Please Help! Same Rpm Slower Speed

[ok3wire]

I have a '98 crownline 210 with a 5.0L V-8, 220hp I/O with a Bravo III lower unit. The engine has 250 hours on it. Once I plane out I typically set 3800 rpm and this usually give me between 35-38 mph (load and water conditions considered), according to the speedometer (not gps). Twice this season I've noticed that my speed had fallen off from this. Yesterday I was running at 4000-4200 rpm and that was barely giving me 30 mph, light load and 1/2 tank of gas. Any suggestions on what to look at? I gut says I'm not far off from something giving out and being stuck on the river. Please help.

 

[HT32BSX115]

Re: Same Rpm Slower Speed

Re: Same Rpm Slower Speed

(assuming your hull is clean and you don't have a severe water leak in the underdeck area) Verify your speedometer with GPS or other speedometer. My Bravo speedometer pickup is so completely plugged it won't work at all and I haven't so far been able to unplug it!

Pitot speedo's are notoriously unreliable anyway.

Regards,

Rick

 

[ok3wire]

Re: Same Rpm Slower Speed

Re: Same Rpm Slower Speed

Thanks for the reply Rick. I am going to verify my speed with my gps next time I take it out. However, my speed drop off has been significant enough to notice without even looking at the speedometer. I will check the pick up though to see if it is clogged at all.

 

[QC]

Re: Same Rpm Slower Speed

Re: Same Rpm Slower Speed

The "guy" is assuming a coupler or prop(s) hub or something else, but those typically fail much more dramatically than what you are reporting. As HT has noted I am much more concerned about what your bottom looks like. To be specific, I have no interest in your bottom, but the boat's bottom condition could do what you describe.

 

[ok3wire]

Re: Same Rpm Slower Speed

Re: Same Rpm Slower Speed

I'll go home and take a close look at it. I've had the boat for a year now and had a surveyor check it out before I bought it. He gave the hull and the rest of the boat, for that matter, a clean bill a health. The bottom has scrapes and scratches in the bottom paint but no damage.

 

[Limited-Time]

Re: Please Help! Same Rpm Slower Speed

Is it possible for a B III to lose drive to one of the props?? I have no idea myself...................thats why I'm asking. :redface:

 

[tommays]

Re: Please Help! Same Rpm Slower Speed

Well

A BIII is pretty fussy about things like water in the drive OIL and will slip if a condition like this exists


And your losing allmost 10 MPH

 

[Bondo]

Re: Please Help! Same Rpm Slower Speed

Ayuh,...

Is this a Dock boat, or a Trailer boat,..??
If it's a Dock boat,... I agree with the Guess that the bottom is Growing things,...
If it's a Trailer boat,... A check of the Fluids is in order...

 

[HT32BSX115]

Re: Please Help! Same Rpm Slower Speed

Have you done a close inspection of your prop? Damge to the blades and/or spun prop?

You spin a prop on a Bravo III and you'll have MUCH BIGGER PROBLEMS!!

(no rubber snubbers in the prop on a BIII it's all metal-to-metal)

 

[Don S]

Re: Please Help! Same Rpm Slower Speed

If the drive is slipping, you aren't going to see a reduction of 10 MPH, you are going to see little speed gain, and a lot of high rpm.

 

[ok3wire]

Re: Please Help! Same Rpm Slower Speed

Thanks for all of the replies. Please keep them coming. To answer some questions, the boat is kept on a trailer and the bottom of the hull appears to be fine. The props are fine as well, just a few very minor knicks.

Don, you made an interesting point about drive slipping and higher rpm's. The engine is rated for 4400-4800 rpm and yesterday when I noticed the decrease in speed I pushed the throttle up. It seemed as if the rpm was wanting to climb up out of this range. I was at 4600 rpm with 1/4 of the throttle left to push. Only a couple of times have I run the engine at WOT and I usually have to back it off just a little to keep it at or under 4800. Is it going to do damage to run it over 4800 if she's willing to give it to me? Like I said though, I typically run it at about 3800. I figure that's more fuel conservative and normally my speed is pretty good.

 

[Don S]

Re: Please Help! Same Rpm Slower Speed

What I was referring to with little speed gain was a lost of rpm and a max speed of maybe 2 to 3 MPH. That's what a slipping Bravo drive will get you.

 

[ok3wire]

Re: Please Help! Same Rpm Slower Speed

Sorry, well then what does that leave me with. My engine seems fine. It runs great at all settings with no lag in response time. My props are not torn up and neither is my hull. So why a sudden loss of 5-8 mph?

The only thing I can think of is that the props are not spinning as fast any more ( I know, master of the obvious ). So if the engine is producing the same power, what is causing it to not get to the props?

 

[Don S]

Re: Please Help! Same Rpm Slower Speed

Just where you have the drive trimmed can make that much difference. Sure hope you aren't using the trim gauge to say "There" is the mark I put the trim at, and then expect that to be accurate each time. Drive trim, trim tab trim, wave/wind conditions all chage constantly and change with fuel load, cargo load, number of people on the boat, and a lot of other things.
Get a GPS for speed and check things, adjust drive trim, trim tabs and boat loading. I think you will find your speed and rpm will agree again.

 

[ok3wire]

Re: Please Help! Same Rpm Slower Speed

BTW, what does it mean to "spin a prop"? Does that mean you stripped some gears in the drive or thrashed your hub? Curious.

 

[ok3wire]

Re: Please Help! Same Rpm Slower Speed

Unfortunately for me, things are never that simple. When I had it out yesterday the wind and water were calm, 1/2 tank of gas (25-30 gals) and a very light load.

As far as trim, I start out with the lower unit full down and once I plane out I will usually give it a couple of pushes of the up trim until I see the mph go up a little.

Please let me know if that is incorrect or just bad technique. Also, tried playing with the trim tabs and that did not help either.

Someone else had mentioned something about water in the gear oil could cause slipping. Is that worth checking?

 

[Mischief Managed]

Re: Please Help! Same Rpm Slower Speed

I suspect your tach is reading wrong.

 

[HT32BSX115]

Re: Please Help! Same Rpm Slower Speed

Most props except a Bravo III prop(s) have a rubber "cushion" in the center of the prop to absorb shock. They also sometimes fail and have to be removed and replaced. Some other props also have (user) removable center hubs that can be replaced.

It's also possible to strike something hard with a prop while running and have it "spin" the hub and not damage the rest of the drive.

With a Bravo III there's no rubber hub. it's all metal to metal in there on both props.

DON'T HIT ANYTHING. It'll be expensive.

BTW, what does it mean to "spin a prop"? Does that mean you stripped some gears in the drive or thrashed your hub? Curious.