Troubleshooting top speed

Chris Patterson · Nov 4, 2015

Did the shaft/coupling mate up flat with the transmission? I'm wondering alignment..

Steve Dion · Nov 4, 2015

Did they align it on the water?

Anonymous 121765 · Nov 4, 2015

Vitor,

I have a question, was any loosening of motor mounts done? If so, it could be your motor is out of alignment. But, you felt no vibration....this is strange. Chris also brought up a good point. Could something have happened to the cutlass bearing as some yards go gorilla trying to pull a shaft & I've seen warped out bearing housings before. But again if so, you would feel vibration.

If having an in-water boat, did anyone jump under to see if the prop was not properly tightened or, can you be certain it's the same prop??

Did your prop pick up some line that may be wrapped on the prop & slowing you down? This happened to me one time where I lost speed & there was NO vibration.

I can't wait to see what the cause was pal. I wish you luck.

CR

Benny17441 · Nov 4, 2015

First determine if there is a real problem. How are you measuring speed, GPS or log meter? Are you getting the same RPM now as you were before? As you probably know GPS measures speed over the bottom and the reading is diminished or enhanced by the effects of wind and current. Measuring speed at two different times under different conditions will yield different results. Best is to perform two runs in opposite directions and use the average as an indication of boat speed. When using a log meter we are measuring speed over water so any effect due to current and wind will be largely nullified and readings taken at two different times should be close to each other. Engine RPM is crucial to boat speed, make sure your engine is attaining its maximum rated RPM. Was the prop just cleaned, painted and reinstalled or could it have been substituted by a different prop which had already been refurbished? A difference in pitch can cause a loss of speed which could be noticeable in the RPM. I'm not a gambling man but would venture to say that your loss of speed is either due to improper comparison of readings before and after or to a loss of RPM.

capta · Nov 4, 2015

Absolutely the only two things I can think of for your problem, if it doesn't have anything to do with the engine directly, or a repitch of the prop, would be the installation of the wrong cutlass bearing. If someone installed a metric one on a fractional shaft, perhaps lubing and forcing it on, this could bind things up and create the problem you are describing. The other is that someone over tightened the stuffing box. Can you turn the shaft by hand at the stuffing box fairly easily, with the engine off and out of gear? If you can't, put a wrench on one of the flange bolts and try. If it won't turn that way, something is binding up in your running gear.

Slow lane · Nov 4, 2015

Had a similar problem quite a few years ago. Dried my boat on the tide to do some unrelated work at last minute decided to change shaft anode. Working in water up to my ass got new one on farther up the shaft than normal. Even in pre gps days I new I had a speed loss and my max rev was up a bit. The point I'm trying to make is sometimes the most trivial change can make a big difference. Hope find your problem with a lot less trouble than I had I only found mine by accident. Never got a good answer as to why it made so much difference to boats performance.

capta · Nov 5, 2015

Vitor souto said:
The speed measured by log meter.did run different directions.( when in gear can't go over 3000 rpm at 6 kts max, but neutral goes 3500 rpm.

Just a note. You will never get full throttle no load rpm's in gear, never. If you were getting 3200, that's pretty good. Your engine shop manual (or the manufacturer) should be able to give you no load max and expected (properly propped) operating top end rpm's.
If you can turn the shaft by hand, there is nothing wrong with your running gear, so move on to the engine and trany. Perhaps if you are using a tach driven rpm gauge there is a problem there, though that would not affect speed. More than anything it sounds like you prop got repitched somehow.
I wish I had more to suggest.

Anonymous 121765 · Nov 5, 2015

Vitor,

Capta & I are on the same page here pal. In addition, the only way a wrong cutlass bearing can be installed is if the ID doesn't match the shaft diameter. If this occurred, you would feel vibration. You did not & nothing was changed in the alignment of engine to cutlass bearing. What are we missing here?

Mistakes happen at boat yards & you need to supervise & be there for the important parts. If you are at work, things can & will go wrong at times. Eyes-on are your only real insurance.

I have to say that there is definitely something else/undiscovered going on here. Only the right size key shafts work as designed. Did they do something different, I don't know. However, when under throttle, if the prop doesn't slip or you feel cavitation/rattle, I have to think the key is not suspect, as your prop is under a constant load @ this juncture.

You didn't have your prop pitch changed, did you log your existing prop size & pitch? If so, there should be stenciling on the hub noting that. If however, the prop was re-pitched at some time, was that also noted or stenciled?

Between Capta & I, something is amiss here pal.
ps: On a good day in calm seas, I can only get 6.5 KTS at best so you should feel blessed.

My bet would be that maybe they put the wrong prop back on & maybe the pitch is different.

Capta & I hope you find the culprit pal.

CR

Benny17441 · Nov 5, 2015

I'm sorry everything points to a prop change, either the prop itself or adjustments made to the pitch. Was an imbalance found on the prop and were adjustments made to correct it? On another note, how long did the entire job take? A few days or a month? This could help ascertain the chances of a possible engine issue developing.

woodster · Nov 5, 2015

Vitor souto said:
Thank you for your support I'll answer some questions but will be on boat just tomorrow.
Motor mount was not done
When shaft was replaced they los original shaft key ,they gave me another ,could these cause?.. a prop misalignment ?
The speed measured by log meter.did run different directions.( when in gear can't go over 3000 rpm at 6 kts max, but neutral goes 3500 rpm.
Prop was just clean and painted
I can turn shaft by hand.
Can a shaft anode cause these problem?

are you getting any white smoke out the exhaust when you are running 3000 rpms if so you may be over propped (too much pitch)

capta · Nov 5, 2015

It occurred to me that the 'lost' prop key might actually be because they misplaced your prop and the new one had a different size key???

woodster · Nov 5, 2015

capta said:
It occurred to me that the 'lost' prop key might actually be because they misplaced your prop and the new one had a different size key???

Not sure how that could because the key way on the shaft didn't change unless they put a larger diamature bore prop with a tapered sleve and taller key

Davidasailor26 · Nov 5, 2015

Vitor souto said:
The speed measured by log meter

Assuming you mean the paddle wheel type I'd consider that something may have changed there to affect your speed readings rather than anything about the engine. Have you had the chance to get the boat under sail to see how the speed readings seem then? I'd recommend checking the accuracy of the meter against a handheld or smartphone gps before putting too much more time into other potential causes.

capta · Nov 5, 2015

woodster said:
Not sure how that could because the key way on the shaft didn't change unless they put a larger diamature bore prop with a tapered sleve and taller key

I've seen (actually had) 'stepped' keys; two different size key ways and a key machined to fit both.