Prop question/help

[Jake Swift]

Hi Everyone,

I am new to boating and wanted to upgrade my prop.

I have a 16 ft Swiftsure Ski boat with a 1979 Johnson 85 hp 2 stroke (Boat and motor weigh approx 1500 lbs).

With the OMC painted stainless steel prop (13 3/8 x 17") I was getting (on GPS) 38 mph peak (avg about 33 mph) at WOT 4900 RPM.

I purchased a Stiletto Advantage Stainless Steel with the guardian hub from iboats in a 13 1/4 x 17". With the same load in the boat I peaked at 40 mph (avg 36 mph) but at WOT 4400 rpm. Is this a normal drop in rpm without changing the pitch???? Is this RPM out of range for my engine (cause problems?)

With a decent load in the boat the WOT dropped to approx 4200 rpm. Another thing was that at WOT and trimmed up for speed the boat wanted to yaw to the left really hard. Is this normal??

Thanks for the help,

Jake

 

[steelespike]

Re: Prop question/help

I don't think your tach is accurate,reading low.38 at 4900 rpm indicates about 3% slip really low 6 to 10% is more normal.
40 at 4400 isn't possible.So you need to verify the tach.
I think you may end up actually at about about 5000 the motor is rated 4500 to 5500.
I have the gear ratio at 2.00:1 speed is gps and prop is 17" So it seems the tach is wrong.
The Yaw to the left did you mean trying to turn left or leaning left?
Is the boat a deep V ? Sometimes prop torque will make the boat lean to the port side.
Some props more than others.
If the motor is too low on the transom it might cause issues.
The anti vent plate(just above the prop) should be about even with the bottom of the boat.
Does the boat respond to up trim?

 

[Jake Swift]

Re: Prop question/help

I don't think your tach is accurate,reading low.38 at 4900 rpm indicates about 3% slip really low 6 to 10% is more normal.
40 at 4400 isn't possible.So you need to verify the tach.
I think you may end up actually at about about 5000 the motor is rated 4500 to 5500.

I am taking to boat into the shop next week. I will have them check the tach. Thanks.

Jake

 

[Jake Swift]

Re: Prop question/help

The Yaw to the left did you mean trying to turn left or leaning left?
Is the boat a deep V ? Sometimes prop torque will make the boat lean to the port side.
Some props more than others.
If the motor is too low on the transom it might cause issues.
The anti vent plate(just above the prop) should be about even with the bottom of the boat.
Does the boat respond to up trim?

Ya I mean it wants to turn left, not leaning. But this is only at WOT and trimmed up to top. With the trim at about 75% to top i can let go of the wheel and it stays straight as an arrow. The anti cavitation plate is within 1" (slightly below) the transom. The boat does respond to trim, up for speed and down for hole shot (as best i understand it).