Length vs Beam - will this cause any problems?

Working on some ideas for an aluminium catamaran to take recreation divers out to dive sites, deck space is king for this sort of thing, so would having a 6 or 6.4 metre beam on an aluminium catamaran cause issues from a naval architecture or stability or sea keeping etc point of view? Or are there any problems at all that having such a wide beam on a 12 metre cat cause?

 

There are boats floating around the world that have a length/beam ratio that varies, for example, from a value of 6 to a value of 1. Each of them will have represented problems that the designer has been able to solve. Why will your boat have special problems?.
So don't worry, calculate correctly what needs to be calculated and apply the correct solutions that will allow your boat to meet its SOR.

 

I guess that is one way to look at it! Another smart way may be to see if can make a beam work that is the same as one of the standard aluminium sheet lengths so as the full length of a standard sheet can be utilised for a frame. Just on that what would be the range of aluminium thicknesses that a frame for a 12 metre power cat would be made from?

 

A 6m beam on a 12m cat isn't particularly wide, so no. No problem.

 

Cheers. I just took a look at standard aluminium sheet sizes here - at around that size they come in 6100 mm long so it is probably going to make sense to have a beam that uses frames of that size if it will work for what is needed as far as deck space goes. Standard sizes are 6100 x 1830 mm for thicknesses up to 10mm, and then 6100 x 1200 for 12mm and up.

 

I'm sorry I can't help you because I have no idea what that range might be. It depends on so many things that I don't know that it would be very unprofessional of me to give you a concrete answer. Take a look, if you want, at similar boats (similar in dimensions, that do the same work as yours, in similar waters, at a similar speed,...) although that will not provide you with valid information either.

 

Don't just look at the length of the piece and the plate. Do a preliminary nesting of several pieces on a plate and see which size gives you the least waste.

 

Cheers - will do!

 

Assuming you have a typical (for that style/type) of vessel, an L/B ration of the demi hull, the only issue you will find will be seakeeping.
The stability, in terms of its statical stability will be high..i.e stiff.
This leads to higher roll accelerations, i.e more violent motions in moderate to rough seas.
You may also wish to consider the wet-deck clearance height too...otherwise you may also incur slamming at slight to moderate conditions.

 

Is this sort of saying that it will be very stable - until suddenly it crosses a certain point where it suddenly violently rolls?

That is something that I am very wary of - having once worked on a cat that had far too little clearance and was awful to work on because of that!

 

No.
What I am saying is that its statical stability - the ability to restore itself after a disturbance - is very high. It takes a lot to "move" it.
This means the roll period will be low. This means whenever you encounter a beam on wave, for example, it will snap back to up right very quickly. This = high roll accelerations = making people seasick.

If you make the midships wetdeck clearance 5% of LWL, and the bow 10% of LWL...it will suit most seastates you wish to run in safely.

 

Just to be sure that I am understanding this - it takes a lot to move it from being very stable? As in it will take a bigger wave or swell to cause things to start rolling around on deck than say a 12 x 4.5 metre cat? (thinking dive boat here as there tends to be a lot of loose items - dive masks, wrist computers, cameras etc etc.

Is there any fairly simple / inexpensive way to dampen that? On thing that is very fortunate is that the run from the dock to the absolute furtherest dive site is about 5 miles, most are less than one nm.

OK cheers - that is good to know to aim for that as what is needed - and it fits in with my desire to have the deck height about level with the dock.

 

Many of the older cats would have approx 50% bean to length ratios. I don't see many problems with increasing this if you pay attention to 2 factors,..
1) provide 2 substantial cross beams constructed in a sandwiched structural manner rather than just sheet of one material.

2) provide an X reinforcement (cabling) within the flooring connecting the hulls,....LINK to come
X-Beam and the Giant https://www.boatdesign.net/threads/x-beam-and-the-giant.14679/
Sail Loading on Rig, Rig Loading on Vessel https://www.boatdesign.net/threads/sail-loading-on-rig-rig-loading-on-vessel.2293/page-4#post-114487

3) I would also consider a central nacelle structure to help minimize under deck pounding. Central Nacelle and Centerboard on CRUISING Cat https://www.boatdesign.net/threads/central-nacelle-and-centerboard-on-cruising-cat.69084/#post-960622

 

Yes, think of it this way.
If you stand with your feet together and you are holding a rope....and you are being pulled, to the side....the force to pull you over is not so large.
If you now, spread your legs to be wider than shoulder width apart...and being pulled again, the same force is now easy to resit. Because the width of your feet provide you with the stability to counter the force.
The same basic principle for a multihull.
So, to make the vessel "roll" requires more of an applied load than if the beam is narrow.

You can dampen it, but it adds drag.
But if you're only going circa 1-5nm out, most likely not worth considering.

 

Many thanks - will take a good look at those! Even though this is for a powered vessel I would imagine that much of the same will apply.