OK, I have put on my flack jacket for this one!!

I have attached a rather basic drawing of a catamaran I am attempting to build with my kids.

Not being an engineer, I could do with some advice on the size of box section aluminium tubing I could get away with as I am keen to keep weight down to a sensible minimum.

The cross beams will be about 5'6" across, the overall boat length is 12' and I plan to use approximately a 10square metre windsurf rig. This is really meant to be a single hander, or one adult plus one child.

Looking at the dimensions I was thinking I could use 3' box section with a 3mm wall thickness. The smaller sections at the end of each cross beam would be 2.5' box section (these are the pieces that fit into the beams running along the length of the hull.)

I am wondering however if I could get away with 2.5' box section for the main beams?

Any suggestions or advice on any other aspect of my project would be appreciated. (yes, very funny, I meant constructive advice )

Thanks
Andy

Andy,
If the beams were attached directly to the hull you could get away with beams 50mm x 50mm with a 2mm wall.
As with the Glider in the attached pic. It is 13ft long and a similar all up sail area.

The 14ft Paper Tiger has 50x50mm beams with 3mm wall and 100sq ft mainsail.

Both boats have a min platform weight of 50kg.

Once you put your beams on posts you create a lot of problems that will add weight to the platform without much benefit. My personal view is you would be better saving the weight.

Then that is just my personal opinion.

Regards,
Phill

i can not wait till my son is old enough so i can build him a glider.

Andy,

There are a ton of available plans out there for cats like this (just talk to Phil above!) and they will keep you from having to be an engineer to build a safe and sturdy boat. Phil is a home builder extrordinaire and while I can't find the link to his website, you really should take a look...Anyone have that link?

oh...wait! duh:

http://www.geocities.com/phillbrander/

Thanks for the advice Phill, those are some excellent designs on your site. I'll take on board what you have said and look at doing away with the hull beams. I initially incorporated these only to give height for the trampoline, as I am using flat hulls. Your comments have given me a few ideas to lighten the design a little.

Jake, I know the sensible thing to do would be to buy plans for a tried and tested design, however, most of the fun for me is in trying to build my own design.

Cheers

Yeah, I can identify with that - Good Luck! and keep us posted!

I had fun as a youngster building a cat with Plywood and sealing it with Fiberglass it was a very simple design as the hulls were esentially triangular Like the H16 and had a little rocker from being narrower at the front. I think a lot could be done if you somewhat copied the H16 and I bet you could even get already built parts like the mast and cross bars, But if you insist on inventing everything then remember that triangles are very strong. (sure you knew that but I felt like adding a coment)

There are many boat plans available for home building, I have seen two that have been produced by some interesting naval architect and that might give you some ideas:

Kohler has a a 14' cat

http://www.ikarus342000.com/SC435page.htm
It might not look that good but it gives you a idea of the project.

He also has a twin rig 15' cat

http://www.ikarus342000.com/DUO425page.htm

The other architect is Richard Woods and he has a few plans for beach catamarans. The easiest plan would probably be the Pixie:


http://www.sailingcatamarans.com/pixie.htm

He also have this plan for a very interesting sailboat, the Stealth:

http://www.sailingcatamarans.com/stealth.htm

Thanks Charles and everyone else for the useful advice.

The remit for my design is for a car toppable cat, that when packed up it can be carried down to the water in one go. I also want all assemblies to clamp together without the use of any bolts. (There was a cat called an ALLEY cat that had a similar design, however there were too many bolts involved, making assembly a little too long winded.)

I am also keen to use windsurfer style planing hulls. Apart from the wish to experiment with such a hull shape, by using flatish hulls, I can pack up the whole assembly so the rigging, rudder etc sandwiches between the two hulls for easy stowage and transport.

The main design problem I am experiencing is in developing a light weight method of raising the trampoline and cross beams to give enough clearance between water and sailor. (This cat will only be used in an estuary so not too much clearance should be required.)

Anyway, I'm having great fun thinking through the individual problems.

By the way I have already built the first hull using a plywood underbelly and Styrofoam SPX that I have shaped. Once happy with the hull shapes I will lay them up with some fibreglass.

And yes, I will be wearing a crash helmet, whistle, flares, radio on the initial test

Andy,
If you want to build a boat like that you could consider raised sections in the hulls at the critical points to support the beams. (This can be done without adding much weight if designed with care and it would be structually much better than post) 12ft surfcats are built this way. The have these raised sections and the tramp is on a 4 sided frame made of round aluminium around 50mm dia and around 3mm wall. The have around 80 sq ft of sail and no dolphin striker. The hulls are symetrical but don't need centreboards. Only drawback is this type of system needs 4 nuts to be done up to assemble the platform.

I had a look for a photo of one unfortunately only have one ofa surcat upside down when I was making some beach wheels for it. I attached it just in case it is of some use.

Regards,
Phill

Hi All

I have finally launched my planning beach cat I have working on over the last 24 months on and off. I posted this on Boatdesign.net a while ago but forgot this thread and the great advice I got from you guys. So, anyway, here are some pictures if you are interested (don't laugh I'm trained in IT not boat design )
The Pink Onion

I know in the pics I need another foot of downhaul on the sail...my friends in the windsurfer forums put me straight on this!

I was trying to achieve the following charecteristics in my design:
a) Car toppable - achieved, although it doesn't pack up quite as cleanly as I had anticipated
b) Quick and simple to rig - achieved, first rig up time from roof to water in 23 mins.
c) Prove whether a 12 foot cat with planing hulls can give an exhilarating ride - achieved, it does - in my biased view
d) power the boat with a standard unstayed windsurfer rig - achieved, certainly seemed to provide enough power with an 8 metre rig
e) Single rudder and no centre board for simplicity and minimal drag. Use of hard chines on the sides to provide lateral resistance - this set up works but due to no dagger boards is tricky to tack.


My first sail was in a steady force 3 on an estuary (this boat is not designed for sailing in choppy sea). From that initial sail, I have learnt much about what works well and what doesn't about my design. Here are the key things:

1.) The concept of windsurfer style planing hulls works. She got up on the plane on a reach and certainly gave the local windsurfers a run for their money (OK, I may be a little exageration here )
2) Windsurfer rigs have lots of potential for small projects like mine
3) A little prior knowledge can go a long way. In my case an understanding of boat balance (Centre of effort and Centre of lateral Resistance) would have served me very well when deciding on the position of the mast

5) She is a pig to tack (hardly surprising I suppose. Although I find that by sinking the transom a little she spins round a little easier.


Anyway, I have had fun building it, and now have a boat with a very shallow draft that gives me quite an exilerating ride, and that my kids love sailing.

What would I do differently from what I have learnt if building another one? I would loose the posts as you guys suggested. To compensate the clearance I would raise the height of the hulls. This would dramatically reduce weight and complexity. Apart from this I would change very little


Anyway I have just had my entry through for the Round Texel Race........

I have just received the go ahead from the missus to start my next project... . I have this obsession with easy to rig car toppable quick boats, so this is going to be a mono hull with racks and stabilizing foils.

Thanks again for the initial advice!

Andy Lyne

Looks like your hulls are buried about a foot into the sand.

-Rob V.
Panama City
Nacra 5.2

Andy,

While I question some of the design aspects (that you yourself already bring up) I think you've got the beginnings of something neat and unique there. A cat-windsurfer bastard child or something. While the hull shape certainly is open to some future development, I can see the sailor sort'a trapezed, sorta windsurfer rigged but flying a hull - looks like it could be a blast!

Maybe it would tack better with windsurf-style centerboards in each hull? It would remain completely beachable. Just a thought, and wonderful job on executing a dream.

I,ve said it before and I'll say it again, its a great boat, I especially like the unstayed windsurfing rig, Do you know at what speed it begins to plane?

Gareth

I wouldn't like to guess at the moment what speed it gets up on the plane as I have only been out a few times, and to be honest I am not experienced enough to give an acurate estimate.....doesn't need to be too windy though.

I quite fancy getting one of those GPS systems to record some speeds. It would be nice also to get a waterproof video camera so I can do some recording on the water, doubt I can afford this till next year though

Looking at some sailing videos, I would be interested to know how they incorporate the GPS readout into the film ....it makes for some really interesting footage when you can see the speed readout.

VERY cool Andy,
Between you and Grob we'll have to change that Heaven/Hell joke to British Engineers.....being heaven.
I think this looks like a blast to play with. Wish I had the time and patience to build something from scratch. I'm very envious.
One question: How is the mast attached to the front beam? Hard to tell from the photo. Seems like a high torque area.

Congratulations!!

Greg

Hi Greg

Thanks for the compliments.

The mast simply slots into a larger tube welded to the cross beam. The mast is reinforced with extra fibre glass in the bottom 1 metre section. This is enough to strengthen the mast without loosing the shape of the sail.

You are dead right, the way I have the mast attached to the cross beam is a very high torque area and I plan to improve this by using shrouds that attach to the mast directly under the boom. I have thought of a simple clip I can use to easly attach the shrouds so it won't affect rigging time much.

As for the shape of the hulls, I wouldn't want to change the under water profile much as it seems to work well. I have small rocker in the front and no rocker aft of the centre line. The hard chines help reduce leway as I don't want to introduce centre plates, dagger boards or skegs,thus cutting out unnecessary drag. It will never be good round a triangular course because of the leaway when headed into wind, but thats not the point of it, its more for blasting up and down on a reach.

I would however introduce some curvature into the top of the hulls. This would give a stronger shape if built out of stressed ply, and I could also build in a little more buoyancy in the bows.

That would be great! Then you could turn the hulls 'upside down', mount the tramploine on the 'bottom', and you would have a normal looking beachcat: flat deck and U-shaped waterline.