weight distribution

Discussion in 'Boat Design' started by gregs, Jun 13, 2007.

  1. gregs
    Joined: May 2007
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    gregs Junior Member

    I am looking for some help. I made a post a while ago about building a floating boat lift similar to a floating dock that could be submerged and lifted again thru the use of an air pump. I made a test platform with 2- 5 gallon buckets and a shop-vac and it worked great. My plan is to use plastic 55 gallon barrels for the chambers lined up in two rows like a pontoon boat. My question is about how the weight is normally distributed in a small power boat. I have a 18' v-hull boat with a 6cyl I/O that weighs 2200 #'s empty. It is an older boat with the big straight six and a 18 gallon fuel tank in the bow. I am thinking that more of the weight is towards the rear so I may need to add extra floatation to that area so that when the lift is floating with the boat on it, it will be level. Also does anyone know where I can find reference to the lumber weights? I am planning on building it mostly from pressure treated pine and need to calculate the weight of the structure.
     
  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Your boat's centre of gravity will probably be forward of the transom by about one-third of its length. If you need a more precise estimate you could either do a lot of math or measure it, either with a load cell or by moving weights around and measuring the changes in trim.
    Depending on species, grade, dryness and treatment, pine tends to weigh from 400 to 550 kilograms per cubic metre. Take the volume of a board (length x width x thickness, all in metres), multiply by 550, and that's more or less what that board will weigh in kilograms.
     
  3. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    If you mean to build with pressure treated pine (the type you use for outdoor construction), the density of the wood will initially be close to 1000 kg/cubic metre (same as water). At least where I live water is used as solvent to carry the toxics into the wood during pressure treatment, so the boards will be wet when you buy them. Whatever wood is below water level will remain wet, but the wood above water will eventually dry out to the densities Matt indicates.
    Erik
     
  4. gregs
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    gregs Junior Member

    I came up with a couple of answers as well for the weight. I found a chart with the weight per foot of different size lumber and also a reference to 35 pounds per cu.ft. So I think that will be close enough to determine the weight of the structure. I am also thinking that it will be wise to add extra floatation (barrells) to the rear 1/3 of the lift to insure that it will support the extra weight of the engine and drive. It will be easier to install them now than later.
     
  5. Poida
    Joined: Apr 2006
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    Poida Senior Member

    G'day Gregs

    There was a post here a while ago pertaining to the use of floatation drums when lifting and may help you to find it. Having said that the search feature has never been much use to me, probably because I can't use it properly and don't want to spend the time finding out.

    Using a model like you have done is great, even when done to great lengths up-sizing can bring additional problems.

    When creating a model you should try to imitate exactly what you have in reality. For example when you pumped air into your test rig, you used a shop vac. This pumps air at low pressure and high volume and can quickly evacuate the water out of a bucket. What you rigged was more or less how an airbag works and the prefered method of lifting.

    Using drums you have to: Get them into position, dragging drums across the ocean floor is no picnic. Make sure the air in and water out is in the right position otherwise you will be pumping the air into one hole and it will be escaping out of the other. Have some means of pressure relief on the drums because as the drums rise the air inside expands and may split the drums (depends on the depth) AND make sure your boat is not in the way when the rig hits the surface as it accelerates as it rises.

    Now for your question on weight distribution. And only an assumption on my part but I feel when a boat is manufactured, it is made to float horizontal, in other words the boat is made distributing the load equally to get it to float straight. Therefore if you distribute your drums around the boat evenly it should float horizontal. This also depends on why the boat sunk, a hole or swamped.

    Anyway have fun experimenting and don't hurt yourself.

    Poida
     
  6. gregs
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    gregs Junior Member

    Great thoughts Poida. What I am actually doing is building a floating boat lift to dock and raise my boat out of the water for storage similiar in design to a floating dock. The concept is to let the air out and submerge it enough to get the boat on and off and then be able to reinflate it so that the entire boat and drive is out of the water. So all the barrels will be mounted to the frame work with a manifold connecting them to the air source. And like you said I need a high volume low pressure blower to displace the water, so I am thinking of something like a electric leaf blower or two. Thanks for the comments so far, I am working on a design for the framework now trying to make it simple and low weight. My other concern is going to be the buyoancy of the wood and what problems it may lead to with it being submerged enough to get the boat back on. I figure the weight of the boat will push it down as it pulls on top of it. My plan is for bunks just like the trailer to cradle and support the boat and may have to extend them out and down to form a "ramp" to assist in getting the boat lined up and start to push the submerged frame down. Thanks, Greg
     
  7. Poida
    Joined: Apr 2006
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    Poida Senior Member

    Well, once again Greg, I've got everything completely wrong.

    So, may I ask if this floating dock is attached to where you live ie you have a waterside property. If you lived here and had such a residence then money would not be an issue.

    Back to the problem, just thinking quickly and probably being wrong again, my previous point about equally distributing the drums I think is still true.

    We are metric here so:
    I would personally make the frame work out of 65 x 35mm x 8 metre about 21/2 x 11/2 Galvanized RHS, bolting all the components together with galvanized bolts. Make up a square framework the size of your boat, but you need the cross members spaced at the front and back so they are about 75% of the measurement of the diameter of the drums so the drums sit under them and all you have to do then is secure them to the framework. This way the securing method is not taking any weight. The width of your frame is sized to your boat as well as the length of your drums so you get an even number of drum lengths across the width.

    Only put the drums at the front and rear. Now configure the frame work as a boat trailer ie. an upright at the front with a winch and boat rollers down the centre.

    Now here's the important bit. If you try and lift the framework up vertically you will have a hell of a job locating it. I would winch it on as you would winch a boat onto a boat trailer. As the weight of the boat hits the rear roller it will naturally force the rear drums down into the water. The framework will slowly level itself as the boat makes it's way onto the frame.

    Of course if you had to you could evecuate some of the air from the rear drums but I don't think it would be neccessary. This means no pump and a lot less complications.

    I would probably secure the drums with building galvanized hoop iron. Used on timber (lumber) framed housing as wind braces.

    Well that's my thought anyway.

    Poida
     
  8. gregs
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    gregs Junior Member

    Yes it will be located at water front property owned by my inlaws and attached to their concrete seawall. I thought about galvanized steel for a framework but am still leaning towards wood. I understand the idea of pulling it up on the dock with a winch like pulling it on a trailer, but how would I get it back off? I want the whole boat out of the water. I think by being able to control the buyoancy of the dock I would be able to control how far submeged it would be which would allow getting the boat on and off the bunks easy. I figure on adding some guide poles allong the side and a bow marker to know when I am on all the way. Keep the ideas coming as I am sure there are things I havent considered yet.
     
  9. Poida
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    Poida Senior Member

    Ah getting it off, what, you want to get it off? lol.
    I knew there was a flaw in my design somewhere.

    The only thing I can think of off the top of my head at the moment is, not float it horizontal have the boat tilyed slightly astern plus when you launch, stand at the back of the boat to give it more tilt so it will roll off.

    Or if you can tilt it so the prop is in the water then drive it off.

    I just feel that letting water in and pumping water out is going to lead to problems. ie. **** getting stuck in the valves

    One of the systems used here are a frame as we have described on a rail track that runs into the water, the boat in secured to the frame then the frame is winched out of the water along the rail track.

    You don't have to use proper rail tracks, steel angle with the "VEE' upwards with "VEE" rollers sitting on that.

    Poida
     

  10. alan white
    Joined: Mar 2007
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    alan white Senior Member

    The boat's CG should be easy to determine. When the boat's in the water, balance the trailer with weight clamped to the tongue end.
    Retrieve the boat but do so by winching to maybe a foot shy of the normal position (maybe more, depending on your particular trailer positioning , i.e., boat-induced tongue weight). After putting the boat to the trailer, take it to level graound, block tires from rolling, and winch the boat slowly forward to where the receiver will jiggle free up and down, then unhitch with a friend assisting, to ensure the boat is freely balancing. The tires (or space between duals) are now under the boat's CG.

    alan
     
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