Problem question cant get on plane

Black Valkyrie

Black Valkyrie

Active Member
Jul 19, 2014
688
Canada
Boat Info
1993 Sundancer 270 DA
Engines
Twin 4.3
Hi long time no post. Interesting problem has arisen.

Had my boat tuned up in the spring. Runs fine. In neutral both engines run freely up to 4300 rpm. Underway port engine runs up to 4300rpm with no change in velocity past 2500 rpm.

Starboard engine underway wont rev past 3200 rpm.

Best speed I can manage is 13 mph instead of a previous top speed of 30mph.

Mechanic says the starboard prop is spun (to me this is illogical) ... to test I changed the starboard prop to my (new) spare and it made zero difference.

I (sadly) dont have a spare port prop and the unit onboard is the old style (no flow torq) and no one local will rebuild so I assume next step is to buy new port prop and change it out.

Question - if the port prop is spun would that prevent the starboard engine from exceeding 3200rpm under load?

Or is there something else more sinister lurking underneath I should be investigating?

Losing confidence in my mechanic.

Thx!
 
If you spin a hub the rpm`s go up. Might b time for a compression test
 
Thx - had that tested when I bought it zero issues. Dont believe it is anything like that. As mentioned before both engines run wonderfully this is an issue when it is in gear and underway. No oil loss or use. No coolant loss or issues. Prob is as described above so more specifically what other mechanisms between the engine and the prop could be the source of lost power?

(That would be IF the port prop is spun, and IF a spun port prop would cause the starboard engine to not be able to rev to full rpm underway)
 
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Just an fyi, revving up in neutral is worthless - it really doesn't tell anything usable. However, you should easily be able to get WAY over 4300 in neutral... or is that just where you stopped? Those engines should, ideally, get into the upper 4,000 range under load.

The only two things between the engine and the prop is the coupler and the prop hub. If either of those go out, as mentioned above, RPM's will sky rocket. That's not what is happening in your case. So unless you've got growth/lines on the hull/running gear or the props are bent/chewed up or something similar, that leads us back to the engine(s) as the issue. It could be anything from needing a tune-up to a loose/failing plug wire to a weak engine.

While a compression test isn't the be-all-end-all, it would be a good idea to do that as it could point to an issue. Don't discount that just because the compression was good back whenever it was that you bought the boat.

Fuel quality/quantity also comes into play. Both engines pull from one tank?

You won't be able to get full RPM's under load until your speed increases.
 
Yes, a spun port hub could be the cause of his. The spun hub allows the port motor to rev without actually providing the thrust that it should. The starboard motor is actually doing most or all of the work and can’t get the boat in plane by itself.

I bet if you only ran the starboard motor, you’d have similar results.......around 3000 rpm and not getting on plane.
 
I would double check the tune up work, as that is the recent change. Are the spark plug wires run to the right plugs? Are they pushed all the way down on the plug? Are the plugs tight? Etc.

If it isn't related to the the tune up, then it sounds like your port prop might be spun, not starboard. You're likely getting high RPMs on port without any "help" from the prop because the hub has let loose. And as other have said, the starboard engine is taking all the extra load. But then the question is "why did the prop hub break loose?" If it worked fine before and you didn't hit something, why all of the sudden would it start to spin?
 
Looks like my port prop has been discontinued? Will have to make some calls tomorrow

P/N qs5057x
 
Had my boat tuned up in the spring. Runs fine. In neutral both engines run freely up to 4300 rpm. Underway port engine runs up to 4300rpm with no change in velocity past 2500 rpm

Sounds more like incorrect plug firing orderr a dead plug
 
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