Hatch Deflection

[Barry]

The OP by his schedule is building a cored hatch so there are two aspects to consider

1) Is the top skin capable of withstanding a load before puncture or failure? and
2) The OP is trying to calculate the amount of deflection of the cored hatch

With this in mind, the deflection is a result of a Load in pounds NOT pressure in psi. Ie a 600 pound force over 1/2 a square inch is 1200 psi and the same load applied over 1 square inch is 600 psi. The deflection is determined by the 600 pound load. (very small variances due to the slightly larger footprint)

The plate deflection calculators, abundant on the internet, require input in say pounds, inches etc to calculate bending stresses and deflection

With the calculators are inputs to the type of plate support at the perimeter. Ie a plate welded around the perimeter will deflect less than a simply supported, ie resting on a sill,

So the goal is to figure out what load/force a 300 pound person, in a dynamic situation , wave slam etc , would produce and apply this force to the center of the hatch to calculate the deflection.

The vertical down force a person will exert on the hatch is a function of the acceleration in the vertical up direction that the boat may be subject to. Say a g force of 3 times
So the force the 300 pound person will create is 900 pounds of force.

Then simply take the measurements of the hatch, put the 900 pound force in the middle, choose simply supported perimeter conditions, assume the guy is standing on one foot, as you can not really guarantee that he is not, and check the deflection.


Or you should be able to enter, your max deflection that you want to allow, the max loading, perimeter conditions, which will give you the numbers to design your laminate/core schedule.

 

[pyoung]

This been a very critical/detailed analysis of something boring, "hatch deflection", which is why I came to boatdesign in the first place. This is exactly the kind of information I was looking for so thank you everyone for their input.

There are a few things driving the overall design.
1) I can say without a doubt that this current layup is more rigid and durable than many other hatch lids I have seen. One production builder (name will be left out) decided to cut cost on their lids. Originally they were inner and and outer molded panels sandwiched together with a hardened polyester putty. They decided to forgo the inner and used only the outer mold with EVA foam on the underside to finish the hatch. Those felt like very soft and to me were some of the worst hatch lids I have ever walked on.

2) The deck layout for the current boat would make it difficult, unsafe and awkward to be standing on a lid in such a situation that would create an amplified loading condition. Not to say it can't or won't ever happen.

3) Puncture failure wasn't my primary concern although that is a good point and certainly something to consider.

4) I am torn on the end conditions. While the lid is simply supported by the rain channel detail, it is also being pinned by hinges and latches. I am getting good deflection numbers with simply supported ends and I believe the hinges and latches will help with the end conditions which would further help the deflection numbers.

5) The foot graph is interesting. As expected we do not stand flat footed which certainly adjusts where the load is being carried underfoot. I think in order to keep things simple, it is a safe bet to apply a point load because we know that our entire body weight is not being carried on one point. If a hatch can pass a point load test it should display even better strength characteristics in application.

6) I will go ahead and say many times, we as designers are asked to do things that certainly are not how we would do it on our personal boat. Not that is is wrong, just seemingly unnecessary or not our style. I throw out this disclaimer ahead of my gutter detail pic for Fallguy because this was largely customer driven. Function is there and its build-able but have to pick your battles sometimes.

Thank you to everyone for their help. We are going to build a panel to check and make sure we are happy with the feel of the hatch before making any RTM molds, I will come back and update once we have done that.