cat design software

[northerncat]

i have sketched out a basic plan for a 6m assymetric powercat, is there any free software out there that lets you plot out assymetric catamaran hulls ?
sean

[rkpshenoy]

This is for all those who have prior knowledge of designing a passenger cat.
I have already done preliminary design of my catamaran design the way conventional ships are designed by following similar steps but for sticking to the required L/B , S/L ratios for catamarans. Having done that i saw that the lines plan i chose is inapplicable to my design. The CB of the hard chine hull form is far below the required value for the displacement calculated.Later i learnt from some other guy in discussion forum that my approach in fixing the dimensions itself is wrong.
If so plz tell me how to fix the main dimensions like planing length..., trim, lift required etc for catamarans.Also if you can suggest me any catamaran or a planing hull specific book it would be gr8.
Id let you know about the details of my project after anybody has replied.
Suggestions of all kind are invited.

[marshmat]

northerncat,
Delftship http://www.delftship.net/index.php and its predecessor Freeship http://sourceforge.net/projects/freeship/ can both handle asymmetric catamarans.
rkpshenoy,
What exactly are you trying to do? Is this a school project of some sort, an actual design, a home-build? How much do you already know/understand?
(As a side note, if you were to clean up your writing style a bit it might be easier to figure out what we can help you with.)

[rkpshenoy]

i am not here to post links with some pictures of boats. i want to get the best out of the discussion
forum.If you take time to understand
and figure out what others are
trying to say i dont mind waiting for
few more days to get a better reply

[marshmat]

rkp, I don't mean to insult you or anything. I'm just asking for more information and a more detailed, clearer statement of what it is you need help with. For instance, you say "The CB of the hard chine hull form is far below the required value for the displacement calculated". The CB (centre of buoyancy) is a point in 3-dimensional space, and has an associated volume/mass value (the displacement), it's not clear what you mean by "below the required value". All catamarans are different, just like all monohulls are different- without knowing your exact situation and what you are trying to build, I can't give any helpful advice.

[rkpshenoy]

Yeah Im sorry matt to write the short form, by CB I mean the block coefficient.
I am done with the GA and LCG (longitudinal center of gravity) ,LCF,VCG
calculations. My main dimensions are something like this

L = 40 m
B = 11.7 m
demi hull breadth = 3.3 m
draft = 1.56 m
block coefficient for hard chine comes out to be 0.37
displacement = 260 tons
now you see thats wat i was talking about.

[marshmat]

Ahh, that makes a lot more sense. There's two things that use that acronym- CB (centre of buoyancy) in capitals and Cb (block coefficient) with a lowercase or subscript "b".
OK then. So at 40 m long, 3.3 m wide, drawing 1.56 m with a block coefficient of 0.37, your cat would be displacing 152 tonnes. To get 260 tonnes with those dimensions you'd want a block coefficient more like 0.63.
For something this heavy, you won't need much deadrise in the hull bottoms (smaller craft need that V-shape to cut through waves when they plane). Do you have a fair bit of deadrise or are the hull bottoms nearly flat?
Another clue to your dilemma might be hiding in your prismatic coefficient and vertical prismatic coefficient. Both will have to be rather large in your case, probably around 0.7 longitudinally and as high as 0.9 vertically (ie. a demi-hull with a fairly square bottom, and hullsides parallel for most of the length except for right at the bow). If those prismatic values are too large, you need to either cut down the weight, or increase any of: length, demi-hull beam, or draught.

[rkpshenoy]

Thats a nice bit of information matt. You understood my dilemma.

No, the values of Cp and Cb are not so high as you said.
They have the values 0.41 longitudinally and 0.614 vertically.
This is my lines plan.
The hydrostatics are obtained by maxsurf.
Im also doing a Savitsky power prediction for the vessel which requires
an average deadrise of around 0.38 degrees which in my case are exceeded.
Well what do you suggest. Can i meddle with the lift generated
while planing to meet the required displacement.
Thanx matt

[marshmat]

Re: prismatics.

They have the values 0.41 longitudinally and 0.614 vertically.

Those values imply a vessel with very fine ends and a significant V-shape over much of the bottom.
If you want your cat to plane well, you will need to get the longitudinal prismatic well into the 0.6-0.7 range. Cp must get larger as speed goes up- and a displacement sailboat is in the 0.55ish range, planing boats are almost all in the 0.6 or 0.7 range. So you need to get more volume in the ends of the hulls. This will have the added effect of bringing the total displacement up closer to your required 260 tonnes.

Can i meddle with the lift generated while planing to meet the required displacement.

No. The boat must be able to sit on her lines at rest. While a planing craft supports much of its weight by dynamic lift while running, it has to support all that weight by displacement at lower speeds.
If your vessel will be on plane a lot of the time, you might look at widening the demi-hulls to meet your design displacement. A long thin hull is very efficient in displacement mode but does not generate planing lift very efficiently.

[rkpshenoy]

Nice that you pointed out the silliest mistake i was going to do by
thinking the vessel is never gonna be at rest.
Now that there has been a shortage of available info
on cats and their design spirals i had to go for a geometrical model
of the cat and its lines plan. How do i fair the lines to get this prismatic values.
Plz suggest me some reading. some books or journals on cats , planing vessels.

[Gilbert]

If you are willing to use the trial and error method and you want to keep the general shape you can just pick a couple of stations, say one at midships and the other at 75% from the stern and move them forward say 10% of the hull length for the midship section and say 15% of the hull length for the other. Refair the lines and see what you get. If the Cp is not high enough move them more yet. I am assuming it has a transom stern.

[marshmat]

I am assuming it has a transom stern.

From the bodyplan shown above it looks like it... but it's awfully hard to get a Cp of 0.41 with a transom. Can we see any more views of the boat?

[rkpshenoy]

No, I havent developed the views after seeing the hydrostatic
values in maxsurf.
Yes, it has a transom stern. May be the chine angle and the deep V form in the fore end is making it have a low Cp.
I guess the method suggested by Gilbert may work, but it looks the method needs manual fairing. Can maxsurf do this variable station spacing..