Correct prop

[Astroglass owner]

Hi newbe here need some help on a 1976 70HP Johnson It came with a 13x17 prop and ran pretty good but tach was bad and replaced it and it only turned 5400 switched to a 13,25 x 15 and it dropped to 5200 wrong size prop?? Boat is a Astroglass 15.5 Ft Bass Boat

 

[Dhadley]

Re: Correct prop

Welcome!!<br /><br />I understand your concern. Dropping pitch should raise rpm not lower it. What brand / style were the 2 props?

 

[Astroglass owner]

Re: Correct prop

the first was unknown this one is Michigan Match both were SS

 

[walleyehed]

Re: Correct prop

I'd guess the 17 to be one of the old flat-blades...No cup. But, a 17 flat should turn close to the same R's as a 15 with cup.<br />Hummmmmmm.....I'll let DH figure this one out. <br />EDIT: If both were SS, then both "should" be cupped...there goes my theory....

 

[sawblade]

Re: Correct prop

???

 

[walleyehed]

Re: Correct prop

Possibly the first prop had been "worked" at some time or another, and is not a true 17...<br />The MWC prop is a std design, but may be lifting the stern more than you need, plowing a bit.<br />I can't think of any other issues that would cause this by just changing props..???....