I had done my Beta testing in the prior post and was now ready to finish the tank to blend in better (visually) on the flying bridge. I finished the holes for the "inspection ports" by epoxying an additional thickness to hold the screws. Then I painted it. Because the weather had cooled off for the year, the paint (Total Boat Wet Edge) took over a week to dry. Grrrrr. I then put a patch of KiwiGrip on the section that will be stepped on. The inspection ports were installed and it was raised onto the flying bridge.
As I said in a prior post, the large inspection ports were mainly because I needed access to the inside of the tank to finish construction using epoxy fillets. Still, it is nice to be able to really look inside and see what the liquid is doing. I'm hoping to do a video so that people can see the liquid in action.
For liquid, I began with water, of course, but wanted to experiment with additions to alter the viscosity. I started with about 160# of fresh water (a little under 20 gallons or 70 liters). This is a little less than I had in my antiroll bag experiment and less than in my first trial with this tank. I think that the finished tank weighs about 40 pounds (I didn't weigh it), so the total weight on the flying bridge is about 200# (90kg).
To begin with altering the viscosity of the water, I used some HPMC powder (hydroxypropyl methylcellulose). This is a weird "food-grade" powder that turns into a slime when mixed with water. Used in some food preparation, it is also used in cosmetics. Very slow to dissolve, it then turns from cloudy water to become a clear "suspicious looking" water. It forms a thixotropic liquid, meaning it has some odd characteristics. If mixed in a high enough concentration, it forms a sticky gel which becomes less viscous when agitated.
Weird as that sounds, you've experienced this yourself. The common example of a thixotropic fluid is ketchup. Turn the bottle upside down and nothing comes out. Shake or wiggle the bottle and the ketchup becomes liquified such that it will pour out. The same is true with some paints (like the stuff I just painted on my antiroll tank. Thick on the brush, but when applied (with a shearing force) it becomes less viscous and spreads easily until the shearing force stops. Then it can stick to a vertical surface without sagging. Folks who use an air sprayer love paint with this characteristic. Anyway, my mixture does stay thicker than water even when agitated, so it will slow down the wave action despite impermanent viscosity.
Just as interesting are the viscosity changes in a liquid based on temperature. Most of us have experienced this with the oil in our car engines. Started up cold, the oil moves slowly and oil pressure can be high. Get the oil hot and viscosity drops, sometimes lowering oil pressure . . . sometimes too low. Water does the same thing. When you pour ice water into a cup, it is five times the viscosity of your boiling-hot water for tea. Even though there is a measurable change, it's not too noticeable. For my tank, I am looking for a viscosity change many times that of simple cold water, but interesting that the viscosity will change from a hot day to a cold day. Will I notice? Probably not. Based on internet tables for viscosity of various liquids, I'm probably looking for something in the range of commonly available light cooking oils.
To begin using the HPMC thickening powder, I added approximately 1/4 cup per gallon. I thought that wouldn't do much, but I was impressed. I didn't buy an expensive viscometer ($2,000). Instead, I bought something similar to a Zahn cup ($8), used to compare the viscosity of liquids, but without an exact viscosity measurement (usually measured in centipose or cP). Dip the little cup into fresh water, lift it out and time the draining through a little hole (10.4 seconds). Dip it into my HPMC mixture, lift it out and time (11.3 seconds.) I could tell the mixture was thicker, but not measured in centipose. One can then use that timing to repeatedly prepare the same viscosity.
Before adding HPMC to the tank, I made a little test batch (which is when I first measured viscosity). Half a spoonful to half a liter. Mixed, mixed, mixed. It doesn't dissolve fast, but it does finally dissolve. It dissolves slower if just left alone (like hours or days). When I poured the HPMC powder into the antiroll tank, I thought I could either stir constantly for 9 hours or simply come back in a few days. I chose the later. More details on my antiroll "slime" when I visit the boat again.
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