RPM vs. Speed (Gearing/Prop pitch)

[Tahiti350]

Hello all,
Newbie to the forum with a question on approx. speed for my boat. I have an '81 Tahiti 20' (looks like a jet hull) with a 325 hp (est) 350 Chevy, and OMC 800 outdrive. I don't have GPS or speedo, but my last good run on glass water (American Lake, Tacoma Wa.) was at 5400 rpm, 14x21 stainless tri-blade, under 1/4 tank fuel, and flat haulin'. I was able to pick up about 300 rpm from my previous best by dialing in the trim. I'm not sure of the gearing in the drive (now appears I have 1.16:1 in the drive (SEE POST BELOW FOR ALL OMC ELECTRIC SHIFT GEAR RATIOS, found on OMC forum). Is there a way to guestimate top speed?
Also wondering if there would be any advantage to taking my exhaust over the transom, but still use the cast iron mainfolds? I know they are heavy (I've had to pull them off twice about 75 lbs each) and less weight would be more speed. I have 3 1/2" outlets currently dumping out through the drive (Quiet, just listen to that motor suck), but have tubing that will clamp on the outlet flanges and clear the transom. I know noise is a concern at a lot of lakes, but will this help it breathe any better? I know headers are available, but with more areas refusing access for noise, and Oregon's Header law, I hate to spend the $800 and not be able to use them. Thoughts from the "pro's"?
Tahiti350

[jackpunx]

with out knowing your gears its really hard to estimate.. I'll guess its something like a 1.50 so.. we'll go from there.
64.4 MPH

[Tahiti350]

Any idea what gearing options there were for this drive? I have tried finding info, but have not found any web sites that could help. I know I could wrench the motor over and count the output, but I am currently on the other side of the world pushing tin.
Tahiti350

[jackpunx]

I did some research and what I read didnt make any sence to me.. Someone else should be able to help you
OMC 800 outdrive 14/26 gears for 5.7 L motor.. I think it came standard.. I dont know what 14/26 is I also saw lowers that were 19/21 I think...
Anyway..
What is pushing tin and why are you half way around the world?

[Tahiti350]

Pushing tin is a term for keeping aircraft moving, I'm Active Duty Air Force mechanic, Deployed "over there".
Gear ratio's work out as:
14/26 = 1.857
19/21= 1.105
I know it sure likes the big prop for top end, but makes a nice cruiser with a 141/2 /19 prop.

[TIMINATOR]

With a stock motor the exhaust usually won't help much, but when you put in the Comp 270 cam, performer rpm and AFB(625), you will need it. TIMINATOR

[Tahiti350]

I don't know the cam specs, but its seems pretty mild, light RV grind maybe, but I pulled the Torquer off last summer and put the Performer RPM (non-airgap) on with the Edelbrock Carter Carb. Haven't had a chance to run the combination due to work, but hoping it will help with the slight flat spot I had just off idle with the Torquer. Am hoping to recam when I get home, but might pull the motor and check everything out, upgrade the oil pump and pan, etc, at the same time.
I've had the boat for about 5 years, and, other than normal maintnenance, I haven't had to do much to it. Worse problem was the fuel tank (non-stock) leaking and having fuel trailing thru the boat while trying to get back to the dock. Still need to replace the carpet from that fun little ride.
Only other "learning opportunity" was forgeting to close the drain valves on the block when I launched, then trying to figure out why I was taking on so much water ("hey, there shouldn't be water running up the floor when I decellerate!!!"). That one cost me $50 to repair my aluminum prop after I slid it up the ramp with the drive down. I figured the prop was easier to replace than the motor, if it went under before I could get the trailer under it.

[Tahiti350]

All,
Here are the gear ratios for the OMC electric shift models (which include my 800):
120hp- 16:21(upper) 1.92:1(overall)
140hp- 19:20(upper) 1.61:1(overall)
165hp- 21:19(upper) 1.39:1(overall)
175,190hp- 21:19(upper) 1.39:1(overall)
225-245hp- 21:16(upper) 1.16:1(overall)
The lower gearcase ratios for all electric shift models are 15:23.
I just found this on another forum, hope this is of help to others.
With this info does anyone have formulas to figure out the RPM/gear ratio/prop pitch equation to get an approximate MPH. If so here are my best run numbers: 5400 rpm, 1.16:1 gearing (?), and 14x21 stainless tri-blade.
I was racing a PWC that had previously been radared at 72 mph, and slowly outran him on a long run. I know conditions change top end, but am hoping that I may have actually been over 70MPH.
Tahiti350

[spectras only]

Tahiti, based on your specs given,with a hull weight approx 2600 lbs 19' waterline [ not hull length] your speed came about 69.12 MPH. The program recommends a 13X17 prop.With your present 14X21 you have a better holeshot but probably around 64-65MPH as others mentioned.If your boat is close to 3000lbs ,the speed numbers will drop.Buy a Garmin GPS that will tell no lie :D .

[racer]

Tahiti350 your final ratio is not 1.16 to 1 I will look it up this next week.

[spectras only]

The OMC 800 5.7L had a 1.41:1 ratio.With this ratio it works out to 64.34 MPH.

[Tahiti350]

So the info I pulled from the other forum is not correct? I'll check the actual ratio on the boat when I get back to the states in Mid January. Thanks for the help. I need to check the prop again, I think it is actually a 14X21+ (twist blade).
Not sure I understand how a smaller, less aggressive blade would increase top end though. There's a lot I still need to learn. I have 15x17, and 14 1/2/19, both of which seem to pull better down lower, but let the motor wind too high for my taste. I use the 15x17 for pulling skiers, and it pulls 2 skiers comfortably at 2800-3000 rpm. The 14X21 loses a litle bottom end, but definately top ends better than the others.
I was told elsewhere (by someone who "knows") that I needed to increase the pitch until the motor would not pull past about 4800 rpm.
Thanks again for the help.
Tahiti350

[jackpunx]

The OMC 800 5.7L had a 1.41:1 ratio.With this ratio it works out to 64.34 MPH.
I guess I was close.. :D

[burtandnancy]

Here is a formula that may help you:
rpm x pitch x ratio x .00094697 = theoretical speed, minus slip. you will have to find your own slip factor. Use radar or GPS to get your actual speed. If you have a Bravo with 1.50 gears, use .667 for ratio, if you have 1.36 gears, use .735. Hope this helps...

[75spectra]

The 800 series drive was not electric shift in 81. They changed to a mechanical push pull cable in 79.

[Tahiti350]

Thanks to all for the info.
Spectra,
I'm not sure what year the drive is (believe the boat may have been a conglomeration project), I know the lower is not the original for the upper (different colors, and striping), but I do have 2 wires going out thru the gimble to the drive, and was told by the mech at the boat yard where I bought it that they were for the electric shift . Guess I still have some investgation and research to do when I get home in January. Actual ratio will be the first factor to find out. Are there serial or id markings on each part of the drive that I could use to ID what I have???
Gary B.
"Tahiti350"

[mirvin]

So how come my 5.7 pushing a VP290 with a 21p prop is barely finding 50mph??? That's at about 5500 rpm.
I have no idea what my gear ratio is........1988 21 foot Crusader (in my avatar). I think it's a bit on the heavy side.
mirvin :cool:

[spectras only]

Mirvin, with a max 260HP SAE, and a heavy 21' boat 50 MPH sounds about right :mix:

[mirvin]

Mirvin, with a max 260HP SAE, and a heavy 21' boat 50 MPH sounds about right :mix:
That's what I was thinking...... :rollside: I keep thinking about messing with it but it runs so darn good and hey, it's a VP 290 for cripes sake. That thing gets hairy at about 45mph. I'd hate to see what it'll do at 70 :notam:
Thanks spectras :rollside:
mirvin