Length vs Beam - will this cause any problems?

Great analogy - that helps heaps!

Probably not - the nearest dive sites are just a thousand feet from the dock, most are about 1 nm or less.

 

Your l/b ratio is more about each hull.

The total width is less important than the width of each hull, sort of.,

I’m only pointing this out generically. You’ve got an expert responding, so I have little else to offer. Just that l/b is more on a per hull basis. The total width of the 12m cat can probably be pretty wide. I have a pretty nice aft deck about 16’ wide on a 10-11m cat, but my boat had other issues Ad Hoc helped me with.

There are other things to keep in mind for a dive cat. Loading it up with lotsa stuff means you need ample displacement over vessel weight, for one.

 

OK cheers! This one will certainly have some weight on board at time as it will be used by technical divers with all of their gear - rebreathers, extra bailout tanks etc etc.

 

I recommend you do an estimate of the maximum weight of persons and gear on the aft deck.

My catamaran has two outboards. Your power means will affect boarding for divers. My catamaran has really terrible boarding out of the water. I’m sure someone has managed a way, but to do so may drive the beam up higher; not sure.

 

I'm hoping that the vessel design will be suited to having twin Hamilton LTX36 jets with Yanmar or similar engines driving them.

I've just started a spreadsheet to calculate weights that will give me a low, medium and max figures - will be for 12 divers, and a maximum crew of four which will include dive guides.

 

Pax n crew of 16 and gear is a lot. This would be way too much for my boat. Your hulls will need to be much wider than mine; for example. This will push down l/b ratio. I would only be able to take 4 max.

I’m not a naval architect, but I’d think your hulls would need to be designed with reserve in mind as well.

I don’t know how a good designer determine ppi or immersion, but I assumed it would be done to deal with expected loads.

If you have 4 crew and 12 divers, what are they bringing to the ship? Harpooned fish or bottom treasures, etc. Dive guides are also divers? So even more gear..

 

The naval architect that I am going to be going with has a 12 metre catamaran ferry that takes 50 passengers - several of these are in service. I haven't progressed my weight calculations very far since last posting as I've been tied up with a few other things - will be tomorrow's job now.

That is a good point - I will ask him about that, the regs/standards might specify something about this.

Fortunately I'm pretty safe with regards to this - the diving is in a marine park that has no known wrecks - so the buggers had better not being bringing anything back but photos!

 

A passenger catamaran, 12 m long, has to be very poorly designed to not allow 16 people (plus their equipment), let's say 1450 kg, on board.

 

depends on l/b .. right?

or rather, affects beam, no?

 

Of course, everything depends on these relationships, and on many other variables. But, as I said before, that size of boat should have no problem carrying 16 people (and many more) on board.

 

I’d like to see some real 12M designs capable of many passengers, just to expand my understanding of how these boats are different from mine. I am guessing the waterline beam per hull is at least double for the 2 meters extra length. Then these boats are l/b of say 6 versus 10-12? (Not to be confused with total beam of the hulls and bdeck)

 

15 m LOA catamaran.
Deck habitable area, L = 12.3 m
Total beam: 6 m
Hull beam = 1.85 m
79 passengers

 

Nice looking design TANSL.

I think I would use a Tennant CS canoe style hull with the engine in horizontal line with prop located behind the skeg of that hull,...better protection for the prop, and even the potential to perform work on the prop/shaft gear without hauling the boat.

CS Hull Sailing Performance? https://www.boatdesign.net/threads/cs-hull-sailing-performance.35387/page-2#post-799663




Engine Size and placement

keysdisease said: ↑
Gold Coast built at least one of their 53 ft models with 2 x 230HP Cummins power.

Sail or power = 20 knts

http://www.goldcoastyachts.com/catamarans/earlier/Earlier-GC54MS-Kaimaka.pdf
Keep in mind that Cummins is an in-line 6 that will fit in the narrower portions of the hull. Go bigger HP and you will likely have to move the engine further forward to get space beside it to service it....that space further forward may eat into your living space, paricularly if you are using a conventional shaft arrangement. Unless of course you provide a wider hull which defeats your slenderness ratio, and slows you down.