Need Cat advice

Discussion in 'Multihulls' started by GUSRENTIT, Dec 20, 2020.

  1. GUSRENTIT
    Joined: Dec 2020
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    Location: Pueblo, Colorado, USA

    GUSRENTIT Junior Member

    Howdy, I'm the new guy, and very unfamiliar with Cat designs/engineering. So I will be working on a Cat that I got years ago, for free. Its a AMC SeaMoth, 15' sailboat,,, without any sail equipment, but that's ok, as I want to make several modifications, and make a motor Cat. Not knowing the how and why of hull design, does not mean that I don't know what or how to do it. Granted this is a small Cat, not something that you guys tend to talk about, BUT marine theory, should apply to almost any size. I need some basic knowledge, and some not so basic info.
    OK, 1) Why do I keep hearing that sail boats don't make good motor boats, ( most sail boats have motors ), most sail boats are displacement hulls, yes,, not all...
    2) On Cats and most other boats have the Banana hull curve, fore/aft, mainly it sweeps up at the aft, what does that do,,, what is it for ?
    3) What would happen if I took some of that rocker shape out of it by adding/building up the bottom of the aft hull, and adding about a foot to the length of the hull. I realize that it would increase buoyancy aft and make it a bit more nose down, but then again, the added weight of a motor and gear, would lower it back down and increase the water line, and add a bit of drag.
    Thanks in advance,
    Gus
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    G'day Gus, it is possible to convert sailboats into motor boats, and generally speaking, a catamaran is a better candidate than a monohull, especially a ballasted one. But your boat is much smaller than that. The 'banana" shape allows rapid tacking, the boat has little inertia, so it will not lose too much speed doing it. In sheltered situations a small outboard will work quite well, but you will encounter problems heading into waves, as it will hobby-horse noticeably (sailboats are not designed to encounter head seas, and slender hulled catamarans with your "banana" shape, are not very "stiff" in the lengthways direction) and keeping the propeller in the water could become a challenge, too. It is pretty hard to change the hull shape you have, to something ideal for motoring, without major "surgery", as much work as building a boat from scratch, probably. You can't really add much weight to a small catamaran, so don't get too ambitious with alterations. It all depends what your plans are, but if you are content to potter about at modest speeds, leaving pretty much as-is, seems the best course.
     
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  3. GUSRENTIT
    Joined: Dec 2020
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    Location: Pueblo, Colorado, USA

    GUSRENTIT Junior Member

    Mr. Efficiency.
    Thank you so much, I forgot the theoretical "pivot point" of a sail boat. God I hate getting old and forgetting everything I learned more than 50 years ago. LOL,,, It looks like a very symmetrical displacement hull, as it sits, it only weighs 110 LBS (50 KG), much more buoyancy and free board than a Hobie 16, from the looks of it. It will only be used on lakes, and not looking to go very fast. So I thought of adding enough foam and FG to get get rid of some of the rocker and add some length to it, also thought of adding some free board to the fore section, all in all, I would be adding far less weight to it, than the sail rig weighs (not counting the motor). What do you think?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    It is possible you could give the hulls more of a canoe shape, that would still allow easy propulsion, but were you to try and end it in a submerged transom, that would be like a handbrake on it. What size engine do you have in mind ?
     
  5. GUSRENTIT
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    GUSRENTIT Junior Member

     
  6. garydierking
    Joined: Sep 2004
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    garydierking Senior Member

    The first question to answer is how fast do you need to go? I have built a couple of power catamarans. With a 2 hp properly adapted, you can probably travel at 6-7 knots. If you want to go faster, you'd have to modify the after underbody and use a larger motor.
     
  7. GUSRENTIT
    Joined: Dec 2020
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    Location: Pueblo, Colorado, USA

    GUSRENTIT Junior Member

    I was thinking of a small block Chevy V8,,, lol.. No, had in mind a 4 HP 2 cycle.
     
  8. GUSRENTIT
    Joined: Dec 2020
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    Location: Pueblo, Colorado, USA

    GUSRENTIT Junior Member

    garydierking, thanks,
    I thought about an air cooled 2.5 HP, but you folks live at about 6 feet above sea level, I'm at 5100' elevation, and some lakes that I will be playing on, will be at over 10,000', by the time I lose 3% per thousand feet, when I get it in the water, I will owe that poor engine about 2 HP.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, you only want to potter around slowly, I looked at a pic of those boats, there isn't a lot of volume there, so if the are concerned it won't carry as much weight as you would like, what I would do is stick a belt of flexible polyethylene foam high up on each hull, both sides of each hull, but not submerging when running lightly loaded. You will get a bonus of it preventing the boat from sinking.
     
  10. GUSRENTIT
    Joined: Dec 2020
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    Location: Pueblo, Colorado, USA

    GUSRENTIT Junior Member

    Mr. Efficiency.
    Thank you, that makes a lot of sense, besides the thought of sinking and having to swim in real cold water, does not sound like fun.
     
  11. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You should be able to buy that foam fairly readily, it isn't dirt cheap, but even 2" thick would go well toward the objective, it weighs nothing, it would also damp pitching motions, just a question of placement, and fixing.
     
  12. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    For easy reference by other readers, here is a link to some info about the Sea Moth on Sailboat Data -
    https://sailboatdata.com/sailboat/sea-moth-ii

    And a photo from this site -

    sea_moth_photo.jpg

    And a basic GA drawing -

    sea_moth GA drawing.jpg

    Re a power cat conversion, I think that simplicity is best and I would agree with Gary above re a 2 hp O/B motor - you could perhaps build a simple bracket on the aft end of the trampoline framework for it.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd be all over the foam bouyancy belt, just trim it so it doesn't dip under the waterline, 6"x 2" following the sheerline both sides each hull, will give you over 300 pounds. Like a Christmas special, you get safety, pitch damping, even spray deflection, cheapish and easy, minimal added weight. Appearance, well the fish in the lake will get used to it.
     
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  14. GUSRENTIT
    Joined: Dec 2020
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    Location: Pueblo, Colorado, USA

    GUSRENTIT Junior Member

    Bajansailor, Thank you, great picture, I also agree with Gary, but because of my elevation I think I need a 4 HP 2 cycle. I'm at 5100' elevation, and some of the lakes I go to are at over 10,000' elevation, I lose at least 3% HP every 1,000'. I'm not looking to go fast, just fishing, and puttering around, but need enough HP to get back to camp, it's not uncommon, in the high country to have a nice sunny day, and in 10 minutes, it could be gale force winds, with snow and hail, and that is in the summer.
     
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  15. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    You can get high altitude jets for outboard carbs. Not in Australia, though, I bet !
     
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