Engineering Question - flexion of hull

[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

[MicHughV]

Interesting....hmmmm....????

1a) I note that the bracket has T fittings on the four corners, with a possible L shaped T fitting, ie, there is a "lip" that fits " over the sole support.
1b) The T end also looks like it might just be thick and be " embedded" in the sole support. It's difficult to tell if it's (a) or (b) from the photo.

2) The left side of the frame has an H sub-frame, which the right side does not have. That H sub-frame is also located over the tank fill hose.
The left lower leg off the H frame also appears to have 4 holes drilled into it.

I'm wondering if this frame was put there by a previous owner, as it does not appear to be original equipment to my eye. The 4 drilled holes are indicative that something may have hung there or otherwise support a pump or something else.

I'm kinda doubtful it has anything to do with the hull flexing or mast compression, etc, as you would see this support elsewhere as well..that H sub-frame would have been noted on both sides of the mast, if it had anything to do with it, but it is only on one side.

For peace of mind, it would be a simple matter to fabricate and install a temporary truss there, but all the little things I noted above, makes me think that frame doesn't have anything to do with hull flexing.

Most all fiberglass boats flex to some degree due to the various loads the hull has to manage. I have never come across a " frame" before to support hull flexing.

[MicHughV]

getting back to the H sub-frame....the horizontal section of the "H" sub-frame is not centered between the perpendicular legs....it appears to favor the "south" side.

missed this the first time around.....but the horizontal section of the " H" frame also appears to have a hole drilled in the dead center.

finally, I also note what looks like more holes on the left side of the lower leg.

All these holes seem to direct my thinking, that " something" was located there at one time, which seems to have been the purpose of this frame.

[sv_pelagia]

I am in no way knowledgeable about this; take this with a grain of salt:

The "H Frame" seems connected to the floorboards (or boards under the teak floorboards) and not associated with hull. I do not see how this frame has anything to do with structural integrity, flexing, etc.

MicHughV's suggestion that it was once for equipment suspended over bilge, such as a pump, makes more sense.

[VChild]

You are correct, the T fittings in the 4 corners are L shaped and they rest in recessed cutouts in the sole supports. Then screws go in laterally into the supports. I saw the identical bracket on another Lord Nelson so it must be original. The drilled holes in the sub frame are where bolts come up through the frame and overlying floor board to secure the table pedestal to the sole (it’s about a 1 foot square pedestal – see picture). I’m wondering if the only purpose for the frame is to have a secure attachment for the table rather than prevention of hull flexion? With the table pedestal off, that floor board moves and lifts out freely.
I’m thinking the reason for the asymmetry of the H frame is so that it can be removed from around the compression post. Otherwise, no way to get the tank out.
Even if the frame was not meant to prevent flexion, I'm wondering how tight it is being squeezed with the boat in the water. I'm thinking I might need some sort of jack because just building a temporary truss might not overcome the compression forces (it might give in a little).

[VChild]

I guess what worries me is that the floorboard gaps increase by about 1/8" with the boat out of the water.

[MicHughV]

Seeing that table there, (looks really heavy to my eye)...and the table leg support certainly appears to support the theory, that the frame is there to provide support for the table. Supporting the table leg on just the removable floor insert would provide little security for the table in inclement weather, any major heeling motion would send that table flying.

You are correct that having the H sub-frame only on one side does make sliding that frame around the mast possible.

That frame is almost certainly installed after the cabin sole has been installed. Maybe even after the mast has been installed ???

The frame is braced against the cabin sole, not the hull. If it were a hull support it would be braced against the hull.

Always hard to say from a few photo's, but I'm confident that frame is there for the singular purpose of supporting the table.

[MicHughV]

The Lord Nelson 41 is a heavy boat....google sez 30,500 lbs.....add a few more tons of equipment, etc, and yes, it's quite conceivable the hull may flex a little once pulled from the water, as it looses all that water pressure support.

[thinwater]

a. Remove the screws and see if it is tight. It may not be.


b. Place a 1-inch board on each side, aft of the H. Place a 2x4 across, and hammer in wedges in one side to take the load. The H should lift right outout.


c. With the H out, repeat forward of the tank and remove the first brace.



d. Swap tank and reverse the process.


Easy Peasy. Just wood and wedges.

[MARC D]

Just curious: how corrosion occured in your stainless steel tank?

[VChild]

Originally Posted by MicHughV

Seeing that table there, (looks really heavy to my eye)...and the table leg support certainly appears to support the theory, that the frame is there to provide support for the table. Supporting the table leg on just the removable floor insert would provide little security for the table in inclement weather, any major heeling motion would send that table flying.

You are correct that having the H sub-frame only on one side does make sliding that frame around the mast possible.

That frame is almost certainly installed after the cabin sole has been installed. Maybe even after the mast has been installed ???

The frame is braced against the cabin sole, not the hull. If it were a hull support it would be braced against the hull.

Always hard to say from a few photo's, but I'm confident that frame is there for the singular purpose of supporting the table.

All makes perfect sense!

[VChild]

Originally Posted by thinwater

a. Remove the screws and see if it is tight. It may not be.


b. Place a 1-inch board on each side, aft of the H. Place a 2x4 across, and hammer in wedges in one side to take the load. The H should lift right outout.


c. With the H out, repeat forward of the tank and remove the first brace.



d. Swap tank and reverse the process.


Easy Peasy. Just wood and wedges.

Excellent idea!
Thanks

[VChild]

Originally Posted by MARC D

Just curious: how corrosion occured in your stainless steel tank?

Well they lasted 39 years but finally corroded through at the seam welds.

[VChild]

Originally Posted by MicHughV

The Lord Nelson 41 is a heavy boat....google sez 30,500 lbs.....add a few more tons of equipment, etc, and yes, it's quite conceivable the hull may flex a little once pulled from the water, as it looses all that water pressure support.

Yeah, last time she was hauled, the scale read just over 36,000lbs.

Thanks for the advice!

[Pete7]

Originally Posted by MARC D

Just curious: how corrosion occured in your stainless steel tank?

Hmm, I was wondering that too.

Pete