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

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage and a
couple of other things are needed.

I did have the formula on the computer but a recent crash of the system
caused it to be "misplaced"

Many thanks..

http://www.rbbi.com/folders/prop/propcalc.htm



"Barry" wrote in message
...
Hello,
This has probably been flogged to death but I am going to ask anyway.

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage and a
couple of other things are needed.

I did have the formula on the computer but a recent crash of the system
caused it to be "misplaced"

Many thanks..

Thanks Mr Woodchuck


"Woodchuck" wrote in message
.. .
http://www.rbbi.com/folders/prop/propcalc.htm



"Barry" wrote in message
...
Hello,
This has probably been flogged to death but I am going to ask anyway.

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage and
a
couple of other things are needed.

I did have the formula on the computer but a recent crash of the system
caused it to be "misplaced"

Many thanks..

The formula is:

rpm x prop pitch
------------------ x efficiency(slip) = speed mph
1056 x gear ratio

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"Barry" wrote in message
...
Hello,
This has probably been flogged to death but I am going to ask anyway.

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage and a
couple of other things are needed.

I did have the formula on the computer but a recent crash of the system
caused it to be "misplaced"

Many thanks..

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

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage
and a couple of other things are needed.

I did have the formula on the computer but a recent crash of the
system caused it to be "misplaced"

Many thanks..

Woodchuck responded:
http://www.rbbi.com/folders/prop/propcalc.htm

Good link, Woodchuck, thanks. Nice JavaScript app. A caveat, though:
You answer is only going to be as accurate as the "Propeller Slip" figure.

Their suggestion that it's "10 to 20% slower at top speed" has an error
margin of 50%(!) Of course, they go on to say, "This ratio is found by
dividing the actual boat speed by the theoretical boat speed..." Great!
So, if you start with the actual speed, you can calculate the percent slip,
by which you can accurately determine the actual speed. *chuckle*

--
frosty (Circular Reasoning: see 'Circular Reasoning.')

For me, it works very well as my props have about 8% to 10% slip and it's
verified by GPS. But then again I'm running a hi-performance boat over
100mph. The props I have are Mercury stainless steel lab cleavers. Most
average consumer props have alot 15% slip for stainless steel and near 20%
for aluminum. The calculation provides a good starting point when you want
to change pitch, RPM, mph, etc


"frosty" wrote in message
...
Barry asked:
Hello,
This has probably been flogged to death but I am going to ask anyway.

What is the formula for calculating boat speed?

From memory the propeller pitch, engine rpm a reduction for slippage
and a couple of other things are needed.

I did have the formula on the computer but a recent crash of the
system caused it to be "misplaced"

Many thanks..

Woodchuck responded:
http://www.rbbi.com/folders/prop/propcalc.htm

Good link, Woodchuck, thanks. Nice JavaScript app. A caveat, though:
You answer is only going to be as accurate as the "Propeller Slip" figure.

Their suggestion that it's "10 to 20% slower at top speed" has an error
margin of 50%(!) Of course, they go on to say, "This ratio is found by
dividing the actual boat speed by the theoretical boat speed..." Great!
So, if you start with the actual speed, you can calculate the percent
slip,
by which you can accurately determine the actual speed. *chuckle*

--
frosty (Circular Reasoning: see 'Circular Reasoning.')

"Woodchuck" wrote in message
...
For me, it works very well as my props have about 8% to 10% slip and it's
verified by GPS. But then again I'm running a hi-performance boat over
100mph. The props I have are Mercury stainless steel lab cleavers. Most
average consumer props have alot 15% slip for stainless steel and near 20%
for aluminum. The calculation provides a good starting point when you
want to change pitch, RPM, mph, etc

No one has asked the obvious question - why would anyone want to calculate
boat speed this way?

--
Peter Aitken

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.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com

-
"Peter Aitken" wrote in message
om...
"Woodchuck" wrote in message
...
For me, it works very well as my props have about 8% to 10% slip and it's
verified by GPS. But then again I'm running a hi-performance boat over
100mph. The props I have are Mercury stainless steel lab cleavers. Most
average consumer props have alot 15% slip for stainless steel and near
20% for aluminum. The calculation provides a good starting point when
you want to change pitch, RPM, mph, etc

No one has asked the obvious question - why would anyone want to calculate
boat speed this way?

--
Peter Aitken

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?

--
frosty

tony thomas wrote:
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.

"Woodchuck" wrote in message
...
For me, it works very well as my props have about 8% to 10% slip
and it's verified by GPS. But then again I'm running a hi-
performance boat over 100mph. The props I have are Mercury
stainless steel lab cleavers. Most average consumer props have alot
15% slip for stainless steel and near 20% for aluminum. The
calculation provides a good starting point when you want to change
pitch, RPM, mph, etc

"Peter Aitken" wrote in message
om...
No one has asked the obvious question - why would anyone want to
calculate boat speed this way?

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.