SUPERSTRUCTURE
Welcome, I'm glad that you decided to check out this page. I was afraid that you might skip this one 'cause building the superstructure is just like adding on a room or building a garage and everyone knows how to do that...right? Well, there are some differences, especially if you want your vessel to look like a boat instead of a family room or a garage. Of course I'm writing this on the assumption that you're building your superstructure of wood rather than steel. I may add a page later covering steel construction.
Now, I'll admit that the basic framing techniques are the same as for a family room especially when it comes to window and door openings and corner posts and partitions. However, there are some other differences which I feel need to be addressed. I'll try to cover the major ones but, of course, I can't go into a complete dissertation on the techniques of framing.
First is the selection of materials. The easy way out is to go to your local lumber supply and buy a batch of 2 by 4's then go at it. I don't recommend it. Most framing lumber today is spruce and it does not hold fasteners very well (i.e. nails and screws). The cabin of a boat is subject to twisting and vibrations plus severe weather and even an occasional hard jolt...at least for most of us captains. Therefore, I feel, boat cabins should be built with a bit more care than a house or room addition. To this end, I recommend shopping around to find either a good grade of yellow pine or Douglas fir. Either is quite strong and both hold fasteners very well. I also suggest, after assembling with nails, that 'clip angles' be installed at the top and bottom of every third or forth stud using lag bolts. This will prevent the nails from working loose and will keep all joints tight.
Next is room height. I recommend you don't use the standard 8 foot ceilings. This raises both the center of gravity and the 'sail height'. I have found that 80 to 82 inches of ceiling height is adequate because most people who are taller than this are use to banging their heads. Of course, if you are 6' 11" tall, you might want to raise it a bit. Don't forget to allow for the finished floor and ceiling material when calculating your stud length.
If you followed my advice about "no steel decking inside the cabin" and lipped the outer deck up 3 inches all around, then a treated 4x4 makes a good base plate to set the exterior walls on. This 4x4 is bolted to the top of the frames then the bottom plate of the walls lag bolted to it. This arrangement gives 2 inches of wood above the 3 inch steel lip to attach the bottom of the siding. Don't forget to cut a 2x12 inch opening in both plates on each side of the cabin at every hull compartment. A header placed between studs a couple feet above these openings and a grill in the siding makes excellent vents for each hull compartment. With this arrangement, air against either side of the vessel will find its way into the hull and out the other side resulting in excellent ventilation.
At the top of the walls, I like to make the second or doubler plate 3/4's of an inch wider than the top plate and let this project to the outside of the wall. This gives room for a long lag bolt up through it into the upper deck beams. The doubler is also lag bolted to the top plate.
Upper-deck beam size is determined by the distance they have to span. If your cabin is less than 12 feet wide and you are building a double decker, that is a main cabin topped with a pilothouse and perhaps a Texas cabin, then 2x8's are usually enough. You might want to double every third one if you include a Texas cabin and don't forget to install a couple rows of bridging which divide the span into thirds. Also, the beams need to be crowned to prevent water standing on the deck. I generally use 1 1/2 inches. Therefore, a 2x8 would measure 7 1/2 inches in the center and 6 inches at the ends. Of course, if your cabin is more that 12 feet wide or you are adding a third deck, you should use 2x10's for the second deck beam and either 2x8's or 2x6's for the third depending on whether you intend to walk on it or not. Facia boards applied on the end of the deck beams should be made of the same size material as the beams. They, then, will project 1 1/2 inches below the bottom of the beams. Clip angles lag bolted to facia and beam at every third beam helps hold it all together.
Next is the decking for the upper deck. I like to use 3/4 inch tongue and groove treated plywood and attach it with lots of galvanized screws. I then apply an under-layment using 1/4 inch plywood and fasten it with screws spaced 4 inches on center, all directions. I realize this is a lot of screws but you don't want 'bubbles' in your deck. Space the seams so they do not coincide with those in the sub-decking. A special note: I strongly urge you not to use any type of 'chip board' or 'OSB board' anywhere on your vessel. Regardless of manufactures claims, it will not withstand moisture within acceptable limits for use on a boat. To the under-layment, I glue 60 mil EPDM rubber roofing. Extend it down over the facia boards and use a 6 inch lap at the seams. The rubber should be 100 % glued with a special sealer used at the seams. Also, the 'edges' of all seams must be sealed over with 'lap caulk'. This is very important!
Siding, siding, siding......what to use. This is kind of a personal choice thing, but I like horizontal tongue and groove. I sometimes have it special made of clear poplar and sometimes I use 'treated' pine flooring. If you use the flooring, I suggest that you bevel the edges a bit. It makes for a neater job. I strongly urge you not to use aluminum or vinyl siding. This will truly make your boat look like a family room addition. If you must have an 'easier care' surface, consider applying 'pre-painted' aluminum sheets over 3/8 inch or 1/2 inch plywood. Any siding 1/2 inch or more in thickness can be applied directly to the studs.
You have a couple choices on how you install the windows. Aluminum or vinyl windows can be installed either before or after the siding. I prefer to install them before. I think it makes a neater job and they are also easier to trim on the outside. Wooden windows can be installed after the siding, but I remove the standard outside molding and install a fancy one of my own design.
Installing doors requires a bit of fussing around. Most often, standard door frames are not the proper width to match the wall thickness you usually end up with and, most often, they will need to be shortened due to the lower ceiling height. I generally make the frames myself. Also, be careful in selecting your doors. Since there is a good probably that the doors will have to be cut off, choose one that can be cut without seriously affecting its appearance. Of course, you can always build your own doors and that's what I usually do. Also, when establishing your cabin layout, be sure to have at least one door at each end of the cabin. The second worst thing that can happen aboard a boat, next to sinking, is a fire. Always have a way out.
Now to stairways. Many times there's not enough room in the main cabin to install a 'standard' staircase and you end up using something that's more of a 'ships ladder'. Since this stairway is usually a convenience for getting to the pilothouse and back, especially during inclement weather, a good ladder is usually sufficient. However, be sure of a couple of things. One, make sure there's plenty of 'grab rails' or something in general to hold onto when using these stairs or ladder. Two, be sure the opening in the upper deck is as long as the 'run' of the stairs or ladder you're using. Next, be sure you can include an outside set of stairs that, at least, approximate a set of standard stairs. There'll be times you'll need to carry larger loads such as coolers, chairs and the like up these stairs plus guests usually prefer an easy means of getting to and from the second deck. One last item. If you plan to have an elevated pilothouse with a Texas cabin, I suggest running the inside stairs to the Texas then two or three more steps from there to the pilothouse. A set of stairs or ships ladder long enough to go straight to the pilothouse is too long to navigate safely.
I realize this is a very brief coverage of a subject that to be adequately covered would require chapters. Unfortunately, I cannot devote the needed space on this site to include all the necessary information. However, there's one last topic I must touch on and it has two parts. First is control and wiring routing. If you've accepted my suggestion on steering control, then provisions must be made to route the steering shaft. This can be done just below the upper deck beams, in the center of the main cabin with wiring for AC and DC voltages plus throttle and shift cables flanking either side of the shaft and attached to the underside of the beams. These, then, can be boxed around with a shallow 'tunnel' with removable panels at the bottom. The panels allow for servicing the steering shaft bearings and installing or changing the wiring. However, if you wish to have air conditioning, the very best place for the main duct is the same place occupied by the tunnel for the steering and wiring. To incorporate both, I suggest boring three, 2 inch diameter holes on 10 to 12 inch centers in the center of the upper deck beams directly above the tunnel which then becomes the main duct for the air conditioning. The center hole would be for the steering shaft with the others for the AC and DC wiring and control cables. Screw 'duct board' to the bottom of the beams in the tunnel area. This board becomes the top of the duct yet can be removed to gain access to the shaft and wiring. Of course, the bottom of the tunnel still needs to be removable. Cold air outlets for the varies rooms can be installed in the sides of the tunnel/duct.
Well that's about it. I truly hope this helps enlighten you on the subject and is not just enough information to confuse and get you in trouble. If you have specific questions, please get in touch with me and I'll be glad to help.