flat bottom bridgedeck vs other shapes in heavier sea states

[big_dreamin]

I've been slowly reading about and researching about multihulls for awhile, wanting/ wishing/ dreaming out various possible scenarios whether for work or play where I can finally build/ have built/ acquire one.

One design philosophy that I tend to prefer is that keeping it light is best - and one of the best ways to keep it light is to simply have the hulls keep the bridgedeck high enough up and out of the water so that it doesn't TOUCH the waves at all during normally designed-for seastate conditions. Plus going to sea in a bigger boat to begin with - it's not that the seas are too rough, it's that youre challenging them in too small of a boat! This makes more sense to me instead of building it heavy and able to take impacts but then guaranteeing that you'll be smacking every wave that comes by... ruining efficiency, speed, and comfort all at the same time.

However...

It makes me concerned of what happens under sea state conditions where you aren't high enough to stay out of the wave impacts. I am wondering if a multihull designed for say 6 foot seas, and thus 8 feet up, and built with a light flat underside to the bridgedeck becomes under SERIOUS danger if you get caught in 8.5 foot seas on something blowing in unexpectedly on the way back home.

I am wondering if there are shapes or other design choices which could maintain the "standing up and above the waves" strategy while being better able to handle bad weather that you sincerely tried to avoid and didn't plan for but just ended up caught in unexpectedly because the gales of November came early. A Plan C if you will. (since Plan A is build light and never get caught in weather/have a good lifeboat, and Plan B is you build it so heavy it will survive a hurricane killing the philosophy) Just something that would enable it increased ship survivability under "greater than designed for normal use" sea states" to ride over too large waves better and not get swamped - like vee hull like extensions out in front or whatever.


For instance the Incat fast ferry looks like a vee hull over the cat hulls - which I imagine would ride up stormy waves very well. But also would catch on waves instead of riding over them again creating alot more drag under more normal conditions.

 

[Mr Efficiency]

You seem to be broaching the subject of "wavebreakers", a subject about which I have been able to find very little discussion of.

 

[mydauphin]

Everyboat reaches a point where it becomes useless. There are many factors, lenght, depth of prop, air intakes, or bow height. The type of waves, speed, etc.. can get prevent the props or water jet from grabbing the water and providing proplusion. Sorry if I am not giving it a proper name but even on a large boat sometimes your props jsut spin in a storm and youa re lucky to keep your nose point at the waves. The shorter the boat and less draft the worse. It can happen to very large boats, with shallowish drafts. Anyway, it is all part of the design, and at some sea state the captain knows his boat is not going to function right.

 

[Ad Hoc]

I am wondering if a multihull designed for say 6 foot seas, and thus 8 feet up, and built with a light flat underside to the bridgedeck becomes under SERIOUS danger if you get caught in 8.5 foot seas on something blowing in unexpectedly on the way back home.

It is called seamanship and common sense. One slows down and/or changes direction!

 

[CatrigCat]

All cats have waves splashing under the bridgedeck. Sometimes the hulls channel the waves and they slam under the bridgedeck. Im told that catamaran sailors soon get used to the noise.
Good designs slam much less than others.

 

[Richard Woods]

I am not sure if you are considering slamming on catamaran ferries or on leisure boats, but in either case you might find these basic comments useful

http://sailingcatamarans.com/index....performance-questions/108-bridgedeck-slamming

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[redreuben]

Purely from what I've read on here and elsewhere the design features that contribute the most to minimising slamming are good clearance to begin with and a chamfer panel from hull to bridgedeck.

 

[Mr Efficiency]

Purely from what I've read on here and elsewhere the design features that contribute the most to minimising slamming are good clearance to begin with and a chamfer panel from hull to bridgedeck.

Yes, the chamfer seems an obvious help, do you think it would work better than a wide radius ?

 

[redreuben]

Not better just different, depending on the radius of the curve or the angle of the flat panel, the radius will be harder to build whereas the flat panel is easier. Not sure which is stronger in this situation, curves are usually stiffer than flats but here the flat panel benefits from triangulation. Anyone care to share on this ?

 

[Mr Efficiency]

I seem to recall reading somewhere that the chamfer was the best.

 

[Spiv]

Not better just different, depending on the radius of the curve or the angle of the flat panel, the radius will be harder to build whereas the flat panel is easier. Not sure which is stronger in this situation, curves are usually stiffer than flats but here the flat panel benefits from triangulation. Anyone care to share on this ?

Here we must make a distinction between fast power cats and sailing cats, I had several of both.

I spent hours watching under the bridgedeck and took photos and lately, films.

Power cats ride fast and leave a long rooster tail behind; they has been beneath the boat just a few seconds ago.
They DEFINITELY benefit from a double arch design, something like the Manta cats of Australia used to build. I owned one for several years after owning a Shark Cat (and others with flat bottom) and I can tell you the difference was very noticeable.
The flat bottomed fast cats slam harder while the round double arch are always much softer riding on a double cushion of air bubbler made by each hull. That was aided by asymmetric chines.
However, as you can see from the pic, these boat are comparatively narrow from sailing cats, so I firmly believe that you cannot apply the same principles to sailing cats.


Sailing cats cannot 'sail' into the waves, so when sailing the waves can arrive from all different directions (even from behind) and will hit flat surfaces or inclined surfaces as they please...
One important factor is weight, if the cat is light, the waves will lift it earlier, hence decreasing the possibility of slamming.
I had a light (7.7T) 12.5m cat that had only 60 cm clearance, but it slammed continually, bottles and plates would fall of the table, cracks developed in the seating in the saloon etc... So light weight is not sufficient.
I also had a 14.5T, 14.5m cat (Salina 48) that didn't slam so much because she had 85cm clearance. Not enough in my book, but better than 60cm.

Many 'roomarans' builders now use a wave-breaker central nacelle, some extend it all the way to the transom, I have often been wandering about the advantages and must admit that the slamming produced more noise than discomfort or fear of structural damage, in 5y living aboard and crossing oceans, we never had things flying off the nav station or the galley. See Salina pic.

 

[brian eiland]

You might have a look thru a few other discussions on this forum,...
http://www.boatdesign.net/forums/multihulls/center-nacelles-wave-pounding-cats-19139.html

....perhaps do a search on the forum with the word 'nacelle'

 

[brian eiland]

You might also do a google search for 'Domino powercat'

Nice bridgedeck design for ocean going powercat. Also a nice hull design.

 

[big_dreamin]

Everyboat reaches a point where it becomes useless. There are many factors, lenght, depth of prop, air intakes, or bow height. The type of waves, speed, etc.. can get prevent the props or water jet from grabbing the water and providing proplusion. Sorry if I am not giving it a proper name but even on a large boat sometimes your props jsut spin in a storm and youa re lucky to keep your nose point at the waves. The shorter the boat and less draft the worse. It can happen to very large boats, with shallowish drafts. Anyway, it is all part of the design, and at some sea state the captain knows his boat is not going to function right.

Thank you for such direct clarity. Speaking of which, does anybody know of any kind of simulation software that lets you literally model up a boat in 3d and just sail/power it through various simulated sea and wind states just looking for flaws? Like the software X-plane for aviation enthusiasts, where would-be kitplane makers have designed and found flaws under certain conditions that they found occurred in the real prototype without the danger of extreme testing.

It is called seamanship and common sense. One slows down and/or changes direction!

Yes but sometimes reality doesn't cooperate. If I have an unexpected breakdown hundreds of miles out to sea I might find myself in an undesirable situation. I am trying to see whether certain shapes are better than others.

I'm going down a list of "must haves" and various options and i've pretty much nailed down my needs to motor catamarans with a lightweight design and long hull to beam ratios and high bridgedecks planned for eventual oceangoing even though it will mostly be in the Great Lakes for it's first decade. I also know that sometimes just the right design can improve safety/performance/efficiency without an increase in real cost or weight so i'm curious what designs best control or reduce wave slam even just from the perspective of reducing drag further.

 

[Ad Hoc]

Yes but sometimes reality doesn't cooperate. If I have an unexpected breakdown hundreds of miles out to sea I might find myself in an undesirable situation.

Indeed...and that comes down to 2 issues, 1) design and 2) experience.

1) Design, no design can cater for all the "what if" scenarios. But some are better than others. If one is worried about being caught out with a broken down engine, is that the fault of the designer or the owner penny pinching on a cheap knocks off to save costs. This is only part of the bigger picture though.

I also know that sometimes just the right design can improve safety/performance/efficiency without an increase in real cost or weight so i'm curious what designs best control or reduce wave slam even just from the perspective of reducing drag further.

2) Experience.
This is experience of being in such situations as well as "professionalism" in what one whats from their boat and their journey while at sea. Does the vessel have enough life rafts and update certified rafts..yes or no. This is not a design issue per se, it is an experience/knowledge issue, since not everyone will pay for the latest raft. Is there an EPRIB on-board..are there flares, full set of NavComms....all these are available..but at a cost. It does not affect the design, only your wallet. Is there enough insulation on the vessel, to combat those colder climes, again, a cost issue. And so on...

Every professional designer will offer all the usual safety features, but not everyone is willing to pay for it. They often prefer to spend their money on a fancy looking deck or fool themselves that a fancy sounding carbon fibre design will make their design better when in reality any composite will do, if designed correctly...and so on.

Thus safety is only partially a "design" issue. There are usual rules and regulations that all must comply with, but one cannot legislate for idiots and poor seamanship. The safest boat or car in the world will not make YOU safer, if just makes the experience of an unforeseen event less traumatic. But one must then ask the question...how did one get into THAT situation to start with...was it by design or by poor experience!

Going around a 90 degree narrow bend at 90mph in a Tank will be the same experience as in a small compact car. The result may well be different for the passengers depending which vehicle they are in, but, and this is THE important part... the person driving the car has not engaged their brain to realise such a manoeuvre is dangerous to begin with! Ergo design cannot legislate for idiots and wannabes. Just focus on your own skills or lack of them in such situations and let the naval architect take care of the design.