A couple of questions from a new enthusiast

Discussion in 'Multihulls' started by easywake, Nov 11, 2010.

  1. easywake
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    easywake Junior Member

    I am working on my multi-hull learning curve and have a few questions for you folks. The power cat I am thinking of will be in the 35-40 ft range, probably an 18-20 ft beam. Power would be two 25-35hp 4stroke outboards.Epoxy/plywood construction.

    1. Performance wise, would there be a significant difference between a flat bottom sharpie type hull and a rounder type bottom?
    2. I am thinking of putting the engines in wells ahead of the transom between kick up rudders and a protective keel. Is this idea practical?
    3. Thinking along the lines of cruising the great lakes, would it be necessary or prudent to consider any type of steadying sail?

    I realize there are many factors that ultimately dictate the design of cat, but I am looking for some "in general" info here to get my basic concept down to a point where I can take it to a NA for plans.
    Thanks, Russ
     
  2. gonzo
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    gonzo Senior Member

    1. A flat bottom hull will generally pound more.
    2. You will need a rather wide stern to house a well. Sharpie hulls are double ended and won't probably have enough room. Why are you installing rudders on an outboard powered boat?
    3. Steadying sails can dampen roll and give you some emergency propulsion. It may not be a bad idea, but there will be structural issues for you to consider.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    If you are building in plywood then a flat panel chined hull makes the most sense. You need buoyancy aft to compensate for the weight of the engines and to help reduce stern squat. I'd bolt the engines to the transoms, anything else is unnecessarily complicated for no real benefit

    As gonzo says, no need for rudders. And, as most powerboats rely on only one engine, and you will have two, it seems unnecessary to have a sail as a backup and you certainly won't need one to stop the rolling, as you won't roll

    Have you seen my Skoota 20 design and also my 34ft powercat that sounds like it is something close to what you want?

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  4. michael pierzga
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    michael pierzga Senior Member

    And whatever you can do on the design front to hide the ugly outboard motors would be good esthetically and practically. outboard motors hung aft, hobby horsing as the yacht pitches in a seaway, dont live a a happy life. engine thrust over a rudder is also handy when manovering. Give it a go.
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    My input is to watch the windage, especially on the great lakes. Some of the power cats I've seen are skittish in a cross wind, a flat bottom wouldn't help.
     
  6. michael pierzga
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    michael pierzga Senior Member

    A rounded section has more inherent stiffness and strength than a flat section. Rounded sections look better to the eye. The technique described by the west system guys of torturing plywood into a hull shape might be a cool way to conceive and build a small power cat. Perhaps drop an outboard motor thru the bottom , well forward of the stern, to function like a saildrive then steer with a rudder. Would get the weight out of the ends and allow your vessel to function as a little cat motorsailor, with just a little sail for messing around or steading out and hanging a nice sun awning, rain awning for a cruise. . . .


    http://www.westsystem.com/ss/birth-of-the-gougmarans/

    If you Goggle tortured plywood cats you will find plenty of home builders building cats. I had a quick look but didnt see any power cats.
     
  7. catsketcher
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    catsketcher Senior Member

    A couple more things to think about

    Gday Russ

    I would like you to answer a few more questions before you can go much further.

    -Will you build this yourself - if so how handy are you and what materials do you like?
    -What type of cruising will you do?
    -How many people will be on the boat?

    I will give you my resale talk straight up. If you want to save money you will do as you say in your intro and BUY PLANS. 35-40ft is big and the cost of plans will save you the money you spend on them in less wasted material alone. Then when you have to sell the boat you will get tens of thousands more with a proper designers name on the ad.

    A designer has to be careful with flat bottoms. A friend finds his slap a lot offshore - on his sailing cat. So the bows must be well immersed.

    With the power you will be using you will be probably thinking of a 10 knot cruising speed. Resistance is a squared relationship with speed so if you increase the horsepower by ten times you will get about (roughly) three times the speed. The boat should have a totally different hull shape for high speed. The Schionnings here in Oz followed Malcolm Tennant's idea of a straight keeled canoe stern underbody and a flat planing type stern put above. Good for fast speeds but unnecessary at low speeds.

    The displacement cats usually use sailing type hulls as sailing cats usually sail at the speeds you will cruise at - so you have a wide range of designs. In fact you could even build a sailing type design just in case you want to change later.

    As for a rig - I did meet some people who put a mast on their high speed cat. It was short and was thristy on fuel so they put a mast on it to reduce fuel consumption downwind. Friends of mine went from a sailing cat (Perry) to a 10m fast powercat. They motored a lot anyway they said. Funnily enough they went back to another Perry.

    I would like motors hidden but only because I love swimming off the stern of my cat. In the great lakes it is probably too cold for lots of swimming. The last Schionning I saw had them motors in little pods off to one side where it will work less well but a cat you can't swim off is silly in Australia.

    You won't need the sail to steady the boat. Strangely enough she will roll hardest in chop. A cat this size will scissor up and down when you have powerboat wash hits you abeam. Learn to steer slightly into the chop and this effect is drastically reduced. Large waves (swell) will move you but not roll you.

    Get some graph paper and draw up what you want in terms of accommodation, basic set up and basic style. I am really happy with the boat I got when my ideas and Robin Chamberlins agreed. I still have the boat 14 years later and she is still a fab boat in my eyes.

    As for round or flat - Personally I would go both. The easiest part of a boat to build is the part underwater. It gets antifouled and no one ever sees it so it hasn't go to be faired well. This allows the designer to produce exactly the hull shape required. Then I would go flat panel above the waterline. This allows you to easily make the rounded bottom section and then build the rest. Be aware that flat panels underwater may not save you time. With multi chine the panel joiniing gets so complicated it is usually faster to do strip foam.

    I have built two 6 and 7 metre cats this way and really like it. Best thing to do then is the get all of the flat panel pieces computer cut so you can build the interior before the outside. This saves hundreds of hours. So make sure you designer can provide CAD files so you can get a cutter to save you time and money. Don't be tempted to do it yourself. Cad takes huge time to learn.

    Better go to work now

    cheers

    Phil
     
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  8. jamez
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    jamez Senior Member

  9. Richard Woods
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    Richard Woods Woods Designs

    As always Phil made good comments

    I did forget to say that I'd expect you to do 10-12 knots with the engines you mention. Very roughly for that size/weight cat for every knot over 10 knots you'll need an extra 10hp. So going even a little bit quicker is expensive

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  10. michael pierzga
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    michael pierzga Senior Member


  11. easywake
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    easywake Junior Member

    Thank you all for the excellent replies! There is nothing like having experienced people willing to share ideas.

    I am confident in my construction skills and will be working in plywood/epoxy.
    The ultimate goal is to come up with a concept for two to live aboard with accommodations for another couple. We are both tall and carry a bit of ballast, so we will be looking at slightly larger dimensions in our plan. Our cruising will most likely be great lakes or Chesapeake bay in the warmer months then off to Florida (keys) or perhaps a crossing to the Bahamas for the colder months. Comfort and a bright interior are high on the admirals list. The ability to be final assembled at the water would be very desirable as we are about 75 miles from the nearest water big enough to launch her. So many ideas! It's a good thing retirement is still a ways off!

    We plan on building a few smaller boats before we even think of trying anything of this scale.

    Jamez: Thank you VERY much for the power cat design link. Lily is much closer to what I have in mind. I'll be watching her progress.

    Thanks again everyone, and please keep the suggestions coming.
    Russ
     
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