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Thread: UHMW Thread

  1. #1

    Default UHMW Thread

    Thought I'd start a new thread...
    Kevin Turner question
    In an earlier post you said,
    "...did one hull have full or partial UHMW (B/C UHMW will suck ya to the water until you get the nose lifted),..."
    I am wondering how you got that info? The friction co-efficient of UHMW, dry or wet, is substantially less than aluminum, and it is 99% buoyant.
    UHMW added to an existing hull could change hull dynamics. The longitudinals/stringers and lifting strakes might not protrude as much therefore being less effective in providing lift. Keep in mind that UHMW weighs in at 2.6 lbs/sq ft at 1/2" thick. A boat with UHMW on the bottom measuring 6'x18" at 1/2" thick adds 280 lbs to the deadweight.
    I am always looking at ways to use UHMW. Any feedback is appreciated.
    Water is retentive and clings to smooth, curved surfaces. I’ve found hulls with glue on UHMW had a difficult time breaking the waters grip and were slow to plane. A couple of quick port/star turns would shear the waters grip and allow the bow to rise. Once free, it was like a different boat.

    A simple test: Gently dangle a kitchen spoon between your fingers. Now, allow the dangled spoon to barely contact a stream of flowing water (from the sink). The water will wrap itself around the spoon and suck it deeper into the flow.

    The same thing happens with differing hull designs. With no strakes to shear the waters grip, a long sweeping bow with UHMW will stay firmly planted to the water surface until the magical point of release. Once released, she’ll perform much better for ya.

    Still not with you Kevin
    If you have 2 identical hulls, (same weight, width, length, deadrise, longitudinal, lifitng strakes, etc. even the same fat guy driving) one aluminum, one UHMW, the UHMW wins getting out of the water.
    If you are comparing a hull that has been modified, then you no longer have a valid comparison. You would have to figure out the same percentage of protrusion that exists on the original boat and make the modifications.
    We know that a boat in a light chop gets up quicker than off of glassy water. I get the retention part, but all thing being identical (which I don't see how you could pull it off) you have to also measure the drag on the aluminum over the surface area.
    Therefore I do not see how the UHMWPE is the issue here.
    I share your puzzlement...
    Of the UHMW hulls I've built or taken part in building, UHMW has had a adverse handling effect on every one.

    As stated above, if you don't shear the waters grip, the UHMW hull stays sucked to the water.

    In my 240 Sport Jet powered UHMW hull tests, moving 200lbs of sand bags from the stern to the bow, slowed the boat 8 MPH. Using the same hull design, w/o UHMW, moving the weight forward had little effect on speed.

    Another UHMW negative was handling. The UHMW only added 220 lbs to the hull, but it was sluggish & slippery in turns.

  2. #2
    Member JOAT's Avatar
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    Default Consider...

    If your theoretical boat bottom of 6' by 18' started out life as 14g aluminum (0.64" which is actually a bit heavy by many of the factory stamped-out boat standards) then the bottom piece would weigh about 100# (not including structure). So you hang 280# on there to bring it up to 380# for the bottom. (For simplicity, we won't consider any of the structural stuff you need to add when you hang that 280# of plastic on there, but I bet you add another 60-80# of bracing and hardware... if you install it right).

    Now consider if you just built the boat right to start with. Bump up from 14g to 1/8" (0.125") sheet. Now your bottom piece weighs 190# and you don't need plastic.

    If you're planning to be extremely rough on the bottom (e.g. airboats), then start with 3/16" (0.188") sheet for the bottom. Now the bottom sheet weighs 285#, which is still lighter than your temporary plastic bottom add-on. If you can wear out a 3/16" boat bottom, then you need to find a different hobby.

    I've yet to see an add-on plastic sheet boat bottom that did much good... or lasted very long in between repairs.

    Winter is Coming...

    Go GeocacheAlaska!

  3. #3
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    Default Not a UHMW Hull

    As I thought, you used UHMWPE to modify a hull. I don't doubt that some issues came up, but not simply because of the UHMW. You would have had the same issues if you added any sheet material of the same weight and thickness. (assuming it was not as rough as 50 grit sandpaper)
    Probably the only way you can measure what you are saying is on a completely flat air boat hull. One without any lifting strakes or ribs running the length of the boat. (Maybe the old Carolina Skiffs or Bayou Runners too)
    Assuming you know the weight of the sheet you'll add later, you could place that same amount of weight in the hull, dispersed equally, and start tests.
    Then when you add the UHMW you can make a true comparison.
    I have no idea the effect on lift of the strakes and or any design that runs the length of the boat. Race boats tend to try to capture air to get it further underneat the boat. It seems reasonable that anything that captures air is better than nothing.
    Again, it appears that you removed all of the engineering that helped with lift and turning when you modified your hull. Add the same amount of contour back on to the hull and see how it handles.

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    Member Xerophobic's Avatar
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    Default

    Quote Originally Posted by JOAT View Post
    If you can wear out a 3/16" boat bottom, then you need to find a different hobby.
    I need a new hobby I guess .......


    Oh and any air entrapment benefit on a hull travelling less than 60 mph would be too negligable to even measure. Its marginally helpful on most raceboats till about 75+.

    Your typical pleasure/river boat is being supported 100% by the water alone

    Cheers
    Skinny water addict

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    Default air trap

    Xerophobic, I will yield to your expertise on air under hulls. The link I've included is of the style of race boat they use around here. A 50hp motor w/ a 3 man crew moves these boats along at upper 60mph to low 70mph. That is their goal anyway.
    All of the ones I see use air traps. Talking to Captains, there goal is to ride the prop, not the bottom of the boat on the water.
    Nope, not very usable in our applications, but very cool boats. It is a blast to see these guys screaming past the house tucked and leaning into the corners.
    I hope the pictures and videos work for everybody.
    http://www.yukon800.com/photos/main.php?g2_itemId=14

    With no engineering or racing experience myself, it just seems that changing or modifying a hull, then pointing at the material as the problem, just does not make sense. With that said, I am always willing to learn new stuff about boats from all you guys.

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    Member Xerophobic's Avatar
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    Default

    Yup link worked Never seen that style of boat before, what do they call em?

    I think there may be a difference in technical terms with what they are doing . Its not so much trying to trap air as to just break surface tension. Its not so much the "lift" of the air you're looking for, just to break the tension that Kevin is talking about, two very different things. Gaining airlift at low speeds in a boat is extremely difficult

    Of course all boats want to break surface tension, its why they tend to be faster in slight chop, why float planes prefer slight chop, and also why many boat utilize stepped hulls. Unfortunatly for jetboats we want the best of both worlds, smooth delivery into the pump and broken surface tension everywhere else. Not an easy feat by any means, especially when balancing straight line and turning performance.

    I didnt mean to question what you were saying, just stating that air "lift" persay is non existant when looking at recreational boats. Its maybe 20% on a very fast high performance outboard which is nearing triple digits and in terms of high performance jetboats less than that even

    Cheers
    Quote Originally Posted by Ak River Rat View Post
    Xerophobic, I will yield to your expertise on air under hulls. The link I've included is of the style of race boat they use around here. A 50hp motor w/ a 3 man crew moves these boats along at upper 60mph to low 70mph. That is their goal anyway.
    All of the ones I see use air traps. Talking to Captains, there goal is to ride the prop, not the bottom of the boat on the water.
    Nope, not very usable in our applications, but very cool boats. It is a blast to see these guys screaming past the house tucked and leaning into the corners.
    I hope the pictures and videos work for everybody.
    http://www.yukon800.com/photos/main.php?g2_itemId=14

    With no engineering or racing experience myself, it just seems that changing or modifying a hull, then pointing at the material as the problem, just does not make sense. With that said, I am always willing to learn new stuff about boats from all you guys.
    Skinny water addict

  7. #7

    Default

    Thought I had seen or heard of about every type of motorized craft for water made.
    This was not only new to me, but looks really cool!!

  8. #8
    Member Akgramps's Avatar
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    Default

    Quote Originally Posted by Xerophobic View Post
    Yup link worked Never seen that style of boat before, what do they call em?

    I think there may be a difference in technical terms with what they are doing . Its not so much trying to trap air as to just break surface tension. Its not so much the "lift" of the air you're looking for, just to break the tension that Kevin is talking about, two very different things. Gaining airlift at low speeds in a boat is extremely difficult

    Of course all boats want to break surface tension, its why they tend to be faster in slight chop, why float planes prefer slight chop, and also why many boat utilize stepped hulls. Unfortunatly for jetboats we want the best of both worlds, smooth delivery into the pump and broken surface tension everywhere else. Not an easy feat by any means, especially when balancing straight line and turning performance.

    I didnt mean to question what you were saying, just stating that air "lift" persay is non existant when looking at recreational boats. Its maybe 20% on a very fast high performance outboard which is nearing triple digits and in terms of high performance jetboats less than that even

    Cheers
    The Yukon 800 boats have evolved many times over the years, someone would experiment w/ a new hull design and if it worked, well then next year you might see several that looked that way.
    Thats the great part about racing, and to my limited knowledge about boat racing, the Yukon 800 and the boats that participate are rather unique.
    The "air traps" were added as the hulls developed, as an experiment, it was discovered that "trapping air" under the boats does indeed create some lift and speed benefits in racing conditions.
    “Nothing worth doing is easy”
    TR

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    Default yep, pretty neat

    The boats used for racing around here are pretty cool. I mean think about it. Open river racing in a boat that you sit on the bottom of, hanging your butt as far as possible to hold the inside down in sharp turns.
    Years back a friend was on a crew, leaned into a corner in rough water, and his head hit a wave. His helmet was yanked off his head at some 60mph. He lived, but sure hurt like heck.
    Going back to the original question, I still think I am correct in that it is not fair to blame UHMW-PE for lack of lift when the bottom structure of the boat is changed. Anybody want to present a different view?
    And Xero, no offense. I am guessing at a lot of it. And sure was not meaning to be sarcastic or anything. You could be way more right than me.

  10. #10
    Member Xerophobic's Avatar
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    Talking

    Quote Originally Posted by Ak River Rat View Post
    The boats used for racing around here are pretty cool. I mean think about it. Open river racing in a boat that you sit on the bottom of, hanging your butt as far as possible to hold the inside down in sharp turns.
    Years back a friend was on a crew, leaned into a corner in rough water, and his head hit a wave. His helmet was yanked off his head at some 60mph. He lived, but sure hurt like heck.
    Going back to the original question, I still think I am correct in that it is not fair to blame UHMW-PE for lack of lift when the bottom structure of the boat is changed. Anybody want to present a different view?
    And Xero, no offense. I am guessing at a lot of it. And sure was not meaning to be sarcastic or anything. You could be way more right than me.

    No I wasnt trying to question what they are doing either. In a small raceboat like that im sure lift helps and can be generated at those speeds. Im just saying in a pleasure boat its not gonna happen, well.....maybe with a 4x8' sheet of plywood for a roof....

    lol
    Skinny water addict

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    Member alaskabliss's Avatar
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    Default

    Anybody have any info on the spray on coating for our boats?

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