Re: Can't pull a skier.
You’ve got the same problem I do. Watch the thread on 94 Javelin / 140 Evinrude and it may be helpful. This boat is about the same size as your’s and the motor is close in HP. I know back in the day Evinrude was rating HP at the crankshaft and not at the prop so mine is really about 125 real HP. I’m not sure about Mercury and your year model but my instinct says your’s is rated at the prop so you should be at 115 HP. Every boat runs a bit different but they are at least close enough to compare.
I’m running about the same rpm as you with two versions of a 21 pitch prop and neither one is anywhere close. It runs 4,200 rpm max and that’s going to kill the motor. On the other end, I’ve tried a 15 pitch big ear prop and it runs through 6,000 rpm which will eventually blow the motor up. The best one so far is the one I tried yesterday and it’s a 19 pitch (a bit beat up) and it ran about 5,000 with two people in the boat and about 5,200 with just me. My target rpm range is 5,000 – 5,500 on this motor.
The other factor to consider when comparing one rig to the next is the gear box ratio in the motor may not be the same. My 140 has a 2:1 ratio so 5,000 rpm equals 2,500 prop shaft rpm. I’ve got a later model 150 Johnson on another boat that has a 1.87:1 ratio (1 / 1.87 = .5347) so 5,000 rpm x .5347 = 2,674 prop shaft rpm. With all things on two comparable boats being equal except the gear box ratio, the lower 1.87 motor will lean towards a lower pitch prop because it’s turning faster.
If you want to figure speed and rpm on you’re rig then find a prop calculation formula you can input you’re data into and you can get an idea of max speed. There are several on the internet. If you like math then here’s the manual method I use along with an example using a 2:1 gear box and a 19 pitch prop.
5,000 rpm x .5 (2:1 gear box) = 2,500 prop shaft speed. Then, 2,500 rpm x 19 inches per revolution = 47,500 inches of forward movement per minute. Then, 47,500 / 12 inches per foot = 3,958.33 feet per minute. Then 3,958.33 x 60 minutes per hour = 237,500 feet per hour. Then 237,500 / 5280 feet per mile = 45 mph. This is the theoretical speed of this boat and then a slippage factor must be applied of say, 10-15% so the real speed with a good performing prop would be 38 – 40 mph.
I don’t know you’re gear box ratio but I seriously doubt that 23 pitch motor will ever work. 1st, I would google the gear box ratio so you can either run a calculation or just to note the difference between comparable motors/boats. I’m running what I can find so I don’t have to buy all of these props just to test. I’m close enough to know I need a 19 modified down to an 18 or a 17.
I’m guessing if you could find a 19 pitch to try that would be a good place to start. Follow the bit about the exhaust ports in the other thread because they help with hole shot. Also, if you do mostly skiing and tubing then I think a 4-blade would be a good choice. They are known for hole shot, good mid range and good stern lift. They make the boat run easy at tubing and skiing speed I can tell you from experience. You may lose 1-2 mph on top end.
