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How to determine optimum RPM's

Discussion in 'Technical Discussion' started by Chris Schell, Apr 17, 2015.

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  1. Chris Schell, Apr 17, 2015
    #1
    Chris Schell

    Chris Schell New Member

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    I just purchased a 2007 Sabre 34 hardtop express and I am wondering what is the optimum RPM's for maximum efficiency? The boat is equipped with 2 Yanmar 315HP engines. It tops out at 3800 RPM's. Is there a rule of thumb based on the max RPM? Also, how should I run the tabs, I assume using the tabs will actually create drag and as long at I have good enough visibility no tabs would be better or more efficient. I don't believe there is a fuel flow detector and with a 250 gallon tank it will probably take a lot of statistical data to figure out. I will be looking at some upgrades and a fuel flow detector will be one of them, but until I get around to it I would love some advice and would really like to find other owners of the same boat.
  2. RER, Apr 17, 2015
    #2
    RER

    RER Senior Member

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    Is this what you're looking for?...

    http://www.sabreyachts.com/pdf/SL34X_Yanmar_6LPA-STP_315mHp.pdf

    3000 to 3200 RPM looks like cruise to me based on that chart.

    Trim tab position will vary due to load, sea state, direction, and speed. Some boats need a lot of help from tabs to get on plane, some don't. Rule of thumb is trim the boat enough to plane but no more.
    Last edited: Apr 17, 2015
  3. NYCAP123, Apr 17, 2015
    #3
    NYCAP123

    NYCAP123 Senior Member

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    Good chart RER. I keep one similar at the helm of the Hatt. Beats relying on memory.

    Most engine manufacturers today say it's ok to run their motors at 90%. I'm old school and go by the 80% rule. That's more about not abusing the motors than efficiency though. The "fuel flow detector" you refer to is called a "Flow-Scan". Wonderful item, but they cost about 5K per side. Hard to justify when you're dealing with only the difference in consumption between that and an experienced ear.

    Although the act of dropping the tabs creates drag, it actually gives lift to the stern thus reducing drag. Each boat reacts differently to the use of tabs. Most boats I've run do best at about 75-80% tab in flat water, but that will also varies with sea state and load. Generally you want the boat running bow down when heading into the seas as you'll cut it better with the bow than the belly. When running with the waves you'll want the bow running higher to keep from plowing in. You'll also want to vary side to side to compensate for load. What you can do to find the base for your boat is to set it at your cruise RPMs and set the boat on auto pilot. Then adjust your tabs and watch if the speed increases or decreases. If you don't have a tab indicator, it should take about 8-10 seconds to go from full up to full down.

    BTW, I've been on several boats where the tabs were reversed. So look for that. The stbd. control should operate the port tab causing the stbd bow to rise or fall and visa/versa.
  4. Chris Schell, Apr 17, 2015
    #4
    Chris Schell

    Chris Schell New Member

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    RER, Thanks for the chart, come to find out that year, they only shipped 2 of the 34's with the 315HP Yanmars and one of them was hull #009 which the chart refers to, although mine is hull #014, but the data should be close enough. As to the adivce from NYCAP123 on picking an initial RPM and playing with the tabs while monitoring speed is what I was going to do this weekend. I agree $5k for a flow-scan doesn't make sense, although there is no autopilot and $5k for an autopilot makes much more sense based on advice from others and what I have read. Going in a straight line is much more efficient and zig-zagging.
  5. NYCAP123, Apr 17, 2015
    #5
    NYCAP123

    NYCAP123 Senior Member

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    Good investment. Although autopilot generally makes life easier and more enjoyable, it's great in fog. Every time you turn your head you turn the wheel. With nothing but cloud to see it's easy to go in circles while trying to maintain a watch.
    In the mean time just hold the wheel steady while finding your sweet spot.
  6. Capt Ralph, Apr 17, 2015
    #6
    Capt Ralph

    Capt Ralph Senior Member

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    The use of tabs brings the bow down. The determining point is where efficiency of propeller thrust more straight back overcomes the inefficiencies of increased drag. Extermination here will never end, BUT, there IS some payback in reducing the thrust angle. Where???? Again you have to experiment..
    Wind head on, astern, beam and chop will also affect your need on trim tabs. Wife AND mother in-law will also throw any specs off. Constant trim adjustments may be required.
    I am a fan of the Flow-Scan equipment. It is hard to justify the cost. So, more notes, logs and experimentation will be required to answer any efficiency questions.

    Just think like Scotty, While in AquaSpace, some logic must still be applied;;; Toss da in law...
    Warp speed.....
    Last edited: Apr 17, 2015
  7. K1W1, Apr 18, 2015
    #7
    K1W1

    K1W1 Senior Member

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    Flow Scan is one manufacturers brand, there are others that work as well to chose from.