brand new to boating

i just bought a 2006 202 vantage made by vip, it has a 5.0 mpi mercruiser w/ an alpha drive sporting a 14.25 x 19 stainless 4 blade .

i was looking around at different props and found out that my sterndrive could have one of three different gear ratios.

i looked in the operation manual but the information was left blank.

how do i tell what gear ratio i have?

 

It's most likely in the outdrive's serial number. Jot it down and check with a good dealer or research it online.

 

You can put the engine in gear and then turn the engine by hand untill the prop completes 1 turn. Count the number of engine revs.

 

Your Prop is pushing the Boat 19" (less slip of 8-10%) for each revolution, or about 17.3 inches. Slip is based at higher speeds - at or near wide open.

Run the Boat near wide open, say 40MPH and note Engine RPM.(Speed and Engine RPM MUST be noted fairly precisely for this calculation to work)

Calculate Prop RPM From pitch distance
Prop Rpm = [mph (40) x 5280 x 12] divided by 60 x 17.3 = 2442

Divide Engine RPM by 2442 = Gear Ratio

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Jango

 

uscgret/e8,
i have the serial numbers in my manual. where would i look to research it online?

jango,
the only time i tried wot i remember seeing 4900 rpm and the gps said 49.5mph. (my wife doesnt like going fast on water, anything above 30 and the bitching starts)
so [49.5 x 5280 x 12] divided by [60 x 17.3] = 3021.5 prop rpm
so 4900 divided by 3021.5 = 1.621711
the three choices i had for gear ratio on the alpha drive are 1.47:1, 1.62:1, 1.81:1.
looks like i have the 1.62:1 ratio.
jango, your the man.
thanks.

im in the process of making the boat plane out as quick as possible, i have loads of nieces and nephews that love to tube and ski. with 5 adults, three youngsters and two tubes it doesnt plane out as quick as i would like.
last week i learned that having people up front helps the boat plane out quicker.
others have pointed me toward a whale fin and a 5 blade 19 or 18 pitch.
im still learning about pitch, cup and dia. of props.
range of rpm for the 5.0 mpi is 4600 to 5000 so im in the groove with the 19.
not sure if the 18 will put me over 5000 rpm or not.
any advice on the best whale fin out there? or does it even matter?

 

look on the drive right next to the serial number is the gear ratio it will say something like 1.78r or 1.95r this means 1.78 to 1 ratio right hand

 

Hi th3,
Props are still something of a black art, no matter how much cash and computer software gets tossed at them.
A "whale-tail" hydrofoil helps to lift the stern of the boat, forcing the bow down. If the boat has a tendency to stick its bow way up in the air as you accelerate, the whale-tail might help it level out and accelerate more readily. If it already runs level you'll get minimal benefit from this gadget.
Props are generally sized to keep the engine in its ideal max-RPM range at full throttle, with normal load. Generally, a boat tuned for top-end speed will have a smaller diameter, steeper pitch prop; a boat set up for acceleration will have a larger diameter, shallower pitch prop. Going down in pitch generally means you'll go up in blade area, either by increasing diameter or increasing number of blades, in order to keep roughly the same load on the engine at full throttle.
Going to a 5-blade prop, 14x18 or 14.5x18 or so, would probably give a bit better acceleration at the expense of top-end speed. Others with more experience on boats like yours will undoubtedly chime in with more accurate prop estimates. The prop you have is about what I'd expect to see on a boat like that, though.
If you do decide to go into prop tweaking, find a good prop dealer who has a large selection and technicians who actually work on props (as opposed to ordinary salesmen). He won't be the least bit surprised when you find you need to test a few different props, and will help you to analyze the characteristics of each one you try.

 

I'd stay with that prop you have. It sounds like it's doing the job. A lower pitch will get you out of the hole faster, but you'd have to be careful to not over rev it at top speed. 4600-4800 rpm is usually the max suggested. I have seen whale tails/Dolefins, etc. break the cavitation plates on occasion.
If it does, the whole lower casing usually has to be replaced and is exspensive. So, they do work, but it's a gamble.

 

thanks for the information guys.

 

Just a note to those who use a whale fin. As previously mentioned the whale fin pushes up the engine and consiquently pushes the nose (bow) down. Make dam sure you outboard is well tight --it should be bolted really. It will pop off the transom if it not, Bye bye engine!!

 

i was wrong about the rpm.

i just got the boat back from the dealer after some warranty work and took it out for a test run. (oh, by the way i found the 1:62 stamped on the foot)
i though it was 4900rpm but i could only get 4500 rpm and 48mph with a full tank.

the owners manual says wot should be between 4600 and 5000 so im thinking maybe a 14.25 18 pitch 5 blade.

i thought i read somewhere that changing pitch one number will change rpm by 200, does that sound right or is my brain giving me bad info again?

if i went from a 14.25x19 4 blade to a 14.25x19 5 blade, what would the rpm do? loose a few (50 or so)?

if i went from a 14.25x19 4 blade to a 13.75x19 4 blade, what would the rpm do? increase a few (75 or so)?

i guess what i am trying to figure out is every:
inch of pitch = aprox 200 rpm?
1/2 inch of dia.= aprox 75 rpm?
each blade = aprox 50 rpm?

is there some kind of rule of thumb that covers this?

 

1" of Dia change is supposed to be the same as 1" of pitch, abt 200 RPM. It probably is not proportional so 75 RPM for 1/2" is probably close.

Not sure what going from 4 to 5 Blade does, but going from 3 to 4 blade is supposed to be the same as 1" of pitch, 200 RPM. 4 to 5 is probably a little more than 50 RPM. It has to do with total Blade area. You will need to look at D.A.R (Dia, Area Ratio) Most 3 Blades are abt 55%, 4 Blades, 65+%.

By the way, MAX RPM should be obtained with One person (Driver) NO unnecessary gear and MINIMAL Fuel - No more than 1/2 tank. You are probably not that far off, as is, especially if Motor is not broken in yet.