Honda 50 propeller needed

[Mark]

Having damaged my prop rather extensively, I can replace it with the same (11.5 x 10) or try a different size. I can't find much on this forum for the Honda 50 although I thought there was some discussion on the old board. Does anyone have any suggestions?

[Ron]

I'd first try to see if the standard 50hp Honda prop (11.5x10 - your specs) is the best one for the boat. There are a number of web sites around that could help you to mathematically compute what pitch should work best. 10 to 1 sounds like it could be close, but there are too many variables that don't enter the calculation. Weight (over 4600 pounds in your case) is definitely one of them.

The stock Honda 20 prop can't really get much above 4500 rpm, according to a few tests done by me and Dan. I wound up with a 10x7 4 blade that will do 5500 rpm and over 8 knots.

Has anyone replaced a stock Honda 50 prop yet? Those with the Honda 50, what is the highest rpm that you can reach in gear at full throttle? It should be in the 5500 to 6000 rpm range. Much less would indicate that the 10 to 1 pitch is too high, 6000 rpm and a low boat speed (below 14 knots?) would show the opposite.

[Mark]

This is the reply I got back from Michigan Wheel:
Diameter and Pitch: 11.5" x 8"
Style of Propeller: Vortex
Number of Blades: 4
Material: Aluminum
Rotation: RHR
Michigan Part Number: 992401
Suggested List Price: $89.00
Hub Kit needed: 113
Suggested List Price: $34.00

If memory serves, WOT with the old propeller gave about 5000 rpm and 12 knots

[Ron]

With the 4 blades and 8 pitch, it sounds like a high thrust prop. That could possibly help you or anyone else with a 4600+ pound boat with 3 hulls in the water that was not designed to break 16 knots. The 10 pitch on the stock prop is more suited to perhaps 20 to 25 knots of boat speed. If the hull can't do that (and the Telstar can't), then the prop winds up slipping in the water. If WOT on your boat is 5,000 rpm or less, then it can't even get into it's power range. The Honda 20 can't break 4500 with it's 10 pitch stock prop.

Considering the 16 knot potential, maybe a 9 pitch would work a drop better? It would be nice to find a high thrust version with 4 blades. Iv'e got a friend who builds fancy props that have adjustable pitch. I'll send him an email to see what he thinks.

The only real way to find out what works best is to experiment. But that can run up some cost, unless you find some company who's willing to do an exchange.

[trashpad]

If Iremember right, Reboot runs about 5400 WOT but she likes it between 3800 and 4200.

[Ron]

Heard back from my friend, who builds and sells some of the best props around. On a sail boat powered by a gas outboard, the pitch should be around 70% of the diameter. That's a starting point and depending on the boat (displacement, wetted surface, engine torque, gear ratios, etc.), it could vary a drop. For the 20hp Telstar, that gives a 7 inch pitch for a 10 inch blade. That's exactly what I have now and it seems to work in my case. The 50hp Honda with it's 12 1/2 inch blade should have about an 8.75 pitch. I'd go with a 9 inch pitch if I was buying a prop and those specs are available. Four blades look good too, especially in the 3,000 to 4,000 rpm range. This is all a compromise, going too low on pitch would hurt the mid range cruising ability of the boat.

Power boats with outboards, with their lighter weight and higher speeds, should have a pitch greater than 10 inches. Diesels with more torque and lower redlines would also have props with higher pitch.

[Mark]

Thanks Ron

I decided to go with the 11.5 x 8 vortex prop from Michigan wheel - a pitch of almost exactly 70%. My damaged prop (somewhat smaller than the original 11.5 x 10) gave me almost 12 knots at 6000 rpm wot. I'll try out the new one this weekend.

Mark

[Mark]

I have installed the new prop and tested it out in the last couple of weeks. Max speed was 13.1 knots at a WOT of 5000 rpm. We also made 13.1 knots towing a teenager on a kneeboard (stock prop gave 12-13 knots on a good day, pulled a rider at 10 to 11 knots . Estimated fuel consumption was 1.5 to 2 gal per hour compared to the stock prop but this needs more careful evaluation. The new propeller is thus at least as fast as the stock propeller, probably uses less gas but does not get to 6000 rpm.

Mark

[Gene]

I may be mathmatically challenged, but can a prop with the same diameter... less pitch AND less revs create MORE boat speed on the same boat? Is there any way it could not do be able to produce MORE revs?

[Ron]

Gene -

In theory, the pitch represents the distance a prop would screw itself into the water for each rpm. For example, an 8 pitch would walk forward 8 inches each revolution. Since the motor has a set amount of torque, a lower pitch would allow more rpm, with less boat speed as a consequence (less forward motion per revolution). Pushing the pitch too high could have the effect of not allowing the motor to reach reach redline at full throttle because it does not have enough torque to spin the higher pitch prop in the water. Something like a car with manual transmission where 1st gear produces more torque but less speed than 4th gear. The experts all say that the motor should be able to reach maximum rpm at full throttle, but I don't think you have to be at 100 percent. You should be in in the rpm range that produces maximum horsepower. That's probably around 90 percent of redline.

The best props have an adjustable pitch, but these are not available for outboards. My last boat had an Autoprop which automatically changes pitch based on shaft speed and boat speed. Something like an automatic transmission in a car. Which single gear would you order a car with if you did not have multiple speed transmissions available? 1st gear for maximum acceleration to 40 mph or 6th gear for cruising speeds over 75 mph? A single pitched prop is a similar compromise.