Mast Step and Compression Post

[Nekton73]

In a recent post on standing rigging a comment came up that there may be some underlying mast compression issues. It made me think about my own boat and something I’ve been putting off dealing with, so I thought I’d see if anyone here might be able to point me in the right direction of a long term proper fix, if it’s needed.

I’m away from my boat for a while so unable to take pictures or measurements, but have included pics off the net to illustrate what my boat looks like. I’m thinking I’d like to have this all researched, thought out, and planned for when I am finally able to dig in to the project, possibly this fall.




My Grampian 30 mast is deck stepped and since I’ve owned it the compression post has always wiggled easily when the mast is not stepped. I’ve never tried wiggling it with the mast up and the rigging not tensioned, so can’t speak to that, but can say it’s rock solid when the rig is tuned and loose when mast is down.

The header inside the cabin, below the mast, has spider cracking radiating out from the compression point. The mast step above deck perhaps does appear to be slightly depressed, but not so that it jumps at you as clearly deformed, just slightly if you look very closely.




The bilge has a series of stringers laid between each side of the hull and they simply support the floor, except for the one directly under the compression post which I think also bears a lot of the compression load. The stringers do not reach to the bottom of the bilge, they are cut to match the curve of the hull so the load is distributed there and not directly down onto the keel step. The stringers have all been replaced at some point, and a very poor job was done. The ends are badly cut, often resting on the hull in only a couple of places, with the majority of the radius not touching. They are glassed in with tabs, also not well done. The one below the compression post is particularly bad, the tabs show signs of flexing and I don’t think they are providing much support.




As a temporary support I put a jack post of sorts under the stringer directly below the compression post, but this means force applied directly down onto the keel step.

So my general questions:

- In other’s experience, is it normal for a compression post to be slightly loose with the mast not stepped? Should I even be concerned about this? The spider cracking in the header makes me think some amount of deflection is happening, however the bulkhead is carrying some of the load so it hasn’t worried me greatly so far.

- I presume the compression load from the mast really should not be directly down onto the keel step, a la my temporary support solution, but directed to the side via the stringer. Is my jack post a majorly bad idea? If no then can I perhaps jack up the stringer to remove the slop in the post when the rig is not tensioned, or is this just a bandaid?

- I suspect the best answer is to redo the stringer, and I’m prepared to do that, but unlike the others it is not easily accessible as it is trapped under the floor and bulkhead so it will mean some awkward fiddly work. Such is boat life.

- How about the header that distributes the load across the bulkhead. I’m assuming it’s wood encapsulated in fibreglass. I’m not sure if it has balsa core over the top or not. I can drill to find out. My worry is that the bolts holding the mast step down have allowed water to seep in and rot that inner wooden header so that there’s really just the fibreglass for support and strength at this point. Comments/suggestions as to what to look for, diagnosing, fixing if needed? I’m thinking a full on removal and replacement is probably the way to go.

- If I’m going to that extent then I’m thinking I should just remove the step foot altogether (an aluminum U-channel about 6” wide and 24” long, held down by large bolts, 6 I think), cut away the fibreglass top, and re-core at the same time as I do the header. Just get it all done at once.

- Anyone been through this and have thoughts or wisdom to impart for doing it well, making it easy, things to watch for?


Thanks in advance.

[Bowdrie]

It was a bit confusing at first.
"Stringers" are fore-and-aft members.
The parts you're talking about would be, (in common vernacular,) called "Floor timbers".
Anyway, no, there shouldn't be any looseness in the vertical.
There should be at least two or three of the floors/floor timbers, spaced such that the aluminum channel spans all of them.
They do need to fit the hull pretty well, and not depend upon some tabbing to carry the vertical load.
More-or-less standard practice is that the distance of the space that the compression post fits in is a "small" bit less than the length of the post.
Not much, a 1/8th of an inch or so, you're looking to get some "preload" on the post to prevent compression/deflection of the cabin top.
Sometimes some shimming might be needed.

[Nekton73]

Hey, thanks for the reply, apologies for the incorrect terminology.

Not sure what you are referring to when you talk about aluminum channel, or two or three of the floors/floor timbers. I don't have multiple floors, nor aluminum channel.

I included a picture of a Grampian 30 bilge that looks very similar to mine for reference to hopefully make it easier to understand what I am dealing with. They are just plywood timbers(?) that span the bilge. Looks like 3/4ply laminated to make a 1.5" thick piece, cut poorly to match the curvature of the hull, and tabbed in with crappy fiberglass work. The floor (sole?) of the cabin is just 3/4 plywood laid over those timbers(?) and covered with outdoor carpeting. Sounds ugly but actually quite serviceable and doesn't look too bad.

One positive from your comments though is that my compression post has less than 1/8" between it and the header (if that's the right term), really no visible space at all, so perhaps nothing to worry about here. I really only noticed the wiggle because I've leaned against it while inside on the hard. Once the rig is up and tensioned there is no wiggle, everything is solid.

[Benz]

You definitely should check for water ingress at the mast step bolts on the cabintop.
There's no problem with the compression post wiggling when unloaded-- if it won't move when tuned, then the deck is just compressing onto it--no big deal.
The floors (the cross-pieces at the bottom) should be made solid, though. If it's easier to, why not rest everything on the top of the keel, if that's solid? My compression post sits on a scaffolding jack that in turn sits right on the keel. That way it's resting on a rock-solid surface and I can snug it up so it doesn't wiggle when the mast is down.

[Nekton73]

I'd love to do that. It's one of my core questions actually. Is it ok to have the compression force of the mast distributed directly downwards onto the keel stub through a jack post vs. sideways through the floor timber?

Ignoring the poor quality of constructing and tabbing the current timbers it appears they were done this way for a reason instead of just supporting directly against the keel stub. Or is the reason perhaps just to avoid having wooden floor timbers sitting on the bottom of a wet bilge rather than how force is distributed?


I don't have enough room for a scaffold jack but what I am using currently is a giant bolt, like 2" diameter, inverted with a double nut. The head of the bolt sits on the keel stub. The first nut is screwed up so it presses firmly against the timber, and the second locks it into place. Found it in the scrap yard and it fits perfectly in the space with mm to spare.

[MicHughV]

Let's start with the basics.

Your boat is a 30 footer...having done mast compression forces calculations several times before, my seat of the pants guess is that the compression load on your compression post/bulkhead is likely to to be in the 6-8,000 lb range when fully loaded, under sail, etc. That is a significant number, even if off by a few thousand pounds one way or another.

What ever you chose to do needs to keep this in mind as eventually this load will be born by the bottom most point. The trick therefore is to distribute this load over the widest platform you can manage.

Think of it this way, a 100lb woman wearing high heels.....of say 1 square inch surface area, exerts 100 lbs per square inch.
A 200 lb man, wearing shoes of say 72 square inches (two 12 x 3) exerts only 200/72 = 2.78 lbs per square inch.
A significant difference.

[Bowdrie]

Originally Posted by Nekton73

Not sure what you are referring to when you talk about aluminum channel, They are just plywood timbers(?) that span the bilge.
Looks like 3/4ply laminated to make a 1.5" thick piece,
Once the rig is up and tensioned there is no wiggle, everything is solid.

Sorry, I mistook the set-up/location of the channel.
My "gut feeling" is that a single 1-1/2" thick floor is on the light side.
I'd want about twice that, fitted well to the hull curve.
It should be solid without the rig tensioned.
Right, the supporting floor is kept out of the bilge to prevent water from getting into the wood.
I know, "fitted well to the hull curve" is so easy to say, but not so easy to do on a boat that already has an interior and bulkhead(s) fitted.

[Lee Jerry]

Originally Posted by Nekton73

One positive from your comments though is that my compression post has less than 1/8" between it and the header (if that's the right term), really no visible space at all, so perhaps nothing to worry about here. I really only noticed the wiggle because I've leaned against it while inside on the hard. Once the rig is up and tensioned there is no wiggle, everything is solid.

I would be a little bit concerned about the wiggle in the compression post when unloaded. I doubt it was there originally. Therefore it is an indication of some loss of material integrity in the end(s) of the post. Rot, if you will (maybe not exactly, but in effect). It appears to be a wooden post, so not unexpected or uncommon in a wet environment. But, if possible, I think you will want to trim a bit off of the end(s) and insert a new section that makes a tight fit. Currently, the deck is flexing to make that tight fit. This might be acceptable now, but will only get worse.

One way to check: put a straight edge vertically on the adjacent bulkhead when unloaded. Then repeat when the rig is loaded. Is there any increased bowing? If yes, then the bulkhead is taking significant load.

Originally Posted by MicHughV

Think of it this way, a 100lb woman wearing high heels.....of say 1 square inch surface area, exerts 100 lbs per square inch.
A 200 lb man, wearing shoes of say 72 square inches (two 12 x 3) exerts only 200/72 = 2.78 lbs per square inch.
A significant difference.

I think you forgot about the toe of the high heel shoe. She isn't balancing just on the heel. So more like 8-10 sq in total.

[Nekton73]

The post is teak. I have clear view of both ends and absolutely no sign of rot, wet or dry. The bottom sits on top of the plywood floor (sole?) which sits on top of the timber (floor?, I'm so confused), which is a good 12" above the bottom of the bilge which has never (at least as long as I've had the boat) seen water that high, nor is there evidence there ever was.
The timber supporting the post is plywood (as described earlier) and shows no sign of rot, but I am now leaning towards the conclusion that it's either been like this for the life of the boat, or since the timber was replaced.

From the sounds of the replies so far I think I'm best off to just bite the bullet and replace all the stringers/timbers/floors and do them properly. I can scribe the curve and create a template fairly easily, it's just the work to remove the old ones and tabbing, clean the surface, install, glass, etc.

Also I think removing the step and drilling a couple of test holes is probably in the cards. If I find no rot I'll be happy and remount with confidence. If I do find rot I suppose the answer is to drill farther out until I find the extent of it and then replace the whole section.

[Marathon1150]

The mast step in our Beneteau Idylle 1150 failed about 5 or 6 years ago. This was a known problem with this boat but ours was fine until the bilge filled with water and soaked the teak block inside the floor timber (which are hollow GRP square channels bonded to the hull). Long story involving an absentee boat watcher. It may not have been a problem except Beneteau drilled a hole through the block to run some wires and a PO drilled another one to run a hose. I noticed some deck sag and found the situation in the following picture:

[/url]

We pulled the mast, removed the compression post and cut away the bad glass. Ultimately the step was rebuilt with epoxy and tri-radial fibreglass cloth. Many thousands of miles later the step is fine.

When the mast was off we drilled holes into the deck and found that the wood core was all fine but to strengthen the deck we filled a bunch of holes in a pattern around the foot of the mast with some liquid "epoxy" designed to penetrate and strengthen damaged wood.

Good luck, its a big job.

[Nekton73]

Wow, that looks ugly, thankfully mine is not showing visible deformation. I think I'm resigned to the remove and replace solution though, just for peace of mind if nothing else.

Interesting suggestion re the liquid epoxy. I had not heard of this. A quick google search comes up with several products. Do you recall what you used? I would much prefer this option vs removing and re-coring, assuming I find rot. I've already done a larger section in my cockpit sole and on one of my side decks and it's something I'd rather avoid if I have to.

[Marathon1150]

Originally Posted by Nekton73

Wow, that looks ugly, thankfully mine is not showing visible deformation. I think I'm resigned to the remove and replace solution though, just for peace of mind if nothing else.

Do you recall what you used?

I'm pretty sure it was this:

Clear Penetrating Epoxy Sealer

[Nekton73]

Thanks.
Huh, very interesting. Just penetrates into the voids and hardens, replacing the original structure. Definitely going on the option list for small area rot repair.

[Lee Jerry]

Originally Posted by Nekton73

The post is teak. I have clear view of both ends and absolutely no sign of rot, wet or dry. The bottom sits on top of the plywood floor (sole?) which sits on top of the timber (floor?, I'm so confused), which is a good 12" above the bottom of the bilge which has never (at least as long as I've had the boat) seen water that high, nor is there evidence there ever was.
The timber supporting the post is plywood (as described earlier) and shows no sign of rot, but I am now leaning towards the conclusion that it's either been like this for the life of the boat, or since the timber was replaced.

From the sounds of the replies so far I think I'm best off to just bite the bullet and replace all the stringers/timbers/floors and do them properly. I can scribe the curve and create a template fairly easily, it's just the work to remove the old ones and tabbing, clean the surface, install, glass, etc.

Also I think removing the step and drilling a couple of test holes is probably in the cards. If I find no rot I'll be happy and remount with confidence. If I do find rot I suppose the answer is to drill farther out until I find the extent of it and then replace the whole section.

Rot was probably too strong a word (I tried to minimize it). But you know how the ends of a board will "smush" in time. Any little settling over time will loosen the fit. This could be from the post ends or the structure below compressing or wearing just a little bit - anywhere in the post "system." It appears the header is showing signs of this happening (or happened, past tense), where the deck had to bend slightly to meet the new height.

Do you know what the bulkhead looks like with the mast in?

[Nekton73]

I've never put a straight edge on the bulkhead but it does have a sliding door attached to the forward side and the door never binds or rubs, and I can't say I've ever noticed anything unusual about it. Honestly I think the issue is the supporting timber under the post. All evidence points to that.

I was really just trying to get a read on how severe this issue sounded to some other experienced ears, and some good input received so thank you all. My read so far is that this is (thankfully) probably not that serious and if I attend to it now it should be solved for a long time to come (as long as I do it right). I think I'm going to suck it up and replace the support timber. I'll do some investigative drilling up top and repair accordingly depending on what I find. Pretty excited about the penetrating epoxy if it is required, seems like a lot less work than ripping up another section of deck.
Other insights certainly welcome if anything else jumps to mind.