Top vs Bottom Layers of Composite

What works and what doesn't. Share design ideas, references and contacts for paipo board builders.
PhillyViking
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Top vs Bottom Layers of Composite

Unread post by PhillyViking » Thu Apr 26, 2018 5:50 pm

I am finally getting around to making a Paipo. I found this link while thinking about what layers to put on the top vs the bottom of the board.

http://www.bertram31.com/proj/tips/core.htm

According to that article tension strength is set mostly on the bottom skin. The top needs proportionally less skin to get the overall tension strength to prevent buckle and break. However, we also need enough load strength on top to prevent dents and dings.

Most surfboards have more skin on the top than bottom. That makes sense from a load point of view especially for stand up surf boards. But it would actually be stronger from a break and buckle point of view if the extra thickness was on the bottom.

For my board, I am thinking two layers of fabric on the bottom and one one the top. That leaves the top vulnerable to load and impacts like my elbows. I am thinking a top layer of 1/4" cork above the fabric to handle the load & impact stress. I assume cork will absorb the impacts and handle static load. I wanted to use cork anyway to be kind to my body and I just plain like it.

Incidentally, I will be using unidirectional flax linen (w/Entropy Epoxy) with one of the three layers at 90". That way I will get more nose to tail than (unneeded) side to side strength for the same material thickness. I might chicken out an go an extra cross fabric layer on top but I like the theory of using three fabric layers and one cork layer.

Any thoughts on the article or my reasoning?

asier esnal
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Re: Top vs Bottom Layers of Composite

Unread post by asier esnal » Thu Apr 26, 2018 11:54 pm

your observations are correct, the top sheet tends to shrink. the lower one to stretch. but our body gives many hits to the board, the board above needs to hold more stress because of impacts, than the buckling effort

I am in the process of making my first paipo, but I have made many surfboards, not professionally.

In surfboards it is very common to reinforce parts with an extra layer. I attached a picture. for the paipo, you can put a layer on the side of the elbows, you will not add much weight, you will have good flexibility but it will lengthen the life of your paipo

I have surfboards with corecork, it gives very good results as long as you put a fiberglass wing on each side of the nuchoe sandwich. it absorbs the blows very well, in an incredible way and gives good flex.
but do not let the corecork touch the water, it does not rot but it absorbs a lot of water, it adds a lot of weight when it's wet, that water will wet the coke. and it does not provide so many structural advantages

I have a video with a sandwich structure that has given me many joys. the niche of eps has no stringer

fiberglass 4oz
corecork 2mm
fiberglass 4oz
eps
fiberglass 4oz
corecork 2mm
fiberglass 4 oz

my videos of flex demonstration and hardness corecork in the form of sandwich
good luck
https://www.flickr.com/photos/135579363 ... 007081972/

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bgreen
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Re: Top vs Bottom Layers of Composite

Unread post by bgreen » Fri Apr 27, 2018 5:34 am

Do a search on cork - lots of examples. I have a couple of composite boards with cork decks. It's a great feel. My ribs would appreciate this after recently hitting the deck on some takeoffs, on a fibreglass deck.

You also don't need wax.

PhillyViking
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Re: Top vs Bottom Layers of Composite

Unread post by PhillyViking » Fri Apr 27, 2018 8:18 am

Asier, I like that you achieved the right amount of flex while retaining sufficient strength with your layers. Looks like your board would travel well. My board will travel to Costa Rica and Hawaii. I assume your examples use epoxy? Your experience is helpful.

Some say that if you vacuum epoxy into the cork and then sand away the surface epoxy that the water absorbed on the surface is not significant and you get the benefit of going waxless. e,g, http://lostsurfboards.net/c3-black-dart/c3/ If the cork is already dense with binding then vacuum might not be needed. I am going to try the vacuum approach and not use an outer layer of fiberglass cloth on the outermost top.

I still have to decide if I need one or two layers of cloth between the core and outer cork. This board has a stringer so might tip the decision toward one layer of cloth on top. I am using Linen rather than fiberglass cloth. I am hearing conflicting reports on claims that linen of the same weight and weave has more tensile strength than fiberglass. Some claim it does and others say "BS". The next board will be stringerless so will likely tilt the decision to two layers and/or to add more to the rails.

For me, perfection would be a board that duck dived and turned like my wood board with fins but felt like my surf mats. Of course all design is a series of trade-offs so I expect to iterate.

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Re: Top vs Bottom Layers of Composite

Unread post by krusher74 » Wed May 30, 2018 4:42 am

My first question would but what is your aim/need in straying from the standard glassing procedure that's tried and tested?

Standard constructions are (stand up board)

comp: 1 layer 4oz top and bottom.

standard 1 4oz bottom 2 4oz top.

heavy 1 6oz bottom 1 4oz plus 1 6oz top.

I have used both standard and heavy glassing on my paipos, 1 4oz or 1 6oz had always been more than enough strength for the bottom.

With 2 4oz top or 4oz plus 6oz top. both bare , with cork and with 5mm eva foam I have always got pressure dents from elbows.

After testing all the above, my go to is now 1 4oz bottom and 2 4oz top with 5mm eva foam for at least elbows. (poly resin)

I have trued epoxy , its stringer, but found more problem with de-lamination (as it will spring back and life out of an elbow dent) So I prefer the way poly resin will stay in a dent.

Its takes me over 5 years of regular hard surfing to ruin a board. (mostly rock impacts and airlines!!)

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Re: Top vs Bottom Layers of Composite

Unread post by GeoffreyLevens » Wed May 30, 2018 9:46 am

PhillyViking wrote: http://www.bertram31.com/proj/tips/core.htm

According to that article tension strength is set mostly on the bottom skin. The top needs proportionally less skin to get the overall tension strength to prevent buckle and break. However, we also need enough load strength on top to prevent dents and dings.

Most surfboards have more skin on the top than bottom. That makes sense from a load point of view especially for stand up surf boards. But it would actually be stronger from a break and buckle point of view if the extra thickness was on the bottom.
Only hard board I ever bent, a kneelo fish, very obvious that the glass failed on the deck by compression even w/ double top and single bottom. The bottom showed stress fractures from bending i.e. tension/stretching

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Re: Top vs Bottom Layers of Composite

Unread post by rodndtube » Wed May 30, 2018 11:36 am

Lots of things going on here and most of which I can't qualify myself as an expert. Glass and resin are important. Layers are import. The ability to "bend-but-not-break" is important. For my boards I am looking for 1) strength to resist breaking and 2) strength to withstand dings and 3) strength to withstand the pressures, bumps and bruises of airline travel.

I have been double layering glass jobs (with rail wraps) for quite some time especially since the beginning of my Austin boards. Sometimes it is 4+4 deck and 4+4 bottom, sometimes a 6 oz is added in. These boards have been resistant to dinging and suffering pressure points from travails of airline travel. Board breaking/snapping is probably the area of least worry based upon simple observances over the years. My first paipo was built from a heavily fortified Holmesy Sidewinder as most longboards were built in the mid- to late-1960s, probably two layers of 10oz cloth on both sides with wrapped rails. The board snapped in a 4 ft. wave at Bridges, Aguadilla, PR. Granted, Bridges is a powerful wave, but the stresses leading to breakage are easier along a long surface (longboard) compared to a short surface (paipo). Simple test? Compare breaking a 2 inch carrot with breaking a 6 inch carrot from each end. Anyway, my 2x2 layerings adds good strength for the three needs mentioned above. I think the "S" cloth has more stetching strength than "E" cloth, but will let the experts weigh in on this one.

The extra cloth regimen apparently works. Scratches don't become cuts, cuts don't become gouges and many bumps do not become cracks. A friend's Austin, similarly glassed, needed some fin box repairs done and the ding repair guy commented how strong the boxes were and how little relative damage there was. I also super pack my board bags with strong, forgiving foams, along the rails and below and above the board.
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bgreen
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Re: Top vs Bottom Layers of Composite

Unread post by bgreen » Wed May 30, 2018 4:52 pm

Some of my boards have been 3 x 6oz, latest I think are 2 x 6oz top and bottom. These cope well with rocks and travelling. John Galera used to put 2 x 10 oz I think. I'll ask him what his current glass jobs are. I think he has moved to epoxy.

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Re: Top vs Bottom Layers of Composite

Unread post by nomastomas » Thu May 31, 2018 1:21 pm

rodndtube wrote: Compare breaking a 2 inch carrot with breaking a 6 inch carrot from each end. .
Great analogy! I've never found any measurable flex in a sandwich construction prone board with dimensions in the 45" x 22" x 2" range. I would think that the potential for flex would increase as the thickness is reduced. There may be some measurable flex in a 1" thick board, but even then, I'm not sure it could be perceived by the rider. The prone rider spreads the "load" so much more than the stand-up rider. I have found stringers in prone boards to be overkill; just unnecessary weight.

I've found that 2 layers of 4oz e-cloth deck and 1 layer 4oz e-cloth bottom over a 2lb density eps core is pretty impervious to damage. The vulnerability point is the rail. Its difficult to avoid "bumpimg rails" in the crowded breaks of So-Cal. About a year ago I added a second layer of 4oz to the bottom to increase the number of layers protecting the rails (4 layers at the apex of the rail).

When considering elbow dent (or heel dent) protection, I was taught to look to the core not just the skin. A brick wrapped in 2oz cloth will not dent (or float). I use 2lb density eps for durability and ease of shaping. My prone boards weigh about 5lbs, which is light enough. If you're shaping with polyurethane foam, you should consider stringeless "Blue" density foam from US Blanks. If you need a permanent centerline as a reference point, ask to have a centered "glue cut" done in the color of your choice.
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asier esnal
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Re: Top vs Bottom Layers of Composite

Unread post by asier esnal » Fri Jun 01, 2018 3:10 am

my first surfboards were with eps of 25 kg / m3 density 1.5lb / ft3, they do not take hardness or putting many fiberglass fabrics
I have discovered that the super light boards are not good, they float too much and do not let you turn, you do not sing the ridges in the water, the surfboards tend to skid

now I am using 35 kg / m3 density 2.1lb / ft3,

I am very happy with the result, for surfboards I put 2 fabrics in deck according to the needs 5oz x2 or 6oz x2 in the bottom 1 fabric of 6 oz

for my paipos, as they are the first and I do not know the bad treatment that I am going to give them, I have reinforced the recipe. I got 5oz x 2 up with reinforcement on the elbows
and 5oz x 2 down, they have been heavy, but it is also what I want, especially in the red paipo I have seen inertia

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