Tube BeachCat Problems

Hi,

I have a few questions regarding a hull design for a beach cat.
Most of the hull is made from a 3.0m, 56cm(22") diameter plastic pipe. This pipe will be cut of lengthways by 16cm(decreasing diameter by 16cm) creating a flat deck of width 50.6cm. This will leave the pipe with a total displacement of 503 litres. More than enough for one hull of a beach cat? This displacement value will be increased further as the stern (transom) and the bow will have to be built and fitted to the hull. The problem is that the hull will have a totally flat "keel", the same draft the whole length of the hull. What can be expected from this? Centreboards will have to be used I guess. Are there any major set-backs with the totally round shape (except deck) and uniform beam for 3.0m of an approx. 4m boat?

The purpose of this design is to create a rather seaworthy beachcat with low costs and easy, fast building. Preferably it should be able to carry 3 people without loosing all the common characteristics of a boat in water. That should be possible with the displacement, or? And lastly, how deep should the centreboards go? Would a long, rather than deep centreboard be recommended?

Thankful for all replies,

Kai

 

I built my first beach cat in 1977, and my last one for solo (the 5th) in 1991; 5.5 long, 3.2m wide, wing mast (birch plywood and carbon fiber) height 11 m, mylar kevlar mainsail 18 m2 (no jib), home made titanium hardware, special asymetric centerboards inclined of 25 degrees, total weight 91 kg ready to sail. Able to touch the 25 true knots and you can imagine needing a 200 pounds guy in very good physical condition with a sharp mind when the wind is above 10 knots.

But I do also love also simple fun cats for spending the afternoon sailing in the sun in front of the beach moved by a gentke breeze.

You ask a lot of questions , so I'm going to answer in the order;

1/ Total volume of the hull; a safe rule of thumb is have a total volume per hull of about 160 to 220 % of the total displacement (weight of boat plus weight of crew). You can have less like the Hobie 14 or 16 but it's a submarine lacking of power and pitchpoling easily.

If you want a cat going well upwind the LCB must be around 54 to 55 % when the cat is flying on a hull, and until 60 to 62% if you want a fast boy downwind . It's worthy to try to design a hull able to keep the same values while the cat is sailing on two hulls or flying on one hull.

The bow must not be too immersed (from 5 to 10 cm) to not grab too much in the water for good tacking, the max depth must be around 55 to 70 % of the length ( all % measured from the bow). For the stern there are 2 schools; keel flowing gently from the max depth to the stern almost not immersed, or a more step curve from the max depth to a stern immersed of about 5 to 7 cm. The second school gives more powerfull boats able to have a spinaker.

The width of the hull may vary from 20 or 18/1 (length/width) like the Class A to 10/1 for some fat boys. A good cipher is around 15 %. The stern must have a width of about 70 to 80 % of the max width which is located at about 50 to 60% of the length.

A classic good underwater shape is; razor bow going to a U 45 cm from the bow, almost a lightly flattened half circle at the max depth to a flat U at the stern.

For a beach cat just for fun a long keel under the hull is a good solution as centerboards need a lot of attention. The surface of each centerboard under the keel is about 6 to 10 % of the sail's surface. It's good that the govern takes about 25 % of the lateral effort, that gives a more reponsive cat.

A good measure of the width of the centerboard is about 35 to 35 cm with a 10 to 12% thick NACA profile, that gives a easy to make, and strong foil. The length under the keel depends of the surface to get. But I repeat: for a fun cat centerboards are not the best solution.

I pass over the subtilities about the placement of the center of effort of the sails, the rack to give to the mast, and some other ones.

2/ I wrote this lenghty first part as you have now the answer of the other questions. As you see the Cat as you described is very far from the shape obtained after 35 years of design and racing by numerous people. It will be a slow pontoon unable to tack.

3/ It's not worthy to start from a pipe: the addition of the stern and the bow will be painful and the lack of rocker is handicapping.

4/ These fun cats do not stay in water and the most expensive parts are the mast, sails and hardware. So remplacing eventualy a worn or broken hull is not painful nor expensive. Plywood, even the modest meranti skin door of 3 mm at 10 bucks a sheet, 6 oz glass, blue polystirene foam and a few gallons of epoxy resin and dedication are all you need for a simple compounded plywood hull.

The Gougeon Brothers sell the plans of a class A (5.5 m long) in compounded plywood. With a smaller mast ( about 7 meters) and 10 to 12 m2 of sail
it will be a very good fun cat for 350 hours of work. This cat is able to take 1 to 3 persons. Some of my friends built it, and it's far from being slow...

 

Ah, lots of usefull information there. Thanks.
Ok, so the tube idea is realy out of the question. I was just thinking it could be cheap and easy. Plastic tubing is very cheap and quite strong.
Anyway, I now have 2 more questions =)
What is the LCB that is mentioned above?
And where can I find the build plans for the Gougeon Brothers Class A compounded plywood catamaran, I could'nt find anyone selling them on the net, is there some web page?
Depending on the Class A cat above I might go for something like the "Crab Claw" catamaran or come up with something of my own with the help of the info from Ilan Voyager
"Crab Claw" catamaran:
www.rclandsailing.com\catamaran\index.html
If anyone is interested.
I'd not have that rigg on it though, does'nt look like that much fun to me.

cheers,
Kai

 

LCB Longitudinal Center of Buoyancy.
Read some books about sail boat design and multihulls, it's worthy...

The Gougeon Bros ( http://www.gougeon.com/) are famous for their epoxy
resin: send them an Email asking for the plans of Class A cat in compounded plywood. Buy also their book about boat construction; it is gold and the lone describing the compounded plywood in detail.

For a first boat stay with plans. Do not add to the building skills to learn, the task of learning a bit of naval design and after correcting expensively your mistakes of design. Nobody plays Bach's BWV 1043 on violin in 3 lessons...

The page about "Crab Claw" catamaran is unreachable.
Forget about exotic rigs and stay classic, you have already enough work building your cat. Crab Claw sail was used by polynesian (very good boat builders) who had only some wood and vegetal fiber... now we have other materials. Do you use stone tools like 200 000 years ago?

Good luck

 

Crab Claw address

The correct address is www.rclandsailing.com/catamaran

 

Exotic rig it is! Done some sailing and model studies,quite surprising work posted on http://proafile.com/view/weblog/entries/C12 need feedback