Engineering Question - flexion of hull

[Chotu]

All engineering aside, here’s a practical tip from someone that built a boat.

Cut a 2 x 4 or a couple of them and jam them in there before you take out the steel. Put in the steel when you are done, and knock the 2 x 4 out with a hammer. Problem solved.

[VChild]

Originally Posted by Chotu

All engineering aside, here’s a practical tip from someone that built a boat.

Cut a 2 x 4 or a couple of them and jam them in there before you take out the steel. Put in the steel when you are done, and knock the 2 x 4 out with a hammer. Problem solved.

Yeah he had a great idea and all advice is appreciated. I went with this system because of the ability to have precise control over the pressure. Also, not a lot of swinging room for a hammer and wedge. Either way, the H-frame is out now and all is stable. Thanks everyone!

[MicHughV]

Your use of the term " flexion", you questions and observations, your "devices" you came came up with, etc, lead me to believe you are someone with a strong technical background.....hope you post a thread of how this will all finally pan out......

[Searles]

Heavily built vsl ,slacken off rig a little ,see if any movement ,shouldn’t be any or minimal if in the water ,that’s your support ,the tank will not be structural ,can’t see any problems .⛵️⚓️

[VChild]

Originally Posted by Searles

Heavily built vsl ,slacken off rig a little ,see if any movement ,shouldn’t be any or minimal if in the water ,that’s your support ,the tank will not be structural ,can’t see any problems .⛵️⚓️

[VChild]

Originally Posted by MicHughV

Your use of the term " flexion", you questions and observations, your "devices" you came came up with, etc, lead me to believe you are someone with a strong technical background.....hope you post a thread of how this will all finally pan out......

I will definitely post updates on the project.
My background is in the sciences. My daytime job is an anesthesiologist but my interests include 3d CAD design, fabrication, and welding – mostly for fun but occasionally for something useful for my colleagues, friends and myself.

[MicHughV]

I figured as much.....
I'm a structural marine engineer...now retired....but keep my license updated...
Was...and still am....quite well versed with CAD.....what a tool....
Built my first boat from the ground up....38' steel hull and all the other bits and pieces....

So, we have a lot in common....

[rslifkin]

Well installed aluminum tanks last a long time. The key is keeping them dry. I've currently got my 36 year old aluminum fuel tanks (gasoline) moved aside to replace a failed aluminum water tank. The water tank was mounted just above the keel sump, so down pretty low. And it sat flat on a piece of plywood (with limber holes next to it), so of course the bottom of the tank got wet over time and corroded through.

The fuel tanks, however, are well mounted. They sit up out of the bilge with wood spacers (with rubber strips on top) to hold them off the mounting surface, allowing airflow under and around the tanks. Visually, the tanks are in great shape all the way around and I see nothing that concerns me. I expect them to last a lot more years. I've yet to hear of anyone else having issues with a fuel tank on this model boat, so that's a good sign as well.

[wgerstmyer]

Stainless steel, in its several formulations, is NOT corrosion-free, despite the implications in its name. In most applications it could last a long long time but there are several factors that can reduce that lifespan considerably, one of them being saline!

[Icarus]

Originally Posted by VChild

Looking for some advice from any engineers or others who have dealt with this issue.
I will be replacing my original SS water tanks with plastic tanks due to corrosion. In order to remove the tanks, I need to unscrew and remove a steel “H” frame cross member under the floor boards near the mast support (see picture). The other cross members are teak and simply drop into slots in the longitudinal members. They lift out easily. My concern is that the combination of the rig tension and the water pressure on the hull sides (boat is not hauled out) will compress the space between the longitudinal members when the “H” frame is out, making it impossible to get it back into place later. The hull is overbuilt with thick fiberglass and the keel is fat with 11,000 lbs. of poured lead (see picture). The frame is about 20 inches above the floor of the bilge. The bilge space extends somewhat down into the keel. The metal frame is probably at the level where the hull bottom is almost horizontal but near the transition to the vertical keel area. I can’t imagine much flexion happening on that part of the boat. But, when hauled out, the floor boards do fit slightly looser than when the boat is in the water. I’m thinking of concocting some sort of horizontal spreader jack between the longitudinals to unload any possible pressure on the frame before removing it, but there may not be enough space for something like that. Any thoughts on whether this is even a real concern and, if so, other ways to approach it. I was trying to avoid a trip to the yard with a haulout and de-tensioning the rig.
Thanks,
Tom

Looking at the pic this H frame got nothing to to with with the integrity of the hull, all it does its keeping the floor boards in place.
Sitting on it's keel on dry land would most likely distort and stress the hull a great deal more then being afloat.
Hence the floor boards are tightening when sitting on her keel.

[roverhi]

1st, the forces on the hull on the hard are the opposite of what they are in the water. On land the keel force is upward, in the water it's pulling down. Not unusual to have small changes in hull shape or keel attachment.
2nd, slack the rig so the mast is loosely held vertical. That will eliminate almost all the down force except for the weight of the stick. Shouldn't have to worry about that steel apparatus and no problem pulling it up to get the tank out.
3rd, It is hard to figure why it's there in the first place. A lead sled like that should have more than enough structural integrity not to need a floor brace like might be needed in wooden construction. If the brace is just tied into the cabin sole its relying on the tabbing of sole into the hull to carry on the bracing. That tabbing would be in shear which is a pretty weak bond to the hull and those out of sight areas are ones that typically get poorly done so are suspect as actual structural members in the first place. The sole in fiberglass boat are typically designed to handle downward force of people walking on them and any sideward reinforcement is an afterthought plus. Some boats have bonded in structural grids but keeping them bonded to the hull in Bendy boats seems to be a challenge.

I wouldn't worry about the fitment on the hard especially if the rig is slackened.

[valhalla360]

Originally Posted by wgerstmyer

Stainless steel, in its several formulations, is NOT corrosion-free, despite the implications in its name. In most applications it could last a long long time but there are several factors that can reduce that lifespan considerably, one of them being saline!

Yep, lots of people confuse stain-less not stain-free.

PS: As far as holding the hole open, a couple woodworking bar clamps where the jaws can be reversed would be straight forward and give a lot of control over the forces.

[VChild]

Originally Posted by Icarus

Looking at the pic this H frame got nothing to to with with the integrity of the hull, all it does its keeping the floor boards in place.
Sitting on it's keel on dry land would most likely distort and stress the hull a great deal more then being afloat.
Hence the floor boards are tightening when sitting on her keel.

I appreciate your input. My floor boards don’t tighten when sitting on her keel, they actually loosen (presumably from no water pushing on the hull sides). And the H-frame, we recently concluded, was only to give a secure structure to thru-bolt the table pedestal to.
I wasn’t really concerned that hull integrity was relying on the H-frame. My question was really about whether I would be able to get the H-frame back into place without hauling out. The logic being that if the floor boards are tighter in the water, then the supports for the floor boards, which incorporate the H-frame, might be tighter as well. So, the braces I used were just place holders for the H-frame in case the space tended to close in a little. It turned out that the H-frame did have compressive forces on it and would not lift out until the adjustable braces were expanded a little. Anyway, the H-frame is out and all is well. Thanks.

[VChild]

Originally Posted by MicHughV

I figured as much.....
I'm a structural marine engineer...now retired....but keep my license updated...
Was...and still am....quite well versed with CAD.....what a tool....
Built my first boat from the ground up....38' steel hull and all the other bits and pieces....

So, we have a lot in common....

Yes indeed!
I just bought ExactFlat surface flattening software to plug into Rhino3D and it is fascinating!
It will be great for digitally solving cut patterns for a complex dodger shape, bimini, cushions, covers etc.
Very impressive building a 38” steel hull. Congratulations!
Sorry about thread drift.

[VChild]

The tanks are a tight fit coming out. The second one has to slide out of the “tunnel” after the first one comes out. A lot of padding was used to prevent scratching the teak.
I also learned that no matter how clean the rest of your boat is, if you occasionally catch a faint whiff of a musty smell, it may be coming from years of buildup of damp debris under your tanks. It was pretty nasty. I will definitely be scrubbing and painting that part of the bilge to match the rest.

Kudos to Gareth at Dura Weld Plastics in Lake Worth, Florida.
I sent him my CAD drawings for the custom replacement tanks and they are already built. They look great and will be shipped this week.