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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 04-27-2011
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Cabin top sags under mast step

I've seen photos of a Kittiwake 23 where the deck beam beneath the mast had deformed, cracked and bent considerably beneath the mast step, but sadly the photos aren't on-line any longer. I do have a pdf of the photos that I would email to you if you wish.

Note, as the cabin top sags, the shrouds slacken. Re-tensioning the shrouds increases the stress on the cabin top, causing it to sag more. Continuation of the process will crack the underside of the cabin top beneath the mast step as you convert from a deck stepped mast to a keel stepped mast. This what I believe happened in the Kittiwake photos.

There are some threads here , here and here that discuss this issue.

A deck stepped mast needs a beam or bulkhead under the step to carry the loads to the hull sides and or through compression post(s) down to the keel structure. It sounds as though your boat doesn't have a visible beam, or maybe not one at all, but I think that's unlikely. Possibly the post(s)was moved?

The need is to provide, beneath the mast step, proper structural support that will carry the mast compression loading down to the keel structure. This is usually done using a bulkhead (which can also resist and balance out somewhat mast loads and chain plate loads from the shrouds) or a compression post (or two). If the compression post on your boat is offset from the mast, you need to figure out how it was intended that the load be carried down... A post offset fore or aft is structurally inefficient, it results in eccentric loading, lots of bending of the post and also needs a very strong cabin top to resist bending there. Was the post ever relocated? Usually, directly under the mast step, there's a bulkhead, or a beam under the cabin roof at the mast step with compression posts either side of the passageway opening (see how it's done on the Kittiwake for example)

After you determine if the post was ever moved, check inside the cabin top under the mast step for cracking or other damage. A bulkhead, a post, or beam and posts under the mast step would most likely solve your problem and return structural integrity to your boat's hull. Such a beam can be composite aluminium and wood or fg, cambered to match the original camber of the cabin roof, and well tabbed into the roof, and properly supported by posts or bulkhead to the keel. Not too too terribly difficult (mast off) but you might need a beam or bracket at keel level also to pick up the loads at the bottom...

Good luck, let us know what you find and what you do!
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  #12  
Old 04-27-2011
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keep looking you will figure it out





Cals for example sag under the post
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Old 04-27-2011
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I googled bombay clipper 31 and found some interesting information. The manufacturer's brochure shows the compression post and main bulkhead sit pretty much in line but so the compression post is just aft of the mast base. The offset seems to be minimal and should not account for sag. See this link: (http://forum.chaparralboats.com/publ...Clipper-31.pdf)

This link is a short discussion of the issueBombay Clipper 31 Mast/Deck Compression Area - SailboatOwners.com.

There does not appear to be an owners group for the BC31. Maybe an inquiry to "Good Old Boat" magazine can produce some leads. Also since it is the predecessor to Island Packet Yachts, you might inquire with them about this problem and if they have a recommended fix.

Your problem is not unique and several fixes are possible, some more expensive than others. But any fix should procede only after indentifying the underlying problem(s) and then engineering a solution. Since the brochure specifically mentions deck core material for strength and insulation you will need to look for core rot.
Good luck, the BC 31 looks like a great boat for comfortable cruising.
John
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Old 04-27-2011
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Thanks John,
There are several discrepancies in the brochure verses my boat. I beleive the manufacturer made changes as the design was built. I have been on another B31 that is different structurally from mine. My mast is stepped 8 inches aft of my c post and bullwork. The schematic shows the post forward of the bullwork and mine is aft. Go figure. If I do not have core rot I am sure my fix will work. Hopefully I will get it done this week or next. I will try and post pics then. thanks again. Wade
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Old 04-27-2011
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Quote:
Originally Posted by abode View Post
I have a 31 Bombay Clipper 30 yrs old. I have noticed the cabin sole under the mast is sagging. The boat has a compression post that is set ahead about 8 inches of the mast. I have read this problem is not uncommon for many boats. My proposed fix is to attach low on the compression post and build a truss like structure up to the cabin sole with an adjustable length that I can use to jack up the sole to my liking. This places a post under the mast and transfers all weight where it was designed to go. Does anyone foresee a problem with this? W
Not sure about your BC 31 but my cabin top has a wood core. You should have a couple of inspection covers over the galley and in the head that provide access to where the traveler is thru-bolted. Once the covers are removed you should be able to look up and see the bottom side of the wood core easily with a flashlight. Not that the wood core has anything to do with your sagging issue.

What material is your compression post made out of? How many inches behind the forward bulkhead is it? How much sag are you talking about? Does the amount of sag appear to be the same on the exterior cabin top as it is in the interior molded cabin liner? Can you take photos and post them here? Is your post plumb and square with the world? Did you look under the sole at the compression post base to cofirm it's not rotting were it sits on the top of the keel?

I'm somewhat confident that my compression post is directly under the mast but out of curiousity I will take a quick measurement when I'm at my boat tomorrow. The stainless steel compression post was replaced with a wooden post in my boat before I bought it. Above the post a large wooden "plate" was installed that spans out much further than the post itself dispersing the load over a greater area.

Rather than trying to engineer a truss system I would pull the mast and look at installing a second post further aft and more under your mast.
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Old 04-27-2011
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Quote:
Rather than trying to engineer a truss system I would pull the mast and look at installing a second post further aft and more under your mast.
Seconded; as pointed out in my earlier post, a single post is best, place it vertically under the mast step. Two posts equally spaced on around the mast step and with a beam directly under the mast step and spanning between their tops, is as good a solution. Sometimes the beam runs side to side, sometimes front to back, depending on where the posts and mast step are positioned, and what works for the cabin layout.

Avoid a single post offset from the mast step, whether it has a truss to support the step or not. The loads will be off centre from the post, the bending stresses will be made much larger because of the offset, and will cause all kinds of problems... It's called an eccentrically loaded column and is avoided by engineers whenever possible, and when not built very strongly. Easy for you to avoid!

Keep it simple, best: put a post under the mast step, or next best and more work put a second post on the other side of the mast as far away from the step as the first post, with a (likely fore and aft) beam beneath the mast step, across the post tops.
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Old 04-30-2011
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Thanks for all the info guys. I think I have it figured out now. The mast is stepped on a fairly large aluminum? or whatever plate. The compression post is placed below the forward edge of this plate. All is good both high and low at the post. Here is what happened. The boat has a roller furler. When tensioning the back stay to get the fore stay tight the mast was racked to far back. The mast acted as a lever and pressed the aft end of the plate down and depressing the cabin top.The fix will be to remove the furler and shorten the forestay removing the excessive rake and lifting the top back into place. I believe I will then add an additional support with another post directly under the mast. I have never fooled with a roller furler but I guess now is time. I believe it is an older CDI. Thanks for all the help. Far cry do you use a tension gauge to rig? And do you know what my rig should be tensioned to initially. Thanks again, Wade
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Old 05-01-2011
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Quote:
Originally Posted by abode View Post
Thanks for all the info guys. I think I have it figured out now. The mast is stepped on a fairly large aluminum? or whatever plate. The compression post is placed below the forward edge of this plate. All is good both high and low at the post. Here is what happened. The boat has a roller furler. When tensioning the back stay to get the fore stay tight the mast was racked to far back. The mast acted as a lever and pressed the aft end of the plate down and depressing the cabin top.The fix will be to remove the furler and shorten the forestay removing the excessive rake and lifting the top back into place. I believe I will then add an additional support with another post directly under the mast. I have never fooled with a roller furler but I guess now is time. I believe it is an older CDI. Thanks for all the help. Far cry do you use a tension gauge to rig? And do you know what my rig should be tensioned to initially. Thanks again, Wade
Wade, I don't use a tension guage. A very good tutorial can be found here though. Adjusting Your Rig
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Old 05-01-2011
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The cabin top is probably wet, rotting and failing in the section near the mast and the post. The post is supporting the segment above it,
but is not supporting the rotted piece beneath the mast. So the top is flat at the post and sagging at the mast.
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Old 06-08-2012
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Re: cabin top sagging under mast, 31 bombay clipper

Sorry it's been a while, I removed the bulkhead and drillled the top there was no rot. Some water was leaking at the hole for the mast wiring. I made a pipe with adjustable end. placed under the mast and by loosening the rig and extending the pipe raised the top back into original position. Problem solved for about 25 dollars and i like the more open feel of the boat now. The old compresion post was left in place, the new sets back about 8" and is fine. It still rests on the I beam below deck. can't get the pic to upload
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