deck-stepped mast prob

Hello,
I am needing advice on how to create a stronger than original deck-stepped mast cabin roof structure. Before I touch it, could you give me some advice and text references for current well built standard practice? What would a well-built cabin roof, bulkhead, doorway span, look like? The 38' mast on my 28' sailboat was not supported properly. The support beam is tiny, only 1 1/2 inches wide, 2 inches thick, and tapers with the roof arch to 1/2 inch thick over 31 inches of length. There is no post, the mast is over the doorway.
Thanks

 

Hi, West.

You do not give nearly enough information for even a preliminary response. For that I would need at least the following.

1.) the design displacement of your boat, or, better yet the actual, as used, weight.
2.) The Beam of your boat.
3.) The Width between lower shrouds or stays
4.) the height of the lower shrouds or stays, or the angle from vertical that they meet the mast.
5.) If your boat is a center boarder or a keel boat.

The five things I just mentioned are the minimum facts I would need to even come up with a good guess.

Have you seen any evidence that the present structure is not up to the job?

Have the shrouds or stays on the leeward gotten unusually loose when under sail?

Have you seen this doorway sag downward noticeably while under sail?

If the answer to the last two questions is "no.", then the present structure is somehow up to the job. There is no telling how that was managed.

The beam could have metal (probably stainless steel) in it.

The cabin top could be extra thick as well as the bulkhead that has the doorway through it.

There is any number of ways something could be made strong enough and not look strong enough.

 

Unless you have a custom design, it would be simpler if you just provided the year, make and model of the boat. If it's a custom then the design number and designer.

 

Deck stepped mast prob

Sharpii2,
Thanks, I’m new to this
You asked about:

1.)Design displacement: 6200 lbs (attachment: Ajax28Specs.doc)
2.)Beam: 9 ft
3.)Width between lower shrouds: 8 ft starb to port (attachment: RiggingSpecs.doc)
4.)Height of the lower shrouds: 16.2 ft from mast base to tang rivets
The angle from vertical that they meet the mast: 14 degrees.
5.)Keel boat: bolt on lead ballast: 2700 lbs

Have you seen any evidence that the present structure is not up to the job?
Have the shrouds or stays on the leeward gotten unusually loose when under sail?
Shrouds loosening while boat sits at dock, no more threads available on the turnbuckles, salon door started catching slightly at top starboard side after tightening shrouds to correct tension (still pinching despite being on the hard for one year with the mast down), ceiling liner bulge beneath aft end of tabernacle. (attachment: MastStep1.doc)

Have you seen this doorway sag downward noticeably while under sail? No, I’ve only sailed it in less than 5 knots, but cabin roof under the tabernacle is balsa cored (appears to be still dry but should it be solid here?)

The beam could have metal (probably stainless steel) in it. I’ll look again, I believe it is aluminum but it is on the head side of the main bulkhead. It does not support the aft half of the mast (attachment: MastStep1.doc)

The cabin top could be extra thick as well as the bulkhead that has the doorway through it: balsa core is 3/8 thick sandwiched between 2 layers of 1/8 inch glass.

There is any number of ways something could be made strong enough and not look strong enough. Should the load be transferred to a grid of sturdy floors? There is no grid, only one broken floor (spanning the doorway at the main bulkhead) and no longitudinals. (attachment: MastStep2.doc)

Westwind2

 

If it were my boat, I'd cut the bulkhead back to each side a few inches and bond in a replacement opening---- this time with a doorway with a full half circle top and bottom opening. You still have the same headroom and foot room.
Your door's side is so close to the centerline, tha curves in the opening allow transfer of loads through what amount to strong webs. This chould be stiffened with a beefy lamination at the doorway itself (in the form of a glued trim at the opening) of solid wood about 2" x 2" or so, and attachment to the head's wall (assuming the head door is not simply attached to the main bulkhead).
It's not necessary to engineer this structure since it provides support well in excess of the original design. It cannot deform except under tremendous loads.
Any thin member, no matter how engineered, has very little margin prior to failure, and on boats where your life is dependant on things holding together, the design should be able to withstand forces well beyond what the sea can dish out. The suggestion here adds little if any weight and is even pleasant to look at.


Alan

 

Allan, I really like your idea. Does one scarf the replacement opening to the old bulkhead to prevent it from hinging along the vertical join? The top of the existing bulkheads are not tabbed into the roof. If I simply slide the replacement opening up into the ceiling liner rabbet I maintain standing head room. Would this be sufficient? Or does your idea include two laminated beams—one against the roof wide enough to support the tabernacle (fore and aft and side to side) and one of trim, below, around the doorway? As there are no floors, does the top beam also function to prevent the sides of the hull moving inward with shroud tension? If so, would the top laminated beam need to be continued to the chainplates? i.e. to transfer horizontal compression loads between starboard and portside hull section at the chainplates and to transfer vertical compression from the tabernacle to the floor. What do you think? Am I interpreting your idea correctly? I’m imagining this solving the salon door pinching and the heel of the mast coming through the roof in a practical, structurally correct, and aesthetically pleasing solution.
Oh yes, the head door is only attached to the main bulkhead; there is no head wall. The head is merely a corridor between the salon and the v-birth. Do you foresee a requirement for a vertical laminated post or panel here on each side of the main bulkhead where it joins the replacement opening? Thank you for sharing your expertise.
Westwind2

 

Alan W.

 

Alan,
Thank you for advising that I do not need any additional beams.
You wrote: I'm surprised the existing bulkhead is not tabbed in. All the bulkheads remain well tabbed to the hull. However, they were not tabbed originally to the cabin roof; perhaps the factory added the roof/deck/liner assembly later. The fiberglass liner has ¾”grooves that accept the top of the bulkheads.
Alan,: I need clarification with regard to your comment about the replacement doorway (bulkhead) providing adequate cross-sectional (c.s.) support for the compression forces of the mast. I am imagining that I need side to side, and fore/aft, vertical and horizontal cross-sections supporting the mast-step.
RE: Side to side c.s. support of mast-step: 1) Should there be a minimum depth between the top of the semi-circular doorway trim and the cabin roof? Should there be a minimum width between the doorway trim and the adjacent vertical cleat? For example, you mentioned a replacement doorway (bulkhead) width of 36"but also suggested cutting back the existing doorway a few inches on each side. A few inches would provide for a 26” wide replacement bulkhead. Would a 26"bulkhead width suffice for a doorway span of 19 3/8”? I have read that maximum mast compression can be roughly estimated as similar to the displacement of the boat--close to 6500 lbs.in this case.
2) Aesthetically it would be a little disappointing, but would this structural web idea work without the semi-circular bottom door opening? (I am presently imagining how to make this work; but at present I see it blocking access to the leading keel bolt!).
RE: Fore/Aft c.s. support of mast-step: Am I correct in assuming that further c.s. support needs to be applied fore and aft to counter, for example, the mast pumping on a run (or the spinnaker dumping in too strong a wind!)? From the attachment FoxfireMastStep1.doc posted in reply to Sharpii2 earlier in this thread, you noticed that, over the doorway, the mast is balancing (teeter-totter style) on an aluminum fulcrum forward of centre. On the longitudinal centre line of the boat, there is no bulkhead inserted into the ceiling liner “rabbet”, just a piece of ¾” trim that is not supported by anything below. Do you agree that the aft half of the mast and tabernacle needs fore/aft c.s. support?

Regarding the 4” wide, ½” thick aluminum plate bent to the c.s. curve of the cabin roof: Is this structurally better glued “under” or “in” the roof laminate? I have a bulge in the cabin liner below the tabernacle (heel of mast-step). To repair this, I will be going into the roof from the top and could just as easily epoxy this aluminum in either location.

I am new to this. Your advice is greatly appreciated
Westwind2

 

I've addressed design issues like this on several different boats of this general type build, some foam, most balsa cored.

From a structural stand point the mast wasn't provided sufficient compressive support, but more importantly, the deck or coach roof puckered repeatedly as the rig worked fore and aft under different points of sail (no tie rod is appropriate on a deck stepped stick as the forces are very different) and it's effectiveness as a cored structure is destroyed when the skins sheer their bond to the core.

Your current arrangement is just barely suitable for the loads with a sound deck/liner interface in good condision. The bulkhead at the aft end of the head, does a fair job of transmitting the rigging load to the hull shell, but only if the deck is doing it's job. In fact, the port side of the door frame is on the centerline, directly under the forward portion of the mast step base, on the coach roof. It's not a compression post, but helps keep the bulkhead plumb under load.

I suspect your coach roof core is delaminated from the skins and this has caused considerable movement, with time possibly compromised the bulkhead tabbing (lets hope not). The bulkhead doesn't have to be tabbed to the inner liner, but it does have to maintain contact (with the liner/core/exterior deck skin laminate) when the mast is properly tensioned. With a non-crushed core and a good laminate bond, the coach roof is a strong thing, coupled with the full circumference bulkhead able to return to service. Of course, this is with the understanding it's not one of the better ways of holding up your deck stepped stick.

This is a common problem with deck stepped mast, that don't use a compression post to transmit the loads to a below the sole, structural grid, instead relying on a cored deck and bulkhead to retain their relationship with each other. This is common in smaller boats, where offsetting a passageway to permit a real post isn't possible and the case here.

The usually solution is to restore the core integrity or replace the core (portions of it) with a solid laminate (the typical practice now), along the contact patch between the bulkhead and the inner liner. Because you have a bunch of interior stuff you don't want to have to move out of the way, the usual approach is to de-rig the boat and cut the deck. Yep, I know this sucks, but it's a lot simpler then removing the bulkhead and related furniture. Will skillful use of an angle grinder, you can follow the waterways between the textured areas on the coach roof and not make as difficult a job as it sounds. With a large portion of the outer laminate removed (which will be re-used later) you'll see the extent of your delamination. Crushed core, likely moisture damage too.

Replace the core and use a solid laminate across the top of the bulkhead area (prevents future crushing), which is often plywood to quickly build bulk. Bond the deck side laminate back down and fair in the waterways as best as you can. This will ease the problem of trying to match textured areas.

This is major surgery and not a job I'd recommend a novice with little 'glassing experience. Several techniques will be needed to get a solid, good looking result. I recently did a similar job on a friend's beloved SeaWard sloop (his father bought it new). Same issue, same solution, it turned out great (got lucky with the way the waterways were cut into the deck) and now he has to live with a painted deck and cabin top, but a small price to pay for a much more solid boat and he can open the door to the head while on a rough slosh to windward, which he couldn't do before. In fact, his head door was the compression post for a couple of years, giving you an idea how much movement is possible with cored decks, once they delaminate.

I caution against a compression post in the traditional fashion, as well as a tie rod (typically associated with posts). These rely on different engineering principles then the ring frame you have on your boat. You could successfully install one, but the sole will have to be removed and some sort of grid or structural floors installed to transmit loads to the hull shell, which your hull currently doesn't have (at least in the correct locations).

 

Having read PAR's comment, and his having recently repaired a similar problem, you'll get better advice from him with the external surgery. My advice would have involved removing a section of the inner liner to embed (in my case, aluminum), though ply was mentioned as well, to spread the mast load. Not knowing your exact situation, one method will be better than the other, though as said, the job is a serious one.
The idea is to strengthen by rebuilding the pad the tabernacle sits on and then to make sure the bulkhead can carry the load. To do this, a decently thick (e.g., 3/4") plywood bulkhead need only accept the load evenly across a line a foot or more wide (inside, 24" is easily done), and the bulkhead must also stay in column. Hence, the door trim can be structural to that end.
If your door is binding, then your bulkhead is probably point loading to the port side of the opening rather than spreading the load over a wider area, evidence that the stiff core below the mast has failed. The aluminum I suggested was a way to guarantee that no crushing could take place there again. Plywood does this too, if it stays dry, so again, this should be done by an accomplished carpenter.
PAR knows his stuff, so listen carefully to his advice on going into the core from above. Done right, there's little to do below but jack the door opening up to its original height on the port side before the work begins.

Alan

 

PAR,
Thanks for sharing from your experience in the business of yacht restorations.
RE: the deck or coach roof puckered repeatedly as the rig worked fore and aft under different points of sail
It seems this motion has caused my coach roof to fail directly below the aft end of the tabernacle.

RE: going in from the top.
Would you recommend polyester or epoxy resin for the cabin roof fix?
If epoxy is best: Would you recommend duplicating the original layers plus carbon fibre for rigidity and lightness or including in the original layup a curved aluminum plate-- a little more weight but good as an anchor for halyard blocks?

RE: With a non-crushed core and a good laminate bond, the coach roof is a strong thing, coupled with the full circumference bulkhead able to return to service. Of course, this is with the understanding it's not one of the better ways of holding up your deck stepped stick.What is the best way of holding up a deck-stepped stick?

RE: I caution against a compression post in the traditional fashion, as well as a tie rod (typically associated with posts). These rely on different engineering principles then the ring frame you have on your boat. You could successfully install one, but the sole will have to be removed and some sort of grid or structural floors installed to transmit loads to the hull shell, which your hull currently doesn't have (at least in the correct locations)Ideally, I’d like the re-design to both spread the load around the bulkhead and to send it down 2 doorposts to floors. Have you ever run into a solid glass hull design that relies on both the bulkhead method of distributing mast loads to the hull and the post to floor method?
Thank you,
Westwind2

Alan,
After considering both your advice and PARS’ advice I believe that my fix may well require all of your recommendations: The replacement doorway, the replacement coach roof, and the bulkhead cleats as posts. Interestingly, the starboard side of the doorway exhibits crushing, probably, as you identified, because here, there is almost zero cross-sectional support.

RE: The idea is to strengthen by rebuilding the pad the tabernacle sits on and then to make sure the bulkhead can carry the load. To do this, a decently thick (e.g., 3/4") plywood bulkhead need only accept the load evenly across a line a foot or more wide (inside, 24" is easily done), and the bulkhead must also stay in column. Hence, the door trim can be structural to that end. Would a 24”long by ½” thick aluminum replacement doorway arch (veneered with ¼” mahogany ply) stay in column better than a ¾” ply bulkhead with structural trim?


RE: Composite cleats: I imagine transforming these bulkhead cleats into posts by epoxying alternate layers of ¾” veneered mahogany ply and ½ inch aluminum plate.
These “posts” would provide support for the aluminum under the aft end of the tabernacle, thus balancing the fore/aft load over the bulkhead and minimizing PARS’ concern about coach roof puckering.
But, how reliable a bond does epoxy form between aluminum and wood? (i.e. should one through-bolt as well)? Would you recommend through-bolted aluminum channel posts on either side of the bulkhead instead?
Thanks in advance,
Westwind2

 

The whole idea of using aluminum OR a very stiff pad of any kind is to eliminate the need to directly support, for example, the aft end of the tabernacle base. The pad need only be wide enough (maybe six-eight inches) to extend an inch or so aft of the aft tabernacle edge.
Frankly, now, with a better understanding of the problem, I would second PAR's method of using plywood rather than aluminum. Yes, aluminum will bond fine in your application. As said, though, if you're willing to redo the bulkhead section to create an oval doorway, then plywood is fine. The load-carrying characteristics of an oval doorway with thickened edges is extreme relative to your current opening. Simple screwed and glued cleating on the lav side of the bulkhead is enough to bond old to new. Perhaps a three inch wide by 3/4" thick vertical ply cleat with rounded or bevelled edges for appearance, epoxied and screwed together, at port and stbd..
I think you've got some things confused. The main concern here is to stiffen the horizontal pad directly under the deck skin under the tabernacle. Think of this pad as having qualities of not flexing so that the whole pad transmits the load instead of wherever a load is sitting on it. That means the pad is like a bridge--- stiff and able to cross spans.
Now, once the pad is rebuilt (and it's assumed the old one is mush, like a bridge made of wet paper), the span between the sides of the doorway currently installed cannot sag.
The rounded (oval top and bottom) doorway was an idea to simulate an arched bridge base. With such an opening, you don't need to completely rely on the ultimate stiffness of the pad above. It can be far less substantial if desired and still work well. Being a nut for overkill, I would both build in a new pad (plywood as PAR mentioned, from above probably) AND replace the doorway with the oval opening. Now you have better-than-new structure and practically zero chance of future failure.
Forget aluminum for the time being if you are making an oval opening.

 

I wouldn't use aluminum either, firstly it's not as stiff as wood, second the sectional dimensions of an aluminum beam, to carry the loads to the cabin sides and bulkhead may exceed the thickness of the liner/cabin roof.

The first thing you need to do is determine how much delamination has occurred. This can be done by "sounding" the areas. Once a deck starts to move as I suspect it has, large and wholesale areas of inner and outer skin sheering takes place, affecting large portions of the surface, not just directly above the bulkhead or around the deck step.

If the delamination is relatively localized, then you can inject epoxy into the core (which must be very dry). This is a common repair and owner possible. Basically you drill 1/8" holes through the outer skin on 2" centers (yep, it's a lot of holes). Then each hole is injected with slightly thickened epoxy. Ideally the core absorbs the goo, rebonds itself to the skins and all is well with some paint.

If a large area of core is separated from the skins and/or the core is wet or damaged (very common) then one of the skins has to be removed and the core replaced or dried (replaced is easier, faster and a more certain repair).

If you elect (or have to) remove the outer skin, then use a plywood laminate to bridge the span across the cabin top, making sure to tab it well into the cabin sides. The plywood weight, on a boat of this size, wouldn't be much of a concern.

Some pictures would be helpful, but I suspect you'll need a marine carpenter to help you access the extent of the delamination and possibly recommend repair options.

After the deck is restored, the bulkhead will probably do it's job just fine again, assuming the tabbing is solid, intact and the bulkhead isn't damaged.

Design considerations for this type of step arrangement, range from barely suitable to marginally so. The most successful designs I've seen (ones I've employed myself) have a X bulkhead under the step, which makes for interesting interior accommodations, but is being practiced more and more in modern production yachts. The best method is to use a compression post, with tie rod and a structural grid or bridged floor timbers. You're offset doorway offers this opportunity, but there has to be a grid or set of floors under the compression/tie rod assembly for it to be effective. Without the grid or floors there's no way for the compression loads to transmit to the hull shell, which is the whole point of an engineered solution.

In all probability, a restored deck structure will provide a few decades of new life for this particular structural element. Over engineering a doorway/compression post/tie rod assembly doesn't seem to me, to offer enough benefits to warrant ripping out more then you must. Fix the deck, then see where you're at. If you're racing, then a better system should be considered, but if it's just a pleasure boat, enjoy and save the fancy stuff for the folks who have more money then time on their hands.