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Old 18-02-2016, 13:31   #76
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Re: Change to Deck Stepped Mast

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Originally Posted by Adamaris View Post
All,

Thanks very much to all of you for your comments, I appreciate that everyone is giving their opinon for free and like many subjects there is no black and white answer. As I have seen several T37's with deck stepped masts I thought this would be the easiest/ cheapest way around my problem, I have no probem with the fibre glass work or making a compression post but didnt understand the issues with mast strength and lateral loading..

From the consensus it seems that repairing the base of my mast will be the best option. Ive made a few enquiries locally, to riggers and and a local fab shop which specialises in aluminum to see if the base of the mast can be replicated to form a sleeve style repair, all have reserved judgement until they see the mast off the boat. Once I have the profile measurements I'll get more idea of whats feasible.

Anyway thanks for your help again
Post some photos when you start working on it, I bet there are a bunch of us who'd like to see how it turns out!
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Old 18-02-2016, 13:33   #77
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Re: Change to Deck Stepped Mast

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]I suspect that the real reason that masts are keel stepped is that they are cheaper to install and overall can be lighter. However I have noticed that the boats requiring urgent rescue in the Southern Ocean after roll overs generally had keel stepped masts and that the reason they then started to take on water was the large hole ripped in the deck when the mast buckled?
We were dismasted at sea (not due to a rollover, however) and can report that there was no large hole ripped in the deck by our keel stepped mast. In fact, the one roll-over dismasting that I am personally acquainted with did not suffer this injury, either, and that was on a Valiant 40. IME, the typical failure point of a keel stepped mast is well above deck level, leaving a stub standing. The stub, being a hollow tube, will admit water, but is relatively easy to seal up (we used a bucket inverted over the stub).

So, I kinda doubt your statement about the gaping hole... and I suspect that the reason that these Southern Ocean boats tend to have keel stepped masts is that most designers feel that they are better suited to high risk sailing. The latest crop of Volvo type boats are pretty well out of the design picture for cruising boats, and to be honest , i don't know much about these exotic beasts.

Jim
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Old 18-02-2016, 13:48   #78
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Re: Change to Deck Stepped Mast

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The Euler formula for buckling is Critical Buckling Load = C*Pi*Elasticity*Moment of Inertia/L squared. Where C is a factor based upon the end fixture, and L is the length of the column. All else being equal the L is the biggest factor since it is squared
No. L in that context refers to the *unsupported* span of the column. The spreaders, as well as the pinning at the deck, affect how eulers buckling formula would be applied if sizing a mast. All else being equal, a keel stepped mast can actually get away with a smaller, less stiff section than a deck stepped mast of the same height over deck.
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Old 18-02-2016, 14:00   #79
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Re: Change to Deck Stepped Mast

You're all busy proving my point!

UNCIVILISED
"
Okay, Basics: It is a flat out FACT, that a deck stepped mast on a boat Must be of a stronger tube section, than one which is keel stepped.
I only state this, as for some, this info bit, seems to still be unclear."

You can't state that as a fact. Consider the bottom panel of the mast as a column in axial compression only. Again simplify the keel stepped mast as fixed at deck level, then for the same spreader height the slenderness ratio ( which is the measure of resistance to buckling ) reduces by 0.7
compared to a deck stepped mast which is effectively pinned at deck level. But it isn't totally fixed at deck level, it is pinned and there is another much shorter section of column below it, so the reduction in slenderness ration may only be say 0.85. So if I lower the spreaders, I can compensate, and as the slenderness ration is proportional to the square of the column height I only need to reduce the deck to spreader height to square root of 0.85 ie approx 0.92.
Such a change would not radically change the design of a cruising boat rig.
That calculation is theoretical but valid for a general understanding. I am aware that I have ignored any effect caused by bending / lateral loading on the mast. I don't think I can provide a simple explanation of how that affect the two cases we have been considering. A proper analysis would be required.

RaymondR

Your conclusion is wrong, because you are ignoring the change to the factor C caused by the different end fixity. See above

Adamaris

the general advice you have been given - stay with the keel stepped mast - is right. It comes down to how you make the bodge (that is the correct engineering term I believe)
I haven't noticed any dimensions for corrosion damage and extra height for a mast step or length of a spliced on piece of mast.
It is worth making sure that the new mast step is restrained both fore and aft and sideways, if it can move, and the forces are significant, then you loose the structural benefit of keel stepped.
If you have to splice on a new piece to the bottom of the mast, then the devil is in the detail. If you go this route, and sketch up your ideas, PM it to me and I'll help.
I have spend a lifetime designing and engineering structures ( oil rigs ) and the difficult bit is joining the bits of metal together. Different scale, same problem.
I also weld, not to coded standard but good enough for boats ;-)

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Old 18-02-2016, 15:38   #80
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Re: Change to Deck Stepped Mast

Correct that keel stepped mast boats tend to be cheaper to make than deck stepped masted boats. Also, deck stepped mast boats are usually safer and have the ability to lose the mast without breaching the enclosed hull structure. All in all, go with deck stepped. However, deck stepping requires not only a compression post,but the area under the mast but before the compression post must be structurally designed to distribute the mast loading laterally across the coach roof and down to the decks. Usually this is done with a wooden or metal curved bridge between the outer deck and the inner head liner, usually embedded into the fiberglass. Its absolutely imperative to get this design correct otherwise the mast will crush the fiberglass roof and potentially split apart the cabin sides. Not something for the amateur to tackle.

Remember, the mast transfers not only downward loads, but lateral loads: both forwards and sideways. Standing rigging only deals with the upper lateral loads, not the lower lateral loads. Hope that is somewhat clear.
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Old 18-02-2016, 21:29   #81
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Re: Change to Deck Stepped Mast

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I can't think of a boat over 32', besides the Pearson Vanguard that has a deck stepped mast...

We just sold our Hanse 342 which had a deck stepped mast. If I'm not mistaken, several, if not all models (at least their new 415) have deck stepped masts.
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Old 18-02-2016, 22:20   #82
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Re: Change to Deck Stepped Mast

Nah, cheaper and lighter. You just jig saw a hole in the deck, drop the mast through and seat it on the top of the ballast (which on most boats is pretty solid) and whack a bit of bog or clamp on a rubber boot to stop the leaks. A compression post for deck stepping is much more complex to build and install.


Definitely holes in deck, once the mast is broken off above deck level it is doubtful that many waves would immerse the upper end of the stub. A cup full of two pack expanding foam inside would prevent flooding through the stub.
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Old 18-02-2016, 22:56   #83
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Re: Change to Deck Stepped Mast

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We just sold our Hanse 342 which had a deck stepped mast. If I'm not mistaken, several, if not all models (at least their new 415) have deck stepped masts.
Yes, clearly I have not been keeping up on boats with deck stepped masts!
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Old 19-02-2016, 06:02   #84
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Re: Change to Deck Stepped Mast

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Yes, clearly I have not been keeping up on boats with deck stepped masts!
Obviously of little experience.
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Old 20-02-2016, 18:05   #85
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Re: Change to Deck Stepped Mast

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Obviously of little experience.
Yes, clearly I need to get to more boat shows! I haven't been to one in.... a lot of years...
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Old 21-02-2016, 05:54   #86
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Re: Change to Deck Stepped Mast

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Yes, clearly I need to get to more boat shows! I haven't been to one in.... a lot of years...
Don.... An alcoholic that won't allow booze in the house is still an alcoholic...

Leave your wallet/checkbook home next time you hit a boat show... A tip from a fellow boatshowaholic...
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Old 23-02-2016, 12:53   #87
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Re: Change to Deck Stepped Mast

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Don.... An alcoholic that won't allow booze in the house is still an alcoholic...

Leave your wallet/checkbook home next time you hit a boat show... A tip from a fellow boatshowaholic...
I don't get much of a buzz off of boatshow boats... has to be either really new or really old:
https://www.youtube.com/user/LHYDROPTERE
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Old 24-02-2016, 06:06   #88
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Re: Change to Deck Stepped Mast

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I don't get much of a buzz off of boatshow boats... has to be either really new or really old:
https://www.youtube.com/user/LHYDROPTERE
Sweet!

Could you imagine being a trimmer???
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Old 25-02-2016, 08:21   #89
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Re: Change to Deck Stepped Mast

I'd pay big bucks just for a turn at the helm!
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