Fuel economy vs speed

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

  1. gonzo
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    gonzo Senior Member

    You need to go learn about boats. The simplest concepts befuddle you.
     
  2. rattleandbang
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    rattleandbang Junior Member

    Is that fuel economy or fuel consumption? So in a prefect world, I might be better running closer to hull speed than mooching along just above idle. Of course I've read that a gas engine has maximum efficiency at or close to idle. How that translates into fuel economy, I don't know, and if that effect would be swamped by other factors.
     
  3. gonzo
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    gonzo Senior Member

    That is fuel economy. Fuel consumption will go up, but the speed will increase proportionally more after the hump. That is commonly called "the sweet spot"
     
  4. TANSL
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    TANSL Senior Member

    Consumption always increases with the power which tends to increase with speed. But is not the same the speed for lower consumption (which is reached when the boat is stationary), than the lowest possible consumption for a given route. You should specify better what you are trying to achieve and what loading conditions of the boat, sea conditions, etc.
     
  5. rattleandbang
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    rattleandbang Junior Member

    Thanks! What I wanted to know. I'll give it a try.
     
  6. Barry
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    Barry Senior Member

    Go to the Google search engine and type in "images fuel curves boats"
    There are a myriad of graphs. Click on anyone of them and they will usually take you to a website that explains the type of boat and you can perhaps match one close to yours.

    Note that fuel consumption, gallons per hour, is really not very useful as Gonzo and others stated. You are really looking for miles per gallon, or km per litre (or litres per 100 km as per most land vehicles)

    The graph that is the easiest to use, for you application, will have the miles per gallon on the vertical "Y" axis and the speed on the "X" axis.
    You will see that at slower PRACTICAL speeds that the you get the best distance travelled per gallon of fuel.

    Also, you might check Boating Magazine's websites, archives. They have always tested their boats and given graphs of fuel usage wrt speed and might even have your boat or one close to it
     
  7. PAR
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    PAR Yacht Designer/Builder

    The point of emptying the boat to is to establish a base line, which you don't currently have and to find how much loading you're carrying. Most boats (read damn near all) have lots of things aboard that have been long forgotten and are simply ballast, along for the ride. I've seen 4,000 pound boats (about what yours should be) carrying another ton or two in extraneous stuff, most of which they simply didn't realize. I performed this very thing on a 30' cruiser a couple of summers ago. It was a 3.5 ton boat, but the travel lift said it was closer to 6. We unloaded the boat and you could have rebuilt the twin 302's with the spares he had aboard, 3 extra anchors, other than the one in the forepeak locker, even hunks of plywood he used to cover the cockpit once. His boat nearly doubled it's top speed once emptied and this simple act gave him an opportunity to access what was truly needed aboard and what could stay in the dock box, including the two cracked and two perfectly good exhaust manifolds he had stuffed under a settee. After you establish the baseline, you can put stuff back in, but a few WOT blasts around the bay, will offer some telling info, about how this load is affecting the boat (or not).

    With your boat (again) "mooching" along below full plane mode will bring two things - lousy handling and dead engines. Your engines are designed for two modes of operation, idle and WOT. There are instances where you'll motor at just off idle and a bit below WOT, but for the most part, over 90% of the time, the engines will be cranking out near maximum output. Asking them to lug along at displacement speeds, will just foul the plugs (at first) and eventually cause issues. If you want to chug along, you need to regear, so the engines can operate near their optimum output band and provide the speed you desire.

    Lastly, unless you're grossly overweight, your Crusader 220 (305?) should easily get you close to double what you're getting now. I estimate high 20's at the least. This means there's something up. Maybe the engine hasn't any compression, maybe the trans is laying down on you, maybe she's carrying a lot more than you think she is, maybe it's just a tuning thing, maybe a poorly matched prop, but it's likely a combination.
     
  8. rattleandbang
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    rattleandbang Junior Member

    Fair enough, but all this boat is carrying is camping gear and some tools, nothing like what you're describing with the other boat. I only have one functioning fuel tank right now and the second tank if full likely weighs almost as much as my gear aboard. Plus one iron tank was replaced with plastic. I doubt weight is an issue.

    Carb might be an issue. I've had a lot of problems with the original 4GB carb and look to replace it soon. It's possible it's not providing full fuel to the secondaries, although the engine does reach over 3,000 rpm, I suppose that's not necessarily indicative of power produced. You'd need earplugs if you planned on cruising that way anyway.

    And I've not had any problems with handling, although getting in and out of a tight marina without any keel to keep it going where you point it has been a steep learning curve.

    Funny thing about the fuel curves I've found there are several that show steady decrease in fuel economy with speed until the boat plans after which it pulls up again. In those curves lowest RPMs provide the best MPG.
     
  9. PAR
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    PAR Yacht Designer/Builder

    As Gonzo mentioned, the fuel curve will rise, proportionally with engine load and RPM, though displacement speeds, then will hit a wall around 1.3 S/L ratio (about 7.5 MPH on your boat), where the fuel consumption will jump way up, as you push through the bow wave. As you transition through semi plane to full plane mode, the consumption will come down a little, but will still be much higher than displacement mode. As RPM's rise in full plane, you can expect fuel use to rise again.

    Your engine/drivetrain likely has some issues. It could be a bunch of things or just something simple. Rebuilding or replacing the carb is simple and inexpensive enough. Check the distributor and related gear. If it was me, I'd just pull a comprehensive tune up on this old gal, replacing all the consumables, like the cap, rotor, wires, checking things as simple as the throttle cable pulling to full WOT, timing, etc. You should be pulling more than 3k RPM at WOT. A brand new 650 CFM spreadbore is in the $350 range and can solve a lot of issues, if you determine the carb is past a rebuild (not uncommon).
     
  10. gonzo
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    gonzo Senior Member

    The engine should reach about 4200-4400 RPM wide open. If it only does a bit over 3000 there is a problem. It could be the carburetor, or it could be overpropped, etc.
     
  11. rattleandbang
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    rattleandbang Junior Member

    I've never even checked to see if the throttle opens it all the way.

    The damned anodizing has crapped out in the float bowls, leading to whitish junk forming very quickly after I clean it out. My last ditch effort to salvage the beast has been to line the bowls with epoxy. Carbs are expensive up here, an Eldebroke will set me up over $500+, and that's to go at a speed I don't want to. My biggest problem has been the corrosion clogging idle jets!

    Just about to reinstall it and we'll see how it goes.
     
  12. WestVanHan
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    WestVanHan Not a Senior Member

    My past boat,a 50' planing cruiser,would get the same or better mpg at 6-9 knots than 35-40' trawlers..which really annoyed friends with trawlers.
    At 9 and over she'd start swilling the juice until about 14,then the mpg was almost the same from then till 32 knots. I'm talking about .1 to .2 mpg difference between 15 and 32.
    With the engine torque curve/gearing/props she got slightly better mpg at 24 than at 17knots.

    My new boat of similar configuration is much larger than 50',and has similar consumption characteristics in that the difference in mpg from 17 knots through 34 knots is only .1 mpg- from .5 mpg at 17 to .4 mpg at 34 knots.
     
  13. PAR
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    PAR Yacht Designer/Builder

    You can play with a near dead carb all you want, maybe enjoying some success, but frankly, sometimes it's just best to toss the damn thing over the side and put a new one on. Don't buy from Elderbrock, which will be a Performer series 1409 (600 CFM), which is the marine version of the Quadrajet. This is about $450, plus shipping. If you go to Summit or Jegs, you can get free shipping and the cheaper choice is the Jet 33009, which is essentially the same thing (R4 quadrajet), but about $350. The Jet carb comes setup (jetted) for a 350, so it'll be a little rich, which can be tuned down, if necessary. Simply bolting this puppy on will make a huge difference, over one that's proving cantankerous. You might even get better pricing from one of the auto supply stores (Advance, AutoZone, NAPA, etc.), but insure you get the marine carb, not an automotive equivalent. There may also be a remanufactured replacement option, which is usually a lot cheaper.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    The engine had a recent re-build, but the carb is not up to scratch ? I would be checking compression first, to eliminate that.
     

  15. rattleandbang
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    rattleandbang Junior Member

    The carb is original and suffers from corrosion. The engine was rebuilt in 2006 and I doubt the previous owners put 100 hours on it since.

    Tried out the carb today. Idles fine but wide open will only do 3300, and sure enough the secondary isn't fully open, and its not the linkage. These things have a vacuum secondary and I guess it's not getting enough vacuum. Or something. What a racket though, like the apocalypse. I just can't imagine running a 60 year old V8 at those RPMs anyway, even if I didn't care about the fuel consumption. Ran her for a couple of hours at about 1400 RPM and 6.5 knots and she seemed content enough.

    And PAR, you forget where I live. No way to get a new carb in Canada for that price, and especially not with the loonie at $0.69 to the greenback. Lots fewer options here, especially on Vancouver Island. I'm also suspicious of new parts: a few years ago I restored a 1973 motorhome with a Doge 413, and it needed a new carb. I dropped a $500 Holley onto it and it constantly gave me grief with float needles jamming. Manufacturer blamed it on "modern gas".
     
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