Y Class avon best prop?

mlittlejohns · 29 October 2011, 11:22

Being a proud owner of an ex RNLI y class I am looing for the best prop option. I currently run the boat with a mariner 2 stroke with a 9 x 12" prop. This gived me a top end of 16.9 knots with two adults and 50% fuel.

Does any one run a better prop option or run on a 20Hp?

Max... · 29 October 2011, 13:06

Hi, welcome to the forum - I don't own a Y class but I am actively seeking one - be great to see some pics of yours if you can post them?

There are a couple of guys on here with Y Classes so I am sure you will get some advice.

frankc · 30 October 2011, 01:49

Do you know the engine RPM at the conditions you gave? Maybe someone has the same engine on the same boat who could give you an answer, but if not, a rough calculation of the anticipated result of a prop with different pitch could be made. Basically you use the prop shaft RPM and pitch of the prop in calculations and having a starting point of what the engine is doing now would allow a rough estimate of what a change in pitch would do.

It appears your engine is rated for 4,500 to 5,500 RPM with a lower unit gear ratio of 2.25:1.

Using 1 knot is 1.1508 MPH for the knot to MPH conversion.

Using the formula RPM x Ratio x Prop pitch divided by 1056 = MPH

Assuming that engine is turning 5,000 RPM with 10% prop slip.

The lower unit ratio is 1 divided by 2.25 or .4444

5,000 x .4444 x 12 divided by 1056 yields 25.25 MPH with no prop slip. Using 10% slip gives an estimated speed of 22.724 MPH. You reported 16.9 knots which is 19.45 MPH, so you may have a lower engine RPM than 5,000. You may actually need a lesser prop pitch to allow your engine to attain a higher RPM. That would provide more horsepower to possibly be developed.

You seem pretty close, but you really need to tach the engine.

Fenlander · 30 October 2011, 13:47

Great calcs Frank!

With a lighter boat of a similar length (but 2 teens added to your load) and a 15hp 2-stroke Mariner I was seeing 19mph... max of 23mph with 1 adult and 1 teen. I have a 9" prop and the max revs are 6000 for my 2001 Mariner 15, gear ratio 2.0:1.

Using Frank's calc on my setup it would max out at 25.6mph less 10% slip which gives 23.04mph so it seems the 9" is performing 100% to expectations.

12" is a largeish pitch for a 15hp and you may get better overall perfoprmance by dropping to a 9/10/11" but as Frank says you seem to be pretty close with the figures you're seeing already.

BTW are you 100% sure you have a 12" pitch... part of the Mariner prop code can be A12 and I've seen props stamped with just A12 on the boss but they are 9" pitch??

Solas props supplied by Steel Developments only go up to a 11" pitch for your motor.

mlittlejohns · 30 October 2011, 17:02

Thanks all, my prop is stamped A12 P9 and on my last boat that was a much lighter Avon 3.10 ribwas getting 23knots flat out with the same loading and flat water conditions. Am I to assume that the P9 is the pitch and the A12 is the diameter.

The engine is also rated at 15hp at 6000rpm.

chris123 · 30 October 2011, 18:21

I am running a Y boat and Suzuki DF15 4 stroke OB with a 9.25" diameter x 9" pitch prop.

With two adults I am hitting 17 knots at 5500rpm, the bottom end of the recommended max rev range for this OB. The Y boat is a very heavy beastie.

I would be surprised if the Mariner would take a 12" diameter prop, however it is easy enough to measure.

Cheers

Chris

mlittlejohns · 30 October 2011, 18:57

Thanks Chris, I am going too measure the diameter as I do need a replacement prop and I need to make sure I get the right one to suit the boat.

Your right the y boat is a heavy boat but a great one in my view.

frankc · 30 October 2011, 19:32

Fenlander, it looks like you have hit the right prop for your rig. Maybe a slightly larger pitch would do well, but I would be hesitant to try to change it without a sea trial.

That 10% prop slip has been mighty close to correct on the few rigs I have calculated and then water tested.

If you are reading max RPM, be aware that some engines are equipped with an RPM limiter. Sometimes you can improve performance with more prop pitch on those rigs.

You also run into cupped props which can actually operate as well as about an inch larger standard pitch prop. Hit the sweet spot and it is surprising what a difference it can make. Had an old 18 HP years ago and 1/2 inch less prop pitch with a slightly cupped blade made a great improvement in performance.

Fenlander · 30 October 2011, 21:37

>>>Am I to assume that the P9 is the pitch and the A12 is the diameter.

No... A12 is an abbreviated section of the part number** that gives a basic ID of the prop type. It is very likely you will have a 9" dia but it could just be a 9.25". P9 will be a 9" pitch the same as mine and I think about ideal.

**As an example my 2001 15hp Mariner has an original prop and the part number is 828156A12.

chris123 · 30 October 2011, 22:06

Quote:

Originally Posted by mlittlejohns

Your right the y boat is a heavy boat but a great one in my view.

No argument from me

Cheers

Chris

frankc · 31 October 2011, 00:56

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.

Fenlander · 31 October 2011, 09:26

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.

mlittlejohns · 31 October 2011, 12:34

Mine must be a 9 x 9 then.

frankc · 31 October 2011, 14:40

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.

mlittlejohns · 02 November 2011, 12:44

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.

m chappelow · 02 November 2011, 13:08

Quote:

Originally Posted by mlittlejohns

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

Fenlander · 02 November 2011, 14:26

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.

frankc · 02 November 2011, 19:57

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.

Max... · 23 June 2015, 12:58

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.

Fenlander · 24 June 2015, 07:44

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.