New power cat - help please!

Discussion in 'Powerboats' started by foxfish, Aug 2, 2008.

  1. foxfish
    Joined: Jun 2008
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    foxfish Junior Member

    Well I have been looking around at various local boats for inspiration & I came across an "x racing 20' x 7' carbon fibre cat". The boat looks like a conventional power boat that has been cut in half, ie the inner side of the hulls are completely straight, in fact it is an American boat that was once powered by twin 200hp motors & had a top speed of 100mph.
    My questions are - can I fit two 50hp motors & expect the boat to work OK (20 -30mph would be great) & can I use polyester resin on any mods I want to carry out?
    I would like to convert the boat to suit my needs as a open deck centre console boat, I only consider this as the cat is for sale at a very good price (very cheap)
     
  2. Guest625101138

    Guest625101138 Previous Member

    Have you got a photo?

    You could reckon required power would be roughly the cube of speed if it does not get heavier. If original with 400HP did 100mph then 100HP should give around 60mph.

    If you are making big changes to the deck then you would need to determine what role the current deck plays in the structure. It could be critical.

    If existing hull is carbon/epoxy then it is best to stick with these materials for any structural elements.

    Rick W
     
  3. northerncat
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    northerncat Senior Member

    depends on the weight i would think although a carbon 20' stands a good chance of being a light boat
    sean
     
  4. foxfish
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    foxfish Junior Member

    Here is the boat, I actually measured it at 21' 7'6''.
     

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  5. Guest625101138

    Guest625101138 Previous Member

    Fox
    That looks exactly like what you wanted. I would be reluctant to make radical changes to the deck.

    I expect it will be light and it sits low for little windage so it will get along with 100HP. The predicted 60mph might not be far off.

    Just need to make a careful inspection to ensure there is nothing cracking up. You should be able to get some history on the boat.

    Rick W
     
  6. brian eiland
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    brian eiland Senior Member

    New Design Idea

    Dear Foxfish, I've not had time to read your subject threads yet, but I do have an idea to pass before you...its a RIB with a unique hull...but you might not have to build it as a RIB initially.

    Please send me a private email as I don't wish to post it on the forum at this time.

    brian.eiland@gmail.com
     
  7. foxfish
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    foxfish Junior Member

    Still not really going my way! That boat has been for sale for two years but when I phoned the chap he said it has just been sold!!
    Brain message sent.
     
  8. foxfish
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    foxfish Junior Member

    I have been looking at all the cats I can find in my area & it would seem that most of the commercial cats have the same design. The two most common cats, Cheater & Gemini are very similar & their sea keeping is legendary around these parts.
    Do you think I could simply scale down this basic square sided shape & build a 5mt version?
    http://www.geminiworkboats.com/STANDARD_GFO_7_8.pdf
    I would however be using two motors.
     
  9. Guest625101138

    Guest625101138 Previous Member

    These look good shapes for their size suited to moderate speed. Reducing the size by just scaling would result in a deep entry for the type of boat you want and weight further forward than needed.

    I really liked the shape of the black one. If you were going to scale one for a smaller size then I believe this would be better for the speed of boat you want.

    Maybe something in between the work boat and the racer.

    These boats are well regarded in Oz:
    http://www.markhammarine.com.au/cats_pleasure.html

    Rick W.
     
  10. foxfish
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    foxfish Junior Member

    I see where you are comming from Rick but the square sides design is so basic & simple, I could easily build a one-off boat using basic frames & glassed over ply.
    However if you could look at my purposed schematic & point me in the right direction I would really appreciate your help.
    I only really need help with the below deck shape but as my two bottom pics show! however they are purely guess work! Any comments would be a help.
    I will use two 50 hp motors because I have them but smooth ridding is more important that outright speed.
     

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  11. Guest625101138

    Guest625101138 Previous Member

    I would go with a bit more "V" just to help with lift in the turns. Maybe even offset the V like the black hull.

    Soft riding will be a combination of the V and the width of the hull - narrower is softer ride. If you go too narrow with the hull then it will be harder to get on the plane.

    Narrow deep hulls push easily up to maybe 15kts but will be hard to plane. Above this speed for the length you have you want a planing hull.

    You could have asymmetric hull like the black boat with a single bottom piece at 15 degrees and fit planing strake. This may be a simpler build than you currently have.

    Best to make some cardboard models to get an idea of the hull
    development.

    You should shop around for plans of something that has been tested. The structure joining the two hulls needs proper design to counter the torsional loads it is subjected to.

    Rick W.
     
  12. brian eiland
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    brian eiland Senior Member

    Sea Knife, SeaKnife by Peter Payne

    I might suggest you have a look at this hull form for your experiments...shouldn't be too difficult to build:

    ...the 'Sea Knife' concept
    http://www.yachtforums.com/forums/16028-post47.html
    A boat capable of cutting through rough waves at high speed with astonishing stability has a hull provided with a flat planing surface, which in plan is the shape of a thin wedge or delta. The sides of the boat rise upwardly and outwardly with a simple concave curvature from the two edges leading from a knife edge bow at the point of the wedge. The slender wedge shape moving through the water at high speeds develops continuous spray sheets up its sides, which are intercepted by the outwardly curving portions of the hull sides. Spray rails or deflectors may also be utilized to intercept the spray sheet, such deflectors being inclined at a small angle to the bottom of the planing surface. The knife edge bow rises upwardly and forwardly with a concave curvature from the point of the wedge and eventually terminates in a forwardly sloped bow transom. A keel skeg minimizes side-slipping. A stern transom, which rises substantially perpendicular to the trailing edge of the delta or wedge may have a rearwardly extending bustle secured thereto for buoyancy roll stability at low weeds.

    http://www.yachtforums.com/forums/16010-post46.html

    http://www.yachtforums.com/forums/16033-post48.html

    http://www.yachtforums.com/forums/24306-post65.html

    Peter Payne was a very learned mathmatican who had a very real interest in boats as well as other subjects. His attempts to define planing and other hydrodynamic phenomena mathmatically is difficult reading for most of us, but he made very real test on actual prototype vessels to confirm some of his theory.

    To quote a web reference,"Another type of Payne high speed boat achieves results in a different way. The SeaKnife hull form is a supercritical planing hull that cuts through waves instead of riding over them. This reduces the vertical pounding accelerations typical of conventional planing hulls by a factor of more than ten. There are a number of SeaKnife boats in operation and on the drawing boards. Designs include sport and racing boats, and military patrol boats. Another planing hull form is the WaveStrider, which has been produced as a 24-foot boat for the Navy, and is in production as a high speed ferry. (The stealth boat that appears in the new James Bond movie "Tomorrow Never Dies" is a WaveStrider.) He also developed the Air Lubricated Planing Hull (ALPH). A boat of this type is the Hydrotrac, noted as being the world's fastest production boat for a given power. All of his boat designs were tested in the Chesapeake Bay, near Annapolis, and the nearby Severn and Magothy Rivers. High speed planing hull theory receives a detailed presentation in Peter's book, Design of High Speed Boats, Volume 1: Planing. His computer program, BOAT3D, is used by many to model and design high speed boats."


    I will post some pictures sometime near future when I receive something presentable. I happen to have been in the Chesapeake area when he was conducting some of his early test on SeaKnife and I was also working with the Navy at the time. I didn't pay much attention to his 'far out' hull designs at the time, but I recently sought to go back and look at his concepts again as a result of both Bladerunner's work and Payne's concept. Mr Payne passed away a few years ago and I am talking with his widow and his son.
     
  13. foxfish
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    foxfish Junior Member

    Thanks guys I appreciate your replies.
    Brian there are some great pics amongst you links & I enjoyed reading about your findings but that design would be very difficult to copy without any actual plans or dimensions. I have built many boats & trained as a boat builder & although I might well be capable of building such an amazing boat, I dont really want to spend the next two years building complicated compound curves that require mutable formers & tiny strip planking.
    Rick, I have searched the net for weeks now - if I could find suitable plans I would buy them.
    I am confident with the "above water" design & have no worries about the structural integrity it is just the underwater hull shape that concerns me.
    From my very limited experience of building power cats I can conclude that many shapes will in fact work however I would like one that works well.
    My criteria -
    A catamaran design to be very stable at rest & for beaching on small sandy bays, shallow draught & low free-board to avoid fast drifting.
    Twin 50hp motors (not necessary to use full power but I have the motors)
    4.9 x 2.1 is ideal because of mooring charges & plywood sheet size.
    25 - 30 knots speed but able to coupe with a good chop!
    Please persevere with me as I do really appreciate you support.:)
    Fox.
     
  14. Guest625101138

    Guest625101138 Previous Member

    For the speeds you want to achieve you are into a planning hull. The biggest factor on power required will be weight. You can use this calculator to determine the power required:
    http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
    You will need to do each hull as separate calculations. The power required is that on the hull so you need to multiply by say 1.4 to get actual boat power.

    I would not go less than 15 degree dead rise for an ocean hull. I would offset to maximise the lift in a turn. Something like the black boat is my preferred.

    Asymmetric hulls are OK but I would not have odd kinks in the side of the hull. Just nice curves. I would have a shallow for foot to avoid the bow digging in off a wave.

    So the first thing is to work out what the all-up weight is going to be. This will be iterative. It will depend on the type of building material and what load you want to carry. How many people, what gear, 2X 50HP outboards, fuel, basic hull, controls etc.

    You will then be able to determine what speed you will get with the planing hull. I expect it will be max around 25kts. It might be slower if you have a lot of weight to carry. A first approximation for the hull weight would be 500kg.

    Rick W.
     

  15. foxfish
    Joined: Jun 2008
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    foxfish Junior Member

    Thanks Rick, I am going to the Southampton boat show next month (a huge show covering several sq miles) I hope to study some different designs.
    I have found, on the net, quite a few designers offering custom plans for any size boat but it seems to me a tryed & tested design rather than a calculated guess is the way to go? However I enjoy the challenge & I am going to build something one way or another.
     
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