How to practically measure float bouyancy

    The better floats list the lift capacity in pounds. For example the Riffe torpedo float claims 80lb of lift on the surface. Has anyone done a practical test to measure the lift capacity? What is an accurate test?


    This is what I did to test a float's lift, let me know if it's a real test. I a have brass fish scale like this one. Granted it's not as accurate as a digital scale but it doesn't get damaged by submersion and a pound of inaccuracy is not a big deal.



    I went to the local swimming pool, jumped in the deep end, hooked the scale to one of the float's D-rings, held on to the bottom rung of the pool ladder with one arm as I pulled the scale with the float attached to just below the surface. The resulting reading is the float's lift at the surface correct?


    Another question I have; if I'm testing an inflatable float which was inflated by mouth, would the lift at the surface increase if I were to inflate it to a higher pressure with a pump? Or would the higher pressure only produce more lift at depth? Remember the volume of the float on the surface doesn't increase when inflated to higher pressure.

    Two things:
    Your test was in a fresh water pool, the result in salt water will be different but you should be able to do the math and get the salt water results.
    (x lbs lift fw/wt of freshwater per cu ft) = (y lbs lift sw/wt of saltwater per cu ft) if you know 3 of the 4 you can compute the 4th
    Additional inflation should affect the compression rate as the float is towed underwater. This would increase buoyancy/lift. Additional inflation could stress the buoy and cause it to fail.

    Saltwater is more buoyant because of the weight of the salt. Sea water weighs approximately 2.5% more than fresh. Multiply the freshwater buoyancy x 1.025 to convert it to saltwater buoyancy.

    That equation is a little off. Buoyancy is equal to the weight of the liquid displaced minus the weight of the object. If it was just equal to the weight of the water displaced then everything would float. To do your calculation Dan you would set the measured buoyancy equal to the volume of the inflated float times the density of water and then subtract the weight of the float.


    In equation form:


    Buoyancy = V*D - M


    V = volume of float
    D = Density of water
    M = mass of the float


    Using that equation it would be pretty easy to calculate the lift capable from a float. Most have their volumes labeled. Then multiply by 1.02 for saltwater and subtract the weight of the inflated float.


    Your spring test would provide a fairly accurate measurement of the buoyancy of the float as long as you had everything stationary once underwater.


    Additional air pressure i.e. using a pump would slightly decrease (probably not enough to even notice) the buoyancy at the surface but would prevent the float from loosing its volume at depth which is more important than the minimal loss at the surface.

    Quote from Dan

    Thanks. Makes a 1-2lb difference with the available floats.


    What about my test, is it valid?


    yes your test is valid .


    if you inflate to a higher pressure,like 30psi instead of 15psi (assuming the size stays the same )flotation at the surface will stay the about the same ,but at depth flotation will be higher than same float that is at a lower pressure that is crushed .



    as the pressure the float is inflated with increases, the flotation will actually decrease because of the weight of the compressed air. but it will maintain that displacement to the to the atm # its inflated to


    .for example:
    a 10# displacement float inflated to 30psi (2 atm) will still displace 10# at 66'sw at 99' it will displace 5# and at 132' it will displace 2.5#


    most riffe and aimrite floats i see people use are only full ,and really dont have more than 4-5 psi in them ,if a fish takes it past 33' its gone and not coming back
    phil

    Thanks Phil. Then my DIY buoyancy test is more or less accurate. Has anyone tested floats this way and can confirm the manufacturer's stated buoyancy? I will test floats and post the results. The first one being the new Speardiver Zodiac Spearfishing float which measured 45lb of lift at the surface (saltwater), after converting from freshwater and accounting for a 1.25lb lead ballast.



    1b686.jpg


    The Speardiver float is similar to the Aimrite. It measures 45" long by 12.5" wide. For comparison the two Aimrite sizes are:
    Reef float 39" long by 12" wide - stated buoyancy 48lb.
    Bluewater float 48" long by 14.25" wide - stated buoyancy 90lb.


    As you can see the size of the Speardiver float is approximately in between the two Aimrite sizes, and has 45lb of lift, so I'm very curious to see if the reef float which smaller really has 48lb of lift. I really doubt it does.

    I would just neanderthal test it....
    Get a 50 lb dumbell and attach it to the float. Throw in pool. Add or subtract dumbells and 1 lb weightbelt weights until you determine the max floatation. It should be faily close to the math experts calculation, but I prefer to see it in action.:toast:
    I like the size of the new float that Dan is making. The smaller float is only good as a marker float and should not be used for shooting any deep diving fish like amberjacks. I watched a small Aimrite float disapear in to the abyss never to be seen again. It was attached to a custom made spear, Mori Slip Tip, and a 100 foot Riffe bungee. That was a $500 mistake:toast:

    Therein lies the problem G.R. While the Speardiver Zodiac float bridges the gap between the two Aimrite sizes, it's buoyancy is 4lb less than the stated buoyancy of the smaller Aimrite float. I find this hard to believe, the Speardiver Zodiac float is bigger, the construction is the same, so its buoyancy should be greater. I really need to test the small Aimrite float.

    Mike, The float material is a 3 layer construction; Thermoplastic polyurethane / Nylon / Thermoplastic polyurethane. This material is used in the manufacture of inflatable rafts. Thermoplastic polyurethane is referred to in the industry as TPU. The glue used in the construction is TPU specific. My experience with it has been that the bond is very strong. I decided that I will be using one float without a flag and so will not need the ballast and flag pockets. I took the opportunity to see how strong the bond is and simply ripped off the pockets. The bond was very strong and it was hard to separate the materials. As often as not rather than the bond coming apart the material broke but the layers did not fully delaminate and a run was not created in the material, I was using a lot of force. I used a dremel tool with a grinding wheel to even out the broken surface areas of the float which in some places went as deep as the nylon layer. It's clear that even these damaged areas do not compromise the float's ability to hold air. I will post a pic of this later.


    Xan, all the measurements are in the previous posts.


    I tested the 11 Liter Rob Allen hard float today. It has 19lb of lift in freshwater with a 1.5LB lead ballast. So we can round it off to 21lb in saltwater without ballast. The RA float was significantly easier to submerge than the Speardiver inflatable float. It's pretty much what I expected. The RA float will maintain buoyancy when submerged better than the inflatable float. But I'm not sure if at this low buoyancy it's a bigger advantage than 44lb of buoyancy at the surface preventing the fish from sounding in the first place.


    Video using a mask mounted Gopro.
    Rob Allen 11L Float Buoyancy Test.mp4 - YouTube


    In any case the RA float has been my go to float for a few years for boat diving. I haven't shot very big deep sounding fish. But in combination with the float line I've been able to handle everything I've encountered diving South Florida reefs. I like the RA float because I don't need to worry about it getting punctured being tossed around on the boat, and obviously I don't need to inflate it. But discovering the difference in buoyancy is making me have second thoughts. I definitely wouldn't use the smaller RA 9L float.


    A Rob Allen hard float that was reportedly crushed at 60 meters.
    1_zps909becf2.jpg

    Hi Dan!
    Do you already have this Speardiver float for sale?
    Regards
    KWS


    Well the numbers you got for the RA 11 liter float are pretty close and you would be interested to know that the difference in buoyancy between the 11L and the 9L is only about 4 lbs in saltwater. As for your float based on the dimensions you gave should produce a float that its right between the two aimrite floats. I have seen both floats in person and there isnt anything significantly heavier on your float that would change that assumption.


    I could make a better calculation of the theoretical buoyancies of the aimrite float if I had the dimensions of the bladder but from the rough guess from the numbers you gave I dont see a reason why they couldnt make the claimed numbers.


    Whats the max weight on your scale? I ask because the 20lbs seemed about 2/3 down and that would put 40+lbs at the edge of the scales range and Im wondering if your possibly getting the wrong measurement on the speardiver float.

    The scale goes to 50lb. If anyone knows of a digital submersible scale I'll buy it and do the tests again. My guess is that the Aimrite float lift capacity is overstated. But this has to be proven. These floats do not have a separate bladder. I'll say this, it was damn hard to pull the speadiver float below the surface. Maybe my scale is wrong. Do you think a 100lb scale will be more accurate?

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