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300 hpdi reving up but no going over 28mph

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  • 1 Post By noelm

300 hpdi reving up but no going over 28mph

Old 03-17-2019, 03:24 PM
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Default 300 hpdi reving up but no going over 28mph

Hello I have a 2004 255 wellcraft fisherman center council, the boat would always run at 50 mph at around 5800-6k rpms... the lower unit recently blew and I had a mechanic replace it.. he also replaces the vst tank, and spark plugs.. engine itself runs perfect and gets on plain fast but once over about 4000 rpms it seems like there is no speed, the boat will max out at about 28mph at about 3900 and will do 6k rpms with no gain in speed, I replaced prop and have and have tried 3 different prop sizes.. I believe the lower unit was a right hand and changed it to left hand rotation, he explained that would not make a difference before any of this the boat sat on trailer and now it is in the water but has only been in the water for about 2 weeks, has bottom paint.please help in any way someone can I'm stuck at a loss here
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Old 03-17-2019, 03:39 PM
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Need to confirm, you changed from standard rotation to left hand? now speed is low, even with different props? you are using left hand props, and not your standard spinning backwards?
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Old 03-17-2019, 03:39 PM
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Are you saying?
28 mph at 3900 rpms then push throttle to 6000 rpms with no change in speed?

if so
- can only be a slipping prop hub
- you are over revving the engine
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Old 03-17-2019, 03:54 PM
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Read what I posted, we need to know that he is using a left hand prop! and he has tried other props, what's the chances of having other left hand props laying around?
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Old 03-17-2019, 03:54 PM
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That is correct no more speed when excellerating more.. but I just put a brand new prop and hub on. Yes lower unit was changed from a right to a left hand rotation
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Old 03-17-2019, 03:55 PM
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You still didn't tell us if you used left hand props, that's pretty important.
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Old 03-17-2019, 03:59 PM
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By the way, I know the physics with the prop going backwards and the direction of boat travel, BUT we need to KNOW what's what.
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Old 03-17-2019, 04:02 PM
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Sorry yes it is a left rotation prop..
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Old 03-17-2019, 04:13 PM
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OK, now, is the prop the same size as the original one?
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Old 03-17-2019, 04:22 PM
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The original prop did not have markings on it so we played a little guessing game, started with a 17 then a 19 and same speed, 28mph, if u give it more throttle the engine revs up but doesnt move, a 21 slowed the rpms down but did not go any faster and lost the hole shot.. the boat is not water logged and only has about 10 gallons of gas in it and it holds 160 gallons so I kept everything light.. also I checked the bottom of the boat there is a little slime but not enough to slow it down by 20mph
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Old 03-17-2019, 05:59 PM
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Sounds like you are ventilating the prop about 29ish mph. Canít tell you why given all you did is change lower units, but since it is consistent across props itís not a hub issue.

Im assuming the motor was not removed from the boat so it could be higher on the transom?

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Old 03-17-2019, 06:01 PM
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Did you accidentally put on a wrong sized lower unit. Maybe take off a 25" and replace it with a 20"?
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Old 03-17-2019, 06:17 PM
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Originally Posted by KBH View Post
Did you accidentally put on a wrong sized lower unit. Maybe take off a 25" and replace it with a 20"?
that would require the motor to have an extension added or removed.
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Old 03-17-2019, 07:03 PM
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Yes correct, but, without pictures, we don't know what's happened, we can only get information piece by piece by asking questions, agree it sound like a spun hub, but not if a few props have been tried, unless they were all spun! or it's way too small.

Last edited by noelm; 03-17-2019 at 07:20 PM. Reason: added something
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Old 03-17-2019, 10:16 PM
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Hello, thanks for the reply, the motor was never removed and the lower unit is the same shaft.. we verified the gear ration.. the only thing I could think of is the slime on the bottom of the boat, but I cant possibly see it slowing it down that much..
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Old 03-18-2019, 12:53 AM
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How are you measuring boat speed? GPS or pitot or paddle wheel -fed gauge? I guess even without a boat speed measurement , you would notice the difference between 28 mph and 50.

It's just hard to picture the hub is spun if it accelerates fine to 3900 rpm/ 28mph . Is it a pressed in-type rubber hub ? If the prop shaft is spinning at 6000 rpm and the prop is not spinning faster than 3900 , you should be able to see and smell damage to the hub after a run. You can also mark or scribe across the prop outer metal hub through the rubber hub and onto the other side of the metal prop hub. Go for run past where boat speed does not increase and check the hub when you return to see if the line is still intact across prop & rubber hub.

If the prop is ventilating I would think STW would go down as rpm increases past the point of ventilation. That's ...usually.... the case with spun hub as well.
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Old 04-01-2019, 08:39 PM
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Look on YouTube for a video regarding the neutral switch. Sounds allot like what your experiencing. Iím interested in buying a proline 240 with a 05 300 hpdi on it and have been researching these engines and came across that video. I tried posting the link but you apparently need 10 post to add a link and this is my first. Keep me updated!
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