Opinions, re: bluewater rigs

Campylobacter

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So, I've been searching for a boat for long distance, bluewater sailing (not cruising, per se, more along the line of expedition sailing) and it's ticking all of the boxes, except one - it's a deck-stepped mast. The boat is custom; configured and built by the original owner to be seaworthy, so I'm surprised he made that specific decision. 

I've always been strongly biased toward keel-stepped masts, especially for offshore use and am wary that I wouldn't be able to shake being insecure about the strength of the rig when it gets rough. 

Am I making something out of nothing here? It's been a long search and I'm disappointed that I need to start again, but if my assumptions are correct then I need to accept it and move on to other options.

Thoughts? Personal experiences? Many thanks. :mellow:  

 

bgytr

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If the rig is engineered properly for deck step configuration, it shouldn't be a worry.  There's a simple allowance in strenth requirements for deck stepped masts v. keel stepped.  The deck stepped might be a bit heavier, but it can have one big benefit- no leaks.

 

estarzinger

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Am I making something out of nothing here?
yes, you are.

As bgytr says, if it is properly engineered it is just fine.

Deck stepped masts do offer the benefit of often being more water tight, and creating less of a hole if the mast comes down.  And less of a potential cold funnel into the cabin in high latitudes .

edit: I too like keel stepped masts, but I would certainly not discard an otherwise near perfect vessel because it was deck stepped.

 
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Zonker

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Evans has summarized it pretty well. If I had to choose between two otherwise equal boats, I'd pick keel stepped but it's hardly the most important criteria.

I sailed around the world with a deck stepped mast.... wait a second. It was a catamaran. They're all deck stepped. Damn.

 

Panoramix

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Bear in mind that keel stepped is prohibited in most French offshore sailing classes on safety grounds . True a keel stepped mast is more robust assuming that everything else is equal but deck stepped mast are engineered in consequence and you are much less likely to damage the roof with a failing mast that is deck stepped. Many will consider watertightness a higher priority than one less set of spreaders (often the consequence). Even if your boat is insubmersible with watertight compartments (nearly never the case on a cruising boat despite the obvious safety advantages), when green water gets inside you are in real trouble as the boat becomes barely able to operate.

 
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Trovão

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Evans has summarized it pretty well. If I had to choose between two otherwise equal boats, I'd pick keel stepped but it's hardly the most important criteria.

I sailed around the world with a deck stepped mast.... wait a second. It was a catamaran. They're all deck stepped. Damn.
Team Philips must have been keel-stepped... just sayin' :p

 

Jud - s/v Sputnik

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yes, you are.

As bgytr says, if it is properly engineered it is just fine.

Deck stepped masts do offer the benefit of often being more water tight, and creating less of a hole if the mast comes down.  And less of a potential cold funnel into the cabin in high latitudes .

edit: I too like keel stepped masts, but I would certainly not discard an otherwise near perfect vessel because it was deck stepped.


Evans has summarized it pretty well. If I had to choose between two otherwise equal boats, I'd pick keel stepped but it's hardly the most important criteria.

I sailed around the world with a deck stepped mast.... wait a second. It was a catamaran. They're all deck stepped. Damn.
This is good to hear, as I’ve wondered about my own when the going gets rough.  It’s well stayed (inner forestay/aft intermediates, and two backstays; heavy tabernacle with 3/4” bolt.), but I’ve often wondered about the viability of a deck stepped mast.  

I recall Trevor of Iron Bark specifically spec’ing a keel stepped mast for his Wylie 35, which once got me thinking...

BBB79BCE-558F-42BA-8C40-8455DD788333.png

A7840AA1-BC78-47C9-ACB9-982FBD87DCE3.jpeg

 
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Zonker

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Class is in session:

The engineering of keel stepped masts and deck stepped ones is pretty similar.  Deck stepped masts are not as "fixed" a column as keel stepped ones. They have different end conditions and thus different effective lengths. ("L" in the formula below). Pcr = the critical load at which the column will buckle. Let's compare

image.png

where:
E = modulus of elasticity. doesn't vary between mast types. It's a fixed property of the material (aluminum). "How stiff is this material"
I = moment of inertia. a geometric property of the section. "How far away from the middle is the mass" 
L = effective length.  If L = 1 in our example, for a deck stepped mast, L = 0.7 for a keel stepped mast. Trust me on this; you don't want to see the math.
But we're squaring L so L=0.7^2 = 0.49.

So deck stepped has to be about twice as stiff to reach the same critical load. The key point to understand: twice as stiff is not equal to twice as heavy.

Example:

If I have a rectangular hollow tube with dimensions 8" x 5" x 0.125" the moment of inertia in the transverse direction = 14.1 in^4
(this is sideways buckling between the deck and spreaders if you have spreaders)

If I increase this to 9.25 x 6.5 x 0.125" the moment of inertia increases to 28.5 in^4.

So by increasing the mast section just a bit, we can double the moment of inertia.
The weight increase is only ~20%.  But because it's a mast and is way up in the air, it's a lot of negative RM.

So if you're a race boat you'll usually go with keel stepped because it does have big performance gains. (Unless you have lots of RM like an Open 60)

If you're a cheaper builder you'll go for deck stepped because 2m of a metal compression most might be cheaper than 2m of fancy mast extrusion, even if you do have to install flanges on your compression post and bolt it separately.

Class dismissed.

 

estarzinger

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 tabernacle
idk anything at all about tabernacles.  Zonk could probably comment on the 'engineering'/dimensioning.

Can you actually lower/raise the mast using that pivot? Or does it sit on the deck and that pin is simply to secure it?

 
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Veeger

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I think the 'attracton' of a keel stepped mast for offshore work is that presumably you'd have a stump from which to set up a jury rig and self rescue to safe haven.  The watertight integrity argument is compelling however.

 

estarzinger

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Bear in mind that keel stepped is prohibited in most French offshore sailing classes on safety grounds .
I'm curious - how/where specifically is the rule that prohibits it?  It is not in the OSR's, and not in the CE.  Is there a common rule that is somehow written into individual class rules across multiple 'french' classes?  Which classes?

I would be curious to see it; is it somewhere in the class 40 or mini rule? Can you link that?  Edit: I'm just looking at the class 40 rules (2019) and I do not see any prohibition on keel stepping? Do you not consider this a 'French offshore class'?

Is this quintessentially a french 'thing', or do any Italian or british classes use the same language?

I was aware of (strong) preferences on this topic but was not aware of (m)any prohibitions.

 
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accnick

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The boat I built was designed with a deck-stepped mast. I had already owned a larger boat with a deck-stepped mast, but still agonized over it for a boat intended to sail around the world.

During this decision process, I spent the day at Greenwich (UK) making a pilgrimage to, among other things, look at Chichester's Gipsy Moth IV, which was on exhibit there. Looking at the boat, I realized it had a deck-stepped mast.

Never thought twice about it after seeing that.

As others have said, it's just a matter of proper engineering of the mast, its rigging, and the mast support system.

 

DDW

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Zonker covered the engineering. One difference that's been touched on is how graceful is a failure? If a shroud or stay lets go on the deck stepped, it's probably all over the side, but cleanly. On a keel stepped you might have something left to use, but the wreckage might be pretty knarly too. 

Of course unstayed is the way to go, and they are typically keel stepped. If the French have outlawed keel stepped, they've outlawed unstayed too. 

If I liked the boat I wouldn't discount it because of the mast step. 

 

Elegua

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This is good to hear, as I’ve wondered about my own when the going gets rough.  It’s well stayed (inner forestay/aft intermediates, and two backstays; heavy tabernacle with 3/4” bolt.), but I’ve often wondered about the viability of a deck stepped mast.  

I recall Trevor of Iron Bark specifically spec’ing a keel stepped mast for his Wylie 35, which once got me thinking...

View attachment 485956

View attachment 485957
Ooooh...tabernac.....le

 

Jud - s/v Sputnik

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I think the 'attracton' of a keel stepped mast for offshore work is that presumably you'd have a stump from which to set up a jury rig and self rescue to safe haven.  The watertight integrity argument is compelling however.
I randomly met a lady yesterday —well, not too randomly; I was working up my mast and she asked what I was doing, which led to a conversation, and she immediately grasped what I was talking about so I knew immediately she’d ‘done some sailing’— who’d recently come back from all over the Pacific.  She was describing to me a guy on a catamaran who lost his mast recently on the “clipper route” up from Cabo to California (the offshore route north that avoids the horrendous motor/bash to windward, the so-called Baja Bash).  She said he’d been lucky enough not to loose his boom in the dismasting, and had a spare sail or something, so used the boom to make a jury rig and sailed downwind to Hawaii...no keel-stepped stump available, being a catamaran...

 

weightless

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One difference that's been touched on is how graceful is a failure?
If a deck stepped mast comes loose and hops around on the deck until it punches a hole, plunges into the accommodating piercing the owners berth and then falls over the side tearing out a big chunk of deck as it goes, perhaps, just perhaps, the failure is not exquisitely graceful? Okay, that's a remembrance of a story about a rig coming down on a cat that I was told 20 years ago. Maybe it didn't go down just like that or maybe it's the exception to the rule. I suspect odds of a "graceful" failure are better with a deck stepped mast but it wouldn't be a make or break (as it were) point for me. I've sailed offshore on boats with both types.

 
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