MPH vs RPM

[blktout67]

hi everyone i was just trying to figure out what kinda top end speed im getting out of my boat as the mph gauge does not work i have a 2.3 140hp with the cobra out drive on it. speed is not a real big deal to me as i like to cruise at about 1/2 throttle when im not fishing and the family is just out for the day to have fun but lately i have been getting a lot of ? from friends about how fast it will go i just tell them it don't matter to me how fast it go,s i don't very often open it up to wot i just like to cruise and have fun be it letting the kids knee board or tubing or just fishing thanks

 

[IVAZ]

Re: MPH vs RPM

What size boat? What size prop? What RPMs are you turning?
I think this might be in the wrong section of the forum.

 

[QC]

Re: MPH vs RPM

Yeah, I moved this up to boat topics as we try to keep Dockside non-Technical . . .

We really need your prop pitch, gear ratio (we can probably figure that out if it is stock) and we need your RPM at WOT, trimmed correctly. If we guess at prop slip we will get pretty close.

I am going to make a SWAG here. It is a 17 ft. runabout and will run 37.2 MPH with a light load . . .

 

[blktout67]

Re: MPH vs RPM

sorry guys i forgot to put the boat size in the post. QC is correct it is a 17ft capri and the prop im guessing it is stock i don't know very much about the whole boat thing yet im still learning and at wot it pushes about 5500 rpm


on the prop there is a a #
D 3x13.5x15r and it has 3 blades
and it has some writing on that says solas

 

[IVAZ]

Re: MPH vs RPM

QC, What did you have for breakfast? I want some!

5500 RPMs, you are winding the crap out of that engine. Is your tach good?
Do you know your gear ratio?

I'm off to the fishing hole, hopefully someone can help you out.

 

[QC]

Re: MPH vs RPM

OK, here we go . . . I was waaaaaaay off with my speed calc, I am now going to say 34.4 MPH . . .

As far as I can tell you have a 2:1 ratio and a 15 inch pitch (typical). With a semi-conservative 12% slip 5500 RPM puts you at the 34.4. If you are getting 10% slip (pretty good, so that's why I used 12) speed goes up to 35.2 MPH. The prop seems to be dead nuts as that OHC Ford four banger was rated to run between 5200 - 5600 at WOT.

Edit: Dang, I wasn't quick enough . . . I just knew somebody was going to say the tach was off or that you were going to blow her to the moon. The 2.3s are rare, but they spin fast . . .

Edit 2: I had two sausages, link without skin, and a piece of sourdough toast

 

[blktout67]

Re: MPH vs RPM

ok thanks guys for the help i dont know if the tach is right or not so what i guess im going to go out and do is put a test tach on it and see if they are close to each other the guy that rebuilt my carb had the timing way advanced so i had to re-time it i will let you know what the test tach is at later this after noon thanks again for the help

 

[CaptNCamille]

Re: MPH vs RPM

if you have or have acces to a portable gps, bring it on the boat, it will give you an accurate read of your mph...its mote accurate than the speedometer on the boat.

 

[QC]

Re: MPH vs RPM

Personally I think the tach is right, and I never say that . . .

 

[blktout67]

Re: MPH vs RPM

QC what would you set your ideal at ? i have it set now at around 600 rpm when we did the river run after the carb rebuild he had it set at around 1000
i think that is way to hi so i turned the ideal and set the timing at 600 rpms it seems to run and sound a lot better at 600 rpm im i wrong for turning it down to this ideal point

 

[QC]

Re: MPH vs RPM

I'd post that question over in the I/O section. Also you can go to the Adults Only sticky and download a Service mnaul for your engine. It's 45 MB and I didn't feel like waiting . . . My guess is no more than 750 RPM and that should be performed in gear on the water if it is like the Merc procedure, 1000 is deinfitely too high.

Our best 2.3 guy doesn't post much anymore, but there may be some good info in that manual. Somebody may come along who knows the spec as well, but that would be more likely over there than in the non-Engine section

 

[blktout67]

Re: MPH vs RPM

QC i will take that 35 mph like i said i dont like running wot anyways it is a waste of fuel and it beats the motor up so im cool with just the cruising thing thanks for all the help for the guys that replied to the post have a gr8t day

 

[IVAZ]

Re: MPH vs RPM

Edit: Dang, I wasn't quick enough . . . I just knew somebody was going to say the tach was off or that you were going to blow her to the moon. The 2.3s are rare, but they spin fast . . .

That little Ford sounds like a real screamer.

 

[MikDee]

Re: MPH vs RPM

According to my fairly accurate, simple, quick, formula:

5500rpm divided by 2 (gear ratio) = 2750 prop rpm X 14 pitch (15 minus 1)= 38.5mph, minus 10% slip = 34.65mph! Pretty close, no? Waddaya think?

 

[QC]

Re: MPH vs RPM

Yeah Mik, your guestimate calc seems to work pretty well. Where does the 15 minus 1 come from? I recall discussing this with you once before, but don't recall that piece of it . . .

 

[MikDee]

Re: MPH vs RPM

Yeah Mik, your guestimate calc seems to work pretty well. Where does the 15 minus 1 come from? I recall discussing this with you once before, but don't recall that piece of it . . .

It's all part of the formula that I found yrs ago in a boating handbook, for a simple, quick, formula it has amazed me for yrs

Engine RPM, divided by the gear ratio X the pitch minus 1 = theoretical speed
minus estimated slip, usually avg. 10-20% = Top speed

 

[blktout67]

Re: MPH vs RPM

thanks everyone for the help on this you guys are gr8t i just have one more ? should i change the prop out for a bigger one or leave well enough alone

 

[QC]

Re: MPH vs RPM

The prop seems to be dead nuts as that OHC Ford four banger was rated to run between 5200 - 5600 at WOT

I still believe this assuming the tach is correct which I think it is . . .

 

[Kev144]

Re: MPH vs RPM

According to my fairly accurate, simple, quick, formula:

5500rpm divided by 2 (gear ratio) = 2750 prop rpm X 14 pitch (15 minus 1)= 38.5mph, minus 10% slip = 34.65mph! Pretty close, no? Waddaya think?

Jeeze that formula puts my Charger DL with a 2.5L (260 HP) merc at 135 mph est!

Too bad it only does about 92 mph

 

[MikDee]

Re: MPH vs RPM

Jeeze that formula puts my Charger DL with a 2.5L (260 HP) merc at 135 mph est!

Too bad it only does about 92 mph

Then your probably not using it right