Hull types

Mark26

Active member
Joined
17 Nov 2017
Messages
361
Location
Penton Hook
Visit site
We have displacement hulls and plaining hulls, and of course a myriad of hybrid designs in-between.

How would one define a catamaran ?

They have two hulls which sit in the water and have sharp bows to move through it in the same way as a displacement hull does, so how come the max hull speed rule doesn’t appear to apply?
 

Chris_d

Well-known member
Joined
15 Jun 2001
Messages
4,667
Location
Oxfordshire
Visit site
Don't know the technical answer but I've always understood it's because the displacement is shared between two hulls. The normal equation for displacement speed assumes a certain length to beam ratio, but a cats hulls are very narrow so it has a different multiplier and can effectively go much faster for a given length than a mono hull.
 

Mark26

Active member
Joined
17 Nov 2017
Messages
361
Location
Penton Hook
Visit site
Thank you.
It is an interesting subject.

I was thinking about the max hull speed of my own boat: my thoughts turned to ships and bulbous bows, and then I thought, “hang on a cotton-picking minute; what about high speed catamarans, such as the Poole - Jersey car ferry? They are not plaining hulls and they are not hydrofoils unlike their monohull cousins”

I though there were two basic types: Displacement and Plaining.
Catamarans makes three ?
 

Chris_d

Well-known member
Joined
15 Jun 2001
Messages
4,667
Location
Oxfordshire
Visit site
Well maybe not, Catamaran's come with displacement and planing hulls as well, best way to think of say a 30ft displacement Cat is as a 60ft long thin displacement hull cut in half and put side by side, so hull speed almost the same as the 60ft mono but only 30ft long, and very wide:)
 

JumbleDuck

Well-known member
Joined
8 Aug 2013
Messages
24,167
Location
SW Scotland
Visit site
Don't know the technical answer but I've always understood it's because the displacement is shared between two hulls. The normal equation for displacement speed assumes a certain length to beam ratio, but a cats hulls are very narrow so it has a different multiplier and can effectively go much faster for a given length than a mono hull.
The normal equation takes no account of hull shape at all. It's simply the speed at which the wavelength of the bow wave is equal to the hull length. In "deep" water - typically more than ten times deeper than the wave amplitude - the wave velocity is sqrt(length x gravity / 2 x pi): put in value for gravity and pi, convert the units and you get the formula.

Two things, as far as I know, mean this doesn't apply to catamarans. First of all, it assumes that at higher speeds the stern squats down into the bow wave, but long, thin hulls don't do this as much. Secondly there is interference between the two bow waves.
 

Windy_Stu

Member
Joined
11 Nov 2007
Messages
119
Location
Chichester
Visit site
I just sold my 35ft Cat....no idea how u workout theoretical max speeds...but what i can say is wind on aft of beam no mono of similar length would get close :) downside was load her up and she defo lost loads of performance much more weight sensitive than a mono. I thought it was something to do with the canoe type hull shape. Strange u dont see many on inland waterways...very low draft, and our had prop in the middle quite well protected,
 
Top