Fuel economy vs speed

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

  1. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    Keep in mind that a powerboat's fuel consumption is typically measured in gallons per hour as opposed to miles per gallon. At best cruising speed, you will be hard pressed to find a gas powered boat that does better than 1.5 MPG. Diesels are typically a bit better at 2 MPG. At 3 MPG I'd be throwing a party! Also keep in mind that by best cruise speed, I am referring to up on plane in the sweet spot as was previously discussed. What kind of RPM's are you spinning at 6.5 knots?
     
  2. rattleandbang
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    rattleandbang Junior Member

    About 1400 RPM
     
  3. sdowney717
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    sdowney717 Senior Member

    What epoxy did you use on the fuel bowl?

    That is really good fuel mileage.

    About the 4GC, the spring tension on the air valve is adjustable.
    You can loosen it, but I bet if it opens all the way when the engine can not suck the correct CFM air rate, the engine will bog badly or stall out.
    Variable CFM carbs are designed to be placed on engines of differing CFM capacity, the carb is supposed to adjust itself to the engine it runs on.
    It does that at a WOT setting by the spring setting of the air valve. The genius is it keeps the air fuel ratio in the proper range at WOT for use on a variety of engines. So the carb matches itself to the ability of the engine to flow air.
     
  4. Barry
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    Barry Senior Member

    As I disagreed with Pars comment above that "your engines are designed for two modes of operation, almost WOT and idle, I called one of our suppliers for marine engines to see if there was any validity to his comment.

    A Chevy small block comes to the marine engine builder with stronger valve springs, Roller rockers, stronger rods, different valves, double induction hardened seats, marine cam and a stainless insert for the water pump to handle raw water.

    None of these changes make the engine DESIGNED to be ran ONLY at idle and close to WOT

    These changes make the engine more durable, granted but do not support a statement to a recreational boater that you should only run a marine engine at these rpms or you will have "dead engines"

    The engine curves from our suppliers 350 show a maximum horsepower at about 5000 rpm, which by Pars suggestion that you SHOULD run the engines at 4500 rpm or you will have problems. Running at 4500 rpm will shorten the engine life as compared to running at anything off idle to say 3500 rpm
     
  5. rattleandbang
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    rattleandbang Junior Member

    Personally I read that as the engines can be run for long periods at the same high RPM with trashing themselves. I think if you took a basic auto old school V8 and ran it for many, many hours at close to wide open throttle, that you'll burn it out in short order. It makes sense that engines intended for this kind of operation would have to be modified and made more robust for what is really punishing use. Certainly the example 413 industrial I was talking about was very different, with much larger heads, sodium filled auto rotating valves, extra coolant passages and a massive water pump compared to the auto 413. I just don't buy the corollary that therefore they must be run at those speeds. It seems to me that what I'm doing is underutilizing the engine's capacity, and that you could likely drop any old auto block in there and it'd be happy purring along at 1400 rpm. It just wouldn't last doing 4000 RPM for hours at a time.
     
  6. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Lots of gasoline powered boats do better than 1.5 MPG. Here is an example:
    http://yamahaoutboards.com/sites/de...stroke_hpv6_pro_rng-1850reata-f150txr-pro.pdf Many more examples can be found looking through test data at http://yamahaoutboards.com/owner-resources/performance-bulletins

    Or was your statement limited to boats of a certain type and minimum size, powered by engines of a certain type?
     
  7. PAR
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    PAR Yacht Designer/Builder

    You'll ruin this engine if running at 1,400 RPM 90% of the time. You're way off it's power band and essentially "lugging" along. Try running a gen set at 1/3 it's usual RPM levels and see what happens. Do the same thing with a lawn mower or prop driven aircraft engine and see where it gets you.

    This (OP's needs) isn't about how and what the engine is designed tolerate, so much as getting a handle on what going on. If he insists on lugging this engine, he should regear it to take advantage of the power band, after he sorts out it's repair needs, which seem very obvious to me. I'll bet if he pulls the plugs on this engine after everything is tuned to spec, they'll be black and oil/carbon covered, after a quick run at 1,400 RPM.

    The cam grind for this engine is quite different from a "street" grind. The reason is simple, the amount of load this engine takes on, as soon as it's put in gear, which is 70% higher than any automotive application, other than drag racing uphill, pulling a fully loaded trailer. The ignition and other engine elements also receive special "marine" treatment, again to accept the loads expected in the marine environment. It's not moisture related so much as load related. Yes, attention to moisture and vapor control is considered too, but you don't need hardened valve seats for this.

    I'm not suggest you "have to" run an engine at 10% - 15% below WOT, but I am suggesting every reasonable skipper finds the sweet spot, which is just off WOT and this becomes the "cruise" setting. The same is true on a lawn mower that uses a fixed throttle setting (about 85% of WOT) for it's full throttle detent, but when the engine hits heavy load (tall grass) a RPM governor kicks in and offers the WOT setting until this load eases. All stationary engines use this technique, for the same reason. Can you run the engine at greatly reduced RPM's, yep, sure can. Is it wise to expect long life from an engine at 80% load and 1/3 RPM settings, nope. Comparing automotive applications to continuous use applications, just isn't apples to apples.

    In any case, the OP needs to pull all the consumables, replace the carb, check the ignition, etc. and fully tune up the engine, before any reasonable assessment can be made of this engine. Mercruiser has offered the 305 for several decades. The 4.7 is relatively new and I suspect it's the second gen SBC.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Interesting question, I was told by an "expert" once, not to line a fuel tank with epoxy.
     
  9. rattleandbang
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    rattleandbang Junior Member

    I'd have to double check the brand but funny, because epoxy can be used to make fuel tanks - plywood coated in epoxy or straight glass epoxy. My research suggests that it's pretty impervious. Time will tell.

    And as for PAR's fear about my engine, I've been running her like that for 8 months now, and she started with new plugs. I'll pull one and see what they look like.
     
  10. rattleandbang
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    rattleandbang Junior Member

  11. PAR
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    PAR Yacht Designer/Builder

    Epoxy has good solvent resistance, including fuel. A common issue with quads is the well plug and the usual cure is a drop or two of epoxy, before reseating a new one. I don't think an epoxy coating is going to help this carb, because it's machined passages are experiencing the same corrosion issues as the bowl.

    Pulling a plug now, might give you some idea of what's going on, but not much. The best approach is to install a fresh set, then take the boat out and give it several minutes of load. Then you immediately shut down, coast to a stop and pull the plugs for inspection. No puttering back to the dock, then pulling them as much of the "indications" will "fluff off" in the ride back to the dock. The way I do it is a WOT blast out to the center of my local lake, shut down, coast to a stop, then pull the plugs for a look. This gives you an unmolested view of what's up.
     
  12. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Attached is a fuel consumption curve for various engines and boats.
    At low rpm and boat speed you maximize the mpg number
    As you approach the transition speed (some like to call this hump speed) the mpg drops and then increases for speeds of up to around 30 mph.
    After 30mph, but well before WOT, the mpg drops as engine rpm increases.
    Running anywhere near WOT is hard on your engine and harder on your pocket book when you have to refuel
     

    Attached Files:

  13. rattleandbang
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    rattleandbang Junior Member


    What this doesn't account for is the use of stock automotive engines in heavy, luggy applications like motorhomes. The standard offering for my Dodge was a unmolested stock 318, and it was like 10,000 lbs. All drum brakes too :) It generally cruised at below 2,000 rpms.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Again, automotive applications (including RV's) aren't the same as continuous duty applications. There's no real comparison between them. BTW, when your RV cruised below 2k RPM, its engine load was below 20% as well and had gone through a few gear changes as well, which is never the case on a boat.
     

  15. Barry
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    Barry Senior Member

    I would define a continuous duty engine that is installed to drive say a generator that requires a relatively constant horsepower and an engineer would size an engine to deliver this horsepower.
    There is no reason to install a 500 horsepower engine to drive a 200 horsepower pump/generator. ( Depending on the application, there is often a case that can be made to perhaps supply an engine that can say deliver 250- 300 hp at max and only take 200 hp off it due to fuel consumption but that is another thread)

    An example
    The curves that I am using are a 350 chevy engine, naturally aspirated with a 4 barrel carb with a max hp out put of 260 prox and a Berkley 12j pump with an A2 impeller.

    A 3,000 to 3400 pound 12 degree boat with the Chevy would cruise at 30- 32 mph (3000rpm) consuming 10 US gallons per hour. We had been given fuel data from Berkley that said that they use a figure of 10 US Gallons per hour for 100 Horsepower. This in essence said that the engine was producing 100 horsepower at our 30-32 mph. The A2 impellor curve showed that it absorbed about 100 horsepower to confirm our numbers.

    A naturally aspirated 350 is able to produce 210 horsepower at 3000 rpm. So our engines were using about one half, 100hp of the 210 hp available at that rpm.

    We owned for about 18 years a 17,000 pound Silverton with twin 350 Chevy engines.
    The first engines gave us about 2200 hours before corrosion began to cause failures of manifolds, distributors, starters and alternators. These engines probably had 1/3 of the time at about 3400 rpm, cruise at 17 knots, well below WOT, and the rest of the time was spent trolling at about 650 - 700 rpm. They did not die from either this low rpm or from running below much below max WOT rpm that Par is suggesting.
    As we were 8 hours drive from the boat, we decided to repower with EFI, repitch the props and did the same low rpm trolling without any issues.
     
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