How do you design correct Bravo III ratio?

[RexSnr]

I have a 2001 Bayliner Ciera 2855Si with a "dead" 310hp 7.4L MPI/Bravo III with 2.2 ratio and ran 15.75"/14.25" props x 22" pitch. I bought it like this, so have no idea if this combo worked well or if it was original. It only had 310 hours from new and risers had rusted out and leaked water into the engine, so I presume it was set up ok?? Boat weight is 7185 lbs plus if you put max allowable persons plus full fuel, water and tools etc, it would be circa 10,400 lbs maximum, but I have to allow for this. Note: std. engine was 260hp small block, this has optional "big" 310hp big block.
I don't know, but strongly suspect this boat was underpowered. In repowering, I was going to install an 8.2L Mag HO 430hp, but price is scary. I am now considering the 380hp version of the same, but dealer is pushing to use the 6.2L 350hp, which worries me. He has also quoted a 1.81 ratio Bravo III.
Question:
How the hell do you choose the correct Bravo ratio? I have been reading for hours. Dealers 1.81 vs original 2.2 is a worry. How does a boat designer do this? I can find zero info explaining it? Is it a wild guess? Surely there is some engineering knowledge to keep the guesswork out of it?
Does anyone have experience with same weight boat that could steer me to correct engine too? I am not too interested in high speed but will take it. I would like a big engine running slow, (cruise speed say 22-25 knots) to give long life and lowest poss fuel burn (for its' size).
Scared the engine size, the Bravo ratio and props won't be right and guess who pays to have this remedied, after already coughing up lots of $$? Lot to go wrong.
Hope you guys can put me out of my misery?

 

[Scott06]

@msd58 is that the same boat you have ?

 

[msd58]

@msd58 is that the same boat you have ?

Yes

 

[msd58]

Most of - if not all of the 2855's I have come across that have the 7.4 motor have 24" props.... that would include mine. Dealer told me that my boat came with 2.20:1 Bravo 3. He verified that my hull # with Bayliner. A lot of guys who's engines die go with the 6.2 I have not read where they change gear ratios... perhaps a prop pitch change. Obviously an 8.2 MPI Mag would be ideal but the cost is barrier to entry there.. although some do it.

My performance numbers with 2000 7.4 MPI motor 310 HP cruises nicely at 3600-3750 RPMs at around 25-27 mph depending on sea state and load on boat. Never felt that the boat was under powered. Not a speed demon for sure. With a 21* dead rise at the transom it won't be.
Here's a pic. 3500 rpm, full fuel, isenglass enclosure up, solo trip 25 mph

New item by mark dimaio

photos.app.goo.gl

 

[RexSnr]

Most of - if not all of the 2855's I have come across that have the 7.4 motor have 24" props.... that would include mine. Dealer told me that my boat came with 2.20:1 Bravo 3. He verified that my hull # with Bayliner. A lot of guys who's engines die go with the 6.2 I have not read where they change gear ratios... perhaps a prop pitch change. Obviously an 8.2 MPI Mag would be ideal but the cost is barrier to entry there.. although some do it

Thanks very much for the info. You have confirmed mine was stock except for props. Causes me to question the dealer even more now. As a user, and me not ever using the boat,
1/ does your boat plane easily (quickly) and seem correctly powered?
2/ does it feel "spritely" and easily doing its work?
3/ does it carry a "full pax" load easily?
4/ or do you think it could use more power, and if so 350, 380 or 430hp?
5/ are the 6.2 guys happy?
6/ does the 6.2 seem to give good service/fuel economy/reliability/life span?
Thank you very much for the help. These are really rare where I am so info is really scarce.

 

[msd58]

Thanks very much for the info. You have confirmed mine was stock except for props. Causes me to question the dealer even more now. As a user, and me not ever using the boat,
1/ does your boat plane easily (quickly) and seem correctly powered? Yes I always felt that 7.4 is adequate for this model boat. The planning characteristcs are commensurate for 28.5' x 10' boat with single engine.

2/ does it feel "spritely" and easily doing its work? I would label it spritely. My Seadoo I would say yes or my 17' Whaler.

3/ does it carry a "full pay" load easily? I had 5 adults, small kids and gear while carrying less than 1/2 tank of gas and it got up on plane no problem. It's not ski boat.

4/ or do you think it could use more power, and if so 350, 380 or 430hp?

I don't think 350 hp is a big enough jump to justify the expense. Just rebuild your 7.4 Now a 8.2 with 380 or 430 hp is a different story. Of course I would love more power but at what cost? You could just fix your motor and sell the boat and get a twin engine boat will twin 5.7s

5/ are the 6.2 guys happy? Yes they seem to be. Nobody complained about the 6.2

6/ does the 6.2 seem to give good service/fuel economy/reliability/life span? Fuel burn in my opinion relative. Longevity largely depends on maintenance.
Thank you very much for the help. These are really rare where I am so info is really scarce.

 

[Bondo]

I have a 2001 Bayliner Ciera 2855Si with a "dead" 310hp 7.4L MPI/Bravo III with 2.2 ratio and ran 15.75"/14.25" props x 22" pitch. I bought it like this, so have no idea if this combo worked well or if it was original. It only had 310 hours from new and risers had rusted out and leaked water into the engine, so I presume it was set up ok?? Boat weight is 7185 lbs plus if you put max allowable persons plus full fuel, water and tools etc, it would be circa 10,400 lbs maximum, but I have to allow for this. Note: std. engine was 260hp small block, this has optional "big" 310hp big block.
I don't know, but strongly suspect this boat was underpowered. In repowering, I was going to install an 8.2L Mag HO 430hp, but price is scary. I am now considering the 380hp version of the same, but dealer is pushing to use the 6.2L 350hp, which worries me. He has also quoted a 1.81 ratio Bravo III.
Question:
How the hell do you choose the correct Bravo ratio? I have been reading for hours. Dealers 1.81 vs original 2.2 is a worry. How does a boat designer do this? I can find zero info explaining it? Is it a wild guess? Surely there is some engineering knowledge to keep the guesswork out of it?
Does anyone have experience with same weight boat that could steer me to correct engine too? I am not too interested in high speed but will take it. I would like a big engine running slow, (cruise speed say 22-25 knots) to give long life and lowest poss fuel burn (for its' size).
Scared the engine size, the Bravo ratio and props won't be right and guess who pays to have this remedied, after already coughing up lots of $$? Lot to go wrong.
Hope you guys can put me out of my misery?

Ayuh,..... Welcome Aboard,........ Have you had a full in-depth survey done on it,..??..??
At that age/ vintage, I'd be extremely concerned about deep structure rot,...
I believe the backbone of that hull is glassed it wood,.... How about transom rot,..??

 

[Scott06]

Thanks very much for the info. You have confirmed mine was stock except for props. Causes me to question the dealer even more now. As a user, and me not ever using the boat,
1/ does your boat plane easily (quickly) and seem correctly powered?
2/ does it feel "spritely" and easily doing its work?
3/ does it carry a "full pax" load easily?
4/ or do you think it could use more power, and if so 350, 380 or 430hp?
5/ are the 6.2 guys happy?
6/ does the 6.2 seem to give good service/fuel economy/reliability/life span?
Thank you very much for the help. These are really rare where I am so info is really scarce.

Is the drive gone too? If not run it behind what ever engine you can afford.

have also seen the 6.2 as a replacement for the 7.4 you will shed some weight with the small block that makes as much power as the 7.4 .

are you doing this work or is the dealer? If the dealer is doing it they have to make it work and can swap props as needed to get it right. The fact that you are entertaining the 8.1 makes me thing your budget is pretty high. If your original drive works try that and reprop as needed.