On bulkheads and secondary stringers

[Howie]

I am still early in my design but in keeping with the suggestion for a heavier camper oriented boat I am looking into having a single belly tank with some saddle tanks to supplement. I would like to have around 100 gallons on board (ideally more). I am wondering if there is any general guidance around "piercing" the aft cabin bulkhead with a single belly tank. If I were to also run the saddle tanks through this bulkhead I am wondering if there are any structural issues I am creating by reducing the material that connects one side of the boat to the other. I guess this may not be that big of a problem since folks do open rear pilothouses, etc. Similarly, it seems that there is a lot of leeway with secondary stringers...people carve up the belowdecks with all sorts of fish wells, storage etc.

Are there any guidelines for the minimum amount of extra structure below deck needed? Or is it more about supporting the cockpit for walking on and any structure (cabinets,etc) above?

[Brian.Dixon]

The decks are structural and required.  Sub-deck structure is primarily for deck support, not hull structure. The aft house bulkhead is structural and required, but it's of secondary nature as compared to stringers, decks, the hull itself, and the sheer structure.  If you look at the 26' Prince Rupert in the 000x series of drawings carefully, you'll see tankage passing by/through underneath the aft house bulkhead.  A sub-deck bulkhead is not necessary there.

[Dan Boccia]

Great answer from Brian, good to have this structural info/reminder.

I have over 120 gal of storage in two belly tanks, and I could have squeezed a bit more in there with some more careful design.
I'm a big proponent of having dual belly tanks, as this allows me to play around with trim a lot more. So far I always benefit with less weight aft, so I always burn from my aft fuel tank first, improving trim as my trip progresses.

In addition to improving your ability to control trim, I like the simple redundancy this provides. Of course, two tanks is a more expensive option and you lose a little bit of volume because you cannot simply butt the tanks right up next to each other - mine are aluminum, and it needs some space to breathe.

Finally, since you have plenty time, take a close look through all the shapes Moeller offers in rota-molded tanks. These are superior to aluminum in that there are no corrosion issues and they are lighter.

[cj8mule]

Great answer from Brian, good to have this structural info/reminder.

I have over 120 gal of storage in two belly tanks, and I could have squeezed a bit more in there with some more careful design.
I'm a big proponent of having dual belly tanks, as this allows me to play around with trim a lot more. So far I always benefit with less weight aft, so I always burn from my aft fuel tank first, improving trim as my trip progresses.

I agree about having the two tanks Dan, but this is the first time I've read that you empty the rear tank first.  (I might have missed that in your build)  I might run into the same thing with the Kodiak I'm building, but in my head I was thinking it'd be just the opposite. 

Thanks for the real world data Dan.

[Brian.Dixon]

These boats are relatively light for their size and will therefore trim differently based on loading - more so than commercially-built boats that weigh twice as much and burn twice the fuel.  Ask Dan about his foam core (I believe?) construction .. can't remember the list of items that he did this with, but it lightened up the forward half of the boat, hence his remarks on trim and balance by burning off the aft (heavy) tank first to tune the trim.  Other GA's will trim a tad low at the bow and burn the forward tank first in order to tune the trim while on a trip.

[Dan Boccia]

Yep, Brian beat me to it - much of my cabinetry and cabin are built with foam-core rather than plywood, saving about 40% weight in these areas, so although I have a relatively large cabin, it probably weighs less than any other standard 28' GA made of its size. The other thing is that mine is a standard 28' GA, and it appears that it trims differently than the Kodiak.

I would consider doing some of the cabin in foam core again, but I would be much more judicious about where - it's a royal pain in the butt having to through-bolt with flange washers to avoid crushing the foam core, when I could have used screws to screw equipment into plywood.

I love that you're taking the time to think all these details through - it will make a huge difference once you start your build.

[Grady300]

I used 2 takes also for the same reason to be able to help trim the boat as needed. Which includes adding fuel appropriately if I do not need full fuel in either tank. I also pay attention to which tank I want to run on first, it changes based on the days load. I have a 58 gal. that starts at the rear cabin BH and runs aft. The 102 gal. and starts at the same rear BH (2" gap) the runs forward. Fuel management is much the same in a single engine airplane, it's all about planning!!!!