Slamming waves and open bridgedeck crossbeams.

Hello experts and seamen all,
I need help arriving at a concensus in regards to wave slamiming and I would like some advice about design considerations regarding open bridgedeck tramps and crossbeams (I'll address the struct. engr. stuff in due course later). OK, here are my questions:

1. How high above the LWL should the MINIMUM crossbeam bottom of girder/tube be for an open net (no tight woven tramps) bridgedeck at a given Beaufort number?

2. If you had to have a tramp (on or about the center of bouyancy) between the crossbeams, what would the minumum area not to exceed be for a given volume of reserve bouyancy?


To give you an idea of what catamaran I had in mind, my hull length overall would fall between 33 and 36 feet, displacement under 5500 lbs at LWL for coastal cruising with possible offshore cruising of no more than one week max.

Any rules of thumb, yacht design references online and in print I can peruse, formulas, and personal experiences pertaining to the two questions would be very much appreciated.

Thank you,
Bob

 

It's not as simple as that, life never is.

It also depends on the hull spacing, length of boat, absence of knuckle etc.

My rule of thumb is that you should be able to get a conventional inflatable dinghy underneath without it touching the bridgedeck.

So anything over 450mm is good, obviously the higher the better but higher means you have more windage = more weight = less elegance = slower boat. You shouldn't ever need to have more than 900mm unless your boat is very heavy, very narrow or very slow

It's a complicated subject and I doubt if you will find much written down. Best to look at and sail a number of existing catamarans and form your own opinion.

Only rarely is slamming a structural problem, usually it is a comfort one.

Sorry, not sure that this helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hi Richard,
Thank you for replying. Your advice is greatly appreciated as always. I hadn't thought about the dingy part. I''ll use my zodiac clearance as a benchmark given as a minimum. All food for thought.

Best regards,
Bob

 

Hi Bob,

Charles Kanter in his great book "Cruising Catamaran Communique`" explains the various factors affecting slamming: fine bows fat sterns, finesse coefficient, over 'square' design etc; but he gives a general rule of thumb: bridgedeck clearance should be at least 1 inch for every foot of hull centerlines distance.

Therefore, for a 'square' 42' cat the beam max should be 21', the centerlines distance would be approximately 19' and the clearance should be at least 19". That equates to 8%.

I found that to be not enough, my last 12.5m (41') cat had centerlines beam of 6.5m (21') and a bridgedeck clearance of 620mm (24"). She would slam badly, tossing things off the table, we even had damage to the saloon seats..
So, my view (shared by a local cat builder on his 47th cat) is that the same size cat should have a minimum 900mm (35') or 14%.
I was looking on Kurt Hughes web site and most of his 13 to 15m cats have 920mm clearance.

If your cat ends being 35', I would guess a BMax of 18' and CL beam of about 15'; using 'my' min 14% you would need 2.1'.

That is bridgedeck clearance, forebeam should be higher as it is going to hit waves much earlier and before the boat starts to rise.

 

Bob,
there was another thread on this subject here: http://www.boatdesign.net/forums/showthread.php?p=159065#poststop

 

As I said in my early post, bridgedeck slamming is a complicated subject. My 32ft Eclipse design has a CL spacing of 4.4m (14ft 6in approx) and a bridgedeck clearance minimum of 500mm, 20in approx. I sailed my Eclipse over 15,000 miles including hundreds of miles to windward in reinforced trades in the Caribbean. Earlier we had crossed the Bay of Biscay to windward in a November gale. Yet we never had anything that I would call slamming.

I do know what bad bridgedeck slamming is like, however. Like that which we found on the Norseman 43 that I raced on in the Capetown to Rio race. And on a 43ft catamaran delivery I made one February, UK to Canaries. On both these boats it wasn't possible to eat at the table when sailing to windward in any sea. Neither were my design (of course!). And on neither of those boats was it possible to go under the bridgedeck in a dinghy. In fact I found that even swimming under the Norseman was scary (the things you do when becalmed in mid ocean!).

I have found that even a small knuckle makes a huge difference in reducing slamming as the waves are broken up into smaller pieces (in effect). Even more important is the hull spacing. The wider the better. In part because what goes in at the front must come out at the back, so it is the AREA under the bridgedeck that is important. That is why the "Prout Nacelle" doesn't work. There is just no room for the water to get through.

Indeed much of wave slamming is self inflicted, if the boat is too narrow then the bow waves meet, and thus peak, under the bridgedeck instead of further aft. The Solaris designer had a mad idea that this would create "self surfing" and thus speed the boat. Not true of course, instead owner's teeth would literally rattle in any seaway.

These days the most popular boat with a low bridgedeck is the Gemini 105. I have been moored next to one and been kept awake, even in harbour and on a neighbouring boat, because of its bridgedeck slapping. It must be terrible to be actually on board and is certainly a major reason why it isn't an ocean going boat. The earlier Gemini's had a higher bridgedeck so in that respect were more seaworthy.

In short what you need is a wide hull spacing, a good knuckle, a bridgedeck that starts well back and ends well forward (because when pitching the sterns sink and so waves hit aft as well). Lighter boats respond quicker to waves so slamming is much less whatever the bridgedeck height.

As you are interested in an open deck lightweight boat I don't think you will have a problem unless you are going for ultra low freeboard (say less than 450mm (18in). Even on the low freeboard 35ft catamaran Bad Kitty that I raced, and won, the recent Swiftsure didn't slam. But it was very wet!

I would design such a boat to be at least 20ft wide, probably 23ft, but not more. You also have to think how you will power the boat. If an outboard is used then you'll need a nacelle, so some part of the bridgedeck will be close to the water. I try to use a nacelle shaped like the bow of the boat so that it cuts through the water. That seems to work well.

Whatever you do, don't have a flat bottomed nacelle. When water hits there is nowhere for the water to go. Even a few degrees of V allows the water to slide off and dissipate the energy. That is why on my newest bridgedeck cabin catamarans, like the Transit 38 I have drawn a slightly Veed bridgedeck floor rather than a flat one. As a bonus it is also a stiffer solution.

I write more about bridgedecks and lots more on my website.

I hope this helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard Woods....come in Richard Woods

If I was to design a cat that is 22 ft LWL, 10 ft BWL, weighs 2240 pounds and has only a bridgedeck clearance of 1 foot....do you suppose it would slam in 2 foot waves?
Or do you think it would be light enough to respond quickly to the waves? Making a higher bridgedeck would make the boat unstable.

The hulls will be very flared in the tunnel, effectively creating chamfers. This might be good or bad. There is no flaring on the outside of the hulls. The boat must stay within 10 ft beam.

There will be tramps forward and aft.

If you or anyone could have a wild guess as to how bad the boat would slam, that would give me at least some crude perspective. At the moment I have none.

Thanks.

 

I have a 44 foot power cat 2x 250Hp 22Kts 14 tons. The clearance is approx 800mm, the slamming is frightening, I have to come off the plane in 1 meter seas.

It was not something we (the wife) knew about when we bought the cat. It terrifies her and it really bangs although I try to tell myself that it is normal,---so they say,--- it is the worst thing about cats.

One foot is not enough.

 

As requested, despite it being Sunday and my day off, so in haste.

Power cats behave differently to sailing cats. You make bigger compromises designing smaller boats than bigger ones.

I assume you plan a boat with a bridgedeck cabin. If so then your boat will be very similar to the Hirondelle. It also had about 12in bridgedeck clearance. It was OK in sheltered inshore water but slammed when coastal sailing. The Jarcats are also of similar size.

I didn't like the Hirondelle as I thought it was just too small inside for normal sized people.

If you are planning an open deck cat then I see no reason why you cannot have the cockpit a bit higher and avoid the slamming problem. My 22-26ft open deck designs never slam.

I hope this helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks Richard, I'll post my latest and hopefully final design in 'Trying to design my own cat'. If it slams I might just have to accept it. Raising the bridgedeck means raising the passengers.....can't do that too much on a 10 ft wide boat.....bad centre of gravity.

Frosty it's a shame people are not given all the info by the salesman. Hope you still like the boat.

 

Well its not the boats fault realy,--If cats slam then they slam, Ille have to get used to it. When I mention it to other cat owners they look at me surprised and say things like ,--"Oh yes they slam ,--they all do". Now I think about it of coarse they do, they must do.

 

wow! I just got back from a month in sunny San Diego and all the replies were terrific you guys, as always. The personal experience's given as well is also of great value. The aforementioned design rules of thumb gives me confidence in the direction I was thinking of going in my own design. But reality finally did sink in and I ended up forking over $600 for the plans for a trailerable 9 meter open bridgedeck coastal cruiser. It might be all that I desire for those thin waters hereabouts in the Land of Ten Thousand mosquito havens.

Cheers,
BobG

 

Not all cats are born the same.

Frosty,
I had several power cats of different size (23 to 40') and driven others on test runs to try and make up my mind on what to buy. All cats are not the same.
I know this is off the tread subject, but I hope Bob doesn't mind...

What cats have in common is that they (in general) go faster and are more comfortable than monos of same length and power as they (should) ride on a cushion of foam. All my cats did more than 25kn, the Manta 33' did 36kn and it was by far the best.

The idea of a wavebreaker has been used by different design, but the manta carried all the way to the stern and the half bridgedecks were round.
Also, it had asymmetric chines so it would climb on the plane smoothly and react to waves smoothly.

If you are considering doing some mods to your cat, add the wavebreaker, keep it off the waterline and carry it all the way aft. I am sure it will make a big difference, especially at speeds over 20kn.

 

Any effective method for mitigating wave slamming are indeed worth a closer look. Thank you for the illustrations Spiv. They certainly help me visualize how this whole business of how slamming can be addressed in the terms of hull forms chine "stepping" and wave channeling. Submarining of narrow bows are the other thing to bite one in the keister (proly a good thread on that somewhere hereabouts). Incorporating hull and bridgedeck chines and tunnels to help thwart any bouyancy deficiency in, lets say close to shore choppy swells , is most definately one avenue to study.

Cheers,
Bob

 

Hi Bob,

I'm not sure of the type of boat you are refering to. Both Richard and Frosty is right, but their hulls are different. The sailing cat handles waves different from the power cat because the sailing cat has hulls further apart while the motorized cat traps water under the hull more easily. Maybe Frosty should lose some weight

As Richard suggested the bigger the freeboard the better, however there are limits. The cat I'm designing currently showed a funny way of forcing things to be a certain relation to one another... but it depends on the compromises you are willing to make. To name one you have to choose between windiness, and bridgedeck clearance and standing headroom. Real fun to choose, I bet you'll blame yourself later for making some choices one way or another However, getting a feel for it and thinking it all through very thoroughly with some planning and reading up on everything and asking questions helps a lot - as well as add to the confusion

Do have a look at as many boats similar in size as yours. Give you an idea what you'll like and what not as much.