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Old 16-02-2016, 12:00   #31
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Re: Change to Deck Stepped Mast

Cutting off corroded part and sleeving on a new section to replace a couple of feet would be a sound engineered solution. External or internal sleeve will need to be riveted on. Welding aluminum dramatically reduces strength. Mast loading below deck is the highest at deck level and goes down proportionately to the step. Since extruded masts have a constant section the are extremely strong at the step. Using a fabricated sleeve or a piece of well fitted 6061-t6 aluminum pipe for a 2-3 foot extension. This would much stronger than raising the step.


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Old 16-02-2016, 12:15   #32
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Re: Change to Deck Stepped Mast

Welding wont matter as it's just in a low stressed part of the rig down there. Some masts are actually welded in many places including up in the highly stressed area of spreaders etc.
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Old 16-02-2016, 12:18   #33
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Re: Change to Deck Stepped Mast

The rig that was keel stepped should be , if anything, more robust an extrusion than a shorter deck stepped design. So the basic mast should be no problem for deck stepping.
Of course a support post is needed below /under the mast.
That is your main support, other than that you need to get rid of the core in the deck.
But there must be a reason that NA's prefer keel stepped on larger boats, although I've never been able to figure what that was..... as the longer the extrusion, the worse the "slenderness ratio" is.
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Old 16-02-2016, 12:43   #34
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Re: Change to Deck Stepped Mast

Cheetako when you weld aluminum you reduce the strength of the material by 75%. Welding a oval to attach spreader only to the sides of mast in a low stress area is engineered process. If a mast is designed to pivot on the deck (deck stepped) great. But a keel stepped mast is designed to pivot at the keel with lateral support at the deck you take high risk by making it deck stepped.


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Old 16-02-2016, 12:44   #35
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Re: Change to Deck Stepped Mast

I haven't read ALL the posts on this problem so maybe someone has already mentioned this; One thing stands out in my understanding of what you are wanting to resolve and that is CORROSION. Fix the cause of this first, otherwise you will just repeat the failure.
In this day and age with all the new types of shaft seals, there is no need to have water in the bilge. If water is coming in from some other "source" that's a whole different matter and more bucks to fix it ... but fix it you must!
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Old 16-02-2016, 12:55   #36
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Re: Change to Deck Stepped Mast

I can't think of a boat over 32', besides the Pearson Vanguard that has a deck stepped mast...
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Old 16-02-2016, 13:06   #37
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Re: Change to Deck Stepped Mast

Keel stepped masts can be lesser strength than deck stepped. Could be smaller section, lesser wall thickness and/or narrower shroud angles. That's the reason race boats almost always have keel stepped masts. In your case, the lesser scantlings may not hold true as there a lot of misunderstood lore about the strength/advantage of deck versus keel stepped masts. Keel stepped might have been the choice in your boat to quell any misconceptions about that controversy. You'll need to get the dimensions and wall thickness of the mast, number or spreaders and location, spacing of the chainplates from the mast centerline, and wire diameter and take it to an engineer. Bob Perry I understand is approachable if you want someone who has a lot of boats out there and may be the designer of your boat. If he's not, getting hold of the designer would be a good source.

It could be a matter of cutting off the mast and stepping it on the deck or require a new mast section and moving the chainplates outboard. Your local friendly naval architect will tell you the answer.
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Old 16-02-2016, 13:11   #38
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Re: Change to Deck Stepped Mast

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Originally Posted by Sailor Doug View Post
Cheetako when you weld aluminum you reduce the strength of the material by 75%. Welding a oval to attach spreader only to the sides of mast in a low stress area is engineered process. If a mast is designed to pivot on the deck (deck stepped) great. But a keel stepped mast is designed to pivot at the keel with lateral support at the deck you take high risk by making it deck stepped.


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Thanks for your info, I'm an Aerospace welding engineer.
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Old 16-02-2016, 13:19   #39
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Re: Change to Deck Stepped Mast

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Originally Posted by Cheechako View Post
W...Some masts are actually welded in many places including up in the highly stressed area of spreaders etc.
But these welds are usually done under well controlled conditions.

Riveting a sleeve and extension is preferred in the field.
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Old 16-02-2016, 13:50   #40
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Re: Change to Deck Stepped Mast

I have rebuilt the steps to raise the stick twice . Last one was a 49 foot ketch with a 63 foot stick . Built the base up with ligumau vitae , a very hard wood . I was working on Tortola at Nanny Key Yard . The wood butcher and the rigger guided me through it . I do not know how long it will last but that was 29 years ago and it is still working .
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Old 16-02-2016, 14:00   #41
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Re: Change to Deck Stepped Mast

the cost of deck reinforcement would be more than the boat is worth. take the mast out and have the bottom sleeved and cuffed.


if you want a deck step mast, put in a tabernacle
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Old 16-02-2016, 14:30   #42
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Re: Change to Deck Stepped Mast

Quote:
Originally Posted by Cheechako View Post
The rig that was keel stepped should be , if anything, more robust an extrusion than a shorter deck stepped design. So the basic mast should be no problem for deck stepping.
Of course a support post is needed below /under the mast.
That is your main support, other than that you need to get rid of the core in the deck.
But there must be a reason that NA's prefer keel stepped on larger boats, although I've never been able to figure what that was..... as the longer the extrusion, the worse the "slenderness ratio" is.
Oddly enough, the opposite is true, according to any of the relevant literature I've read or experts I've consulted.

It has to do with how buckling failure occurs, and the effect of pinning the mast at both the deck level and the keel.
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Old 16-02-2016, 14:31   #43
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Re: Change to Deck Stepped Mast

I began a project to replace floor in my Islander 37, To find under mast support Original 3 foot piece on steel was almost completely gone. This got past the surveyor to my surprise. There had been put a large stainless deck collar I think at time of Mast and boat being re painted before sale .I used a u-tube to make a plan to re- pocket base with non corrosive material and a lot of Epoxy. one thing I learned was to address original causes for corrosion and make sure mast in bonded properly. and be thankful I had been able to fix this before I had gone forward with floor. Fair winds
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Old 16-02-2016, 14:54   #44
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Re: Change to Deck Stepped Mast

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Thanks for your info, I'm an Aerospace welding engineer.
You are absolutely right, I have been welding for over 60 years. Don't know where some of these folks are getting their information. (If this were true, don't think I would be getting on those 747's.)
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Old 16-02-2016, 15:06   #45
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Re: Change to Deck Stepped Mast

Quote:
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Thanks for your info, I'm an Aerospace welding engineer.
Aren't most if not all aerospace fabrications heat treated after welding to bring back strength?


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