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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Slow boat- prop pitch ?

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ct95949
Captain

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Aruba
300 Posts

Initially Posted - 02/03/2007 : 22:56:24

I did a search on prop pitch and found some good info but still have questions. The boat will only go about 4 1/2 knots with my Honda 8 at full throttle. The prop is a four blade, 9.25 x 8 and I have no problem manuevering in tight spots at idle. I use the moter as little as possible when cruising and if I have to listen to the noise I want to get it over with as quick as possible.
I see that some of you are using a 9 pitch, my boat yard guy suggested re-pitching my prop to a 10. There is only about 3/4" between the blade and the anti-cav plate so he didn't think I should go to a larger diameter.
One more observation: The moter sounds like it's 'winding out' at full throttle as in not a lot of load. Any thoughts ?
Thanks, Craig

'90 SR/WK #6017. PO of '79 SR/SK.

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Dustysailz
1st Mate

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USA
85 Posts

Response Posted - 02/04/2007 : 07:41:59

I'm not certain, but I will watch the responses. I can tell you that my old motor was an 8HP Yachtwin. When I repowered last summer, I got a 9.8 Nissan. As it's new, it has not yet been opened full throttle. However, at approximately 1/3 throttle the boat moves at approx 4 knots. This may not apply to your motor which looks new, but the new motors deliver the stated HP off the prop and don't lose it through the shaft as an older motor will. Just an after thought, is the bottom clean?

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KD4AO
Navigator

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USA
202 Posts

Response Posted - 02/04/2007 : 08:06:17

Before you repitch the prop you need to know if the motor is reaching close to recommended max rpm. If it is not, then you have an under power problem which could be caused by drag or load. Repitching to 10 would only make matters worse. On the other hand if you are over reving them 10 would help. Bob

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 02/04/2007 : 10:09:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by ct95949
 The boat will only go about 4 1/2 knots with my Honda 8 at full throttle.

The motor sounds like it's 'winding out' at full throttle as in not a lot of load. Any thoughts?<hr height="1" noshade id="quote"></blockquote id="quote">

A few years ago, I experimented with a lower pitched prop and it was like I was stuck in first gear (4 knots at max rpm's). After reinstalling the original higher pitched prop, I went back to 5 1/2 knots somewhere between 1/2 and 3/4 throttle.

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ClamBeach
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3072 Posts

Response Posted - 02/04/2007 : 10:55:19

"The boat will only go about 4 1/2 knots with my Honda 8 at full throttle"

Quick question. How are you checking your speed? Knotmeter, GPS or Distance/Time? If it's by knotmeter I'd verify your speed with another method before taking action (i.e. spending money). With GPS or distance/time be sure to use two-way runs to negate the effects of current.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 02/04/2007 : 11:34:39

There's something screwy here... What vintage is the engine? 4 blades suggests the newer (rounded cowl) model--introduced around 2001. Do you have the 25" (XL) shaft? The prop you describe is not the "power thrust" prop but rather the standard pitch, designed more for planing hulls and inflatables. I'm suspicious you have too much pitch, and that it's causing cavitation, which is giving you that "winding out" (?) sound. The Power Thrust prop on my 2002 Power Thrust 8 was 10x6.5, which is designed for a heavy displacement hull moving slowly. But the other issue is whether the prop is deep enough in the water.

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djn
Master Marine Consultant

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USA
1561 Posts

Response Posted - 02/04/2007 : 12:38:42

I'm running a couple year old Johnson 9.9 and see 6kts on my GPS at about 7/8 throtle. The motor really like that setting and if I pull back just a bit more the boat speed falls off fast. Cheers.

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ct95949
Captain

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Aruba
300 Posts

Response Posted - 02/04/2007 : 15:58:15

Clam-I am using a knot meter and have motered with the current and against with the expected differences. I am comaparing my moter speed with sailing in strong wind at 6.5 knots or better. I am planninig on buying an inexpensive GPS and will check the speed with that-thanks.

Dave- the moter is a 2002, I'm not sure about shaft length. What you say about cavitation may be right, it seems like I'm just churning up the water at full throttle. From what I've read on this and other threads reducing the pitch would also reduce the speed. If I increased the diameteer by 3/4" to 10" the blades would be about 3/8" from the plate- would this help much ? Should I go to a three blade? How deep should the top or middle of the prop be in the water?
Thanks,Craig

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Sloop Smitten
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USA
1181 Posts

Response Posted - 02/05/2007 : 09:48:02

Craig,
I don't know if you keep your boat in the water full time so this may not apply to you, but when I first purchased Utopia she would not go past 4.5KPH either. Turned out she had a ball of barnacles on the keel and needed to have the bottom cleaned and repainted. She then could do 6+ KPH. It had nothing to do with the engine.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 02/05/2007 : 11:54:55

Craig: Joe might have something there--are you confident your bottom isn't fouled?

The shaft length is measured from the top of the mounting board to the top of the anti-ventilation plate. The X/L is about 27.5", and the L is about 22.5". I'm pretty sure the lower unit casting is no different between standard and "Power Thrust" Hondas--you should have the same clearance between the shaft and the anti-ventilation plate as I did. My experience was that, at rest and unloaded, the anti-ventilation plate should be at least 4" below the surface--certainly not less than 3". (Mine ended up about 5", which was good for the relatively short chop in Long Island Sound.)

As for the prop, some folks here have tried and then abandoned lower pitched props, but I believe that was on engines that couldn't handle a larger diameter, which compensates somewhat for lower pitch.

Cavitation and ventilation are two different but related things--both cause the prop to spin wildly, but ventilation is the sucking of air down around the prop. The term "cavitation" is now used for the loss of bite due to excessive turbulence generally caused by a prop with too much pitch being pushed to spin too fast for the forward speed of the boat. The standard pitch on an 8hp outboard is intended to allow a boat (like an inflatable) to go something like 15-20 knots. The C-25 can't get past about 6.3, and if you have sufficient torque, you can push the prop to the point that it just starts acting like an egg beater instead of providing rearward thrust. A lower pitch helps to solve that. The Power Thrust prop on the Honda might be a little lower pitch than the ideal, so the engine runs at slightly higher RPMs for any given speed, but the real advantage is the thrust you get by throttling up at very low speeds--stopping or maneuvering around a dock. It makes a huge difference, and I say that having used both types on our C-25. I cruised at 5-5.5 knots running easily between 1/2 and 2/3 throttle (hard to say).

Check your bottom first, check your prop depth, and only then consider a prop change--but I don't think it should be to a higher pitch. I'd suggest you try the Power Thrust prop. Your gear ratio is the same as the P/T model.

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Heartbeat
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USA
161 Posts

Response Posted - 02/05/2007 : 12:49:18

I get 4.5 knots with my (ancient) four horse. Certainly look into the bottom being clean or something. I expect to have my 6 horse fixed this spring and will be back in the 6-6.5 knot range.

Good luck!

-Matt

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 02/05/2007 : 15:08:56

One more thing related to cavitation: Don't pop the engine up to full throttle. When starting up, go no further than half throttle until you're moving along nicely, and then adjust up a little for speed. You'll never, repeat never need full throttle with that engine--it'll practically guarantee cavitation, waste gas, and make a lotta unnecessary noise.

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ct95949
Captain

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Aruba
300 Posts

Response Posted - 02/07/2007 : 18:51:22

I had the bottom painted last September and it has always been pretty clean when I've hauled out. The anti-ventilation plate looks to be 3-4 inches below water.
Dave-the power thrust prop spec is 10 x 7 which as I said before would put the blades pretty close to the plate. Your comments on cavitation are interesting. I'm going to get my GPS set up then try your suggestions. If I still have problems I may try the other prop.
Thanks, Craig

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 02/07/2007 : 21:09:12

Craig, I suspect you'll move along smartly if you don't crank it all the way up. Remember, your theoretical hull speed (the max without planing) is about 6.3 knots, and maximum efficient cruising speed is about 5.5. That last 3/4 knot takes a disproportionate amount of power, fueld, and noise to achieve. And the boat begins to squat and drag the transom, which just adds to the drag. She'll run comfortably between 5 and 5.5 knots.

Edited by - Dave Bristle on 02/07/2007 21:10:48

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