Efficient prop selection

[t_south91]

Hey guys,

I am looking to add another prop to the boat this year and would love some help with this selection.

My current prop is a 4 blade 19p stainless. Great setup for skiing and getting on plane with a full boat and fuel. However, when its just the wife and I during the middle of the week we just like to cruise across the lake. Plus, I have the need for speed....so I want more top end.

With the current prop, if i am cruising at 45mph and hammer down the RPMs shoot up and you can tell the boat wants to go faster, but the prop wont let it. Top speed is right at 50mph.

So, any suggestions on another prop to help me achieve my goal? I was thinking 21-22 pitch and probably a 4 blade. But I would love some outside advice.

Boat is a '93 webbcraft, 21ft open bow, 350 MAG/Alpha 1

Thanks in advance!

 

[Chris1956]

Props do not usually limit your speed. Trim, hull design and power limit your speed. What is the max RPM you see with the 19Pprop?

 

[Starcraft5834]

I echo what Chris was asking........

what are the numbers your getting? @ WOT= what RPM's you seeing? you said you sense your engine wants to give you more speed but your prop is not cooperating... that would indicate your engine is racing perhaps above recommended rpm range of 4600-4800... the opposite would be not reaching recommended rpm range for your engine, sensing the engine is bogging, that would indicate to much prop, which is probably not the case here.. we really need to know your numbers before your going to get any useful information beyond a guess.

 

[steelespike]

Rpm range on the 350 MAG is 4800 to 5200 gear ratio is 1.47. Prop calculator says rpm at 50 mph and 12% slip is 4650.
If it really is only 4650 your not within normal rpm range.You cruise at 45?Is that GPS?
A 3 blade ss with a lot of bow lift might get you more speed Others with mopre experience with big fast boats may be able to pin a prop down.
It would appear that a 19, 20 or 21" might trade off for speed.Going from a 4 blade to 3 blade
We need your lightly loaded wot rpm and GPS speed.

 

[t_south91]

Thanks for all the input.

The WOT on the RPM is unknown because the gauge is not 100% accurate. But, it has always run very strong and never given any issues.

Lightly loaded, the gauge ranges from 5000 - 5600 at WOT depending on how I trim the drive. However, it also says I idle at 1200 - 1400. Which is not true. The boat cruises at 45 GPS confirmed with 2 people and 3/4 fuel at 5/8 - 3/4 of the throttle.

I just didn't know if I could get a little more top end speed by going with a 21p or 22p prop.

 

[steelespike]

If your going to be buying a ss prop at probably $300 + you need to get a working tach.Tach results aren't lineal so idle rpm means nothing as far as wot rpm goes.
Just an indication tach is wrong. So right now we have wot gps speed with no indication about your rpm. or how well the present setup works.
For the health of the motor it needs to rev freely within its rpm range.What little info we have now we're guessing it is below rated rpm.
You need a tach. Get us the the lightly loaded wot rpm and gps speed.So we have a base line.
Without actual numbers you might as well put some props in a barrel in a dark room and pick a prop.
With a working tach you may be able to find a dealer that will let you test props with the understanding your buying.
Probably require a sizable deposit.

 

[t_south91]

Gotcha.

Once season get here, getting the tach fixed is one the items on my laundry list to get done.

Appreciate the input.