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Old 07-25-2018, 03:49 AM   #1
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Default 4200 SCR lacking full power

2002 4200 SCR. I've had the running gear worked on and two new DQX 22x25x2 props installed (originals were Nakashimas). At full throttle my starboard is at 2800 rpm and my port is at 2450 rpm but the boat is only doing 18 knots in calm seas. Cummins gave me a thumbs up on the engines (450Cs 3000rpm at engine) and now its dry docked waiting for a genius to figure out what happened. I've been told the starboard shaft strut is bent but its not confirmed yet but would explain a zero tach at gear forward/reverse because the shaft is difficult to hand turn. The boat gets up quick but fails to go beyond 18 knots. Both shafts were straightened 4 months ago and the shafts were aligned at that time. Any ideas?
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Old 07-25-2018, 10:42 AM   #2
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If the shaft is bent there should be a vibration when in gear. If they were just straightened 4 months ago and it is now bent is the alignment correct? Are the cutlass bearings good? Did you hit anything?
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Old 07-25-2018, 02:32 PM   #3
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I would assume if both shafts were straightened, then a grounding occurred. The other cause might be an improperly placed lifting strap on a travel lift during a haul out.
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Old 08-13-2018, 07:02 PM   #4
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Mike.....I had the shafts straightened, props balances and checked for specs, cutlass bearings changed, and even the starboard strut was repositioned a few millimeters. It was out of the water for three weeks being worked on (I pamper her) and the moment I took her sea, the same results.....nothing had changed. I've done everything I can except change my NEW DQX props back to new Nakashima props. I'm trying to verify that Maxum used Nakashima's in the build.
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Old 08-13-2018, 07:04 PM   #5
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Shrew.......no groundings but the "lift" is an idea. Only thing is that after having the issue, I had it hauled out again and the shaft was fine but upon being returned to water, I still had the problem.
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Old 08-13-2018, 09:41 PM   #6
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When did it last run correctly? What has changed since then, props? Are these news props the same number of blades and pitch?
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Old 08-14-2018, 02:57 AM   #7
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Quote:
Originally Posted by mmwjr View Post
When did it last run correctly? What has changed since then, props? Are these news props the same number of blades and pitch?
I know what you're thinking and if thats the problem I'll be so pissed. After the Nakashima's were replaced (starboard blade sheered off at the hub) with new DQX's under the same specs 22" x 25 pitch x 2" bore Nibral (same specs) I began to notice the difference. In the last week of July I had the props removed and specs checked/confirmed. Identical! Aren't blades supposed to be interchangeable as long as the specs are identical? I can't get Nakashima in Japan to respond to inquiries and Michigan Wheel wants all kinds of boat info before they'll comment.

Why didn't I replace the old props with the same manufacturer? It's a long story about a screw-up at the boatyard.
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Old 08-14-2018, 03:16 AM   #8
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Understand stuff happens but it sounds the the prop change is when the performance was impacted so thatís were I would focus. Does the new ones have the same number of blades as the old ones? What about weight, different materials may have the new ones weighing more.
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Old 08-14-2018, 02:44 PM   #9
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Quote:
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I've had the running gear worked on and two new DQX 22x25x2 props installed (originals were Nakashimas). At full throttle my starboard is at 2800 rpm and my port is at 2450 rpm
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but it sounds the the prop change is when the performance was impacted so thatís were I would focus. Does the new ones have the same number of blades as the old ones? What about weight, different materials may have the new ones weighing more.
If it were the props, wouldn't you expect both sides to be at 2450? What is the max RPM for the engines? Does Stbd reach that RPM?
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Old 08-15-2018, 12:19 AM   #10
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Just had the running gear guy here today and the starboard shaft spins more loose but it's still weaker than the port.....example.....from a stopped position, port forward and starboard back......then port back and starboard forward. This should spin the boat without much varying from the circle however the starboard shaft feel weaker such that the port pushes and pulls stronger taking the boat out of the tight circle. The full throttle test had identical results of port 2600 rpm and starboard 2750 but the top speed was 18.5 knots.....no change. The WOT is 3000 rpm at 28 - 29 knots......supposed to be. I'm working with Michigan Wheel to analyze the current DQX props and the alleged original Nakashima props with the engine and transmission specs. Waiting for my mechanic to retest the rpm at the engine with no load. I like a good mystery but this has been crazy. Props alone shouldn't be able to cause a 10 knot drop in performance, right?
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