Power Catamaran Design

[DHMzip]

I need some help with power catamaran hull design. I am a NA student, and am trying to find information about hull shape and resistance prediction of power catamarans. Does anybody have any info/ can point me in the direction of some papers on catamaran hull shapes, catamaran bow shapes.

I am mainly interested in hull types for high semi-planing speeds (Fn 0.5-0.9).

I have discovered some helpful papers on predicting catamaran resistance by Molland. so That covers some of the resistance side.

I would like to find out more information regarding bow shapes (ie. wave piercing, ax bow, reverse bow, bulbous, z-bow, etc), and how to solve issues such as reserve buoyancy.

thanks,
Dave

[Ad Hoc]

There are many good technical papers about regarding this, Molland’s are very good. However the only real issue you need to focus on is the length displacement ratio. If you get to grips with this, really understand it, then you’re laughing

Basically once you delve into this you soon begin to realise that hull shape plays such an insignificant role that it is not worth worrying about. There will be many that so….oh yes it does!! Those with computer programs will tell you as such, because it is something that can of course be measured by many decimal places. But it doesn’t in real design. So read up and fully appreciate the length displacement ratio and all its implications.

A summary is given here:
Dare to Say No

With regards to bow shapes, you can see here too if you like:
Wave-piercers - marketing myth or design ingenuity?

But basically, doesn’t affect one jot. Again, read the posts and then if you’re a naval arch student you’ll understand why from analysis of the mechanisms that really affect pitch and heave, rather than a ‘faith based’ approach. The "gains" are very minor to be insignificant.

[Leo Lazauskas]

Originally Posted by DHMzip

I need some help with power catamaran hull design. I am a NA student, and am trying to find information about hull shape and resistance prediction of power catamarans. Does anybody have any info/ can point me in the direction of some papers on catamaran hull shapes, catamaran bow shapes.

Which paper by Molland do you have?
I highly recommend Ship Science Report 71 that he wrote with Wellicome and Couser.

You can do some interesting preliminary work using that report.
For example, plot the maximum total (calm-water) resistance
for the three main groups of slenderness ratio, L/D^(1/3)
at the same demihull separation ratio.

My program "Michlet" includes the 10 models used in that report.
You can fiddle around with the bow shape and then see how much
difference it makes to the maximum total resistance.

Form drag is a bit trickier to capture and, in fact, it does not always improve
correlations of experiments with theory.
A resource that might be useful to you is the thesis by Simon Robards:
"The hydrodynamics of high-speed transom-stern vessels "
http://unsworks.unsw.edu.au/vital/ac.../unsworks:3426

Many different hull forms are considered and regression equations are
given for the form drag, IIRC.

Unfortunately, Robards' regression equation contains about 23
parameters, so it is cumbersome to use. For the NPL series, I obtained
very similar results using a simple function of the slenderness ratio.
See the post (and attachments) in the second post at:
Flotilla Demo 2.07

Best of luck with your studies!
Leo.

[DHMzip]

thanks so much for the help.

here is a preliminary lines plan of one demi hull. let me know what people think. comments for improvement are welcome.

~Dave

[Alik]

You need more flare or slight chine at bow. With this hull, ride will be very wet.
Also how to laminate such a narrow entry?

[DHMzip]

thanks for the comment.

The construction will be strip plank wood epoxy most likely.

Also I am going to play around with different bow shapes: wave piercing, z-bow and axe bow. Most likely i will insert either a chine forward or spray rail in order to divert the spray.

~Dave

[Richard Woods]

I would agree with Alik. Also, even in strip plank the bow area is going to be tricky to build for what benefit??

How long is your boat? Did you see my Skoota 20 videos??

Richard Woods of Woods Designs

www.sailingcatamarans.com

[DHMzip]

The boat will be around 75ft long.

no i have not seen your videos yet.

~Dave

[DHMzip]

Update:

I have developed my design a bit. The section shapes of the demi hulls are now much more semicircular in order to reduce WSA. I am going to solve the lack of reserve buoyancy issue with either a bulbous belly in the tunnel or tons of flare at the topsides.

I am still researching the pros and cons of different bow shapes.

Does anybody have any leads to info on Axe bows?

thanks again for the help so far.
~Dave