Which foam for first attempt with small trimaran float build?

I'm looking at botching another float on my ISO test rig and want to test out building with foam as I've only previously used stitch and glue plywood.

I'm tooling around with shapes and have the basic shape and volume I want to test which is attached. 5m long 35cm wide at widest and 50cm tall.





I'll knock out the stations and the strong back but I am wondering the best foam to use and the thickness?

I've spoken with a local supplier and they are suggesting either Airex T92 or Tricast 6 which are both approx 100kg / m3 so much lighter than the ply.

Does anyone have any thoughts which would be best to use on this?

Also what thickness would be best on the float then on the main hull which will be 5m long and approx one metre wide?

I'd like to use this float as a tester for getting the method worked out before I go to any great expense and effort with the main hull so any thoughts appreciated.

 

Polyurethane foam is easy to work and can be a lot cheaper. You rely on the outer glass skin and deck join, stringers etc for strength . Old Multi is currently on this subject. There's a couple of other threads current on this , one in boat building ..polystyrene.

 

Basically, the lower the density of the foam, the more structure engineering needed.

 

Thanks for the replies, chaps.
I've attached the Airex product list, If I were to use the Airex T92 any thoughts on the thickness to try?
I've done various things in ply previously and been amazed at the strength achieved with 3 and 4 mm okoume ply but I'm basically looking for a nicer shape more than anything so my wife will come out on it with me!
She hates my ISO test bed trimaran even though it is quite lethally quick...

 

Engineering is your friend.

Suggestions from people you have no knowledge of their capabilities are just like trusting a Ouija board.

Have you ever looked at the construction of an old Hobie 16? At least you know they work for 15 years or so.

The ama shape you show looks a lot like an old plywood Tornado in section shapes. Those were made from 4mm Okoume. It's completely unknown if your shape could be made from tortured ply like the Tornado.
A glass/foam sandwich Tornado had 1/4" foam - as measured just a few minutes ago.

Good luck

 

For such a size 1/4" 90kg/m3 airex as on the Tornado. Reticulated PVC foam is the best on multis.
The rather high density is beneficial as the amount of resin absorption is better controlled and the structure is far stronger.
Forget polyurethane foam it's not structural and soaks lots of resin and after water. When getting aged it will decompose in powder.
Forget too light foam, there is an important factor of peeling and buckling skin in a sandwich composite. A too light foam will delaminate under stress.
A small cata has very high local stresses induced by the hellsman and crew. Whatever the size of the small boat a 220 pounds guy on one foot while on trapeze will stress locally the deck and sides. It's the minimal requirement of strength.
You can get better shapes that those shown.
The first one looks like an old 70ties tortured Class A plywood. The center of volume is too forward, the cat will be unable to sail correctly "downwind". It lacks of volume in the middle and aft sections.
The wide shape has a too fat transom with a curious inflexion at the waterline.
The first trials while designing are never good...

 

"they are suggesting either Airex T92 or Tricast 6 which are both approx 100kg / m3 so much lighter than the ply."
Well, yes, of course.
But then, you aren't adding in the big unknown of the 'glass "scantlings" to keep it intact. That light foam will split fast if you load it up with bad glass design.

The only strategy you can adopt if you don't want to hire a professional engineer is to buy the plans of a similar boat and use their specifications.

 

In this size in the old times 30 some years ago, 3 mm 3 equal plies okoumé premium marine plywood was used. Plus some structure inside. The weight by m2 of the skin was plywood 1.5 kg, inside epoxy resin coatings 0.25 kg, outside 160 gr satin glass, 0.160 kg epoxy plus finishing coatings anf fillings around 0.25 kg. A total of 2.3 to 2.4 kg m2.
Working hard with a good scale a complete 5.5 m long hull, 0.31 m wide, 0.52 high at the max, 0.24 m transom width was obtained complete with the hull hardware around 17-18 kg, and a complete 18m2 catamaran 18 feet long, 10 feet wide with a 30 feet mast was from 89 kg to 96 kg. The 90 kg one is with carbon beams and spi pole, titanium bolts and screws and a very carefully chosen sailing hardware. Even the battens of the sail were weighted...With less expensive materials you got around 110-115 kg, and that was already pretty good.
Such a plywood cat is pretty fragile to impacts. You have to handle it delicately and have a custom trailer.
To beat this weight with sandwich you have to go to carbon fiber. A glass epoxy sandwich normally built will be around 135-140 kg, as I have seen, but it's very resilient and rather durable.
A very good 4mm plywood tornado was around 150 kg as it has a complex interior structure. For comparison a 18 feet Hobie was a good 190 kg and more.
"The only strategy you can adopt if you don't want to hire a professional engineer is to buy the plans of a similar boat and use their specifications." a very good advice. You'll get also a lot of useful details.

 

The main hull of my trimaran (Broomstick) is similar in size & shape to the floats you're considering, so it might help you to know what materials & methods I used.

However, I'm making no claims that what I chose to do was optimal in any way. While I put a lot of effort into the design & estimated performance, my structural method was strictly "eyeball engineering", and priority in materials was given to what was readily available locally.

Here's a photo of the hull midway through construction.

I used the foam from this source (I believe it was thickness =1" & density =4.0 lbs/cu ft, even though that combination isn't shown.)
LAST-A-FOAM | Aircraft Spruce https://www.aircraftspruce.com/catalog/cmpages/lastafoam.php?recfer=26364

As you can see, the flat shape of the sides allowed me to use one large panel per side, but the round bottom required strip-planking. After sanding and fairing, I added 2 layers of 8 oz/yd fiberglass/epoxy on the outside, and 1 layer on the inside. After decking & painting, the hull weighed about 50 lbs.

For the final structure, I'm sure the foam could have been thinner. However, the strips and panels were extremely flexible, even with temporary frames every 12", so I think it would have been much more difficult to build using thinner foam.

I also had to add some 3mm plywood in areas of the deck where I would be stepping frequently, in order to prevent denting & delamination.

I haven't had any problems with the strength of the hull, but it is mainly an experimental platform for my foiling experiments and probably not as robust as a typical daysailor.

Overall, I was pleased with this construction method, but I have to admit, it was pretty tedious. For the floats, I opted to use stitch & glue plywood construction.

I'll be glad to fill in more of the details, if you think they would help you.

​

 

Doug,

Thanks very much for that detailed example. - everything I do is pretty much "eyeball engineering" as well so gladdened and amused to hear that phrase : )

If you have any further info would be greatly appreciated by me and can only be useful for others as well as there are many admirers of your work on Broomstick - I love those videos you have made.

 

Just had several days out racing on a Beneteau First 40 in the Taittinger Regatta on the Isle of Wight.

Mildly odd experience as never been on a boat like that and with 11 onboard and being a bit shambolic reminds me why I like small fast boats!

Anyway, Ilan Voyager, if you have some suggestions on the shapes they would be welcome - I can always post the Freeship files. If something half decent can be forged together it might be useful for someone else?

Also your comment about "designing" this from my perspective might be slightly over stating the case : )

I spent about an hour on Freeship then a morning making a full sized prototype in cardboard - pictures attached! before spending about 30 minutes revising it as the prototype main hull appeared to be too wide.

The curious inflexion on the side was where I was looking at flaring out the topsides a bit and haven't smoothed it in yet - currently it is all about just getting some approximate sizes and volumes.

Four things I would ask if I could relate to your comments
1st "The first one looks like an old 70ties tortured Class A plywood. The center of volume is too forward, the cat will be unable to sail correctly "downwind". It lacks of volume in the middle and aft sections."

This isn't a cat - it will be a trimaran - only using the rig from my cat initially at least.

I'm simply looking at upping the volume up front as I've seen how hard pressed the Spitfire F16 gets when we are pushing it hard. Even with the mast really raked back hard and we are still completely burying it so it seemed a bit more was worth trying.

The main volume is directly under the front beam and then further forward.

The cost of making a float to test and break is pretty low so having something will give me a lot of useful feedback on likely weights I can build to as well as performance etc but if you have suggestions on changing it most welcome.


The test float here was just botched up as I had about 2 spare sheets of ply lying around and just put a bit more volume up front.

This gets crushed in to the water under load when we were pressing it hard so I know some extra volume needed over all from that. The hull was pretty flat so took some heavy smacks when landing back on the water, so now want to spend a bit of effort to get some thing rounder and generally better shaped.

In the background is the test bed ISO - I'll make up the new float and run it one side with this other one on the other to get some test data from it - get an idea how the different shapes and volumes work before spending some proper time getting something final together.



Here looking at how it might work with folding and rough width for trailering.

The main hull will be a bit higher with an indentation for the floats to nestle in a little closer.

The verticals beams on the floats here won't be as tall as the float will be taller - again this basic testing on the ISO will help sort that. Having upward curved beams and a vertical join to the float will be useful to reduce the collisions with the sizeable chop here - beams hitting the water will be wet, messy and slow.

I've got a crap load of cardboard I can mock things up with as I've just had a massive delivery of gabion cages so can use it with a few £'s of tape to get an better view.

The 2nd thing was with regard to making it particularly light. Initially I am more concerned about getting a rounded nicer shape than my first chined hull on the ISO and seeing how it goes together. I'm not that fussed about weight initially while I test building with foam but it will stand me in good stead for the final hull builds.

3rd things was about the foam thickness - the tornado you mention is 1/4 inch thick so approx 6.35mm. 10mm is approx 100kg airex is approx 25/64th of an inch so circa 50% thicker so assuming I can do any sort of fibreglassing work I should get something vaguely usable. If it breaks then I'll make it stronger.

4th is about loads on the decks when trapezing - if I am single or double handed sailing it I/we will only be trapezing on the outer tramp rail if needed , definitely not standing on the float decks anywhere. The deck up front would be covered with a ply capping with multiple light ring frames to ensure it can be walked on when having to deal with the inevitable **** ups going up front.

Ilan, and anyone else, I'd be very interested in anything you would like to add - perhaps we could come up with an open source larger Weta alternative?

 

I've received some sample foam from the supplier so I now at least have some understanding of it.

and another view


Weighing these out to double check, these sheets comes in at 100kg / m3 for SX and 130kg /m3.

A sheet of 4mm ply I have used before comes in at 2.33kg / m2 and for the test I need 6 sqm so 14kg.

Made in Airex SX it will come in at 6kg and made of non SX will come in at 8kg (before composites).

The SX will need more epoxy filler so I assume weight between the 2 will come out reasonably equal so for this first float it will be interesting to see how it comes out.

I'm going to build a quick ply chined float for the test bed to see how it works for approx shape and volume and can then refine the float shape / volume ready for building a final foam float.

4mm okoume ply is costing £47 for a 2.5m x 1.2m sheet plus delivery and the T92 costs exactly the same.

I need to confirm a price for the SX as there must be a premium but I assume I would only use that for the rounded parts of the hull.