Correct Prop for 1975 70Hp Johnson

[harley909]

Model # 70ESL75B 70HP Johnson motor<br />Looking for opinion and information on the correct prop for above noted. Present prop is marked "Michigan 001005" (or 001003 as it is unclear). It measures 13inches in diameter. The motor has 13 splines on the prop shaft.The motor is on a 15ft TriHull. <br />I really need someone to recommend what prop should be on the motor. Inparticular diameter and pitch. My present prop is damaged.<br />Thanks

 

[Jack Shellac]

Re: Correct Prop for 1975 70Hp Johnson

According to the approximation charts, it should be about a 13 1/4 x 17. This is very general, however and it would help to know the WOT RPMS your motor would turn with the old prop. The tri-hull has a lot of wetted surface [more friction] and the general recommendation might be wrong for it. My charts may be wrong, but I believe the 1003 was a 13 1/4 x 17 and the 1005 was a 12 3/4 x 21. Try to look closer and see which one is right for your prop.

 

[harley909]

Re: Correct Prop for 1975 70Hp Johnson

The number on the prop is 011003 now that I have had a real hard look at it. The diameter is definitely 13 1/4 so I have to assume it is the 13 1/4 x 17 prop.<br />The boat handled OK (for speed and power) with one adult and small kids (2 at 70lbs each) but it died when cornering (lost power big time).<br />I don't have a tach, but I think it would get up to 30-35 mph with one adult and 2 kids. <br />I could pull up a 250lb man skiing and I did pull up a 340lb man on a tube, but in both cases it was hard to plane out and it definitely would not plane out with 2 adults and 2 kids pulling these.<br />Is the 13 1/4 x 17 the best prop for this?

 

[Jack Shellac]

Re: Correct Prop for 1975 70Hp Johnson

If it's losing power cornering, it's probably ventilating, meaning air is getting in the stream of water going to the prop. This is usually a sign that the motor is mounted too high. The anti-ventilation plate should be about level with the bottom of the boat. The only sure way to tell if you've got the right prop is a tachometer. The motor should operate in the upper end of the maximum RPM range at wide open throttle specified by the manufacturer. Otherwise, you're just guessing. If you've got to have a prop right away, go with the kind that's already on there since it's generally OK.