Origami Metal Boat Construction

[dskira]

Originally Posted by Brent Swain

The 3/16th tank top is the equivalent of a fully welded horizontal bulkhead , structurally. Some have asked me "Why not put in a removable stainless tank?" When you do that you make the area under the tank completely inaccessible and have no way of knowing what is going on under it until it becomes problem. With the built in tank, and a clear port in the inspection pate, you can see the hull( tank bottom) any time by simply lifting a floor board.
The top of the built in tank adds a huge amount of structural strength to the ends and baffle, making the equivalent of a huge floor, to support the keels, something which a removable tank doesn't do. The joint between the tank top and the hull is an extremely strong point , as strong as or stronger than the centreline, which lets you end the keel webs there, tied in for and aft with longitudinal gussets, along the tank top- hull seam.Without the tank top, the webs would have to reach all the way to the centre line, greatly extending their span, and making it hard to use the space as tankage, and if it is not used as tankage, then tankage has to go somewhere else, taking up valuable storage space. The area under the floor is not too useful for anything but tankage . Other spaces are far more usefull for storage.

Brent, here lies the problem: not horizontal bulkhead, you need vertical bulkhead or at least some floors to bring the whole hull together, mostly with the projection of the two lateral keel.
It will no take much space, they are easy to weld and costy nothing as material. can be find in the scrap left over.
The top of the tank, by been horizontal, can't be close to the same transmission of strength as a floor. It will voble in bad circonstance if not taking care by a vertical element and transmit the strength to the frame, which trasmit to the frame/beam gusset. The whole structure should be one, every piece having its role.
Daniel

[Brent Swain]

It is the triangulation of the tank top with the hull plate , the fore and aft curvature of the hull , keel sides, and the tank top, which give it vertical strength. If we were talking about straight hull surfaces, straight keel sides and straight tank sides, then your hinge theory would be valid. They are not straight ,which changes the whole dynamic.
The built in tank is necessary to support twin keels , halfway up the bottom plate , halfway between the centreline and chine. My single keeler, having a keel top 12 feet long and 18 inches wide, with a lot of fore and aft curve in both the keel and the hull shape, makes floors there uneccessary, but that is still the best place to put the tank. One client with a single keeler, made the whole tank top removable , set on stainless angles. The tank top itself was stainless . He had a cheap source.

[dskira]

Originally Posted by Brent Swain

It is the triangulation of the tank top with the hull plate , the fore and aft curvature of the hull , keel sides, and the tank top, which give it vertical strength. If we were talking about straight hull surfaces, straight keel sides and straight tank sides, then your hinge theory would be valid. They are not straight ,which changes the whole dynamic.
The built in tank is necessary to support twin keels , halfway up the bottom plate , halfway between the centreline and chine. My single keeler, having a keel top 12 feet long and 18 inches wide, with a lot of fore and aft curve in both the keel and the hull shape, makes floors there uneccessary, but that is still the best place to put the tank. One client with a single keeler, made the whole tank top removable , set on stainless angles. The tank top itself was stainless . He had a cheap source.

That I find the problem, the tank do not support the lateral keel as you see on your design.
You have to put a floor on top of the tank from one frame to the other.
As you see you have a hard and soft spot at your gusset since it take care only on one part of the hull at a time. the hull and frames should be reunited with a floor to conteract the soft and hard point, diffusing the strength all around. The top of that tank is not structural regarding the lateral keel in this drawing. It is quite a rupture point along the plating. Mostly if the tanks have baffles which in that case will strengthen the hull to much compere to the week spot at the side top of the tank.
Perhaps you have an other drawing showing the same spot with a floor on top of the tank.
Daniel

[s/v 'Faith']

Just a point that may help clear up confusion for those following the discussion.

The 'floor' Dan is talking about is not like a floor you stand on. It is a shipwrights term for the transverse members that the sole sits on. The tank top that brent is saying is appropiate is not the same structual member Dan says is necessary.

Here you go;

Floor or Floor Timber - A transverse structural member lying across the keel and tying the frames on either side of the keel together. The central futtock or futtocks of a sawn frame, lying across the
keel. Floor timbers join both sides of a vessel together and make up the substructure for external keel fastenings, engine beds, and mast steps.

[SaltyMonkey]

s/v 'Faith' - thank you. I appreciate this.

This term I cognitively blended as the word "frame". We were referring to the same thing, yet I took "floor" for something else.

[dskira]

Originally Posted by s/v 'Faith'

Just a point that may help clear up confusion for those following the discussion.

The 'floor' Dan is talking about is not like a floor you stand on. It is a shipwrights term for the transverse members that the sole sits on. The tank top that brent is saying is appropiate is not the same structual member Dan says is necessary.

Here you go;

Thank you, I realy apreciate your clarification. It help a lot.
I should have been more clear.
Thanks again.
Daniel

[SaltyMonkey]

Here are some pictures from the MOM site that shows the area in question. You will see the pseudo-"floor" or in this case what I consider really a "frame" or "mesh" across the two keels lending support. I personally don't think closing the triangle between the keels is necessary due to the slight arch from the curvature of the hull as well as the latitudinal ribs or framing supported by the stringers, but that's what we are debating? If these ribs were I-beams or T-braces it would add the same level of support but that would be overkill.

[pressuredrop]

brent, i love your boats, but do you force that giant anchor windlass on all your customers?

[SaltyMonkey]

Some additional notes:

- It appears that the "ribs" here are not welded at the V or the stringers. I assume this is to allow for expansion rather than drainage, as the later pictures show fairing and tarring filling in these gaps?

- If these were welded together with a solid triangular "floor" would not there be expansion issues?

- I do not see where the hypothetical single point of failure would be that is the concern in the original counter to the design. Maybe someone can point out the exact point of contact where the force might cause an issue?

- However, I still feel somewhat nervous about having a singular weld running the entire longitudinal length of the hull and a zipper FX that might occur at the keel.

[dskira]

I think some framing in the side will be a good way to bring the whole structure at the same level of strength without having soft/strong spot, and to transmit the stress from the deck edge to the keel.
Mostly on a hard chine type of design, the hard spot could be diffused easely.
Floors, even small will be a good addition to realy tied the frames together mostly when the frames are in charge of the lateral keel.
I don't understand the pillar on the right going staright to the lower plating. Perhaps is a temporary support.

[SaltyMonkey]

The pillar looks permanent, as you can see later that it is covered in epoxy-tar.

Another note

- Lets not forget that steel property has no grain and is strong uniformly in all directions, unlike wood or fiberglass mat. Forgot what this word is called.

[spiritbaer]

Hello, I am almost certain that the pillar you are seeing is part of the mast support. It terminates were the cabin sides start. A half inch plate comes down the cabin side, across the cabin roof and then tied into the opposite side. The plate is welded into the pillar. The pillar is schedule 40 pipe.

[SaltyMonkey]

spiritbaer, you are right. Come to think of it, I have also seen this on another french steel boat recently.

[GordMay]

Originally Posted by SaltyMonkey

... Lets not forget that steel property has no grain and is strong uniformly in all directions, unlike wood or fiberglass mat. Forgot what this word is called.

An isotropic material has uniform properties in all directions.

I believe that normal steel does have a grain (I'm no expert).

[spiritbaer]

Hello, never heard of regular or structural steel having grain, however I do know that light gage 20 to 26 gage 316 s.s. there is a surface grain. This type of finish is used mostly in commercial kitchen and architectural work. The pieces are laid out and fabricated following that grain, this is to make it easier for polishing the finish product. A good example of this would be s.s. appliances. Not all s.s. has this type of finish, there is a big selection of finishes.