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WOT RPMs not up to standards-gctid735157

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    WOT RPMs not up to standards-gctid735157

    I have 2001 4788 with twin Cummins 370s. I have 4 blade 24" Nakishima Props. The port prop is pitched at 22" and the starboard is 21.5".

    the reason for the 1/2" difference is my starboard engine had higher exhaust temperatures and pitching at 21.5" brought the exhaust temp down by 150 degrees.

    I have Floscan digital tachometers that were recently tested for accuracy. With full fuel, full water and full of supplies I put the throttles to 2100 rpm and then eased the port throttle up to full and reached 2688. I pulled it back to 2100 rpm and eased the starboard up to full throttle which was 2768 rpm.

    Under load on the water I believe the benchmark is to get 2900 - 3000 rpm.If I was running the engines at 80% of their capacity at 3000 rpm would be 2400 rpm.

    But if I am only getting 2688 80% would be 2150 and starboard with 2768 rpm 80% would be 2214 rpm.

    My conclusion is that I should not cruise at more than 2150 except for shorter bursts when needed.

    Last year I was getting 2800 on port and 2900 on starboard. I usually cruise at 1700 rpm 10 knts.

    I am going to talk to the prop shop tomorrow and see what he says about any adjustments that could be made to get higher WOT. Or maybe this isn't a big deal.

    What WOT numbers are others getting? What rpm are others cruising at?
    Jerome Robbins
    Vice Commodore, Fidalgo Yacht Club
    Anacortes, WA

    2001 Bayliner 4788
    Twin 370 Cummins

    Previously owned:1994 Bayliner Ciera 2855 454 V8-7.4L
    1994 Bayliner Classic 2252 V6-5.2L

    #2
    I have the same boat with 24x23 props and can get 2900 rpm (as measured by Maretron digital tach) when the props and bottom are clean. If you got those numbers just last year but now getting only 2700, I'd quick haul the boat and check for fouling on the running gear.

    I know I should shoot for 3000-3100 at WOT and plan to either repitch or replace the props to 24x22 at the next major out-of-water opportunity.
    Evan
    2001 Bayliner 4788 "Fifty / Fifty II"
    League City, TX

    Comment


      #3
      Hi Jerome, three quick points.

      First, the WOT test should be done with both engines at the same time - not with one at 2100 RPM. Try with both engines at WOT and see what you get.

      Second, I didn't have 370's but I don't believe you are limited to 80% of you RPM for cruise. I know on the Cummins 330, the limit was 200 RPM below WOT rating of 2800. With that said, we never cruised at 2600RPM - if we were doing fast cruise we were at about 2400, which gave about 15-15.5kts.

      Third, 10kts is getting into the inefficient cruise speed range for the 4788, where you are well past hull speed and starting to push water, but not yet up on the plane. If your looking to save fuel you'd be much better off at about 8-8.5kts, and if you want to go faster, you might as well get up above about 14.5ish kts where you start to plane. Your nmpg on plane isn't much more than what you get in between hull speed and plane, and your engine loading is better.
      Mike
      "Allante I" Rayburn 75
      Previous: '97 4788

      Comment


        #4
        When is the last time the hull was cleaned? You did not mention the use of tabs at any of readings.

        Being loaded down as you were will bring RPM's down. Provided you run like that most of the time, everything else is in tune, hull bottom does not have any growth on it a change in props might be in order.
        Phil, Vicky, Ashleigh & Sydney
        1998 3055 Ciera
        (yes, a 1998)
        Previous boat: 1993 3055
        Dream boat: 70' Azimut or Astondoa 72
        Sea Doo XP
        Sea Doo GTI SE
        Life is short. Boats are cool.
        The family that plays together stays together.
        Vice Commodore: Bellevue Yacht Club

        Comment


          #5
          Jerome - 2000 4788 w/ 370s (2,550 hrs) and 24 X 22 props. Below is table of my WOT runs over the past few years . Like everyone says - I know that ideally should be getting 3100+. We spend most of cruising time at 1400 - 1600 RPM; occasional 2400 - 2500 (

          Date WOT RPM Kts Notes

          1/14/2016 3000 20.2 2 POB; 87% fuel; 10% water; 10% holding (WOT = 3000 both engines)

          11/28/2015 3025 20.1 WOT; 2 POB; 91% fuel; 25% water; 10% holding; Bottom recently cleaned by diver

          9/6/2014 3000 20.2 WOT; 3 POB; 56% fuel; 40 % water; 0 holding

          12/28/2013 2950 19.6 2 POB; 60% fuel; Near full water

          9/9/2013 2925 19.2 2 POB; 31% fuel; WOT: P=2900; Stbd = 2950

          7/26/2013 2937.5 19.4 6 POB; 45% fuel; Bottom/running gear cleaned by diver on 7/24. With wind. P=2925 RPM S=2950 RPM; tender on board; 1/2 water; no blackwater.

          7/19/2013 2850 17.2 5 POB; 50% fuel; WOT

          3/15/2013 3025 20.7 3 POB; 93% fuel; No water; No black water; tender aboard; Clean bottom & running gearWOT Port=3000; Stbd=3050

          11/4/2012 2962.5 19.4 2 POB; Full fuel; clean bottom; No tender. WOT

          10/10/2012 3000 19.7 4 POB; Clean bottom & running gear (diver cleaned 10/8/2012); no tender; 50% fuel; WOT

          9/3/2012 2850 17.7 4 POB; 63% fuel. WOT: P=2825; S=2850

          6/13/2012 2950 19.7 3 POB; 50% fuel. WOT

          11/20/2011 2900 18.7 2 POB; 95% fuel; WOT

          10/15/2011 2900 17.9 2 POB; 75% fuel; WOT

          7/30/2011 2900 17.3 WOT (P=2850; S=2900) 6 POB 47% fuel. Fouled bottom.

          1/10/2011 3000 21.7 Sea Trial. 4 POB. 1/4 fuel. No load. WOT
          Steve Taylor
          2000 Bayliner 4788
          Nighthawk I
          New Bern, NC

          Comment


            #6
            "vinomaker" post=735157 wrote:
            I have 2001 4788 with twin Cummins 370s. I have 4 blade 24" Nakishima Props. The port prop is pitched at 22" and the starboard is 21.5".

            the reason for the 1/2" difference is my starboard engine had higher exhaust temperatures and pitching at 21.5" brought the exhaust temp down by 150 degrees.
            I would address this problem first. This is just asking for other troubles down the road... err water.
            Aquatic Muse
            Mount Vernon, WA
            MMSI: 367498870
            '79 Bayliner Santiago w/ Mercruiser 470 power and drive

            Comment


              #7
              The bottom and gear are clean. It has been in dry dock for the winter. I will put her back in the water in 10 days and I will try WOT with both engines at the same time.
              Jerome Robbins
              Vice Commodore, Fidalgo Yacht Club
              Anacortes, WA

              2001 Bayliner 4788
              Twin 370 Cummins

              Previously owned:1994 Bayliner Ciera 2855 454 V8-7.4L
              1994 Bayliner Classic 2252 V6-5.2L

              Comment


                #8
                "vinomaker" post=735157 wrote:
                I have 2001 4788 with twin Cummins 370s. I have 4 blade 24" Nakishima Props. The port prop is pitched at 22" and the starboard is 21.5".

                the reason for the 1/2" difference is my starboard engine had higher exhaust temperatures and pitching at 21.5" brought the exhaust temp down by 150 degrees.

                I have Floscan digital tachometers that were recently tested for accuracy. With full fuel, full water and full of supplies I put the throttles to 2100 rpm and then eased the port throttle up to full and reached 2688. I pulled it back to 2100 rpm and eased the starboard up to full throttle which was 2768 rpm.

                Under load on the water I believe the benchmark is to get 2900 - 3000 rpm.If I was running the engines at 80% of their capacity at 3000 rpm would be 2400 rpm.

                But if I am only getting 2688 80% would be 2150 and starboard with 2768 rpm 80% would be 2214 rpm.

                My conclusion is that I should not cruise at more than 2150 except for shorter bursts when needed.

                Last year I was getting 2800 on port and 2900 on starboard. I usually cruise at 1700 rpm 10 knts.

                I am going to talk to the prop shop tomorrow and see what he says about any adjustments that could be made to get higher WOT. Or maybe this isn't a big deal.

                What WOT numbers are others getting? What rpm are others cruising at?
                WOT measurements should be made at WOT on BOTH ENGINES AT THE SAME TIME. With those measurents then you should be able to st least cruise at 80 % of those numbers or even a bit higher.
                Started boating 1965
                Bayliners owned: 26 Victoria, 28 Bounty, 32, 38, and 47 since 1996

                Comment


                  #9
                  Vinomaker, you may have already, but check the throttle levellers on the engines. Make sure the level is hitting the stops and into the spring tensioners. Both my throttle cables were loose. I gained 100+ RPM on one side and 300+ RPM on the other by simply re-adjusitng the throttle cables. It's a two minute check that can have amazing results.
                  1997 Bayliner 4788 "Mostly Harmless"
                  Sold: 1990 Bayliner 3888

                  Comment


                    #10
                    "spt7566" post=735171 wrote:
                    Jerome - 2000 4788 w/ 370s (2,550 hrs) and 24 X 22 props. Below is table of my WOT runs over the past few years . Like everyone says - I know that ideally should be getting 3100+. We spend most of cruising time at 1400 - 1600 RPM; occasional 2400 - 2500 (

                    Date WOT RPM Kts Notes

                    1/14/2016 3000 20.2 2 POB; 87% fuel; 10% water; 10% holding (WOT = 3000 both engines)

                    11/28/2015 3025 20.1 WOT; 2 POB; 91% fuel; 25% water; 10% holding; Bottom recently cleaned by diver

                    9/6/2014 3000 20.2 WOT; 3 POB; 56% fuel; 40 % water; 0 holding

                    12/28/2013 2950 19.6 2 POB; 60% fuel; Near full water

                    9/9/2013 2925 19.2 2 POB; 31% fuel; WOT: P=2900; Stbd = 2950

                    7/26/2013 2937.5 19.4 6 POB; 45% fuel; Bottom/running gear cleaned by diver on 7/24. With wind. P=2925 RPM S=2950 RPM; tender on board; 1/2 water; no blackwater.

                    7/19/2013 2850 17.2 5 POB; 50% fuel; WOT

                    3/15/2013 3025 20.7 3 POB; 93% fuel; No water; No black water; tender aboard; Clean bottom & running gearWOT Port=3000; Stbd=3050

                    11/4/2012 2962.5 19.4 2 POB; Full fuel; clean bottom; No tender. WOT

                    10/10/2012 3000 19.7 4 POB; Clean bottom & running gear (diver cleaned 10/8/2012); no tender; 50% fuel; WOT

                    9/3/2012 2850 17.7 4 POB; 63% fuel. WOT: P=2825; S=2850

                    6/13/2012 2950 19.7 3 POB; 50% fuel. WOT

                    11/20/2011 2900 18.7 2 POB; 95% fuel; WOT

                    10/15/2011 2900 17.9 2 POB; 75% fuel; WOT

                    7/30/2011 2900 17.3 WOT (P=2850; S=2900) 6 POB 47% fuel. Fouled bottom.

                    1/10/2011 3000 21.7 Sea Trial. 4 POB. 1/4 fuel. No load. WOT
                    I virtually duplicate this, with the same props, except our top speed is faster. Our best is 3050, at 25.3 mph (GPS) with a decent current. Most of the time with 50% fuel, water we are @ 2950 at about 22 knots, slack current.

                    I will likely go to a 21" pitch as well, but we are seriously considering underhull/hull extension so I want to wait and see where I am with these props first.

                    Comment


                      #11
                      We just went to 21" pitch. These were new props. Same Nakashima as OEM, just custom cast for us to 21" pitch.

                      The results were about as expected. RPM is up to well over 2900, and best is that EGT is below 900. In fact I can't get it even up to 900 no matter how hard I work the engines.

                      KEVIN SANDERS
                      4788 LISAS WAY - SEWARD ALASKA

                      Comment


                        #12
                        +1 on checking the throttle stops! We adjusted ours and that solved part of the rpm problem.

                        We had 24x24 Nakimshima when we bought the boat and repitched them to 24x21 right away. We are heavy and have 370's, underhulls and hull extension. We have rebuilt turbos and serviced engine water side. I have calibrated the tachs and get right at 3000 rpm WOT on both with bottom and props reasonably clean. My WOT EGT is 1050 on P and 950 on S. The next items on our list to to address are injectors and valve adjust. We will see what results these final adjustments make before making any additional prop changes at next haul out. My goal is 3150 rpm and egt of 900 max under worst case loading.
                        2000 4788 w Cummins 370's, underhulls, swim step hull extension
                        12' Rendova center console with 40HP Yamaha
                        MV Kia Orana
                        Currently Alameda CA

                        Comment


                          #13
                          I did a WOT test under full load and the Starboard hit 3025 rpm and the port was 2975 rpm. Our mechanic felt that 3000 rpm or close was a good benchmark.

                          Exhaust temps were Port 800 boost 32 and Starboard 1000 boost 28. Speed 22.7 knts.

                          Do these numbers look good to others?
                          Jerome Robbins
                          Vice Commodore, Fidalgo Yacht Club
                          Anacortes, WA

                          2001 Bayliner 4788
                          Twin 370 Cummins

                          Previously owned:1994 Bayliner Ciera 2855 454 V8-7.4L
                          1994 Bayliner Classic 2252 V6-5.2L

                          Comment


                            #14
                            "spt7566" post=735171 wrote:
                            Jerome - 2000 4788 w/ 370s (2,550 hrs) and 24 X 22 props. Below is table of my WOT runs over the past few years . Like everyone says - I know that ideally should be getting 3100+. We spend most of cruising time at 1400 - 1600 RPM; occasional 2400 - 2500 (

                            Date WOT RPM Kts Notes

                            1/14/2016 3000 20.2 2 POB; 87% fuel; 10% water; 10% holding (WOT = 3000 both engines)

                            11/28/2015 3025 20.1 WOT; 2 POB; 91% fuel; 25% water; 10% holding; Bottom recently cleaned by diver

                            9/6/2014 3000 20.2 WOT; 3 POB; 56% fuel; 40 % water; 0 holding

                            12/28/2013 2950 19.6 2 POB; 60% fuel; Near full water

                            9/9/2013 2925 19.2 2 POB; 31% fuel; WOT: P=2900; Stbd = 2950

                            7/26/2013 2937.5 19.4 6 POB; 45% fuel; Bottom/running gear cleaned by diver on 7/24. With wind. P=2925 RPM S=2950 RPM; tender on board; 1/2 water; no blackwater.

                            7/19/2013 2850 17.2 5 POB; 50% fuel; WOT

                            3/15/2013 3025 20.7 3 POB; 93% fuel; No water; No black water; tender aboard; Clean bottom & running gearWOT Port=3000; Stbd=3050

                            11/4/2012 2962.5 19.4 2 POB; Full fuel; clean bottom; No tender. WOT

                            10/10/2012 3000 19.7 4 POB; Clean bottom & running gear (diver cleaned 10/8/2012); no tender; 50% fuel; WOT

                            9/3/2012 2850 17.7 4 POB; 63% fuel. WOT: P=2825; S=2850

                            6/13/2012 2950 19.7 3 POB; 50% fuel. WOT

                            11/20/2011 2900 18.7 2 POB; 95% fuel; WOT

                            10/15/2011 2900 17.9 2 POB; 75% fuel; WOT

                            7/30/2011 2900 17.3 WOT (P=2850; S=2900) 6 POB 47% fuel. Fouled bottom.

                            1/10/2011 3000 21.7 Sea Trial. 4 POB. 1/4 fuel. No load. WOT
                            How are you getting such accurate % numbers for fuel?

                            Derek
                            Family Boater
                            1997 Bayliner 4788 w/330 Cummins - Phoenix
                            (past) 1987 Bayliner 3218 w/135 Hino - True Story

                            Comment


                              #15
                              My 2007 490 came with 330's which I upgraded to 380's. My surveyor noted the props as 24 x 25. My engine temperatures are always perfect. My boat is probably average in added weight and bottom cleaned monthly. I never have a problem with top end. Cruising at 18-20 mph in moderate seas I think was about 2500 rpm. Never any smoke. I cannot remember exactly, but it seems that after the upgrade the WOT went from 2800+/- to 3050+/-. I have less than 300 hours on each engine and rarely run at higher speeds for more than an hour. I am having my diver double check the props.

                              Reviewing the forum, 24x25 seems like an odd pitch and they are OEM. If I all the above is correct, should I be concerned about overloading the engines and reduce the pitch? I only have standard gauges at this time and tachs have not been calibrated. Comments? (Some of the posts have me a little worried about loads and exhaust temps...)
                              Lee
                              2007 Meridian 490
                              2007 Meridian 368
                              1996 2855 Cierra
                              1978 Caribbean 26'
                              Oceanside, CA

                              Comment

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