I dont want to calculate speed. I asked the question this way because I knew
someone out there had the formula.
I want to ensure that my new boat has the correct prop fitted. I can have
the speed recorded by GPS, I will know the prop pitch and gearbox ratio and
maximum engine rpm. With all of that I can calculate slip and make a
decision on the prop.


"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

"tony thomas" wrote in message
news:kFyfe.67380$c24.3956@attbi_s72...
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

Wayne is correct.

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

-
"Wayne.B" wrote in message
...
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.

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.

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

-
"Peter Aitken" wrote in message
om...
"tony thomas" wrote in message
news:kFyfe.67380$c24.3956@attbi_s72...
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

"tony thomas" wrote in message
news:N_Tfe.68273$NU4.55368@attbi_s22...
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

Tony, did you ever sell your 21' Eliminator yet? I sold my STV Euroski and
purchased a 1990 21 Skater! Just about ready to run the 300PM after a
complete overhaul.




"tony thomas" wrote in message
news:kFyfe.67380$c24.3956@attbi_s72...
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

Have a guy coming from Nashville Wed to look at it. I think it is sold. I
just can't use it right now so I don't want it laying around.

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

-
"Woodchuck" wrote in message
...
Tony, did you ever sell your 21' Eliminator yet? I sold my STV Euroski and
purchased a 1990 21 Skater! Just about ready to run the 300PM after a
complete overhaul.




"tony thomas" wrote in message
news:kFyfe.67380$c24.3956@attbi_s72...
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

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.