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C-25 SR/FK: Outboard regular length

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Mountaineer62
1st Mate

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USA
33 Posts

Initially Posted - 12/31/2011 : 09:12:02

Am new owner of '82 C-25 with 9.9 hp Evinrude. Not experienced with o/b's, I sense the shaft is too short; the prop in flat seas appears approx. 10-12 inches below the surface. In a ten-hour delivery it gave the sense of lacking good thrust. (The bottom is not clean; I recognize fouled botton as a deterrent to boat speed and will have it cleaned and sanded in the Spring.)
I would appreciate some thoughts on whether a long shaft is desired for this fin keel vessel in contraswt to the existing standard length motor.
Located on Miles River, St, Michaels, MD.

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Davy J
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Response Posted - 12/31/2011 : 09:19:02

Most people are using an extra-long 25" shaft. Even with the XL shaft the prop will still come out of the water in larger waves.

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OJ
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Response Posted - 12/31/2011 : 09:37:07

Hi Bob, welcome and congrats on taking delivery of your (new-to-you?) boat.

I don't equate thrust with prop depth or bottom smoothness. The high thrust models have larger, 3 bladed props.

As Davy J notes below, long shafts help in heavier seas - where pitching causes average length shafts to come out of water.

Does this help?

St. Michaels! Lovely area!

Edited by - OJ on 12/31/2011 09:38:37

John Russell
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Response Posted - 12/31/2011 : 10:21:39

I have a 25" shaft on my Tohatsu. It's mounted on a Garhauer mount which lowers it further into the water. I'd guess (I've never really thought about it that much) that the prop is still no more than a foot under the water line. Is the motor original to the boat? That could be the issue more than depth.

Welcome to the forum and the Association. Take the time to become a member of the Association if you haven't already. The $22 dues will support this forum and the volumes of information and advice here.

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Steve Milby
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Response Posted - 12/31/2011 : 11:06:25

If the prop is immersed 10-12 inches, I'd say that's about the norm. The principal way you can tell if the prop is too short is if the engine revs up frequently, because the prop lifts out of the water. When you're going to windward in very choppy water, it's normal for any outboard engine to lift it's prop out of the water occasionally.

If the engine in fact has a standard length shaft, it's probably too short, because sailboats typically need a long shaft. I'd suggest you compare the length of the shaft to an engine that you know to be long shaft. Nevertheless, if the prop stays immersed in the water, even when going to windward in choppy water, the length is probably OK. If the problem is simply that the boat is slower than it should be, the foul bottom is more likely the reason.

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panhead1948
Captain

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Response Posted - 12/31/2011 : 11:11:52

Hi Bob
Welcome. I was in St. Michaels maybe five years ago beautiful area. Also went to the Ram's Head to see Dave Mason. I have a 7.5 Honda outboard with a 23"shaft. The bracket on my 84 has several holes so it can be moved up or down th help adjust the height.

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Voyager
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Response Posted - 12/31/2011 : 17:22:35

I'd agree with most that a long shaft engine is preferable, but if you're never in big seas on the river, then you can probably make out fine.

Trick is, you want the prop to stay in the water, while the head of the engine should remain out of the water ;-)

In certain cases, if your prop is not sufficiently under water, you will notice cavitation, or air mixing in with the water coming out of the prop. Cavitation will reduce the efficiency of the engine.

That said, you will not be able to make the boat go much faster than 90% of hull speed, no matter how efficient your prop or engine is.

Edited by - on

GaryB
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Response Posted - 01/01/2012 : 01:09:22

Measure from the bottom side of the mounting bracket where it fits over the transom down to the lower cavitation plate just above the prop. If it's a long shaft that measurement will be approx. 20 inches. If it measures approx. 25 inches it is an extra long (XL) shaft which is preferred by most. If it measures shorter than either of the 2 above it is a standard shaft.

When you mention it feels like it lacks thrust I assume you mean it accelerates slower and/or cruises slower than you would expect for a given throttle setting. If so, that could be due to any of the reasons listed in the previous threads above and in my opinion is most likely due to a dirty bottom or the wrong prop.

Also, if you're used to a regular power boat you might feel a C25 is a bit under-powered (it is!). You've got 9.9 hp pushing at least 5000 pounds of boat if not more.

With a clean bottom and the correct prop a 9.9 should push a C25 to 5 or 5.5 knots at well less than 1/2 throttle. I have a 8 hp Suzuki and it will push my boat to 5 knots at just under 1/2 throttle. If I open it up to full throttle the stern of the boat squats and I top out at 6 or 6.25 knots and burn a ton more gas.

Edited by - on

Mountaineer62
1st Mate

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33 Posts

Response Posted - 01/01/2012 : 09:50:39

Thnaks for all the replies. My prior experience involved inboards( Atomic 4 and diesels) on largeer boats. Advancing age suggests this smaller vessel will be easier to handle when solo sailing. Will do some more measuring and observations to det3ermine if any change is necessary. I do like the electric start!

Edited by - on

Stinkpotter
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Djibouti
9080 Posts

Response Posted - 01/01/2012 : 09:58:34

I'm mostly familiar with Hondas, which measure about 17", 22.5", and 27.75" respectively for standard, long and extra-long shafts as Gary describes. Honda refers to them as 15", 20" and 25"--that's the recommended height of the top of the transom above the waterline for a powerboat. If your anti-ventilation plate is 10" below the surface at rest, I'd speculate it's lower than many of ours here, and may well be an extra-long.

Older 2-strokes with small, higher-pitched props don't have the acceleration and stopping "bite" that the newer 4-strokes with larger, lower-pitched "high thrust" props have, but 9.9 is plenty of HP--any more is just adding unnecessary weight. But 5000 lbs. is a hunk... My first rule of docking is "Never approach a dock any faster than you're willing to hit it." Also, 5.5 is about max cruising speed--you can push her close to hull-speed, but it takes a lot of noise and gas as the stern squats.

Welcome! Before my wife died, we were looking hard at moving to Annapolis until my retirement, and then to Oxford or St. Michaels for the duration. Wonderful area!

Edited by - Stinkpotter on 01/01/2012 10:17:53

Joe Diver
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Response Posted - 01/03/2012 : 06:48:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
My first rule of docking is "Never approach a dock any faster than you're willing to hit it."
<hr height="1" noshade id="quote"></blockquote id="quote">

+1 Some of the best boating advice I've been given so far....

Edited by - Joe Diver on 01/03/2012 06:49:09

tbosch
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Response Posted - 01/03/2012 : 11:11:30

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
 ...Older 2-strokes with small, higher-pitched props don't have the acceleration and stopping "bite" that the newer 4-strokes with larger, lower-pitched "high thrust" props have, but 9.9 is plenty of HP--any more is just adding unnecessary weight....
<hr height="1" noshade id="quote"></blockquote id="quote">

I've got a similar setup, older 2-stroke with a 9.5x10 prop. There is room for a bigger prop and I'm considering swapping it out before spring. Is there any reason that I couldn't put as big a prop as will fit and reduce the pitch by a couple/few inches? How does 2/4 stroke affect the situation?

My goal is to gain some thrust in reverse and travel efficiently over long distances. I suspect that the prop that came with my motor was intended for a small planing fishing boat, not a 5000# displacement hull. Any wisdom to share?

Edited by - on

OJ
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Response Posted - 01/03/2012 : 11:28:04

Edited by - on

dlucier
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Response Posted - 01/03/2012 : 12:21:08

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by tbosch
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
 ...Older 2-strokes with small, higher-pitched props don't have the acceleration and stopping "bite" that the newer 4-strokes with larger, lower-pitched "high thrust" props have, but 9.9 is plenty of HP--any more is just adding unnecessary weight....
<hr height="1" noshade id="quote"></blockquote id="quote">

I've got a similar setup, older 2-stroke with a 9.5x10 prop. There is room for a bigger prop and I'm considering swapping it out before spring. Is there any reason that I couldn't put as big a prop as will fit and reduce the pitch by a couple/few inches? How does 2/4 stroke affect the situation?

My goal is to gain some thrust in reverse and travel efficiently over long distances. I suspect that the prop that came with my motor was intended for a small planing fishing boat, not a 5000# displacement hull. Any wisdom to share?
<hr height="1" noshade id="quote"></blockquote id="quote">

By changing to a lower pitch prop, you might get better grip at low speeds, but your long distance fuel economy will probably suffer.

A few years ago, I replaced the original 9" x 9" prop on my 8hp two stroke with a lower pitch, 9 3/4" x 6.5" (Mickey Mouse) prop. I thought I'd see a significant performance boost around the docks, but since I don't use a whole lotta outboard in the marina anyway (usually no more than just above idle and I don't power stop), there wasn't any noticeable difference in maneuvering at low speed. The real difference came when operating in open water at speed. With the new low pitch prop, the engine RPM's increased significantly resulting in my fuel economy being cut in half and to top it off, my top speed also decreased a knot or so. Needless to say, I put the old prop back on.

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Stinkpotter
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Djibouti
9080 Posts

Response Posted - 01/03/2012 : 12:36:23

Todd: My high-thrust 8 hp Honda (4-stroke) had a 10" 4-blade prop with 6.5" pitch. On an 8-10 hp two stroke, I'd go down to at least 8", maybe 7" pitch and either 4 blades or a little larger diameter, but I'd probably ask the manufacturer first. If fuel consumption is a concern, below 7" pitch might sacrifice more mpg than you want.

The Honda had another feature that I'm sure helped reverse thrust: When backing, the exhaust would vent from the lower unit away from the prop, rather than through the prop hub. This kept the exhaust from turning the water around the prop into foam. All I can say is popping it into reverse and giving it a little burst would stop my C-25 like it had run into a pillow, especially compared to the standard-propped Honda 8 it replaced. It cruised at 5-5.5 knots at maybe just above half throttle. I never, ever opened it up.

Edited by - Stinkpotter on 01/03/2012 12:39:24

tbosch
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163 Posts

Response Posted - 01/04/2012 : 11:14:48

Thanks for the responses, very helpful. My issue is primarily having no power in reverse. I don't rely on reverse to stop the boat, although it would be helpful in minimizing the figurative an literal "impact" of those occasional learning experiences :) But even backing out of a slip or away from the fuel dock is futile. The attendant usually asks if I'm still in neutral and gives me a push... Exhaust through the prop is probably the issue and I'm hoping a bigger prop will help. I'm not expecting a miracle but some bite would be better than none.

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dmpilc
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Response Posted - 01/04/2012 : 12:00:23

With thru-the-prop hub exhaust, you don't want to use a lot of throttle backing up. Backing out of the slip, give it gas gradually and you should see better results. Wind coming from the stern will, of course, make it more difficult. Ideally, you want the prop deep enough so that if a crew member needs to go forward while motoring, the flat plate above the prop (cavitation plate) will not come out of the water.
When in the slip with the motor lowered to running position, how far is it from the top of the motor mount board to the water surface and how far is it from the motor mount board to the cavitation plate?

Edited by - dmpilc on 01/04/2012 12:06:40

tbosch
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Response Posted - 01/04/2012 : 16:20:33

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dmpilc
 With thru-the-prop hub exhaust, you don't want to use a lot of throttle backing up. Backing out of the slip, give it gas gradually and you should see better results. Wind coming from the stern will, of course, make it more difficult. Ideally, you want the prop deep enough so that if a crew member needs to go forward while motoring, the flat plate above the prop (cavitation plate) will not come out of the water.
When in the slip with the motor lowered to running position, how far is it from the top of the motor mount board to the water surface and how far is it from the motor mount board to the cavitation plate?
<hr height="1" noshade id="quote"></blockquote id="quote">

Sorry but I don't have those measurements. I live in the land of hard water... I can tell you, though, that when in the upper of the 2 down positions the cavitation plate is below water (I've never seen it at or above the surface). At the lowest of the down positions the bottom of the mount board is not far above the water.

Edited by - on

dmpilc
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Response Posted - 01/04/2012 : 17:12:45

Sounds like the shaft length is ok.

Edited by - on

dlucier
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Response Posted - 01/05/2012 : 19:40:29

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
 The Honda had another feature that I'm sure helped reverse thrust: When backing, the exhaust would vent from the lower unit away from the prop, rather than through the prop hub. This kept the exhaust from turning the water around the prop into foam. All I can say is popping it into reverse and giving it a little burst would stop my C-25 like it had run into a pillow, especially compared to the standard-propped Honda 8 it replaced. It cruised at 5-5.5 knots at maybe just above half throttle. I never, ever opened it up.
<hr height="1" noshade id="quote"></blockquote id="quote">

Sounds exactly how my 8hp two stroke Mercury operates including the exhaust diverter when in reverse. It's 17 years old now and starts with an easy half pull. The way this thing runs, it'll probably outlive me.

Edited by - on

dmpilc
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Response Posted - 01/09/2012 : 09:31:07

Hey Don, did you sell a main to someone in Canada? There's a C25, location Ontario, just listed in the Swap Meet showing a pic with your sail number, #2151. The boat is listed as a 78 model, but someone commented that the sail number put it around 1980. I looked up the sail number in the old roster and came up with your name.

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dlucier
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Response Posted - 01/09/2012 : 11:41:14

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dmpilc
 Hey Don, did you sell a main to someone in Canada? There's a C25, location Ontario, just listed in the Swap Meet showing a pic with your sail number, #2151. The boat is listed as a 78 model, but someone commented that the sail number put it around 1980. I looked up the sail number in the old roster and came up with your name.
<hr height="1" noshade id="quote"></blockquote id="quote">

Yeah, that's my old main. I sold it to a guy from Windsor so either the boat or the sail changed locations. Coincidentally, I purchased my current main, and cruising spinnaker, from a guy in Windsor who lost his C25 (sail number 1159) in a marina fire. The photo below shows the old number before I changed it to mine (2151).

Edited by - dlucier on 01/09/2012 11:46:27

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