# catamarans vertical clearance

Discussion in 'Boat Design' started by Brian.Lin, Nov 27, 2017.

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### Brian.LinJunior Member

dear Sir,
how to decide the catamarans vertical clearance between waterline and wetdeck?
I try to search information on www.
I find following simple formula.
a. DNV rule. HL=0.22*Lwl*(0.3-0.0008*Lwl)
b. documents from THE SECOND CHESAPEAKE POWER BOAT SYMPOSIUM.
Ⅰ、Displacement catamarans : t=(0.045…0.060)L
Ⅱ、Planning catamarans: t=(0.02…0.03)L

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### jorgepeaseSenior Member

Sailing?
Everyone always says don't compromise bridgedeck clearance but I see some big names at around 30" while other performance cats above 47" regardless of length.

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### goodwilltoallSenior Member

Min. 1'-0" clearance (no obtructions) for every 10'-0" Beam OA.

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### JSLSenior Member

A guide I got from a well known multihull designer (power and sail) was minimum of
Disp. hull = 6 1/2% of the DWL (ie: 30' DWL = 1.875')
Planing hull = 3 1/4% of the DWL

As a check I looked at a 65' power cat (225 passenger tour boat, semi-disp hull form- cruise 8-10 knots) I did several years ago. Wet deck freeboard was 3.0' on a 60' DWL = 5%. Boat does harbor tours so water is usually calm.

Last edited: Nov 28, 2017
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### Mr EfficiencySenior Member

These formulae are a guide, I suppose, but other factors affecting the calculation would be area of operation, slimness of hulls, vertical taper of hulls, shape of intersection of hulls with the wet deck, and provision of wave breakers. Or at least, I "imagine".

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### JSLSenior Member

Correct

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### TANSLSenior Member

They are a guide indeed, there is no rule that requires a minimum value of the air gap but, depending on the navigation area, Lwl, ... and the air gap you will have to reinforce more or less the wet deck.

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Depends on the type of catamaran, its duty, role, location of operations, what is it: yacht, passenger, crew boat, fast ferry, and of course sea conditions...etc etc

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### Ajmal Cosmanstudent

Can someone please accurately define the Air Gap with some reference please?

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### Ajmal Cosmanstudent

Hi Tansl you are very knowledgeable about catamaran can you please help me for i need a reference and a good definition of the air gap ?

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### TANSLSenior Member

Air Gap definition

#### Attached Files:

• ###### Air Gap.pdf
File size:
1.1 MB
Views:
66
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### garydierkingSenior Member

If you can't row your dinghy under it, don't go to sea in it.

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### Mr EfficiencySenior Member

Limbo rowing ?

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### AlikSenior Member

Min values are, for power cats:
- displacement 4-6% of length
- planing 2-3% of length
These values are for middle, need rise at bow at fwd 25% of wet deck
The values above are minimum, higher values are recommended

Another consideration: 1/2 of anticipated operational wave height, for middle section.

For sailing cats, obviously 'as much as possible', however high clearance will result excessive windage. I would say 800-1000mm for 40', up to 1200mm for 60' cat is good reference.

My paper on power catamaran design, available from RINA:
Nazarov A. SMALL CATAMARANS: DESIGN APPROACHES AND CASE STUDIES//Trans RINA, Vol 157, Part B1, Intl J Small Craft Tech, Jan-Jun 2015

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### tom28571Senior Member

I don't know what the formula should be but do know that if you get it wrong, you're in for a lifetime of misery.

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