Bulkheads

[Geebert]

Hello all,
Ive been slowly going over my new to me 1969 sportfish and doing some light demo and removal of parts needing attention.
I can tell someone along the way has done a poor job of a couple repairs. One of those is bulkhead repair.

My question is, are the bulkheads structurally necessary, or are they just dividing the hull into fuel, engine, salon, divisions.
If they're serving the purpose of transverse stringers, then the repair will need to be more involved than just bilge dividers.
Thanks for any insight you guys may have.

[Ironworker]

They are structurally necessary. Some bulkheads are more important than others.

[Geebert]

If they're structural, and they're totally rotten, what damage or failure would you expect to see from being run with failed bulkheads?
I've seen hull failure from bad stringers, never from bad bulkheads

[Carl]

My understanding is the bulkheads in the 31 Bertram are Not structural, our boats rely on the hull and stringers.


That said...on a 31 boat with a bridge, I'd expect bulkheads to have a hand in suppporting that structure and weight.

[Geebert]

Thats what i thought, Carl.
If they were performing a primary support roll, I'd think you'd see hull failures of some kind because it's so common for the bulkheads to be rotten.
Mine definitely need replaced, but i don't see the need to overbuild them, unless they are carrying some substantial load.
Im thinking single sheet of 1/2" marine ply with one, maybe 2 layers 1708 front and back.
I'll probably use poly to encapsulate the whole thing, then epoxy bed and tab to the hull and stringers.

I wasn't going to go back with any wood at all, but Material prices have me thinking otherwise
Thank you guys for the responses.

[Carl]

I believe the biggest issue with replacing is keeping a gap between the bulkhead and the hull so as not to create a hard spot on the hull as it flexes. I think I saw that on Captain Pats Building Tips page...it's been years since read and redid mine.

What had me thinking and thought to bring up is when you mentioned epoxy bed the bulkhead



Going better than the original is not a bad idea. But then I think, the original did last a good 30--40, 50 years. How much longer do I really need it to last? Cost of marine ply is high, but composites are even higher, plus then add the cost of shipping a large sheet. I hate to say cost is a factor, but for me, it definitely was and continues to be.

If going for more strength, for example adding more weight to the bridge or trying to lighten the boat that is a different story.


I think of my buddy who overbuilds everything. He built a really nice bar in his basement. As usual, he went "WAY" overboard, maybe thinking everyone would be dancing on his bar. Ridiculous size lumber with HD deck screws everywhere...a few years later his wife wanted to remodel, he had a hell of a time breaking that thing apart ripping apart while snapping a shoulder ligament in the process.

Good enough and adequate are words with a bad connotation...but perhaps "purpose-built" better fits the bill.

[Geebert]

Im glad i brought it up. I was going to bed them to the hull.
Where do you find Captian Pat's building tips?
Is it on the old site?

I think i found it, but couldn't find anything about bulkheads

[Amberjack]

That said...on a 31 boat with a bridge, I'd expect bulkheads to have a hand in suppporting that structure and weight.
[/quote]



I'm pretty sure that's the purpose of the heavy aluminum band joining the aft bulkhead to the bridge deck on my FBC. It spans the entire width of the boat.

[Carl]

Oops, I was wrong, it's not in the Building Tips section.

Below seems to be the post I recall.




Post by CaptPatrick » Thu Aug 19, 2010 7:18 am
Rocky,

You're break test was done on the wrong plane... Try the strength test using, instead of the 1/2" cross section, the 6" longitudinal direction.

Coosa type board is quite rigid on it's own, but like all composite cores, becomes exceptionally stronger when captured between two fiberglass faces. (See: Basic Composite Structure and Benefits of Core Materials)

Unlike a load bearing wall in a wood framed house, the structures being supported on a 31 Bertram are very light and have been structurally designed to carry much of their own weight with rather minimal additional support. I wouldn't install a Coosa panel without facing both sides without glassing them up, or at least epoxy laminating the face(s) with hard laminate. If for no other reason, the ability to easily paint the surface.

With regards to the carrying weight under the gunnel, the maximum live load directly above the bulkhead probably won't exceed 200 lbs per square inch. In the pilaster sections, the load from the bridge is being distributed over a larger area, and in the case of the Flybridge Cruiser & Express cruiser, the load is also be distributed across the full aft bulkhead.

Even at only 1/2" and without fiberglass faces, once a Coosa bulkhead is tabbed in, there would be adequate structural support.

All of my aft bulkheads have been laminated plywood. Two 1/2" panels epoxied together with heavy weight hard laminate epoxied to the resulting faces. While the same could be done with Coosa board, saving a lot of weight and being totally rot proof, I would probably continue to use my plywood technique...

Remember, with whatever materials you decide on for the new bulkhead, all intersetions with the hull sides and gunnels must be done in a manner that will not create hard spots that will lead to structural fatigue.

Br,

Patrick
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Post by Rocky » Thu Aug 19, 2010 10:33 am
Very helpfull Capt., thanks. I do have on either side of hull a good 1/4" gap between bulkhead and hull, just not at the top of bulkhead were it is not tabbed by Bertram but gunnel resting on top edge of bulkhead.I've read the basic composite structure tips, and sounds like Coosa would have to be laminated minimum two 1/2" pieces together, and minimum epoxied the outsides for strength/painting. I would do the same with ply so, no real labor time benefits for Coosa it seems. Would you tab in the top of bulkhead under gunnel were bertram did not, and not have the top edge of bulkhead touching under gunnel? (To retain that gap all the way around bulkhead).
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Post by CaptPatrick » Thu Aug 19, 2010 1:18 pm

gunnel resting on top edge of bulkhead

Tabbed or otherwise, that contact is a hard spot. On the boat I just shipped off, both of those contact spots had caused the gunnels to crack completely through. Every time the boat slams down in a sea that bulkhead contact acted like a chisel.

Relieve a solid 1/2" of space there and epoxy a broad piece of material, (Coosa Board, plywood, PVC foam Board, etc.), to under side of the gunnel to spread the load and protect the fiberglass. Don't tab it, just let it ride on the top of the bulkhead. I used H80 Divinycell on the Hancock's boat.
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[Ironworker]

Interesting conversation. One of the issues that I have corrected with my boat was the port side gunnel had sagged over the years and a half ass repair was made by bolting in a piece of 3" aluminum flatbar under the gunnel.

Also, when these boat were originally built there was a lot of cold polyester glass joints in the construction process. It doesn't take much to pry these tabs up.

In regards to creating a hard spot, It would seem to me that if you just have one and the rest of the boat is flexing around that spot then you may have some fatigue, however if the entire boat is stiffened and the hull is uniformly sharing the load is highly unlikely any portion will be over stressed.

In my opinion the original construction with the cold layups may be the root cause of some of the over stressing concerns.

When these boats were built, I doubt there was consideration of adding towers and some of the other amenities we are adding to these boats today.

[Geebert]

Aaaaahhh.
I have two existing bulges from the bulkhead pushing up on the gunnels.
The PO had replaced the bulkhead, or someone along the way had,
Obviously not original.
I bet that's why the bulges are there. The bulkhead is glassed all the way around.

Im so glad i said something before i repeated the error.
Thank you so much for the response Carl!

[Geebert]

These stress cracks are on both sides of the hull directly over the bulkheads.
I bet also caused by not leaving clearance.
I can't thank you enough for the info, Carl.

Those bulges had me worried the hull was flexing too much, pressing the bulkhead into the gunnels.
I hadn't considered the bulkheads were not allowing the boat to flex as intended

[Carl]

I am only passing on information I heard here. Captain Patrick is the man who had and shared the knowledge.


My abilities lay elsewhere so I defer to those who know. What Captain Patrick said makes lots of sense, it’s the bend a coat hanger in one spot, it fatigues and breaks thing. Then Rick makes a great point of not having a weak spot to allow fatigue to develop…makes sense too…just don’t leave out a spot. In both cases there is always a weak link. Odd thought is both scenarios create that spot.

[Rocky]

Hi Guys!

Like this:











And the work was retained by Pearson Marine when they redid my 31 (which was sold to them).

[Tony Meola]

nice work Rocky.

[Rocky]

Thanks Tony,
I owe it all to Capt Pat of course, and these great bunch of guys on this site!

[Carl]

Rocky-

That is some truly top-shelf workmanship !!

Carl

[Ironworker]

I agree. Great job Rocky.

Eventually, I'll get around to posting pics of my complete rebuild.

[Rocky]

Thanks guys!

[Rawleigh]

My understanding is that the rear bullhead in the cabin is not necessary, as many came without the bulkhead. i do not remember seeing any additional reinforcements on the open ones, but it has been a while since I have been on one set up that way.

[Ironworker]

My understanding is that the rear bullhead in the cabin is not necessary, as many came without the bulkhead. i do not remember seeing any additional reinforcements on the open ones, but it has been a while since I have been on one set up that way.

I consider the engine bulkheads the most important of all in the flybridge model. The weight of the engines plus that is just aft of the contact part of the boat with the water at cruising speeds is where the max stress is being placed on the hull. Coupled with old engine beds that may have rot then these bulkheads are even more important.

[Tony Meola]

My understanding is that the rear bullhead in the cabin is not necessary, as many came without the bulkhead. i do not remember seeing any additional reinforcements on the open ones, but it has been a while since I have been on one set up that way.

I remember Capt. Pat saying if a fly bridge cruiser was converted to a sport fish, reinforcement was needed due to the elimination of the bulkhead.

Rick

I will reference Capt. Pat again. He said many times that the longitudinal stringers were enough to support the hull and that the wood used in them was basically to help form them, rather than for strength. The main function of the bulkhead seems to be to support the deck.

[Ironworker]

Tony,

I agree with your statement. The fiberglass in the stringers is pretty thick, it varies from 3/16" to 1/4". The hull has various thicknesses of glass. In the forward keel its almost 3/4" thick and towards the stern it is 3/8 to 1/2".

My issue regarding the structure of the hull and the stringers is not that it can't support itself. When these boats were designed it was a different time. I'd bet most people are using these boats far differently with more equipment than in the 60's and 70's. Heavier weights, faster speeds, towers, etc.

Also you have to factor in the age of the boats. Everything wears out. I'm ripped out a bunch of tabbing on my stringers. I found the most effective tool to do so was a 12" pry bar. Once I got under the tab it would peel right up. Reference my comment about cold layups in the earlier post.

I'm rebuilding my boat from the hull up. Replacing some stringers, reinforcing others, retabbing etc. All the bulkheads are being replaced.

Here is a photo of my forward cabin bulkhead being installed this weekend. Its a composite of 1/2" and 3/4" coosa laminated with 1808 biaxial glass between the two. The bulkhead was epoxy puttied to the hull then tabbed. It will get a covering sheet of glass tomorrow. Note this bulkhead is picking up the forward weight of a tower. I currently have 9' in the cabin between my engine BH and the fwd BH in the photo. I moved the engine BH aft by 5" and moved the FWD BH by 2'.

[Ironworker]

BTW, notice the shelf has been cut away. That was very thick glass and provided structurally rigidity for the flybridge and distributing the load between the two gunnels. Its gone so I have to take in consideration of the additional loading on the gunnels and bow hence the reason for the BH.

[Geebert]

I dont know what we're using today that's heavier than what these boats came with.
3208s , 2stroke Detroits, iron head, intake and block 440s, iron transmissions
Vs
Yanmar, 5.9/6.7 cummins, aluminum head and intake 454s and aluminum case transmissions.
Even modern bateries are lighter than they used to be.
And the generators of the 60s were on another level, compared to today.
And most of the time these boats are refurbished, the plywood and mahogany is pulled out and composites are put back, making them lighter still.
Our electronics and sound systems, maybe, but i remember my grandpa's bottom finder that had a paper roll and polygraph needles that drew lines for the bottom, and the loran was so heavy, i assumed it had a steel case and id bet they weighed as much or more than a garmin and some sub woofers

Id bet, overall, these boats are getting lighter than they intended.

[Tony Meola]

Not sure they are getting lighter.

When full of Diesel they are definitly heavier. Diesel weighs more than gas. When we redid the strut pads and shaft logs we reinforced that area. We used a 3/4 inch solid fiberglass pad, then had to mix up resin to get it to stick to the hull then added cloth to lay over it to finish the job. Probably close to an inch of extra glass by the time we were done stretching over a 2 x 4 area.

The Cummins 6BTA with Transmissions came in about the same weight as the old big blocks. I would say, when full of fuel, we weigh more now than before. Not by much but we do.

You don't want them to light, the heavier the boat, the better the ride.

I was told that one of the best riding 31's was the ones that had the 3208 Cats in them.

[Ironworker]

Weight Distribution is probably the most important factor in the ride, IMO.