rob vanharten

If I have a 25 pitch mirage plus prop spinning at 5600 rpm's, with a 1.5 ratio drive, about how fast would I be traveling?

picklenjim

79.5 at 10% slip.

http://www.marksmarineinc.com/prop_calc.aspx

GPS your speed and you can figure what actual slip is. Be sure your tach is right on.

hawk232

I am not familiar with boats that big and what kind of prop slip numbers are typical for a boat that size. Just playing around I put in 13% slip (high for the boats im used to) and it comes back to 88mph

calculator:
http://www.mercuryracing.com/propell...calculator.php

edit: actually after pickle posted his numbers i started looking into it. the merc numbers are way off of other calculators. this is the site i normally use http://www.rbbi.com/folders/prop/propcalc.htm and it seems to be more accurate at 76.9mph with 13% slip

picklenjim

I think you left the slip field blank hawk.

CigDaze

25 / 12 (inches/feet) / 5280 (feet/mile) * 60 (minutes/hour) * 5600 (revs/minute) / 1.5 (engine revs/prop revs) * 0.90 (efficiency(1.00 - 10% prop slip)) = 79.5mph.

Or more simply:

(Pitch * RPMS * Efficiency) / (Ratio * 1056)

hawk232

no i entered .13 for slip, i wonder if the merc calculator cant work backwards like that. at any rate i updated my post with info from the site i normally use!

CigDaze

It works fine. You have to also enter your prop slip guess, otherwise it assumes 0% slip.

rob vanharten

Thanks, picking up boat Saturday with the new power. Was really hoping for upper 70's to 80mph. I think I will be flurting with that 80mph mark!

glassdave

iTrader: (1)

I got a pretty cool prop calculator app for my Droid from Orange Dreams, works great. What are some general rules of thumb for a slip ratio?

CigDaze

On my heavy, Old School V-bottom I see 13-14%