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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Propeller related question

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dblitz
Navigator

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240 Posts

Initially Posted - 04/13/2011 : 15:35:34

I have an 86 fin keel standard rig with a 10 hp Universal inboard.
The engine has a 2 blade 11RH7 prop. The boat doesn't really make headway when it is going against current and against wind. Thinking about changing to a 3 blade prop. The questions are:
What speed should the boat do when it is at or near redline rpm?
What props do those of you with an inboard have?
Would a 3 blade also improve backing up and prop walk?

Daniel 86 "Solar Wind" #5339 C-25 FK/SR I/B Diesel

Edited by - on

TRogers
1st Mate

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USA
90 Posts

Response Posted - 04/13/2011 : 18:16:36

Started to respond and noticed INboard, not outboard. I wouldn't think there should be a dramatic difference?

I have a 9.9 HP outboard with a three blade prop and that is more than enough to hit close to hull sped of 6.4 knots. Of course if the current is doing 6.4 knots against you, you won't go anywhere anyway.

Edited by - on

Steve Milby
Past Commodore

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USA
5908 Posts

Response Posted - 04/13/2011 : 18:40:49

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dblitz
 I have an 86 fin keel standard rig with a 10 hp Universal inboard.
The engine has a 2 blade 11RH7 prop. The boat doesn't really make headway when it is going against current and against wind. Thinking about changing to a 3 blade prop. The questions are:
What speed should the boat do when it is at or near redline rpm?
What props do those of you with an inboard have?
Would a 3 blade also improve backing up and prop walk?

<hr height="1" noshade id="quote"></blockquote id="quote">I don't have much experience with a diesel C25, but my C&C 35 has a 2 blade prop, and my mechanic tells me a 3 blade prop will increase maneuverability and improve reverse, and increase speed and fuel economy. Personally, I think even a 2 blade prop creates significant drag, considering the size and weight of the C25, and I'd be concerned that a 3 blade prop would cause the C25's performance under sail to suffer severely.

If your boat can't drive to windward under power against strong wind and waves, or currents, that's normal. I was doing over 7 kts with the engine wide open on a 38' boat against what was apparently a 7+ kt current, and the boat came to a stop. I have had to give up trying to motor to windward on a boat against about 20 kt winds, because we made such slow progress that we wouldn't have had enough fuel to make our destination. Sailboats often don't go as well under power as they do under sail.

Edited by - on

Stinkpotter
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Djibouti
9087 Posts

Response Posted - 04/13/2011 : 19:17:31

Going against a current, the current plus your speed over the bottom will not exceed 6.3 knots, no matter the engine, because your speed through the water can't exceed hull speed (unless you can get your bow up over your bow wave).

That said, a 10hp inboard should be able to push a C-25 through almost any wind conditions, and in fact do it better than an outboard (because of less cavitation when the boat pitches in the seas). My 8 hp outboard could make 3+ knots into 30+ winds and steep Long Island Sound chop, at well below full throttle. (Full throttle would just increase cavitation.) If the current had been 3 knots against us, we'd be standing still and there'd be nothing we could do about it. We didn't race, but we daysailed frequently with the outboard in the water (with a low-pitch, 4-blade prop), and the boat moved along fine. Just lazy I guess...

Your inboard "redline" is well below our outboards', but you should reach hull-speed below it with no wind or current. Adverse current will slow you absolutely, knot-for-knot. Wind might slow you somewhat, but you should have enough to punch through it and some big chop. I think I'd give the bigger prop a shot.

Prop walk tends to increase as pitch increases--the higher pitch throws more water laterally off the prop. More blades and/or a larger prop with the same or less pitch gives you more forward thrust with somewhat less prop-walk. But more blades, larger diameter, and/or lower pitch creates more drag. Everything's relative, and almost everything is a compromise.

Edited by - Stinkpotter on 04/13/2011 19:26:27

Arlington
Navigator

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USA
196 Posts

Response Posted - 06/10/2011 : 23:03:37

Hey Daniel, what conclusion did you reach concerning the prop. My surveyor told me the same thing, recommended replacing the 2 blade with a 3 blade. As a new bee, I am also finding that I have a million questions. Welcome aboard, you are in good company!

Edited by - Arlington on 06/10/2011 23:06:28

Tomas Kruska
Admiral

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Czech Republic
522 Posts

Response Posted - 06/10/2011 : 23:37:39

I have saildrive with 2blade folding prop (don't know the parameters but pretty big blades), but my brother with the same motor and similar boat has 3blad folding prop and he can go a little bit faster even I'm faster on sails.

I will try to test the maximal speed this weekend.

Edited by - on

Tomas Kruska
Admiral

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Czech Republic
522 Posts

Response Posted - 06/13/2011 : 01:53:17

Here are is my weekend test with two adults and one baby: 5.3 knots @ 3100 min-1 (measured by GPS and onboard speed meter, flow current was calculated)

Would be nice to have 6.3 knots

Edited by - Tomas Kruska on 06/13/2011 02:00:56

Tomas Kruska
Admiral

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Czech Republic
522 Posts

Response Posted - 06/13/2011 : 02:06:49

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by TRogers
I wouldn't think there should be a dramatic difference?
<hr height="1" noshade id="quote"></blockquote id="quote">

Inboard diesel with saildrive is 130kg (286 lbs) and your outboard is around 52kg (114 lbs). That's a noticeable difference I would say.

Edited by - on

trptelf
Navigator

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USA
113 Posts

Response Posted - 06/13/2011 : 04:36:16

We have an Evinrude 9.9 and need a new prop. There are many different sizes listed as parts for this motor. How do we choose the right one? How are they different? I hope its okay to post this question here, it seems related.
0398126
PROPELLER, 10 x 7,3 blade
$97.33 $74.90 1

51 0387389
PROPELLER, 9-1/4 x 8
$97.33 $74.90 1

51 0389892
PROPELLER, 9-3/8 x 9 SST
$252.60 $194.37 1

51 0386669
PROPELLER, 10 x 5
$80.00 $80.00 1

51 0386670
PROPELLER, 9 x 10
$68.50 $68.50 1

51 0386118
PROPELLER, 9-1/2 x 10
$97.33 $74.90 1

Edited by - on

cshaw
Captain

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USA
460 Posts

Response Posted - 06/13/2011 : 06:05:45

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by trptelf
 We have an Evinrude 9.9 and need a new prop. There are many different sizes listed as parts for this motor. How do we choose the right one? How are they different? 51 0386669
PROPELLER, 10 x 5
$80.00 $80.00 1

<hr height="1" noshade id="quote"></blockquote id="quote">
I have a 1976 9.9 electric start long shaft, and bought a 10 x 5 three bladed prop when I initially got the motor so I would have enough "power" to push the boat. It has been a really good combination for me.

The numbers are dia x pitch. Pitch is simply how far the prop would move forward in one turn if there was no slippage. Small numbers have more power, but less speed through the water.

I have used mine to bust through 4-5 ft breaking waves (yes, I was getting a lot of cavitation too!), and to pull a shrimp net behind the boat. I can get about 6-6.25 knots in average small waves (no shrimp net or sails), and it burns a little over 1/2 gallon/hr at my normal powering speed of about 5-5.5 knots, I rarely run the motor at full throttle (usually only when I am scared!!). When we would do longer races off shore in California, we would power non-stop at 5.5 knots for 24hrs at a time to come back up to Ventura from LA and points south and two 6 gallon gas tanks would always get us home (with the main up as a steadying sail and for some added push). I have also towed in a lot of boats with it (largest was a 35 ft cabin cruiser).

Look on your old prop (usually someplace on the hub) to see what its dia/pitch are. Might be hard to find but its usually stamped or cast into the hub someplace.

Chuck

Edited by - cshaw on 06/13/2011 06:10:13

Stinkpotter
Master Marine Consultant

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Djibouti
9087 Posts

Response Posted - 06/13/2011 : 06:54:05

Bill, 9x10 is for getting a little inflatable on plane. 10x5 is, as Chuck describes, half the pitch. Like the low-range gearing in a truck is for pulling stumps, this one is for pushing heavy displacement hulls to relatively slow speeds and stopping them quickly. I think my "Power Thrust" Honda 8 (4-stroke) had a 10x6.5 4-bladed prop. It was a stump-puller, with the same kind of performance as Chuck describes. 5.5 knot cruising speed was at somewhere between half and 2/3 throttle, eating a little over 1/2 gallon per hour. It would punch through anything and stop on a dime. And the lower pitch generates somewhat less "prop-walk" (sideways thrust).

All that said, since you asked, I'll toss in the recommendation that a question like this be posted as a new topic (next time

)--since the original question has to do with inboard diesels, which are a whole different animal--lower RPMs, etc. Thus, the answers (opinions) won't get confused with the questions.

Edited by - Stinkpotter on 06/13/2011 06:56:33

trptelf
Navigator

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USA
113 Posts

Response Posted - 06/14/2011 : 05:09:04

Thanks for the info.
Sorry for posting here. I didn't realize prop issues were specific to inboard or outboard.

Erin

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