Would that catamaran ever sail?

A heavier well designed mono-hull will blow away a poorly designed catamaran. I have done it.

There is a local waterfest at a large lake community where people enter boats that cost less than $50 to build. Most build crude rafts, many are boxy catamarans, with large tarp sails. I built a nicely designed 14 ft mono-hull with salvaged lumber, we outdistanced the next closet boat by two to one. My than teenaged daughter was my crew, we passed the finish line, we dropped the head sail and just for fun tacked back out onto course (it was a down wind race), circled the rest of the contestants, and than sailed back to the finish line. I scaled down the K-18 hull to 14 ft so it would be able to carry it cartop.

So I do not need to prove that a well designed, and even heavier mono hull, will out preform a much larger poorly designed cat. It has already been done.

 

"Just because you say that it has been done isn't proof that it happened"... blah, blah, blah...
... repeat ad nauseam...


Is this thread helping anyone?

 

It'll keep going as long as someone is silly enough to keep feeding it.

 

Flat bottom, two chines, displace less water than any other design.
""The two chine hull (B), with a flat bottom and nearly vertical sides, was the first hard chine design to achieve widespread use. This design provides far more stability than the single chine hull, with minimum draft and a large cargo capacity."
http://en.wikipedia.org/wiki/Chine_(boating)
http://en.wikipedia.org/wiki/Draft_(hull)

Less draft, less displacement. Less draft, also means less wetted area. That means less drag, according to Froude number.
"For over 100 years, it was realized that FRICTION varied with the wetted surface and the square of speed (SV²) and mildly affected by length."
http://smalltridesign.com/Trimaran-Articles/Boat-Resistance.html

A 2000 lbs curved shape, moves slower through water than a 50 lbs rectangular shape using the same force.

No, I agree, it is better that we stop talking about it. We cant agree on the F=m.a formula and how mass and displacement is a more important thing, lets stop talking about it.

 

I am doing it purely for my own entertainment. I know its pointless. Yes dear internet overlords I have a problem - I do reply to trolls. Sorry.

Displacement - as in you use is not based on a shape but weight. So weather your 100lb structure is a sphere, a shurinken, box or a boat it will displace the amount of water, it does it solely based on its weight. A box compared to any kind of streamlined body will weigh MORE for similar size/strength structure - (of course you want to compare to a fixed keel seaworthy cruiser, I'll get back to that.)

I think Emerson's comment "displaces more water" means that when traveling it makes more water move. It forces bigger amount of water to dodge the **** out of the way compared to sensible smooth design. And where ever there is a corner the water needs to move fast and in practice it does it in very turbulent manner. Thus F=ma is relevant - it not the mass of the boat that is relevant in the earlier explanation but the mass and the acceleration of the water. You probably understood that in the 1st place but chose to be dense.

Wetted area - A sphere has the least amount of surface are for any shape. Smooth shapes are like that. A box will have significantly more surface area for given volume compared to a typical streamlined boat - even one made of plywood with hard chines.

Well its an idiotic question. Is it easier to pull a shoebox on a string compared to a Boeing 747?. Ahahh!!! that means that a shoebox is more aerodynamic!. (Prove me wrong!!!)
Comparing a 50lb box to a 2000lb boat is a stupid premise. You cannot compare the use and function of the two.
But relevant to your claim: you would be surprised how little power a smooth shaped boat needs to move at slow speeds. It of course needs more time OR bigger force to ACCELERATE but to just travel its actually possible that the smooth boat takes less force to travel than your box of 50lb + a passenger. Check that drag coefficient chart again. Streamlined shape vs. box is in totally different magnitude so yes much smaller shitty shape can have more resistance than better shaped drastically larger object.

btw. the formulas assume that you are using somewhat sensible boat shaped objects. You can't have 17ft parachute or a funnel and estimate its resistance by a 100 year old formula for calculating boats' resistance. They didn't take idiots into account when they wrote the formulas.

And nobody suggested you should build a 2000lb displacement day cruiser as a solution for your need to get in the water. How on earth you got from milk carton shaped catamaran hulls to comparing a box shaped composite 50lb "canoe" to that boat is beyond me.

Now with the time spent on this thread you could have butt jointed 2 plywood sheets together (with a block on side for strength, drawn the shapes and sawn the plywood. But something tells me you have never built anything. And you are very unlikely to ever do it either.

 

You justified the extra weight you added to the other design as the weight of passengers, were you not planning on riding on your boat? It's all a matter of fair comparison, honest comparison. It's dishonest to compare a small multihull to a small monohull, you still lose that comparison but you have a dishonest advantage. The multihull has far fewer amenities and less space, it's apples to oranges. Not a 50 lb craft though, no your design is going to weigh several hundred pounds, probably more than a thousand, because you need to use lots of reinforcement, because it's such a poor design.

An honest comparison is one between your multihul design and a good multihull design. That's the one you need to deal with, not apples to oranges but apples to apples. It doesn't take long to sort out which apple is the wormy one.

 

yes but it WILL sail, end of story... - so does any floating object with a sail...

Stability - the hull shape doesnt matter for stability of catamaran

Weight - why dont you work out the weight of your proposed hull and how you will build it - do you know how to work this out? i suspect not, otherwise you would soon realize that a curved shape will be lighter...

 

The original question was, 'Would that catamaran ever sail?' And the answer is 'no, because it would never be built.'

I think anyone with enough basic skills to build it would know better....

 

http://rebelcat.com/Rebelcat5-morepics.html

This is not a "proper" boat design. There are ways that it could be improved. However, any improvements would require a non-trivial amount of extra work and skill.

The Rebel cat is what you should be comparing your design to.

Is your design going to be easier to build? Not by a long shot, you're will probably take 3-4 times as much time to build.

Will yours be cheaper? No, it will cost considerably more.

Will yours be lighter? No, it will be several times as heavy.

Will yours hold more? Probably not, your design is going to have huge stress loads.

Will yours go faster? Never in a million years.

Will yours be stronger? Nope, again, square joints don't handle stress well.

Round is good.

 

I have been to many gatherings of small boats and boaters. It seems, always, that here is at least one comedian at such gatherings.

I have seen what is claimed to be the cheapest sailboat possible. The boat consists of a truck inner tube, a crude bunch of sticks lashed to the top, a broomstick for a mast and garbage bags for sails. Does it sail. Yes indeed it does. Not very well of course but it will sail, as demonstrated by said comedian amidst the obligatory cold beer and laughter.

This week is the annual Cedar Key (Florida) boat bash. There will be hundreds of small boats there. Boats of all descriptions including perhaps a hundred home builts, some amateur inspired and designed, some really strange rigs, some amazingly rudimentary ones, some very sophisticated ones, some will be multis, some monos, sailing canoes, and all sorts of other UFOs (unidentified floating objects).

I will buy the OP a chicken dinner if there is even one boat out of the many hundreds that does not have at least some curves in the sides, and nary a one will have a triangle stuck to the end of a box.

 

Shame on me for helping to keep this train wreck going. I'll hate myself in the morning....

But a while back, John was posting quotes that said good things about flat bottom boats. Unfortunately, I don't think he understands what the term means. When someone says a hull is flat bottomed, they don't mean it's a planar surface like a sheet of glass, or like John's box with a triangle on one end. They mean it's flat in cross section only, rather than being vee'd, muti-chined or rounded.

Blue Rose, the canoe my son and I built, is a typical example. It's flat athwartships, but has a strong rocker from end to end. The rocker is flatter towards the stern, because sides are only pulled into a transom instead of a second stem (yes, I've built them that way too...). And of course the sides wound up with a rolling flare, which created more very nice curves.

It's about as simple a boat as you can build. The solid wood sides were bent around a temporary mold, and screwed to a stem at one end and a transom at the other end. Then some frames were inserted, before nailing on a plywood bottom. And frankly, the frames might have been overkill. The bottom and the gunwales alone would probably have kept the boat's shape - especially if we had kept the center frame and just ditched the other two. But it isn't a racing craft, and I don't mind a modest amount of overkill in a boat that will probably see years of use, abuse and neglect.

The point is that we wound up with all sorts of cool curves - and it certainly wasn't because we worked our butts off to create those curves, instead of assembling flats with corners. They just showed up naturally, when we used a flared temporary mold in the center and cut both ends of the side boards at an angle.

Look at this picture, John. Why would you fight to build a boat with planar surfaces and right angles, when it's so much easier to produce pretty and functional curves instead?

 

That is quite the handsome paintjob on Blue Rose. I'd like to suggest that perhaps she should be considered a dory rather than a canoe.

 

She's definitely a kissing cousin to a dory. But given her dimensions (roughly 15' overall, 3' extreme beam, and 2 1/2' beam at the chines), she's too skinny to qualify as one. It might be a better fit to classify her as a miniature sharpie....

But let's not go there. Otherwise I'll start adding sails, a centerboard and a yoked rudder - and eventually I won't have time to fish. I'll be too busy reliving my youth: screaming across Lake Elsinore or Lake Hemet while standing on the gunwale leaning back against the knotted tiller lines in my hands - seeing how long I can last before the wind throws a sudden lull or gust at me, and dumps me into the water on one side of the boat or the other.... all the while thanking God and the textile industry for fuzzy towels.

Been there; done that; enjoyed it. But I'm years older and lazier now, and I still have these foolish dreams of someday becoming a competent fisherman - if only I can come up with the right boat and the right pole.

 

Many many old dories are that narrow, the ones that they would pull the seats from and stack on board ships for instance. I think the difference (other than the chines) is that a canoe must be formed with curves that are carved or steamed, the dory has sprung curves.

 

Cheaper, lighter, easier.

If I wanted to build the lightest and cheapest, there are many ways.

Ferrocement, cheap, it is not light but easy.

Skin-on-frame construction.

I could glue some layers of foam making it thick enough, making a 17 feet rectangle, cut the ends to make the triangle and fiberglass it.

Plywood is still the best way because it needs some weight. If it is too light, it will easy cause planing http://en.wikipedia.org/wiki/Planing_(boat) and it will be difficult to streer it. The flat, very flat bottom is the shape that causes less displacement, any other form using the same length and width will cause more displacement, that is why barges have that bottom. It distributes weight better if the area is the same between two geometries. The shape of your regular sailboat is that because it lowers the center of gravity to the lowest point in the keel, making it very seaworthy and difficult to capsize it, like a cork in the water.



All that doesnt matter, because we know a rectangular hull moves through water like a curved hull with a lot of displacement and that works and people sail everywhere like that so that is good enough. In 50 years in the future, people will make hulls that moves through water better than any hull designed today but knowing that, will you stop sailing the curved hulls of today? No, you compensate by using more sail area or more hp in your motor today to move the same amout of weight through the fluid. Same thing of a rectangular hull, only compensate with less displacement or more sail area. A simple thing.

It is easier, specially if you grow on complexity. People take years building boats because often enough, bulkheads and other parts never fit so easily. There is a lot of shaping when you build a boat, the transom, the bow. Do you think the furniture today is mostly in a rectangular form because of what reason? It is easier to mass produce, it is easier to describe the parts, it is much easier to fit everything together. It is just much easier.