Bravo 3 Prop selection - other options?

[Steve51]

Hi
I have a Mercruiser Bravo 3 drive and I have been speaking to the owner of an almost identical boat and same engine, his boat was tested with a set of test props and then the engine running data ran through the mercury prop selector tool and they came to a conclusion of 21" props.

This gave the boat about 35knts top speed and a lovely cruise at 20knts at about 40 litres / hour. This was with a medium load and clean hull.

I would be happy with that … however this is my first single engine duo prop where in the past I had twin engines. So just wondering if there were other options I should look into and what to expect if I could say sacrifice a little top speed for low speed maneuverability or stick to what works. Forgive me as I am a complete novice as far as prop selection goes and the Mercruiser prop selector doesn’t seem to work for me.

Thank you

 

[Scott Danforth]

define

an almost identical boat and same engine,

unless its the exact same boat model with the exact same amount of gear on board, and the same amount of bottom fouling, the boats can not be compared to each other.

can you answer the 6 questions in the prop sticky?

 

[Steve51]

define

unless its the exact same boat model with the exact same amount of gear on board, and the same amount of bottom fouling, the boats can not be compared to each other.

can you answer the 6 questions in the prop sticky?

Hi apologies will look at that. The difference in weight dimensions etc. is identical, however what I don't know but trying to find out is the drive gear ratio.

 

[Steve51]

Hi apologies will look at that. The difference in weight dimensions etc. is identical, however what I don't know but trying to find out is the drive gear ratio.

Hi, I would also say that this is a complete new engine and drive installation as part of a renovation project and as such have no meaningful data to go on.
It would be handy to find a prop selector that works as the Mercruiser one doesn't even have my drive gear ratio as part of the drop down list.

 

[alldodge]

Your going to like the B3 in docking, the props bite at slow speed. Your not able to spin the boat, there is no side walking in reverse

Get a prop where your motor reaches max WOT with your regular load (people, fuel, gear, water) and your good

 

[StewartL]

Since your buddy’s tested 21 props work well for speed and cruise, that’s a solid starting point. If you want better low-speed maneuverability, you could try a slightly lower pitch like 20 it’ll improve acceleration and docking control but cost you a few knots top end. Stick with Mercruiser’s Bravo 3-specific props for best performance. If you’re not hitting ideal WOT RPM check your manual, adjust pitch accordingly. Otherwise, if 21 does the job, no need to overcomplicate it.

 

[tpenfield]

What makes you think that the current prop set is not right for your boat? What is on your boat now and what is your maximum RPM and speed?

Playing prop sets on a B3 gets expensive.

 

[Steve51]

What makes you think that the current prop set is not right for your boat? What is on your boat now and what is your maximum RPM and speed?

Playing prop sets on a B3 gets expensive.

Hi .. complete new installation so no props yet and working out what would be best. The 21's on the identical boat and setup could be the way to go. I could try those for a season and if needed, update to another set. It will not hurt to have a spare set. My worry is the loading, as I cannot confirm the "other" boats set up - gear ratio.

 

[tpenfield]

Hi .. complete new installation so no props yet and working out what would be best. The 21's on the identical boat and setup could be the way to go. I could try those for a season and if needed, update to another set. It will not hurt to have a spare set. My worry is the loading, as I cannot confirm the "other" boats set up - gear ratio.

Mercury has a prop selector tool that can get you in the ballpark . . . maybe within a pitch or 2 . . .

https://www.mercurymarine.com/us/en/propellers/mercury-propeller-selector

 

[jlh3rd]

you're not looking at your buddy's data properly. It's not speed that determines the correct prop, it's WOT rpm's along with the engine's rpm limits.
What is your buddy's rpm @WOT ? That's the important question we're trying to get you to understand.