Sea Trials

Discussion in 'Boat Design' started by Deering, Oct 12, 2018.

  1. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I read through your original thread.
    Good on ya Deering.
    My hat is off to you.
    PS I gotta tell you, I love those canoe ends with props!
    (How big are the props?)
     
  2. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Thanks. 22” diameter by 20” pitch. 4 blades.

    The hope is that the keel and skeg under the prop will give it good protection from the big debris we have floating around here, as well as the riverbank I park it on.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Did you attempt to work out the PC...propulsive coefficient ...for your arrangement from the sea trials, with that prop?
     
  4. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I am curious about how high your centers are versus the beam.

    It looks like you have good clearance.

    What is your draft and can you beach with the bulbs?
     
  5. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    No, I did not. How would I do that, and how could I utilize that information? I did capture speed, fuel consumption, and RPM at a range of speeds. Would that be enough information to work with?
     
  6. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    I really could use higher clearance for the tunnel. Because I adapted a planing hull it wasn’t optimal. My sea trial conditions didn’t allow for adequate testing of that element. Typically I’ve found with the cats that I’ve owned is that if I’m experiencing pounding in head seas I veer off about 15 degrees and that generally resolves it. Forces me to ‘tack’ a bit but the penalty isn’t that significant. Hopefully this hull will perform similarly.

    Draft is 32 inches, lowest point being the skeg that runs under the prop/rudder. Yes, I can beach with the bow extensions. The boat lives on the tidal flats during the winter and is quite happy there.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    When you calculated your expected speeds, did you do this by establishing the resistance, or via other means?
     
  8. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    By establishing wave making and wetted surface resistance for one of the hulls. The estimated DL (one hull) is about 61. LWL is 51 ft.
    I did not attempt to factor in hull interaction resistance - I couldn’t find definitive, consistent guidance on that from the multiple sources I consulted other than a general “wider is better”. A Shuttleworth design of a very similar DL and dimensions found a 10% increase in resistance over what would be predicted from a single hull, so that’s what I used as an assumption.

    Horsepower usage from sea trials aligned pretty closely with what the resistance calculations predicted.
     
  9. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Ok.
    Without an accurate EHP measurement/prediction, naked hull resistance, it will be somewhat tricky to calculate. But if she performed as you expected, that is good enough :cool:
     

  10. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    I didn’t have time on my one short sea trial to take detailed measurements across the entire range of performance, but the trend was that at the lower speeds I came in right about on my predictions for engine RPM and fuel burn. But as the speed increased the actual vs predicted performance got better and better - i.e. almost 15% less fuel usage for a given speed.

    Top speed was 2 kts faster than predicted too, and there was no apparent leveling out of speed - if I had more power it seemed that the speed would have continued to increase.

    But I have to caveat those results. I was running lighter than the assumed displacement in my estimates. I would expect an additional ton of fuel/water/payload for full cruising so that will probably push the performance to align with predicted. And I only have a few solid data points from my sea trials where I captured velocity, RPM, and fuel consumption, so I don’t want to draw too much from that. My main focus during the trials was to see if the boat would break in half or some other catastrophic event might occur. Further testing will have to wait for spring.
     
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