Prop Question

[Frycztech]

I'm have trouble understanding how to choose a prop.

I Have a 1986 21' Greaty White walk around.

It has a 1999 mercury 150EFI outboard.

I wanted to change the prob since the one that is on there is a little beat up and i wanted to put a new one on and keep that one as a spare.

The prop i have now says J 15 x 17P on the inside on the hub. The outside says 48 832828A45 17P

I'm happy with this prop the way it takes off i wanted to know if there was another prop that would be similar and give me a better top end speed.

 

[Islandbrah]

Re: Prop Question

What is your RPM at top speed? the pitch (the 17p) determines travel distance per revolution. your engine should redline around 5600-5800 RPM. When selecting a prop each degree of pitch will either raise or drop your top RPM by about 200 as the pitch goes down the RPM goes up. The 15 is the diameter of the prop. if you are at the top of your redline then thats all you got unless you goto a bigger engine.

 

[Frycztech]

Re: Prop Question

my rpm at top speed is about 5200

 

[Islandbrah]

Re: Prop Question

A 15p would put you in the middle of the range assuming all has been done to correct this current selection, and you can trim up or down from there. But you say the current is beat up and that may cause excess drag resulting in lower top rpm. also if your prop is cupped you can have some of that removed. If you have a friend that has a 15 x 17p clean prop i would suggest trying that before purchasing a new one. are you trimming up the motor at plane to the point before it cavitates? i usually gain 300-400 rpm just by trimming up just before the prop "blows out".

 

[hwsiii]

Re: Prop Question

Fry, how fast are you going at 5,200 and is it trimmed up just before cavitation starts and is that speed by GPS.


H

 

[Frycztech]

Re: Prop Question

Yes im using gps for speed and its about 27-29knots i have tryed trimming but it just seems to slow the boat down.

 

[hwsiii]

Re: Prop Question

Fry, my first assumption is that your motor needs to be raised on the transom at least one notch up and you need a 4 blade prop to decrease your prop slip. I show your maximum factory RPM to be 5,600 RPM and a gear ratio of 1.87:1, please verify this. At this time your prop slip is 26% and that is too high, even though GW are heavy. I am pretty sure I know what prop you need but I need more information.

Prop Slip

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[Frycztech]

Re: Prop Question

Sorry im a little confused what info do you need? What is meant by prop slip?

 

[hwsiii]

Re: Prop Question

Fry, think of a prop as a screw that every time it makes one revolution it theoretically should move the boat 17" forward,but because it is in water and not a solid it doesn't move that 17" yours slips 26% which means it only moves forward about 12.5" instead of the 17" it potentially could move forward. The right prop for a boat should only have a prop slip of 8 to about 12% in my opinion.
The information I need from you is in the posting above with numbers 1 thru 19.


H

 

[hwsiii]

Re: Prop Question

Fry, think of a prop as a screw that every time it makes one revolution it theoretically should move the boat 17" forward,but because it is in water and not a solid it doesn't move that 17" yours slips 26% which means it only moves forward about 12.5" instead of the 17" it potentially could move forward. The right prop for a boat should only have a prop slip of 8 to about 12% in my opinion.
The information I need from you is in the posting above with numbers 1 thru 19.


H