Strong enough???

I am having a steel catamaran built 18.5m X 7.5m about 22 tons deadweight. The architect is planning 6mm steel for pontoons and deck. This will be a cruise boat operating in the A ndaman Sea, and I plan to beach her on a regular basis. I would feel better if I knew the pontoons are not going to crush in a light surf with all that weight on her.
Thankyou in advance for your input

 

DanB welcome to the forum; of interest had me a 10m monohull some years back with 4mm steel hull, she took the ground often with no problems (I'm a crap driver!). But your problem is more concerned with the stiffening, sometime known as the ribs etc. you'll need a substantial backup system of ribs, stringers etc! After all an animal with no ribs is called a 'jellyfish' - think about it, makes sense. Can't see any problem with 6mm for that size of vessel, after all purpose made military Landing Ships whose main aim in life is to hit the beach, normally only have a shell plate thickness of 10 to 12mm and they come in at 40 to 50m and 1000 plus tonnes

 

On 18,5 mtrs LOA, 6mm will do nicely for the topsides, and 7 for the keelplates; make sure that the scantlings/frames and stringers/longitudinals are placed with the correct interval. For the deck, 5mm will be satisfactory, superstructure 4mm.

 

Some other ideas:

• angle the first meter or so of the bow to around the same angle of a typical beach in your area. Assuming you use a sharp bow, then widen it out to a flater surface at the bottom of the curve and back to the begining of the part of the pontoon where the shape remains constant
• your greater risk will be with 'bending' of the entire hull as the bow hits the beach and gets lifted up. This is where the "ribs and stringer" come into play. For sure run some kind of upside down 'T' bar along the full length along the bottom of (inside) of the pontoon. One more (can be ligher) at every quadrant will also be good.
• The ribs you will need for sure near the bow, to reduce denting (the wider the space between the ribs and stringer, the deeper the dent could be). Behind the bow area the can be wider spaced. Depending on the diameter (say if its 75cm or so), you might not need any after that (if you stick with the 6 mil steel), but I can not say for sure.
• Put a waterproof bulkhead in where your bow shape ends and the contsant pontoon shape begins. That way if you do hole the thing, you will not sink... A few more along the lenght would not hurt.
• If you have big diameter pontoons, then consider adding some water and air tight hatches, so you can at least pump out any water, but perhaps go in to do repairs (no smuggling allowed!) (and if they are that big, consider some ribs...)
• regarding the stingers, etc, be sure to include holes in them against the hull, so any condensation water can drain to a cental

rust proof pump out station.

These are just my ideas, I am not a pro, so use them as you see fit. Pay a real yacht designer to be sure... cheers!

 

Humm. that first point was not very clear...
What I was trying to say, is, assuming you have a sharpish bow, then where the sharp, first part (more vertical part) of the bow curves around to more horizontal - where the pontoon will hit the beach - you should quickly flatten the shape out to bring as much surface area in contact with the beach as possible... this will reduce the point stresses, and reduce the chance of crumbling, etc. Also you will not dig in as deep and so can push off easier...

 

With a steel hull you'll have issues with grinding paint off if it sits on the beach in the surf for any length of time. Maybe you can install "beaching pads" on the bows made of HDPE that can be replaced as they wear out.

How about some more details on this boat? Is it planned for commercial tourist cruising or personal? How will you access the beach - drop-down ramp? How will you power it? Sounds interesting.

 

Big Thankyou

This was my first post, and I extreamly impressed with the replies. This is also my first boat. I sold everything I own in Canada, drew a picture of my dream boat and bought a one way ticket to Thailand. I am told my budget is very tight/too tight. I now have a very expensive english speaking navel architect, he seems like he knows what he is doing, but he suggested a few ideas that were totally stupid, even to me. We are to start construction in a couple of weeks. As you can see I need all the help I can get, and I see that I am in the right place for it
P.S. plenty more questions to come