Catamaran design

[Manie B]

Hi guys,
brigde deck clearance on small 30 to 38 ft cats,
how much of an issue is it really?

If you only had 300mm = 1ft of clearance what is the real problem - slamming = noise??? or is there a handling problem that goes with it - as the whole boat starts to float upwards on the large flat under surface.

Many different builders have as many different claims, but windage would be the same its just that you are closer to the water.

[Alik]

Clearance depends on speed - for planning cats lower clearance is acceptable.

I would say for 30-35' displacement cat (designed for sea) minimum clearance is 500-600 mm in operation condition. Lower clearance results decrease of performance, slamming loads on structure, unpleasant pitching motions and noise. For 'displacement-high' Froude numbers immersion of stern will probably cause situation when bridgedeck at stern is touching water. Result -decrease of speed and poor control, especially on following waves.

For planning cat of 30-35' clearance 200-300 mm at rest is proper. For seaworthy cat the bridgedeck should be above WL to avoid 'riding on tunnel' and wetness.

For catamaran, clearance of bridgedeck is the main factor limiting seaworthness, so...

[Manie B]

Alik thanks for the reply, i am thinking in terms of a sailing cruising cat at 4 to 8 knots, family style, not racing.

For catamaran, clearance of bridgedeck is the main factor limiting seaworthness, so...

this is what i am not really clear on, why do they say that.

[northerncat]

i think they term seaworthiness as being enjoyment of sailing in slightly adverse conditions which doesnt happen in low bridgedeck catamarans due to the often mentioned slamming
sean

[CTMD]

Having drawn a lot of cats over the years, yes wing deck height is important, however, it is often used as a very emotive argument by sales people.

Structurally a low wingdeck is acceptable but it will need to be engineered to take additional slamming loads.

Visually a low wingdeck height will usually result in reduced freeboard and therefor a "sleeker" looking design.

Aerodynamically the lower wingdeck will be better for the reasons mentioned above which will make it easier to tack in windy conditions.

The common main argument against a low wingdeck is slamming and while this is a problem its not a major one. A bigger problem is actually "hydro-lock" (for want of a better term). when you have a low windeck there will be times when the waves end up running along the bottom of the deck. This causes a huge increase in wetted surface and the boat can just about come to a stand still. I've seen people add steps etc to the bottoms of wingdecks of both power and sailing boats to try and fix this issue but it usually only has limited success.

So the big question, what height do you need.........

Obviously the answer is dependant on vessel size and intended usage but I'd recommend approx 4.5-5% of length waterline on the size you are talking about (30-38ft). When cats get big (50+ feet) this can usually be reduced to 4-4.5%.

Note for non-planning powercats slamming is usually considered not to be an issue by the survey authorities if the wingdeck is greater than 5% LWL above the static waterline.

[Alik]

Note for non-planning powercats slamming is usually considered not to be an issue by the survey authorities if the wingdeck is greater than 5% LWL above the static waterline.

For ISO12215-5 it is an issue. Bridgedeck clearance is compared with height of wave for category, not with length of boat.

CTMD is correct about meaning of clearance for marketing. But more important is longitudinal profile of the bridgedeck - raise in front and aft are desirable to minimize effect of pitching.

Unfortunately, many cats are now designed for marinas, not for the sea. We tend to design cats with higher bridgedecks, probably having less 'sleek' apperance than others. But aerodymanic resistance due to increased height of boat is much less than added resistance due to low bridgedeck on seaway.

[Alik]

Originally Posted by Manie B

this is what i am not really clear on, why do they say that.

I mean seaworthiness is a ability to function in certain weather conditions. For instance if one has to reduce speed or change track because of slamming, or passengers start feeling uncomfortable - these are the limitations of functionality.