Understanding gear ratio's bravo 3

Shabah180

Petty Officer 2nd Class
Joined
Apr 16, 2016
Messages
167
I dont plan on changing anything on my bravo 3, maybe its sensless information im asking for. Lol.
With so many different prop pitches, what is the purpose of mercruiser having a range of gear ratio's? I have 2.20. 23/30 upper and 16/27 lower.
So if a boat beside had a 2:1 the differece in gears is very minor.
Im just curious what determines during a boat build, which gear ratio to put on and why they exist?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
The prop pitch is done in inches. A 21 pitch prop with zero slip moves 21 inches with each revolution. So were talking about moving a boat inches per second which turns into mph. So if you have lets say a 24 pitch prop on your 2.2 drive, and change it to a 2.0, you will probably need to change to a 17 or 19 pitch prop.

As an example, my boat changed from 425 HP to 530HP so during the build we thought I should get a 1.81 drive. Well the extra HP could not push my boat out of the hole, would not even get on plane even with 22 pitch B3. So I removed the new 1.81 lower and installed the old 2.0 drive and she came right up with 24 pitch.

The builder wants to put a boat in the middle of the available prop pitches. This will allow the user the ability to change to fix their needs
 

GA_Boater

Honorary Moderator Emeritus
Joined
May 24, 2011
Messages
49,038
The motor in front of the outdrive determines the ratio used. 4 cylinder, 6 cylinder and 8 cylinder motors have ratios determined and outdrives built by Mercury.Marine. Boat builders don't change the ratios if the power train is installed in a light or heavy boat or installed in a runabout or a houseboat. Different diameters and pitches are used to match the drive train to the boat.

AllDodge's situation isn't the norm for a production boat builder, more like a custom builder.
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
As an example, my boat changed from 425 HP to 530HP so during the build we thought I should get a 1.81 drive. Well the extra HP could not push my boat out of the hole, would not even get on plane even with 22 pitch B3.
That was very likely due to the new engine not having any additional torque capability (at lower RPM) over the original engine.
 

Shabah180

Petty Officer 2nd Class
Joined
Apr 16, 2016
Messages
167
The motor in front of the outdrive determines the ratio used. 4 cylinder, 6 cylinder and 8 cylinder motors have ratios determined and outdrives built by Mercury.Marine. Boat builders don't change the ratios if the power train is installed in a light or heavy boat or installed in a runabout or a houseboat. Different diameters and pitches are used to match the drive train to the boat.

AllDodge's situation isn't the norm for a production boat builder, more like a custom builder.

Yes thats right.. but he did say the builders try to gear so the props can be adjusted to match the boaters style. Its just puzzeling, you can have a baja and a crownline roughly same lengths and same power but different gear ratios. Same style drives.
But i guess they are there, to be able to work with the vast amounts of hulls, style of boats etc.

My boat was a "build" im trying to understand why they paired a 2.20 b-3 with a 350 on an 18 footer.
Like all dodge said,they try to keep it in the middle, id say im at the higher end of the prop pitch table. 26's im can push it to 5000-5100 light, 28p hills marine, 4500 light.
So when the previous owner did his upgrades with the techs, how, do they determine " well shes going to need a 2.20 to run right!"
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
That was very likely due to the new engine not having any additional torque capability (at lower RPM) over the original engine.

No the new motor has peak torque at 617 FT LB, the standard 415 HP 502 is just under 500 FT LB.

Do need to make a correction on my previous statement. The boat would get up on plane going to a 22 pitch prop, but it took it a while and really struggled. The old Motor with the 2.0 drive came up on plane OK, but struggled with 6 people aboard. The same drive and 24 pitch 4-blade (same as 26 3-blade) come s up quick even with the same 6 folks and even quicker with just two of us.

The issue I ran into is the builder did not know as much as he thought he did. If we do the math
A motor turning 4000 rpm (zero slip)

A 2.0 drive in one minute a 24 pitch prop travels 4000/2.0 = 2000x24 = 48000 inches / 12 = 4000 feet or 45 MPH
A 1.8 drive in one minute a 24 pitch prop travels 4000/1.8 = 2222x24 = 53328 inches /12 = 4444 feet or 50 MPH

Now a boat which did not weight as much as mine would "probably" have an easier time, but 11K LB is a lot to get up. My top speed with the 2.0 is 47.6 MPH, but I'm starting to over rev, but that's another subject I'm working on

Shabah, your dealing with hind site. If you have a hill marine 26 pitch 4-blade the it would be the same as a 24 3-blade. Turning that kind of rpm would tell me you are on the high end. If someone was building your boat in mass amounts with the same power setup, they would have different ratios and found the mid. I would say you would probably work well with a 2.0 and 4-blade in the 22 to 24 range.

In most cases I have found thru many prop changes and ratios over the last few years :facepalm: is more pitch increases slip. While the increase in slip may be minimal its there. Most do not see it because they are not working with a heavy boat, but then again maybe what I'm seeing is just because its heavy
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Yes thats right.. but he did say the builders try to gear so the props can be adjusted to match the boaters style. Its just puzzeling, you can have a baja and a crownline roughly same lengths and same power but different gear ratios. Same style drives.
But i guess they are there, to be able to work with the vast amounts of hulls, style of boats etc.

My boat was a "build" im trying to understand why they paired a 2.20 b-3 with a 350 on an 18 footer.
Like all dodge said,they try to keep it in the middle, id say im at the higher end of the prop pitch table. 26's im can push it to 5000-5100 light, 28p hills marine, 4500 light.
So when the previous owner did his upgrades with the techs, how, do they determine " well shes going to need a 2.20 to run right!"

I think 5000-5100 is near or at the "top" of the WOT range for your engine (5200 being the "rev" limit)

If you can turn 26's at 5100, you're really "propped" just right with that drive. (your props are turning 2318 RPM at WOT)

OTOH, if you had a 2.00:1 drive, to get 2318 you would have to turn 4600 RPM. (top RPM for a carbureted 454 by the way)

Also, there is probably an upper limit on how fast (RPM) you should turn a (normal) prop before it starts losing HP due to drag. RPM above that doesn't get you much other than more fuel flow. [Racing props, surface props etc don't necessarily fall under these rules)]

Because of the above, if you were actually near or at that limit, you're boat would perform better if you went to a 2.00:1 drive and dropped down to a lower pitch (24/26 instead of 26/28 etc) I don't think you're anywhere near that upper limit though...........

My FourWInns 211 Liberator for example has a 1.81:1 drive and I can turn 26p props at a 4600 ENG RPM and 2541 prop RPM at 57 mph......... (rev limited at 4700)
which according to http://www.go-fast.com/Prop_Slip_Calculator.htm and results in about 10% slip

If I were to switch to a built 454 that had a LOT more HP, and could run 5000+ RPM, I would likely be able to go about 61mph (assuming I still only had 10% slip) using the same props. BUt I think I would be able to turn 28's or even 30's

30p props at 5000 RPM would yield 70 MPH (assuming the same slip which is probably NOT realistic AND A LOT faster than I really want that thing to go!!!!!)

And a word about slip..... And you're right AllDodge. Slip increases with RPM (because it's DRAG) and with heavier boats, the slip INCREASE with RPM is even larger.


No the new motor has peak torque at 617 FT LB, the standard 415 HP 502 is just under 500 FT LB.
I have to ask... Was that assumed or was that measured on a dynomometer?
 
Last edited:

Shabah180

Petty Officer 2nd Class
Joined
Apr 16, 2016
Messages
167
Yss sir, she will spin 3x3's 26p me, wife, half tank, to 5000, 60 mph. I thought 454 carb was 4800? My carbed pre vortech 350 is 4800.

Those are both really good reads! Theoretical speed calculations etc are pretty interesting. Prop revolutions / ratio /pitch/ to determine performance is the technical stuff thats good to learn. Theres no fish stories its all just laid out by math lol!

So the lower the ratio the more she will spin at a given rpm, of the same prop pitch.

See? This is why i dont ask questions in different forums, maaan ive read some doozie advice that could easily put someone whos learning in a bad place lmao!

Thanks for putting it there fellas at least i have something to refer back to. Unreal the how much you guys know! đź‘Ť
 

Shabah180

Petty Officer 2nd Class
Joined
Apr 16, 2016
Messages
167
So, with the math you did, is it theoretically possible for a 21p prop at 4800 rpm with the available gear ratio's in an alpha to move whatevers in front of it 60mph?
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
So, with the math you did, is it theoretically possible for a 21p prop at 4800 rpm with the available gear ratio's in an alpha to move whatevers in front of it 60mph?

No theory necessary.............My FourWinns 211 Liberator was actually available with a 454 + Alpha drive. The Alpha that was used behind the 454 was a 1.36:1 ratio.
Using http://www.go-fast.com/Prop_Slip_Calculator.htm with 1.36:1 ratio, 4800 RPM, 21p and 15% slip, you get 60 mph

Since my 211 went 60 MPH with the King Kobra drive (1.43:1) with a 20p (cupped 19) running at about 5000, there's no doubt in my mind that it would have done it with the optional 454/Alpha. Problem was, there were more than a few reports of the 1.36:1 Alpha suffering some failures running behind a 454. Which is sort of why Mercury Marine developed the Bravo
 

Shabah180

Petty Officer 2nd Class
Joined
Apr 16, 2016
Messages
167
Thanks all doge for doing up that diagram picture with my engine last year for dry block winterization! After more reading about the inlet and supply hoses to the t-stat, i took my time and it really gave me chance to start understanding how the cooling system works, and they way the water moves thru the engine.
Not a single issue after i opened up after storage, and its gets to -30, -40 up here in winters.
So thanks to all dodge, bondo, and don.s for putting THE RIGHT info out there, hopefully people arnt to stubborn to learn.
Why force anti freeze thru the block when all you have to do is let gravity donthe work! Lol
( i blew out lines with shop vac)
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
I thought 454 carb was 4800?

I used 4000 rpm as a baseline just for example in calculations

So, with the math you did, is it theoretically possible for a 21p prop at 4800 rpm with the available gear ratio's in an alpha to move whatevers in front of it 60mph?

Agree, as HT mentioned, it does happen, so long as other things are not holding you back. My Rinker will do right at 60 with 300 HP (under 10 percent slip), but my Formula will only make it close to 50 with 530 HP (16 percent slip). My formula needs more blade area. If it had two SBC it would make it above 50
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
I used 4000 rpm as a baseline just for example in calculations



Agree, as HT mentioned, it does happen, so long as other things are not holding you back. My Rinker will do right at 60 with 300 HP (under 10 percent slip),
:thumb:



Your 300hp (propshaft rated) Rinker is similar in size/WT to my 1987 Four Winns.

The original OMG 460 KC was rated at 340 (crankshaft) HP and the also available 454/Alpha was rated at 330 (crankshaft) hp. The 1997 7.4L + Bravo that I installed in 2006 was rated by Mercury at 300 Crankshaft HP. so it's no surprise that it performs similarly. I presume that I would get a few MPH more top speed with the Bravo I (and before I installed it I was actually looking for a Bravo ONE.....I just found the B3 for a "song" and couldn't turn it down!

If I had it to do over, I would still get a Bravo III (or Volvo DUO-Prop) every time!! even if it is a tiny few mph slower.........only because of FAR better holeshot and slow speed performance.........

It's probably worth mentioning that low slip percentages are usually due to very low (hull) drag (zero slip results in the theoretical speed in the prop-slip calculator)

Small and/or relatively lightly loaded planing type boats will have the lowest slip where very heavy cruisers etc will have very high slip numbers regardless of the number of blades in the water.

A dual contra-rotating prop (Bravo III, DUO-PROP) drive will always have lower slip vs a single prop drive and though I haven't tried the 4x4 props, I do have the 4x3 24p props on it right now and I cannot tell any difference.

Have either of you tried the 4x4's?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
Have either of you tried the 4x4's?

Yes, currently have them on the formula now (22 pitch hill marine) and they do very well. After removing all the bottom paint and repainting with epoxy I'm over reving a bit now. So I need to remove them and put my 3x3 26 pitch back on and see if I can reach 50 MPH
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Yes, currently have them on the formula now (22 pitch hill marine) and they do very well. After removing all the bottom paint and repainting with epoxy I'm over reving a bit now. So I need to remove them and put my 3x3 26 pitch back on and see if I can reach 50 MPH

Do you think they improve the holeshot enough to make them worth it and (I presume) they slowed you down a bit ?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
Do you think they improve the holeshot enough to make them worth it and (I presume) they slowed you down a bit ?

They bite a tad better then the 3 blade, but if it was not for the weight of my boat I probably would not have tried. My Formula even on full plane I'm still drafting at least a foot and a bit more. Where some can trim up a bit more to pick the nose up, I cannot do that. My trim is slightly up off flat when cruising, but has to go down and stay almost flat true with the hull for WOT. If I trim up a bit I'll start to porpoise a tad. Now with the 3 blade and a little less lift I can trim up a tad more but I'm drafting more water at the stern.

Wouldn't mind trying 24 pitch 4x4 again with the fresh hull, I sent them back because I couldn't get the rpm high enough prior. The 4 blades give you more lift but everything has trade offs. My 26 pitch might just be the ticket. Might try removing the 125 feet of anchor chain and anchor to see if that helps :rolleyes:

My Rinker is a B1 and I wouldn't put a 4 blade on it. The 21 pitch Mirage works quite well for me

BTW Shabah180 where drifting off your topic, so say the word and we'll stop :D
 
Top