13ft, 20hp boat for light chop

Discussion in 'Boat Design' started by possum, Aug 14, 2020.

  1. possum
    Joined: Aug 2020
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    Location: Ga

    possum Junior Member

    Hello folks,

    I currently have a 9.8hp (85lb) on a Lowe 1256 (12ft by 56”, 105lb, “vee style” aluminum boat). It is nearly perfect. Of course I want more, faster, and more work/projects. It beats us up in minor river/bay chop (and I have to go quite slow), as the vee quickly transitions to essentially flat bottom. Typical use is joy rides with wife and small kid, to the sandbar or other local destinations. Not a fisherman etc.

    Please help me find an existing design, or discuss features for a new design for the following purpose and requirements:

    - Optimize for 20hp outboard, tiller, pull start (sub 100lbs)

    -Approx 13ft by 5ft max. I drag the boat up onto a floating dock for storage with block and tackle. Say 150lbs goal weight.

    -stitch and glue plywood/epoxy for ease and speed of build. I’m ok with other methods (frames/strongback) if they make this possible. Prefer no tunnel, no pad, only simple hull shape. I am fairly handy but want something simple. I’ve built a Yost skin on frame kayak from a table of offsets.

    Current boat goes almost 20mph once re-propped. Would like to go high 20s in calm conditions.

    Design elements I’m considering:

    Moderately deep vee: if a jet ski can have 22.5 degrees of deadrise, can I get away with 12-15 degrees (or more?) from transom to about the middle of the boat, and sharpen from there going forward?

    Bench seating rear and middle. Perhaps deck the bow to give some dry storage (not to stand on). Gas tank under middle seat or forward. No battery.

    I would love self bailing floor, or at least raised slats to keep stuff (wife purse, beach towels) dry, but that would add weight and raise CG. Not sure if it’s feasible.

    Garvey style may be the best option but frankly it doesn’t please my eye so I’m not doing it.

    comparisons:

    Bateau fs14 and fs14ls- flatter bottom than I want to try, a bit long for my storage space.

    Glen L bingo - 12 degree bottom is probably about what I need, but funky tumble home-like chime at the gunnl, decked/cockpit layout, remote steering, doesn’t fit my open/utility boat idea and adds weight. May need more vertical sides above the chine and more freeboard to keep water out in an open boat.

    am I missing other good designs to use?

    Is my idea totally hairbrained and impossible?

    thanks for your thoughts and feedback.
     
  2. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    https://www.spirainternational.com/study/EnsenadaStudy.pdf

    About 2.5X your target weight, but that extra weight will help smooth ride, but otherwise meets your SOR, and 2x3 frames will make it easy to add removable non-structural benches and bow covers. Engine horsepower rating is a match.
     
  3. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Last time I looked there are no 20hp OBs under 100 lbs, Yamaha just came out with a 25 at 126 lbs though.

    The old Johnson/Evinrude line had 18/20 hp motor that was I under 100 lbs. That was in the 60s.

    It will be tough getting to the high 20s because as you design the boat with more deadrise it will require more power to push it.

    It will also weigh more, this makes it ride smoother but will further reduce your speed.
     
  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Normal bare hull weight for what you describe would be about 350 pounds, either all glass or stitch and glue. The scantlings go up as you get bigger, and as you get faster. Thirteen feet is a bit small for what you describe. A fifteen footer that weighs 450 pounds fitted with a 20 hp would work well. Low twenties is what these motors are set up for from the factory. That would be with just one aboard. Probably make about 16 knots with the family in decent conditions.

    For comparison, a top of the line Kevlar and foam 17' flats boat weighs 420 pounds and does 36 mph with a 60hp motor.

    A Mako 16' skiff with a 40hp, which would suit you needs well, has a package wt of 1750 pounds.

    An Edgewater 158 CS (15') fitted with a Yamaha F 70 outboard can hit 33 knots under ideal conditions. It seems to weight about 1200 pounds and is beamier than you want, but is suitable for your use.

    If you want to get to the high 20s with a Yamaha 20, you are stuck with a flat bottomed jon boat and only then in ideal conditions. A stripped down 14' jon weighs about 200 pounds.

    So you need to rethink your weight limits and either your speed or power requirements. A 6" bay chop will keep most little boats home or off plane.
     
    Last edited: Aug 15, 2020
    bajansailor likes this.
  5. possum
    Joined: Aug 2020
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    Location: Ga

    possum Junior Member

    Appreciate the thoughts. Tohatsu and mercury (well, Tohatsu makes the merc) make pull start 4 stroke 20s under 100 lbs. They are efi, too. It’s an exciting motor which is why I want to find/design a boat for it.

    spiro’s 12 ft mini panga is 170lb, 10hp

    spiro 14 is 265Lb and 15hp (25 remote)

    bateau FS 12 is 160lb and 15hp

    bateau fs14ls is 190lb and 15-25hp, and actually 13.5ft long

    bateau fs14 is 220lb and 15hp. He says 27mph with 15hp.

    Glen l Bingo is 12’8”, 240 lb and 40hp (!!!)

    Clearly 150lbs is wishful thinking on my part if I’m going with a deeper vee than those boats, but I wonder if a 13 could be done around 200lbs since I don’t need deck structure, fish boxes, etc.
     
  6. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Just looked at the new Tohatsu (and others made by them) 20 and it is under 100 lbs.

    I had a 13' wooden ski boat when I was growing up. It had a 35hp Evinrude and did about 30 mph. You could put 4 people in and cruise along in the low 20s.

    It was a simple and light boat, but was far heavier than your target. It also had a relatively flat bottom that was typical of the time.
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Too many factors acting against each other here, to get a 13 foot boat to cruise in the high 20's (mph) with reasonable comfort in chop, you would require a deep-vee, with probably 40 hp, and a fair bit of weight, or it would be alarmingly "tippy" at rest. Something like this glass boat, called a Cootacraft Little Ram.

    Coota.jpg
     
    hoytedow likes this.
  8. possum
    Joined: Aug 2020
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    Location: Ga

    possum Junior Member

    That’s a super cool boat.

    I have no delusions about high 20s in chop, only delusions about going that fast in flat water, ha.

    again, perhaps I’m dreaming of the impossible but I know there must be something better than my current boat under 200lbs. The Hull weight limit is pretty strict since I drag it onto my floating dock for storage.
     
  9. possum
    Joined: Aug 2020
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    possum Junior Member

    Perhaps I need something more like this, which the legendary PAR (may he rest in peace) posted some time back. Not sure if there are more detailed specs or plans floating around out there that could be lengthened, or if it was intended for ply construction.
     

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  10. Squidly-Diddly
    Joined: Sep 2007
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    Location: SF bay

    Squidly-Diddly Senior Member

    how about adding a row of flotation around your floating dock? Got any pics of that? Seems like adding 200lbs of flotation would be cheap and easy, and really open up your options to cheap, durable ready made boats.
     
  11. philSweet
    Joined: May 2008
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    philSweet Senior Member

    If it's the launch/retrieval problem that limits your weight, there are tiny little cheap anchor winches that would entirely solve this problem. Cost about $200 for 100# line pull with 50' of line. Rigged to a triple purchase, that's about 250 pounds of pull over 14' of usable distance. I'm pretty sure I can get a 500# boat onto a floating dock with that. They draw about 12 amps, which is within the alternator range of the boat, but you want some minimal battery support as well for buffering, like a motorcycle battery. But now we can talk about a 325 pound hull with a 100 pound motor and 75 pounds of fuel and kit, and you can build a bay boat with that.

    small boat winch
     
  12. possum
    Joined: Aug 2020
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    Location: Ga

    possum Junior Member

    I like the ideas but unfortunately for several reasons I’m limited on what I can do to the floater. Environmental regs prevent expanding it or adding on, living along a high wake/traffic area with 7ft tide swings (heavy boat on the dock may add enough momentum to kill my pilings), and I want my girls to be able to man handle the boat in my absence.

    I can certainly switch to a strap type winch as seen on trailers to make hauling a boat up easier than a block and tackle. Plus I just love the elegance of power to weight ratios (being a former air cooled vw hot rodder).

    Regarding electric winches, my plan is a battery-free boat. On my current Alum boat I take a small lithium cell to plug into my LED nav lights if going at night, and have 2 sets of emergency nav lights on board if I get caught out late for some reason.

    I already have a 22ft center console on a boat lift but sliding a utility boat on and off the dock in 30 seconds sure beats using the big boat, so I use the little boat easily 5-10x more often.
     
  13. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    Maybe upgrade the material$ and reduce the weight of Spiro 14. I hear the general Panga concept is very sturdy work boats able to take beating hauling lots of heavy fish at high speed in choppy waters AND built with cheap, local, low quality lumber. But I wouldn't be the guy to ask as far as exactly how to lighten up the design.
     

  14. Matt Gent
    Joined: Feb 2012
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    Location: Treasure Coast, FL

    Matt Gent Junior Member

    I’m after something similar. Jon boat functionality with a better ride and more refined hull form and appearance. I’d like to be around 14-6 x 5. Also to be stored by pulling up on a floating dock.
    The Suzuki 20 is 97lb in 15” manual start version.
    I think I’ll draw my own and build it in foam, following the guidance of Chris Morejohn on the microskiff forum. At your size I think a <200lb hull is achievable. A simple boat doesn’t need all the features that the production boats mentioned have.
    Other similar boats are the Tango 14 and Salt FRS 15 and Bateau OB 15. Each may be a little bigger than you want, but you can scale or adjust the design to meet your needs. I like the concept of a little deeper V like the OB15 with some healthy chine flats for lift, transitioning to spray rails fwd.

    Another reference would be scaling down the B&B Eco18/30hp to 13.5/20hp. It has a nice chop-eating entry.
     
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