Y Class avon best prop?

[mlittlejohns]

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...]

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]

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]

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]

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]

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]

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]

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]

>>>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]

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