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