Power boat design for economy.

Discussion in 'Boat Design' started by Frosty, Apr 11, 2011.

  1. Frosty

    Frosty Previous Member

    I keep telling this to mass. He has a completed but cant get away just because of a mast.

    In my sailing experience masts are rarely used, but handy to hold the radar up .
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Sounds like several issues all at once?

    Not forgetting the Nav lights!! :eek:
  3. masalai
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    masalai masalai

    No, - - The nav lights are not on the mast nor is the anchor light, wind sensors, or radio antennas... Sheesh - pickey, pickey.. Does not one look at the pretty pictures I post ? ? ? - - I gotta save up for fuel, increase the fuel tankage, get a watermaker and relocate the aft berths about 15 inches lower... Then I should be ready to go - - some 3200 plus miles... How long would it take a wealthy person like you to prepare your boat for a voyage of that distance? - Around half with no reliable fuel supply stops available - I will hopefully be able to carry some 1700 litres when leaving Cairns...

    P & S lights are attached to the frame of the forward PV panels, the anchor light is on the tube alloy frame aft which also carries radio X 2 antennae, and wind instrument... Aft white steaming light comes on with P & S, and is midships high on the aft bulkhead extension, just below the roof-line, all in full compliance...
  4. JorgenBeyer
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    JorgenBeyer JorgenBeyer

  5. erik818
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    erik818 Senior Member

    I rechecked my calculations and I still believe that I've fed Michlet with the correct inputs to have the total water resitance correctly calculated. I enclose the input file in case someone wants to check.

    For a cat with 11.5 m long and 0.8 m wide hulls, each displacing 2 tonnes for at total of 4 tonnes, I get Rt = 1.583 kN at 5 m/s = 7.915 kW. This roughly corresponds to 10.6 Hp at 10 knots. It's fairly easy to run michlet if you have the offsets for your hulls, and each hull is symetric. Wider hulls will increase the resistance.

    I think that the main explanations for the difference between practical experienced Hp and my calculations is the propeller efficiency and a difference between advertised Hp and actual shaft Hp. The propeller efficiency doesn't become 70% just because I assume it.

    From the discussions on this thread I'm reinforced in my conviction that I should have a diesel inboard with a straight shaft, low propeller rpm and a large diameter propeller.


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  6. groper
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    groper Senior Member

    yes and in addition to your propeller efficiency and hull resistance, dont forget to include sinkage and trim (how do you know what these will be?) and dont forget your appendage drag (rudders etc) and aerodynamic drag of the ship above the water.

    This will get you close to the total resistance and thus required HP in dead calm water...
  7. Richard Woods
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    Richard Woods Woods Designs

    Remember that "dead calm" on a powerboat really means a following wind of say 12 knots

    All I was trying to say in my previous posts was, don't buy a 15hp engine and expect to do 12 knots. Use the figures given by others to adjust the scale of your graph

    Richard Woods of Woods Designs

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  8. masalai
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    masalai masalai

    Hi RW,
    Fully concur, Although the numbers expressed may address and reflect the "calm water - no wind" condition it behoves us all to remember more adverse conditions and respond appropriately...
    - - The original design specifications for my 'kit boat', (a Bob Oram 39C modified with approval), were for twin 9.9hp Yanmar outboards...
    - - I was weight restrained in fitting a pair of 20hp Nanni sail-drive systems but still achieve 10knots in "calm water - no wind - no current" tests at cruise settings (near peak torque)... and that demands a clean bottom and correctly functioning systems...
  9. philSweet
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    philSweet Senior Member

    When I went down to my boat last time I had a new neighbor and chatted to him and exchanged phone numbers and such. He bought an older but well pedigreed 30' racer from a guy who got Alzheimer's. All the seller could tell the buyer was "It's out there somewhere, go find it.":eek: Buyer paid $500. After he found the thing ,it cost him $2000 so far and he thinks another $1000 more will do it. It's got seven racing sails and everything is top of the line. It had sat for five years though. He's recruited a crew who are kicking in a couple grand each per year to maintain the thing and race it. So I think he will start his first race a few dollars ahead of the game.
  10. Frosty

    Frosty Previous Member

    Ive seen free boats on a number of occasions, I heard of one in Guam where they set out to see thye world and got into a storm , she was on a p-lne back shortly after and he gave it away to be with her, awwww tyrue love.

    And I guy at the yard got a little 26 boat given at Changi sailing club in Singapore, diesel engine, little galley, 2 berth, sails everything, by afternoon it was up and running.
  11. WestVanHan
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    WestVanHan Not a Senior Member

    Mas FYI

    -1.7 nm/litre @ 10 knots
    -3.14 km/litre @ 10 knots
    -3.14 km/litre @ 18.5 kmh

    So you are using 5.9 litres an hour with both engines,2.95 litres an hour per engine.

    Going to your post #107 and charts, using diesel's weight at 850 grams per litre, at 3000 rpm it's using 195 grams/hp/hr that would be 230 ml/hp/hr.

    So if you are using 2.95 litres an hour,divvy that by 230 ml and you come up with 12.8 hp.
  12. masalai
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    masalai masalai

    That 10knots is with both engines? - - as 3000rpm each I measured 3Litre/hour each and that gave 10knots in trials immediately after fitting the new propellers... on one engine 6knots at 3000rpm...

    - - Thanks very much, I am well pleased with the performance... So I am using about half the theoretical engine power to achieve comfortable cruise... After the winter season on the hard I will see what she will do flat chat (briefly) and test her capability to hold a good sea and surf :eek: :D :D .....

    That could be interesting on the run Cairns to Samarai with a good following sea.....
  13. WestVanHan
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    WestVanHan Not a Senior Member

    That is 12.8 hp per engine....
  14. masalai
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    masalai masalai

    That - I presume is 'at-engine hp' and concurs with continuous rated output at 3000rpm.... 3600rpm is max rpm... After winter layup and on a clean bum I will be interested in testing peak rpm and what speed, but not for long, as the fuel consumption/hour increases substantially...

    Thanks again westie...

  15. sottorf
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    sottorf member

    For your size of boat a HYSUCAT will not work. It takes special care to make a HYSUCAT work for speeds equiavalent to FnV < 2.2.

    Talking about aerodynamics, the aerdynamic drag on your boat at that speed is probably about 5% to the total resistance. Not much point designing an aerodynmaic superstrucutre if at best you can save 1% in power...
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