i need to understand gear ratios.

in doing a re fit, and dont fully understand drive gear ratios.
350 hp, b1 or b3 drive. engine 5000. rpm (it looks like b1 at this point!) approx 3200 lbs
i want to get an even ratio.... that means i dont want to overload the motor and underdrive the leg. or vice versa! i want high enough top end but dont want to labor the motor or drive.
please help me understand all variables.

thanks
oops

Re: i need to understand gear ratios.

The leg ratio is the number of revolutions of the engine to one revolution of the propeller. For V8s the usual ratio is 1.5:1, unless you are at high altitude. The leg ratio is fixed and difficult to change (without stripping the leg and changing the gears). Once the power package is installed in the boat a prop test is done to determine the correct propeller. This is the one that will allow the engine to achieve the WOT (Wide Open Throttle) revs specified by the manufacturer, usually around 4500-5000rpm. If the engine can't make the specified RPM (with a normally loaded boat) then the propeller needs to be changed to one with less pitch. Pitch is measured in inches, the number of inches the propeller will move with one revolution, eg 17", without allowing for any slip. If the engine is running faster than spec at WOT then a prop of higher pitch needs to be fitted.

Hope this helps.

Chris.................

Re: i need to understand gear ratios.

thanks chris
that helps.

the b3 i have accuired. has a 2.4 ratio.
now i knoe from riding my bycicle. that 10 th gear is hard to start out in.
if the b3 has a 2.4 ratio. wont that make my motor work like the dickens?
if a 1.5 to 1 (overdrive) is normal would i be better off with a smaller gear ratio. (less than 2.4) less hard on the engine. i want the load to be more or less equal. so one part is less over taxed, and less failure.

am i thinking correct. or have i had too much corona and missed somthing?

thanks
oops

Re: i need to understand gear ratios.

if a 1.5 to 1 (overdrive) is normal would i be better off with a smaller gear ratio. (less than 2.4) less hard on the engine.

Click to expand...

Basically,.......... You've got it Backwards..........

A 2:1 turns Much Easier than a 1.5:1...........

Re: i need to understand gear ratios.

have i had too much corona and missed somthing?

Click to expand...

Yep!!

Bond-o.... stop beating me by half a minute, please!!!

The prop in the water is not at all like a bicycle or car. Cars and bikes at low speed have exactly the same 'ratio'. Props in water slip more at low speed, effectively reducing the load at low speed.

Example... my boat... at 2000rpm I'm doing 8 knots... at 3000rpm I'm doing a gentle 20knots and at 5000rpm I'm hanging on for my life at 41knots. (not really, it's a lot of fun at 35knots +)

Chris...........

Re: i need to understand gear ratios.

Bond-o.... stop beating me by half a minute, please!!!

Click to expand...

Ayuh,.........

Good Morning Chris,........
Or,......
I guess it'll be Good Evening for You,....................


Btw,........ That 1 was 6 minutes...............

Re: i need to understand gear ratios.

Bond-o said:

Btw,........ That 1 was 6 minutes...............

Click to expand...

Took me that long to type that one up....

Re: i need to understand gear ratios.

oops, you'll not be able to get props with a high enough pitch if you use a B3 with a 2.4 ratio in your boat. Bravo 3 propsets are only available from 20" to 30" in 2" increments.
For example, if your boat had come new with a 350hp motor and a B3, it would have had a 2.0 to 1 drive ratio, maximum. And it would have used 28 pitch propsets. It might have even gotten a 1.81 ratio drive and still used 28s.
For example, my Chaparral 2135 allegedly weighs 3400lbs, has 280hp, a 2.0 B3 drive, and uses 26p propsets. If I installed your 2.4 drive, I doubt a set of 30s would keep my rpms under 5000.

Re: i need to understand gear ratios.

my guys here, (the mechanics)
said i was on drugz

i bought it cheep! 2 grand for a new b3.
i now know i need a b1.
im going to go over 300 hp on a 19 footer.

all your help is invaluable!

oops!

Re: i need to understand gear ratios.

oops! said:

my guys here, (the mechanics)
said i was on drugz

i bought it cheep! 2 grand for a new b3.
i now know i need a b1.
im going to go over 300 hp on a 19 footer.

all your help is invaluable!

oops!

Click to expand...

Has a death-wish!!