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Old 02-04-2010, 01:24   #1
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Adding Support Post Under Mast

I have a Bristol 27 sailboat, it has a deck stepped mast, and there is no post directly underneath it going to the keel. Instead there is a very fat beam going across the boat under the mast (Actually slightly forward of the mast), and the beam is well supported on either side by mahogany.

I noticed the cabin is slightly flattening under the mast, and a crack in the fiberglass is forming. I think this is because of years of compression from the mast. I imagine it is still very strong, but I have some concern.

I am wondering if it would make sense to add a post (probably painted steel pipe since I can get this free) which I could fix in place and form a solid fit going to the keel... or maybe this is not needed after all?
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Old 02-04-2010, 01:33   #2
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If you are seeing deformation and cracking where the mast is deck stepped then I would not hesitate to shore it up as you suggest.

It could last for years as is... but why have that concern in the back of your mind the next time you are pumping into a short head sea under full press?
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Old 02-04-2010, 01:36   #3
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Aluminum tube would work, as would wood. Diameter of either should be about 1/15th the length. Probably around 5cm.
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Old 02-04-2010, 07:59   #4
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I'm surprised the beam is forward of the mast rather than under. Is that normal? Wonder if it was moved at some time? If David ("FloridaWriter":resident Bristol guru) is looking in, he might have the answer to that...
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Old 02-04-2010, 12:00   #5
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The problem with the beam, is it would make it difficult to move between. There isnt a lot of space. Maybe instead I could use 2 beams going to either side forming a triangle, but giving me more space to move under.

As for the diameter... I thought a larger diameter pipe would be stronger to shearing or bending, but for compression, isnt the thickness or overall weight the determining factor?
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Old 10-04-2010, 23:22   #6
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Yes, the tube 'weight' gives it the necessary compressive strength. The diameter keeps it from buckling. 15:1 is a general thickness ratio that won't buckle if both ends are basically fixed. I kinda presumed that such a tube, in aluminum, would have plenty of compressive strength for your needs. Aluminum is exceedingly strong in compression. If somehow you locate a tube made like a beercan my advice won't apply.
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Old 11-04-2010, 05:47   #7
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The B27's I remember all had plywood partitions under that beam. Am I confused? Well supported plywood is really strong in compression too - if it's well supported in turn. I'd go on a rot search. If some sections of the supporting system have rotted that would account for the sag - and certainly need fixing. There's usually a reason for something like this besides "years".

Although it's a pain, I'd also unstep the mast and poke around under the mast step.

If there's no rot then you could still add a support post but you don't need to ruin the cabin. The forces just aren't that great in this boat. Some 3/4" ply, a 2x4, or a 1" pipe would all be fine if it's supported along it's length by the existing structure so it doesn't come out of column. You don't need to stick it right under the mast step. I would not use plain steel. It will have rust spots in a year and be a constant headache.

Carl
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Old 13-04-2010, 13:41   #8
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Originally Posted by geckosenator View Post
I have a Bristol 27 sailboat, it has a deck stepped mast... I noticed the cabin is slightly flattening under the mast, and a crack in the fiberglass is forming. I think this is because of years of compression from the mast. I imagine it is still very strong, but I have some concern....
From what I’ve read (not done !!), it sounds like Carl may be right on the money for this boat… I’ve got a B24 with a very similar arrangement and although I’ve not detected any significant sag in the area on mine, I’m going to pull down the mast for some unrelated reasons and expect to basically inspect, reinforce and buttress similarly… In my case, there isn't room for a post, but if the bulkhead passes muster, I still plan to reinforce with oak arch and carry the compression loads down either side of the off-center doorway… since I don’t know the state of the deck yet, I’m expecting to have a good look-see there with (perhaps) some upgrade through the decking, between the mast-step and the bullhead… doesn’t appear too technical to me (in my case), but a fair amount of elbow grease – depending…
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Old 13-04-2010, 14:41   #9
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The cross beam sitting on top of the bulkheads is rotted. Water gets in through the wiring run for the mast light. This beam is the mast step support. If the deck is flexed, it's rotted.
Had to replace this on a B24 I owned long ago.
It was not a fun job but not so bad either. You just need to like eating fiberglass for awhile.

Glad to give you a run down on the procedure if you need.

John
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Old 14-04-2010, 06:44   #10
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Glad to give you a run down on the procedure if you need...

John
I'd be grateful to learn anything you think would be helpful in this; or feel free to email me: dcstrng1213 att yahoo dot com
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Old 14-04-2010, 13:52   #11
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Larry,

I'll put something together and shoot it to you. Basically trying to shore up aft or forward of the cross member will not be ideal, but better than nothing. Shoring up the bottom of the cross member will do nothing as the top and center of the piece is probably soggy.
You can run a drill into it and see if the shavings come out wet. If good just epoxy the hole up and away you go. If wet, then you need to make the change.

I'll try to get something to you in the next couple of days. Beating my brains over tax forms at the moment.

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Old 14-04-2010, 13:59   #12
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painted steel pipe would work .. my old boat had a 3" SS pipe as a mast support.
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Old 18-04-2010, 09:23   #13
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Alrighty then, taxes are filed so here's what I did and some thoughts if you just want something to last a few years;

The cross beam is a ply laminate that transfers the mast loads to the (ply) bulkheads, therefore down through the boat where it is tabbed in and the area it sits in the keel. The bulkheads also serve as the chain plate attach points. Any water coming through the deck at the chain plates will deteriorate the bulkheads at the bolt holes in the bulkheads.
While you have the mast down, pull the chainplate bolts in the bulkhead and check for soft wood. If none, you have lived an honorable life and have been rewarded. If soft wood is found, determine how far out from the hole until you get into good ply. I took a hole saw and enlarged the bolt holes to remove the soft wood, made plywood plugs and installed with epoxy compound and re-drilled the holes in the new plugs. Then I added a strap to the forward side of the bulkhead to pick up the existing chain plate attach points and extended further down about 18 in and installed additional through bolts into known good wood. The strap allowed additional support to be sure the plugs would not try to twist or work themselves out that just an area washer might have done. (this was instead of changing the bulkheads as the bottoms and the rest of the wood was sound).
On the cross beam;
I sawed out the beam, laminated up 5 layers of 3/4 marine ply (I think it was 3/4, you just need to measure the inside dimensions of the old glassed in structure), wrapped that in glass, installed with microballoons to take up any gaps from the deck to crossbeam and glassed the whole thing to the bulkheads and deck. (nasty amount of sanding and glassing require but as close to original as possible)
If you are looking to do something that does not require complete replacement, you might try to cut out a section of the aft wall of the crossbeam, dig out the wet ply and do an insert ply block then put a post under that. (re-glassing the hole made in the crossbeam) (You must have good solid bearing surfaces from deck to support). Or even just cut out the beam section and run the post to the bottom of the deck. (with this method you will need to work out how the mast step plate is going to be bolted through the deck and into what to keep it stable.

Good luck, The Bristol is a tough boat so you can get away with a lot if it's not pushed with or without repairs.

John
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Old 18-04-2010, 20:37   #14
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Grat advice from jkd - Well done!

My only addition here is if you are considering a compression post, you need to consider the structure at the hull upon which the compression post will transfer its loads.

I presume the boat was designed with a cross beam, and wes not dsigned to sag. Adding a post or shoring the beam would be a short term fix for what might be a rotting beam problem.

My boat has a compression post that sits on a molded beam that the keel mounts to. I agree having the compression post takes up interior space I wish I had...
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Old 19-04-2010, 12:11   #15
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Any water coming through the deck at the chain plates will deteriorate the bulkheads at the bolt holes in the bulkheads.
While you have the mast down, pull the chainplate bolts in the bulkhead and check for soft wood. If none, you have lived an honorable life and have been rewarded... Good luck, The Bristol is a tough boat so you can get away with a lot if it's not pushed with or without repairs.

Super, that helps clarify things considerably and I’ve copied it to my 3-ring instruction notebook…

I agree on the B24 – generally a tough little chunk… I’m a recent owner of our B24 in the past few years, but clearly a former owner had chain-plate problems because the outer foot or so of the bulkhead has been replaced and glassed, with newer chainplates hefty enough to hoist half the marina – they are heavier than I recall on my Irwin 42… however, some of that will probably be for naught if I get my way, because I’m planning to go to external chainplates and double lower shrouds; notwithstaning about twenty pounds of epoxy in the outer edges of the bulkhead, the deck continues to leak and the pass-through continues to be a source of weeping, and local deck delaminating (although not particularly structural, still not reassuring...).

In any case, since the mast in coming down anyway, I want to spend some quality time with the mast step and where the loads go…
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