What prop size do I use?
 

I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

There is no way to know what is the correct prop without knowing how many
rpms your turning at wide open throttle. You select prop based on max rpms.
You want the engine to run in the power band. This will be different
depending on the boat the motor is mounted on. A heavy boat will require a
smaller prop than a lighter boat.
However, a 14 diameter prop on a 50 hp engine is definetly on the large
side. I am surprised it even clears the cavitation plate or fits the
splines. Check the prop size again. My guess is it is actually an 11-3/4 x
17.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

As Tony said, you need to judge it based on your RPMs at wide open throttle
(WOT). You need to find the specs for your engine and see what the RPM range
is at WOT. If your boat currently revs higher than the spec, you need to
increase the pitch of the prop. A general rule of thumb is that each number
increase of pitch will reduce WOT by about 200rpm.

I bought a boat last year that ran at 5500rpm WOT. The manual said 4400-4800
was optimal. I suspect that the previous owner bought a lower pitch throttle
to pull a good sized skier out of the water. I put a 23 pitch (from a 19) on
the boat, and it purrs along at about 4600rpm WOT...perfect.

Now, if we could just have variable pitched props like airplanes, all would
be good with the boating world. g

--Mike

"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

Actually they do make variable pitch props. They are very expensive
compared to regular props (even more than a high performance prop), require
maintenance on the springs and cams, and are less efficient on top end due
to blow by at the hub. However, they do work for overall use on a runabout.
Most people just won't spend the money to get one.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"mgg" wrote in message
om...
As Tony said, you need to judge it based on your RPMs at wide open
throttle (WOT). You need to find the specs for your engine and see what
the RPM range is at WOT. If your boat currently revs higher than the spec,
you need to increase the pitch of the prop. A general rule of thumb is
that each number increase of pitch will reduce WOT by about 200rpm.

I bought a boat last year that ran at 5500rpm WOT. The manual said
4400-4800 was optimal. I suspect that the previous owner bought a lower
pitch throttle to pull a good sized skier out of the water. I put a 23
pitch (from a 19) on the boat, and it purrs along at about 4600rpm
WOT...perfect.

Now, if we could just have variable pitched props like airplanes, all
would be good with the boating world. g

--Mike

"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

Well, ya learn something every day. Thanks for the info!

--Mike

"tony thomas" wrote in message
news:yu6%d.88068$Ze3.87319@attbi_s51...
Actually they do make variable pitch props. They are very expensive
compared to regular props (even more than a high performance prop),
require maintenance on the springs and cams, and are less efficient on top
end due to blow by at the hub. However, they do work for overall use on a
runabout. Most people just won't spend the money to get one.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"mgg" wrote in message
om...
As Tony said, you need to judge it based on your RPMs at wide open
throttle (WOT). You need to find the specs for your engine and see what
the RPM range is at WOT. If your boat currently revs higher than the
spec, you need to increase the pitch of the prop. A general rule of thumb
is that each number increase of pitch will reduce WOT by about 200rpm.

I bought a boat last year that ran at 5500rpm WOT. The manual said
4400-4800 was optimal. I suspect that the previous owner bought a lower
pitch throttle to pull a good sized skier out of the water. I put a 23
pitch (from a 19) on the boat, and it purrs along at about 4600rpm
WOT...perfect.

Now, if we could just have variable pitched props like airplanes, all
would be good with the boating world. g

--Mike

"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

I'll be damned. Just googled it, and there they are. However, they aren't
variable by the driver like they are in airplanes. I didn't think of it
being automated which is why I didn't think they existed for boats.

Thanks again.

--Mike

"tony thomas" wrote in message
news:yu6%d.88068$Ze3.87319@attbi_s51...
Actually they do make variable pitch props. They are very expensive
compared to regular props (even more than a high performance prop),
require maintenance on the springs and cams, and are less efficient on top
end due to blow by at the hub. However, they do work for overall use on a
runabout. Most people just won't spend the money to get one.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"mgg" wrote in message
om...
As Tony said, you need to judge it based on your RPMs at wide open
throttle (WOT). You need to find the specs for your engine and see what
the RPM range is at WOT. If your boat currently revs higher than the
spec, you need to increase the pitch of the prop. A general rule of thumb
is that each number increase of pitch will reduce WOT by about 200rpm.

I bought a boat last year that ran at 5500rpm WOT. The manual said
4400-4800 was optimal. I suspect that the previous owner bought a lower
pitch throttle to pull a good sized skier out of the water. I put a 23
pitch (from a 19) on the boat, and it purrs along at about 4600rpm
WOT...perfect.

Now, if we could just have variable pitched props like airplanes, all
would be good with the boating world. g

--Mike

"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

mgg wrote:
As Tony said, you need to judge it based on your RPMs at wide open
throttle
(WOT). You need to find the specs for your engine and see what the
RPM range
is at WOT. If your boat currently revs higher than the spec, you need
to
increase the pitch of the prop. A general rule of thumb is that each
number
increase of pitch will reduce WOT by about 200rpm.

I bought a boat last year that ran at 5500rpm WOT. The manual said
4400-4800
was optimal. I suspect that the previous owner bought a lower pitch
throttle
to pull a good sized skier out of the water. I put a 23 pitch (from a
19) on
the boat, and it purrs along at about 4600rpm WOT...perfect.

Now, if we could just have variable pitched props like airplanes, all
would
be good with the boating world. g

--Mike

"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat
with
a 1972 johnson 50, I beleive the prop that it came with is
incorrect.
It was a 14" by 17 pitch all the literature suggests that is a
wrong
prop but I can not find any guess on what size prop should be used.
Any
help wouold be nice. Thanks
Wes


The prop that was on the boat a 14x17 barely was using the bandwidth,
it peaked at a slower speed probably 17 or 20 mph but was barley
working the engine it didnt even sound like it was getting to WOT
recomended for the boat engine combo is a 13 1/4 x 17
wes

Wes used the 14 x 17 but it seemed to not use the whole bandwidth, the
boat seemed slow and the motor wouldn't reach WOT I suspect it needed a
21 or 23 the recommened for this boat size and motor is a 13 1/4 x 17
max RPM for the johnson is 5000-5500
wes

Wes,

If the boat seems slow, and will not reach the recommended WOT range of
5000-5500 rpm, then you need a lower pitched prop, not a higher pitch. What
rpm do you see now at WOT?

--Mike

"hdb" wrote in message
oups.com...
Wes used the 14 x 17 but it seemed to not use the whole bandwidth, the
boat seemed slow and the motor wouldn't reach WOT I suspect it needed a
21 or 23 the recommened for this boat size and motor is a 13 1/4 x 17
max RPM for the johnson is 5000-5500
wes

My tach is not really working but if I had to take a guess I would
probably say somewhere near 3000 or 3300 RPM
Wes

Go with the recommended diameter and pitch prop. If you're running 1500-2000
rpm below recommended WOT rpm's you're putting a tremendous amount of torque
load on the engine.

The numbers still don't fully compute, though. You're either giving the
wrong specs for your current prop, or the engine has more problems thatn
just the wrong diameter prop.


"hdb" wrote in message
ups.com...
My tach is not really working but if I had to take a guess I would
probably say somewhere near 3000 or 3300 RPM
Wes

On Sun, 20 Mar 2005 03:35:51 GMT, "tony thomas"
wrote:

~` snippage ~~

There is no way to know what is the correct prop without knowing how many
rpms your turning at wide open throttle. You select prop based on max rpms.
You want the engine to run in the power band.

Now this is interesting. I've always assumed that the power band is
exactly that - a range of RPM in which the real horsepower is being
generated - it would follow that the power band would fall off going
towards WOT rather than being the peak of the band.

Yes/No?

Later,

Tom

hdb wrote:
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice.

The "right" prop depends on what you want the boat to do. For example,
many people that like to waterski place more emphasis on being able to
accelerate under load, and the right prop for this costs a bit of top
end speed & fuel economy.

As others have said, a good guide is your engines RPM at wide open
throttle. Give it at least 5 minutes to wind up, water is incompressible
and it will take a while for it to reach true top speed. If the engine
is not reaching it's rated RPM then you need less pitch; if it winds up
quickly and doesn't make good top speed, you need more.

It may be that your boat has gained weight over the years and the right
prop back then has too much pitch now.

Hope this helps
Doug King

hdb wrote:
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes


That's a "large gearcase" motor... and it is in fact 14" diameter. You
are rated at 50 HP @ 5500 rpm, the pitch should be 15" (which is
actually 13 3/4" diameter from the factory) for that application, which
should land you within the 5000-6000 rpm operating range for that motor.

OMC #391198

or

Michigan #011002

You should see 30-33 mph with that combo.

Rob

On a car engine the max hp may be at 3000 rpms but red line is 5000.
On an outboard the rated hp is at the rated max rpm. For example. A 1990
Evinrude/Johnson 200 was rated at 5000 rpms. However at 6200 rpms the
engine would crank out 220 hp or more.
Every engine is different but in general a 2 stroke makes its max hp on the
top end. A 4 stroke makes its max hp at a rpm specified by the cam profile.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
"Short Wave Sportfishing" wrote in message
...
On Sun, 20 Mar 2005 03:35:51 GMT, "tony thomas"
wrote:

~` snippage ~~

There is no way to know what is the correct prop without knowing how many
rpms your turning at wide open throttle. You select prop based on max
rpms.
You want the engine to run in the power band.

Now this is interesting. I've always assumed that the power band is
exactly that - a range of RPM in which the real horsepower is being
generated - it would follow that the power band would fall off going
towards WOT rather than being the peak of the band.

Yes/No?

Later,

Tom

On Sun, 20 Mar 2005 16:09:18 GMT, "tony thomas"
wrote:

~~ snippage ~~

Every engine is different but in general a 2 stroke makes its max hp on the
top end. A 4 stroke makes its max hp at a rpm specified by the cam profile

I should have thought of the way two strokes make power - your right
of course.

D'Oh!!!

Later,

Tom

On 19 Mar 2005 19:21:37 -0800, "hdb" wrote:

I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

Run a 15x16.

krause

you should never be giving out any advice to boaters on this group or at
your favorite bar or where ever. Not when there is an ounce of safety
involved.

Stick to your Bush bashing, insulting others obviously more intelligent than
you and stick to your meds and the little darling (your wife, 20 years your
junior) or try and get your children back krause after they left you
promising never to associate again AS YOU POSTED YOURSELF.

krause you own so many boats, so many marinas, so much land, so many wives,
how can you be sure you are giving the right advice,,,,, at your age krause
I am sure one of your yachts at your marina must be looking like the rest of
them,,,,

Stick to your bush bashing krause,,,











"HarryKrause" wrote in message
...
On 19 Mar 2005 19:21:37 -0800, "hdb" wrote:

I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes

Run a 15x16.