Bravo 3 XR changing ratio from 1.81 to 1.65

[espen22]

Hi, i have an bravo 3XR drive, with the 1.81 gear ratio.

What is it that makes the lower gear kit so much more expensive for this ratio ?

The propellers need to be changed and so does the lower gear kit, can i use the 1.65 gear kit that is cheaper and go for lower pitch propellers ?

 

[Mischief Managed]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

You need to change the lower gears and upper gears to go from 1.81 to 1.65.

 

[HT32BSX115]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

Hi, i have an bravo 3XR drive, with the 1.81 gear ratio.

What is it that makes the lower gear kit so much more expensive for this ratio ?

The propellers need to be changed and so does the lower gear kit, can i use the 1.65 gear kit that is cheaper and go for lower pitch propellers ?

I have a 1.81:1 in my boat.

You might be a little more specific on what you have exactly.....

boat (length, wt)

Engine, prop pitch etc.....

Are you trying to get more speed with a different engine etc?

a 1.81:1 drive with 28p Props in a semi-go-fast boat will yield close to 70 MPH at 5200 RPM...

 

[espen22]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

I have a 1.81:1 in my boat.

You might be a little more specific on what you have exactly.....

boat (length, wt)

Engine, prop pitch etc.....

Are you trying to get more speed with a different engine etc?

a 1.81:1 drive with 28p Props in a semi-go-fast boat will yield close to 70 MPH at 5200 RPM...

Sorry for the lack of info.

Boat : powerquest 240 sx
engine : qsd 350 (dieselengine)
Propellers are bravo 3 type (3+3)with 28" pitch

I want to get more speed, the diesel maxes out on 3800 rpm and top speed is around 60 mph with that gear ratio.

Looking at mercruiser part catalogue I cannot find any XR drive gears with different ratios, it seems like only the lower unit has different ratios on XR.
Anyone knows why 1.81 is 3 times the price of the other gear kits?

 

[HT32BSX115]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

With a max governed engine speed of only 3800 RPM,

I would want a 1.36:1 ratio. With only 270hp you may only be able to swing 24 or 26 pitch props on that boat. You'll just have to experiment.

 

[Mischief Managed]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

I think it would be easier and cheaper just to install 30" props and see if you get any more speed.

 

[espen22]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

I think it would be easier and cheaper just to install 30" props and see if you get any more speed.

they dont twist them to 30".
28" is maximum when running a 3 unit.

There is some mhp gains on changing to 3xr 4+3 setup.

 

[HT32BSX115]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

they dont twist them to 30".
28" is maximum when running a 3 unit.

There is some mhp gains on changing to 3xr 4+3 setup.

30p 3-blade Bravo III props actually ARE available. I have seen more than one set on EBAY for sale

There's also a company now that manufacturers 4x4 props for the Bravo III drives.

Did your engine/drive combination come new with a 1.81:1 Bravo III?or did someone cobble that engine and drive together and install it in that particular boat?


Mercury designs their drives and propellers to operate at specific maximum RPM ranges for maximum efficiency.

For example, A 300HP 5.7L (gas) V-8 has a recommended RPM range of approx 4400-5000 RPM. Using a (2.0-2.2:1 ratio) Bravo III drive and suitable prop-set, you get propeller RPM ranges of 2200-2500 and 2000-2272 RPM respectively.

It appears that Mercury may have intended the Bravo III propset to have an upper limit of around 2500

A single prop (Bravo I ) with a 1.5:1 ratio (@5000 eng RPM) would produce a "max" prop RPM of about 3300 RPM .................stock Bravo III installations aren't usually setup to produce prop RPM's that high.


Since your particular engine has a governed maximum speed of 3800, in order to achieve approx the same max propeller RPM, you would need a ratio *around* 1.7-1.9:1 With 1.65 being optimum to give you approx 2300 prop-RPM

If you used a 1.36 ratio, you could conceivably achieve a maximum propeller RPM of approx 2800. That might be too high producing either excessive vibration or cavitation at the higher speeds.

Your best bet might be going with a 1.65:1 drive as you indicated.


A 1.65-to-1 drive, 30p props and 3800 (using the prop-slip calc below) would yield approx 60 MPH with 10% slip.
Prop Slip Calculator

That engine would certainly achieve "Hole-Shot" RPM's with ease due to the high available (low RPM) torque.

Using the 4x3 or new 4x4 props, you may be able to limit the upper RPM to just below governed maximum engine speed but it will require experimentation. Most propshops will allow you try different propsets as long as you buy one of them.


Please post your test results! Be sure to use either GPS or a calibrated speedometer and an accurate tachometer.



Cheers,


Rick

 

[Don S]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

Guys, he has a Diesel Engine, not gas.

Boat : powerquest 240 sx
engine : qsd 350 (dieselengine)
Propellers are bravo 3 type (3+3)with 28" pitch

Don't use gas engine HP ratings and think it's the same as a diesel. It's not. Torque is the key word here.
Moving to the diesel forum so things don't get confused again.

 

[QC]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

I disagree. It is still a horsepower discussion. Top speed always is. I also agree with Rick, it is propshaft RPM that is more likely in play.

 

[HT32BSX115]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

I disagree. It is still a horsepower discussion. Top speed always is. I also agree with Rick, it is propshaft RPM that is more likely in play.

I'm absolutely aware that it's a diesel........ (I'd LOVE to have that QSD4.2-350 HO in my boat!!! OR the VW TDI Cummins MerCruiser Diesel | TDI 4.2L which Cummins evidently supplies......)

BUT to get the same (top speed) performance I get with my 7.4L V-8, I would be running a 1.65 drive and 30p Props.

It is entirely ALL about prop RPM..............It doesn't matter what type of engine is turning the drive...... 3800 eng RPM with a 1.65:1 ratio will result in 2300 prop RPM.......assuming it will turn that RPM with a given prop-set pitch

My 7.4L V-8 @ 4600 engine RPM with the 1.81 ratio yields 2540 Prop RPM

And yeah.......... it should be in the diesel forum.......

 

[espen22]

Re: Bravo 3 XR changing ratio from 1.81 to 1.65

Hi Guys

A lot of wery good replies here

Reason why this isnt posted in the DIESEL forum is simply because i started this tread to find out why the bravo 3xr gearkit with 1.81 ratio is so expensive.
And to find out if its possible to put in a 1.65 gear kit in the lower unit on a 1.81 drive. (hence the title)
The top gear on all Xr has same ratio on part list
The bottom gears are 1.65, 1.81 and 2.0 if i dont remember wrong.

Thanks for all answers so far