30% slip at WOT

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  • 30% slip at WOT

    Looking for some help.

    The boat is a 2003 Sea Hunt 232 with a 2002 Yamaha F225. The prop is a Power Tech YM200MQS3R21P. I have lost some top end speed. This past weekend I measured top speed (GPS) at 39mph (not knots) at 5500 rpm (maybe 40mph accounting for the current). The Faria tach is mechanical but only 2 years old and should be fairly accurate. That was WOT. This gives me about ~30% slip. The load was 3 guys, t-top, 100lbs ice, fishing gear, 75% full tank (~75 gallons) of E10.

    When I bought the boat 5 years ago, it was hitting close to 50 mph. The three different Yamaha test reports I have for this hull and motor show a top speed just shy of 50mph with a 19 pitch, 20 pitch and 21 pitch. The 21 pitch was 47.8 at 5500 and 49.1 at 5700(WOT). I am running around 500 pounds heavier than the stated load in the performance reports.

    A couple of years ago I got a stray rope wound up in the prop. I suspect that this has caused some minor slipping in the hub. I have marked the prop and hub and will check next time out to see if the alignments change.

    A couple of questions:

    1 - How much speed loss can be expected from adding a clamshell water intake to the bottom of the hull?
    2 - How much speed loss can be expected form a larger transom mount transducer (Airmar TM260)?
    3 - When I replaced the corroded exhaust components, I went ahead and raised the motor to mounting height #2 to match the Yamaha reports. Could this have caused a loss in speed? How can I determine the optimum height?
    4 - I have a Lowrance LSS2 transducer mounted directly to the bottom of the hull directly in front of the lower unit. I know this might be causing some turbulence at the prop, but how much speed loss can really be expected? Should I lower the motor to get the prop into cleaner water?

    Unfortunately, I never looked at the change in top speed when each of these changes were made.

    Thanks for any help.
    Last edited by Tainted; 07-13-2015, 09:28 AM.

  • #2
    1. If you put a clamshell water pickup inline with the motor, it can cause the prop to loose grip badly, and cause your issue.
    A properly located water pickup might drop your top end 1 mph.

    2. While that Airmar transducer looks huge, it should really only be skimming the water, so I'd say 1 or 2 mph from that thing.
    3, If the prop is not cupped much, and doesn't have very good traction, raising the motor could cause some speed loss.
    4. To test that theory, I'd loosen the transducer and bring it up out of the water flow, and go make a test run, see what happens.

    I'd say there is a very good chance something inline with the motor is causing dirty water which is causing this horrible slipping.
    Probably that water pickup, or the transducer or both.

    Lowering the motor might be an option, but try to take that transducer out of the water flow for a test first.

    Ken Reeves
    [email protected]


    • #3
      Thanks Ken.

      1) The clamshell is off to the side, and not inline with the prop.
      2) The TM260 transducer is actually barely touching the water, for cooling purposes only, as I only use that one when off plane.
      3) Can you tell me if this prop has adequate cupping? PowertechYM200MQS3R21P
      4) The side scan transducer is screwed and glued directly to the bottom of the hull (and not the transom), so moving it for a test is not easily done. And the current location gives me a side scan return at 30mph, which is phenomenal....so.....I might just lower the motor instead. Does lowering the motor one hole have any negative consequences besides increased drag?


      • #4
        One more thought...I moved two batteries from the stern to the center console. Could this possible detract from top speed?


        • #5
          That might be a little bit of it. But I would be surprised if it amounted to more then 1 or 2 mph of your speed loss.

          Ken Reeves
          [email protected]


          • #6
            Tainted, what did you decided to try or go with?


            • #7

              I've been attacking this slowly.

              Lowering the motor a hole didn't really make any difference in the speed. It felt like it became sluggish and stern heavy. I even wondered for a second if I had put the drain plug in. It seemed to ride more bow up with no trim, and porpoised more easily as I trimmed up. So I will be raising the motor back up.

              I put a few holes in the backside of the clamshell intake. No difference there....not that I expected much.

              Before I lowered the motor, I threw on my spare just for comparison. With an aluminum Michigan Match 19" (101028 for Yamaha), I was matching the performance report for the Yamaha Saltwater Series 2 19" at 4000rpm, but slipping more at higher speed:

              RPM - My Aluminum 19" - Yamaha SSII 19" bulletin
              4000 - 28 - 28
              4500 - 32 - 35
              5000 - 35 - 39
              5500 - 39.5 - 43.6
              5800WOT - 40.5 - ??
              6100WOT - ?? - 49.5

              Is my aluminum flexing at higher speeds?

              Am I right in thinking that if I am loaded too heavily for a particular prop, it will slip more at high speeds? Could this partially be what is going on with my 21" Powertech MQS? (I wouldn't be surprised if my boat is 500 lbs heavier now than when I bought it...between 3 extra batteries, trolling motor, tackle, coolers, misc.)

              After researching what others have done to prop this boat, I am thinking 21" is too much pitch, and 19" is the way to go. And I really like the way the 19" felt. It felt as if I had better throttle control and pickup. Could I expect better results than the above aluminum prop by going with stainless? Any recommendations out of your line up? OFS? MQS? Thunderbolt?

              I don't care so much about the top speed. My goal is to run fairly loaded at ~40mph/5000rpm. (38/9 is fine)
              Last edited by Tainted; 09-29-2015, 04:02 PM.


              • #8
                I'd try a 19 Thunderbolt. good consistent all around 3 blade.

                I have them for testing.

                Ken Reeves
                [email protected]


                • #9
                  Following up on this thread.

                  Ken is the man! After testing the Thunderbolt 19, He put me into a Thunderbolt 17. I gained 6mph on the top end, increased my fuel economy by 30%, and lowered my slip from 30% to 10% at WOT. Even more amazing is the slip remains at 10~11% from 24mph through WOT of 45mph.

                  THANKS KEN!!!


                  • #10
                    Glad to hear !!

                    Ken Reeves
                    [email protected]