Bayliner 2858 Flybridge

[Stephen Tarry]

Hi Guys, I am new to Boats and seem to have dived in really deeply.
I have a Bayliner 2858 with a 6.5 ltre GM Diesel installed, the leg im assured is a "diesel" bravo 3 but not sure of the gearing? She has a 28P Mercury Marine Pitch Duo Prop Photo Attached.

The Engine runs upto 3900-4006rpm before the governor kicks in,but with no load on the prop!

WOT runs at 1800 - 1900 rpm giving a top speed of 10knots??

The engine runs ok! no signs of working hard.
The previous owner had the same problem with speed, but didnt check the fuel lines for blockages? and instead changed the Prop from 22p to the current 28P I have renewed the fuel lines and filter system, and she now starts and runs well sounding like the power is there, when she is opened up the bow lifts out and she feels like she wants to really go? but doesnt get over 10knots.
Bayliner 2585 Magica - YouTube

Its know its easy to asume that replacing the prop will cure the problem but I would like and value your comments, before starting out on an expensive course of action that may not be the right one to take.

The videos are on U Tube under stephen tarry 2858 magica and Bayliner Ciera 2858 Engine Rpm test.

Thanks for reading the above I hope you can help

 

[jestor68]

Re: Bayliner 2858 Flybridge

Your Bravo III X drive would have a 1.50 or 1.65 gear ratio. It is unlikely, given the boat size, that it would have the 1.36 gear ratio.

I ran your numbers through the Mercury Marine propeller selector, and it comes up with 19" pitch with either of the aforementioned gear ratios.

It gives a calculated weight of 8227 lbs(boat's dry wt is 7220 with a 300hp gas engine). With a 19 pitch, it says you should do 3500 rpm @ 34.8 mph.

So it seems that the previous prop choices were not the best.

Using the old tried and true average of 150-200 rpm change per inch of pitch change; it seems that dropping 9 inches of pitch should get you in the range of 3150-3600 rpm. So Mercury Marines estimate of 3500 rpm with the 19 pitch appears to be right on.

 

[Stephen Tarry]

Re: Bayliner 2858 Flybridge

Hi Jestor,
We have finally managed to get the Ratio of the leg, we have two turns per engine to one turn per prop. (2:1) Ratio ?
The RPM is at 3600 - 3800 with No Load on the Prop.
BHP is still unknown but we have averaged that out at 180BHP.
The 6.5 GM V8 seems to have between 150 - 240 BHP but I have no way to distinguish which or what it could be.
the 28" pitch lfts her up as if she wants to go but she just sits at the same speed, 10mph @ 1800 - 1900RPM