Mast base flange. How to fit deck curvature?

[Chotu]

Ok, despite two very reputable guys saying so, ease is definitely not the answer I was expecting.

To me, cutting out a square from a piece of plastic and then hitting it with a grinder which takes all of maybe 15 minutes or half an hour is a LOT easier than a multi layer lamination project. Not even counting all the chemicals and expenses involved in the goo approach. That’s hours of work and a lot of money in resin and glass to make a block.

[coopec43]

Go along to a local Plastics Jobber/Sign Maker and rummage through their recycle bin. There is all sorts of plastic (nylon, PVC, Acrylic,?) sheeting there that would do the job. (Tell them you are making up a Christmas present for your Grand Daughter and you'll get it for nothing)

[coopec43]

Originally Posted by Chotu

I am finishing up the design of my mast base right now.

It is not very easy or accurate to have the fabricator build a curvature into the mast base itself.

Instead, I would like to use some kind of deck plate below the mast base to account for the slight curvature. It’s maybe 1/4 of an inch of curvature. Similar to installing a deck hatch.

My question is, what material should I use for this deck plate?

Wood seems easy, but of course that is the worst thing you could use since it will rot. not a good place to have things rotting away. I have been using PVC plastic for deck hatches, but, I think that is too weak. It is kind of soft. It will probably crush under the forces of the rigging.

Chotu

This is just one of the many pieces of "plastic" that I have taken from recycle bins. It is white and I am sure it wouldn't be brittle.

[Chotu]

Originally Posted by coopec43

Chotu

This is just one of the many pieces of "plastic" that I have taken from recycle bins. It is white and I am sure it wouldn't be brittle.

Good lord.

it never ceases to amaze me how privileged you guys are in Australia when it comes to materials. Wow.

first, you have any number of premade panels available. All sorts of fantastic things that we don’t have.

Second, I don’t have dumpsters full of amazing off cuts of plastic like that.

I’m very impressed.

But I guess if I go find a signmaker dumpster, which would be a few hours of work, I could find something like that.

[coopec43]

Originally Posted by Chotu

Good lord.

it never ceases to amaze me how privileged you guys are in Australia when it comes to materials. Wow.

first, you have any number of premade panels available. All sorts of fantastic things that we don’t have.

Second, I don’t have dumpsters full of amazing off cuts of plastic like that.

I’m very impressed.

But I guess if I go find a signmaker dumpster, which would be a few hours of work, I could find something like that.

If I lived well away from sources of materials I'd lay up a fiberglass mast base of Polyester resin and alternate layers of Chopped Strand Matt/Woven Rovings. I'm sure I could do that in an afternoon using off-cuts of glass. Here I've made up stanchion bases (which are much smaller of course)

[Franziska]

Delrin is your friend.

Very tough.
Bolt it through with a flat metal plate on top.

[coopec43]

Chotu

If you take traveling time/cost into account it might pay to check what's available online. Maybe along these lines?

https://www.ebay.com.au/itm/29410506...9c2b3331190a87


Roughly where are you located in the US?

[chris95040]

Mcmaster has a zillion different plastic plate materials and data about their strength. Some are made with polyester resin as opposed to epoxy.

I probably would laminate a handful of thinner plates together around a form to make the base as opposed to just hogging one out with a grinder.

[Chotu]

Originally Posted by Franziska

Delrin is your friend.

Very tough.
Bolt it through with a flat metal plate on top.

that’s definitely a good one. I have used it before. My rudder bushings are made out of it.

[wingssail]

Chotu, No, I am not an structural engineer, but common sense and experience with mast steps leads me to think this is not a place for plastic.

With the size of boat you are assembling, the size of the rig, and on top of that the extra compression force which results from being a high performance catamaran, I don't see any plastic as being appropriate.

If I was doing this I would have an aluminum plate fabricated, at least 3/4" thick, and have curved supports welded under it to fit the shape of the cabin top. Further, I would have another plate welded or bolted on the top of the plate to fit into the bottom of the mast to hold it in place.

You could ask an engineer to calculate the compression force on the mast and compare that to the compression resistance of the various materials you are considering.

This crude drawing represents what I would have fabricated if I was doing it.

[Chotu]

Originally Posted by wingssail

Chotu, No, I am not an structural engineer, but common sense and experience with mast steps leads me to think this is not a place for plastic.

With the size of boat you are assembling, the size of the rig, and on top of that the extra compression force which results from being a high performance catamaran, I don't see any plastic as being appropriate.

If I was doing this I would have an aluminum plate fabricated, at least 3/4" thick, and have curved supports welded under it to fit the shape of the cabin top. Further, I would have another plate welded or bolted on the top of the plate to fit into the bottom of the mast to hold it in place.

You could ask an engineer to calculate the compression force on the mast and compare that to the compression resistance of the various materials you are considering.

This crude drawing represents what I would have fabricated if I was doing it.

thank you for the drawing. That thing already exists. I already have a mast step. That’s basically what you drew. Mine is flat though. It needs to sit on a slightly curved deck. So, it needs a trim ring type thing with a flat surface on one side to connect to what you drew and a slightly curved side to connect to the deck.

Just a spacer.

The force on it can’t exceed 18,000 pounds. It's 24x24 (actually 28x28 at the contact point).

28x28 is 784 square inches.
18,000 lbs is way WAY more force than it could ever see, but let's use it.

That's 22.9 pounds per square inch compressive force.

22.9psi. More than that and the boat is upside down already or the rig already broke off.

so, the material just needs to stand up to 22.9 psi. Let’s rounded up to 23 psi.

i’m not sure why you are saying we need an engineer to figure that out. seems pretty easy I just figured it out in this post.

so the plastic just has to be able to stand up to 23 psi. Or... 0.16MPa.

That’s pretty much every plastic there is.

and probably the most hilarious thing to come out of the math above?

just sliding a piece of my foam that I used for the core of the boat under there is enough. Ha ha ha. Crazy, but accurate (see attached pic of foam properties).

Until you forced me to do the math, I never realized the forces were so low. My gut feeling was exactly the same as yours. So it’s a really good thing that you made that post. Thank you. Now I know I don’t have to worry about it so much.
.

[deltaten]

Call around to kitchen remoders. Check on Corian cot off/outs. See , if given the di.ensions and rad of curve, if they will laminate and sculpt for you.
One posting listed 28" X 28" X1/2"... could this be right?

[Chotu]

Originally Posted by deltaten

Call around to kitchen remoders. Check on Corian cot off/outs. See , if given the di.ensions and rad of curve, if they will laminate and sculpt for you.
One posting listed 28" X 28" X1/2"... could this be right?

that is correct.

it’s a custom mast step. i’ll take a picture of it later when it’s installed with the mast on it.

[AndyOz]

Originally Posted by Chotu

Just a spacer.

The force on it can’t exceed 18,000 pounds. It's 24x24 (actually 28x28 at the contact point).

28x28 is 784 square inches.
18,000 lbs is way WAY more force than it could ever see, but let's use it.

That's 22.9 pounds per square inch compressive force.

22.9psi. More than that and the boat is upside down already or the rig already broke off.

so, the material just needs to stand up to 22.9 psi. Let’s rounded up to 23 psi.

...

Until you forced me to do the math, I never realized the forces were so low. My gut feeling was exactly the same as yours. So it’s a really good thing that you made that post. Thank you. Now I know I don’t have to worry about it so much.
.

The problem is that the force won't be uniform over the area, so the calculation is incorrect. The maximum pressure will be straight under the mast section, not the bottom of the plate. The mast area is significantly smaller, so the pressure will be significantly higher.

There's also problems with creep with some plastics. Even Delrin (see https://www.spacematdb.com/spacemat/...ign%20info.pdf).

[Chotu]

Originally Posted by AndyOz

The problem is that the force won't be uniform over the area, so the calculation is incorrect. The maximum pressure will be straight under the mast section, not the bottom of the plate. The mast area is significantly smaller, so the pressure will be significantly higher.

There's also problems with creep with some plastics. Even Delrin (see https://www.spacematdb.com/spacemat/...ign%20info.pdf).

I don’t think the metal plate above the plastic will be bending. Are you sure about this?

In any event, assuming a cooked lasagna noodle is used instead of the the mast step plate, the pressure is around the outer 26 inches, not in the middle. The custom step is a metal box to help elevate the mast.

A recalculation assuming no base plate on the metal box (a wide open box bottom with 2" flanged edges to bolt through) is as follows:

26" x 2" = 52 (x2 for each side) = 104 sqin
24" x 2" = 48 (x2 f9r each side) = 96 sqin

= 200 square inches with 18,000 lbs on it.

That's 90psi assuming there is no metal base plate all. Or .62 MPa.

All polmer sheeting is orders of magnitude stronger in compression than 0.62MPa.

even with a revised calculation assuming there is no metal plate whatsoever, and just a flange around the edge of the box, I could still use the 80kg foam core material of my boat (but not the 60kg flavor).

So essentially any sheet of plastic still works, most of them by orders of magnitude.