wbeaton
Commander
- Joined
- Jul 30, 2006
- Messages
- 2,332
Everyone always asks how to get more power out of their old Johnson/Evinrude 7.5 hp motors. As side from proper boat set-up and weight distribution correct propping is the only way to acheive better performance. It has commonly been mentioned that switching to the smaller 2 blade 8x7 prop of the 5.5 hp or an aftermarket 3 blade prop will improve performance. I decided to test out this theory.
I took out a 1954 Fleetwin today and tested three different props. My girlfriend also helped in the testing so I have two different sets of results.
The props used were a 2 blade 8 x 7.5 prop from a 5.5 hp, the standard 2 blade 8 x 8.5 prop of the 7.5 hp and a bronze 3 blade 8 x 7 Michigan Wheel prop.
The boat is a 12.3" deep aluminium utility. The load included the operator at the stern and a full 3 gallon tank in the bow. My weight is 200 lbs. I'm smart enough not to try and guess my girlfriends weight! Let's just say average female weight.
The results with me were as follows:
2 blade 5.5 hp prop = 13.2 kmh/8.2 mph
2 balde 7.5 hp prop = 15 kmh/9.3 mph
3 blade 7.5 hp prop = 18 kmh/11.2 mph
Her results:
2 blade 5.5 hp prop = 17.6 kmh/10.9 mph
2 balde 7.5 hp prop = 20.5 kmh/12.7 mph
3 blade 7.5 hp prop = 19.2 kmh/11.9 mph
The 5.5 hp prop performed the worst of the three. You could hear the engine over reving and WOT speed was significantly less. It did get to plane faster with me in the boat than the 7.5 hp prop. Well, that's not entirely true. I didn't exactly achieve plane, but it got to pre-plane faster.
The 7.5 hp prop was better suited to the lighter load of my girlfriend. She much preferred it over the others. However, with me in the boat it was a dog to get on plane.
The 3 blade prop only slightly underperformed for my girlfriend, but it worked great for me. My final speed was significantly faster and I got on plane much quicker.
Conclusion - for heavy loads the 3 blade 8 x 7 prop can't be beat and closely rivals the stock 2 blade 8 x 8.5 prop for lighter loads.
Attached is a photo of the girlfriend running the stock 7.5 hp prop.
A little video of it on youTube entitled "1954 Fleetwin Running on a boat"
http://www.youtube.com/watch?v=P1_JCS_1UcI
I took out a 1954 Fleetwin today and tested three different props. My girlfriend also helped in the testing so I have two different sets of results.
The props used were a 2 blade 8 x 7.5 prop from a 5.5 hp, the standard 2 blade 8 x 8.5 prop of the 7.5 hp and a bronze 3 blade 8 x 7 Michigan Wheel prop.
The boat is a 12.3" deep aluminium utility. The load included the operator at the stern and a full 3 gallon tank in the bow. My weight is 200 lbs. I'm smart enough not to try and guess my girlfriends weight! Let's just say average female weight.
The results with me were as follows:
2 blade 5.5 hp prop = 13.2 kmh/8.2 mph
2 balde 7.5 hp prop = 15 kmh/9.3 mph
3 blade 7.5 hp prop = 18 kmh/11.2 mph
Her results:
2 blade 5.5 hp prop = 17.6 kmh/10.9 mph
2 balde 7.5 hp prop = 20.5 kmh/12.7 mph
3 blade 7.5 hp prop = 19.2 kmh/11.9 mph
The 5.5 hp prop performed the worst of the three. You could hear the engine over reving and WOT speed was significantly less. It did get to plane faster with me in the boat than the 7.5 hp prop. Well, that's not entirely true. I didn't exactly achieve plane, but it got to pre-plane faster.
The 7.5 hp prop was better suited to the lighter load of my girlfriend. She much preferred it over the others. However, with me in the boat it was a dog to get on plane.
The 3 blade prop only slightly underperformed for my girlfriend, but it worked great for me. My final speed was significantly faster and I got on plane much quicker.
Conclusion - for heavy loads the 3 blade 8 x 7 prop can't be beat and closely rivals the stock 2 blade 8 x 8.5 prop for lighter loads.
Attached is a photo of the girlfriend running the stock 7.5 hp prop.
A little video of it on youTube entitled "1954 Fleetwin Running on a boat"
http://www.youtube.com/watch?v=P1_JCS_1UcI
