Efficient Power cat designs

Discussion in 'Powerboats' started by catalyst, Oct 7, 2009.

  1. catalyst
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    catalyst Junior Member

    Efficient Power cat designs (is this one any good?)

    Hi

    I am new to this forum so hello everyone!

    I am looking around for an efficient catamaran design for commercial use (sea fishing).

    I have seen various designs on the web and in the water here in the UK, but got thoroughly confused.
    I am not against outboard powering as I can see the advantage of the ease of maintenance and the fact of reduced initial outlay for the boat, but I recognise the potential for greater efficiency with a diesel set up.

    What do you guys think of the planing v displacement debate with respect to commercial vessels? There seem to be far more planing designs than displacement ones, so is this because they are better / more efficient?

    One thing of interest I found recently via a fishing website in the UK was a company called BWSeaCat Ltd, who seem to be producing a very smart looking cat of about the right size etc for me.
    Before I go ringing them and getting further down the line, can any of you with expertise in this area cast their eyes over their website and give me the inside track as to whether this design is any good or not?

    I would be most appreciative for your help and any feedbck you can give on this.

    Thanks for reading.

    Bob.
     
  2. southatlantic
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    southatlantic Junior Member

    Hello Bob,

    If you are talking about larger (> 35 ft) boats, there is no contest: Displacement cats are much more fuel efficient.
    I refer you to the excellent site by the late Malcolm Tennant for a thorough discussion. You will find a lot of material on the subject there.
    http://www.tennantdesign.co.nz/

    All boats are compromises and one disadvantage of the very efficient displacement designs with an individual hull length/beam ratio of 14:1 or so is that the engines are usually "shoehorned" in, with limited room for maintenance.

    The most efficient planning cats are the ones using Hysucat foils, which are ideal for asymmetric uls but can also be employed with symmetric hulls. See their site
    http://www.hysucraft.com/Default.aspx
     
  3. catalyst
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    catalyst Junior Member

    Thanks for that. Had a look on those sites and yes very informative. Seem to be mainly pleasure craft though with some ferries etc, but no fishing boats as such.
    Not interested in foils as they would get fouled up too quick and plus my loads could be anywhere on the deck so they would need constant adjustment to work right.
    As a point of interest there is also a discussion on the BWSeaCat site, which I didn't notice first time round, about their design and its conception etc. I imagine some of you could pick it apart but for a numpty like me it did make a lot of the things I had been wondering about a lot clearer.

    The size of cat I am talking about is just under 10m (its because of the way our fishing licenses work), and they can mould the hull to anything from 8m to 11m as standard in the same mould apparently.
    Does anyone have anything to add about this design specifically. Looks good on the screen and on U-Tube, but like I say I'm notan expert by any means and was wondering if someone out there who was could have a look and give me their opinion. The more feedback I get the better informed I will be if and when I contact them.

    Regards
    Bob
     
  4. southatlantic
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    southatlantic Junior Member

    Bob,

    I am not familiar with BWSeaCat. I did look at the site and video. Agree the sea trial looked very good, but those were very calm seas... Also, most builders need to learn from experience - which means making mistakes! Having said that, the quotes in the site were all sound.
    Commercial fishing cats are not all that common, perhaps because even a displacement cat needs to be light to perform well, in order to avoid bridge slamming and inefficiency due to immersion of a wider section of the hull. But I am not familiar with the kind of fishing you do, so weight may or may not be a problem.
    A couple of additional resources. Safehaven Marine, in Ireland, makes a sturdy line of work cats called the Wildcat 36. They are 10.7m, but I would think Frank could find a way to chop down 80 cm somewhere for you... Here is a rather more lively sea trial
    http://www.youtube.com/watch?v=Vuemirsn6LY

    And here are the specs:
    http://www.safehavenmarine.com/SPECIFICATION WILDCAT 36 Angling version.htm

    Finally, there is a lot of material on powercats on this site, though much of it is for pleasure craft:
    http://powercatamaran.typepad.com/homeport/

    Good luck in your hunt for the perfect fishing cat!
     
  5. baeckmo
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    baeckmo Hydrodynamics

    What kind of fishing are you aiming at? Traps, net, longline, trawling, seineing......? How long trips, singlehanded or with a crew?

    There are a number of pitfalls when trying to convert leasure cats for commercial fishing. A couple of years ago my company was involved in a successful development of fishing/workboat cats together with our national centre for fishing. The fishing vessel must be capable of providing a safe working platform with soft movements at low speed, for handling of fishing gear, as well as giving a smooth ride when heading towards "the hungry market" with the catch.

    And it has to do so in weather conditions that most leasureboats never face or just barely survive. One lesson after tank testing was that for workboats up to about 12-15 m, asymmetrical hulls had better seakeeping qualities and survival chances in really bad conditions. Also if length/beam ratio was too low (ie too beamy overall) , the vessel movements would be very tiring, even awkward due to a "corkscrewing" motion; pitching movements seemed to be too close in frequency to rolling.
     
  6. catalyst
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    catalyst Junior Member

    Hi

    Thanks for that. I work pots for crab and lobster. Need to move a lot of gear and or catch sometimes hence the need for a lot of space on deck.

    This cat I am looking at seems not too wide at 4.2m and 11m long, but it is symmetric hulls as far as I can see.

    I am not interested in converting a leisure cat dor commercial use, rather to purchase a properly designed commercial cat to start with. This one doesn't look like a leisure boat to me.

    Regards
    Bob
     
  7. baeckmo
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    baeckmo Hydrodynamics

    I strongly recommend that you compile a list of the gear you are going to have onboard! Then estimate the total weight including ice, fuel, extra water and lube oil, tools, your personal gear (don't forget the beer and sandwiches), a reasonable catch, including the "storm nets" full of wet algae, and then add some further 15 % for the bits and pieces that somehow find their way onboard.

    Then arrange a testing run with a corresponding load!!!! I mention this because the hull shape of the BW cat is very sensitive to loading, both in terms of all up weight and WHERE it is placed. The very fine stems make the boat soft in pitching, ie it will react to minor changes in load position with great changes in trim!

    We have seen a number of very slender designs for commercial duty over the years, the concept is "reinvented" periodically. They are fine, as long as they are still light and the wave amplitudes are limited, but they all have a load limit. When loaded above this limit, the resistance is sharply increasing, and the seaworthiness is reduced, in some cases to dangerous levels.

    The video of the blu/white 11 m cat is taken in smooth water and the boat is obviously just finished and "clean" from equipment, ie it is lighter than it will ever be in "real life"! There is a scene where the boat is crossing the wash of "the photographer's boat". The wash is very low, but the 11 m "light" cat is dipping one bow in the through in a way that makes me worry about broaching in anything like normal "october seas"!

    With these lines, there is little reserve buoyancy when the boat is pitching bow-down in a following sea. In order to have a safe restoring pitching moment, the center of buoyancy must move forward as the hull is pitching nose-down, AND the center of waterplane area must do the same to provide some dynamic pitch-up moment to assist.

    The small waterplane area means that a small transfer of load from one side to the other, may result in a big change of draft. The consequence of this is that the boat may behave ok with its load evenly shared by both hulls in smooth sea, but the dynamic loads in a seaway may "overload" one hull, while the other is unloaded periodically. The difference in behaviour between the overloaded and the light hull will make the boat unstable on course.

    So, there are limitations to every design; if the total weight of your paraphernalia is below the critical load limit for this type of hull, it will give you a very good working platform. But if your test run with the full load according to your spec shows any sign of instable behaviour, particularly in a following sea, then stay away and look for a slightly "fatter" hullshape!
     
  8. baeckmo
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    baeckmo Hydrodynamics

    ......and, looking at BW's pics from ongoing jobs; their propeller tunnels have a shape that isn't "best practice". They will cause serious disturbance to the propeller inflow, reducing prop efficiency and increasing noice and vibrations.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Since you are in the location, go look at SouthBoats in Cowes. These are excellent boats.
     
  10. catalyst
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    catalyst Junior Member

    That's interesting. I thought they looked quite smooth and rounded etc and also they look quite long. Won't this help smooth water to the prop rather than disturbing it? What's the theory here?


    Regards
    Bob
     
  11. catalyst
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    catalyst Junior Member

    Hi

    Yes thanks fo that. I would have a look but they are a little too big for me and a little too expensive. I agree that they would be hard to beat though.

    Regards
    Bob
     
    Last edited: Oct 12, 2009
  12. baeckmo
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    baeckmo Hydrodynamics

    In the mid-eighties there were close to 40 cats fishing commercially in the Bay of Biscay, from Brest down to La Rochelle. Most of them built locally in aluminium in sizes 8 to 15 m. Bigger units (24 m and up) were produced in a yard in Les Sables d'Olonne (can't remember the name now). This area is not too distant from your location; I think you might be able to locate a number of collegues with cat experience there, who could point you in the right direction for a decent builder!

    I don't know what changes the fishing in that area has seen lately, but back then you often saw the cat crews minding their boats in good weather, when the monohulls were fishing. When the conditions went worse, the cats went out; they got better pay for their catch then, because many of the monohulls had to stay in harbour ...... !
     
  13. catalyst
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    catalyst Junior Member

    Hi

    Thanks for all that Baeckmo.

    Going back to the tunnel design issue breifly. I have read the other thread that you have contributed to on here, where you are trying to rectify the performance issue on that chaps boat. He has tunnels, but they are totally different from the ones on this cat.
    I have seen your drawing of what you consider to be the best shape for a tunnel and I can see that this is different from the cat one which tapers forward whereas yours flattens off.
    Can you explain in lay mans terms why the cat one is bad and yours is better?
    Is the SeaCat one that bad that the boat won't work?
    They are talking about a pair of 320HP Nanni diesels with conventional shafts for the first boat, giving a cruise speed of 18-20Kn and a sprint in the region of 30Kn. Is this acheivable do you think with the set up as is on the website?
    I only ask because those sorts of numbers would be what I was looking for.

    On the plus side their build quality (albeit from photos) seems excellent. I will try to visit them next week and get a feel for the design up close. I can also book a trip on the demo boat.

    If you could answer my questions I would be most grateful as forewarned is forearmed!

    Thanks
    Bob
     
  14. baeckmo
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    baeckmo Hydrodynamics

    The shape of a tunnel for a normal, noncavitating and nonventilating propeller (cavitating and vented props behave differently!) must reflect the shape of the streamlines of the incoming flow. A common misunderstanding about the inflow to a propeller is the assumption that it somehow approaches the prop (or jet intake, for that matter!) as if it were following an imaginary pipe of approximately the same diameter as the propeller. It is not!!

    If we first study the situation at bollard pull, a free (not tunnelled) propeller creates a low-pressure region into which fluid is moving from all directions; a sc "hydraulic sink". As there is no inflow from above (the hull is there), the sideways inflow is further increased. When the vessel is gaining speed, the incoming flow is directed more and more axially, but it always has a transverse, or radial velocity component. That is fluid reaching the propeller not only axially, but from the sides.

    The amount of transverse flow is varying with propeller load. At high power, low speed pulling, water may enter the propeller disc 45 or even 60 degrees inclined from the axial direction. The same prop running light will pick its working fluid from an area some 15 to 20 percent bigger than the propeller disc area. In your example with the Nanni engines, and say 22" propellers, the inflow area when running light at 28 knots would correspond to about 24" in diameter.

    Now, if you look at many incorrect tunnel designs, including the BW's and CDK's tunnels, the designer has created a shape that forces a substantial part of the incoming flow to pass from the hull bottom into the tunnel via the tunnel side edges, which have far too small radii. This creates turbulence which effectively destroys the dynamic energy in the flow, which is now to be added by the propeller/engine, ie your fuel bill. The flow in the tunnel will have a considerably lower mean velocity than the free fluid, so that the propeller blades will work with very differing "approach angles" (angle of attack) during the rotation. This makes it impossible to match the propeller to the flow correctly; the result is low efficiency and high levels of vibration.

    As for the outlet side, the tunnel should be adapted to the contraction of the flow after the prop. The fluid velocity is increased by the action of the prop; same mass flow means smaller "streamtube" area. The major direction of the outgoing flow is in the shaft direction, with a slightly less, inclination, due to the influence of the forward speed. If the tunnel roof is made horizontal f.i., there will be a pocket of low pressure just after the propeller disc, causing a reverse flow from the tunnel exit into the propeller. With tunnel exit exposed to the ambient air, this air will be sucked into the propeller from behind, which ruins the working capacity of a propeller that is designed for submerged operation. This is the main problem of many tunnel designs (you read about CDK's....). The exit section of the BW tunnel seems to be fairly OK, judged by the pictures, provided the propeller is not positioned too close to the tunnel end.

    Rudder should preferably be placed outside the tunnel exit. But with a slightly increased exit length (tunnel must then cover the whole length of the rudder plus some 15 % of rudder length); it will still work.

    A correctly designed tunnel can actually improve overall efficiency slightly, but unfortunately, there are far too many lousy designers around........ Now, will the BW tunnel work? Probably yes; I have seen worse, but with high inlet losses!

    So good luck, hope the reasoning may give an insight (sorry, couldn't make it in less words.....) and remember that working boats should be tested in bad ("normal") weather and with realistic loads, otherwise you end up with a goat instead of a boat!
     
    Last edited: Oct 11, 2009
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  15. baeckmo
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    baeckmo Hydrodynamics

    Well, Bob, we are curious; did you manage to get a demo trip with a decent load onboard in realistic weather conditions????
     
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