have done it so are there any real advantages?
Tempting though
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Old 21-12-2009, 14:18   #4
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Yes , I have seen a mast the same as this ! It is fitted to a cat of about 50 ft ,but it was not a type known to me , it is in the Greek Ionian Islands and looks like a liveaboard cruiser . The mast was fascinating , I think I counted 20 shrouds to the deck which seemed to cancel out the benefits of the open mast structure ? It made me think what a innovative idea , looked very simple to build and very strong as the stresses would be dissipated throughout the whole structure and less on small parts of it . Hopefully someone will come along soon and tell us about it ....
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Old 21-12-2009, 14:36   #5
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I think it would very negatively affect the laminar flow. You don't see anything like this on aircraft wings except when they intentionally raise the spoilers right after the wheels hit the ground. I think it would noticeably impact your upwind performance.

As far as being put on a cat, the initial load on the sails on a cat is greater than that of monohull because of a cats greater initial righting moment. This must be taken into consideration. You would need a stronger radio tower type mast.
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Old 22-12-2009, 04:00   #6
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Quote:
Originally Posted by David M View Post
I think it would very negatively affect the laminar flow. You don't see anything like this on aircraft wings except when they intentionally raise the spoilers right after the wheels hit the ground. I think it would noticeably impact your upwind performance.

As far as being put on a cat, the initial load on the sails on a cat is greater than that of monohull because of a cats greater initial righting moment. This must be taken into consideration. You would need a stronger radio tower type mast.
Upwind will be a disaster but on the other hand downwind will for sure be better !!!
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Old 22-12-2009, 07:57   #7
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isn't every weld a potential stress fracture in the making?
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Old 22-12-2009, 22:53   #8
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Thank you for the feedback

Well looks like the consensus is pretty negative. The post about every weld is a potential failure is a good point. But these were done by a pro, there are lots of alum welds on other boats I've seen and this whole steel boat is a series of welds. Maybe there is just too much load on a cat's mast for this idea to work.

Again I am not an engineer. Its just a cool idea my neighbor had and has actually sailed twice to the Queen Charlotte Islands and around Vancouver Island. Thats a few hundred ocean miles anyway. Besides DIY for a few days labor and less than $500 is always appealing. The vertical tubes are not 1.5" diameter they are 1.25", schedule 40 alum pipe.

Regarding the comment about not seeing this on airplanes, (other than Wright bros era models..... hmmmm). Is high speed air flow science applicable to a low speed sailing rig? I do admit that its possible that all those tubes would create a lot of little mini turbulant areas. But would this be worse than the turbulance created by a solid mast that is not rotated to the wind? Again I'm not an engineer, I'm just asking for your thoughts.

Regarding the comment about another boats dozens of shrouds does not apply here. This mast is rigged pretty normal with 2 spreaders and 3 chain plates and shrouds per side.

Fast cat I love your work and am honored you have commented. Was yoour concern about the air flow/windward performance or the rig's strength and stability?

I certainly won't use it his unless I have an engineer and sailmaker take a look. To me it looks cool, is practical, (unless it turns out to be prone to failure of course), and other than all the welds it seems to embody the KISS principal.

Again thanks for your input. Any one else want to chime in.

Eric
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Old 31-12-2009, 23:11   #9
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Yes on a cat that I pegged to be about 45’(twas a bit distant to be sure).

On the subject of strength next time you’re in a big city look up at the nearest building being built. Yes that’s right the crane’s boom is made of a triangle frame of tube section and the mast is made of a square frame of tube section and angle which is internally tensioned with chains and cable to restrict its sway as the boom slews.

Strength can be engineered in quite easily, the strength critical application of a crane should satisfy most critics.

The question is whether the round sections of mast are just a hand me down from when wood was the cheapest method.

Of course there is the aesthetic value but if that’s not a concern then perhaps the noise factor would be
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Old 01-01-2010, 00:46   #10
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Just from looking around, a lot of structures on land are built like that and there are plenty of racecar frames built much like that as well but all of the things that we build that are made to move through air or water and support a large stress are either a hollow tube or have some sort of a skin like a racecar body or an airplane's skin. It's probably strong enough given that the materials are an appropriate size & alloy but the problem will be with the amount of wind that it catches.
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Old 01-01-2010, 16:28   #11
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Eureka!!! Someone has finally solved the problem of noise from the main halyard slapping the mast! You'll never hear that above the wind noise through that lattice structure....
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Old 01-01-2010, 22:35   #12
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It may even be quieter who knows?
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Old 01-01-2010, 23:40   #13
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WOW! That's a lot of welding. This guy must have had a lot of spare time on his hands.
Being aluminum every weld is a potential crack unless it were heat threated (stress relieved). The properties change in aluminum after welding.

As a metal smith I would dump it as soon as I could find another mast. I'd be more comfortable with an old hollow wood mast.

It sure would be EZ to climb.
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Old 02-01-2010, 02:23   #14
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Old 02-01-2010, 02:46   #15
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This one has been around since 1985, from what i have heard she has circumnavigated twice and done Australia/UK/Australia once.
She lives in Hobart Tasmainia, so the Southern Ocean is her backyard/playground, while asthetically the rig may not be to easy on the eye, by all accounts it performs very well.

By the way, these are steel not alloy...


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