72 Evinrude 25hp WOT and prop selection

[Captain Caveman]

I've got a 1972 Evinrude Sportster at a stated 25hp. From what I understand, these motors are really 18hp and it feels like even less. My prop is pretty chewed up so I think a new one would make a difference.<br /><br />What is the recommended WOT for my motor? I don't have a tach and no interest in buying one just so that I can buy a prop. Would it make sense to just stick with the diameter and pitch that's on there now?<br /><br />Materials - I don't want to pay for aluminum but boat on a very rocky lake. Is it worth staying with an aluminum prop or would a composite make sense? Will the prop flex with my motor size really make that much of a difference?<br /><br />Thanks in advance for the help.

 

[BF]

Re: 72 Evinrude 25hp WOT and prop selection

Hi,<br /><br />I've got a '70 sportster 25 hp... I'm guessing your '72 is the same. If not someone will come by and correct me. They are really 25 hp's but they share the same block as the 18 hp... there were differences between the 2 motors... I'm no expert, but I think they had different pistons, flywheel and diff's in the carb.<br /><br />Standard prop was a 9x10" 3 blade, optional was 9x9. The correct one for you depends on what kind of boat you have it on. I run a 9x9 on mine on a 16' aluminum fishing boat.... planes the boat no problem with the whole famn damily inside. Even pulls my kids around in a tube with no problem. I do have a higher pitch prop, either a 10" (or maybe 11" don't recall?), and it might be a bit faster when I'm alone in the boat, but you can tell the motor is labouring harder when there's more weight in the boat. That's why I keep it in the boat as a spare, but leave the 9x9 on it.<br /><br />If yours is the same motor as mine (22 cu in displacement), I'm told they like to make their power at highter rpm's so should be propped accordingly. Rated horsepower is at 5500. Sounds like either you are overpropped now, or the motors not healthy, or you're asking too much of it (on a heavy boat) BTW, you could get a prop shop to fix your current prop. Usually cost less than buying another. The guy I use usually charges me <$40. Even if you do buy another, you could get your old one fixed so it could be used. Maybe you'd want to buy one with a different pitch so you could have a choice of pitch depending on how heavy the load is.<br /><br />good luck.<br /><br />Brent

 

[Captain Caveman]

Re: 72 Evinrude 25hp WOT and prop selection

Wow, that's some excellent info. It might be a good idea to keep that extra prop up at the cabin just in case.<br /><br />The probably about pitch is that I have to boat into the cabin, meaning that I boat in and out with heavy loads of gear and people and then have light loads when I go out fishing. It sounds like the 9x9 might work best in that situation.

 

[BF]

Re: 72 Evinrude 25hp WOT and prop selection

that's exactly my situation... a big 16' alum boat that is actually pretty light and easy to plane when it's just me... but when you get 4 adults, 2 tanks of gas, some gear and a dog, that's a different story. The 9x9 seems like the right thing. I don't have a tach either, but I don't think my rpm's are overly high at WOT with a light load. It's a lot slower with a heavy load, but I haven't yet had a load that it wouldn't plane. <br /><br />BTW, you'd get more responses if you put more info about the motor in the subject line for you post.. like,<br /><br />"72 25 hp, what prop pitch?"<br /><br />Most folks on these boards have particular models that they're more familiar with, I think you missed some input from some of the helpful folks that helped me with my 25 hp before.<br /><br />And talkin' about spares, you should pick up some spare shear pins (drive pins) if you don't have 'em already. Keep them in the boat.<br /><br />Good luck.

 

[Captain Caveman]

Re: 72 Evinrude 25hp WOT and prop selection

Thanks again. Mine is a 16' flat bottom Fisher and with four guys in it, won't get up on plane. It pushes water so I don't even try. I think that's too rough on the motor.<br /><br />Hopefully the new prop will work.<br /><br />Extra shear pins are a must on my lake. At fishing opener this year I blew the pin right away. With 2ft swells it took about 40 minutes to get the new one in!

 

[BF]

Re: 72 Evinrude 25hp WOT and prop selection

ya, sounds like my set up. I think 9x9 would work best. What pitch is the old prop? Usually says either on one of the blades near the hub, or on the back side rim under the rear cone.<br /><br />good luck!

 

[G DANE]

Re: 72 Evinrude 25hp WOT and prop selection

Those 25 Hp's are 25 HP rated from the cranckshaft torque, and compared to newer motors, they put out about 21-22 HP on propshaft if in fine condition. The 22 cid block, although a fine design, cant deliver same torque as the later models. They are good, relyable, steady, quiet and smooth running motors. Only weakness apart from the 33 Years they have been in service, is a tendency to corrode at outher edge of sleeves, causing head gaskets to blow often, when operated in salt water. Make sure your pionts are set right, as your spark advance will get less and less as the points wear where the cam opens them. New adjustment up to 0.020 will freshen everything up again, if ignition components are fine. My suggestion would be the 9" pitch too, or even 8, maybe. Most propshops will allow you a test ride, and later swap to another pitch.

 

[Captain Caveman]

Re: 72 Evinrude 25hp WOT and prop selection

The old prop is a 9x10, which you indicated was the standard for this motor. I'll probably try the 9" pitch first, and then maybe the 8".<br /><br />So to understand this correctly, less pitch equals more get-up-and-go but less high-end speed. More pitch equals more high-end speed but less get-up-and-go?

 

[G DANE]

Re: 72 Evinrude 25hp WOT and prop selection

Not quite right , if you have too low pitch, motor will never reach the rpm it is rated at. A 25 HP will only put out 20 HP if it is at 4500 RPM, but 25 HP at 5000 RPM, you will have to find the sweet spot, it really matters.

 

[BF]

Re: 72 Evinrude 25hp WOT and prop selection

running too high a pitch is sorta like trying to start a standard transmission car from a stop in 3rd gear while your pointed up hill. Just too much load for the motor to make the rpm's to get to it's power curve.<br /><br />With some weight in my boat, it's quite a bit faster with the 9x9 prop than the higher pitch one. And lugging the motor with a heavy load at low rpm's is really no good for it. Better that it's prop'd to reach it's power curve.<br /><br />An 8 would likely be better for really heavy loads, but you might reach too high WOT rpm's when the boats empty... Of course you could just throttle back a bit, or switch your 10 back on. 9x9 seems like the best compromise for my rig. <br /><br />good luck