Transition from plow to plane

Discussion in 'Powerboats' started by Tim Lamp, Feb 15, 2019 at 1:41 PM.

  1. Tim Lamp
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    Tim Lamp New Member

    Observing RPM, MPH, GPH, and MPG data on planing power boats there is often a significant jump in speed when transitioning from plowing to planing. Throughout the plowing spectrum speed and RPM's are generally linear, and likewise when planing. For instance, for every 500 RPM increase, speed increases by around 2-5 MPH. But there is a point where the same incremental increase in RPM's, e.g. 500, takes the boat from plow to plane and the speed increases exponentially, say 15 MPH. I understand why this is the case, but am wondering what the practical implications are when sea trialing a boat for different purposes. For example, in some situations, e.g. heavy seas, the optimal speed might be 15 MPH, but a particular boat may go from 10 MPH at 3,000 RPM's plowing, to 25 MPH at 3,500 RPM, up on plane, making it impossible to hold the speed and have control of the vessel at 15 MPH.

    Has this been discussed anywhere in this forum? Has anyone found any particular types or combinations of hulls, deadrise, power-to-weight ratios, or power-to-length ratios, beam, etc. that minimize this effect?

    Thanks.
     
  2. HJS
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    HJS Member

    This is what every seriously working designer works with all the time. It is important to optimize the various input parameters against each other so that the result is optimal, no compromises.
    No guesses, just calculations.
     
    Last edited: Feb 15, 2019 at 3:35 PM
  3. DCockey
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    DCockey Senior Member

    It sounds like you are interested in the "hump" in resistance as a boat starts to plane. The magnitude of the hump is strongly influenced by the weight of the boat relative to it's length. With similar hull shapes a heavier boat for it's length will usually have a larger hump. Another factor is the longitudinal position of the center of gravity of the boat. The further aft the center of gravity the larger the hump. My impression is lower deadrise angle typically results in a smaller hump if everything else is "equal". Power does not influence the magnitude of the hump but the magnitude of the hump may be a factor in deciding on how much power to install.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Having plenty of (in) trim range in your drives, is probably the single most helpful thing. Generously sized tabs may help, but you don't get much lift from them in the early stages of the "hump".
     
  5. Yellowjacket
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    Yellowjacket Senior Member

    The difference in the boat speed to engine speed relationship has to do with prop slip... At low speeds and loads there isn't much slip since the loading is low and the prop not doing much work. As the speed and load increase and the boat approaches hump speed slip goes up because the loading is high and the speed is still low. Once you get over the hump and speed jumps, the amount of water that is passing through the prop disk area (think of the prop swept area as a disk), is higher so the prop loading (surface pressures on the blades) drops back down and slip is decreased. So what you are seeing is really just slip. In a perfect world with no slip, the engine speed would correlate with boat speed from idle to top speed. In the hump range, prop slip is at its highest value because of the high load and the lower boat speed. Now if you change props, or move the CG forward, or trim the motor differently, or add or change trim tab position, you can change the power demand vs speed relationship, and change the amount of engine speed vs forward speed you observe. But what you are seeing is really just slip and the amount can be calculated based on pitch and engine rpm at a given speed and compared that with the theoretical speed for the same pitch and rpm. If you take that data you'll see slip going to a maximum near the hump speed and then dropping off as you go faster than that.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I think he probably realises it is a high resistance stage for a planing boat, but wants to smooth out the curve a bit. It is not a bad idea to run a slightly lower pitch, and maybe more prop blades, if routinely running at backed-off speeds. Failing that, get a good power cat, and backing off is rarely necessary !
     
  7. Tim Lamp
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    Tim Lamp New Member

    Thanks all. Great input and you all have provided the context I was looking for to do further research and decide on my next boat. I am starting to compile a lot of performance data across different manufacturers, like Grady White and Robalo, or whatever I can get the data on (GW posts all of their performance data on line for each model and various engine set ups), and a few different styles of boats, primarily Center Consoles and Express Cabins, because I think I would prefer a CC but the wife wants the option to get out of the weather. We intend to do mostly coastal cruising in the FL Gulf Coast area, less than 20 miles off shore, unless I decide to head to Key West or Bahamas, or do the Great Bend some day, in which case I may just want a different boat... All that said, so far I like to profile of the GW Canyon 376 with triple Yamaha 300's as follows: 2500 RPM: 11.8 MPH; 3000 RPM: 20.4 MPH; 3500 RPM: 29.1 MPH. So far most boats I have looked at are pretty consistent before and after the hump/plane with one big jump in speed of around +15 MPH at the 3000-4000 RPM area. You're either doing 10 MPR or 25 MPR but you can't go 15 MPR. But the Canyon 376 has two small bumps in speed, so like what Mr. Efficiency said, it smooths out the curve a bit. This is all very preliminary, but I wanted to give you all more insight into what I'm trying to accomplish and what I'm looking for in a boat. And while the Canyon 376 looks like an awesome boat, it's really out of my price range. I'm trying to stay around the $150,000 budget. When did boats start costing as much as airplanes? Cheers!
     

  8. Tim Lamp
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    Tim Lamp New Member

    And I should add that Gallons Per Hour and/or Miles Per Gallon are of course big drivers!
     
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