Hi,

Moving along with the Engine Project for Far Cove...

Decided on a fixed 3-blade Campbell Sailor for the new engine (actually my
finances decided for me!). But what size?

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

VicProp (Main dealer for Campbell Sailor props) recommends 14x12.

gpcprop.com (generic prop-sizing) recommends 15x10.

Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the new
one)

Bob McCulluch of TechNotes, Mainsheel Magazine (highly respected, apparently
knows more about the Mk I than Catalina!) recommended a 15x10 for the ORIGINAL
M25.

I think 14in is a good diameter - I'm concerned that a bigger prop means more
drag. I'm leaning towards 14x10 but...??

Lloyd Sumpter
"Far Cove" Catalina 36

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

You might consider paying some attention to this recommendation. This guy
realizes that you will be knocking on his door *first* if you're unhappy with
the final combo.




Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the
new
one)

Also likely a well-founded recommendation. About agrees with your engine
supplier.



I think 14in is a good diameter - I'm concerned that a bigger prop means more
drag. I'm leaning towards 14x10 but...??

If the original engine was 5 HP (about 20%) less than the new one, it wouldn't
seem unreasonable that your new setup can handle an extra inch of pitch.

You can worry a thing like this to death.
Most of the recommendations are pretty close, so if you want to stay with a 14"
prop the 9 vs 10-inch pitch won't make the difference between satisfactory
results and
anything dramatically less.

On Mon, 29 Dec 2003 19:18:31 -0800, "Lloyd Sumpter"
wrote:
Hi,

Moving along with the Engine Project for Far Cove...

Decided on a fixed 3-blade Campbell Sailor for the new engine (actually my
finances decided for me!). But what size?

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

VicProp (Main dealer for Campbell Sailor props) recommends 14x12.

gpcprop.com (generic prop-sizing) recommends 15x10.

Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the new
one)

Bob McCulluch of TechNotes, Mainsheel Magazine (highly respected, apparently
knows more about the Mk I than Catalina!) recommended a 15x10 for the ORIGINAL
M25.

I think 14in is a good diameter - I'm concerned that a bigger prop means more
drag. I'm leaning towards 14x10 but...??

================================================== ===========

The prop diameter (the first number) is determined by the amount of
blade tip clearance with the hull. For best performance under power,
you want the biggest diameter possible which still gives sufficient
clearance. Too small a tip clearance results in excessive vibration
so 14 inches is probably the best bet based on Catalina's sizing.

You can back into pitch (the second number) by knowing something about
the power curve of your engine, the RPMs you want to cruise at, and
the transmission gear ratio. Let's assume your hull speed is
something like 6.5 knots, and that's where you'd like to cruise under
power, in favorable conditions. Let's also assume that your prop
slippage ratio under those conditions is about 80%. Next, we need to
know at what RPM your motor develops peak power and then subtract 15%
for a reasonable cruising RPM. The Universal M-25XPB is rated for 26
hp at 3,000 RPM and has a 2.05 reduction gear. Derating by 15%, we
get a cruising RPM of 2,550 yielding a speed at the prop of 1244 RPM.
At 6.5 kts you need to move the boat at 7,900 inches per minute (9,500
inches allowing for 20% prop slip). Dividing 9500 inches by a shaft
RPM of 1244, we get a pitch requirement of 8 inches (14 x 8). If slip
turns out to be a little higher, or you want to nudge up your speed a
bit more, you could probably justify a 14 x 9.

Hope that helps.

On Tue, 30 Dec 2003 04:00:50 +0000, Gould 0738 wrote:

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

You might consider paying some attention to this recommendation. This guy
realizes that you will be knocking on his door *first* if you're unhappy with
the final combo.

Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the
new one)

Also likely a well-founded recommendation. About agrees with your engine
supplier.



I think 14in is a good diameter - I'm concerned that a bigger prop means more
drag. I'm leaning towards 14x10 but...??

If the original engine was 5 HP (about 20%) less than the new one, it wouldn't
seem unreasonable that your new setup can handle an extra inch of pitch.

You can worry a thing like this to death. Most of the recommendations are pretty
close, so if you want to stay with a 14" prop the 9 vs 10-inch pitch won't make
the difference between satisfactory results and
anything dramatically less.

Ya know Chuck, you might just be right: I may be overthinking this. My concern
comes from my belief that the single most important factor in getting the most
out of your engine is pitch (followed by prop diameter).

As you say, I'd like to lean towards Robert, but he seems out of line with the
rest of the estimates. But I think I can convince him to let me go with 14x10,
mentioning that I can re-pitch in April/May when I do my "annual" scrape'n'paint
haulout. But you're right: I may not see ANY difference between 9 and 10.

Lloyd Sumpter
"Far Cove" Catalina 36

On Tue, 30 Dec 2003 04:09:13 +0000, Wayne. wrote:

On Mon, 29 Dec 2003 19:18:31 -0800, "Lloyd Sumpter" wrote:
Hi,

Moving along with the Engine Project for Far Cove...

Decided on a fixed 3-blade Campbell Sailor for the new engine (actually my
finances decided for me!). But what size?


================================================== ===========

The prop diameter (the first number) is determined by the amount of blade tip
clearance with the hull. For best performance under power, you want the biggest
diameter possible which still gives sufficient clearance. Too small a tip
clearance results in excessive vibration so 14 inches is probably the best bet
based on Catalina's sizing.

You can back into pitch (the second number) by knowing something about the power
curve of your engine, the RPMs you want to cruise at, and the transmission gear
ratio. Let's assume your hull speed is something like 6.5 knots, and that's
where you'd like to cruise under power, in favorable conditions. Let's also
assume that your prop slippage ratio under those conditions is about 80%. Next,
we need to know at what RPM your motor develops peak power and then subtract 15%
for a reasonable cruising RPM. The Universal M-25XPB is rated for 26 hp at
3,000 RPM and has a 2.05 reduction gear. Derating by 15%, we get a cruising RPM
of 2,550 yielding a speed at the prop of 1244 RPM. At 6.5 kts you need to move
the boat at 7,900 inches per minute (9,500 inches allowing for 20% prop slip).
Dividing 9500 inches by a shaft RPM of 1244, we get a pitch requirement of 8
inches (14 x 8). If slip turns out to be a little higher, or you want to nudge
up your speed a bit more, you could probably justify a 14 x 9.

Hope that helps.

Thanks - interesting analysis!

My hull speed is actually around 7.5 knots, but 6.5 at 2500 IS my target cruise.
I suspect slippage is higher at these speeds, so I'm still leaning towards 10in
pitch. Working backwards: a shaft RPM of about 1220 and a speed of 7,900
inches per minute at 10-in pitch gives a slip of 35% - within reason as we
approach hull speed.

Lloyd

Lloyd Sumpter wrote:

On Tue, 30 Dec 2003 04:00:50 +0000, Gould 0738 wrote:

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

You might consider paying some attention to this recommendation. This guy
realizes that you will be knocking on his door *first* if you're unhappy with
the final combo.




Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the
new
one)

Also likely a well-founded recommendation. About agrees with your engine
supplier.


The problem of this Concept is: Where'd the Extra HP go? If I'm turning
the same prop at the same rpm, what's the use of getting an extra 5 hp? I
believe I need at least another inch, which is why I'm bothered by
Robert's recommendation. I'm basically paying $10 for 5hp - I want to SEE
it!

Lloyd



It's just an inch, Lloyd. She won't even notice.



--
Email sent to is never read.

On Tue, 30 Dec 2003 04:00:50 +0000, Gould 0738 wrote:

Robert at Atomic Engine Service (where I'm buying the engine and prop)
recommends 14x8 (grudgingly 14x9).

You might consider paying some attention to this recommendation. This guy
realizes that you will be knocking on his door *first* if you're unhappy with
the final combo.




Catalina recommends 14x9 for the ORIGINAL M25 (21hp compared to 26hp for the
new
one)

Also likely a well-founded recommendation. About agrees with your engine
supplier.


The problem of this Concept is: Where'd the Extra HP go? If I'm turning
the same prop at the same rpm, what's the use of getting an extra 5 hp? I
believe I need at least another inch, which is why I'm bothered by
Robert's recommendation. I'm basically paying $10 for 5hp - I want to SEE
it!

Lloyd

Thanks - interesting analysis!

My hull speed is actually around 7.5 knots, but 6.5 at 2500 IS my target cruise.
I suspect slippage is higher at these speeds, so I'm still leaning towards 10in
pitch. Working backwards: a shaft RPM of about 1220 and a speed of 7,900
inches per minute at 10-in pitch gives a slip of 35% - within reason as we
approach hull speed.
======================================

Lloyd, the risk of going to too high a pitch is lugging the engine.
If you try the 14 x 10 prop and you can't get to 3,000 RPM at wide
open throttle or get black smoke in the exhaust at WOT, the engine is
lugging. If so, I'd strongly recommend getting the it re-pitched back
to 14 x 9, or have an agreement in place for exchanging the prop. You
don't want to risk damaging that brand new engine! 3-bladed props
have significantly less slip than 2-bladed.

On Tue, 30 Dec 2003 17:17:04 +0000, Wayne B wrote:

Thanks - interesting analysis!

My hull speed is actually around 7.5 knots, but 6.5 at 2500 IS my target
cruise. I suspect slippage is higher at these speeds, so I'm still leaning
towards 10in pitch. Working backwards: a shaft RPM of about 1220 and a speed
of 7,900 inches per minute at 10-in pitch gives a slip of 35% - within reason
as we approach hull speed.
======================================

Lloyd, the risk of going to too high a pitch is lugging the engine. If you try
the 14 x 10 prop and you can't get to 3,000 RPM at wide open throttle or get
black smoke in the exhaust at WOT, the engine is lugging. If so, I'd strongly
recommend getting the it re-pitched back to 14 x 9, or have an agreement in
place for exchanging the prop. You don't want to risk damaging that brand new
engine! 3-bladed props have significantly less slip than 2-bladed.

Understood. The engine and prop will be installed in late Jan, and that will
give me a few outings to check WOT RPM before the annual scrape'n'paint in
April. If it's wrong, I can change it then. I doubt a few hrs would damage the
engine. Anyway, more likely it's underpropped, which is harder to determine.

BUT...since 14x9 is the recommended prop for the 21hp M25, if the 26hp XPB can't
make full rpm with at least a 14x10, there's something wrong! If I wanted an
engine that could only turn a 14x9, I would have just rebuilt the M25.

Also understand about 3 vs 2 blades - All of what I said referred to a 3-blade.
(for example, Catalina recommends a 15x10 2-blade for the M25)

Lloyd Sumpter
"Far Cove" Catalina 36