Building a boat around 18hp.

Discussion in 'Boat Design' started by Deadeye, Jan 10, 2012.

  1. Deadeye
    Joined: Jan 2009
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    Location: BC, Canada

    Deadeye Bender of Nails

    Hi all,

    I have recently come into a restored 1960 Evinrude 18hp FasTwin.
    I am looking at designs to build to hang it on. In the usual conflict of design requirements, I am looking for a megayacht that will burn a thimble of fuel per hr and do 40kn, :p.

    What I would like to build is a fishing boat with a cuddy for solo weekending with an occasional passenger for day trips. I am on the west coast of BC, so the nastiest seas are blocked by Vancouver Island and I am prudent enough to run for cover when the weather kicks up.
    I am not expecting anything faster than a few knots while trolling off of downriggers so a trawler hull's low power and seaworthiness are a big plus.
    I am comfortable building in both glass and plywood.

    I don't have enough experience with all of the myriad designs out here, so I thought I'd ask for your opinions.
    Thx
     
  2. gonzo
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    gonzo Senior Member

    If you want it to plane, the rule of thumb is 40lbs per HP. That gives you 720lbs loaded.
     
  3. rasorinc
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    rasorinc Senior Member

  4. Submarine Tom

    Submarine Tom Previous Member

    "A few knots", 18Hp = displacement speeds, so any old fish boat will do but your seating may be limited. Are you okay with WOT on that motor?

    -Tom
     
  5. Yobarnacle
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    Yobarnacle Senior Member

  6. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    You'll have to look for a pretty lightweight and a rather low-deadrise design. Assuming a 5 m boat (around 16 ft), you'll have to keep the total weight of the vessel (hull, motor, crew, gasoline, provisions and various deck items) at or below 500 kgs (1150 lbs). And even then I don't believe you'll manage to get more than 16-18 kts at WOT. Of course, that's only if the old Evinrude has all 18 horses still alive in the barn, which is not guaranteed... :)
    Imho, you need a RIB.
     
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  7. Yobarnacle
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    Yobarnacle Senior Member

  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    This one claims 42 km/h (23 kts) with the same motor. But it is a very simple, lightweight and open-deck 14 ft skiff, probably around or less than 250 kg (500-550 lbs - Gonzo was pretty close) with one person on board. A boat intended for protected waters only. I wouldn't venture too far from the coast with that one (a RIB is safer):
    http://www.youtube.com/watch?v=gkFnMrDZarw&feature=related
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

  10. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Its only 18hp. Id Build a Garvey or a Cheasapeake crabbing skiff or a Sharptown barge.

    I had a Johnson Seahorse on mine.

    Paint her white with a green strake.
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Build a RedWing or something similar. I have a few displacement cruisers that also fit the bill easily. As far as a full plane mode cruiser, well you'll probably need more power then that old 18 HP will provide, but you can putter around on the 18, until you save up for a 40 HP, which opens up your design options considerably.
     
  12. philSweet
    Joined: May 2008
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    philSweet Senior Member

    An 18hp of that vintage shouldn't have any problem with a 16 foot skiff. They rated differently back then. I've seen kids waterski behind a '57 10hp Johnson. I like the Redwing also, but I would be tempted to put a warped plane V bottom on her and get her to plane, at least when just one is along.
     
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The whole back half of RedWing would need to be redesigned to make it plane and frankly this is way over the top for the average back yard builder. There are plenty of putt-putt designs and small (less then 20') warped bottom cruisers to select from, without butchering up a displacement hull in hopes it might plane. An 18 HP of that vintage might produce as much as 25 HP, but also considering it's age, is probably well worn, so 18 HP is likely a stretch for it's actual output.
     
  14. Deadeye
    Joined: Jan 2009
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    Deadeye Bender of Nails

    Thanks for the great replies !
    I'll get through them in the next little while - there are so many designs out there that it's good to have some suggestions to help narrow it down.

    I was in a bit of a rush this morning, so here's an example of the motor I need to put a boat in front of:
    http://www.youtube.com/watch?v=zINg415St9Q&feature=related
    Mine has the square fiberglass cowl and I think it's the last year they ran at 24:1 instead of 50:1

    Exactly !

    Given that I live pretty close to where I like to fish, I see myself running flat out for maybe an hour or two and the rest of the day spent trolling at a few knots, so that's where I'd like to be running all day at around 80% throttle.
    My thinking is that ought to let me push a bit bigger boat than one would normally hang an o/b this size onto since I'm really not looking for what most other guys are.
    That increased size ought to let me get a cuddy and a weekend's worth of gear in (for one). Now, I have NO problems with 'spartan' to keep the weight down - I'm looking for a dry place to sleep up off the sole and out of the wind. The hypothetical BOM for this (based on my past builds) will probably be something like DF marine ply for the hull and foam core for the cabintop.

    That 21' Redwing looks like about the best candidate I've seen so far - I'm a sucker for 'traditional' boats.
    I just downloaded the study plans and they're printing as we speak...

    Thx again all.
     

  15. Wynand N
    Joined: Oct 2004
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    Wynand N Retired Steelboatbuilder

    On older motors the HP was derived at the flywheel compared to modern motors rated at the propeller.
    This is the very reason that a newer engine will outperform a 70's or earlier engine of the same HP by far...
     
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