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Catalina/Capri 25/250 Sailor's Forums

Catalina 250 Specific Forum

Prop Pitch

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doublereefed
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167 Posts

Initially Posted - 08/09/2016 : 15:02:29

Folks, pickup up from the lack of performance in reverse problem I'm having...

I read up on all things propellor. Clearly the prop pitch makes a huge difference, with tradeoffs. I have that smaller 6HP engine, but looking at the Mercury prop options there are several pitches. One is actually labelled "high reverse thrust". The obvious tradeoff would be top end speed.

Does anyone have any advice on pitch, given that I obviously have a smaller engine than most of you?

BTW, I am driving over to the boat tonight to find out what diameter and pitch is on there now.

Thanks,

-Richard

'95 C250 WB #61
Midway, UT

Edited by - doublereefed on 08/09/2016 15:14:11

doublereefed
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167 Posts

Response Posted - 08/09/2016 : 20:30:54

Here are some photos. This sure looks like a long shaft to me, would you agree? There's nothing I can find in the manual or related to the serial number to know if this engine is consider a long shaft.

Does anyone know what the stamped numbers mean? A 12? I presume 9P is a pitch of 9. What is that L? Left maybe... but left in what sense? The prop obviously rotates to the right... clockwise.

'95 C250 WB #61
Midway, UT

TEM58
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USA
113 Posts

Response Posted - 08/09/2016 : 21:11:29

Richard, appearances can be deceiving on outboards. The best way to accurately determine shaft length is by using the serial number of the motor. There are several websites that will help you decode it, but I have found iboats.com to be very useful figuring out an old Honda I have.

I think you are on the right track to consider a different prop--much cheaper than a new outboard!

Tim M
�Perfect Match II�
2003 C350 #35
Cruising FL
PO "Wine Down"
2000 C250 WK #453
PO "Perfect Match"
1983 C25 SR/SK #3932
Lake Belton
Belton, TX

GaryB
Master Marine Consultant

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4275 Posts

Response Posted - 08/10/2016 : 20:34:46

Looking at what's stamped on the prop I'm guessing a 9" diameter (seems small) and 12" pitch but to be honest I think someone didn't know what they were doing when they stamped it so those numbers could be meaningless. Also, looking at the actual prop I think the diameter could be higher.

It appears to be right hand rotation as viewed from the back of the boat looking forward.

Check out this site for more details.

http://propellerhub.com/diameter.html
http://propellerhub.com/pitch.html
http://propellerhub.com/rotation.html

Compare these RH pictures with your prop and it's definitely RH rotation -- >> https://www.mercurymarine.com/en/us/propellers/all/?rotation[]=RH

Using Mercury's propeller finder and guessing at the age of your motor here's the available prop pitches for a 6HP motor. If the numbers stamped on your prop are even close to accurate I'm betting yours is a 12" pitch.

https://www.mercurymarine.com/en/us/propellers/sport-series/black-max/

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GaryB
Andiamo
'89 SR/WK #5862
Kemah,TX

Edited by - GaryB on 08/10/2016 21:06:16

Stinkpotter
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Djibouti
9013 Posts

Response Posted - 08/10/2016 : 21:16:24

Shaft lengths are measured from the top of the mounting bracket (or transom) to the top of the anti-ventilation plate above the prop. Short is generally a a couple of inches over 15", Long is over 20" (like 22.5"), and Extra Long is over 25" (like 27.5"). In the photos, that motor looks like a Long or X-Long. And the plate appears to be over 6" below the waterline, which is pretty good.

I suspect your reverse performance is mostly due to exhaust gas around the prop--a common problem with thru-prop-hub exhaust, which my Honda (and maybe some others) seemed to fix. A lower-pitch prop might not be a good solution overall if it isn't larger in diameter or have four blades. (For "high thrust", Yamaha uses a larger diameter; Honda uses four blades.) Otherwise, especially with a 6hp, you might give up more forward speed than you want except at wide-open, while not gaining much advantage in reverse. But I have no experience with a 6hp on a 25' keelboat.

BTW, the "high thrust" prop on my Honda 8 was 10" diameter, 6.5" pitch, and four blades. (Standard was 9.25" X 8" X 4 blades).

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 08/10/2016 21:29:14

doublereefed
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Response Posted - 08/25/2016 : 10:13:48

Reporting back...

I installed a new 3-blade 7" pitch prop, it's night and day. I can actually work up some decent speed in reverse.

HOWEVER, on comparing the new with the old, it is clear that the old prop was bent to heck. In comparing the smooth arc forms of the new prop, the old prop took a hit and about an 1"- 1 1/2" of the outside edge of each blade was bent flat or forward. That has to be a power killer. Not to mention wondering what kind of shock loads the engine took...

SO, inconclusive if the 7" vs 9" made the difference or the new blades. I am pushing at hull speed no problem into headwinds so I'm not lacking on forward speed. Will troll eBay for a 9" cheap... just to satisfy my curiosity.

The shaft isn't quite long enough, which is too bad. The prop clears the water sometimes with a rolling sea or heeling due to turning. But, it will work.

Now I have to repair the generator. No juice.

That's my report, for anyone else that comes along with a prop issue on a small outboard...

'95 C250 WB #61
Midway, UT

Edited by - doublereefed on 08/25/2016 12:22:42

Stinkpotter
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Djibouti
9013 Posts

Response Posted - 08/25/2016 : 18:25:03

The distortion you report could be most of the problem. It turns your prop into an egg-beater. Add to that gases from a thru-hub exhaust, and you end up with whipped cream around the prop--particularly in reverse where the gasses are sucked back into the prop.

But if your forward performance is adequate for your needs, I suspect the lower pitch is your better choice. The standard prop on most small outboards is not intended to push a multiple-ton displacement hull, forward or backward. In general, 6 hp is a dinghy motor, even if it has the inherent power to drive a very big boat to a hydrodynamics-limited speed. If you find the standard pitch prop, your observations could be very interesting!

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

doublereefed
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Response Posted - 10/17/2016 : 15:07:47

Reporting back after using the 7" for the season. It has made a huge difference in reverse, and I have not problem pushing at hull speed into strong headwinds for dropping main, etc. The lower pitch has also given me a much better 1- and 1+ mph trolling speed. I'm probably running at higher rpms when motoring to a destination, but it's not full throttle and the engine isn't rev'ing abnormally. Just a bit of info if anybody else ever runs into this sort of issue.

Best,

-Richard

'95 C250 WB #61
Midway, UT

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