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arsenuts411

RPM to MPH?

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arsenuts411

Anyone have the monsoon engine and happen to know what speed the speedometer reads at 2500 rpm. I was talking to a guy on another forum and was telling him that at 2500rpms that my speedometer reads about 30. He was saying that his is about 21 mph. I was wondering if mine or his was off. I am not sure if this also depends on the boat length and weight.

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skicrave

It's not possible to know that. Even though your transmission is a 1:1 gear ratio, everything from altitude to temperature, humidity, crew weight and how clean the hull is will effect the boats speed.

Get a GPS if you need to know how fast you're going to calibrate your speedo.

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arsenuts411

makes alot of sense i guess I wasn't sure if it was even possible to determine that. But i guess there is alot of factors that can effect speed compared to someone else.

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crescentbar
It's not possible to know that. Even though your transmission is a 1:1 gear ratio, everything from altitude to temperature, humidity, crew weight and how clean the hull is will effect the boats speed.

Get a GPS if you need to know how fast you're going to calibrate your speedo.

Prop size, pitch will also effect the speed. What skicrave said about the GPS...

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stonelx

You could also use a ski course if you have one near by, and time it. (If you don't have access to GPS)

and, IIRC you can even do this without a course.

There was some mathematical procedure you could do with 2 land markings.

Pick 2 land markings at least a 1000 feet apart.

Drive at one speed say 25mph and time it between the 2 points.

Then drive it again at say 35mph and time it between the 2 points.

The difference in time, and difference in speed (10mph) should give you distance.

Then you can calculate the real speed your going and adjust your speedo.

...or something like that, perhaps someone can correct me?

Edited by stonelx

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Beau

And if you run in moving rivers, like me, current will affect actual speed vs relative speed.

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jgouveia3
You could also use a ski course if you have one near by, and time it. (If you don't have access to GPS)

and, IIRC you can even do this without a course.

There was some mathematical procedure you could do with 2 land markings.

Pick 2 land markings at least a 1000 feet apart.

Drive at one speed say 25mph and time it between the 2 points.

Then drive it again at say 35mph and time it between the 2 points.

The difference in time, and difference in speed (10mph) should give you distance.

Then you can calculate the real speed your going and adjust your speedo.

...or something like that, perhaps someone can correct me?

this is exactly what I was looking for a few weeks ago. there was a thread about setting speedos, and that is how you do it, without actually knowing the distance.

also, 2500 rpms doing 30 mph seems awfully fast, unless you have something like a 24pitch prop...

Edited by jgouveia3

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UWSkier

Agree with what's posted above. GPS is your best bet. Even unloaded though, there's no way you're going 30 at 2500 RPM. Probably more like 22-24 MPH.

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arsenuts411

Its a stock prop I am not sure what pitch or make it is to be honest. For some reason acme rings a bell in my head

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jgouveia3
Its a stock prop I am not sure what pitch or make it is to be honest. For some reason acme rings a bell in my head

i was actually being a bit sarcastic about the 24 pitch prop, they are not common on most boats, just go fast boats...

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