I have a question for all of you experienced with both foams. Do you make you XTR boards a bit thinner? If you are, can you tell me how much by on shortboards?
hi, i sort of came to an idea awhile ago ,i don/t know if i can explain it too well but this is what i think,
its easier to think of a dingy with a plimsol line on it in one sense , two dingys of different weights but the exact shapes would require different weights to float to the plimsol line, so,
if you take a board that has 30 litre volume xtr, eps or pu ,if the finished wieghts are all the same they will all have the same floatation,
because in reality they will all be different what i thought about to equalise the float or the weight the board will support was ,
30 litre volume of water (imagine fresh water to make it easy) weighs 30 kgs ,
the pu board say weighs 3.5 kgs ,so to make the board not float ,not sink but just to suspend in water you would have to add 26.5 kgs of weight,
if the xtr board weighed 2.5 kg you would have to add weight of 27.5 kgs,
so to make the board in xtr that has the same float as the pu you would need to create a volume that would require 26,5 kgs of weight to suspend it, the same as the pu
because the new lower volumed board would be slightly lighter just say it weighed 2.4 kgs ,
so 2.4kgs plus the 26.5 kgs weight need to suspend = 28.9 kgs or litres of volume,
so to get the same float you would need a board in xtr of 28.9 litres ,
whether you take length or width or thickness is up to you, or whether you can get the accuracy is another thing, pete
First, let me say that that both boards (XTR and PU) seem to float pretty much flat on top ofthe water, so displacement is negligable on a board without a rider. What wetsuit you wear has a greater bearing on how a board floats you than what foam the core’s made of. That being said, most shapers around here make their EPS/XTR boards up to an 1/8 thinner. They believe that XTR is more bouyant, so less foam is needed. Aside from that being totally dead wrong (bouyancy is a function of volume), I do the same thing, but to give a bit more flex to a board with a heavier, stiffer glass schedule, not because it floats better.
XTR/XPS Same stuff
I made over 300 boards out of XPS and I found that they required slightly less foam volume: Factor of 96.33% or 3.67% less volume.
Depending on the length of the board to take say 1/8" out of the thickness is pretty random. In some cases it only needs to be a 1/16" less.
I know this seems anal to some however when your dealing with surfers who surf at a level of performance the slightest variances do matter.
**PeteUK: **Your accessment is quite acurrate however what size board are you referring to? A PU at 3.5 KGs = 7.72 LBs? Are you talking about longboards?
PU Shortboards (HPSB) custom (not for shops) with a double 4oz deck and single 4oz bottom should weight no more than 2.38 KGs = 5.25 LBS. I aim for 2.27 KGs = 5.0 LBs for a 6’0". I can do the same board in XPS with the same glass schedule and it will be stronger and last 4 times longer and weight in at 2.15KGs = 4.75 LBs. The problem with the XPS is that they are hard to process. They come in billets of 96" x 24" x 5.25" you cut two blanks our of one billet then your glue the blank the Stringer which should have your rocker profile. The cost is much higher than a PU Blank plus Epoxy resin increases the cost by 21%. Sure the board last 4x longer however surfers in general want a board as cheap as possible that’s way PU/PE construction will never go away.
Plus I think that DOW is going to discontiue producing XPS?
Could you image if they made a moulded XPS like Marko EPS?
hi nj , i knew straight away this would cause a reaction, read again what you have wrote,
“They believe that XTR is more bouyant, so less foam is needed. Aside from that being totally dead wrong( bouyancy is a function of volume)”
bouyancy is a funtion of volume and weight , think of a lead board and a hollow one filled with helium , both the same volume, the weight is the prime factor
xtr is about 2lb cubic ft , pu about 3 1/2 lb, so a lighter board of the same volume wil support more weight, there is no argument there ,
what you weigh or whether you wear a wetsuit obviously affects things but that comes down to your own preference of what you prefare in chosen bouyancy, but if you want a board of exactly the same bouyancy of your favourite pu then you have to go smaller in xtr or eps because both will be lighter than the pu ,
again whether you can feel the difference because it is only slightly smaller i really don/t know, pete
hi surfding , i only used the weights as an example ,just nice round figures to make it easy, i have a couple of extruded foam blanks that i am going to try out , one is ixps foam ,i think its made in china and the other is from another surfboard maker here in the uk ,i don/t know where that one comes from,
it seems an ideal foam other than its really difficult to shape , it tears easily , i have allmost sanded the shape out of the foam , water proof , ultra light, the finished blank weighs 2 lb for a 6 3 x 18 1/2 and pure white, pete
I totally understand what you’re saying, and agree. I’m just being picky about the use of the term “bouyancy,” which people use interchangably with “flotation.” Bouyancy deals only with displacement. Flotation deals with bouyancy and weight.
I’m just a nut when it comes to the semantics… just ignore me!
I found it extremely easy to shape actually. However you do need to be carefull not to over shape. Once you get the shape you desire do a light sand over the whole blank with 60 grit. It will feel a little hairy (Fuzzy).
That helps prevent delam. Creates a better bond.
I like the way the boards made from XTR (XPS) are Hydrophobic.
The foam has more memory than PU or EPS.
Well, if we are going to get picky. Buoyancy is a function of the density of the medium(water, air) and the density of the object( board, balloon). Density is a calculation. Mass(not weight) divided my volume. I’m sure some others can come in here and get way pickier than that. Water has a density of 1gram/cm3 or 1.0. Less than that and it floats in water whether it’s a tennis ball or and aircraft carrier.
It’s cool that the xtr stuff doesn’t soak up water and some of the builders have figured a way to minimize delam problems. I wonder if it’s manufacturing is comprable to eps in terms of toxic outcomes or lack thereof. Doesn’t it have some sort of other plastic mixed into the foam? Mike
OK…I can’t help it ('cause I’m retarded like that)…
Buoyancy is a force that acts on an object, whether it sinks or floats. Yes, it’s a function of the density of the fluid, but not the density of the object, only the volume of the object, which determines it’s displacement. Mass has no effect on buoyancy, as buoyancy only relates to the force that’s acted on by the fluid. Greater volume results in a greater total buyoant force, and the greater the density of the fluid, the greater the buoyant force.
That’s why shapers who reduce the volumes of E/XPS boards actually reduce their buoyancy - these boards become less buoyant, not more buoyant.
However, because they are lighter to begin with, they may float better, because there’s less mass.
Somebody stop me…
That is a good question for Surfteach!
He has years of experience in this area.
It’s great to see an intelligent discussion of volume, bouyancy, flotation, etc. I’ve been talking to people for 25 years about this and you wouldn’t believe the number of times I’ve been dismissed with the comment ''it has to float more, it’s got more air in it!". At least no one here is taking that position. But then you’re all engineers or scientists or teachers. You guys know you can’t argue with physics.
Rooster:
Somebody called?
To answer your question, there is no other plastic in XPS, only polystyrene. The blowing agent is pentane gas and this however
is often left trapped in the closed cells of the foam and can outgas when the foam gets hot. I saw a Merrick xps prototype a couple of years ago that had pin holes drilled up and down the board to help vent the board. The key to using this foam in an unvented shape is to keep it cool!!!
The toxic outcomes of XPS & EPS are the same. Here in Cali the factories are required to collect the air coming off the machines and extracting the pentane from it so it doesn’t go into the atmo. Both foams and not biodegradable and have a very long half life. I think that most boards end up in a land fill just like PU. If a board lasts longer, that just means that you put 1 board in a land fill instead of 3. If broken or old polystyrene boards were collected, then they could be recycled properly but I don’t know of any shop or manufacturer that is doing that yet!
I hope those pin holes are strategically placed or they’re going to get covered in wax.