24 ft "picnic cruiser" designing

I don't think Ponkala's boat needs leaking fuel for being risky. When you see the interior its pretty clearly designed home like not marine environment like. The design looks like there is a lot of weight high up with relatively narrow hull. Add huge windage and waves with high wind might be enough.

And yes Finnfoam can be found in different compressive strengths (shear strength is same for all) - even the heavy grade is quite cheap. I have studied the option to use it for a small boat. I think it might be decent solution for certain uses but for an expensive massive project I think its a bad idea.

Hull is not that big part of the total cost. And its quite a bit easier to add kitchen hooks or more serious attchment points to plywood vs. Xps.

 

"And in the nightmare scenario, solvents or fuel infiltrates the EPS core and liquifies it !"

Not a problem with Jorma's "Shalom". It's a Solar electric vessel.

Also see #10 & #13

http://www.boatdesign.net/forums/materials/inexpensive-lightweight-core-interior-uses-37233.html

jussi 2 also from Finland, looked into XPS.

http://www.boatdesign.net/forums/search.php?searchid=3792189

 

The feasibility of EPS as a core, is bound to the need to use epoxy. It would seem to me to be limited to very light structures, or the extra resin cost will negate the price advantage with the core, or at least sharply reduce it. The compressive properties then becomes the critical matter, with thin skins, and the EPS is not so good as a more expensive alternative. Maybe with Finland being so cold, the EPS scores as an insulator !

 

The Finnfoam that Jorma used was between 4 & 6 inches thick. His build pages are N/A now, (in English that is) but he was shaping each block much like Inuit shape blocks of snow for igloos. http://shalom.ponkala.net/pages/fi/shalom-vene.php
I remembered that the material was used under railway tracks. See link.

http://www.finnfoam.lt/client/frontpage//english.pdf

 

Okay ... I searched a bit and realized that it wasn't a recent thread but one I'd recently been reading.

Oops.

Anyhoo, http://www.boatdesign.net/forums/boat-design/duck-punt-twist-42466.html, from ought-12.

So, yes, a form of polystyrene.

The foam in question is XPS and has the end grain like structure running from face to face, presumably to help compression strength. He notes this grain structure results in edges that don't sand well, comparing it to sanding end grain balsa along the edges.

"High compression strength" boasts in ads looks like it will require some product research. 300 kpa (ClimaFoam) may sound like a lot, and 700 even more for HD-X, but that's only 43.5 or 101.5 psi respectfully. Dow's HD-X sure looks the better of the two. Note: not a representation of real, exhaustive research.

For comparison end grain balsa has compression strength over 1800 psi.

Edit: a not-random pondering ... strength aside do these materials have any memory so that they return to their shape or do the just crush? For floor insulation I'd hope the former ... a natural concern for me because I'm a big guy (a stocky 6'4" and even when I had under 10% body fat I weighed 270#), with thankfully big feet (16EEEE).

Edit #2: has there ever been a thread about comparing the properties of different core materials not limited to the expensive stuff (iow, including the kind of materials like XPS or HD-X above)? For example, I recently read that Divinycell has poor crush resistance making it less suitable for places that may suffer impact like the hull.

 

I just bought a lot of 2nd grade from the factory (ie slight measurement variance, compressive strength guaranteed to be 300kpa even though it might be from a 700kpa run).
That was 2"/5cm stuff. Its pretty good, you can walk on it without any worries of leaving foot prints, and yes it does bounce back pretty good. The surface is very nice, especially on the thinner (~1") boards. You can bend those thinner panels to an extent but not much. Still it would seem that using the boards as boards would make more sense (over the thick block idea), be more efficient and save a ton of epoxy.

and I am talking about the Finnfoam though I doubt there is huge difference between brands.

anyway EPS and XPS are not that much alike so mr Efficiency if you keep on thinking regular EPS then you need to read about XPS. Your criticism might still be valid but the 2 are totally different products.

 

I kinda figured the stronger stuff wouldn't bend all that well. What about using vertical foam stripping (link below) to help achieve more rounded forms with the XPS?

http://www.farriermarine.com/pdf/Foamstr.pdf

You may not be able to achieve the same amount of curviness as with the pricy proper boat building foams but it might still get you some of that nice proverbial flare forward and tumble home aft.

Also, since you mention the better finish on the thinner foams, which presumably bend better, I wonder if you could judiciously use layers of that to form greater curvature of the required thickness? Kinda like making laminated bent wood frames.

Also, great thinking to buy seconds!

Have you considered rigging to a small test stand to separate the stronger material you have and reserve it for where it'll do the most good? Since impact resilience should be a good indicator here's a design for a Charpy rig http://blogs.osc-ib.com/2014/08/ib-...-student-project-in-materials-and-structures/

You might have to sacrifice a small bit of each board but if there's actual variations in strength ... that aren't negligible ... it may be worth it.

 

I am waiting for specs from http://www.sundolitt.co.uk/download.aspx?object_id=9C89DE5D1D2D448AA01E6964CF5F7C56

 

Right, well, I shall have to research that better, thanks for the tip. I love cored boats generally, they are quiet and vibrations are much less. I just run into this mental block, with polystyrene, and especially needing epoxy, which makes it less attractive than it might otherwise be.

 

"And in the nightmare scenario, solvents or fuel infiltrates the EPS core and liquifies it !"

The Navies and Pilot boats of the world have been using AIREX for 40 or 50 years ,,,,,

 

It's a PVC foam, Fred, and about the worst it will do is soften in the tropical sun a little. And it is pretty expensive, as they all are (PVC's)

 

Thank you for good comments!

My idea of the structure is to make a solid hull. I mean that there wouldn't be any empty spaces or chambers inside the hull. That is why it's good to have a cheap core material.
It is also possible to combine different core materials. At most critical places core could be divinycel or similar and at noncritical places it could be XPS.

"And in the nightmare scenario, solvents or fuel infiltrates the EPS core and liquifies it !"
- This will be an electric boat too but I've planned to have a range extender, like a 4-stroke generator.

About the resin... Epoxy is expensive but it has many advantages. It is stronger and more water-resistant than polyester. Somehow I'm a bit scared that polyester resin will get wet.

Plywood must be used anywhere I need to attach anything, like railings. But I don't like to use plywood or wood as a structural material because sooner or later it will get wet. I've an old Bayliner and almost all wooden parts are rotted and replaced.

Pericles, I know this Ponkala's Shalom project quite well and it is an inspiration for my project.

Rurudyne, That foam striping method looks interesting. I've planned to do the most difficult shapes with saw, knife and sandpaper. XPS is really easy to shape with basic tools.

 

it is pretty easy to shape but the coarse surface will eat a lot of epoxy = weight and ca$h

 

I've been thinking about the design and this is what I came out with. It's a combination from those I showed in the first post.

I've also been thinking about some details and the layout.

Layout is a combination from a sailing boat and a daycruiser. Front cabin has a bed for two and a chemical toilet under it. Social area has two sofas for three person each. Driver's seat is at the end of the sofa and it is foldable. The table between the sofas divides into two parts. The tables rotates around their legs. Pentry is at the rear. Sink is on the sb. side and cooker on the bb. side. The sink and the cooker has foldable cover/extra levels.

There will be a turnstile at the bow to make boarding more easy. It has also a built in ladder. The ladder needs some kind of sliding chassis that I've not designed yet.

 

Dear friend, have you taken into account the points of flooding ?. How do you solve this serious problem?