DIY cheap "tube-cat" design

Hi, I'm new to this forum (and boatdesign) but lately I've been inspired to build my own cat. I'm hoping to get it done by the summer.

I tried a couple of Hobie Rotomolds in Key West this easter (Wave, Getaway) and I was impressed by how responsive and easy-handling they were. (I've had some experience with Lasers and some recreational cruising with some guys from my university.) After seeing the "rebel-cat" online (link) I decided I'd have a go at designing and building a cheap, safe and (hopefully)fun beach cat myself.

I intend to buy two pieces of 5 meter (16'5") 355 or 400 mm (14"/16") diam plastic tube. These will be stiffened by using "bulkheads" of high-density PS (styrofoam) and PU foam filler (SikaBOOM). This will give me a sturdy, easy to make (almost no poly/epoxy/fiber mess) and cheap hull. Cost per hull will be 180 - 450 USD (depending on tube material and diameter)

I'll heat the plastic (with hot-air or water) to be able to "pinch" the end closed, making the bow. (think of a drinking-straw folded over itself, or bent 90 degrees). If I do this at both ends, I'll have a nice sharp bow and aft. This eases the fitting of a rudder (and possibly flow?)

This is the first of 3 concepts I've made so far.

Buoyancy for each hull will be 300-400 kg (dep on diam). The total weight will be around 200 kg this allows for 4 adults to be carried safely (400mm):

total displ. = 800kg
weight of boat = 200kg
safety factor = 2

(800 - 200) / 2 = 300 kg

75kg * 4 = 300 kg

I've attached some images as well:

What do you guys think?
I'm concerned about the bow (looks like an axe-bow), especially that it is the lowest point of the hull.

Also, no rocker (or rather inverse rocker) could make this a b**ch to turn.

 

Concept # 2

This is basically the same at the first, but with a Styrofoam plug in the rear covered with epoxy/fiber to make a transom.

This reduces the thermo-forming work by 50% and adds some buoyancy in the rear. However it could mean more drag...

 

Concept #3

This is a refinement of concept 2

I've cut the bow at 45 degrees to reduce the bow sticking down so low.
Will this reduce the wave-piercing abilities of the hull?

Also I've tried to add some rocker by squeezing the aft opening into an oval shape (thermoforming, like the bow).

Here are some pics:

 

Now, I'm pretty sure this hull will need a skeg or a daggerboard to have any up-wind performance to speak of. Sailed singlehanded it will be floating mainly on the reatively flat bottom (ca 1/3 displacement ) I'm leaning towards skegs as it's easier to make and use (and adds some protection), and I won't have to make slots in the hulls (and thereby possibly weakening them).

here's a drawing and a render:

 

Naka; you are being very inventive and you seem to know enough about the math involved. Good for you.

As clever as your idea is, I Do not think that it has enough merit to justify the build. Here are my objections............
* Cylindrical sections are not the best shape for a cat hull or any hull except a raft.
* Your estimated cost per hull is more than a conventional boat would cost.
* You have estimated boat weight at 200kg. That is much too heavy for a small catamaran if you expect decent performance.
* You are correct that the boat will be difficult to turn. That will will not only frustrate you but it may be dangerous in certain circumstances.
* the cylinder section will cause complications when attaching crossbeams.
* A 16.5 foot cat is not big enough for four persons to enjoy.

You live in Scandinavia where you can source some very fine boat building wood and some very nice plywood. Why would you not take advantage of your good fortune and dismiss the plastic in favor of a much better building material. You can easily get plans for a wooden catamaran that is a proven design. Such a boat might very well cost less than the plastic thing. It will certainly perform better. In the long run the wooden boat may be easier to build as well. It is a certainty that the wooden boat will have more value if and when you decide to sell it.

 

@ messabout
Thanks for your insights!

My goal for this build was to get to done quick-and-easy so I could get a feel for rigging, sailing etc. and build some experience on the lake. Then, I'll use the experiences from this build to build a more "serious" boat in a year or two. I need to get me some sailing experience and find a suitable place to build it.

Funny you should mention plywood, my plan is to build this "serious" boat out of plywood (cat or mono, not sure yet).

With regards to the weight: I recon each hull would be around 60 kg (75 tops)
I'm not sure what the rig/tramp/rudders will weigh, but I'm hoping I'll get it under 50kg total. (I have no experience, so I really don't know)
For reference, the Hobie Getaway (16ft) is 170kg, and I thought it handled OK.

Four people is a stretch, I know. The way I see it, it'll be sailed singlehanded or with a crew of two 9 out of 10 times. However I like the fact that it could hold 4 in a pinch.

The crossmembers will be an issue, that's true. I haven't quite figured this one out yet.

The directional stability could very well be a hazard if this boat is pushed. My biggest fear is that the "drooping" bow is a broach-wating-to-happen.. Frankly that is the single most off-putting feature of this entire design.

Where I see this build having merit is in the fact that I'll probably be on the lake quicker than with a ply-design, it does not have the same requirements with regards to indoor work-area and the fact that it'll be cheaper than anything on the market here in scandinavia. The Hobie Getaway is nearly 20000 USD over here. A 70's Hobie 16 is around 5500 USD.

I guess I'm back to the drawing board on this one.. That bow/hull is a little troubling..

BTW: here's a link to the rebel-cat I mentioned. Similar build, but narrower and longer hulls. However, looks like it'll barely hold two (sits very low with one)..
http://youtube.com/watch?v=uvw8t0TTLPs

 

As long as you're happy to accept that the limitations of this construction mean that you'll never get a high performance boat, you can make a seaworthy craft which sails. I think you're basically on the right lines, but you need to think simpler.

First, if you cut a V out of one side of a tube, then you can re-join it to form an (approximate) cone. This will do nicely aft.

If you cut a V from top and bottom of the tube which is the same with as the tube's diameter, you can rejoin it to give an acceptable pointy bit at the front.

The geometry of these will define you bow and stern shape. The cross-beams need solid sockets to slide or bolt into, and here I would suggest using a plywood bulkhead slid into the hull and fixed appropriately. Remember that the rigidity that you have in the mounting and cross beams will be the defining structural factor in how the boat sails.

It certainly should be pretty quick building. The things which will take the time will be rudders and rig. Do you have a donor vessel for these? Also, because it's a cat, stick something buoyant at the top of the mast, otherwise if you capsize you'll never right it.

Best of luck,

Tim B.

 

There have been multitudes of attempts to do essentially the same thing you wish for at least 40 years or more. I've never seen a satisfactory boat built from PVC pipe other than rafts and amas for low budget outriggers. The reasons are many but many are those Tim outlined. That said, your proposal is the most promising and inventive seen but is still an attempt to make a silk purse from a sow's ear. That's American for using junk to make anything of real value. It's a waste of your obvious talent that can be put toward a boat you can be proud of.

 

Thanks for your feedback, guys!

@Tim: The idea of cutting Vs is a good one, thanks! Im'm looking at both PP, PE and PVC right now. Of these PVC is the heaviest and most expensive but also the stiffest and easiest to form (can be glued). PE and PP pipes are cheaper, not as stiff, somewhat lighter and much more flexible (not britle like PVC). The big downside is that they aren't gluable and have to be welded or sealed in another way...

Anyone had any experience with either PE or PP?

@Tom:
I've already come to terms with the fact that this thing will not be a performance craft. I'm just so eager to get out on the lake.. (and my girlfriend really want me to build one) Besides, I think it would make a nice hand-me-down to less experienced sailors when I'm bored with it.

In the long run I'd like to build something worthwhile. I've been looking at Prindle and Nacra designs, and something along the lines of an F18-type cat will probably be more in line with my long term plans. However, they all look so high-tech that I'm not sure if my manufacturing skill is quite up to the challenge (yet anyways). Could I make something in this performance-class with Stitch-and-glue or would I have to lay it down in fiber and glass?

For now I'm focussing on this "toy" though. As a lot of different people will be using (and abusing) it (girlfriend, "in-laws", friends), I'm trying to focus on safety and affordability rather than performance.

I'm thinking a 6m aluminum pole will do nicely as a mast (maybe I'll stiffen it up with some fiber and glass if needed) Also I'd like to make a battened sail (around 10-12 m2) and rig it boomless (for safety). Will this be an OK power to weight ratio?

I have one question, though, the rebelcat has extremely slender hulls (its 21 ft long and each hull is 10-12" wide) with a fineness ratio of over 21/1.
The concept I have sketched will be only have Rf=12.5 - 14.5 wil this cripple performance compared to the rebelcat?

Thanks for the feedback and support!

Tom from Norway

 

Tom; Permit me to argue that you can build a better boat with plywood. You can do it just as quickly as you can with pipe and with a lot less hassle. The only advantage with pipe is that you will not need to paint it. If fugly and crude is satisfactory then i'll shut up. (but not yet...see rant below)

Consider building plywood boxes of rectangular or square sections. Pinch the ends together and give the boxes some rocker. Bending ply is easy and it will tend to create fair curves. Now with a bit of forethought you can build in some bulkheads that will give you a secure place to attach cross beams. You can double up on the sections where your chain plates and fore bridle will be. (how would you do that inside a chunk of pipe? The boxy sections will help cancel some of the leeway. The pipe will not. You can even make the boxes assymetric and do away with the need for boards. Ala Hobie et al. That would save board building time. If you choose assy planform then make the hulls symetric about the midship line. That way you will need only one building form. You will not be constrained by pipe dimensions and you can make the boxes as tall or as wide as you want them. The boat will be drier, more structurally sound, dead simple to build, and surely a better performer, and less likely to produce a lee bow trip.

If you want to really rush the job you could consider a free standing mast. Just build a vertical box into one (or both) of the hulls and jam the mast in the hole. Now the sail will be offset but that will work. The down side is that while one down wind tack will show near neutral helm, the other tack will have heavy helm. With the 10 square meter sail that you mention the difference will be manageable. Incidentally you may be able to find a used International canoe sail which is 10 square meters. Ten meters is pretty small for this boat but it will be enough to make good fun. And on a day when the wind is up it will be more than fun. One other thing about the ply build is that you can have your girlfriend participate in the finish work. Girls seem to enjoy painting things to make them pretty. They even do it to themselves.

Boats built of pipe are irreverant in the extreme. Beware the Norse gods who frown on such behavior.

In the future, when you consider a more sophisticated boat, take a look at the Tornado. You can get plans for tortured ply construction for that boat. As you may know, the Tornado is a spectacular performer.

 

Starting to reconsider, feeling peer pressure

@messabout

Hmmm, you make a compelling case there..

OK, I'll consider plywood as it's the easiest way to get the all-important rocker, AND it'll make a much better boat with minimal extra effort (according to you). More importantly, I don't want to upset the norse gods

As I have no experience with stich/glue and plywood I have some questions:

-----------------
a) What thickness of ply should I use?

b) What quality of plywood am I looking for? (Okoume is quite expensive, are there viable, cheaper alternatives?)

c) How much of a V should the hull have and should it taper to flat at the stern?? (from what I've read deep V is not preferred in cat hulls due to drag, however I imagine needing some to get an "edge" if I am to do without skegs or foils)

d) Hull beam. Can I go with the 400mm for a 5-6m boat? where do I put maximum beam (forward or backward of midship?)

e) should the boxes be square sided or is a slight trapezoid shape better (a little wider at gunwhale than at the chine)

e) Will I need to glass the inside/outside or will just epoxy do (apart form the seams, of course)

Based on this i can make a model of the new hullshape and post it here for review..

Again, thanks for your insights and comments, I'm pretty much a noob at boatbuilding, so anyone is very welcome to share their views/experiences/cirtique!

Tom.

 

Hi Tom (from Norway )

I started almost the same way you did !!! I had a similar idea, and then I ended up here, same as you. (are we exactly the same )

Not much I can add to the advice you were given. I would like to say your pipes are going to be too small. You mention the girl friend... if you want her to enjoy it then it has to have some comforts females just won't get along without. She'll rough it out there with you a few times, and then you either will start going it alone or the boat is going to become a white lonely elephant.

I would suggest you look at a cat with hulls, (7 -) 8m long, 1m200 high and 1m wide and an overall beam of say 4m as a minimum. You can fit a port a potty in there, a mini kitchen and basic berthing. If things are unpleasant it offers a retreat and bad weather will affect you a lot less. Trust me, you have an excellent chance getting cought in some bad weather.

Such a rig will also offer a weekend away on the water...

The boat in a week thing hasn't taken off. You could of course shop around and just buy a nice boat. It'll get you on the water in even less time, and without the headaches.

Good luck.

 

BTW, download Freeship off the net and you can draw it in there...

 

@ Fanie

Great minds .. and such.

To your proposal. The cat will be moored at my in-laws' place in Sweden. The property has a small shoreline to one of the lakes, so I could make it a big one. However the weather one the lake is rarely very rough, and a bigger boat is in my future (in a couple of years, though). For now I'm just after a handy-sized cat that I (or someone else) can take out and have some fun.

My GF has sailed dinghies (Laser primarily) more than I have, so I'm not worried that she won't join me for a bit of inspired cruising. However this cat absolutely needs to be managable singlehanded as well. I foresee a lot of solo sailing as well cruising with a friend (or maybe 2).

The Freeship software is great. I've been using SolidWorks so far, which is not optimal. Thank you for the tip! I also foud the CARENE2008 great for making chined hulls to import into Freeship.

 

Hey everyone! I'm playing around with the CARENE/FREE!ship software and I have some questions. Where do I put maximum beam?

When I put it at midship (3m) CARENE shows a waterline angle of -1.8 to -2.9 degrees (bow rising with increased displacement)

When I put it at 1/4 aft (1.5m) CARENE shows a waterline angle of -0.5 to -2.2 degrees (bow rising with increased displacement)

Shouldn't this be as flat (close to 0) as possible?
What causes this? My hullshape?

OK, I screwed up the CARENE sim. I got it now...

I'll have some pics for you guys later today.

Here's a pic from FREE!ship.
My new hull should be easy to put together (some precision cutting required, but I'm up to it) 6 pieces per hull, but its symmetric, so I guess i COULD screw 4 plates together and be done with only 3 cutots

I'm planning on adding bulkheads at 3m (center and widest point), 4.5m and 1m

I'm thinking the 1m and 4.5m bulkheads will support the crossmembers.
Anyone see any problems with this design? Sure looks a helluva lot better than the tube.. This looks like a proper sailboat hull.

LOA = 5.75 m
Draft set to 20cm wich equates to a displacement of 315 kg
Beam = 0.524 m

Fineness ratio = 11