can someone explain something to me

Discussion in 'Boat Design' started by boatenthusiast, May 13, 2016.

  1. boatenthusiast
    Joined: Jul 2015
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    boatenthusiast Junior Member

    Marksman27g.jpg

    above i have attached a image of a boat sitting basically on the ground,i found this while looking at boat plans from hartley. can someone explain to me how they move this boat to the water, it perplexes me on so many levels, i just cant figure it out, someone care to tell me how they would start to move this boat?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Any crane operator would have the problem sorted pretty easily. Those wooden blocks it sits on allow room to slip some straps under it.
     
  3. boatenthusiast
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    boatenthusiast Junior Member

    Thanks, Super Efficient post.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

  5. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    Give me some rollers, planks of wood and a come-along and I'll get that boat into the water by myself

    Launching is always easier than it looks. But tell helpers "it's my boat we will do it MY way"

    two of my own boats shown here

    What always perplexes me is how babies are born....

    RW
     

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  6. moTthediesel
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    moTthediesel Junior Member

    Two railroad jacks and a bunch of blocking.....
     
  7. Barry
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    Barry Senior Member

    I would be worried about supporting a boat that might weigh 15000 pounds on a couple of rollers. Say at some point the rear roller sees 2/3 of the weight or 10,000 pounds and this load is concentrated on the top tangent point of the roller and the curve of the keel, the stress becomes extraordinarily high.

    If you have a 10,000 pound load working over 1 square inch the stress is 10,000 psi. If you cut the area to 1/4 square inch the stress is 40,000 psi. When you have a round roller contacting the curve of the hull, theoretically you would have an infinitely small area and the local stress could climb dramatically. Drop the area to 1/8 square inch and the stress climbs to 80,000 psi


    The theory of this type of contact (Hertzian Contact Stresses), ball bearings being an example, is that there is enough elastic deformation at the interface to increase the area which reduces the stress (psi) to allowable limits. If the material has a very high modulus of elasticity, ie does not deform much with a load applied, there might not be enough deformation to keep the stress within limits.

    Without any deformation, the area of contact will be almost molecular, ie infinitely small and therefore the stresses infinitely high, and above failure of the material

    A quick overview of the theory is below.
    https://en.wikipedia.org/wiki/Contact_mechanics

    A calculator to run some numbers

    http://www.amesweb.info/HertzianContact/HertzianContact.aspx

    I am not saying that you could not do it but there is a chance that the hull could see localized damage, gel coat cracking, delamination that might not be evident.

    Slinging would be the safest way so long as you knew where the hull could take the weight.
     
  8. Barry
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    Barry Senior Member

    I ran some numbers through the calculator but had to guess at some due to the fact that the diameters of the hull keel and roller are not known, or the type of roller and the strength of the fibreglass and the localized stress is about 90,000 psi.
    Which is probably above the limit for a fibreglass keel
    Rough numbers, quite a few guesses, but even if I am out by a factor of 2, you can see that the localized stress is quite high and that is the point that I wanted to make
     
  9. WestVanHan
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    WestVanHan Not a Senior Member

    1 person likes this.
  10. PAR
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    PAR Yacht Designer/Builder

    It's unlikely they used a travel lift to place that boat, but instead a forklift, instead.

    [​IMG]

    They have specialized lifts for this (shown above) or just extensions on a regular fork lift. If you look at the image posted, you'll see the lumber the Hartley is sitting on, is placed well inboard, ideally under the bilge stringers or engine beds, so the forks can slide under and lift under the outboard bilge stringer or chine logs.
     
  11. whitepointer23

    whitepointer23 Previous Member

    Its ply not glass. Rollers are very common way of moving boats. We used to use a heap of inflatable rollers . As long as there are enough rollers its fine.
     
  12. Barry
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    Barry Senior Member

    Inflatable rollers would work as they will deform to carry the weight. And because they deflect due to load, you can put many in line, as the videos showed, and all of the rollers would carry some load and even things out.

    A 15,000 boat with 2 ridged rollers could result in damage.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Rollers aren't a safe nor reliable way of moving things. Some situations they're helpful, but usually on difficult surfaces, like a beach. Things tend to get away from you with rollers too, particularly on inclines or very uneven surfaces. Also with the boat sitting on those longitudinal bits of lumber, you'd have to jack one end of the yachts to remove them, or you'd have to deflate them and pull them out, before they got pinched under the boat. If you only had to move it a couple of times, maybe some rollers, but look at that building. High ceilings, tall stacks of wood, pallets stacked 15' high, likely needing a forklift to place them there, so I'll bet on the forklift instead.

    [​IMG]

    A regular forklift with fork extensions, very common. I've also seen skid loaders pick up boat and it's trailer sideways in one shot, to move them around the yard. Insurance companies hate forklifts, because of the damage they can do in the wrong operator's hands. I'm pretty sure an insurance company would deny a claim, if rollers were used.
     
  14. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    Barry

    No thats not how I would do it. When I said "planks" I meant something like 20ft x 8in x 2in

    Its a bit of a fiddle jacking it all in place but once done you have: on the ground two long planks, on those the rollers laid across them. Then two more planks on top. Then the boat, well supported by chocks and sitting on the top planks exactly as though it was on a concrete floor, so no excessive point loads

    in effect I have made a roller bearing slide. I can then push the boat along with my hand. I have found that it is tricky to stop it moving because of the momentum

    of course I'd really need two sets of planks each longer than the boat so I can keep it moving without rejacking

    thats how we launched the boat shown in the first photo above. In this photo you can see the boat in our garden a few hours earlier. In the back ground you can just see the water behind the trees, about 10ft below a small cliff. We had to pull the boat out backwards, turn in 90deg and then slide it over the bank. All done by four people (2 men 2 women) no cranes, forklifts, heavy lifting needed. No damage - and the only stress was in our heads

    that has to be how they built Stonehenge and the pyramids. The films showing hundreds of slaves pulling on rollers just sitting in sand is not how to do it!

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

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  15. rwatson
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    rwatson Senior Member

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