Fuel economy vs speed

Discussion in 'Powerboats' started by rattleandbang, Jan 11, 2016.

  1. rattleandbang
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    rattleandbang Junior Member

    I hope this isn't a beaten to death topic, but search didn't help me much. I'm an ex-sailor recently switched to a 1957 27' cruiser locally built that looks very similar to Chris Craft/Owens of the period. She is powered by a single Crusader 220, which is a small block Chevy. Wide open she'll do about 16 knots, but due to engine/exhaust noise, fuel consumption and the fact I'm used to taking it slow, We generally putt along about 1000 RPM which gives about 5 knots depending on sea conditions.
    I enjoy that she uses only about a gallon an hour at that speed, but I realize that I might be fooling myself, and wonder what higher speeds would return, because of course though I'll burn more fuel per hour, I'll get there sooner. It's very difficult to simply learn by doing in my area because sea state, winds, and currents vary enormously and comparing various trips for fuel consumption is haphazard at best with these confounding variables. I know that once one exceeds hull speed fuel consumption goes through the roof, but below hull speed is there a rule of thumb in regards to speed and economy? Is slower always better?
     
  2. gonzo
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    gonzo Senior Member

    Actually, low planing speeds can be more economic than dragging the stern. The only accurate way to find out is with a reliable speedometer or GPS and a fuel flow meter.
     
  3. rattleandbang
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    rattleandbang Junior Member

    She won't plane. Her bow goes skyward, for sure, but she doesn't lift up and out and take off, just pushes more water. I was told she was a displacement hull when I bought her.
     
  4. gonzo
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    gonzo Senior Member

    Can you post a photo? I can't think of any late 50's Chris Craft that was a displacement hull.
     
  5. rattleandbang
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    rattleandbang Junior Member

  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Might be a tired old engine, it ain't a displacement hull doing 16 knots. May be overweight too.
     
  7. daiquiri
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    daiquiri Engineering and Design

  8. gonzo
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    gonzo Senior Member

    I believe it should be a Delta Conic design.
     
  9. rattleandbang
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    rattleandbang Junior Member

    Sorry, no photos of below. Engine isn't tired, recent rebuild, 4 barrel Rochester and a mean cam. But she's a substantial boat with only one V8; I don't see how she was ever intended to plane. She'd need a lot more power. But plane or not, the original question still stands: does it make much of a difference in efficiency/fuel consumption as long as one doesn't exceed hull speed? Or is there no rule of thumb about that? Her "sweet spot" if there is such a thing seems to be about 7 knots. After that vibration and noise and wake really seems to climb - which I suppose isn't surprising given hull speed...
     
  10. daiquiri
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    daiquiri Engineering and Design

  11. rattleandbang
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    rattleandbang Junior Member

    Right, but the difference is that Owens has a twin engine needed to bring it up on plane, while mine doesn't...And while I don't have her underwater profile, she is much deeper in the forefoot than aft; the bilge pump is right forward and any water collects there, while the aft areas flatten out and are dry, although that's where the engine and iron tanks are.
     
  12. PAR
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    PAR Yacht Designer/Builder

    That's a typical warped bottom of the era, though Chris Craft and Owens used fairly different shapes, on their hard chine design, both were relatively easy to plane off with modest power.

    That hull is burdened a bit with top hamper and its associated weight, but with 200 HP she should easily see the high 20 MPH bracket, possible the low 30's. I have a 1960 Chris Craft, though a sea skiff hull form, quite similar and her 185 HP 283 drives her into the mid 30's.

    It's likely you have some issues. The first thing to do is empty the boat. I mean everything that's not physically screwed down should be removed, every locker, every extra dock line, every cushion, everything removed from the boat. Next weigh the boat, which can be done at the local land fill, concrete block manufacture, actually lots of places. This will give you a baseline to compare (she should be in the 2 ton range). Next launch the boat and see how much better she does in a light condition.

    Your assumptions aren't appropriate, nor accurate. The Owens, doesn't need twins to get on plane, it was just a common option back then, because engine reliability wasn't what it is today, not to mention the huge maneuverability improvement with a twin setup. The bilge pump is properly located forward, just aft of the forefoot. This is the low point when the boat is static. Underway you could have one aft if desired. I've installed both on my Chris.

    These double wedge hull forms are very efficient and hop up on plane quickly in smooth water. They don't have enough deadrise to tolerate rough water service, without pounding badly, but the wise skipper throttles down a bit, to get the forefoot to engage and slice the chop, easing the harshness. Since you're not even getting into a full plane regime, I'd suspect more than just extra anchors, etc. Have the boat hauled and closely examine the bottom planks, particularly around the engine and transom, which I'll suspect are oil and water soaked. This can add a bunch of weight. I've seen soaked boats take up 20% of the boat's displacement in weight gain, though this would be an exception, 10% wouldn't be unreasonable.

    Your boat will always have an immersed transom, regardless of speed. It's the nature of this hull form. Other than huge modifications to the bottom shapes, you're married to a full plane mode hull form. Simply put, fuel efficiency and maneuverability will suck at low speed, compared to something specifically designed to run at displacement or semi displacement speeds. Once up on plane, this particular set of hull shapes, is about as efficient as you can get (there are some slightly better, but not many) and of course the more you push the boat's speed, the more you have to pay at the fuel pump. This is true of most hull forms.
     
  13. rattleandbang
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    rattleandbang Junior Member

    I appreciate your sentiments, although I'm not clear what was "inappropriate" about my post. Having said that, the boat loaded as she is now is how I need her to be loaded; if she'll plane empty is no use to me, nor would I want to even if she could, due to the noise, vibration, and fuel consumption factors as I said earlier. My original question was fuel economy vs speed when traveling below hull speed. I find cruising the Gulf Islands at a slow pace is much preferred to going like a bat out of hell anyway.
     
  14. gonzo
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    gonzo Senior Member

    Fuel economy increase sharply up to "hull speed" and then decreases once the boat get up. The curve depends on the design, but in a hull like yours there will be a hump. To answer your original question: no, slower is not always better economy. However, if you want a range of speed close to 5kt, it is possible to calculate, but it will cost a lot more than a fuel meter. You can also use a small gas tank to calculate how much fuel you use.
     

  15. TANSL
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    TANSL Senior Member

    I find very difficult to understand all this. Any drawing of that curve, please?
     
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