what gear ratio and prop do I need?

[fritz95]

Hey guys..and girls.

Just bought a 1989 Carver Allegra 2587. its got a single 7.4L merc with a missing outdrive. I've done a lot of reading, and pretty sure I want a Bravo 1 outdrive. my question is what gear ratio should I be getting. and what pitch prop? I realize testing is the ultimate answer but I don't even know where to start. I can't find any information on my specific boat as its very rare(not sure if I like that fact or not). My best guess after reading many threads is a 1.81/1 gear with a 24 pitch prop. but I could be way off base. boat dry is 5100 lbs. 28 loa.

 

[Don S]

Re: what gear ratio and prop do I need?

Below is some information from the Merc application manual. I would think you should be looking for a Bravo II, not a I, with a 1.50 Gear ratio.
That is a very common drive found on larger/heavier cruisers of that era.

Bravo One
The Bravo One drive is designed for higher horsepower engines, and is intended for boats
that operate at 39 knots (45 MPH) or above. The use of high‑strength, precision?forged
gears allows a smaller gear housing torpedo to be utilized for superior hydrodynamics and
optimum efficiency. The Bravo One can accommodate up to a 40.6 cm (16 in.) diameter
propeller. The standard drive can be used for either right‑ or left‑hand rotation on dual
installations.

Bravo Two
Larger, heavier boats that operate at 39 knots (45 MPH) and below require more thrust to
get them moving quickly and to keep them moving at slower engine speeds for optimum
fuel economy. For these type of applications, Mercury recommends the use of the Bravo
Two drive. A larger gearcase, shafts, bearings, and gears provide unequalled durability
and allow the Bravo Two to accept up to a 50.8 cm (20 in.) diameter propeller. The
additional thrust not only improves acceleration and cruise performance, but also affords
excellent reverse thrust for greater stopping power and maneuverability. The Bravo Two
is the drive of choice for many big cruisers and houseboats.

 

[scutly]

Re: what gear ratio and prop do I need?

you might want to rethink your options and do some research on a bravo 3. your boat is a tad smaller and lighter than mine and in my opinion your only 2 choices are a bravo 1 or a bravo 3. from my understanding, bravo 2 drives are mostly used more on heavy large houseboats and other displacement type non-planing hulls. my buddies boat is a 27' searay sundancer we both have a 7.4 carbed engine. he has the bravo 1 and i have the bravo 3. we both weigh over 6000 lbs. my boat outperforms his considerably in planing time (hole-shot) low speed maneuverability by a whole lot and really shows a difference in reverse mobility while docking. my boat docks like a car in reverse.

 

[fritz95]

Re: what gear ratio and prop do I need?

what bravo three ratio do you have? what pitch props?
I just picked up a great deal on a bravo 3 with 2.00/1 but I don't think that the right ratio

 

[HT32BSX115]

Re: what gear ratio and prop do I need?

That's a pretty big heavy boat.

I have a 1.81:1 "III" and 26p props in my 21ft Four Winns and have 24p props which perform very closely with the 26's.

A boat your size would probably perform better with a 2.00 or 2.20:1 Bravo III and 24's.

If you have a "deal" on a good 2.00:1 Bravo III, I would get it.

Even with a 2.00:1 drive, you may need to go to 22p props. (4600 rpm, 22p, 2.00:1 [15% slip] =~ 41mph) http://www.go-fast.com/Prop_Slip_Calculator.htm You might even be better off with a 2.20:1 drive.

Using 2.20:1 I also get 41mph using 24p props at 4600 RPM........I am not sure that your 28ft boat will go that fast at WOT either.......

I get 57mph at 4600 with my 1.81:1 + 26p props [and 10% slip] but my boat is 2000+lbs lighter! and 7ft shorter

 

[dubs283]

Re: what gear ratio and prop do I need?

if you really want to do your homework you can get the hull id # and call Carver and find out what they rigged the boat with

or mercruiser may even have a record of what drive they sold the package with to Carver