Structural Integrity without bulkheads?

Hello all,

My boat, a 1961 swiftsure 33, needs a complete restoration of the interior, which means I will be changing the layout to better suit my needs. The main bulkheads in the boat are rather displeasing to me because they make the fore and aft sections of the interior feel very small and cramped, especially given the rather narrow beam. Does anyone who has experience with boat design know if there's a good way to keep the structural integrity of the hull while simultaneously removing at least a large portion of the bulkheads to open up the boat a bit? Im thinking possibly vertical and horizontal hardwood stringers? Or perhaps steam bending a hardwood frame that takes the shape of the outline of the bulkhead, similar to the frames of a wooden boat, just glassed to the inside of the hull. I simply have no idea what kind of forces are actually acting on the bulkhead and how much support is really necessary to keep the hull sound. I'm not in any way dead set on this, but I just wanted to put out an inquiry to see if my ideas are at all feasible. Thanks for any help.

 

You'll get lots of opinions, but you need a Naval Architect.
I'd believe you could do something, at a huge increase in weight.

 

It certainly seems feasible to replace them with frames that are properly engineered, to open up the interior. Being a structural element, that integrity has to be preserved. It would seem wise to leave the existing bulkheads in place, install the replacements in close proximity to them, and only remove them when the new parts are all fitted. Alternatively, it may be possible, depending on what you have in place, beef up the perimeter of the bulkheads, then cut away the rest of it. But you need to be sure strength is retained.

 

Knowing the layout of this particular boat, I think is would be an obvious thing to assume, major reduction or removal of athwart partitions, would be catastrophic structurally to this boat. It's a bit like asking, "I have a car and would like to remove the chassis, do you see any problems with this approuch?". Really?

Yes, the hull shell could be supported with framing, though you'll lose even more internal volume, making it feel even more cramped with 3" to 4" removed for the interior spaces, because of frames. The hull layup is a heavy one, but was engineered, unlike many of her era. The main bulkhead under the mast, could be converted to a ring frame, which would open the interior a bit, but this would likely be heavier than the current arrangement, given the compression loads it must endure. The galley and head/V berth partitions could be dramatically cut back, though you'd lose the functionality of the enclosed head, the locker opposite and possibly some issues at the icebox and the ends of the settees. The aft cabin bulkhead obviously needs to remain intact, if only to keep your socks dry, when down below.

Can this be opened up, yep, sure can. I'd recommend foam cored sandwich construction, to make similar partitions and bulkheads, but reduce weight and visual imposition when in the cabin. You can guess at it, but I'd agree, you should have the laminates engineered, to retain structural integrity. The interior is a pretty traditional one for the CCA class of yacht she is. She's not overly narrow by these standards, though compaired to a modern, wide butted craft, probably so. This type of project is quite extensive and will have a negative impact on resale value, unless exceptional, better than production line results, can be performed.

 

OK, PAR, your being familiar with the boat, would it be feasible, as I mentioned above, to beef up the perimeter of the existing bulkheads, and cut away the remainder ?

 

You can space some framing on relatively equal centers along it's length, including some hefty partners, etc. and call it a day, with out all but the main bulkhead (under the mast) and the aft cabin bulkhead, which if tied to a new frame could be a light cored thing. This will be heavier than what he's got and will intrude into the elbow room within the yacht, particularly if he wants a ceiling to cover those frames. The bulkheads and partitions (going on memory) are 3/8" and 1/2" plywood, well tabbed to the hull shell so pretty light and quite stiff. You can cut some holes I suppose, though the main bulkhead backs up the head and an opposite side hanging locker, making both less than functional afterward. The same is true of the forward bulkhead. The cabin has a few partitions, most related to the galley/ice box/nav table areas. This would be the only place I'd consider "opening" things up, if it was my boat and would have the least impact on structural integrity. There is a locker on the forward end of the galley that could go away with little difficulty and open the inside up a bit, but much more than this and you're playing with major structural elements and things are going to move or become much less functional, such as wall free hanging lockers and heads.

 

Large openings in ring frames can be reinforced by adding uni directional ropes between sandwich laminates or uni tapes around the edges. So for example, a new ring frame can be built so the internal space is more open by building a ring frame with unidirectional glass fibers running around the perimeter closest the hull.
These can be layed up onto the hull by placing them beside existing bulkheads or frame and heavily tabbed onto, or by incorporating them into a new frame which is pre made and then offered into place. The internal edges of the frame are also treated with the same process and then taped around the edge.
The result is a glass ibeam which is in the form of a more or less doughnut shaped ring frame. Theres a bit of work involved but not that bad if you really want to do this.

Another approach if you need to support large compression load of a mast or similar is to build a composite arch beneath the deck, spanning between the topsides. Again, large amounts of unidirectional fibers between ply or sandwich webs with -+45deg fibers. I had a beam like this on a previous catamaran - which was design to take a mast on top of a cabin on a 40ft catamaran. The laminates in the compression and tension caps were 20mm thick x 100mm width (mostly uni tapes) and the span of the arched beam was approx 4.5 meters between cabin sides of the bridgedeck cabin walls. The 2 webs of the arch box beam had a depth of approx 8 inches in the center and increased dramatically as they blended into the cabin sides. I think the design load was around 4 tonnes and would have had a very small deflection limited design condition.

Its all doable with clever engineering...

 

So Groper,
Did you ever make a design trade between a simple bulkhead and your complex, expensive ring frame in terms of weight?

 

No- it was already built by the previous owner, i didnt build it. The design and drawings came with the boat tho.
The weight would have been higher for sure, but it was to acheive a goal of having a completely open space underneath the mast inside the cabin. A compression post or similar would have impeded the otherwise open plan space. It also provided support for the very large roof span...

 

Given the headroom in this yacht, sacrificing 8" it, for a box beam or otherwise, isn't practical. As mentioned, the interior is limited in what can come out, unless wholesale restructuring and remodel is planned.

 

Well if one is prepared to do the work- head room is not lost either- at least not in the center. The box arch beam was constructed such that the bottom of the webs in the center of the arch was flush with the ceiling as can be seen in these old photos I found of her;

The box arch beam protruded above the cabin roof externally and the mast was to be stepped on this raised arch section in the center. The arch tapered out toward the cabin sides so that all the protrusion was then inside, below the roof line, as it approached the cabin sides and the web depth increased also.
It was built to take a mast aft rig - For clarity, I was in the process of changing the whole thing back to its original Bermudan sloop rig and didn't use this arch beam for the mast - you can see a new compression post I built just behind the front windows to step the mast in its forward position.
This is all way overkill for the boat the OP asked - it was designed to take a massive load compared to what the OP is asking. None the less - ring frames can be designed to be very strong in composite provided the increased cost and complexity is acceptable...

PS - PAR - ive been meaning to ask you about your macgregor 65 - ive been thinking about the one for sale in seattle - time to go cruising again

 

If you can afford the beam protruding above the cabin top, well you're right, not an issue, though aesthetically, pretty Mickey Moused. This boat has a single skin coach roof, so pretty hard to hide a deep web beam. I'd think a laminate would work better, maybe some metal to apply a belt and suspenders approuch.

The Mac 65 has received a bad rep over the years, but unjustly so. It's fast, damn fast and weatherly, though you do have to manage headsails to keep her on her feet and not get over powered. All tall rigs on skinny hulls are like this, so not a problem, unless you want to sail over 95% of her potential most of the time. Those that say it doesn't go upwind haven't sailed one. I currently have a standing bet of a bottle of Glenfiddich to anyone that can beat me with a similarly sized boat. A Santa Cruz 50 MKII came close last year, but upwind he got clobbered and I overtook him handley. I have the MKI which is lighter, taller, deeper and way faster than the pilothouse MKII version. The pilothouse version is more tolerable for a cruiser, being shallower, not as tall and more comfortable below with the raised house. They all suffer from the usual things of this era, so considering replacing all the hardware, which will have broken or will be near, so at this age. I just replaced the engine.