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Old 30 October 2011, 20:56   #11
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Are we talking 20 or 15 HP? Looking at prop charts, you definetly could have a 11 or 12 in prop pitch with 20 HP, check out:

Mariner 20C (THRU PROP. EXHAUST) PROP CHART Outboard Motor Parts

You could put your model and ser number in that site and find out a lot about what props were available for that engine.

It appears that diameters run from 10 to 10 3/4 in and pitches run from 10 thru 15 inches.

Looked at the Mercury site and they seem to be using the same equation used in my example to determine prop expectations.

My guess would be that you have an 11 inch pitch, but that is just a wild guess at best.
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Old 31 October 2011, 05:26   #12
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Frank my ideas were based on the guy's post on page 1 where he said...


...my prop is stamped A12 P9...The engine is also rated at 15hp at 6000rpm.


His prop is stamped the same as the 9" dia/9" pitch on my 15hp Mariner.
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Old 31 October 2011, 08:34   #13
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Mine must be a 9 x 9 then.
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Old 31 October 2011, 10:40   #14
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Fenlander, looks like you have the right information by golly. The first post referenced a 20 HP, but the second spelled out the rating which I overlooked. While on the web, I looked at 20 HP motor props and could not quite match up a similar model number.

According to info from the web, the 15 is rated to operate between 4750 and 5500 RPM with a 1.84:1 (.5435) gear ratio and a prop pitch range of 6 to 10 inches. Your rig is operating on the high RPM side, but the OP's engine could possibly do with a slightly smaller pitch or actually be correct. A tach would sure answer the question, and if it were my rig, I would definetly tach it.

Keeping an engine in the proper rpm range creates less wear.
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Old 02 November 2011, 08:44   #15
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I have just sourced and purchaced the mercury replacement and it was a 9 x 9 after all. When you look at a new one in the box you soon realise how worn and tired your old prop is after a few years use.
Thanks all for your comments.
I am still looking out for if any one runs a 20hp on thier Y boat.
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Old 02 November 2011, 09:08   #16
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Quote:
Originally Posted by mlittlejohns View Post
I have just sourced and purchaced the mercury replacement and it was a 9 x 9 after all. When you look at a new one in the box you soon realise how worn and tired your old prop is after a few years use.
Thanks all for your comments.
I am still looking out for if any one runs a 20hp on thier Y boat.
early zodiac 10ft 6" y boats used 20hp evinrudes but the later avon ones use 15s ,
though i dont see any problems in using a 2o hp on one as they are a really heavy duty boat compared to the rest,
at the moment i just use an 8 hp 4 stroke tohatsu longshaft on mine ,last time we were out going alongside another boat gave a top speed of 17 mph with me and my 11y/old lad on board .

mart
,,,ps get some pics posted up
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Old 02 November 2011, 10:26   #17
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Seems the problem (if it's a problem) is craft around 3-4m with a 10 or 15hp are very weight sensitive. A standard 9" pitch is pretty close to ideal if you carry up to 4 adults but if you only have 1 adult plus a child then you could probably take the pitch up to an 11" for max speed over 25mph.

But then conditions are rarely good enough for that sort of speed in a SIB so I'm happy to stick with the 9" pitch as a good balance.
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Old 02 November 2011, 15:57   #18
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You are right that a slightly undersize pitch is not a problem if you keep from running wide open with a light load. It also develops more thrust quicker than a larger prop which can be very helpful in a sea way.

Just for kicks I looked up the test figures on our 14 ft fiberglass boat with a 15 HP Yamaha two stroke. It has a 0.4815 gear ratio and is approved from 4500 to 5500 RPM. Total weight approx 1,000 lbs.

I had always heard that a 1 inch pitch change caused roughly a 500 rpm difference in engine RPM. Evidently that is a pretty fair rule of thumb.

9 3/4 in pitch provided about 23 mph with 5200 to 5600 rpm. The 9 in pitch provided about 22.5 mph with 6000 to 6200 rpm. It is held in reserve for when we load the boat heavilly. You run off the horsepower curve when you over rev the engine, so these figures are in agreement with that.

The same boat with a 25 HP motor develops about 26.5 mph. The extra 10 HP yields 3 or 4 mph with a large increase in fuel consumption. There is a downside in going for more horsepower than you need on a boat.

The engine was furnished with a 11 in pitch prop which would have really taxed the engine had it been used.

A 9.8 HP two stroke planes the boat nicely and provides about 16/18 MPH in calm water for even less fuel useage. The engine does not do so well in a chop however.

So the gentleman with the 15 who is considering going to a 20 may be dissapointed, but he would have to try one to see for sure.
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Old 23 June 2015, 08:58   #19
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Bumping this up and apologies for that but as it contains so much useful info in similar engines/loads I wanted to keep it all together....

...with the new question that can I gain anything by changing the prop on my 2006 Mercury/Mariner 15?

It's the standard prop that comes with the engine (will post up prop designation ASAP) and on my lightly loaded Aerotec 380 gets us up to around 17 kts touching 18 part of the time, pops up on the plane nicely.

Typical load is me (70kg), son (30kg), fuel (20kg) and Pelicase full of gear + ropes (20kg) - so about 140 kg all in.

Usage is a bit of towing toys, the odd longer coastal trip but mostly given this year's weather just blatting about in the waves.

A bit more top end would be nice but at the same time, pick-up is important so don't want to lose that.

[edit] forgot to add we did 'TinyTach' the engine last year when setting up ideal height/trim and I think max rpm attained was around 5200 but will ideally re-check this in calmer waters this year.
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Old 24 June 2015, 03:44   #20
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I've only ever had a standard 9" prop on the Mariner/Mercury 15 outboards and that's what's on the Mercury 10 I've just bought too.

I did buy a brand new Mercury spare for my first Mariner 15 in case we snagged a rock in Scotland and ruined holiday boating but after a bit of thought chickened out of experimenting with an alternative and bought another 9.

From your revs info and the theory of prop sizing, despite your light payload compared to many, it would seem you could go down a pitch slightly to allow the revs to achieve a greater maximum?? At 5200 you are in the lower end of the maker's quoted 5-6000rpm.
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