prop size

[rhuble64]

i have a 120 v4 rude. the prop is a 13 3/4 x 15. the new prop that came with the boat is a 13x19. what is the difference, and is it a problem? the old prop is dinged up a bit. need to change. thanks

 

[hwsiii]

Re: prop size

Rhuble, without more information no one can help you and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as I can.

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 forgoing 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

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

 

[rhuble64]

Re: prop size

hwsiii, WOW, heres some info for you, best i can do. 1982 proline, 20ft x 9ft +/- 1-2 persons, 30-40 gals,v bottom center console fishingboat,1988 120hp rude v4,33mph @ 5500 rpm the boat plate reads 7persons or 1000 lbs,1800 lbs person,motor,gear, 170hp max , the prop on the boat has minor rock damage, i have a new prop. thought the new prop would be better than the damaged one, heard that a damaged prop may cause problems . the prop is 13 3/4 x 15 the new is 13 x 19 just dont want to create problems. thanks for your help!!!

 

[hwsiii]

Re: prop size

Rhuble, if your gear ratio is 2.25:1, verify this, I show you have a prop slip of 5%, which is exceedingly good. If you are changing from a 15" pitch to a 19" pitch prop of the same make and model you can expect your RPM to go down from 5,500 RPM to about 4,400 RPM and you will be lugging your engine which is not good for it. I can't say how much your RPM are being swayed by the rock damage, but that is about what you should expect.

Prop Change

H

 

[rhuble64]

Re: prop size

thanks, your formula is quite accurate. having the damaged prop inspected by pro and looking into a new one. thanks again for saving my motor from the stress!!! tightlines!!!

 

[muchco]

Re: prop size

Just to add my info show the 1988 120 V4 having a 2:1 gear ratio. If this is correct 5500/2*15*60/5280/12= 39.1 theoretica. 33.0/39.1= 15.6% slip which appears to be more realistic for your setup.

 

[hwsiii]

Re: prop size

Muchco, you are right it does sound more realistic, but he didn't say it was a GPS reading and I take non GPS speeds as close only. LOL


H

 

[muchco]

Re: prop size

hwsiii, I concure. He should make sure the speed is verified by GPS and confirm the gear ratio. But without the tach, diagnosing the setup would be like trying to push a car up hill with rope.
Rhuble if you can get or confirm this info, we should be able to nail this down.