Prop/rudder optimization for longe range single engine displacement vessel 30k lb displacement

Discussion in 'Hydrodynamics and Aerodynamics' started by KeithO, Sep 13, 2019.

  1. KeithO
    Joined: Jul 2019
    Posts: 112
    Likes: 11, Points: 18
    Location: Michigan

    KeithO Senior Member

    I started this thread originally with a question regarding a Yanmar pod drive, but the more I investigated it, the more convinced I was that a pod drive was not the solution. Especially so since the pod only allows thrust vectoring at low speeds with the rudder locked straight ahead and control with a joystick. Had thrust vectoring been allowed at any speed, the pod could have eliminated the rudder and associated appendage drag at cruise speed. The leg of the pod would probably be a perfectly fine rudder at cruise speed of 7-8kt.

    The potential bot for this application comes in 2 basic forms:
    1) A converted sailboat in the 50-55 ft length category, which would have the needed internal space for the needed 1000+ gal of fuel. Best case scenario , one could obtain such a boat after a dismasting or one that needs a lot of work to the rig and sails, since that would lower the cost significantly.

    2) Building a new George Buehler style multichine hull intended for this purpose.

    Option 1 would require keel modifications to reduce ballast and most likely a re-power, unless it had a fairly current engine. It is likely to need a different transmission ratio for a bigger prop since that would be the primary means of propulsion and sailboats tend to care more about prop drag when sailing than having the most efficient prop for motoring.

    Option 2 would have a drive train layout intended for the purpose but not neccersarily optimized for drag. There are multiple potential modifications for improved propulsion efficiency, and one has to decide which of any options to pursue. I will make a separate post listing the options based on those listed as options by major ship builders.
     
  2. Deering
    Joined: Feb 2005
    Posts: 477
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    Location: Juneau, Alaska

    Deering Senior Member

    What is your objective with this boat? What do you want to do with it?
     
  3. KeithO
    Joined: Jul 2019
    Posts: 112
    Likes: 11, Points: 18
    Location: Michigan

    KeithO Senior Member

    Deering, this is something along the lines of "voyaging under power". Fuel capacity to go from Panama to Hawaii with reserves. 50-60 ft water line length. Beam around 12ft. Inboard diesel of about 80hp. Cruise speed in eco mode would be 6.5kts with the ability to go a little faster against a tidal race.

    I have gone round and round on the range issue and it seems that feedback on this forum is 4000 miles for a powerboat making long pacific crossings. Given a target fuel consumption of 3nm/gal that means 1333 gal of fuel which is about 9000lb in fuel weight, + the fuel tanks themselves.

    Idlewild had a capacity for 1000 gal of fuel and apparently the consumption was closer to 4nm/gal but she only had a 55hp engine and she may have been run pretty slow to get those numbers. She was built to set a few records and thats what they did at a heck of a cost for the portage down the peace river nearly 2000 miles. I don't think I will be going with an aluminum hull, too expensive. I think she also set a record for the longest un-refueled non stop voyage too.
     

  4. Joakim
    Joined: Apr 2004
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    Location: Finland

    Joakim Senior Member

    You should be able to do much better with a sailboat hull or similar. E.g. Swan 65 S&S (50' LWL) is about 32 tonnes and should do 6.5 knots in calm with 15 kW power. The consumption would be 5 l/h and thus about 0.8 l/nm or about 5 nm/gal. Your planned displacement is less than half of that.

    Here's a smaller boat with the same as your target displacement, but only 40' LWL. https://muldermotoren.nl/media/wysiwyg/Muldermotoren/PDF/Folding_prop_test_Volvo_Penta.pdf
    It's consumption was 3.5 l/h at 6 knots and 6 l/h at 7.1 knots with the best propeller. Would be less than 5 l/h at 6.5 knots.
     
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