More speed and rpm, prop size needed?

[wizbiki]

hey guys here are the stats
2008 stejcraft pro skier
6.2m long 2.4m beam
18 degrees dead rise
5.7ltr mercruiser 315hp
approximately 1750kg as tested

3 blade 13" 13' = 4000rpm @ 40mph

4 blade acme 12.5" 12.5' = 4200rpm @ 42mph

heres the milliondollar questions!
manufacturers recomendation for wot is between 4600-5000rpm
i wanted 2 props for the boat
A) 3 blade that will rev out to 4800-4900rpm and 50mph or faster
B) 4 blade that will rev to 4600rpm and still carry the down low torque

i was just wondering is the prop sizing i currently have incorrect? because i feel the boat is not acheiving its full potential? what sizes should they be changed too, to be able to acheive the performance im after?

thanks for any help and feedback, i tried to give all the info needed
thanks, luke

 

[Bondo]

Re: More speed and rpm, prop size needed?

3 blade 13" 13' = 4000rpm @ 40mph

4 blade acme 12.5" 12.5' = 4200rpm @ 42mph
i was just wondering is the prop sizing i currently have incorrect?

Ayuh,... I'd say so,.... Props are measured in Inches,...

I've No idea what's on it now....

You have a 5.7l Merc. but what drive,..??
Alpha 1 maybe,..??
If so, the Alpha 1s don't run 13"x13" props....Muchless 13"x 13 Feet...

My 1st Guess is it should be runnin' something like a 14"x 19"...

 

[Silvertip]

Re: More speed and rpm, prop size needed?

I too feel you either read the numbers on the prop incorrectly, the tach is very inaccurate, speed is very inaccurate or all three. That combination with a 13P prop of any diameter should have the engine screaming for help and it should easily over rev. That prop, except for 1/2 inch diameter puts it in 40/50 HP Johnson/Evinrude outboard territory on a heavy walleye boat and since I've tested that combination, it is too little pitch in that application. So -- step 1 is to repeat the equipment check paying particular attention to the prop. Then have the tach verified. Borrow a GPS for speed.

Next -- IF and I repeat IF, the prop you have is indeed a 13 x 13 (which I doubt) then to increase revs to 4800 you would need to DECREASE pitch even more by about 20% which gets you into the 10 - 11P range which is absurd. I think you also need to make sure this engine is not tired and is properly tuned. All the prop swaps imaginable cannot compensate for a weak or improperly tuned engine.

Next -- DECREASING pitch raises rpm but it gains you 0% in speed since the net effect is the prop must spin faster to move the boat the same distance it did before. So decreasing pitch by 20% does not RAISE speed by 20%. The only way speed increases is IF the boat is rediculously overpropped (way too much pitch) and the boat simply cannot get out of its own way.

Next -- picking a speed target and then looking for a prop is the wrong approach. You choose a prop that allows the engine to spin at the manufacturers recommended WOT rpm with what you consider an average load. Speed will be whatever that prop provides. It may be over the target or it may be under.

So again, you really need to investigate what you really have. Post back and perhaps then we can make some reasonable suggestions. The data you gave us to work with is very conflicting.

 

[PROPer]

Re: More speed and rpm, prop size needed?

wizbiki, Is this a staight inboard? The prop sizes would make much more sense if that is the case. I think you need to visit your local prop shop. They will be able to tweak your props to get you closer to your desired performance, becuase you are going to be hard pressed to find a wide range of options for pitch and diameter. But check out what iboats has to offer first!
http://www.iboats.com/Inboard-Propellers/dm/view_id.360476--**********.432899460--*******.980449932

 

[Silvertip]

Re: More speed and rpm, prop size needed?

Missed the "ProSkier" so ignore my comments if this is indeed an inboard.

 

[steelespike]

Re: More speed and rpm, prop size needed?

I can't speak specifically to your boat,I'll assume it is direct drive(no gear reduction) I've owned several vintage inboards. A 18' 53 Chris Craft Rivera with 105 hp. I don't remember prop detals but it ran something like a 13 X 13 and turned its max rpm of 3,600.with strong acceleration and good speed.32-35 It was about 1500 to 1800 lbs with very little deadrise.Yours is about twice as heavy and lots of deadrise but it still seems that at 3 times the hp yours would turn more rpm and closer to 50.Especially with a 13 X 13.
Does it have tabs and does it laydown really flat at speed?
My other inboard runabout circa 1940 15' 1200 lbs a 95hp basically flat bottom ran something like a 12.5 X 13 with excellent hole shot and good speed no details.
It just doesn't seem you would need to drop pitch for more rpm.
Is the motor in excellent tune with good compression? And again does the boat seem to lay down really flat?

 

[wizbiki]

Re: More speed and rpm, prop size needed?

hey guys thanks for the info,

it sits flat on the water at speed, being designed as a ski boat, not a wake boat. the prop sizing i have given is correct and speed has been gps verified. tomorrow im going to ask about my tacho and get the guys working on it to check it and make sure its working accurately.

after test running it the other night after its 100hr service. it started back firing on the water and making a knocking noise.

upon further inspection by the mechanic, he found one spark plug faulty, the carby faulty, and idling too low.

going to take it out this weekend and try the prop again and see if the results have changed with a properly tuned and operational boat.

thanks for the feedback guys, i'll let you know how round two goes after the weekend.
cheers, luke

 

[kenmyfam]

Re: More speed and rpm, prop size needed?

I think you found something with the bad plug etc.
Verify tach as suggested but first and foremost before investing in different props make certain that the motor is running at peak efficiency, your loading is good and that you do not have a waterlogged hull.

 

[steelespike]

Re: More speed and rpm, prop size needed?

You were likely running on 7 cylinders and perhaps fuel delivery problems as well.
I hope the "knock" was just a preignition problem.