catamaran question

I was being asked to do a concept design of a daysailing catamaran, the precise structural design wil be done by professional designer. This boat are going to be used as a training, vessel for local sailing school. AS I am a mono guy I have some questions. The design should be:
1. ACAP (as cheap as posible)
2. AFAP (as fast as possible)

My first Idea was to stop designing this boat and start looking for used multi, but it came out that the boat it self and transportation of it would be few times more expensive then local building the simmilar boat.

So to meet the design criterias I am looking forward to a plywood cat. To keep it fast and as safe as a cat can be I think it should have long and narow hulls (L/B ratio somethere between 15-20). Most probably the boat will be built by a profeesional carpenter with help of youngsters and enthusiasts in weekends. So the boatbuilding time is crucial for overall price Of this boat. I think I have stated all things that involves in this boats design. So that I came up with is.

7-10m long
3-6m wide
no accomodation, flatbottomed, plywood catamaran.

I have few questions:
How will the flat bottom involve the performance of this boat?
How will the long narrow hulls involve the seaworhines of this boat?

 

Wharram....

For about $20-$30 dollars US the Wharram Design book is full of inspiration and great 'cat' ideas'. Where better to get a sense of what works and how things should look. The construction methods and interior layouts are very practical and thought provoking.

 

I was on a cat once that was ply with a flat bottom. for the life of me i cant remember the design. it was stitch and glue. each hull built in three flat sheets that were built on a flat bench then glasswd while still flat. no fairing required. then basically cut out with a jigsaw and the bottom and sides stitched,glued and seams glassed. very quick construction. beams put in. this one had a soft canvas and clear bridgedeck housing which kept it light. ran a single center mounted rudder and centerboard. at about 36 foot drew about a foot of water. modest rig but boy did it smoke along.took nothing to get it up into the teens. I beleive it may have been a british designer.
bazza

 

sketch image
L 8m
B 3.5m
Displacement 0.806m3
draft 0.2/1.7
Lcb 3.758 m
Prismatic coefficient 0.6269

 

In the 1960's there was a successful 16 ft. design by Lindsay Cunningham from Australia called the "Quickcat". It was simple and all plywood with deep hulls and a narrow flat bottom. The centreboard (one only) was carried in the centre and was a swivel up design which would work well for sail training students. I believe a flat bottom is fine but needs to kept narrower than the deck. As the overall weight will be quite low for the hull length and width, a narrower beam will make for a seaworthy craft, providing there is enough depth near the bow and there is flare in the topsides to provide reserve buoyancy when the leeward hull is pressed in strong winds.

 

I understand thing about reserve buoyancy. to keep constructions imple i decided to keep tha flat bottom as vide as deck, but I designed the boat in the way that it is like 7 m catamaran with 1m aded in front

 

It will be faster as you requested in your initial posting, with the flat bottom considerably narrower than deck width and more sea kindly. Your version will pound heavily in small waves as the hulls will not be heavily loaded. It is not much harder to construct this way. The centreboards in your drawing are way too far forward although I understand this picture is not to scale.

 

"Shared Lift"

The boards are definitely not too far forward if the rudder and foil areas are varied from the "normal"; the ARC 21 is an example of a cat design utilizing the concept of "shared lift" which utilizes a smaller than normal daggerboard located forward of the front crossbeam and larger than normal rudder; the designer feels that there are distinct advantages- see post #2 here:
Catamaran Centreboard Location - Boat Design Forums
Address:http://boatdesign.net/forums/showthread.php?t=10302

 

The search for simplicity

Nice little bit there from Aqaurius.

Probably overlooked because the philosophy is so elegantly expressed, the following quote from Bill Roberts, the designer of the ARC21, has firmly gripped the wheel of the on-going arguments to make boats more complex in the search for thrills and speed.

"Removing the complexity of rigging and controls allows the sailor to spend more time focused on making the boat go fast and less time trying to decide which unnecessary adjustments to make. Getting the sailors head out of the shadow of complex rigging and into the clean air of simplicity will not only produce a faster sailor, but a better sailor."

More of us should be doing this very thing as we haggle over the need for yet another complex addition to a boat in order to make it faster. Clearly, the thinking line for a faster, more efficient boat does not always lie in the direction of more "stuff" to fill-up the available deck space.

Great link.

In the photo below, you can just see the top of the daggerboard just forward of the main beam mount.

 

I will re design the hulls, and see what will I get. Maybe sbd has sugestions how to build simmilar boat like I am trying to design, examples? maybe I am going the wrong way? :?

 

Couldn't agree with this more, I really like this guys design philosophy, he used to post on the catsailor.com forum, but haven't heard from him for a while. I think he got tired of some of the abuse he got, which is a shame as I think we could learn alot from him.

Gareth Roberts
(Not related)

 

I have replaced, the dagerboards, and started to design narrower bottom of the hulls. Actualy I am redesigning the hull shape. Unfortunately These days I am traveling a lot so I can't post any sketches.

 

A far as I know, the G-Force 36 molds are for sale for $1600. This boat uses a deep-V symmetrical hull design with no boards. It has quite a bit of rocker to it for maneuverability. It was designed as a race boat and is very quick, but the design also means that it is by no means fragile and has a comfortable motion. A local day-charterer here uses one because it requires less maintenance than the Stiletto 30 it replaced. PM me for the phone number. I'm not POSITIVE that the ad is for the G-FORCE, but the picture sure looks like it.

 

what If I redesigned the hulls, in the way that they are longer and narrower, It would be sth like wave piercing hull. Will this work.

 

Maybe you should look at the K design web site. There are some simple to build plans available. You might find the design there that fulfills almost all your requirements.
http://www.ikarus342000.com/