Designing a 16' power cat

Discussion in 'Boat Design' started by SouthernComfort, Jul 2, 2011.

  1. SouthernComfort
    Joined: Jun 2011
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    SouthernComfort Hillbilly Engineer

    Hey everyone, new to the forums. I've learned alot in the short time i've been browsing around. I want to build a small cat, 16', for camping aboard, as well as carrying enough adult beverages, camping supplies, food, and ice to a campsite on the river or lake. Will never see saltwater, just lakes and rivers. Would like to use the stitch and glue method. Trailerable width. Seating for 4, utilizing a bench seat in the front, that will fold and convert the cabin into a sleepable bed. Very basic interior, small stereo, led strips for interior lighting, no heat or air, but seat heaters from an automobile arent out of the question.
    My basic idea was 2 canoes as the hulls, with a small enclosed cabin area in the middle. Dimensions 16'x8'. 2' wide flat bottom hulls for shallow draft and maximum storage, very simple design. The hulls would be enclosed, with hatches along the top to allow access to storage. As for the hull height, somewhere in the 4' area, to allow plenty of storage, as well as giving the profile I desire. I havent built a scale model yet to get an idea of what its going to draft. I'm planning on building a 1"=1' scale model of the boat of balsa wood, and using pennies to determine proper weight distribution. I am in need of help in a certain area. Determining the curve of the bottom of my hull. I notice alot of boats have thicker hulls that taper out towards the edges. How do I determind the proper curve of the bottom? I'm estimating a 1000lb to 1200lb payload when fully loaded for a weekend trip with 4 people.
    For power I'm planning on a 25hp outboard, mounted 3' in from the rear, in between the two hulls. I know its more power than I need to reach my desired cruising speed, but in case I need to get out of the way or race a storm back to dock, i'd like to have the power on reserve. Should i run a long shaft to keep the prop below the turbulent water created by the two hulls? I might be thinking too much into this for such a small boat. I need a tunnel height of 11 inches according to a calculator I found. I'm also wanting around 42" of headroom. Once I get a small model built and determine a way to measure weight to scale, i can get a much better idea of the tunnel height from the bottom of the hulls. What effect would a 3" tall keel starting at the front and running aroud 60% of the length and tapering off have? Would this make a noticeable difference in tracking and ease of driveability? I have attached 2 images, my basic ideas. Not exactly to scale, but close. I appreciate any advice and criticism.
     

    Attached Files:

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  2. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    I couldn't open your drawing files.

    Try pasting into MS "Paint" and then attaching those files.

    Click my handle to see how that comes out on this forum....it shows a medium sized thumbnail which gets bigger when clicked.
     
  3. rasorinc
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    rasorinc Senior Member

  4. gonzo
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    gonzo Senior Member

    I think you are reasonable in the payload. One of the most common mistakes is in being too optimistic on the weights. Pick up Gerr's book on boat design. It is an easy step by step method. Good luck
     
  5. SouthernComfort
    Joined: Jun 2011
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    SouthernComfort Hillbilly Engineer

    I apologize for attaching the images in the wrong format. The side view is very choppy, as I was adjusting lines in corel draw, which i'm not very efficient at as of now. Here are the pictures of the top, side, and hull.
     

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  6. SouthernComfort
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    SouthernComfort Hillbilly Engineer

    Rasorinc, could I use foot controls similiar to those found in kayaks?
     
  7. rasorinc
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    rasorinc Senior Member

    I have never seen a kayak with foot controls so I do not know your meaning and what you want to control with your feet.
     
  8. srimes
    Joined: Sep 2008
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    srimes Senior Member

    check out
    http://slidercat.com

    leave off the sails and mount the outboard. 25 is too much for this boat though.
     
  9. SouthernComfort
    Joined: Jun 2011
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    SouthernComfort Hillbilly Engineer

    Here is something along the lines of what i'm talking about. Using a simple mechanical linkage i could operate the rudder with foot pedals. Like driving a tractor with seperate rear brakes.
     

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  10. messabout
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    messabout Senior Member

    SC; are you totally committed to buiding a cat? If so, why a cat? If you use something similar to two canoes, your speed will be limited to 5 or 6 MPH, so the 25HP motor is way too much. In order to absorb all that power you'll need a hell for strong transom/motor mount. That'll add a lot of weight for not a good enough reason. The boat you describe does not increase speed potential in proportion to horse power. You could get along just as well with a 6 or 8 HP motor.

    If you think that you might need to go faster, you will need to design planing hulls. But with planing hulls, your low speed power demand will be higher than for displacement hulls such as the canoe, and you'll pull a substantial wake behind the hulls when in displacement mode. That's a no no in no wake zones.

    If your boat is to displace 1200 pounds, your speed length ratio will be most unfavorable for a play boat. Your cat needs to be in the 24 foot range to reduce the S/L ratio to a decent number. Murphys law will almost surely arrange for your the weight estimate to be too low.

    Please forgive me for raining on your parade.

    You would do much better to consider a 16 foot monohull for the purpose stated. Here I assume that you want to keep the boat under the sixteen foot limit. In Florida the registration cost for a boat more than 16 feet is much higher than for a boat of 16 feet or less. Then there are storage considerations, trailering constraints, and cost are all in favor of the 16 foot, or less, boat.

    Take a look at some of the monohulls in the 16 foot range. They have a ton more people space than a cat of similar length. They are easier and probably less expensive to build. If you want to have a small boat with load capacity, then the monohull is clearly the smarter choice. You want a distinctive boat that works pretty well, has space for people, and goes pretty good with the 25, take a look at the Bolger Micro trawler, 14-6" x 7-11'. OK that's a character boat but it works very well, is cheap and oh so easy to build, and has a proven success record. There are a lot more established designs that will serve you better than the too short cat. Matter of fact more than a few of our forum people are very competant boat designers. Par,Tad and several others have developed and sell plans for good little boats. I swear to you that you will save time money, and disappointment if you use a proven plan set done by a knowledgeable pro.
     
  11. SouthernComfort
    Joined: Jun 2011
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    SouthernComfort Hillbilly Engineer

    Hey messabout, thanks for the advice. Definately something I will consider. I chose the catamaran style because i'm more comfortable on a pontoon style boat, for the kind of boating I do, the slow ride. Nothin like a good booze cruise (with a designated driver) on a saturday night, never stopping, just barely moving along the river with friends and beautiful scenery. Also fishing the rivers and creeks, usually just floating down the river. I'm making a scale model of the cat at the moment. Next i'll make the monohull and compare the two to my needs. Thanks again
     
  12. Richard Woods
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    Richard Woods Woods Designs

    Not sure why this was moved from the multihull pages

    You might like to look at my Skoota 20 (actually 19ft 3in) power cat which, apart from the length, seems to match your requirements.

    I don't think you'll get the load carrying you want on a shorter boat - have you ever seem a 16ft pontoon boat? Sketch 4 people in your drawing and you'll see how small a 16ft boat is

    But if you do go that route I suggest a max engine of 9.9hp. A 4-6hp would be more sensible. Or even an electric Torquedo

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. SouthernComfort
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    SouthernComfort Hillbilly Engineer

    Hey Richard, thanks for responding. I've looked at your Skoota dozens of times, and watched the videos. I really like the design, and that thing moves. It seems relatively simple. Are study plans available? What is the maximum load, and what does it draft at full weight? Thanks for the reply. I read more on your page, and I wasn't aware of the tent that makes another room at night. Seems perfect for camping.
     
  14. messabout
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    messabout Senior Member

    SC; you are in the process of doing a model of one inch to the foot scale. Too small for meaningful waterborne analysis. The all up weight of the small model must be only 11.1 ounces if it is to simulate the 1200 pound full sized boat.

    Here is a homely method for calculating simulated displacements for models. one to one scale is 1 to 12 or one twelveth scale. Take the cube of the scale, 12^3 which is 1728. Divide the weight of the full sized boat by that number .....1200/1728= 0.694 pounds...= 11.1 ounces. Now try that with a 2 inch to the foot scale which is one sixth scale. Take the cube of six which is 216 Divide 1200/216= 5.555 pounds. One and one half inch scale is one eighth...8^3=512 1200/512= 2.34 pounds.....etc.

    The bigger model is much more practical because it does not require the degree of precision of smaller ones. Consider that a dimensional difference of less than 0.083 inches is equivalent to one inch on the full size boat when you use one inch scale. Thus moving weights around in a tiny model requires great dimensional care. Also the little model will be unrealisticly affected by the tiniest ripples in the water where you are testing. Besides that the larger model is easier to build and you will have more choices of material because the material weight will not be as critical.

    Although I like multihulls, I would not think of them as the most appropriate for the uses that you describe. The matter of creature comfort mostly. You do not intend to flip the boat but if that ever happens, the cat is far more difficult to right. A mono can be self righting with a little design forethought.
     

  15. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    We are talking about powerboats here.

    How many powerboats (excluding Coastguard cutters, RNLI lifeboats etc) are self-righting?? I think the answer is none. Certainly no fishing boat, trawler yacht or speed boat is.

    Years ago the Wolfson Unit did some tank test work on powerboats in big broaching waves for a client of Lock Crowther's. I have seen the resulting videos and I think the report is available on line. Basically a monohull powerboat was easy to capsize whereas try as they could they never managed to capsize the power cat

    To Southern Comfort. You can download a free studyplan from my website if you go to Designs and then Powercats. Draft is about 12in, the outboard leg is the deepest point. Speed will obviously be affected by load. Four people is the sensible maximum. Hope that helped

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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