Deep V continuous rocker proa. version 2.0
I forgot the rub-rail just below the gunwale.
36 m 80X40X5 mm rectangular tubing 300 kg.
That will leave 1000 kg for the rest instead of 1300 kg.
As I said. It is more of a huge canoe or kayak than a yacht.
Cheers,
Johannes
Johannes,
There does not seem to be any allowance in your loadings for:
At least the minimum “we” of 2 people at 80kg each and all their clothes and personal effects, say 200kg total.
Partial loads carried by the vaka (the rest being supported by the ama) of the akas and any cockpits/ decking and trampolines in between etc. Say 300 kg.
Rudders foils and steering system. Say 150 kg.
Life raft (or tender).Say 40kg.
Anchors and chain. Say 60kg.
Batteries. Say 50 kg.
Masts and spars. Say 100kg.
Auxiliary engine or generator. Say 100kg.
TOTAL: 1000 kg without any contingency.
The above loadings are optimistic and the list is not comprehensive, but that still leaves you only 2 up in an unfaired uninsulated and unpainted 18 metre long cabin-less leepod-less interior-less boat without any fuel, sails, sheets, ropes, fenders, bunks, utensils, water, food, spares, tools, wiring and electronics and a host of other basic gear you know you will need for your long expeditions into far away remote cold, wet and dangerous places.
Rob
I agree on your calculations/estimates.
This will make the boat have the intended weight empty. Every boat will be lower in the water and heavier when loaded down with cargo. I don’t think Russell Brown includes 1500 kg of cargo/food/water/etc when he says Jzerro weights 3200 pounds. When fully loaded down with everything we need for cruising I believe it will weight somewhere around 4000 - 4500 kg, depending on where we are cruising and how long time we have planed to cruise.
Think of it more like extended kayaking then a big roomy yacht. I don’t plan on having running water, a big kitchen or anything like that. Think of this proa more like a very long Wharram Tiki 21/26.
As I said earlier in this tread, this is a work in progress and I see that there are not a lot of margin. I have been drawing several versions of this proa, and I have not yet made up my mind about how big it is going to be.
I want to make it lighter, and I see several areas where I can make it lighter, without sacrificing safety and structural strength. I really don’t want to use too thin plates as the sides are only curved in one direction and needs all the stiffness they can get, without resorting to a to complex inner structure of many stringers and frames.
I tend to make it larger for every iteration, but I try really hard to keep the size and weight down. It is very much a moving target, and I am still learning more and more.
Cheers,
Johannes
The vaka has a designed displacement of about 3500 kg, so there is 1300 kg left for paint, insulation, interior, leepod, food, water, rig and all other stuff needed for cruising.
I apologise, I obviously misread your post.
I have been reading about aluminum vs steel and I keep on getting impressed by the strength of modern steel.
I have read Micheal Kasten articles about aluminum vs steel and I don’t agree with alu being stronger for a given weight than steel.
The pictures below are from Micheal Kastens article “Designing boat structure” and from SSAB Domex 650MC data-sheet. Michael Kasten writes that 5083 alu has a as welded strength of 165 N/mm2 and the steel has 650 N/mm2 yield-strength. 5083 has a density of 2,65 kg/dm3 and steel 7,85 kg/dm3
This means you have to use 3,94 X thickness alu compared to steel for a given yield strength. This is 33% heavier than the steel plate. I know the alu plate would have a much higher stiffness and require less frames and stringers, so it might even out a bit in real world applications. But I can’t really see how modern high strength steels could be heavier than aluminum. Steel will also have a superior abbresion resistance and a much higher fatigue resistance compared to aluminum. I know there are stronger alloys of aluminum, but there is also stronger alloys of steel.
SSAB Hardox 450 has an average yield strength of between 1100 - 1300 N/mm2. It is what they used in the container they trashed in the video I posted a couple of posts back.
Cheers,
Johannes
I have read Micheal Kasten articles about aluminum vs steel and I don’t agree with alu being stronger for a given weight than steel.
What you aren’t taking into account is that bending stress is heavily influenced by area moment of inertia. If you were talking about pure tensile stress, you would be absolutely correct because the stress would be force divided by cross sectional area. But when you bend something, the stiffer it is (in terms of MOI, not E) the lower the maximum stress will be. The classic mathematical model for bending stress in a beam is this equation which says that the maximum stress is equal to the bending moment times the maximum distance above or below the neutral axis, divided by the area moment of inertia of the beam cross section. For a piece of sheet metal, the maximum distance is going to be half the thickness, and the moment of inertia will be proportional to the thickness cubed. Which means that the stresses are proportional to thickness squared. Which would mean aluminum hull plating at twice the thickness and 0.67% the weight of high strength steel.
For a piece of sheet metal, the maximum distance is going to be half the thickness, and the moment of inertia will be proportional to the thickness cubed. Which means that the stresses are proportional to thickness squared. Which would mean aluminum hull plating at twice the thickness and 0.67% the weight of high strength steel.
Is this really applicable on a boat hull? The loads on the hull is a global bending (lifting the hull from two points with a large distance between) and waves slaming into the hull. The global bending loads will be concentrated to the corners of the hull (keel and gunwale) and the waves slaming will be absorbed by the frames which are much stiffer du to the greater local thickness. Between the frames the plates will mostly see tension and not much bending loads.
Compare this with a skin on frame construction where the frames resist only compression and the skin only tension (simplified), since the skin does not have any stiffness to counter any bending loads.
I am not an engineer, I am trying to learn this stuff so if I am wrong or misunderstand anything please tell me soo.
Cheers,
Johannes
Is this really applicable on a boat hull? The loads on the hull is a global bending (lifting the hull from two points with a large distance between) and waves slaming into the hull. The global bending loads will be concentrated to the corners of the hull (keel and gunwale) and the waves slaming will be absorbed by the frames which are much stiffer du to the greater local thickness. Between the frames the plates will mostly see tension and not much bending loads.
Yes, it is, and your loading scenario is very wrong. The external loads on a hull (aside from hitting something pointy) are all pressures, either hydrostatic or hydrodynamic. Hydrostatic pressure is buoyancy; water molecules pushing against the hull, the combined force of which is equal and opposite the weight of the vessel. Hydrodynamic pressure is (basically) Newton’s Third Law, the hull is pushing and pulling on the water and the water is pushing and pulling right back. In either case, pressure is a distributed force; it acts across an area and in a boat, that area is the hull plating. The hull frames connect the two side of the hull together to create a load path for the opposing hull plates to push against each other and whatever is being carried around in the hull. But wherever there isn’t a straight direct and rigid connection between those opposing forces there will be a bending moment, even in the frames. And since the force on the hull is distributed across every square millimeter of hull surface, you would need to fill the entire hull with concrete to completely prevent a bending moment. Literally every cubic millimeter of hull plating or stringer that isn’t directly adjacent to the weld bead to a bulkhead is in some amount of bending, and where it is welded to a frame it’s in less bending because of the increased local bending stiffness, but it’s still in bending.
Think on this for a moment: If hull plating sees predominantly tension stress, what is the purpose of stringers? and what causes oil-canning?
Compare this with a skin on frame construction where the frames resist only compression and the skin only tension (simplified), since the skin does not have any stiffness to counter any bending loads.
The skin has almost zero bending stiffness, so it bends. But as it bends, it needs to stretch so the stiffness of the skin in tension keeps it from bending too far in. The skin is acting in the same way as the skin of a pressure vessel or the concrete arch of a bridge. The bent skin is counteracting the pressure purely as an arch in tension. The ribs and stringers of the frame OTOH are almost constantly subject to bending moments.
If you built a metal boat with similarly flimsy hull plating the skin would also bend and take up the load as an arch in tension. People commonly refer to that as oil-canning.
http://wharrambuilders.ning.com/forum/topics/aluminium-wharram-
http://wharrambuilders.ning.com/forum/topics/aluminum-cat
Something similar being discussed elsewhere….....
Thanks Alex and old greg!
I might not agree entirely as the bent steel plate has the shape of an arch and as far as I understand an arch gets its strength and stiffness by changing bending loads to compressive loads. I think your explanation is correct for a flat area, but if the steel plate is bent, you get closer and closer to the arch shape and its properties. If you make a ball of steel plate there would not really be any bending loads at all. I can easily break a thin piece of egg shell between my fingers, but I can not easily crush an egg with my fingers.
To keep my threads easy to read and easy to find I started a new thread about my latest steel vaka scale model.
Steel deep V proa
Cheers,
Johannes
Today I tried to sail my green plywood deep V proa with my Gibbons/Dierking rig.
The wind is blowing 20 - 25 knots with gusts up to 30 knots.
I could not control the boat at all. When laying on the ground it was near to go airborne, and in the water it just skidded sideways without any control. I tried to adjust the sail and move the rudder as far aft as possible but the drag in the rig seemed to overpower the rudder and the lift of the hull. I could get it to sail a couple of seconds dead downwind before it turned up into the wind and stalled again. I should have tested my steel proa instead. I believe the weight and depth of the hull would make it much easier to control.
I might interpret this wrong, but I get the impression that a very lightweight multihull is much harder to control in high windstrengths. I did not load it down with anything as I have to throw it into the water and it will just scoop up a lot of water if it is to heavy.
I really don`t like the idea of a lightweight boat that will glide down sideways when hit by a big breaking wave. I prefer control and speed in heavy weather even if that means lots of green water over deck.
Rolf Bjelke and Deborah Shapiro of Northern Light http://www.northernlightsail.se/index.html who have been sailing the southern Oceans for many years much prefer a boat that is easy to control and fast when there is strong winds.
It was very interesting to hear about there adventures, and their storm tactics, and watch their boat handle waves that would scare the living daylight out of me.
No pictures today.
Cheers,
Johannes