Good cruising rpm?

zing1402

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  • Jul 13, 2020
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    yorba linda
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    nick
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    I have a 19' cc with a 115 merc four stroke originally the prop was way too small 20 mph was around 5500 rpm. i jumped way up in size and it dropped a bunch but was adding a big bait tank so i wanted to see what it did with the new tank. Now with a full bait tank it runs 22-23 mph at around 4200 rpm its getting great mileage around 5mpg but it still planes right away and 4200 seams kinda high or am i just use to inbord motors? thanks for any input
     
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    NaClAddict

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  • Jul 18, 2008
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    Seems high. I would talk to a prop shop. They can get you pretty close. I’m running a heavy 20’ with a 150. Usually get up on plane around 3-3200 rpm’s.

    5 mpg is amazing. Planing at 22 mph is about right. Sounds to me like you have an under pitched prop. Could be placing strain on your motor on the low end.

    Are you trimming your motor? It makes a world of difference. When you’re on plane, adjust motor trim until rpm’s jump a few hundred rpm. That’s your sweet spot.
     
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    zing1402

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    Seems high. I would talk to a prop shop. They can get you pretty close. I’m running a heavy 20’ with a 150. Usually get up on plane around 3-3200 rpm’s.

    5 mpg is amazing. Planing at 22 mph is about right. Sounds to me like you have an under pitched prop. Could be placing strain on your motor on the low end.

    Are you trimming your motor? It makes a world of difference. When you’re on plane, adjust motor trim until rpm’s jump a few hundred rpm. That’s your sweet spot.
    It jumps up on plane around 8mph I have tried trimming the motor but it starts to porpoise so leaving it all the way down gives me the best ride until I get trim tabs
     
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    whosmatt

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    Feb 25, 2011
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    Prop for WOT RPM. Take the boat out with a light load and jam it to WOT and adjust the trim. If you hit the rev limiter or are going over the max recommended RPM you are under-propped. If you can't make it to the low end of the max RPM range you're over-propped. If you're in-between, it's time to start tuning. My personal strategy is to prop for max RPM with a light load, so that when I'm heavily loaded I'll still be able to reach the recommended RPM range.

    As an example, let's say your recommended RPM range at WOT is 5100-5900 (it will be in the manual or on the side of the motor). With a light load, shoot for somewhere around 5800. It will drop somewhat once you've got gear and bait and ice and people on board, but it shouldn't drop below 5100.

    As for cruising RPM, it's different for each boat/motor combo but mostly the sweet spot is around 4000 RPM for fuel economy.
     
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    NaClAddict

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    I believe 5-5.5k top end for my yammy 4. 4200 top makes me think something is way off. Our new motor had similar issues when installed. Things worked fine but numbers could have been better. Less pitch and voila!

    previous owners wanted super fast idle speed.
     
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    NaClAddict

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    All the way down could be killing mileage and putting unnecessary wear on boat and motor. When the rpm’s jump is mileage sweet spot. Adjust trim and weight distribution to match seas. Bow down for stability is one end of the spectrum but it decreases speed and fuel economy. Too porpoisey for better economy is the other end.

    Too much bow down can bog down your rpm’s. Trimming your engine while on plane will diagnose this. Tiny incremental adjustments only.
     
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    Garrison

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    May 21, 2008
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    After determining the boat is properly propped, it will tell you what a good cruising RPM is. In my experience most boats are supplied by the factory over propped for our application. They are tested unladen from, bodies, ice, bait tanks and enclosures.

    A boat is properly propped when it can reach full RPM (hit the rev limiter) with a full load, many owners of work boats with diesels prefer to reach the rev limiter at 80-90% load for long term reliability. This will allow the engine to work within its design parameters without overloading and causing unneeded stress. It will give you the best efficiency at cruise and allow the boat to get on plane faster. The advantages are numerous compared to the small price you will pay in loss of unloaded top end speed that is never used.

    Garrison
     
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    zing1402

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    After determining the boat is properly propped, it will tell you what a good cruising RPM is. In my experience most boats are supplied by the factory over propped for our application. They are tested unladen from, bodies, ice, bait tanks and enclosures.

    A boat is properly propped when it can reach full RPM (hit the rev limiter) with a full load, many owners of work boats with diesels prefer to reach the rev limiter at 80-90% load for long term reliability. This will allow the engine to work within its design parameters without overloading and causing unneeded stress. It will give you the best efficiency at cruise and allow the boat to get on plane faster. The advantages are numerous compared to the small price you will pay in loss of unloaded top end speed that is never used.

    Garrison
    yeah im gonna bump up the prop size and see how it does im not looking to go any faster just want to not be beating the shit out of the motor all the time.
     
    NaClAddict
    NaClAddict
    I think you want less pitch (travel distance through water with 1 rotation) in order to reach around 5k. Look up your engine’s top end first.
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    NaClAddict

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    It’s not that it’s a big deal either way. I think you’re headed the wrong way. Less pitch, not more. And rarely is “all the way down“ properly trimmed. If you don’t trim your boat right it can bog rpm’s regardless of pitch.

    You usually can’t add a bigger prop on an outboard. You can add blades though.
     
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    DOGHOUSE26

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    Feb 22, 2004
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    Best Cruising Speed as far as Engine wear is 75-80% of WOT.
    5500 WOT = 4125-4400 Cruising RPM

    If you follow whosmatt's advice and find your loaded WOT with the proper props, then you can find your best Cruising Speed.
     
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    NaClAddict

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    One call to a prop shop will put you pretty close. Info is usually free. RPM’s at a given speed still have too many variables. Do you know your current prop pitch? Testing has so many variables, try to do it in calm waters, not open seas.

    Matt’s method is spot on.
     
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    sickcat

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    I agree with Garrison - do your max RPM runs with the boat loaded how you use it most. Full bait tank, weight to simulate people usually carried etc.

    Underpropped does not hurt the engine - overpropped does.
     
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