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Yanmar 315 rpm

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  • Yanmar 315 rpm

    At the NY boatshow,I wandered into the Yanmar rep. area and talked to them about the somewhat dissappointing speed of my 06 25 inboard. The factory rep asked me at what rpm I run the boat and I told him I was advised to run at 400 off the top and he assured me that proper continuous speed should be @ 3800rpm.He even handed me an engine fact sheet which indeed rates cont. output at 3600rpm. I am sharing this as I know my new cruise will be faster as I have occasionally fast cruised my boat and now should be seeing 23-25 kts. depending on load.

  • #2
    Yanmar !!!!!!!!!!!!

    You & I have the same c.c.
    When the engine was replaced did they give you the new ~~~6lpa-stp-2 ~~~ engine ??? I am trying to find some one that has this in 25' C.C. ??? I have only 24 psi of boost on the gauge that we installed on the engine ! We were told that it should be near 37 psi ????
    Can you tell me what you have.... When we got the C.C. AT TOWEN CREEK & WERE TESTING , we got up to 35 mpi , Now the new motor will not hit 30mpi ???/ You can run that yanmar all day at the top rpm theu are built to run @4.150 rpm & are turned down to 39.50 rpm.... For most of our running I will run at 36-3600 rpm... Then run her at all you can get !!!! Not sure why yur C.C. IS SLOW ???
    Let me know what you got~~ Thanks -- Norm
    25' C.C.~~YANMAR /INBOARD !!!!!


    • #3
      Don't get carried away with the 3600+ high rpm cruise unless you are able to load the boat down fully and see 3950-4000 WOT using a calibrated tach or a photo tach.

      It's not the rpms that matter at your cruise, it's the load. If you have a Floscan and can measure fuel flow, you can use that as a guide. You would want to keep to 80% or below load. So if you burn 16 gph at WOT, you would want to run at or below whatever rpms translates into about 12.8 gph. It might be 3600, it might be 3400. The point is you can't just assume it is okay to run at 3600 all day unless you verify that your load is what Yanmar intends it to be at 3600.

      Remember that the 6LP is a recreational, high output motor derived from a Toyota automobile/ light truck engine. Pushing a car down the highway at 70 mph, it might be running at 25-30% load so you are asking it to do a lot pushing your boat along for hours running at 80% load. They are good motors but if you overload it, you will kill it very quickly.


      • #4
        Bluefin makes an excellent point that is often misunderstood about Max Continuous ratings. Yanmar states that the 6LP-STE Max Continuous rating is not to exceed 3600 rpms AND 255HP. One cannot assume that every boat is propped and/or loaded such that you don't exceed 255 HP continuously. Since we are unable to measure HP as we operate the next best thing is to measure fuel as the parameter to monitor. If you don't have accurate fuel flow measuring capability then I would strongly suggest this:

        Prop the boat under fully loaded conditions including all gear, fuel, water and persons such that the engine(s) will achieve right about 4000 rpm at WOT. Now you are perfectly safe to run 3600 continuously.

        If you are over-propped all bets are off and you will be at risk to greatly shorten the life of these engines running 3600 continuously. I truly believe that the worst mistake you can make with any marine diesel is to prop them such that they cannot make full RPM plus. If you call Mack Boring or Mastry they will tell you to prop a 6LP at 4000 rpm WOT.

        I also would strongly suggest the installation of an EGT gauge (Exhaust Gas Temperature) pyrometer. They should be standard equipment on any marine diesel but are not on many. You will be very surprised to see how fast EGT will climb as you approach near 80% load. I know Bluefin has some EGT numbers from a 6LP in a 25 CC maybe he would post them? Remember, these engines have aluminum pistons and piston damage from running over recommended EGTs is a sure way to kill an engine, real fast. You can destroy a 6LP in about five minutes of EGT over 1250 degrees F.
        Senior Member
        Last edited by Patriot; 01-13-2008, 04:51 PM.
        2001 CC 28
        Yanmar 300's


        • #5
          My boat is propped @ 3950 and this is with a full load.I believe that with proper maint.these engines are built for that speed and should show negative results from doing so. What is surprising is to hear that a similiarly equipped boat was seeing 35 kts.I can only assume that it has an outdrive and is lightly loaded.


          • #6

            If a 25CC was going 35 Kts you can be sure it was an outdrive. In straight inboard configuration the shaft angle kills top end speed. Duo-prop outdrives also take a huge advantage due to the total prop area and they run with much less prop slip than an IB will.
            2001 CC 28
            Yanmar 300's