more questions on powercat design

[northerncat]

i was planning on using 9mm bulkheads, if im using 6mm ply for the skin can i have them at 1m centres or do i need to have them closer ? im planning on having 4 main bulkheads that stretch across the boat 2 at the fron and 2 at the rear with 2 of these being 9mm and 2 of these being18mm and about 6 bulkheads that will just sit in the hulls but not go the width of the boat
also would i need to have 19*19mm stringers every 200 to add stiffness or could i skip these as i dont want to amke 2 heavy a boat
sean

[northerncat]

oh and also would i be better to have the fuel tanks in the middle of the boat or nearer the fron to balance out the weight a little
sean

[Pericles]

Personally, I would not set out to sea with any epoxy marine ply hull thinner than 25 mm. Why?

I wish to grow older, disgracefully!

Pericles

[cookiesa]

Northerncat is only looking at a 6m vessel, hull thickness is in line with several designers recommendations. On a 40ft vessel I'm right with you though

[cookiesa]

Just a thought, why not use full width bulkheads and cut holes in the extras to lighten them ? That is my plan above the waterline. I'm looking at every 60cm having a full width bulkhead in 9mm however above the waterline I am intending to lighten them through cutting holes. Keeping in mind my rear three bulkheads above deck level will only be about 20cm wide.

[northerncat]

when you say full widtth do you mean from one side of the boat to the other or just filling the hulls, i was planning to have bulkheads after the main ones at 1 m centres but am concerned that this is to far apart with 6 m ply.

as for why not run them full width i want to have a clean and clear empty deck apart from th2 rear bheads which will form a seat and the front 2 bulkheads that will form a step
sean

[cookiesa]

Yep full width/beam. They effectively fill the hulls and then on each side come up the height of the deck. Centre is cut out and cockpit floor will sit on top of these.

Please see attached pic (sorry it is so crude, It was done in Publisher and obviously not to scale. These are effectively the bulkheads where the cockpit is (between helm and rear of cockpit)

[northerncat]

so your bridgedeck will be 2 pieces of ply then one underneath, bulkheads on top and false floor over the top of these?
sean

[cookiesa]

I think so, (I think were on the same wave ) LOL

There is the hull (the roof of the tunnel) then the bulkhead running across the boat. There will also be stringers running fore and aft. (These will be rebated in to the bulkhead so the top of the stringers will be even with the bulkhead "tops" The floor of the cockpit will sit on this. The side pieces you see will have the hull on the outside with the deck running on top of the thinner part of the bulkhead.

The Bulkhead in front of the "helm" position will be similar except it will only have a cutout between the "helm" and "passenger" side. There will be a step up here (to allow for the curve on the tunnel). From here you will be able to stand on this step, open the middle section of the windscreen for handling the anchor. (I have a friend with a seafearer that you go in to a cuddy cabin (bent over) and have to lift a hatch right in the bow which is very awkward, this put me off a cuddy with a top opening hatch especially considering the limit on height with the bridgedeck)

Each side of this will be lockers.

Do you have CADSTD? (It's free) If so I can send you a basic outline

[northerncat]

ah hs this is where you and i differ im planning on putting stringers underneath the bridgedeck(glassed on like on my big cat) and only having one layer of 9 mm for my bridgedeck, my concern is making my front and rear bulkheads strong enough, i was going to go with 2 188mm bulkheads and 2 9mm bulkheads, i may even put some glass unis in them and ill download cadstd as id like to see your outlines, mine are only on paper at this stage
sean

[northerncat]

more on bridgedeck, 9mm ply is quite strong stuff and i figure with a layer of glass(maybe even biaxial) top and bottom this will only improve the strength
its the twisting im worried about although on my 40 ft cat i only have 3 structural bulheads and the first of these at the front is only 18mm thick
sean

[cookiesa]

So your stringers are effectively "external"? And the topside of the hull is effectively the deck?

Twisting, could be a problem I think too. Power V Sail forces aside the 40ft cat probably won't be twisted by wave action to the same extent a 6m power cat will just because it can span more waves rather than riding each one. (More of a problem for me again with the smaller length.)

I will also gain some reinforcement with the cab/cuddy style front which will be almst full width.

[northerncat]

yeah i will have external stringers underneath the bridgedeck they should have a minimal effect in breaking up any brudgedeck slapping waves as well), and the topside will be the deck, as for the twisting im chasing som info as to whether my bulkheads are going to be thick enough, im also planning on have a 250mm rail running along the topside and this will be enclosed allowing me to foam fill the sidedecks for better floatation and hopefully adding some stiffness into the hull as well las providing a nice seat for sitting on the side while fishing, i also thought that this might provide a nice convenient place for rod storage if i make a rod sized length cutout
sean

[Pericles]

I often wondered why the Tasmania built InCat fast ferries have a bow shape under the bridge deck, suspended above the sea, by the wave piercing hulls. I am sure you can find the pictures. http://www.tomw.net.au/2004/tas2004/index.shtml

The reason are twofold. The first is to provide buoyancy to prevent pitch poling should the hulls go too deep in very rough seas and to a lesser extent, to reduce the sound of wave slap. The other reason is to be opened to allow vehicles to roro.

It occurs to me that you could re-create this shape, much as a Tolman lower hull is built and bond it to the under bridge components. http://www.backporchboat.org/Hull002.html

This would provide additional internal volume, plus strengthen the whole vessel and prevent twistiing.

Please don't chew my ears off, if these words do not meet with your approval.

Regards,

Pericles

[northerncat]

its an interesting idea but im basically trying not to add anything on the boat that i dont need to so as to keep the weight down im more looking at preventing twist by making a thicker bulkhead, im just not sure that 18mm is thick enough, im considering using 2 sheets of nine mm and seperating them with a timber 42*19 frame i was kinda hoping someone would chime and say yeah ive built 6 m boats before and 18mm is heaps for a bulkhead
sean