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post #11 of 33 Old 06-11-2018
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Re: Friend's "clever" idea to cut mast off at deck...

My recollection is that the Wavelength 24 is a double spreader rig with a comparatively light 'MORC' spar. It would be possible to cut it at the deck but it would not be easy. You would want to build a compression post at that had a plate at the top that was reinforced to transfer moment through the deck, yu would want to glass over the opening and beef up the deck in that area to distribute the loads, and you would want to weld a moment connected baseplate on the bottom of the mast. When the mast was erected, you would end up bolting the two plates together to transfer the loads.If you want to see what that looks like, take a look at a Laser 28.

Its a lot of work that is beyond anything that I would ever want to do to a great boat like the Wavelength.

For what it was worth, when I was thinking of sailing my boat to Europe I had considered doing something similar with my mast so that I could jettison, it offshore before it sank the boat if the mast went over for some reason.

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post #12 of 33 Old 06-11-2018
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Re: Friend's "clever" idea to cut mast off at deck...

Quote:
Originally Posted by paulinnanaimo View Post
Jeff

I'm sure that your input is correct, but you seem to be assuming that the man isn't addressing the issues. If the materials are adequate the rig can be made safe. This is a trailerable 26' boat, not a 50' ocean racer.
I am assuming from the fact that the person is considering cutting the mast off roughly a foot off the deck, that he does not understand the engineering issues involved since that makes the solution much harder from an engineering perspective, let alone a practicality and aesthetic issue. And if the boat is anything like the Pacific Seacraft that the OP mentioned, looking at a 27 foot Pacific Seacraft Orion for example, that boat displaces virtually the same weight as my 38 footer, so this is not a casual issue.

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post #13 of 33 Old 06-11-2018
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Re: Friend's "clever" idea to cut mast off at deck...

Can we have a picture. I'd love to see how he executed this.
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Re: Friend's "clever" idea to cut mast off at deck...

Jeff

This is another case where clarity is lacking. The OP says the boat is a trailerable 26' something and my read is that the OP owns the Pacific Seacraft…your interpretation is different. Also, the poster puts a negative spin on the friend's abilities, he could very well be a talented handy person, I think most junk yard owners are. If the boat owner is fitting a sturdy mast sleeve on a light weight trailer sailor there should not be a problem. If he is using sheet metal to secure the mast of a very heavy boat things are obviously much different. As I said, the first post is not clear but I am assuming the junk yard guy knows what he is doing.
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Re: Friend's "clever" idea to cut mast off at deck...

Not to worry, Jeff. Another hose clamp will fix'er right
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post #16 of 33 Old 06-11-2018
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Re: Friend's "clever" idea to cut mast off at deck...

Quote:
Originally Posted by paulinnanaimo View Post
Jeff

This is another case where clarity is lacking. The OP says the boat is a trailerable 26' something and my read is that the OP owns the Pacific Seacraft…your interpretation is different. Also, the poster puts a negative spin on the friend's abilities, he could very well be a talented handy person, I think most junk yard owners are. If the boat owner is fitting a sturdy mast sleeve on a light weight trailer sailor there should not be a problem. If he is using sheet metal to secure the mast of a very heavy boat things are obviously much different. As I said, the first post is not clear but I am assuming the junk yard guy knows what he is doing.
I see what your saying. I guess we will find out more if and when the OP shows back up.

Jeff


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post #17 of 33 Old 06-11-2018
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Re: Friend's "clever" idea to cut mast off at deck...

I'm a structural engineer, and I agree with Jeff that the mast is likely to be designed as a continuous beam, taking advantage of reverse bending (contraflexure in engineering parlance) to optimise the section. Introducing a hinge right at the top of the deck will certainly increase bending in the mast above deck. However, the reversed moment dies out quite quickly above the deck, so that cutting the mast higher is actually better than putting the hinge right at the deck level! (I do agree that it will look horrid though).

As regards making a moment connection, I think that's what the tinkerer is doing with the "sleeve" on his tabernacle. It will be interesting to find out more. Hope the OP can follow up with photos.

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Re: Friend's "clever" idea to cut mast off at deck...

Quote:
Originally Posted by Atlas View Post
Introducing a hinge right at the top of the deck will certainly increase bending in the mast above deck. However, the reversed moment dies out quite quickly above the deck, so that cutting the mast higher is actually better than putting the hinge right at the deck level! (I do agree that it will look horrid though).

As regards making a moment connection, I think that's what the tinkerer is doing with the "sleeve" on his tabernacle. It will be interesting to find out more. Hope the OP can follow up with photos.
I would like to respectfully comment on this section of your post. Because the section of the mast below the deck is so short as compared to the panels above, it would be tempting to think of the mast as having near full fixity at the deck level. Therefore the normal expectation would be that the maximum moment would be at the deck and would taper pretty quickly above the deck transitioning from negative moment to a positive moment (if uniformly loaded roughly somewhere near the quarter point of the span between the deck and the shrouds).

But because of the point loads of the boom, and vang, (and the couple that they form) and the triangular loads from the sail, the finite element analysis generated moment distribution curves for masts that I have seen, show that the actual max moment occurs above the deck line in the area between the boom and the deck. This zone also experiences the maximum torsion, with max sheer occurring at the deck line.

That is why I believe that placing a hinge point above the deck would place the hinge in the zone with the maximum bending moment and maximum torsion. I suppose that these could be addressed by a long and strong enough sleeve into both parts of the mast, with each of which welded to a bolting plate (maybe gusseted bolting plates) that would allow a transfer of the loads.

This would look something like the photo below which is the top of the jack post on a Laser 28. The base of the mast has a similar fitting, and they are bolted through the deck which is reinforced at this area to take the loads.
[IMG]Laser 28 jack post interior[/IMG]

Respectfully,
Jeff


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Last edited by Jeff_H; 06-11-2018 at 02:39 PM.
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post #19 of 33 Old 06-11-2018
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Re: Friend's "clever" idea to cut mast off at deck...

I don’t think it takes extraordinary intelligence to fully contemplate the obvious. You don’t cut your mast in half. Your friend needs a different boat and there are tons of them available for near nothing. Put the saw down.


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Re: Friend's "clever" idea to cut mast off at deck...

A lawn chair, a cooler of beers, coupla cameras streaming live on YouTube...Why discourage creative authors from entertaining the rest?

Of course, once that boat has been customized, it will have a resale value of less than zero. It might be simpler to ignore the engineering brilliance, and just let the guy know he'll save money by buying a boat that already has a deck-stepped mast on it.
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