New Solas Amita 4 blade for 17' whaler and 70 hp johnson 2 stroke?

[ChrisMA]

I have a stock OMC 13.25 x 15 - 3 blade aluminum prop on my 70 hp 2 stroke Johnson motor. With 2 people in the boat I am hitting 6200 RPM and around 31 MPH. Max RPM should be around 5500 RPM for this motor. Would a 4 blade Solas Amita in 12.75 x 17" pitch give even better low and mid range performance while keeping the RPMs lower at full throttle or is this too much of a change?

Thanks

Chris

 

[steelespike]

Re: New Solas Amita 4 blade for 17' whaler and 70 hp johnson 2 stroke?

Not knowing the year of your motor I can't be sure of the rpm; but my indication is 5,800.
I think your tach may be reading high I think your speed should approach 40 at 6,200.
Is your speed by gps? You should verify your tach. Borrow a shop tach or get an induction or an optical tach.Yes a 4 blade should lower your rpm 300 to 500 rpm.
The increase in pitch could temper the low to mid range performance but still should be an improvement.

 

[ChrisMA]

Re: New Solas Amita 4 blade for 17' whaler and 70 hp johnson 2 stroke?

Engine is a 1995. I don't think my boat will go 40 with only a 70 hp motor, it weighs around 1500 loaded. I have verified the tach with an inductive tach, it reads fine.

Are you suggesting that I should go from a 3 to 4 blade prop without changing pitch? I would rather be slightly underpropped and have better acceleration and mid-range performance.

Chris

 

[steelespike]

Re: New Solas Amita 4 blade for 17' whaler and 70 hp johnson 2 stroke?

I was saying going to a 17 4 blade could lower your rpm 300-500 rpm
A 2" increase in pitch should decrease about 300-400 and going to a 4 blade for 100-200 rpm drop.

 

[hwsiii]

Re: New Solas Amita 4 blade for 17' whaler and 70 hp johnson 2 stroke?

Chris,

Spike is usually very good when he makes his recommendations. But if you really want to get
the maximum speed and performance out of your setup then we both definitely need more information. We need gps readings in two different directions so we can average the speeds. You also need
to send some pictures of the stern of the boat with the motor in the normal running position, to
see if it is too low on the transom and creating excess drag and slowing you down needlessly. The anti ventilaion plate should be about 1 to 2 inches above the bottom of the boat.

Motor Transom Height (PICTURES) Just click on it to make it bigger.




I ran some tenative numbers with the information you provided and I show your prop slip is too
high, in my opinion. By changing to the Amita, in the same pitch, that problem will go away and
your acceleration and mid range will improve, Dramatically. If you would like a little more speed, then the 17" Amita would be a good choice and it will give you about the same acceleration you have now, with a couple of more miles per hour top speed..

I show your gear ratio is 2.42:1, please verify this. if this is wrong then all my numbers are wrong.

New numbers (PICTURE) Just click on it to make it bigger.




H