Need Help Designing a Catamaran

I've been asking questions about it, but feel guilty for making so many threads, so I'm consolidating my present and future questions about this boat to this one thread.

I'm designing, and hopefully building, a catamaran that will have to displace around 580 kg (1280 lbs) which includes the crew and food and maybe some rods and bait. It will have a crew of six, be about 20 feet long and have a beam of around 10 feet. The beam of a hull will be around 1.5 feet. It is going to be for cruising around on weekends, but I'd still like it to go as fast as it can (while still being comfortable) . It won't have a cabin, just a trampoline (which will probably be wooden beams, and not a trampoline.) I have not yet decided on the type of sail rig, but I'm leaning towards something like a Hobie or F18. It's going to be launched from the beach, so has to be able to get through the surf on a windy day. It will be built out of plywood, with stitch-and-glue methods. All the dimensions are still theoretical and may be changed if there's reason to change them.

I'm new to boat design, so if I'm thinking along strange and amateurish paths, please tell me.
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So, here's my first question in the thread:

Why are the hulls of some racing catamarans very converse, and some almost not converse at all?

By converse I mean like the bottom of a rocking chair, if I'm using the wrong term or if there's a better one please tell me.

For example:
http://www.cat44.com/images/034_31.JPG
http://www.f18-international.org/images/DartHawk03.jpg

The Hobie's bow starts from a point and curves gradually down, then back up again until it gets cut off by the rudder.
The F18's bow is a vertical line that gets fatter and rounder until it reaches the centerboard, then it gets skinnier but stays round.

My guess is that a Hobie would tend to go over a wave more than through it, and an F18 would cut through it rather than go over it. Am I right?

Including any I haven't mentioned, which hull shapes and characteristics are best for my boat as I've described it above?

Sorry for so much text, and thank you for your responses.

 

You are pretty much correct here, but the main difference between the two hulls you are showing is to do with the way they cope with leeway, (the sideways drift caused by the wind). The F18 has a daggerbaord, the hobie does not and has assymetric hulls. A third way to cope with this is to have symmetric hulls with a fixed keel.

The hobie banana shaped hulls are a good design for surf but do not go upwind as efficiently as a centreboard cat. But with a centreboard cat you have to remember to raise and lower the boards when you leave or arrive at the beach.

As for which is better, when launching off a beach with surf I would go for either assymetric hulls or a fixed keel like the hobie getaway.


http://www.hobiecat.com/sailing/models_getaway.html

All the best

Gareth
www.fourhulls.com

 

Thanks, I didn't realize that 16's had asymetric hulls. Do you mean they're shaped like \| |/ ?

Wow. I looked around the Hobie site, and found that the Hobie 16 has almost the same beam length as my design, longer hulls (16' ), and it's "maximum load" is 800 lbs. I guess I'm going to re-do the hulls .

What about a longer and slightly wider version of this: http://www.geocities.com/g_catmultihulls/gcat5-0.html ?

When measuring the beam of a catamaran, it's portmost side of the port hull to the starboardmost side of the starboard hull, right?

Grob, I looked at your website and that X-Cat looks really neat! Are there benefits to having four hulls on the water? Disadvantages? It seems like it would decrease WSA but perhaps get a little drag from turbulence? It's probably more stable, too.

 

The hobie 16 hulls are |/\| not \||/.

There was an article from a magazine on thebeachcats.com a while back on how to build a Hobie16 type hull

see
http://www.thebeachcats.com/modules.php?set_albumName=Tornado-Building-Plans&op=modload&name=gallery&file=index&include=view_album.php

http://www.thebeachcats.com/modules.php?set_albumName=Catamaran-Building-Plans&op=modload&name=gallery&file=index&include=view_album.php

correct

The advantages of the fourhulls, is that stability is greatly increased, and WSA is decreased, there is probably some drag penalty associated with having more bows and sterns but WSA is more dominant in a high speed cat. And trimarans don't seem to suffer from this compared to cats.

All the best

Gareth
www.fourhulls.com

 

The Hobie is notoriously weight sensitive, due to its very low volume hulls. It's also a bit of a pitcher AFAIK because of the rocker. The Hawk is a much newer and faster design, with more U shaped hull sections which give much less wetted surface for the same volume. I think you'll find rocker has moved aft over the years as having less rocker forward and more aft allows the boat to tack faster.

Check out the Tornado shape, still a good one after all these years. A Class cats are much lighter and very different but also very advanced, look them up.

Perhaps best of all, check out the Capricorn lines on

http://digilander.libero.it/mikan1/

Two or three Capricorns got into the top 10 at the F18 worlds and none had an all-pro crew or had much experience in the class.

Grob, why are four hulls less likely to nosedive? Is it really good to have four comparatively short and fat hulls? Don't you get separation problems at those sterns? And how can the boat be as light as an A Class cat without being equally demanding to construct and care for? It's 75% of the weight of the lightest product 16 in the world, and they are pushing the practical limits in terms of hardware and have kevlar hulls. How do you get a "fun boat" to weigh so much less than a racing 16 built by an A class champ?

 

CT 249 is right if you decide to go with centreboards, and there are plans for that on the site I gave you.


The hulls are less likely to nosedive as there is more bouyancy up front, see below, both these hulls are displacing the same volume at the same pitching angle, so you can see why the 16ft 4hull is less prone to pitchpolling than the 18ft Acat. Yes there are seperation problems associated with having more sterns, but that is offset by the lower WSA.

We are not really lighter than the current best in class racing boats. When we use similar construction technique to the A class we come out about the same weight, when we go for a more robust design we come out about the same as an F16.

The web site is quite out of date, the weight of the boat is now up to around 100kg, about the same as the current lightweight F16's. This change is due to a different hull construction , the original hulls were based on a similar construction to the A cats. Also the original boat used carbon beams and we are now using aluminium.

But if I answer your queries based on the original 75kg estimate. It takes about 25% less surface area to get a certain volume out of four short fat hulls than 2 long thin hulls. So you save 25% less weight on the hulls, but they are only part of the story, also the rig is lighter.

The other major innovation is the use of a new hull construction technique. We use a thermoformed Polyproylene/glass fibre material. This has the advantages of being as lightweight as the best of the current composite boats while having very much increased durability and impact resistance.

This is not a new process, it has been used by some white water Kayak manufacturers and the Royal Navy RIB's, it also used on bus bumpers!, but we beleive we are the first sail boat manufacturer to use this technology.


Gareth
www.fourhulls.com

 

Gareth,

Has your folding beachcat been sailing yet? I've been following your project as much as possible.

 

Free Pirate,

Your 20x10 dimensions are common in the beachcat catamaran world. The classic Tornado is 20x10 and the Shark is a folding 20x10 all wood design with a solid deck.

I have a full set of plans for a wooden Tornado on my site, you can browse them here Tornado Catamaran Building Plans

Six crew on a 20 foot catamaran is a lot unless you make the hulls much more substantial. Maybe something like the Reynolds 21 would carry the weight.

 

Damon,

Sorry I missed this post, Yes it has sailed and showed some good potential, there is still some work to do and we hope to be back on the water soon, as a new set of hulls arrived yesterday, I will keep you posted.

As I said previously I haven't updated the website for a while but one of the first things I will do is provide a link to your great site.

All the best

Gareth

 

Lower Wetted Surface Area (WSA)?

Gareth,

I reviewed your website with some interest. Congrats on bringing an new concept to market!

I am curious regarding the assertion that WSA is smaller with four hulls vs two. I can see that by separating a typical cat hull into two sections that have some fore and aft space between them, one can increase fore and aft stability without increasing WSA. However, displacement of the design does not increase.

If we set aside the question of four and aft stability for a momenet, and compare two boats of equal displacement, one a typical cat and the other one of your four hulls design - the cat would offer lower WSA. The same would be true when comparing a cat to a monohull. A given displacement can be achieved with fewer hulls that offer a lower WSA. Do you agree?

Please understand that I don't mean to criticize the four hull concept, I simply want to fully understand the assertion that wetted surface is lower with four hulls.

Thanks,

Randy

 

Randy,

Thanks for the question. I know my statement is not intuitive, it can seem that two volumes may have a higher WSA than one and this is true when the compared volumes are the same shape. This is why a cat has lower WSA when flying a hull.

However, the ratio of Displacement to wetted surface area has a lot to do with the shape of a hull, the shape with the lowest WSA for a given displacement is a sphere or half a sphere. Therefore the closest you can get in shape to a sphere the lower the WSA will be.

For a given displacement a 5m long cat has to be very long and thin, usually with a beam of around 5% of the length, when you have a four hull configuratoin, the hulls are shorter and fatter, so you get a beam which is say 12% of the length. Therefore you are closer to the shape of a sphere and so have lower WSA.

The situation gets even better when you get out of trim for and aft, i.e. as a cat pitchpolls its WSA usually increases as it has more of the thinner bow in the water, but with a 4 hull design it decreases.

Indeed it is not always true that a monohull has a lower WSA than a catamaran, it depends upon there relative shapes, there was a thread on this ealrier on this site.

If you want to prove this to yourself you can do some calcs using ellipsoids (3D ellipses) with a spreadsheet.

By the way we have not brought the design to market just yet, but we are pretty close now.

All the best

Gareth
www.fourhulls.com

 

need urgent help in my design

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.