calculating boat speed

On Sun, 8 May 2005 20:38:12 -0700, "frosty" wrote:

>Does the hull impact the efficiency of the propeller?
>
>That is, if a certain motor with a certain propeller pushes
>a fiberglass boat at 90% efficiency, will that same motor/
>prop drop to 85% (say) efficiency behind a pontoon boat?
>
>Or, would it just turn that much slower, at the same
>efficiency?

===========================================

Both. A heavier or less efficient hull will have more drag
(resistance through the water). This will result in more prop
slippage, and the torque required at the prop will increase causing
the engine to develop lower RPMs at the same throttle setting.
 >> Stay informed about: calculating boat speed 

"tony thomas" wrote in message

> 1. If you are wanting to know what a different prop will do for your
> speed. Most know that a 2" pitch change will be about 300 rpms.
> 2. If you want to know how accurate your speedo is. I have found that
> most aluminum props are about 85% efficient. Performance SS is about 90%.
> A High performance prop like a Merc Lab, Mazco, etc.. are about 92%.
>
> These are just examples. I use the formula all the time. When someone
> says they have a 150hp engine that is turning 5500 rpms w/ a 22 pitch prop
> and running 80 mph you know they are lying. 61.27 is theoritical and w/ a
> good prop at 90% they would be at 55 mph.
> Now a lot of speedometers will show 80 mph. Without a GPS the formula is
> the only way to really know.
>
> --

I understand, but it still seems to me that the prop slippage is the real
unknown. If you change props then your top RPM will change too, right? If
you put more or less weight on the boat, or clean the hull, then things will
change too. With handheld GPS so cheap it seems strange to me to use an iffy
formulas to calculate speed.


--
Peter Aitken
 >> Stay informed about: calculating boat speed 

"tony thomas" wrote in message

> Prop slippage is the real unknown if you have measured everything else.
> However, as Barry stated he wanted to verify that his slippage was in an
> acceptable range. No other way to do it than the formula.
>
> --

Yes, that makes perfect sense.



--
Peter Aitken
 >> Stay informed about: calculating boat speed 

On Sat, 7 May 2005 18:16:37 +1000, "Barry"
wrote:

>Hello,
>This has probably been flogged to death but I am going to ask anyway.

Put a speedometer on your boat, or use a GPS.

I used a GPS to find out my driveway is 3.2 miles long.
 >> Stay informed about: calculating boat speed