Weight/Balance

Discussion in 'Boat Design' started by tinhorn, Aug 1, 2012.

  1. tinhorn
    Joined: Jan 2008
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    Location: Massachusetts South Shore.

    tinhorn Senior Member

    So I decided this summer to pull this little red boat out of mothballs and make it work. It's powered by a 36#-thrust Minn Kota. (May upgrade to a 55# if this project really works.) Problem is, I find a lot of your boat engineering terms confusing, so I hope you'll help me out here.

    I need to calculate the optimal waterline (weight/balance, right?). I've determined 18 pounds of feet and 175 pounds of derierre (plus life jacket). The Minn Kota motor weighs 12 pounds.

    I haven't narrowed down the batteries yet--I intend to use a pair. Group 24s weigh roughly 43 lbs each; 27s are about 53 lbs; 31s appear to weigh 70-75 lbs apiece. My criteria is being able to play all day without running them below 50%. The final choice will likely depend upon the best deal I can find on Craigslist.

    As you can see, I have a lot of room for positioning batteries. I just need to anchor them so that they'll properly trim the boat. Hmm...if they were moveable and the trim line was marked on the hull, the boat could be trimmed for lightweights who might want to borrow it.

    So, in summary, is it possible to calculate the correct waterline when the boat's at rest? And the batteries would be better near the centerline than spread out, right?
     

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    Last edited: Aug 1, 2012
  2. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    what do you mean "optimal water line"? If you are trying to determine how big a battery you can install without submerging the deck, you also need to know the weight of the empty hull, the weight of the motor, plus crew, batteries and cargo. This is known as the "displacement weight", that is the weight of water it must displace for it to stay afloat. Once you know the total weight, you will have to determine the volume of the hull below the gunwale line. there are number of ways to do that but most will estimate the volume using geometry and actual measurements. If the weight of the volume of water below the gunwale is more than the total weight, you are good to go. Fresh water has a weight of 62.4 pounds per cubic foot, sea water is a bit heavier.
     
  3. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    The boat looks remarkably like a boat described in a 50 year old How to build 20 boats magazine. The magazine called it "the Thing". It went like a bat with about 10 HP. Built with two sheets of ply. Your red boat appears to be about the same but with a jazzy deck and cockpit.

    The red boat appears to be a planeing hull form. It probably needs 5 or 6 HP to get it up if it not too heavily loaded. The electric motors you mention are not going to plane the boat. So you will be operating in displacement mode and that particular hull is not ideally suited for that kind of operation. Nonetheless it will move along with either of the electrics. The operating time will depend largely on amperage draw. If you run the electrics slowly you will get a longer duration than if you try to run fast.

    About trim..... just load the boat so that it runs reasonably level and call it good. Be conservative about the weight that you ask it to carry.

    I suggest that you get the book titled: Electric Boats.... by Douglas Little. The publisher is International Marine Camden Maine. Plenty of good council in that book.
     
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  4. tinhorn
    Joined: Jan 2008
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    Location: Massachusetts South Shore.

    tinhorn Senior Member

    Thanks. I only need to go fast enough to keep up with kayakers, some of whom are young, smart-aleck hot rodders. I'll trim it to keep the front 20% or so out of the water.

    An I/O Minn Kota. Kind of funny when you think about it. Found the book on Amazon--it's on my Wish List.
     
  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Gee, that upper deck mold looks familiar.
     

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  6. tinhorn
    Joined: Jan 2008
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    Location: Massachusetts South Shore.

    tinhorn Senior Member

    1 person likes this.
  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Trimming the bow up will slow you down, not improve your speed. In fact, you'll be hard pressed to keep up with kayaks, as the motors and props you'll be using are designed for trolling speeds of about 3 - 4 MPH. A hot rod kayak can easily exceed this.
     
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  8. Sleepy Dragonfly
    Joined: Apr 2018
    Posts: 1
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    Location: Dacula, Ga.

    Sleepy Dragonfly New Member

    Hey TinHorn!

    I got your craft's sister!

    I call her "The Damselfly".
     

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  9. David J Ritchie
    Joined: Jan 2018
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    Location: Victoria BC Canada

    David J Ritchie Junior Member

    Sorry OP this won't help you much

    Wild hull lines look like very nice, looks like a spaceship

    Discloser i love electric drives but I really want to see you drop the guts of a small standing jetski in that bad boy
     

  10. BlueBell
    Joined: May 2017
    Posts: 426
    Likes: 32, Points: 28
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I agree with DJR.
     
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