justinVIP

I am putting a 350 Mag MPI in my 2102 VIP Victory. What is a safe RPM to run this motor? Is it strictly 5000? Where can I find the drive ratio of the outdrive? I see a lot of people talking about a prop calcualtor, and taking into consideration slippage 15% etc... I use formula's on my snowmobile for gearing and clutching for a 500 ft drag at X RPM at X MPH etc... but don't know jack about props etc... Any info would be awesome! Thanks, J.

JaayTeee

The gear ratio is usually on the
l/h side decal on the upper unit.

If it say's 1.5R, that means it's a
1.5:1 reduction drive.

Yours may say 1.47R, which is
pretty close to a 1.5 ratio.

I don't know the exact RPM the
rev limiter kicks in on a 350 mag mpi,
but an "educated guess" would be 5100,
5000 RPM's should be a fine target rpm.

Here's 1 type of speed calculator:

Engine rpm divided by gear ratio, multiplied
by prop pitch( in inches) divided by 1056,
this gives you "theoretical speed", then factor in your slip for true speed.

Example: If your theoretical was 60, your slip is 10%
your true speed would be 54.

Hope this helps.

jt

justinVIP

Thanks a lot. I appreciate your help. Now...why couldn't I put in a 1.34(ex.) drive ratio, and expect to gain about 7 MPH? lack of HP? would it require a prop change, etc... I know that just sounds too easy. If it were that easy, more people would just change the drive ratio. Any suggestions???

JaayTeee

You could put a lower numerical
gear drive on, but you would have
to reduce prop pitch, unless you increased
your power.

Generally speaking, as to drive ratio's,
I've found it more efficient to turn a larger prop
slower(1.5R), than to turn a smaller prop faster(1.36R)


jt

justinVIP

makes sense....thanks again. If I got a prop that I was happy with, good rPM and speed, would there be any gains in getting the prop labbed for less slippage etc.? what else does this do to the props performance? Sorry for all the ?'s. New to this...but always wanting to learn more. I've found forums to be great places to learn, if you are careful in who you listen to.......J.

justinVIP

I ran some quick #'s to see what would be better......

running a bigger pitch prop and losing some RPM, or, trying to keep the RPM at exactly where it should be. If figured the latter would make more sense, but the numbers came out differently... help me explain this....I know this is all theoretical.

all at 1.5 ratio and 10% slippage

21p @ 5000= 59.6MPH
23p @ 5000= 65.3MPH
23p @ 4900= 64MPH
23p @ 4800= 63.3MPH
23p @ 4700= 62MPH
23p @ 4600=60.7MPH
23p @ 4500= 60 MPH
24p @ 4700= 64.1MPH

Would going from a 21 to a 23 pitch prop, on a 21' boat with a 350 mag mpi, drop more than 500 RPM?

JaayTeee

Usually,
1" of pitch equals 200 RPM.

From my experiences, labbing allows you
to go up one prop size, and still keep the
same RPM's on top as the smaller prop
ran.

jt

justinVIP

Thanks again jt.... any more info from anyone would be great... Thanks, J. This has been a lot of help in understanding how prop size etc affects RPM. If my motor has a peak HP at 5000, I wonder how it would operate at 4800 RPM with a 23p labbed prop. In theory it should be able to run about 62.7MPH with a labbed prop. Does that sound reasonable?

justinVIP

jt, what did you mean by one prop size? from 21 to 23?

JaayTeee

Correct.


Also,
Not to rain on your parade, but
a realistic slip figure for most
v hulls is in the15% range.

Which would take it down to 59.2

I just used 10% for an example.


jt