Anchor locker notch plate and bolt sizing

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Feb 25, 2020.

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Ahh.. so this is the front of the boat not the stern then?... hmmm... that wasn't clear to me.

In which case you do need to account for a localised slamming load.

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fallguySenior Member

yes, this is my fear

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Ok - NOW I understand.

Apply a nominal load of 100kN/m2

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fallguySenior Member

Okay, this is rational now, but also adds some confusion for me.

If I apply a load of 100kN/m^2 or 14.5 psi to the front portion of the cantilever section; that force is applied to an area 47" by 102" or 4794 square inches. That is a load of 70,000 pounds. !!! The cantilever section is about 36"x102", so a bit less.

Redacted for error

For right now, help me size the bolts that hold the cabin onto the beam. I used 5 x 3/8-16 316 stainless bolts with a tensile rating of 70,000 psi. (Blue arrow bolts in above pic)

I calculate the bolts tensile rating at 5455 pounds each; there are 5 for say 27000 pounds. If I am correctly applying the 70,000 pounds as the Y portion at 23,941, I have enough bolts to avoid a failure. If not, then I would need like 13 bolts. This would result in weakening my beam. So, instead, I would need to lashings.

To keep things simple, I will wait to hear back from you before discussong the cantilever breaking off.

Thanks!

Last edited: Feb 28, 2020
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fallguySenior Member

Also, that slamming load at 70,000 pounds is about 9 times the vessels total weight. I figured something like 5 times the weight going with my gut...but if the vessel is going forward and slamming into a head sea; I suspect it is something of a brick wall. The angle of approach of the cabin bottom must have relevance, right?

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I dont understand imperial units.

47" = 1.194m
102" = 2.591m
Thus, Area = 3.094m2
Thus force is 309.4 kN..... or 31.5 tonne.

I can only do that with a layout of the arrangement on a dwg. Without dwgs... it is all speculative.

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fallguySenior Member

According to my work; my bolts are insufficient to hold the cabin onto the beam and would fail at 120kN.

of course, this begs me to review the beams themselves for semantics

there is no way I am putting 8 more holes in the beam...so I will need an alternative method to tie the cabin onto the beam.... good news is we have some space to do so

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fallguySenior Member

Of course, this still does not deal with the cantilever; it must be strong enough to take the slamming load.

AH-this is not a bluewater boat, but Category B, so I want to also know how you chose the 100kn force. I assume that is a slamming load standard of sorts...

My aft beam for the cabin would also be subject to potential slamming. I only used 4 bolts back there...our access is much more difficult....perhaps I need to strap the beams from underneath...

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fallguySenior Member

I am consulting with Richard now for a solution. I had really hoped to stop pestering the man.

Thanks.

Let me know if any of the assumptions are wrong.

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fallguySenior Member

Mockup of a possible solution. Mockup is light gauge steel. This would effectively act on the beam bottom versus the fastenings above.

The strapping must be wide enough so it does not kink the beam.

otherwise Wharram type lashings may work; we need to build a lashing point on the cabin

Last edited: Feb 28, 2020
11. Joined: Oct 2008
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Ok.. although not explicitly clear - I assume this is a catamaran vessel you're building - Hence the beam?

If so, when those beams will be doing more than just holding that extension. Hence my request for dwgs to get a better picture of the global and local structural load paths.

Yes, taken from many different vessels i've designed, it is a "reasonable" medium value - based upon several factors.

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I will freely admit I am a barbarian not understanding the finer points of NA. Confronted with the problem of a large cutout under a beam I would simply fit spacers made out of hardwood (or solid glass) at regular intervals and bolt the two sides together using big bolts or allthread. On the insides there would be a need for glued in backing plates (or one continuous board) and washers. If the beam is a mast section as pictured I would install sections of HDPE sailtrack between the spacers and beam. That way I could either slide the beam out and have it positively located or take the beam off with the spacers attached. If sliding out is acceptable I would probably even glue the spacers in, but that depends how one pictures boat disassembly to happen, since the central pod could not be taken off while in the water.
How big a bolt to use? Well as big as the space allows, and that space looks like one could confortably install 30+mm diameter bolts and not weaken the spacers. How many bolts? Don't know how wide the deck is but something like 2-3/m seems appropriate.

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fallguySenior Member

Thank you very much for the wisdom. While I installed the light cover plate: which required sitting face to the impact area to a head sea; I had a bit of a moment where I got a bit nervous about the cabin attachments and the strength of the cantilever. I am convinced I have a good solution where slamming loads would impart to the bottom of the beam instead of 5 light bolts on top. A 4" wide piece of strapping welded to a 1/4" side plate will have 5" of contact to the beam on each side. The slamming load would be shared by 12 bolts in the plywood and imparted on 40 square inches of beam and we will easily achieve the 100 kN/m^2 rating.

The middle beam is still only setup to support 21000 psi..I might be able to install two straps under the beam that would prevent a slamming load elongating or breaking the four bolts. The slamming loads persist on the cockpit. I need to evaluate them there as well. However, they are probably less than slamming into a head sea. I had hoped to use light hardware there; that may be a bit wrong.

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fallguySenior Member

I truly wish I followed you.

The masts are set into the hulls and then the cabin is craned onto the masts.

perhaps you could finger sketch the idea

The middle beam lays on its side and I need to resolve a wee bit of better load managing there as well.

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fallguySenior Member

I will draw up my solution tomorrow.

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