Racing Powerboat Prop Selection

Discussion in 'Props' started by A Frayed Knot, Jul 25, 2019.

  1. A Frayed Knot
    Joined: Jul 2019
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    Location: Southampton

    A Frayed Knot New Member

    I am a University Student studying Naval engineering working on an electric driven high performance planing racing powerboat (8m in length). I am trying to find a suitable propeller design for this craft. From modelling resistance I am looking to achieve 4kN if thrust with an input engine power of around 250kW. With these inputs the expected hull speed is around 50kts. I am basing the motor setup off a 250Hp Honda outboard engine.
    I have studied KT-KQ Propeller charts however I can only find the wageningen b series charts online. For a racing craft I am struggling to find actual propeller design information/
    specifications.

    From modelling and assumptions I have a Wake Factor of 0.02, Thrust deduction factor of 0.09 and a relative rotative efficiency of 0.96.
    I have assumed a 4 bladed 35cm diameter (20') prop with an RPM of 3000. However this gives me a Prop advance coefficient of J = 2.2. This is a much larger value of J than is plotted on the b series charts.
    Changing the propeller RPM to 6000 seams very unreasonable however I have not been able to find information to confirm or deny this.
    What is a reasonable range of RPM's a propeller would experience?
    What is a suitable diameter, Pitch and BAR? And how did you come to this conclusion?
    Is there any KT-KQ data for more high speed props?

    Any help or pointers would be much appreciated
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    There is a discussion on super-cavitating propellers in the 1988 edition of PNA (Vol II, not sure where it falls in the new multi-volume edition) as well as surface piercing propellers. The references list the DTMB Report 1637. A quick net search yields these two papers.
    https://apps.dtic.mil/dtic/tr/fulltext/u2/297120.pdf
    https://apps.dtic.mil/dtic/tr/fulltext/u2/p012090.pdf

    Edit to add, I assumed that you would be needing a super-cavitating prop, though really you would have to work the numbers as explained in PNA.
     
  3. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    You're going where standard propeller theory and CFD modeling really isn't going to give you any good results. Part of the reason is that surfacing props are running in a mixture of air and water and there is much more empirical learning that has been applied over the years. Surfacing props really started with outboard racing in the early 50's and thousands of man hours were spent testing and developing props that optimized performance of small boats with moderate horsepower motors. As offshore racing progressed, they started with outdrives and eventually went to surfacing props and drives. All of these have been highly developed over the years. For these reasons you're going to find that these props don't fit standard metrics. We team on some programs with an expert designer of conventional propellers and waterjets and I asked him if he could do an analysis on my racing runabout surfacing prop.. He simply said "go to the experts that are making these, they know what works and how to design it, it simply doesn't comport to any of our normal metrics"... You would be best to go to someone like Craig Dewalt, or Ron Hill, these guys know what works and how to do what you want. These people aren't writing technical papers because their livelihood is based on figuring out what works and what doesn't, and it doesn't get shared... You might try to do some analysis on something you buy but as I said it isn't going to look like anything that is conventional.
     
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  4. A Frayed Knot
    Joined: Jul 2019
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    Location: Southampton

    A Frayed Knot New Member

    Thank you both so much for your responses. I haven't quite found the specific answers I needed yet however these have opened up some good threads to follow!
     

  5. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    To give you an idea of size and area and rpm, an APBA "D Stock" class runabout uses a prop that is 6.5 inches in diameter and spins it at about 7400 rpm. These are 3 bladed surfacing props with the shaft about 3/4 of an inch below the surface. At that 7400 rpm they should be in the mid to high 70's with a max of maybe 80 mph These motors are making 55 hp at the prop shaft. These props measure usually around 12 inches of pitch or a bit less.

    The 45ss tunnel hull boats are running similar rpm and more power and these guys are using 8 inch props. Those engines make about 74 hp at the crank, so deduct some for the gearbox and you probably have 65 hp at the propshaft. They are running in the mid80's and those props measure in the 12 to 12.5 inch pitch range.

    One of the issues with both of these rigs is getting them on a plane. Typically that is a struggle as the prop ventilates and sometime it limits how high you can run the prop. An 8 inch diameter 45ss prop on a "D" class boat will plane much more easily than a 6.5 inch "D" prop. That is likely to be a big issue in that you will find that with batteries you're going to have a lot more weight and it's going to be much harder to get on plane. If you use an outboard setup you can use the power tilt to tuck the motor a lot more when you're getting on plane and that gets the prop deeper and that helps a lot.

    Attached is a picture of a 45ss prop, this one is exactly 8 inches in diameter.
     

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