partlowr

Mercury Marine Mirage Plus 17p

Trash

Then your outdrive ratio is a big question.

partlowr

Thanks for everyone's input. I put a call into Mercury Marine yesterday and spoke with a support tech/engineer that spent some time talking to me. He did some calculations and came up with recommending an 18 or 19 pitch for my set up. He asked if I tow wake boards or towable's much and said if I do to go with the 18 and if I want more top speed go with the 19. He said that going with the 19 pitch will drop my revs to 4820 and the 18 should be right around 5000. He said based on me hitting 58 at 4950rpm with the 17 I should be able to hit at least 60mph with either the 18 or 19, the 19 having the highest top speed, but with the 19 I would be sacrificing holeshot performance. This is all new and confusing to me so I have to ask you guys if this info he gave me makes sense and sounds accurate? Since I might occasionally tow a person or two on a tube and 60mph is a respectable speed for a boat like mine I am leaning towards the 18. Any last input or words of wisdom before I order up my new prop today? Thanks again everyone.

Trash

I would be very careful about assuming going with a higher pitch prop will net you a higher top end, particularly with a small block. They may not have the torque or HP at the desired rpm. It would also be nice to narrow down cam and head data so we have a rough idea of where your hp/tq might be.

You need to find out exactly what your OD ratio is before you do anything else.

partlowr

He had me furnish him with the serial number stamped on the outdrive and I believe he said it was a 1.4somthing. He said there is always a possibility that it could have been modified at some point by the previous owner. Is there another way I can find out my OD ratio?

Mr Maine

1.47 is the common ratio. Basically every 1.5 turns of the engine, the prop should rotate once. Put a socket on the harmonic balancer and turn the engine 1.5 times by hand and see if the prop rotates one turn (shifter in gear). If the outdrive is off even easier.

Something is up, has the prop been worked to up the pitch to 19?

dbkski

iTrader: (1)

In order to get accurate information from the prop slip/speed calculator the inputs must be valid.
My guess is that your old stock tachometer is inaccurate. However as noted above the outdrive
ratio and prop pitch must be verified.

When I was first doing prop testing I had three analog tachs all read differently. When I installed a
digital tach my numbers started to make sense and I was able to tweak my set-up properly.

Blueabyss

Is it a bravo drive or alpha drive? If its a Bravo I would try a 4 blade porp. If Alpha, I would stick with the 3 blade prop. You wil "Probably" get a little better fuel economy with the 4 blade 18p.
Cheap enough to try
http://www.boatersland.com/qspqa2184...D5caAn5n8P8HAQ

partlowr

Thanks again for everyone's input. I had not even considered that my tach my be off so that is now an obvious place to start looking. I am biting the bullet and made an appointment to bring the boat to a local boat stores service department, they are going to make sure my prop is true, make sure there is nothing slipping in my trans or outdrive, check my tach accuracy and determine if I need a new prop. to optimize my performance on the water. It's early March so boat season is almost upon us here in the Chicago area

Blueabyss

I don't even know where to start on this. Did they tell you your outdrive is slipping. If its and alpha then that is in no way possible. If its a bravo , I still dont think thats possible. A cone clutch in a bravo might slip when hammering to get on plane if glazed but not on top end. It would cost less to buy an alum prop for trial and replace the tach.

Chris