Slipperlaunch electric pod engine power

Discussion in 'Boat Design' started by serdarbas, Mar 30, 2024.

  1. serdarbas
    Joined: Nov 2023
    Posts: 9
    Likes: 1, Points: 3
    Location: rotterdam

    serdarbas Junior Member

    Hello;

    I'm currently building a slipperlaunch and I decided to opt for an electric pod drive. I would like to have a relatively fast cruising speed but I don't have theoretical knowledge regarding propulsion calculations.

    upload_2024-3-30_16-39-41.png

    The waterline is 7.4 meters (24 foot). The hull speed calculator tells me the max speed as 6.6 knots but I don't know how much hull speed applicable to this design.

    I have options between 6hp to 25hp pod selections. Is there a limit where the extra hp added has almost no benefit on max speed?

    upload_2024-3-30_16-50-30.png
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

  3. serdarbas
    Joined: Nov 2023
    Posts: 9
    Likes: 1, Points: 3
    Location: rotterdam

    serdarbas Junior Member

    Are you serious?
    I'm always grateful to the people who knows way more than I do that chooses to take their time to share their knowledge to help me.
    But the answer you have given is borderline narcissistic?

    Did you seriously threw a book at me to read and understand which is like the image below?
    I'm just a novice boatbuilder guy who respectfully asks for a basic questions regarding how much HP would people here recommend?
    You probably was well aware from my question that I'm a novice boat builder, yet you simply took your time just show off your advanced knowledge while being 0% helpful?

    I'm an ophthalmologist. Imagine that you complain about your dry eyes and I give you a link to a advanced medical book without any further comment. How would you feel?

    upload_2024-3-30_20-49-24.png
     
  4. Dolfiman
    Joined: Aug 2017
    Posts: 1,523
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    Location: France

    Dolfiman Senior Member

    I can propose you my formulations for just an order of magnitude, in quote #10 here below :
    7.5m powerboat design i have been working on https://www.boatdesign.net/threads/7-5m-powerboat-design-i-have-been-working-on.67808/#post-942188
    >>>
    V = 502,56 * (0,087-0,0015 Beta)^0,5 * (P/m)^0,5
    V = Speed in Knots , Beta = Deadrise angle (°), P = Power installed in Kw , m = weight in kg


    The above formulation being valid as long as you are at least over the lowest planing speed that you can also estimate by :
    V = 7,2 LCG/ Bc^0,5
    LCG : longitudinal center of gravity from aft (m)
    Bc : chine beam aft (m)
    V = Speed (knots)

    A first numerical approach to illustrate :
    For your boat, let.s suppose your LCG at 2.8 m (~ 38% L 7,4 m) and Bc ~ 2,2 m
    >> Minimum planing speed ~ 13,6 knots

    Let’s suppose your weight in charge m = 1000 kg and the aft deadrise 7° (like for the Rascal of which results I used to calibrate the formulation) :
    With 6 hp = 4, 5 kW >> V = 9,3 knots not significant as you are lower the minimum planing speed
    With 25 hp = 18,6 kW >> V = 19 knots
     
  5. serdarbas
    Joined: Nov 2023
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    Location: rotterdam

    serdarbas Junior Member

    This was really helpful, thank you!
     
  6. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Since you're already building there must be plans. The questions are:
    What engine was specified in them? How much fuel was supposed to be on board? What was the design speed? What's the boats light ship displacement and the maximum loaded displacement? What do you consider to be a "relatively fast cruising speed"? How much time should the battery last at cruising speed?

    When going electric the problem isn't one of power, it's the weight of the batteries for the required endurance. For example let's say your boat was designed along the lines of the originals, so 4 persons, one 25kW gasoline engine and 50l of fuel. That's 4x75kg =300kg + 120kg + 50kg = 470kg. If you fit a 25kW electric motor and wish to power it for one hour you get: motor 30-50kg + 25kWh battery 330-350kg = 360-400kg. This means that if you want the same speed as the gasoline version your 4 person boat just transformed itself into a one person boat.
    You can of course fit a 50kW engine and embark the passengers while keeping the speed, but then you can only drive for 30 minutes. Or you can significantly lower the speed, and you get more endurance. Or you keep the design displacement with whatever motor+battery fits in the weight limit and live with the speed you can get.

    To answer your initial question yes, there is a point where adding horsepower has no benefits, its when the boat becomes unsafe to handle.
     
    BlueBell, C. Dog and bajansailor like this.
  7. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Serdabas, please understand that Jehardiman was not being narcissistic with his remarks and the reference. He was only trying to be helpful. He is one of the several credentialed Naval Architects who participate in this forum. I will reply with a rudimentary explanation about boat types and in so doing I certainly do not mean to question your intelligence.

    The boat that you have shown us is a "planing" hull. It is not a "displacement" hull. Hull speed approximations apply only to displacement hulls. A planing hull is designed to use pressure on the bottom of the boat so as to raise it up upward in the water so that it is just skimming over the surface. The pressure is caused by raising the bow slightly so that the oncoming water will create a pressure on the bottom. A displacement hull is designed in such a way that it does not rise and skim over the water. It merely pushes water out of the way as it moves along. Displacement boats are generally pretty slow while planing boats can be very fast.

    A boat, similar to the one you have shown, will need a considerable amount of power to make it plane. It will be very inefficient when not in planing mode because it has design features that cause much drag when going slowly. There is a relationship between the total weight of the boat and all its' cargo. That relationship partially determines how much power you will need to make the boat perform as intended. There are other variables such as bottom shape and bottom area that play into the equation.

    Your picture of the partially completed boat shows that you are an accomplished craftsman. Kudos to you for what appears to be fine work.
     
    bajansailor likes this.

  8. Dolfiman
    Joined: Aug 2017
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    Location: France

    Dolfiman Senior Member

    I put a correction (I forgot the electrical option of your project) . The formulation is based on installed « combustion » engine, but for an electrical engine, 1 kW « electrical » is roughly equivalent to 1,5 kW « combustion » in terms of power (and ~ 2,2 in terms of torque) . So the figures for the example become :
    With 6 hp « electrical » = 4, 5 kW = 6,75 kW « combustion » >> V = 11,4 knots still not significant as you are lower the minimum planing speed
    With 25 hp « electrical » = 18,6 kW = 27,9 kW « combustion » >> V = 23 knots
     
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