# What hull will be most efficient between 8-12 kts?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Oct 9, 2021.

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### fallguySenior Member

You have not calculated rotational forces.

Now, I am not an expert on these, but they were highly discussed about my boat and build. I can look to see if I can find any threads.

In a catamaran, a wave can lift the back of the boat on one corner while an opposing or same force hits the other corner and the resultant loads try to twist the boat apart.

Your tubes cannot exit a single skin 6 oz laminate. The hulls will be torn apart going through a single surf.

I wish I had the c.v. to advise you further. I only understand the concept and not how to engineer the fix for you. In my boat, for example, we set the beam sockets into 12# laminate so lifting or falling loads are not tearing or crushing light foam.

Conceptually, you would need to put a circle of very strong laminate around the tops of the aluminum. Something like ??! 1/4-3/8" thick solid glass or a large pad of heavier marine foam that gradually tapers to a lighter skin... not sure a 6 oz skin can ever work on 35psi foam....which is another red flag based on my experience.

Don't get me wrong. I build Dierking amas for a canoe with 6 oz glass on 30psi foam, but they are only 5' long and not the main hull. Due to their length, they rarely experience rotational forces. I can promise you, they are not good enough for your plan. Maybe 12 oz may work, but also not my forte'. Just see trouble here.

The skin thickness is insufficient.

@rxcomposite is a wise laminate designer and basically low shear foams require more laminate. This is the ugly weight tradeoff of boat hulls.

plywood hulls are high strength and can work with light skins

light foams are weak and require stronger skins

As for pontoon hulls....there is a reason people don't take them out in rough seas. What is a rough sea? 6 footers are about all I care to bother with...I have been out in 10 footers far offshore, but your boat needs to handle 6 footers

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### fallguySenior Member

His athwart is 1/4", but this boat will be twisted apart.

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### dustmanSenior Member

The largest span would be 12', give me an expected load over what area and I'll calculate. An example: 2000lb load over the middle 3rd of that span gives me a safety factor of 1.6.

The fiberglass and foam are going to provide significant additional strength as well as distribute the loads over a broad portion of the tube.

I just can't imagine what scenario I could get into on this trip that would impose such high loads that you seem to be envisioning.

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### fallguySenior Member

read up on the way Wharram's work...they are actually capable of twisting because the hulls and lashings are strong enough

your lashing is skin and foam and the light tubing will not stand up to enough load

The 4" square tube that is 1/8" wall will experience a cantilever force iirc. The issue is not athwart.

Just because the boat is light doesn't mean it won't experience the forces of the sea.

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### dustmanSenior Member

If the fiberglass breaks where the crossmember tube enters the hull what will happen? The way I see it nothing will happen because the structure is not reliant on that connection. To me what matters is the connection between the crossmember and the spine, and connection between the spine and the foam. If the spine twists a lot I can imagine the foam becoming detached from it.

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### dustmanSenior Member

My lashing is 20' of tube and foam.

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### dustmanSenior Member

You are talking about the force acting on the ends of the spine beyond the crossmembers? Such as slamming or wave strikes? Or just regular cycling? You are arguing that I should increase the wall thickness or diameter of the tube?

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### Mr EfficiencySenior Member

Anything is likely to happen to the foam with that tube inside it putting loads on it, if it starts getting loose in there it could rapidly start moving around busting it all up, the problem is exacerbated by a catamaran being subjected to twisting loads, with the foam taking the load

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### fallguySenior Member

What would happen to this boat as drawn is the athwart members are stronger than the hulls. The boat would encounter uneven loading and the athwart members would apply load to the skin and foam intersections and tear or crush them. Then as that got more and more open; the loads would transfer to the 1/8" tubing and the tubes would bend just near the plylons. How soon I cannot say. It is all intuitive for me.

I would also be a bit worried about the simple load case bending the hulls.

Aluminum does not tolerate bending really.

@Ad Hoc is an excellent cat designer who posts here. Unfortunately, he doesn't step into these much because he has been around enough to know you can't tell people much.

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### fallguySenior Member

No. Your lashing is effectively the intersection of the skin at the pylon/beam interface.

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### dustmanSenior Member

So the answer would be to make the frame extremely rigid?

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### Mr EfficiencySenior Member

You will arrive at the point where the longitudinal "frame" might as well be the hull. Like those alloy log pontoons. The skin is just too fragile for the rough and tumble of long=term use anyway

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### fallguySenior Member

Right now. I must sleep.

Your design has problems that I can understand intuitively.

You have to understand the forces on the skin are significant and as Mr E points out, at some point, if you build the aluminum chasis heavy emough to deal with the skin problem; then you will be at an ally hull weight anyhow.

As drawn, the boat will not work. The reason I know is because light foams are not even strong enough to hold a screw, let alone a pylon and lever forces.

The sea provides the force, not the hull weight.

gnite for now

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### Mr EfficiencySenior Member

You could beef up the glass where the cross frames enter, you could beef up the glass on the bottom, and outside of each hull, to give some bump resistance, you could install a bigger longitudinal that won't flex much, but the weight will have increased. It just seems easier to build a hull by more conventional means

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### trip the light fandangoSenior Member

I don't have cash to splash on boats either.
You could buy a 2nd hand 20 ft catamaran, preferably a bit longer and do exactly what you are describing, cut the mast down to 3/4 its original, set up a way to drop and raise it easily, get reefing points sown in to the shortened sail and only use it if your motor conks out and set up a good boom tent and comfortable light seating when travelling by motor for shade and comfort , camp on it. The aft beam will need some thought to carry the 9.9 motor [ or 15 if 2 stroke] and weight distribution will be important, get a good anchor. Carefully glass in some watertight hatches close to the centre of each hull.
Attach waterproof bags to the forward beam that can't wash off.
You only need to look up what others have brought up and fought for on this forum to see why what you propose has serious issues. I'm one of them ..ha , your travelling light idea sounds excellent and eminently sensible.

If you are convinced that you need to add something original and unique build 3ft extensions to the bows,...and welcome to a whole new rabbit hole world if you do.. regards.

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