RPM vs. MPH ?

[HELLERBOY]

At WOT I miss my ideal RPM by 700rpm I'm thinking of putting on a prop with 3" less pitch (guessing I'll get 200rpm per inch) If so I'll get the boat running in the ideal RPM range,Is there a way to tell (or sorta guess)what MPH I can get by adding the 600rpm,before I spend the money to prop the boat?
thanks for all and any help,HELLERBOY

[smoothie]

You can use this calculator (http://www.boatpropellers.com/propcalc.html).at the bottom of the page.

[MOP]

There is a great prop calculator on the site below!

http://www.go-fast.com/

[Staffan Berg]

This is a site that "Murphy" talked about some days ago.

I think this explains everyting.
http://www.rundquist.com/intro_repair.htm

[blackhawk]

Check your tach first. My VDO was off by 500 rpm.

[Rootsy]

i'll second that Blackhawk.. my stock VDO was off 300 vs a calibrated photo tachometer... the VDO read high... the new Autometer is dang near dead nuts... nothing like the insecurity of not being able to trust your gauges frown

[Craig]

Any experiences of tachs reading too HIGH? That would be a bummer too.

[CnV & Family]

There are no stupid questions right? How DO you calibrate a tach?

[Craig]

Oops! I see that is what Jamie said, Too High. I thought you both meant too LOW. I guess either is a bummer. Reading too low = possibly bad for yer engine, reading too high = boring because you may think you're maxed out when you have some room for improvement left!

[Rootsy]

a calibrated instrument is one whose reading is compared to a KNOWN quanity. if the instrument reading does not match the master it is then adjusted so that it does. it is then said to be calibrated.

[CnV & Family]

Probably should have been more specific Jamie...

Maybe I should have asked were to go or how should I test to see how accurate my tach is vs a calibrated one?

sheesh, i hate feeling so ignorant sometimes

[Rootsy]

i don't own one i just "borrow" one when i need it which isn't very often.... nice to have large corporate backing sometimes... even when they don't realize it wink

[blackhawk]

Mine read too low. When my tach was reading 4400-4500 my motor was actually spinning about 5000. I ran the boat a whole year at 4500 thinking I was overpropped. Grabbed a digital tach that goes around a plug wire, put it on number 1 cylinder and checked it against my tach and then adjusted mine. The best way to do it is to actually send it out and get it "calibrated" or find someone that has the equipment.

[MOP]

I saw digital tach mentioned once in the post, they are the only trust worthy tachs. Anologs bounce read high & low. I've been trying to find a match for my gauges, I bay tested boats nearly 40 years and found alot of tachs off. Our test panels which pluged into the factory harness's were all digital and calibrated which allowed us to verify all the gauges readings. Anyone runnig a hyper mill should invest in a digital tach.

[Rootsy]

digital or analog, doesn't matter, both may be off by a percentage greater than the established instruments precision which is a +/- percentage which is given by the manufacturer. the only way to know for sure that the instrument is correct is to compare it to a master which is KNOWN to be correct or attach it to a source of a KNOWN speed and verify that the instruments reading falls within the maufacturers tolerance for that instrument... filtering, noise and a lot of other fun stuff can happen in a signal, not to mention that all electronics are not created equal... a calibrated instrument will generally be recertified at given periods in order to guarantee it has not drifed beyond it's tolerance.