Displacement Catamaran Design

[islander]

Nice looking design:


http://www.noahthompsondesign.com/NT...-Catamaran.asp

[tom28571]

I agree. Decent looking power cats are rare. I like the propulsion layout.

[aranda1984]

It is nice to see a good loking multihull design.

Most attempts at a power multihull is barely more then a house boat on pontoons, no wonder people have a negative opinion about them.

[Mr Efficiency]

In Fiji it is "time" for democratic, non-racial government, after slipping into reverse gear with coups etc.. Suggest name change for boat to "Island Time".

[islander]

Yeah,nice cockpit...although a huge one!!

[islander]

I have never been on a displacement cat before.How is the ride,stability and performance in rough waters?

[Mr Efficiency]

I have never been on one (displacement power cat ) either, but would expect a bit of hobby-horsing going into the sea, obviously in the traditional sailing cat you rarely pointed that direction, and the rig helped to dampen those oscillations.

[Willallison]

Originally Posted by tom28571

I agree. Decent looking power cats are rare. I like the propulsion layout.

Hmm.... it's a slightly different take to 'usual', isn't it... the props and rudders a somewhat further fwd than you normally find them, and the aft 'bustle' (flat section aft of the canoe-body) is also much larger than other typical displacement cat hulls....

I'm often a little dissapointed by the amount of room in the saloon of these 'mid-sized' cats. With just a single helm seat, galley and dinette on offer, there's less accomodation here than my own (considerably smaller) 9.8 x 2.8m monohull. Sure, there's space elsewhere that I couldn't hope to match - like the sleeping accomodation, nice wide side-decks and monstrous aft cockpit...but then so there should be given that it's almost three times as big (in terms of area).
Don't get me wrong - I love displacement powercats and I agree, this is a pretty good looking one - and the GA is obviously designed to meet a certain SOR... but still....

[DHMzip]

That is definitely not a displacement cat. 18kts on a 47ft WL gives a Fn of 0.8 definitely in the Semi Displacement range.

I am sure the boat rides nice, especially in choppy seas. the cockpit is big because its a FISHING boat.

[erik818]

DHMzip,
If there is no significant lift from anything but displacement of water then the boat is a full displacement boat whatever the Fn. At least that is the definition I have agreed with myself upon.

Erik

[DHMzip]

@ erik818

"significant lift from anything but displacement of water" - please explain.

Dont all semiplaning and planing boats generate lift from the water?
Displacement boats hardly generate any form of dynamic lift by moving through the water.

if this boat had a full displacement hull form and is going at speeds of 24kts, the HP requirement would be much much higher.

[Willallison]

DHM - most of the 'rules' that apply to traditional moderately beamy monohulls go out the window when it comes to the very slender hullforms employed in displacement cats. There's quite a bit of info out there... if you google displacement or displaning catamaran hullforms you should find plenty of good reading on the subject...

[DHMzip]

Thank you Willallison,

I am well aware of the difference between catamarans and monohulls. I have done plenty of research on fast displacement hull forms of power catamarans. It is one thing for an 80m fast ferry to still go 30kts in "displacement" mode, where as this boat is a 14m fishing boat.

[aranda1984]

With multiuhulls that are light enough, and if the L/B ratio is just at optimum, if the distance between the hulls is just right, if at maximum displacement the wetted surface is at minimum, you can exceed the hull speed.. you can make them surf on the water, under power or sail!
The hulls must be designed such a way as to prevent squatting!

... One more fact: if the hulls are long enough to straddle two or more wave crests, you will feel like being on a cruise ship!?!
(Exccuse me for stretching the truth just a bit!)
By the way, hobby horseing can be the sign of a not so well designed multihull, usually a result of badly placed pontoons.
(Also, too heavy superstructure, too short pontoons, too fat entry into the waves, badly placed C of G. etc...)

The super light ones will surf with a fraction of the horse power needed for a same length monohull!

Search the net for "Ultra Thin Hull" (UTH) Triamarans.

The US navy, plus the Aussies and Kiwis made some so fast, that they will boggle your mind! (Note, these nicknames have been used as an endearment!!!)

They can do highway speeds without generating high waves.
All you can see is the surface foam left behind!

Talking about multuhuulls ...most people will think of the short cats and tries that you can pull up the beach.

Think in terms of well over 75' to 150'! That is a good beginning!
Talking about a short beach cat and making judgements on multihulls is like comparing the original mini to a stretched Caddy limo!

Regards,

Stephen

[erik818]

DHM,
What I mean is that if the hull does not generate any significant dynamic lift, then it is a displacement hull. Long and narrow displacement hulls can be pushed way past hull speed (Fn 0.4) with reasonably low power. Typically the optimal L/B ratio is something like 20:1. The problem with one 14 m long and 0.7 m wide hull is to make a useful boat out of it. The solution to this problem is a multihull.

Comparing Fiji Time with the smaller hulls I've toyed with in my computer, 300 Hp seems reasonable for 21 kn and 12 tons. I believe however that 21 kn is past the speeds where two narrow 14 m hulls is the most efficient choice for a 12 tons boat. Reduce the speed to 10 kn - 15 kn and Fiji Time will be in it's most favourable speed zone when comparing to other types of hulls.

Erik