Propeller Horsepower

[rbruce63]

Was my model 2003 Chrysler 20 HP really a 15 HP through the propeller after a 3% power loss?

 

[The Marine Doctor]

Re: Propeller Horsepower

It is hard to say whether you lost hp or gained by cheating with the gear ratio.<br /><br />TMD

 

[rbruce63]

Re: Propeller Horsepower

TMD:<br /><br />But I haven't been cheating with the gear ratios. They are standard in my motor and I have only used the motor for 20 hours the equivalent of my boating experience and my father 5 hours or less back in 1969. The motor was left resting for 30 years with oil in its carbureter throat and in the cylinders and some came down through the exhaust. <br /><br />However, what strikes me the most is that cu.in displacement increased from the 1974 to the 1981 motor catalog displayed at The Chrysler Marine Site without increasing horsepower, tha is the 1974 25 HP is the 1979 20 HP in Chrysler. <br /><br />Could this mean that the 3% loss in propeller horse power common in reduction gears meant that before 1979 Chrysler measured horse power at the drive shaft and not the propeller shaft? <br /><br />That is really my question.

 

[The Marine Doctor]

Re: Propeller Horsepower

I am not saying you cheated the HP but the manufacturer might have.<br /><br />As for HP ratings...they did not change until 1984, this is when HP was measured on the P/shaft instead of the crank.<br /><br />TMD

 

[steelespike]

Re: Propeller Horsepower

15 at the prop would work out to about a 25% difference.3% would work out to about 19.4hp.at<br />the prop.

 

[rbruce63]

Re: Propeller Horsepower

Spike:<br /><br />Pardon my math. <br /><br />What could be the reason for the displacement increase from 19.96 cu.in. to 28.7 cu.in. without the increase in horsepower of the more modern 20 HP's from Chrysler?

 

[The Marine Doctor]

Re: Propeller Horsepower

This was done so they could make a 25hp and a 30hp....though I am not 100% sure the 30hp was that block...but most likely it was.<br /><br />Commonality of parts.<br /><br />TMD

 

[rbruce63]

Re: Propeller Horsepower

TMD<br /><br />The displacement of my Model 2003 1969 20 HP is 19.96 cu. in. A 1974 30 HP was 29.9 cu. in and a 25 HP was 28.57 cu. in and from there on the 20 HP had the same displacement of the 25 HP of 1974 (source Hurrikain.com). <br /><br />I would speculate that the real deal was to increase displacement in order to improve the pickup of the boat with a larger diameter propeller or higher pitch such as in the propeller of the 25 HP had which is 8 1/2" diameter X 10" pitch.

 

[The Marine Doctor]

Re: Propeller Horsepower

Yes..they used the same pistons and block. It was cheaper for Chysler to do this rather than use 2 different blocks.<br /><br />TMD