Tabernacle / bulkhead failures & fixes . .

Welcome to PocketShip.net! This bulletin board is for builders of the Chesapeake Light Craft-John C. Harris "PocketShip" design, a 15-foot micro cruiser sailboat built from a kit or plans.

For more information on PocketShip, click here: http://www.clcboats.com/pocketship

This site gathers PocketShip builders in one place. Here you can share photos, tips, questions, and---eventually---your sailing adventures in PocketShip! CLC will also post design updates and tips here as they come up.

We'll try to knock down spam as quickly as possible.

Moderator: John C. Harris

Forum rules
Spam or commercial posts will be deleted.
This is a civil forum: no flames or drunken tirades.
Please stay on-topic.
PocketShip's Web Page: http://www.clcboats.com/pocketship
If you need CLC customer service: http://www.clcboats.com/forms/contact_us.html
We'll try to delete spam as soon as it appears.

Tabernacle / bulkhead failures & fixes . .

Postby tattoo on Mon Feb 09, 2015 11:29 am

Fellow builders --
My Tattoo has experienced two such failures. Both happened under strong winds. The first probably on a downwind reach when I heard a noise that I thought was something falling in the cabin. The second occurred a year later. On both, we kept sailing in strong winds unaware of the damage which was not noticed until Tattoo was recovered onto her trailer.

My first repair was inadequate and probably amounted only to cosmetics without strengthening the original construction. Inspection showed that the cleat along the forward underside edge of the cabin-roof failed (not the glue joint) -- probably because it was made of cheap pine. I also noticed that I hadn't wrapped the cabin-roof/bulkhead edge with fiberglass. I didn't try to replace the cleat with stronger wood, thinking that the fiberglass wrap (over the edge of the bulkhead/cabin roof interface) would sufficiently strengthen the joint. And I also thought the SS L-bracket (inside on the cabin-roof centerline) would add the necessary additional strength. Obviously, I was wrong! The repaired bulkhead was probably weaker than when originally installed. The L-bracket didn't add any strength -- it just bent. However, it may have limited the damage. The following photos show the damage and repair procedures.

Before repair.JPG
Showing the crack at the tabernacle and on the port-side of the bulkhead.
Before repair.JPG (131.63 KiB) Viewed 6836 times

Failure closeup.JPG
As viewed from starboard.
Failure closeup.JPG (120.15 KiB) Viewed 6850 times

Regluing setup.JPG
The cover for the sliding hatch had to be removed so that the bulkhead could be forced back into place.
Regluing setup.JPG (171.25 KiB) Viewed 6837 times

In "replies" to my own posting I'll add further photos and comments.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Tabernacle / bulkhead failures & fixes . .

Postby tattoo on Mon Feb 09, 2015 11:57 am

Continued . .
Here are three more photos of the repairs.

The main lesson that I learned from the failure of my repairs to prevent a repeat: Even excellent repairs to a cracked piece of any plywood will never restore its original strength. I should have realized that if I had thought carefully about it. Not only that, but a "re-glued" joint will also not have its original strength UNLESS the faying surfaces can be "got at" and the old glue removed down to bare wood.

Repaired before fiberglass wrap.JPG
The port-side bulkhead crack was repaired at the same time the bulkhead/cabin-roof joint was re-glued.
Repaired before fiberglass wrap.JPG (121.56 KiB) Viewed 6832 times

Fiberglass wraps.JPG
I thought the FG wrap over the edge of the bulkhead/roof joint would do the job. Wrong!
Fiberglass wraps.JPG (151.87 KiB) Viewed 6820 times

L bracket inadequate fix.JPG
The L-bracket was made of stainless steel, 1/8th inch thick. It just bent.
L bracket inadequate fix.JPG (115.25 KiB) Viewed 6817 times

In my reply to Craig's posting (Removable Tabernacle) I opined that the failure was self limiting. That is, even when sailing in strong winds AFTER the failure, the damage didn't seem to expand or get much worse. At least not to the point that it was noticed by the skipper or crew. The shrouds probably held back the mast from going much more forward. As stated in my reply -- I think the mast "bowed" out forward just a little. But the joint won't give "just a little." It won't give at all!

In another reply I'll show the details of my repairs to the 2nd failure.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Tabernacle / bulkhead failures & fixes . .

Postby tattoo on Mon Feb 09, 2015 1:11 pm

Continued re 2nd failure . .
Three photos for the 2nd [improved] repair.

A year later -- another nearly identical failure. Clearly, my first repair was merely cosmetic. This time I decided to laminate a doubler on the inside of the bulkhead. I decided on two doublers, laminated together for additional stiffness. This time around I realized that just one doubler might only restore (or just slightly increase) the strength of the original 3/8" bulkhead. So far in several strong winds -- no repeat!

After 2nd flr.JPG
Notice that this failure is almost exactly like to first one.
After 2nd flr.JPG (159.36 KiB) Viewed 6811 times

Failure fix resized.jpg
Cross section of planned doublers.
Failure fix resized.jpg (77.26 KiB) Viewed 6820 times

Prep for doubler lmntn to bulkhead.jpg
The outside of the bulkhead has been repaired and glued back into position.
Prep for doubler lmntn to bulkhead.jpg (53.35 KiB) Viewed 6826 times

The screws seen on the front of the bulkhead will pass thru the bulkhead and into the 3/4" thick doubler -- pulling it tightly against the inside of the bulkhead after the faying surfaces have been coated with thickened epoxy. The screw-heads slightly countersunk filled over before painting.

The remaining 3 photos of the repair will be in the next "reply" to this posting.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Tabernacle / bulkhead failures & fixes . .

Postby tattoo on Mon Feb 09, 2015 1:38 pm

Continued re 2nd failure . .
Final three photos of the repair.

The final repair (of the 2nd failure) has held up in occasional strong winds. I think it's a permanent fix. Since I don't quite understand the forces that caused the bulkhead to fail and because this is not the only PocketShip to experience a nearly identical failure mode -- it would seem prudent that repairs of similar failures should incorporate a pair of doublers. And probably at least one doubler to stiffen the bulkhead on new construction. The intrusion into the cabin is minimal, not interfering with any space requirements. I consider the additional weight minimal.

Inside prep for doubler lamination.jpg
The inside bulkhead surface (to be under the large doubler) was taken down to bare wood.
Inside prep for doubler lamination.jpg (61.25 KiB) Viewed 6811 times

Doubler ready for lamination.jpg
The large doubler fitted up under the cabin roof and ready for lamination to bulkhead.
Doubler ready for lamination.jpg (57.65 KiB) Viewed 6806 times

Both doublers installed.jpg
The pair of doublers ready for painting.
Both doublers installed.jpg (52.98 KiB) Viewed 6807 times

The smaller doubler was laminated to the larger one by means of thickened epoxy with 7 screws and the top tabernacle bolt providing the pressure for a proper lamination.

Opinions and comment/critiques regarding the failures and repairs made would be welcome.

By the way, Tattoo is still for sale.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Tabernacle / bulkhead failures & fixes . .

Postby craig on Mon Feb 09, 2015 2:06 pm

Pete,

Wow, I didn't realize your first failure ripped the bulkhead. In a way, it's very reassuring that failure in that case was self-limiting. Even with dramatically reduces strength of that bulkhead after failing, the mast didn't fall forward.

I agree with your analysis. There seems to be two problems.

1) 9mm ply reinforced with 1 sheet of fiberglass is not sufficient to withstand forces from a bendable mast, loose shrouds, etc. That was your first failure. In my case, the bulkhead clearly flexed but didn't give way, though whether this is because of lower winds in my case or reduced mast flex is unknown. As you said, preventative maintenance is to install a doubler inside the cabin to increase bulkhead strength.

2) The bulkhead/cabin roof joint can fail. In your case the lack of fiberglass cloth was to blame. In my case, the bulkhead2/cleat joint failed when the bulkhead flexed outward. Now I only have the epoxy between the bulkhead plywood top surface and the roof, and two layers of fiberglass cloth over the joint, to hold the bulkhead/roof together. To properly repair this I need to secure the bulkhead/roof joint. I will install the backer (for Problem #1) and screw through the cabin roof as you described. I think that's the single most important repair I can make.

Not related to this failure, but can you (and others) comment on how to attach the boom gooseneck to the mast? I used four #6 3/4" screws, but should I have through-bolted them? I'm concerned these will pull out during high wind.
Titania, launched January 2015
craig
 
Posts: 253
Joined: Fri Feb 21, 2014 1:04 pm
Location: Chapel Hill, NC

Re: Tabernacle / bulkhead failures & fixes . .

Postby tattoo on Mon Feb 09, 2015 3:51 pm

Craig,
I'd replace the goose neck screws with fatter ones, #10 or even #12. The length is ok -- but I'd go to 7/8 th if you can find them. You meant attached to the tabernacle -- which should be 3/4 " thick. If a 7/8 th pokes thru a little, use a washer or two under the screw heads. Remember, the boom is always being pushed into the tabernacle. Use bedding compound under the goose neck. The fatter screws will resist any side thrust.

My shrouds were not loose -- at least on the 2nd failure, because I had pulled them as tight as I could. I was careful to slack off the forestay and jammed the mast foot close up against the bottom-back of the tabernacle. When tightening the shrouds the top of the mast was flexing aft as tight as I could pull the shrouds. That aft flex just about disappeared when the forestay was pulled tight. No matter how tight you get them, the leeward shroud will be slack when sailing on the wind and probably on a shallow reach as well. That's because the boat's hull flexes a little. And it doesn't take much flex to see some slack on the leeward side.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Tabernacle / bulkhead failures & fixes . .

Postby craig on Tue Feb 10, 2015 9:38 am

My mistake, you're right, I mean't attached to tabernacle. Length was my original concern, but I'll go back and check whether I can fit a #8 or higher in the hole. It was my recollection that they were made for a #6. Thanks,

Craig
Titania, launched January 2015
craig
 
Posts: 253
Joined: Fri Feb 21, 2014 1:04 pm
Location: Chapel Hill, NC

Re: Tabernacle / bulkhead failures & fixes . .

Postby John C. Harris on Sat Feb 14, 2015 10:53 am

What worries most about this thread is this simple fact:

YOU WILL NOT DAMAGE your tabernacle if your shrouds are tight enough. I repeat: if your shrouds are loose, I don't care if your tabernacle is welded titanium, that 16-foot lever of a mast will twist the tabernacle off the bulkhead.

In photos people send me, I see lots of boats with the lee shroud hanging slack under sail. These are the boats that are going to damage their tabernacles. It is _SO_EASY_ to make sure your shrouds are tight enough to take the loads off the tabernacle, and so hard to fix the tabernacle if you break it...

PocketShip #1 has the flimsiest tabernacle of any PocketShip. It was built very lightly, of 3/4" softwood; I increased the scantlings when I wrote the plans. That boat has been absolutely thrashed, actually abused sometimes, in heavy air. There is no sign of damage to the tabernacle. My shrouds are tight. And it's not something you have to monitor and fret about. Since mine was launched in May 2008, I have tightened the chainplate lashings exactly once.

Adjust your shrouds so that the mast can't bend forward enough to break the tabernacle. It's just that simple. An ounce of prevention here, folks.
John C. Harris
 
Posts: 182
Joined: Thu Feb 05, 2009 12:39 pm

Re: Tabernacle / bulkhead failures & fixes . .

Postby John C. Harris on Sat Feb 14, 2015 10:57 am

>>>>No matter how tight you get them, the leeward shroud will be slack when sailing on the wind and probably on a shallow reach as well. That's because the boat's hull flexes a little. And it doesn't take much flex to see some slack on the leeward side.>>>>

Okay, maybe the lee shroud will be just a little slack. But not because the "hull flexes a little." PocketShip's hull is a monolithic, monocoque structure. What DOES flex a little is the mast, enough to account for a small amount of slack in the lee shroud.
John C. Harris
 
Posts: 182
Joined: Thu Feb 05, 2009 12:39 pm

Re: Tabernacle / bulkhead failures & fixes . .

Postby herbt on Wed Feb 18, 2015 5:01 pm

Another thing to keep in mind is to be carefull about keeping the mast centered during the raising and lowering process, any sideways movement with that huge lever of a mast could easily torque the tabernacle and cause damage that could go unoticed until under sail. Make sure the mast is centered and properly stowed during transport. I think this could be more of a problem with tabernacles that are not permanently bonded and bolted to the bulkhead.

Herb
herbt
 
Posts: 55
Joined: Sun Jul 12, 2009 6:14 pm

Next

Return to PocketShip Forum

Who is online

Users browsing this forum: No registered users and 7 guests