Fuel economy vs speed

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

  1. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    I hear you. Because of course the RV tranny's purpose is to match the appropriate power output of the engine with various speeds of the vehicle, keeping the engine close to an ideal RPM through dramatically changing loads.

    I think I'm starting to understand. So to use yet another inappropriate auto example, driving a planing hull at displacement speeds and low RPM, is equivalent to operating a heavy truck in 5th gear at ten miles an hour? Is this what you meant by lugging? Does the boat really have such a heavy load on a 200hp engine doing only hull speed?
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,302
    Likes: 227, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I don't think you have a problem unless you push it to the point where wave-making becomes more noticeable, there may be as little as one knot between doing it easy, and being put under strain. 5 knots shouldn't cause any dramas.
     
  3. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    Yeah, maybe it's more when trying to do 8 or 10 knots at 2000 RPMs and the bow is skyward pushing a wall of water. You can really feel it working then. But at 1400 RPM the throttle is just barely open, which suggests it's not working at all. My experience of lugging is when the load greatly exceeds the power the engine is capable of producing at that RPM. Generally you have full throttle but very little response, and RPMs slower than they should be at the throttle setting.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,302
    Likes: 227, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Wind and waves will change the equation, of course. And running a planing hull at slow speeds is not really using it to best effect. I'd be wanting to get to the bottom of what is stopping your boat planing, myself.
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Unlike a car (or RV) once your boat is at the speed desired, you're not backing off the throttle. A car gets to any desired speed and the throttle setting, is dramatically different than what was needed, to get it to that speed. Cruising along the highway, you're just barely depressing the pedal, which is enough to maintain speed. This isn't the case with continuous duty cycle applications, because the load doesn't change, unless you want to go faster or slower.

    Just because the throttle is barely open, doesn't mean the load is light, it just means you have a lot more potential available, with the engine and it's gearing.

    Admittedly, running at 6 MPH with a drive train capable of 30 MPH, means it's not busting it's butt much, but it does mean it's way out of it's ideal operating range, which generally should be avoided. At around 7 MPH, assuming a 25' LWL, you're starting to hit the "wall" and a lot more power is required to punch through the hump.

    Barry has hit the point I've been trying to make. In this application (the OP's) he's using a 220 HP engine in an application where a 20 - 30 HP engine would do. Will the 220 HP engine hold up, yeah, probably, but it's not going to like it much and fuel use will be considerably less what a reasonable 20 - 30 HP outboard might be. Ideally, he can change gearing to make the engine/drive more efficient for the different performance envelop or just hang an outboard, tossing the half a ton of SBC and drive, which would dramatically improve maneuverability and load capacity too.

    I too, think all this puppy needs is someone to run through the standard tune up procedures and fix it's ills, which would seem to be a deteriorating carb, though possibly a few other things as well. Then he can have his cake and eat it too, with 30 MPH capability when needed.
     
  6. 7228sedan
    Joined: Nov 2009
    Posts: 314
    Likes: 6, Points: 18, Legacy Rep: 60
    Location: New Jersey USA

    7228sedan Senior Member

    I was not referring to modern outboards that's for sure :) What is being accomplished today is completely amazing.

    I was referring to inboard or stern drive powered cruising type planing hulls.
     
  7. missinginaction
    Joined: Aug 2007
    Posts: 719
    Likes: 53, Points: 28, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    I've been reading through this thread and maybe I missed something. I see a lot of discussion about the boats engine but I'm seeing nothing about the propeller.

    Rattle, I'm wondering....if you're getting 5 knots at 1000 rpm would it be reasonable to consider changing your prop pitch a bit. It seems to me that 1000 rpm is just a bit off idle speed. I spend most of my time cruising at displacement speeds as you do. The difference is I'm running at 1200 to 1600 rpm or so. I find that 1500 rpm is the most efficient slow cruise speed for my little rig. It's also smoother on the engine, you can just feel it. Certainly not the rpm's that PAR would like to see, but not idling either. I would think that a competent prop shop could get your rpm up a little bit while keeping you in the comfort zone regarding vibration and noise.

    I'm pretty anal about engine maintenance. I'm running one step hotter on my plugs than the manufacturers spec and I'm not having any issues with fouling. I do have a new carb though, so that probably helps too. I hope things get better up there for you economically. I caught that in one of your earlier posts and understand.

    I run a 25 1/2 foot 1973 Silverton, single 302 Ford, v-drive, weighs about 8,000 lbs. The engine is an original Palmer Model 220 from 1973 and still runs fine, about 1100 total hours running time.

    Good Luck,

    MIA
     
  8. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    I've changed my MO since I started this thread, which after all, was about finding the most economical way of operating my boat. I now run at hull speed, which puts me around 1500 RPM. I've never measure my LWL, but I was imagining around 24' so I looked at 6.5-7 knots max. Of course all that is dependent on the tides around here which easily run a couple of knots. If I insist on 7 knots, but I have a 2.5 knot current against me, obviously I'll be burning a lot more fuel than I should be. Same if I'm fighting wind/chop. So I'll look to keep her around that 1500 Rpm unless I have an urgent need not to.
    Having said that, the boat's much louder at 1500 vs 1000, and hours of that is tiresome. Currently there's no muffling at all and I'm going to have to do something about that. As well as a new carb etc. The fun never stops.
     
  9. missinginaction
    Joined: Aug 2007
    Posts: 719
    Likes: 53, Points: 28, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Oh God yes, mufflers. I remember my old cast iron mufflers. Made in 1973 they were heavy, really heavy and didn't do much. Start saving up. There are some really quiet mufflers out there. Based on what you're writing you and I are kindred spirits. We like it quiet.
    I dock next to an older gentleman who is a live aboard. I like to go out early, sometimes right around dawn. He loves my boat because I never wake him up.
    I spent more than I wanted to but it was worth it. These would be almost silent at the RPM you like to run.

    http://www.centekindustries.com/vernatonemkiiround.html
     
  10. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    Mine has nothing but straight pipes. When I fire her up it scatters gulls throughout the marina. My boat is known as the loud one. Scrares the crap out of folks in nearby boats not expecting it. It's a rough muscle car sound which sounds cool for the first 5 minutes but after that it's just a PITA.
     
    1 person likes this.

  11. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    'HULL SPEED " or the sq rt of the LWL is an expensive place to operate.

    The SQ RT of the LWL times 1.1 or 1.15 may reduce the power required by half.

    This will not mean the fuel burn will be cut in half as the internal operating HP requirements will not change much.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.