efficient 10m displacement powercat (build thread)

Discussion in 'Boatbuilding' started by groper, Apr 15, 2012.

  1. groper
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    groper Senior Member

    There is a bit of trim / sqat when underway. It seems to occur at a speed of around 9-10 knots and over. How many degrees this is i cannot easily measure but is the reason i kept the buttock lines flat all the way to the transom. The amount of trim and sinkage is fairly modest compared to most other boats ive seen... You should be able to judge the amount from the previous videos ive put up...

    As your previous question - has it lived up to my expectations? - Yes, im quite happy with how she is performing. It just eats up the light chop and runs very smoothly in all but pretty extreme wave conditions (which all boats would struggle to maintain speed in) and im quite pleased with the fuel consunption considering its an 11mx5m platform which is comfortably cruising at 20kts... If you look at the sea state in the last photo above - we can cruise at full speed in those conditions almost without feeling it, mostly just the heave from the ground swell, the chop is basically undetectable... Most other boats this size would be using alot more fuel for the same speed and comfort... i caught up and passed a planing trailer boat heading home in a following sea on our last trip, they were all bouncing around their cockpit like ping pong balls whilst we were sipping on our wine glasses and eating cheese platters :p
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    That must be gratifying. You could go for a run up to TI in that one, subject to fuel reserves. Not sure I'd want to return into a stiff South-Easter though.
     
  3. groper
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    groper Senior Member

    We have a trip planned up to PNG later this year... Still a few more things to sort out before we go - the watermaker being the main thing. I still have to wire up the dive compressor and variable speed drive which runs it along with a bunch of other details which are slowly being attended to when work is not getting in the way...
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Jolly well done groper. My hat off to you. It is not easy doing all this yourself and you have mate. :p


    Just changes the frequency of encounter and thus the response.
     
  5. groper
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    groper Senior Member

    Had her out in a bit rougher conditions today...nothing too crazy but she loves to eat up the chop :) This vid in head seas @ 18kts...

     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Goes well. Does engine trim adjustment make any difference to the ride ? That seemed faster than 18 knots.
     
  7. Barra
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    Barra Junior Member

    Well done groper, appears to go well.
    The little kiddies sitting out front makes me wonder if you understand the potential hazards.
     
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  8. groper
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    groper Senior Member

    To be honest I havnt touched the engine trim yet. I doubt it would have much effect tho as the hull doesn't rise up onto the plane or generate much dynamic lift from what I can tell.

    The 750mm static trim bridge deck clearance probably increases to around 850mm at the front underway and makes it possible to punch through some decent size chop.

    It was definately only 18kts Mr E. There was close to 20kts of head wind so that's why it probably seemed quicker...
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I thought some in-trim might improve the ride in head seas. And please keep the kids safe !
     
  10. groper
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    groper Senior Member

    Well beleive it or not, i managed to find a little time between work and fun to finish a bit more of the interior - only the settee lid and slide out pantry drawers to go...

    [​IMG]

    And heres a shot from a different perspective :D

    [​IMG]

    And one more showing how we use it :D

    [​IMG]
     
  11. groper
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    groper Senior Member

    heres a couple more vids which show a little more about what she does and how she goes... As to the fuel consumption - on our last trip of 240NM we used 350L of fuel - 1.45L/Nautical Mile for the entire journey. It was mixed with good weather, bad weather, slow cruise, fast cruise, and some trolling for fish along the way. The majority of the distance was covered at 16-18kts tho.

    After checking the odometer on the GPS - we have so far travelled 642NM and i have put 1100L of fuel into her thus far - 1.71L / NM. We have noticed better efficiency after changing to the 14.25x17 4blade "overdrive props" (turned down V6 hub size) from solas - which we had on the last trip.

    This first vid shows us going straight over one of the coral reefs at high tide so that we didnt have to navigate an extra 10 miles to go around it... we had about 0.5m under the keel...



    And this one simply shows it running in a following sea @ 20kts...
     
  12. Jim Caldwell
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    Jim Caldwell Senior Member

    2.66NM per gallon, pretty good!!
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Groper

    Have you done 'official' sea trials...to validate your powering predictions...and also arrive at a PC for your whole system?
     
  14. Richard Woods
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    Richard Woods Woods Designs


  15. groper
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    groper Senior Member

    Richard,

    I had a marine surveyor sign off on the build as per AS1799 Australian standard for recreational boats. He had a look at the beginning and also did a stability test with people as weights after launching her. With his report we could obtain insurance certificates and an Australian builders plate with my name as the builder. The marine surveyor is the "competent person" which ensures that the boat is built to the standard, he gave me a copy of the standard at the beginning of the build and simply said - follow this and all will good. It's upto me as the builder to ensure that the correct laminates and everything was installed as per the standard. Once launched and the stability was measured, he worked out the max POB and loading numbers for the builders plate. It's registered in QLD for 12 people max and max total loading of 1750kg which is on the builders plate and must not be exceeded or the water police can book me... a competent person can be 1 of 3 people here, either a NA, shipwright, or marine surveyor. Total cost for his involvement was about 1000 dollars...
    Yes , I'm Located In Far North QLD East coast, Great Barrier reef area.


    Ad-hoc,
    Don't know what you mean mate ? I'm not a pro remember :) Arrive at a PC????
    My powering predictions were a little off, maybe 10% more than expected. - id hoped it would be a little quicker than what she acheived, but "knowing" what fudge factors to use is very difficult as data on this type of boat/ motor configuration is not commonly published.
    For example, i think it was "Alik" who suggested i use a propeller efficiency of 65% for outboard motors. So thats what i used knowing his competence, but have no idea how close that really is and a few percent makes a big difference to the calculations. Same goes for the windage calculation - what coefficient to use for the frontal area wind drag calculation? Again, small differences make all the difference. I also ignored the drag from the outboard motor legs - i didnt include the appendage drag - my oversight. And of course the dreaded transom drag was only an approximation offered by Michlet - which leo admits as being rather "approximate"....Thats why at the end of the day, empirical methods such as what size and brand motors they are and the generic data for the fuel consumption at given RPM is probbaly more accurate. Ive done all i can as far the props go i think - going to the largest diameter that will fit and maximum blade area available is all i can do. If you allow a couple of percent either way in the above calcs, and also add the appendage drag of the motor legs, then id say i was pretty damn close...
     
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