Square surfaces catamaran hull

[ASM]

Dear All,

Excuse my ignorance and lack of boatdesign understanding, but I have a small question regarding a underwater hull design for an 8 m catamaran.
I like square looks and the simplicity of it to make it out of simple flat surface panels. Attached is a picture of the hull in simple line form. Maximum width = 800 mm, length is 8 m. The outward surface of the cat hull is the one with just one angle. The cat should be a semi displacement, so max. 15 - 20 knts.
Would this somehow work or am I totally off ?

[yipster]

when you make it watertight it probably floats but why not a hard chine design

[ASM]

The hard chine is above the side view piece, the hull side panel after the chine are a little angled inboard, the chines could be horizontal or slightly angled upwards. So the side view picture is the section right under the chine.

I read a forum about an alu square trimaran, just a simple triangle wedge in front and square box mid to aft. It worked, a little like the flat bottom types.

[upchurchmr]

Anything will float, look at a Puddle Duck. They have even been sailed by masochists, but why would you waste your money?

Anything with a sail will go somewhere, maybe not to weather, probably not fast, I suspect you can forget 15 to 20 knots unless you have a good sized motor ($$). Why not make a decent hull which takes less power (motor or sail) and would go upwind as a sailboat?

Sorry for being no help, but square boats are ugly. Even tug boats are typically not square and they have lots of power.

[ASM]

POint taken, but square boats are not necessarily ugly. I should not say square boats, but angled surfaces, a bit like stealth. In order to avoid curved lines nuderwater to connect to straight lines above water, I would like to see whether there is such a big loss in performance/ handling / economy when using a 'square' hul like above. I have the feeling it is not that hugof a difference, but my 'design' might need some basic tuning. There are no rigourous angles on the hull, part from the bow, which might need to be altered. The L/B ratio is 10, so a nice figure for catamaran I believe and the hull centers are 3.5 m apart.... and I do not want a planning hull

[Richard Woods]

I assume you want a power cat, not a sailing one?

I agree straight lines are not necessarily ugly, see my comments on Simple Boats on my FAQs page

It's only a rough analogy. But if you have a garden hose the water will run out easily, even if it is coiled up. However lay it out flat and add a couple of kinks and the water won't come out at all. Explanation: Water doesn't like going round sharp edges.

So try to avoid chines in the direction of the water flow. Fortunately a catamaran hull is long and thin so it is easy to make the whole hull as one long flat panel. It will still bend round the bulkheads. After all you only need to curve a 8m long panel 400mm, and you'd be building in eg 6mm ply.

Hope that helps

Richard Woods of Woods Designs

www.sailingcatamarans.com (and a few powercats now)

[upchurchmr]

Richard Woods certainly knows a lot more about boats than I do, but lets drop the references to stealth. That I know a lot about. The only flat angled stealth aircraft ever to go into production has been withdrawn from service, it obviously wasn't a very good aircraft. Look for flat surfaces on the B2, you won't find any. Sharp edged flat panels cause lots of drag. Unless you want to hide from radar lets forget stealth as an excuse for flat panel boats. Stealth comes at a tremendous cost ($ and performance) you and I can't afford.

[messabout]

The sketches that you have drawn do not represent a good idea, Athwartship abrupt changes are a no-no. The only boats that have that kind of surface changes are barges and shanty boats. Both those types are slow and splashy.

If the boat is to be a sailboat then the asymetrical plan view is OK. If it is to be a power boat there is little or no advantage to asymetry. About the smooth underwater curves.......They are at least as easy to build as the box like shape that you have shown.

If low powered, you will certainly want to minimize wetted surface. The box shape is the worst that you can contrive in terms of wet surface.

Sorry to be so negative, I'm just telling it like it is.

[ASM]

Thank you all for the comments, negative is not necessarily bad, I need to learn from it. The stealth idea was just to highlight the sort of form I want for the vessel, not that I want it to be stealthy !

Question: The outer surface panels do have only a very small angled step. How about if I curved the underside and the inner sides (the ones not visable)... see attached lines... Good idea ?

[ASM]

My thoughts of possibilities also came from this link: http://www.duckflatwoodenboats.com/mainpages/mundoo.php

See Mundoo express... 2x35 Hp, 25 knots, simple surfaces, flat underbody, L/B 1:4 (yes I know it is a monohull....) and yet 2500 kg....

[messabout]

Yes the revised lines are much better than the original. They still need some work but you are moving in the right direction. Mundoo is a typical sharpie style boat. It has no hard corners except at the chines (the intersection of the bottom and sides).

You can find many other creditable designs that have hard chines and some of them have plumb sides. Google Bolger boats, Ruel Parker boats, etc.

[ASM]

Thanks messabout... I have made the hulls to 1:12 now, so a little less volume in the fwd section by drecreasing the max width by 150 mm. I will do a simple volume calc to see what the displacement is and if the WL is in the area I would like.

[rwatson]

Originally Posted by ASM

My thoughts of possibilities also came from this link: http://www.duckflatwoodenboats.com/mainpages/mundoo.php

See Mundoo express... 2x35 Hp, 25 knots, simple surfaces, flat underbody, L/B 1:4 (yes I know it is a monohull....) and yet 2500 kg....

I am sorry to dissapoint you, but the Munddo Express is a figment of the web sites imagination. They have been asked to take it off the site, but this hasnt eventiated.

I have spoken to the designer because i was interested, but there are no plans, and no working versions of that boat.

Pity, because it would be a good idea

[ancient kayaker]

It’s difficult to understand why you would want this. Even if you ignore the esthetics, there are still many reasons why boats are made with curves. Here are just a few:

Strength: a curved surfaces gain considerable strength and stiffness from the curvature. Compare an inflated balloon to a deflated one; it’s not the pressure that creates the stiffness, that just holds the shape, but the shape is doing the work.

Drag; a flat surface will flex, creating noise and absorbing energy. This will increase drag to an even greater degree than the turbulence at edges which cross the flow of water. Flat underwater panels will become concave under pressure; a boat with such a bottom is "hogged" and it spoils everything from handling to performance to economy to sea-worthiness and sea-keeping.

Ease of building and design: it is easier to bend a continuous plank around a curve than to join it at a corner. The sheer curve in the plan view with a correctly flare angle can automatically generate an elegant and functional sheerline when the plank has straight edges. This is far simpler than the scheme you propose.

Handling: this is, perhaps the hardest thing to predict, but if you throw a notchy hull into a turn at high speed you may run into severe instability. Of course, this has been known to happen even with curved hulls; nobody said design is easy.

[ASM]

Thanks everyone for the comments... pitty to hear the mundoo express was a fake, looked promissing and simple.
as for the hull design for me, I think I am gone stick with curves line all the way for the inward hull sides. Attached a little more insight in the design, including WL, so it might make more sense. PLease be informaed that I do not want top speed, just a max ~18 knots, which will seldomly be used. please note the centre deck will be 450 mm above WL, The WL doe snot touch the sheer ángle' at 2/3 aft of the vessel.