Anchor locker notch plate and bolt sizing

3/16" aluminum 6061 plate to close off beam socket notch

View of star side of cabin with anchor locker forward. Anchor locker is cantilevered about 3 feet past beam. 12mm ply core with 17 oz biax each side. Taping incomplete.

Beam socket with beam section shown. Taping incomplete.
Side view starboard side beam notch.



Okay, so I am adding a plate to the bottom of the beam notch socket on the demountable cat I am building.

I sort of winged it and now am wondering if I under thought it.

The reason for the original addition of the plate is to provide a little more support and never worry about the anchor locker shearing off upward if the boat is coming down hard into a wave coming up. I don't actually think it would shear fully off, but I am a little worried about cracking something and if seas were real bad; repeated cracking could result in a catastrophe.

The opening is 9" or so and the plate is 3/16 6061 aluminum. There are 9 bolts each aide with tensile ratings of 70,000 psi. I overbored and filled the cores to 1/2" and then glass taped each side of the holes. The core is plascore which has no shear. The glass tapes each side and the glass laminates and the overbore provides all the shear strength for the core. There is about 100 oz of glass each side or more. 4x1708 or more..

I calculate the shear of each bolt at 1200 pounds. I calculate the tensile of the plate at 1800 pounds. I used 40,000 psi tensile and used 0.183 x 0.250 for the shear area which is how much each bolt would need to push out to fail. I calculated a tensile rating of the cabin onto the beam at 28000 pounds using 5 x 3/8-16 bolts.

There are 9 bolts on each side of the plate. To shear, all 9 bolts on one side would need to blow out. Or a force of 10800# which is about only twice the vessel weight. Of course, the plate is not the only thing working. The cabin sides also work against such a force.

This craft typically does not launch out of the water and will not exceed speeds of 30mph.

So, are my now edited calcs close?

What forces should I be trying to battle? The angle of the cat cabin bottom is 24" y and 65" x.

If I double the number of fastenings in the plate, would it help?

My gutcheck wants 20-40 tons of help, not 5 tons.

The reason I used 3/16 plate is to lower the weight of the panel and reduce fairing, but now am wondering if I need to go to stainless plate instead.

I probably should have asked Richard to spec this thing, but we are close to the end and I hate to be a bother all the time.

Here is another view of the cabin bottom front prior to plate addition.

 

I don't want the cabin to crack here if we hit a head sea or breaking wave hard.

 

We decided we can add some really strong strapping here. Then any wave that would try to bend the light ally plate would have a resistance force and that strapping would also help minimize the likelihood of cracking the cantilevered section.

 

Be nice to see some dwgs and views, to establish the load paths and exactly the arrangement - rather than assuming from just the images.

 

The front of the boat or the cat bottom is 65" in the x with a 24" height in the y. The bdeck clearance is about 17". I expect the angle to be a little bit more; perhaps like 25" versus 24" on the actual water as we are known to be a bit stern heavy and won't necessarily ballast it out unless it is really awful looking. The bows of the two hulls are very knife like, so they would tend to cut the water more than lift, and so the cabin bottom edge is very likely going to hit now and then under right conditions. The cabin bottom has a slight vee; about 3-5", honestly can't recall at the moment, but designed to part the waves some. I don't have a cross section of the hulls/cabin bottom, but I probably could come up with something if needed.

I did some rudimentary calculations and we can install an 8" wide strapping under the cabin beam. That strapping could be made with 1/4" plate and like 6 - 3/4" bolts, so we could get real strength from the strapping I show on post #3. The strapping would go through the plywood, which also has decent compressive strength. And we have permanent access to the areas that would be bolted. We would probably be able to epoxy the nuts in and use Loctite or lock washers on the plate.

I will see if I can find another picture to help.

 

Here are a couple more pictures of the arrangement.

view from inside the cabin showing the cabin resting on the beam which is under the plywood cross member. I used plywood core for the side panels to add strength vs foam
pic of the cantilever section and cabin base on the beam; this gives a sense of hull location; there is a plywood and timber jig at the bottom and the side timbers are a jig we used for the wall attachments to close off plascore, so our walls are on the sides, if you will

the whole thing is all apart now to finish the other hull and build the cabin to full height

here is a straight on view of the cabin; you can see our window panelsare above the cantilevered area a bit...windows to be, that is...the cabin is off the beams here, but you can see the socket notch

 

I did some more calculations with strapping on the sides under the beam. If I use 1/8" stainless steel strapping, and use 6 bolts at 3/4" each, I would be able to achieve tensile ratings on the strapping plates of 39,000 pounds, and shear ratings on the bolts of 65,000 pounds. A little unmatched, but a LOT better than the cover plate ratings. I am a little unmatched because I happen to have some 1/8" stainless 316 in stock. I could also just double the plate up I suppose, although a bit odd. If I go purchase some thicker gauge steel, I could do a little better, with weight penalties. All these ratings add, though. The strapping, the plywood, and the cover plate would all add. They are not a single weak point in the system. The only place I don't get an addition is on each side of the plate, so 6 bolts would be on each side of the strapping and 9 bolts on each side of the bottom plate. The only question now is how to determine the forces on the cantilever section under a worst case scenario. At 5 times the mass of the entire craft, we are at 40,000 pounds, which is the strapping only. I think I would be okay blasting off straight into a breaking surf with that....but I honestly don't know how to do the force calc.

 

...ps...

Don't worry if you don't see glass tapes. The pictures are taken at various stages. Everything is getting taped very well.

And, yes, the timber we used for our dolly broke one day while I was standing on the cantilevered section. I got quite a surprise. The back timber is an LVL, but it cost $50. And I had hoped a regular timber would work, was wrong.... the cabin is pretty light at this point, but gonna get heavier...

 

Also, the tensile ratings on the 5 bolts that hold the front beam notch in place are only 28,000 pounds(see the pic with the gloves and you can see the 5 bolts). So, another reason to understand the maximum forces that could happen here is to evaluate whether the 5 bolts are enough or if I need to add a few more. This would actually be weaker than the rest of the system, which would mean the cabin would be more likely to break off, than the cantilever shear or crack.... I just don't know how to calculate the forces that might act on the cantilevered section properly. It is probably like a deceleration force or something. The boat hits the wave, and the boat decelerates. If the cabin broke off, the strapping plate at the notch sides, would probably prevent a catastrophe.

 

This is the closest I have to a drawing.

Disregard this drawing.

Not to scale.

 

I assume the notch seats on the crossbeam, right? How far is the deepest part from the designed waterline (wet deck height) and how far is the notch to the bow? These are the forces you will encounter. The front part for hitting the ahead waves and the bottom part from wave slopping. The strength depends on the sea state and speed you are going to operate the craft (operating envelope). Say 15 knots on sea state 3?

 

The distance from the top of the notch to water I need to measure and am offsite now.

The max speed of the vessel is 25 knots. The worst condition would be a breaking surf taken head on. We may slow to 12-15 knots, but I would like to design for max vessel speeds just in case.

I will get you the numbers later today. Thanks.

 

Okay, so here are some numbers.

The top of the front crossbeam is 1332mm above dwl.

The bow is about 2 meters forward of the beam center at dwl.

The front of the cantilever is about one meter ahead of the beam center.

The leading edge of the cantilever is about 1212mm above the waterline.

The beam socket top is 500mm above the cover plate on center of beam. I will add a rough sketch shortly.

 

I realize my beam is drawn in the hull socket and not shown in the cabin socket well...hope you can see past the crudeness of the drawing..