20 MPH Slow!

othanku · Aug 10, 2010

Trying to help a friend with her boat, its a 2001 Bow Rider 21ft. It has the 5.0 EFI Engine and Alpha One Gen II? Outdrive. The boat starts and idles fine, gets up on plane (a little slow in my opinion with 1 person) and then just flattens out at about 27-28 MPH. The tach says around 4800 RPM and its sounds about right. Any ideas on this cause? I cant seem to find gear info, but from what ive read 1.4 - 1.5 seems to be ball park. Has a 21 SS pitch prop with no dings. Boat has sat in marina and does have dirty bottom, but not 20MPH worth! Any help would be appreciated.

Matt89GT · Aug 10, 2010

Re: 20 MPH Slow!

4,000 RPM's WOT? you wanna shoot for 4600 to 4800 RPM's at WOT, with the 21 pitch I think your a bit over propped which is bad and will cause other problems, I think you should drop down to a 19 pitch, say like a 14.5 x 19 and make sure the bottom is cleaned because that too will drown the performance of any yacht , just sayin

othanku · Aug 10, 2010

Re: 20 MPH Slow!

Sorry the RPM'S were at 4800 not 4000

Is it safe to assume its something in the outdrive? Wouldnt a spun prop hub cause the engine to over rev, which it isnt at all. Not familiar with outdrives, but is a drive or engaging cable a possibility? I would hate to see her heading right to a $2000 bill at a dealer if i can help her check some things. She thought tune up, but i cant see an engine that starts and runs that good as being a "speed" culprit. The trim is working.

NelsonQ · Aug 10, 2010

Re: 20 MPH Slow!

28MPH????? I'd look at the prop first and see exactly what you've said about the possibility of a spun hub. Seems like a good starting point.

Matt89GT · Aug 10, 2010

Re: 20 MPH Slow!

I must not be able to read, I seen where you did put 4800 however I would start by cleaning the bottom of the boat off you might be suprised how much a dirty bottom will slow a boat down. Then you might mark the center of the prop with a line and run the boat, then check the line if thats ok and after cleaning the bottom and its still doin the same thing I would check the coupler.

Brew2 · Aug 10, 2010

Re: 20 MPH Slow!

It might be stating the obvious, but are you sure the speedometer reading is accurate?

I'd double check with a GPS or possibly another boat pacing you. Perhaps the speedo is out by 10 mph and the dirty bottom accounts for the other 10 mph.

Bondo · Aug 10, 2010

Re: 20 MPH Slow!

Boat has sat in marina and does have dirty bottom, but not 20MPH worth!

Ayuh,.... I bet your Wrong on that assumption....

Clean the Hull....

othanku · Aug 10, 2010

Re: 20 MPH Slow!

Okay, thanks I'll go ahead and clean the hull..mostly just green fur on it...

Yes i was out with another boat who left me standing still...said they took it up to 49 MPH it was a Monterey Montura 21 with 5.0 Penta 2003 model, and by the rate in which i flatlined and they kept going, i think the speed-o is "close" anyways. I appreciate the input and im trying to get as much as possible as i have it out of the water and on a trailer and its 3 hours away back and forth to do any test.

Heres what she just e-mailed me,
Early 08 Mark tunes the boat for Spring, pulls it out and lists it for sale. Mark moves to London.
Late Fall 08 Jeff and I buy it. Store it on the trailer for a few months
Jan 09 Boat is put in the water. Very little usage - but goes fast!
June 09 Jeff uses boat - claims its "slow"
May 2010 Julie takes over the boat and we start using it. Its slower than (#*$(#&&#!!!

180shabah · Aug 10, 2010

Re: 20 MPH Slow!

Ditto - clean the hull...

EddiePetty · Aug 10, 2010

Re: 20 MPH Slow!

...has anyone mentioned cleaning the hull?????

othanku · Aug 10, 2010

Re: 20 MPH Slow!

Not to challenge anyone's knowledge here, but seriously? Your going to say that a shallow light green fuzz from sitting in a fresh water marina is going to slow down a 2001 305 EFI boat with one person in it, 20 MPH, yet gets it on plane within reason?

180shabah · Aug 10, 2010

Re: 20 MPH Slow!

Drag increases exponentially with speed.

That fuzz would have little to no effect on planing ability and could absolutely kill 10-15 mph from the top end.

add that back to the 30mph your gettin' now and it is a huge difference.

Is it possible that you have more than one issue? Sure. But why not start with the cheap (free) easy stuff, before spending a lot of money on diagnostics and parts???

FWIW - if the boat is going to sit in the water all year, bottom paint is a must. Get an ablative paint - it is "self cleaning"...

libellav15 · Aug 11, 2010

Re: 20 MPH Slow!

othanku said:
Not to challenge anyone's knowledge here, but seriously? Your going to say that a shallow light green fuzz from sitting in a fresh water marina is going to slow down a 2001 305 EFI boat with one person in it, 20 MPH, yet gets it on plane within reason?

no-one is suggesting you rebuild the Taj Mahal!

just clean the hull and we will all KNOW the answer

othanku · Aug 11, 2010

Re: 20 MPH Slow!

Im a little amazed here by the "jumping on-board" here of the clean the hull statements. I'm pretty sure we all understand the basic principle of drag, and that sounds great good and fine, but the reality is, that this boat would be dragging a small forrest underneath to slow down this boat by that much. I've known boats with sea worthy "dirty" hulls that have slowed down the speed by 7 + or more miles per hour. But your not going to convince me that a 2001 21ft 5.0 with a 21 prop Sea Ray bow rider is slowing down that much because of light green fuzz. I dont care how much "experience" you have, you know its not the reason...

Bondo · Aug 11, 2010

Re: 20 MPH Slow!

But your not going to convince me that a 2001 21ft 5.0 with a 21 prop Sea Ray bow rider is slowing down that much because of light green fuzz.

Ayuh,... Go Clean it, 'n try it....

Then We can discuss the results...

Cuyose · Aug 11, 2010

Re: 20 MPH Slow!

You're probably right, I wouldn't worry about cleaning the hull, your motor is shot, I would get a new one.

John_S · Aug 11, 2010

Re: 20 MPH Slow!

Well, if that engine can get to 4800 rpms with a dirty hull, wait until you clean it! The slip is greater than 50% !!!! Start with checking the tach.

180shabah · Aug 11, 2010

Re: 20 MPH Slow!

Sorry to bore you with unmanly tasks like bottom cleaning.

YOUR #'s were 4800rpm and 27-8mph with prop in perfect condition. That means a propslip of 50-60% depending on gear ratio in the drive (1.5 0r 1.6).. A weak engine does NOT increase prop slip. Prop damage, debris and dirty hulls DO. You say prop is good. Guess what that leaves? The dirty hull and trash.

One other item that I dismissed was the prop hub - if the engine is holding a steady 4800 and a steady speed - the hub is holding. When they let go, you would expect to rpms climb rapidly as speed drops.

180shabah · Aug 11, 2010

Re: 20 MPH Slow!

imported_John_S said:
Well, if that engine can get to 4800 rpms with a dirty hull, wait until you clean it! The slip is greater than 50% !!!! Start with checking the tach.

I was thinking more along the lines of the hull growth disturbing the waterflow enough for the prop to lose bite, and landing at 4800 was just a coincidence.

othanku · Aug 11, 2010

Re: 20 MPH Slow!

Well i guess i should have checked before i asked for any input. My bad, but still interesting to hear this great speed loss due to the boat having a small amount of growth. Or maybe just the fact that one dist cap wire was barely on/in at all, and made just enough contact to start and run fine, but when it got to the higher RPM's and under load it was basically just dragging a cylinder along. Spark plug confirmed that, and the others nicely gapped at .025 instead of .045 helped peak spark. But then if you do have an inaccurate tach perhaps, it would make sense that it wasnt really 4800 RPM. Or maybe the pitch being 21 on the prop, when i do take it out again will actually put the engine over the 5K RPM range, and it IS accurate. Must be a "pretty shot" motor to not even tell it was running on 7 and some change! Feel like putting the growth back on just to show it might have only made a few miles per hour difference..but hey...I guess we'll never know. unless someone else grows green and loses 20mph! ;o)

20 MPH Slow!

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