The 'How to Build a Water Ski at Home' Thread

[AdamCord]

So I've been able to get some work done this week in building the mold.

 

I started by wet sanding the plug with 400, then 800, then 1500, then 2000 grit sandpaper. After the 2000 it's lookin pretty good:

 

Next I need to wax the plug. I'm using this Partall paste wax. I've used this before with success but there are other options such as a chemical release or using PVA release film. I'm slightly concerned about using this wax because it's pretty cold in my garage and I normally use it in warmer weather. Hopefully it works the way it's supposed to...

 

Wax on

 

Wax off

 

I did 4 coats of wax. Next up it's time to put down the tooling gel coat. I bought all my materials for the mold from US Composites. They're pretty cheap and their stuff is pretty good. This gel coat is made specifically for making molds. The color is supposed to contrast with the color of your plug so you can easily see how thick it is. I brushed this on with a paint brush in one pretty thick coat.

 

 

I let this cure overnight. In a perfect world one would wait till the gel coat sets up to a point where it's hard, but still slightly tacky before the next step of laying reinforcement. I missed that window so I sanded the gel coat with 400 grit to help with the bond to the resin.

 

Now it's time to lay down the fiberglass. I'm using a lightweight fiberglass chop strand mat (CSM) and polyester tooling resin. The CSM conforms to the curves well. It's important to try and get every last bubble out of the first layer and to make sure the fiberglass goes all the way down into the corner around the ski. Otherwise the gel coat could crack off later as it's pretty brittle.

 

I did 4 layers of CSM, two light and two heavy. I'll have to go look at what weights I used. These were done separately to let the previous layer mostly cure each time. Lastly to add some rigidity to the mold I used strips of cardboard, laminated them to the mold, then covered the mold and cardboard with one last layer of CSM. The thickness of the cardboard adds quite a bit of stiffness to the mold. If this were a "production" mold I wouldn't use cardboard, but for what I need this will work just fine.

 

 

Once this is all cured I'll be able to pop the mold from the plug. Fingers crossed....

[AdamCord]

@Stevie Boy if you're talking about making the tunnel deeper I'd say the Sixam isn't the best ski to do that with. You'll essentially make the tail ride deeper and slide less (two things the Sixam already does). You'll most likely end up with a very stable, slow offside turn and an onside that is completely horrible as the ski won't want to rotate.

 

@DW yes thank god we don't have to go through that...

[eleeski]

Adam, perhaps you could mold in some plates you could get bolts through and attach to some scaffolding so you could fine adjust the rocker. Just looking down the road for when you might want to experiment.

 

@Stevie Boy‌ If you just want to stiffen up the Sixam, try adding a layer of carbon unitdirectional to the top. Sand with 80grit to get a good bond and use a quality epoxy and the ski will get stiffer. I used that trick on a pair of snow skis. Worked!

 

Eric

[AdamCord]

@eleeski good point I'll probably build some kind of jig that allows me to adjust the rocker.

 

Also I stiffen skis that way all the time. In fact I think everyone should do it! If you have access to a flex tester you can really dial in a ski that way. I normally remove the section of the top graphic I'm going to add to though.

 

Of course you need to start with a softer ski to begin with. What I do is add uni carbon to the top and make it a bit too stiff, then sand it down until I get the flex exactly where I want it. Many pros do this and the public is none the wiser because the area they've worked on is hidden under their bindings. Of course you don't want to sand into the ski's original laminate and compromise the integrity of the ski, so be careful with that if anyone wants to try it :)

[Stevie Boy]

@eleeski thanks for that little gem, might have to have a look at that.

[Stevie Boy]

Back to the building of Adams ski, I guess normally the core is machined and is placed in the mould, what are you planning to do there ?

[AdamCord]

@Stevie Boy you're correct normally the core would be machined if this were a production environment. That's the way all modern high end skis are made. But it wasn't too long ago that the cores were cast using Polyurethane foam. That's still the way most mid and low range skis are made, and even the high end Connelly skis used this method up until last year. The PU cores end up a bit heavier than modern PVC type cores (about 1/2 lb) but the ski will be tougher and generally handle abuse better, so it's a trade off. The ski I was riding most of this year (and I PB'd on) weighed almost 8lbs so I'm not one to worry about a little extra weight. ;)

 

What I will do is cast the core in the mold I've built to end up with a core that fits perfectly. I'll go into detail about how I handle offsets, inserts, etc. once I get to that step.

[Steven_Haines]

Wow! 8#... I bet that handles rough conditions well!

 

[AdamCord]

Maybe a bit better but it doesn't seem to make a huge difference. There are only two times when I can tell it's too heavy. One is when I make a mistake where the tip comes up at the end of the turn and I struggle to correct and get the ski back in the water. Luckily with the right fin setup this is a rare occurrence. The 2nd is when I drop in at the end of the lake and the ski feels like it's pulling me under water. I never knew how much the ski helps you float until I had a ski that practically sinks.

[Chef23]

@AdamCord the third time would be when you fall in a tournament and need to walk back in carrying your ski.

[Horton]

Ski weight is like politics or religion. You believe what you believe and those who disagree are wrong.

[eleeski]

Eight pounds? That sounds like the baby's weight. If it is ski weight, Adam hasn't seen the light.

 

@Horton‌ is wrong just on general grounds. @MS is right on general grounds. Unless they happen to be reversed for some religious or political reason.

 

Eric

[AdamCord]

@eleeski I once saw CP run 3@43 (in practice) on a prototype I built that weighed ~7-8lbs. I think that ski was mostly bondo! He also ran 41 several times in tournaments on that ski :D

 

Weight is pretty far down the list of important design characteristics... ;)

[Horton]

@AdamCord‌ they're not going to convince @eleeski‌. He worships at the Church of Skis must be light as possible

 

It is like that old joke about teaching a pig to sing.

 

 

[eleeski]

@Horton‌ Who are you calling a singer?

 

Obviously weight is not the most important feature. Waterskis must float by rule so they can't be TOO heavy. And 8 pounds is less than 5% of my body weight. Realistically, the ski's weight is a minor factor relative to the forces involved. Still, for performance I don't want to carry any useless weight anywhere - either on my belly or my feet.

 

Modern composites allow the strength requirements to be met at a greatly reduced weight. I agree that the last pound makes little difference but the first four is probably noticeable. Since I have never seen any lead bolted onto a ski, I don't believe that extra weight is performance enhancing.

 

If my ski doesn't break, it's too heavy and I will remove material on the next build. Hallelujah!

 

Eric

[DanE]

@eleeski‌ HO just bolted down some metal on their latest tournament ski.

[mwetskier]

@eleeski -i get your idea that a very light weight ski performs ' better ' because its lower mass makes it more responsive to input where as a heavy ski may be a little sluggish or slower to respond to input. but one thing you may not be considering is that a ski is incapable of filtering good input from bad input so anything a super light weight ski does to amplify good technique it can also do to amplify bad technique.

 

no doubt a very light weight ski probably benefits a highly skilled skier to some extent but i think the principles that make that true *also* dictate that a heavier ski may benefit a less skilled skier by not making minor errors greatly affect how the ski reacts. to me a good analogy would be a high end sports car vs a big old 4 door cadillac. not every driver is capable of handling a sports car and unskilled drivers get killed all the time trying because the highly responsive car amplifies their small blunders and sends them to their doom. the same might be the case for highly responsive skis though hopefully with less dire consequences.

[thager]

@eleeski‌ Your competition is very happy you use your own homebuilt skis. Keep it going!!!

[Horton]

@thager‌ best post ever!

[eleeski]

Sorry about the thread hijack. I enjoy the engineering of light weight. That's my problem. Adam has his personal drivers and it's fine if weight is not one of them. I will give him a hard time if he has to add floats to keep the ski from sinking but he'll still outski me...

 

Eric

[Horton]

Love you @eleeski‌

[AdamCord]

@eleeski this ski I'm building should be around 3.5lbs if my calculations are correct using this core material, so I don't think it'll be a deal breaker.

[AdamCord]

So my fears about the wax I used were confirmed...some of the paint that was on the plug stuck to the gel coat on the mold surface. I'm guessing the fact that it was ~35 F in my garage when I put the wax on had something to do with that. Next time I'll use a true mold release or bring the plug inside to apply the wax.

 

 

Luckily the paint is soluble in acetone while the fully cured polyester gel coat is not, so all it took was a good helping of acetone and a little elbow grease to get the paint off. I trimmed the edges with a jigsaw.

 

 

There were a few spots where the gel coat chipped that I'll need to fix before moving on to the next step so I'll get some photos of that repair process.

 

The surface finish is pretty good overall but it'll still take a little wet sanding and polishing to get the "shine" where I want it. I'd much rather have the skis come out of the mold shiny and then be able to add texture with sandpaper if I want than the other way around. Smoothing/polishing skis sucks, I have better things to do with my life. B)

[eleeski]

I've had better luck with the mold release when I didn't buff it out at all. Makes the finish kind of gross to deal with. Carb cleaner and a couple paper towels removes enough wax that it sands properly. Spray mold release hasn't worked for me.

 

I hope you used gloves and a respirator with the acetone.

 

3.5 pounds is fine - unless you add double Wileys.

 

Beautiful mold. A work of art.

 

Eric

[AdamCord]

**WARNING** Nerdy Content To Follow

 

So let's talk about strength vs. weight on skis. This could probably go into a different thread so maybe we can get @Horton to move it later.

 

In my skiing career I've broken a LOT of skis. I for some reason was blessed with the ability to pretty much break skis on command, which is not as much fun as it sounds. I'd guess I've broken maybe 15-20 skis in the last 10 years. It's not fun and it ruins your set at best, worst case you injure myself. I've been pretty lucky to only have gotten a few sprained ankles, but I would very much prefer to build a ski that's indestructible. The Elite skis we did at Obrien were basically indestructible, and to this day I've never heard of one breaking, so I'm proud of that. :D

 

When it comes to a high end ski there are three basic materials that all skis share in common. Those are carbon fiber, epoxy resin, and a foam core. You skimp on any one of these and you're going to have a light ski that's just not going to last long. I might as well mention now that "breaking down" is really a thing, and skis that are very light will tend to do that more quickly too.

 

So let's run through a quick example. This is an estimation for the ski I'm going to build. I've pulled this data from the computer since I've designed it in CAD as well.

 

Ski volume: 220 cubic inches / .1273 cubic feet / 3.6 liters

Ski surface area: 813 square inches / .6273 square yards / .5245 square meters

 

The foam I plan to use is 8lbs per cubic foot. A normal production ski will run a foam that is ~5-6lbs. I have to run a heavier foam since I use PU foam and not PVC, which has to be machined and can't be cast. In order to achieve similar structural properties to PVC, the PU needs to be more dense.

 

So I know my core will weigh: 8 X .1273 = 1.02lbs

 

As an estimate I'm guessing I'll use 2 layers of unidirectional carbon fiber and one layer of a weaved carbon fiber on both the top and the bottom of the ski. Each layer weighs 6oz 12oz/square yard. This is a guess right now since I'll need to adjust it to get the flex right, but this will be close.

 

So I know my carbon will weigh 12oz X 3 layers X .6273 square yards = 22.58 oz or 1.41lbs.

 

Now with resin infusion, which is my method of choice, one would normally estimate about a 66/100 resin to fiber ratio. Because we are building with a core, however, we will use more. That's because some resin will be used to create a strong bond to the core. To account for that we will assume about a 70/100 resin to fiber ratio by weight.

 

So I know my resin will weigh 22.58oz X .7 = 15.81 oz or .99lbs.

 

Therefore the basic ski will weigh 1.02 + 1.41 + .99 = 3.42lbs

 

Now that's before including things like inserts and graphic sheets, which would add a few more ounces (I'm not going to use any graphic sheets).

 

So, you ask, how could some production skis be so much lighter than this (2.5-3lbs)? Well let's see...

We know they will use a lighter core. The lightest you can feasibly go without compromising shear, compression and tensile strength is about 5lbs / cubic foot using today's tech. That would be either PVC or something similar to it like Gurit structural foams. When a ski flexes there is an incredible amount of shear across the foam core, so using a light or weak core is not a good idea if you want the ski to last.

Assuming the carbon and resin are the same as in my example and the ski volume is the same, the 5lb/ft^3 core will weigh 5 X .1273 = .637lbs, which saves you .383lbs. So my ski would weigh about 3.04lbs. BUT the less dense core will soak up more resin because there is a bigger gap between the "cells" on the surface. So if we estimate 1oz of extra resin to account for that, the real weight is 3.1lbs.

 

So the core gets us closer but not close enough. I guess we'll have to remove either carbon or resin or both. If we remove carbon the ski will get softer. The only way to keep the ski stiffness up while removing carbon is to make the ski thicker. Of course that means adding more volume back into the core so that weight will go up. We could also use a different process like prepreg instead of resin infusion. Prepregs with such a thin laminate will be around 55% resin content. You can buy lower resin content prepregs but they are designed for thick laminates where there are lots and lots of layers, like what would be in an aircraft spar.

 

At 55% resin content our resin weight would be 22.58oz X .55 = 12.42 oz or .78lbs. So that's a nice reduction but it doesn't account for resin used to bond to the core. That is about 2oz so our weight is back up to .905lbs.

 

So with the lightweight core and prepreg, our ski weighs .637 + 1.41 + .905 = 2.95 lbs before graphics and inserts. Adding in a few ounces for those get us back up over 3lbs.

 

I could go into detail about failure modes, shear stress, fatigue, temperature effects, etc but I won't torture you any longer. Anyone who's still reading at this point deserves a medal (and is probably a nerd :p ).

 

So basically my point is this. Any ski that is built with carbon and a foam core that weighs much less than 3lbs is either very soft or small, or it WILL either break or break down quickly. Fully grown men don't need to be riding a ski like that. I'm not trying to bash any manufacturers, just to educate the consumer about what they might be spending thousands of dollars on. Also I wanted to show @eleeski where I'm coming from ;). There isn't any magic material being used in water skis that allows you to shave significant amounts of weight, so any marketing you hear claiming otherwise is bull. :#

 

[Stevie Boy]

@AdamCord so basically, we should ignore the weight of a ski when looking to purchase and not pay loadsa dollars, just to go lighter, it might not be value for money, really good explanation.

 

I will look forward to the info about flex and how to achieve the sort of flex you might be looking for, I have to say Flex is a mystery to me, it's the one thing I have not got into yet.

[eleeski]

Adam, excellent analysis. Thanks.

 

With bindings that can weigh a couple pounds each, heavy wetsuits and the weight gain kids inevitably cause, going after a few grams of weight loss on a ski is silly. But since I am prone to ridiculous ideas, I can see some wasted weight in your layup.

 

I use only one ply at the front end of the ski. The only load there is from a handle ding. And weight at the tip seems to be most noticeable.

 

I spot reinforce with unitdirectional strips - not full cover layers. I also spot reinforce around the inserts with kevlar (I hate kevlar structurally but it works well with the inserts). Lots of cutting and hand placement for a small weight effect.

 

When I used a lighter core, I strategically slit the core and wedged some glass in the slit to essentially make a reinforcing rib in the core. While I'm using the 6 pound PVC now, a mixed density core is one of the experiments I'm planning.

 

Lastly, I use boron fibers on the top skin. Boron has been around for decades and is known for its strength in compression. Ridiculously expensive for the amount of weight saved.

 

I work hard on keeping my binding weights down. Nascar drilled out plates or no plates at all. Light Intuition liners in either light hardshells or Radar boots. Thinking of the whole package.

 

I don't often weigh my skis so I can't say exactly how much each step saves. But my trick ski with binding (no liner) is close to a factory blank weight. Of course, "I can't sell a ski like that" is a valid observation - and I don't try to sell my skis.

 

@rawly‌ donated an Elite to UCLA's ski team. Nice light ski holding up well to the abuse of being a team ski. But @Than_Bogan‌ gave me a "broken down" Goode that I felt skied well and is UCLA's other abused team ski. Both skis are reasonably light and durable. Way better than the old heavy and inferior VTC they replaced.

 

Magic IS available through hard work and expensive materials to reduce a ski's weight. Claiming you will ski better because of tiny weight differences is "bull" for sure.

 

Eric

[Jordan]

Quite curious, given your description of the importance of the core in creating a strong long lasting laminate, (and the lack of importance of weight) why you made a hollow ski when you were at Mapple?

 

Obviously there are weight savings available, and other ways to increase strength, but I would be interested in your thoughts.

[AdamCord]

@Jordan it's not so much about the core but about the ability to stabilize the top and bottom skins of the ski. If you're using a foam core it better be strong and the carbon laminates need to be well bonded to it. Otherwise you'll get things like localized buckling or distortions in the surface that compromise the structure.

 

This can also be done without a core by using supporting ribs. This is also the way the Warp is built. You have to run a slightly thicker laminate so you gain some weight there, but you can use very lightweight ribs that are much lighter than a foam core. In the end a hollow ski can be a bit lighter than a ski with a core. My main reason for not building hollow skis at this point is because they are just really hard to build, not because of a strength issue. Using a core just makes it way easier.

[Chef23]

@AdamCord is there a benefit to a PVC core vs PU core besides weight?

[Dirt]

@Horton @AdamCord I have really enjoyed this thread. It has been very educational. Have you thought about crowdfunding this project? I would be in for $20 to see the project continue. Maybe a small BOS sponsorship. @AdamCord please send me your paypal info.

[Horton]

Ooooo lord now @AdamCord‌ is back in the ski business

[AdamCord]

@Chef23 some of the better PVC type cores like Gurit foams are better structurally and also they are dimensionally more stable in extreme temperatures.

 

@Dirt I appreciate the offer! I am willing to accept payment in beer when we meet :)

 

@Horton NO.

[JAS]

@AdamCord‌ A new ski design touts dampening with metal plate. Considering the relatively stiff nature of waterskis and the constantly changing load dynamics, merit or hype?

[skiray]

I agree with @Dirt. For me this is the best thread ever. I can not wait to build a ski. Not to build a better mouse trap. I'm planning on sticking with my Mapple. I just want to build one to see what I can learn from it. (And it's going to look great on the wall) thanks @AdamCord. If you change your mind, I'm all in on a donation. Think of it as a diaper fund.

[Horton]

Yes @AdamCord‌ take donations. Set the money aside to pay for the ski you are going to build me plus the darn electric mountain bike you never built me.

[eleeski]

@Chef23‌ Urethane cores tend to break down when flexed. Urethane crushes to powder. So a properly engineered new ski can change and either break or flex improperly. While it is an excellent core material it doesn't last forever. My urethane cored skis didn't last more than a year of hard use failing with core delaminations.

 

PVC will flex many more cycles without changing. PVC is much more resilient. I was able to use a lighter layup. The skis would break when overloaded but that was not time dependent. Typically the PVC core would break with the top skin.

 

When Clark foam quit supplying urethane core, PVC foams replaced urethane in many applications. For me, that was a good thing. Still, the honeycomb filled with casting foam (urethane?) has always intrigued me. Hopefully Adam will give specifics on his casting foam.

 

@JAS‌ I remember when most skis had aluminum molded in. While metal in a ski can work well in a ski, whether it's needed to give a real advantage on the water is debatable. Marketing for sure is involved. I don't plan to add metal to my skis. Hmm, maybe boron aluminum...

 

Eric

[AdamCord]

@JAS quick story: 4 or 5 years ago all the wakeboarders were going on and on about how adding this certain wood stringer to their cores made the boards "better". I think the wood was paulownia. I built two identical skis, one with a normal Divinycell core, and the other with 2 wood stringers built into the core running tip to tail. The skis had identical layups and flex #s. The only obvious difference was that the ski with the wood was WAY less dampened than the normal ski. If I held the ski up with two fingers and smacked it with the palm of my hand, it would vibrate like a tuning fork for a loooooong time. The normal ski was much more dampened and if I did the same thing the vibration would die out almost immediately.

 

I set the fins up exactly the same way and took them to the lake. Then I rode them back to back to try and see what the difference was. At my opening line length when the water was glass and I wasn't pushing on the ski at all I couldn't tell a difference. But if there was even a slight texture to the water or if I was at a shorter line length and pushing on a ski there was a BIG difference! The ski with the stringers would do this really funky rebound at the end of the turn and the tip would shoot up out of the water. I was at least a pass worse on the undampened ski, if not more. I was really surprised at the difference.

 

So to answer your question: Is there something to increasing the dampening properties of the ski? Absolutely. Do I think the metal weight will help? I really don't know. It certainly could but it would have to be designed right. The way to find out if it works or not - pay attention to their pro skiers. If they use it, it works. If they take it off, it's hype.

[AdamCord]

Ha thanks @skiray I'm glad you're enjoying it.

 

@Horton this may be the one ski on the planet that you can't have for free ;)

[ForrestGump]

This has been very interesting. Mike Munn and I were chatting earlier this spring about building some skis based on his favorite ski of all time.

 

BTW... congrats on the wee one, Adam.

[Chef23]

I could see how a wakeboard that was less damp might be a good thing if it helped you get more spring and height off the wake. That obviously wouldn't be a good thing in a slalom ski.

[ALPJr]

Very interesting thread. Does anyone know, or can anyone say what processes and or materials allow Connelly skis to be durable enough for a lifetime breakage warranty?

[AdamCord]

@ALPJr does Connelly offer a lifetime warranty? That's pretty impressive.

 

Assuming they still make their skis the same way (last time I was in their factory was 2011), there are a few things they do that make their ski more durable than others. First off they use some fiberglass in the layup. I'm not sure what there ideas are behind it but I know that it helps with a few things. First it will help with impacts as fiberglass is not as brittle as carbon fiber. Also the laminate being thicker will drastically reduce chances of the ski breaking via buckling, which is by far the biggest cause of ski breaks.

 

Also they use a different bottom material. Most companies print their graphic on a polyester fabric, then soak that in resin and lay it up with the rest of the ski. There's nothing wrong with that method and it works well. What Connelly does differently though is they actually print the graphic onto a thin sheet of plastic, then they vacuum form it to the shape of the ski. That plastic then goes into the mold before the ski is pressed. So there is basically a protective layer of plastic on the bottom of the ski similar to a snow ski. It all adds a little weight but it's pretty cool how long those skis last.

[AdamCord]

Ok I managed to get a little work done. Thanks to my wax problem that occurred because of the cold temps I had one spot where the gel coat chipped pretty badly at the back of the ski cavity. It's actually pretty easy to fix:

 

I didn't get a good shot before I started but it was the back left edge of the ski where it rounds to the tail. I used masking tape to mask the area and to also act to reshape that area:

 

Then fill the area with more gel coat:

 

 

Once it's cured I sand away the excess:

 

And voila! Like it never happened

 

So the next part is going to be tricky. Casting the core. I've never done this without the assistance of a press before so we're stepping into the unknown here. I have a plan though and I've consulted @adamhcaldwell who is the master of DIY cores, so hopefully it works out. I'll document it either way ;)

[swervelake]

How do you do resin infusion with no transfer media in the mold? is there enough space between the mold and core that you do not need any resin transfer media and peel ply? But then again I could be misunderstanding something. Thanks for this thread, I've been making trick skis and will most likely build a slalom or two this winter.

[AdamCord]

@swervelake I'm planning to use some clever placement of flow media and feed lines outside of the part to draw the resin flow front down the length of the ski from the tail to the tip. By putting the feed spiral tubing on the sides of the ski I am theoretically only flowing 1/2 the width of the ski. The resin will flow from the sides and meet in the middle. By taking advantage of the fact that the tail is narrower than the tip and by placing the spiral feed lines in a "V" shape on the mold, the resin will meet first in the tail. Then that "meeting" point will move up the ski from the tail to the tip, theoretically trapping no dry spots. Clear as mud?

[kstateskier]

intriguing...

[swervelake]

@AdamCord‌ Ya that sounds like it may work! thanks, I'm in snow ski building mode right now but as it starts to warm up i will begin this project.

[AdamCord]

Yeah I can't take credit, this is @tap's idea. I plan to blame him if it doesn't work.

[tap]

The physics are sound. If it all goes south I'm claiming operator error.