I'm trying to come up with an ideal prop pitch. My dealer is of No
help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a
16' planing dory. My original prop was 9" pitch which pushed the boat
about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by
GPS by the way). Just listening to the revs seems a little low to me
and sure enough, I calculate 2904 rpm with my 8" prop without
considering slippage. Does anybody know how much I should add for
slippage? Recommended RPM is between 5000-6000 so I need to get 'em on
up there.
.... or is there a better way to do this without a tach? (thinking
maybe I should just break down and buy one).
Any suggestions appreciated.
Thanks in advance
Rick

Rick,
You left out the reduction to the prop shaft of 2.08 in the current
version. That leaves you over 6k with the 8".

Remember: The Honda vs everyother little bike discussion = Hondas
don't sound as fast as any 2-stoke when, in fact, are reving highter.

Matt Colie

lid wrote:
I'm trying to come up with an ideal prop pitch. My dealer is of No
help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a
16' planing dory. My original prop was 9" pitch which pushed the boat
about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by
GPS by the way). Just listening to the revs seems a little low to me
and sure enough, I calculate 2904 rpm with my 8" prop without
considering slippage. Does anybody know how much I should add for
slippage? Recommended RPM is between 5000-6000 so I need to get 'em on
up there.
... or is there a better way to do this without a tach? (thinking
maybe I should just break down and buy one).
Any suggestions appreciated.
Thanks in advance
Rick

Doh!!
You're exactly right!
Thank you kind sir. Issue resolved!
Rick

Matt Colie wrote:
You left out the reduction to the prop shaft of 2.08 in the current
version. That leaves you over 6k with the 8".

Remember: The Honda vs everyother little bike discussion = Hondas
don't sound as fast as any 2-stoke when, in fact, are reving highter.

Matt Colie

wrote:
I'm trying to come up with an ideal prop pitch. My dealer is of No
help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a
16' planing dory. My original prop was 9" pitch which pushed the boat
about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by
GPS by the way). Just listening to the revs seems a little low to me
and sure enough, I calculate 2904 rpm with my 8" prop without
considering slippage. Does anybody know how much I should add for
slippage? Recommended RPM is between 5000-6000 so I need to get 'em on
up there.
... or is there a better way to do this without a tach? (thinking
maybe I should just break down and buy one).
Any suggestions appreciated.
Thanks in advance
Rick

On Tue, 07 Mar 2006 11:26:51 -0600, lid wrote:

I'm trying to come up with an ideal prop pitch. My dealer is of No
help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a
16' planing dory. My original prop was 9" pitch which pushed the boat
about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by
GPS by the way). Just listening to the revs seems a little low to me
and sure enough, I calculate 2904 rpm with my 8" prop without
considering slippage. Does anybody know how much I should add for
slippage? Recommended RPM is between 5000-6000 so I need to get 'em on
up there.
... or is there a better way to do this without a tach? (thinking
maybe I should just break down and buy one).
Any suggestions appreciated.
Thanks in advance
Rick

Let's just chew the rag.
If you want the motor to spin at 5500 rpm at some speed, what speed
would it be? Wild guess: knowing 3200rpm puts you at 22 mph
so 5500rpm would provide about 55/32 as much power = 172 %
cube root of 1.72 is 1.20 so the target speed is 26 mph.

What pitch is called for?
1/5500 (minute/rev) X 26/60 (miles/minute)X 1760 (yards/mile) X 3
(feet/yard) X 12 (inches/foot) = 5 ( inches/rev)
no slip plus 10% for slip = 5.5 inch pitch.
Ah but now, what is the appropriate diameter?
Enough to absorb the power, of course!
Wild guess 8 3/4 in to 9 1/2 in.
(Can't recall the trades for diameter...)

Your mileage may undoubtedly vary.

Brian Whatcott Altus OK

You mean I just wasted all of six minutes on faulty data?
Bah, humbug!
:-)

Brian W

On Tue, 07 Mar 2006 15:01:53 -0500, Matt Colie
wrote:

Rick,
You left out the reduction to the prop shaft of 2.08 in the current
version. That leaves you over 6k with the 8".

Remember: The Honda vs everyother little bike discussion = Hondas
don't sound as fast as any 2-stoke when, in fact, are reving highter.

Matt Colie

wrote:
I'm trying to come up with an ideal prop pitch. My dealer is of No
help. And I have no tach. It's a 20hp Honda with 9.25" dia prop on a
16' planing dory. My original prop was 9" pitch which pushed the boat
about 17mph. So I went to an 8" pitch which pushes it 22mph (speed by
GPS by the way). Just listening to the revs seems a little low to me
and sure enough, I calculate 2904 rpm with my 8" prop without
considering slippage. Does anybody know how much I should add for
slippage? Recommended RPM is between 5000-6000 so I need to get 'em on
up there.
... or is there a better way to do this without a tach? (thinking
maybe I should just break down and buy one).
Any suggestions appreciated.
Thanks in advance
Rick

It sounds as if you have solved your problem, but if a tach is what you
are after, I have had good luck with a $24 unit sold by Tower Hobbies
for tuning model airplane engines.
you would have to put a piece of tape on the flywheel to mimic an
airplane prop.

I did this on my yanmar diesel and it worked nicely. Discovered that
the tach in my instrument panel was off by quite a bit.

good luck,
Andy

"andy" wrote:
if a tach is what you
are after, I have had good luck with a $24 unit sold by Tower Hobbies
for tuning model airplane engines.
you would have to put a piece of tape on the flywheel to mimic an
airplane prop.

Thanks. Yep, this prob solved but the tach idea looks interesting. If
I understand correctly, it would work by running the motor with the
cover off and pointing the tach at the flywheel? .... and what kind
of tape? If it works the way I understand it, you could measure rpms
of practically ANYthing with it. Is that right?

By the way, they sell three tachs. I read up on only one.

Rick