one more prop question

[2182]

so i started out w/ a 14" by 19 pitch, was getting 36-37 mph at 4000 rpms, which was phenomenal(w/ a medium load). problem was i couldnt break 5050 rpm (at 40 mph, light load). so i went to oswald propellors (great guys by the way) here in charleston, told them what was going on, and got a 15x16 prop for her. now i am getting 5600 rpms, which i wanted (and 44mph) but at 4000 rpms, boat runs 29 mph. so my thought was go w/ the original, trade the 3 mph top end for a 37 mph cruise anyday. prop guy said its bad for a motor to not have 5500 rpms at top end. i want a second (or third) opinion from you guys. dont want to damage a new motor but hate to lose 7 mph at cruise. any thoughts on this are appreciated.

[76Red18]

If you have that 60 degree jonnyrude, you want 5500 minimum with a full load; run that motor loose. De-carb it and then de-carb it again. Use carbon gaurd on top of that and that motor will keep you happy!

[Tiny]

I've always thought of it like riding a bike, uphill. Imagine trying to ride uphill when you're geared too low...it takes a lot more strain to make headway, if you had a lower gear it would be a lot easier/less stressful on you. Over propping is a similar situation, in a boat.

[eggsuckindog]

That is awfully big Dia prop, possibly see if you can find a 14x17 - also there is a guy named Ken at Prop Gods - he does alot of work around with Fla Sportsman guys, very knowledgeable and you can switch props. probably google would work if not let me know

[Bushwacker]

You don't say what motor you're running, but Tiny makes a good analogy. Experienced bike riders will tell you that you'll wipe out your knee joints if you ride in too high a gear!

I'd say you're definitely lugging the motor with only 5000 rpm at WOT. As a mechanical engineer, I would say this is the worst thing you can do to any motor! At all throttle settings, it increases cylinder pressure, temperature and loads on the pistons, rings and bearings. That higher temperature will also tend to burn exhaust valves in 4 stroke engines. Even if you spin the flatter prop faster to get same cruise speed, I suspect the cylinder pressures and temps will be lower, so easier on the engine. Much better to let them spin up to full design speed with lighter loads. The 2-strokes have all needle bearings, so high rpm won't hurt them if they get enough oil. (Be wary of fuel starvation, like a plugged jet, on a pre-mix engine, since that's oil starvation too!)

I think the biggest danger with lugging an engine is that it's more octane sensitive and prone to detonation/pre-ignition (folks call it "pinging" in their car engines; the mixture explodes prematurely during compression stroke instead of burning following controlled ignition . . . what you hear is the valves rattling from the explosion!) If you're running ethanol gas, you should know that it's the ethanol that provides the octane. However if you get enough moisture in the fuel, the ethanol will combine with it and separate out, called "phase separation". When that happens the 87+ octane fuel you thought you had becomes something like 80 octane or less! If you have a 4-stroke, you'll at least hear the valves rattling , but if it's a 2-stroke, there are no valves to rattle, so the first indication you have it may be when it blows a hole in a piston!

Because the consequence of phase separation can be so severe, it's worth some extra effort to prevent and/or detect it. ALWAYS use a fuel stabilizer made for ethanol fuels, and run a good Racor-type 10 micron water separating filter with a bowl on it and check it frequently for water. The 2008 & later E-Tec's (and probably most newer motors) have a water sensor in the fuel system and they have a kit for installing it on older motors. If you find much water in the filter, need to ask why, because you may be at risk for phase separation! Denny

[2182]

i have a 150 carbed johnson. was out today heavy load 5200 rpm wot. i may have to go smaller still. the guy i got the motor from said 5800, youre saying 5800, local prop guy is the only one convinced the one i have is right. i do all the ethanol treatments and change fuel water separator every two months in summer, four in summer so im not all that worried but there is still some scary stuff in that post. thanks for the info everybody.

[2182]

found a new ss prop 14x15 on ebay for under $200. running the 15x16 now, 5200 heavy load wot. will that get me up to 5500 heavy load? or close enough to make it a good buy? local shop doesnt have any used or new smaller than what i have to try...

[BigLew]

Sounds like it might. Approximately 200 RPM/inch and an inch less in the diameter- it could workout to be 300RPM, which is your target. Send Fr. Frank an PM and get his thoughts. If it is better than what you have now, you'll still have your current one as a spare, or you could maybe look for another between the two and sell one of them and still have a spare.