Puzzled by prop performance

[sharps45]

I've been trying to find the right prop for my 2000 23' deckboat, 5400 lbs fully loaded, 5.7L Volvo Penta, 250 HP. Most of my playing is at high altitude, 5800 ft and above, and I get pretty good results with a 15X17P 3 blade alum. I also tried a 14 1/2X19 3 blade alum, but it didn't give me any hole shot. (see previous posting http://forums.iboats.com/showthread.php?t=201984 . Just got back from Lake Powell, 3600 feet, tried both props-

14 1/2X19P no hole at all, WOT 3000 rpm, wouldn't even plane
15X17 hole shot would pull arms off, but top end nearly the same as at high altitude 35MPH @ 4100

I expected the 19 to perform much better down there, especially since the 17 is almost over propped. Both are below expected by the Volvo Prop guide, but the 19 was really disappointing. Could I have spun it?

I'm happy with the 17 around home, because I don't need any top speed there. But when I go to the big water, I need both hole and top end.

Where do I go next? Nearest dealer is 90 miles one way, but he will take back a prop if it doesn't run well (after I buy it first, where do you guys find dealers who loan props?) Does a 4 blade cupped SS fit in the picture?

I am still looking at the smart tabs, but I'd probably need another prop besides, and $'s do count for some of us. thanks for the advice.

PS. I really am trying to learn this stuff, so if I keep reasking the same stuff please bear with me. thanks again

 

[Silvertip]

Re: Puzzled by prop performance

Prop diameter has less effect on performance than pitch so try to stay consistent there. As pitch changes, diameter changes slightly but usually just a 1/4 inch or so up or down for each number change and some don't change at all. Select pitch and then take whatever diameter that series of props provides for your drive. With that load, I'm guessing you need to be down in the 15P range. If all you can get out of the engine is 4100 with either a 17 or 19P the rig is definitely over propped. But I would also have a full tune-up or at least make sure the engine is performing properly. Prop changes will not fix a poorly performing engine.

 

[kwikrnu]

Re: Puzzled by prop performance

Changing nothing except for brand (michigan to quick silver)and diameter (from 15.5x13 down to 14x13) my rpms went up 600 and my speed 2 mph.

 

[sharps45]

Re: Puzzled by prop performance

I've scheduled the engine for a lookover, so we'll see about that end of it. They're both Michigan props. I tried a 17 pitch Hustler (vented) and it didn't help at all either. The fellow I bought the boat from claims he ran it at Powell with the 19 pitch prop and got 45 MPH. My nearest dealer says to try a 15P four blade stainless, but the price is a little scary. I'll know more after the mech looks at the motor.
I guess i'm still a little confused on the whole rpm/wot thing. Do you run the wot test just to check peak prop performance?, because you don't run that way all the time on the lake. Don't you try to find that 'sweet spot' where ride, handling, fuel consumption, etc balance out best? Will it hurt to run the 17P if it is a little overpropped? I don't usually run all out because at home the lakes are only a couple of miles long, and at Powell the channel is usually too choppy. thanks

 

[Tail_Gunner]

Re: Puzzled by prop performance

Prop diameter has less effect on performance than pitch so try to stay consistent there. As pitch changes, diameter changes slightly but usually just a 1/4 inch or so up or down for each number change and some don't change at all. Select pitch and then take whatever diameter that series of props provides for your drive. With that load, I'm guessing you need to be down in the 15P range. If all you can get out of the engine is 4100 with either a 17 or 19P the rig is definitely over propped. But I would also have a full tune-up or at least make sure the engine is performing properly. Prop changes will not fix a poorly performing engine.

Sound advice there, when your tune up is done take a run @ WOT use a gps to meaure your speed and while there make sure your tach is calibrated, if the tech's raise a eyebrow at that just tell them to do there job........ Sorry about that last statement ive just had a run in with a few and had to bust a few crown's.

 

[Silvertip]

Re: Puzzled by prop performance

Prop selection is not an exact science. Each boat, each engine, altitude, load etc., affect an engines performance. Your boat has no transmission like a car so it is starting out in high gear all the time. To prevent engine damage from lugging or over-reving, the proper prop is required. To do that you need to make several wide open throttle runs with an average load. The proper prop will allow the engine to run within the manufacturers maximum recommended rpm band. For your engine that would be 4600 RPM give or take a little. If the engine easily exceeds that, the prop has too little pitch. Increases of 1-inch in pitch will lower engine rpm by about 200 - 250 RPM). Conversely, if the engine fails to reach the recommended WOT rpm, the engine is overpropped and therefore lugging and not efficient. You therefore decrease pitch to raise RPM. Changing props however is the LAST thing that should be done to correct a performance issue. Clean the hull as fouled hulls scrub off a ton of speed and hole shot. Make sure the engine is in a proper state of tune. Props can't fix a poorly running engine. While you don't run wide open all the time, lugging is not just a wide open issue, if it lugs wide open it lugs at cruise speed as well. Conversely, if it overrevs at WOT it will run faster than it needs to to maintain slower speeds. All of this affects fuel economy.

 

[sabastianunf]

Re: Puzzled by prop performance

Yea- check the engine performance if everything is running good then see my thread - 4 blade 4 sale -that may be the prop for you. (I know shamless plug-but it may well be the best bet.)