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Thread: New Mercury Spitfire 4 Blade Prop

  1. #11
    Join Date
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    The fewer blades a prop has the more efficient it is. A single blade prop would be ideal, except that it would be out of balance, which causes horrible things. That said, there is something else that one should keep in mind:

    A prop which has a bigger "bite" may not be a good thing. If the engine cannot develop its rated rpms it will not develop its rated power. An engine's HP ratting is at a particular rpm. E.g., an engine rated at 100 HP @ 5,000 rpms will not develop 100 HP, even at WOT, if the prop limits the engine to 4,500 rpms. (There's no way to determine the exact power without looking at a chart of the engine's power, rpms & manifold pressure)

    A 100HP engine, spinning @ 5,000 rpms WOT (max manifold pressure) will develop 100HP. That same engine, with a different prop which only lets the engine spin at 4,500 rpms WOT (again, max manifold pressure) is certainly not developing 100HP. I mention manifold pressure (not a gauge on normally aspirated engines, turbo or supercharged engines have such a gauge) because when one puts a prop on an engine which takes a bigger "bite" of water (& not allowing the engine to come up to it's rated rpms) its like (to use an automotive analogy) putting the engine in a higher gear or constantly going up a hill. This increases the manifold pressure. It's harder on the engine & detonation (think valves pinging on a car going up a hill) can occur more easily. Also, an engine with such a prop does not tend to come up to whatever power it will generate as quick. (Many cars can take off in 3rd gear if floored, but it won't reach it's max rpms as quick as if one goes thru the gears) Full power on demand is a bit slower, but on a boat it won't be as dramatic as the 3rd gear example I used to highlight the difference.

    Now, on a normally aspirated engine which is detuned, such as a boat engine, you can get by with having a prop which limits the engine's rated rpms. But, be aware that it will wear the engine out faster than having the rated prop on the engine as the engine will always be under more stress (higher manifold pressure for the rpms) compared to what the rated prop would be. I'm not contradicting myself from when I said that a boat engine will run at its max rpms all day long, a boat motor can certainly do that. However, a boat engine which is always ran WOT will not last as long as one used more gently. The engine won't burn up immediately like running a 500 hp automotive engine (unless it's something like 800 cubic inches), but one may only get 1,500 hours out of it vs. 2,500 hours (ex., I don't know what the recommended time between overhauls on boat engines are. Most light aircraft engines have a 2,000 TBO, although I've had Lycomings apart which had 3,000 hrs and looked fine)

    Sorry to prattle on. Also sorry to use so many parenthesis's, but they were on sale & I've got another 6 lbs of them to use up.
    1994 Tracker Party Cruiser
    115 hp Merc, 2 stroke

  2. #12
    Drive2Boat Guest

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    Very good info. It certainly sounds as if you know of what you bespeak. It just makes me wonder why engine manufacturers, give such a wide range for max operating range. My 2012 Merc 90 4 stroke has a range of 5000-6000 rpm's.How about give your take on the correctness of my prop choice. I originally had a 3 blade 15" prop on my 23" pontoon. It turned 5950 rpm's and ran 26-27 mph with a light load. I never ran it loaded. I switched to a 15" 4 blade Spitfire. With the same light load, I ran the same speed at about 5400-5450 rpm's. Last weekend we had probably about an extra 900 lbs of load aboard totally about 1250-1300 lbs of load. Speed dropped way down to about 20 mph while the rpm's dropped to about 5200. What's your evaluation?

  3. #13
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    Put the 3-blade back on Danny.
    Last edited by cwag911; 08-18-2013 at 02:37 PM.
    Carl & Suzi
    Denver,NC
    2007 Bennington 2275rl tri-toon
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  4. #14
    Join Date
    Jul 2006
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    I also appreciate you insight and input belercous. Can you tell us anything about the differences in the way "lugging" affects 2 strokes and 4 strokes?

    I agree with Carl, Danny you should put the three blade back on.
    2006 Forester 19 Fish (new deck and carpet, Pontoonstuff interior, 2019)
    1996 Mercury 50 ELPT4S
    1983 Sea Nymph FM171 Striper (complete rebuild from hull up, 2014)
    1985 Johnson 70 J70ELCO

    Raystown Country, PA

  5. #15
    Join Date
    Jan 2012
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    Ok, This is a test right.?
    Lift is needed, here is why....it's the load.. ( weight )
    Attached Thumbnails need lift.....jpg  
    Last edited by funtooner; 08-20-2013 at 05:53 AM.
    Malcolm ( Goofy )
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    " Keep a fire in your heart and a smile on your face, for you never know when your leaving this place"

  6. #16
    Join Date
    May 2013
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    Edwardsville, IL
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    If a different prop keeps the RPM's in the engine manufacturer's suggested range, no worries.

    Drive2Boat: Your new prop choice does not allow your engine to develop full power since it can't develop it's max rated RPMs. With an unloaded/lightly loaded boat you probably get better mileage per gallon of gas. As you've noticed, with a fully loaded boat, your engine can't make it's full power since it can't get it's full RPMs. Think of it like your car is stuck in 3rd gear.: It'll run fine with only the driver in it, but without much low end take-off power. And you'll get better gas mileage than if it was stuck in 2nd or 1st gear. Now, add your whole famn damily to the car. It doesn't want to move very good at all, and certainly won't go as fast as when it has only the driver in it. The engine cannot develop full power. If the car was stuck in 1st gear, you'd get up to speed real quick, even when fully loaded since the engine would be able to develop full power. But your fuel mileage wouldn't be so great.

    A boat engine's prop is like a transmission. (adjust prop ratios accordingly, I'm just using examples) A 12:1 prop will get better fuel mileage than a 10:1 (or lower) prop, but it won't allow the engine to have full power on demand. With a 12:1 prop, Idle to WOT will take longer for the engine to come up to max RPM's (if at all) than a 10:1 prop. A 10:1 prop will be quicker from idle to max RPMs @ WOT, and can do so with a heavy load, but it won't get as good of fuel mileage. As with everything in engineering, there are trade-offs to be made. Have a light boat & want speed or better mileage? Use a steep pitch prop. Have a heavy boat & want instant power on demand? Use a shallower pitch prop. Want both better mileage, speed & full power on demand? Can't be done. A bigger engine can get you power on demand & more speed, but with less fuel mileage. Most boat/engine combinations come from the factory with a "happy medium," Not as fast or good of mileage as possible, but when the boat's fully loaded the engine won't be so lugged down that it cannot (eventually) develop max power. I agree with cwag911; put the 3 blade prop back on. Or don't load the boat so full.

    Moser: In my experience, lugging a 2-stroke engine is a bit worse than lugging a 4-stroke engine, so far as developing full power is concerned. But it really depends upon a particular engine's bore v. stroke ratio. An under-square engine (longer stroke than bore dia. Those engines are called "strokers") can handle being lugged more than an over-square engine. An over-square engine gets it's power from it's RPMs, whereas an under-square engine doesn't need to spin as fast to develop it's power. Under-square engines have more low-end torque. Think of it like this: a Harley engine doesn't spin very fast, but can pull a heavy load from an idle to WOT without bogging. That's torque. Now, take a 2-stroke dirt bike with the same load & try that. The engine will bog down since it doesn't have low-end torque, but rather gets its power from RPMs. If it cannot develop its RPMs, it cannot develop its power. The 2 stroke engine can be rated at the same HP as the Harley engine, but the Harley engine (a stroker) doesn't need to spin as fast to develop its power, whereas the 2 stroke needs to be revved to have its power. The strange thing is that a 2-stroke engine can have 1/2 the displacement of an equally rated 4-stroke engine, but the 2 stroke needs to be on its "happy" range (usually near WOT) to develop its power, whereas a 4-stroke engine isn't that sensitive.

    When I used to ride dirt bikes I learned real quick that if one bogged a 2-stroke dirt bike 1/2 way up a hill it was game-over. If I didn't down-shift the engine would die. And if I did down-shift & the engine came back to life the bike was likely to flip over. 4-stroke dirt bikes were way more forgiving than 2-strokes, but weren't as fast on level ground. Both types had the same HP rating, it was just that 2-strokes needed the RPMs, while 4-strokes had their power at lower/mid range. 4-stroke engines are much more fuel-efficient than 2-stroke engines. But 2-stroke engines, if geared right, can develop power much quicker than a 4-stroke engine since they fire every time the piston makes comes to the top whereas 4-strokes only fire on every other revolution. (FYI: a "stroke" is an up or down movement of the piston. E.g., a downward movement of the piston is 1 stroke, and an upward movement of the piston is stroke #2.)

    2-stroke engines, at the same HP rating, are way lighter than 4-stroke engines. There's a lot less moving parts on 2-stroke engines as well. 2-stroke engines, when geared right, can develop full power quicker than 4-stroke engines (of the same bore v. stroke configuration), but if not geared (on boats this means prop) right, do not fare so well. 2-stroke engines, having fewer moving parts, also are more reliable than 4-stroke engines, but they also oil-foul spark plugs more often. But they're easier to work on. While I'm not a professional boat mechanic (I used to fix airplanes for a living; pretty much everything on piston engine planes, including a few dozen engine overhauls. Not to mention all the dirt bikes, lawn mowers, weedwhips, chainsaws, cars, etc. I've repaired too.), every professional boat mechanic I've talked to has said they wouldn't have a 4-stroke engine on their own boat. FWIW

    Oops, almost forgot. Lugging an engine can cause detonation/pre-ignition. That's the sound one used to hear on older engines going up a hill with a load, or when using too low octane gas. Some would call it "valve-clatter" (which it was too), but that is very hard on an engine & will soon destroy it. One won't hear the "valve-clatter" on 2-stroke engines (no valves, or not the same type anyway), and if one is not near the engine it likely won't be heard, but a sign is a loss of power, closely followed by an engine blowing a head gasket. If you're lucky. Often they just melt into scrap metal.
    Last edited by belercous; 08-22-2013 at 09:24 PM. Reason: apology
    1994 Tracker Party Cruiser
    115 hp Merc, 2 stroke

  7. #17
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    Sorry, I tried to edit my last post. I'm sorry that I prattled on so & gave a dissertation. I probably could have cut out a lot of extraneous BS. I apologize for being so verbose. (I happened to pick up a bunch of extra words cheap at a garage sale. Sorry to unload them here. I do have a lot of extra nouns in case anyone's needing some. I'll let them go cheap, too.)
    1994 Tracker Party Cruiser
    115 hp Merc, 2 stroke

  8. #18
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    May 2009
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    No worries (from me). I personally skipped some of the verbosity the first time through since I was not the one looking for input to my problem. That said, I plan to go back and read your response more thoroughly when I have the time. I think we often get into the habit of responding with sound bites (since that is what we often get from the media) but what we really need is a thought-out, well-written response such as you have given.

    Thanks.

    p.s. - If I find myself needing some of those nouns, I'll shoot you a PM.
    Rick
    2010 Premier SunSation LTD 225
    PTX performance package (triple log)
    220hp 4.3L mpi Merc
    Beautiful southwestern Ohio

  9. #19
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    I skimmed it the first time through but went back and read it all later and I agree with Rick, thought out and well written. My experince has been much as you described, and lugging a 2 stroke has proven to be a costly mistake for me. I put a 90 OMC off of a deck boat I had over on to my old Manitou 24 foot (19 inch tubes) way back in the day. I had no experience with props or motor swaps, I just wanted to go faster and just took the boat out and flogged it wide open. She ran along for about 5 minutes sounding like she wasn't getting enough gas and then just locked up. Costly lesson learned. Now you will always hear me advise folks to prop their boats to run in the upper range of their recommended WOT RPM. Thanks again for explaining why we want to do that and what happens if we don't.
    2006 Forester 19 Fish (new deck and carpet, Pontoonstuff interior, 2019)
    1996 Mercury 50 ELPT4S
    1983 Sea Nymph FM171 Striper (complete rebuild from hull up, 2014)
    1985 Johnson 70 J70ELCO

    Raystown Country, PA

  10. #20
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    Colonial Heights VA
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    My opinion of running the Mercury Spitfire 4 blade prop on my 24' pontoon. Spitfire 11" x 13.8" 4-blade on a 2014 60 HP Bigfoot mounted on a custom 24' pontoon. Top speed light loaded is 23 mph at 6000 RPM. With a 2000lb load it runs 19.5 MPH GPS at 5600 rpm. When the price comes down on the Mercury Smart Gauge I will report some fuel burn figures. Best guess is I am getting better than 5 mpg with a 500lb load at an 18 mph cruise speed. Plans for a custom center pod are in the works to try to increase efficiency.
    Loyd
    Retired US Army
    2013 24' Custom Pontoon
    2014 60HP Mercury Big Foot

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