Prop questions.

LIGHTFOOT

Seaman
Joined
Oct 22, 2008
Messages
50
I have a 75 Johnson Stinger with a 13 1/4X17 prop. I ruined it last week. In the iboats prop section, it lists props with small, or large gearcase. How can I tell which one I have for sure (mine looks huge). Also, is this the best size and pitch prop for my boat/motor combo? I have a 15 foot Tri-Hull boat. This seems to be VERY doggy out of the hole and tops out at 35 mph on GPS. Thanks.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Prop questions.

Lightfoot, welcome to Iboats prop forum and we will try to help you as much as we can but 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, if you would prefer to not go to this much trouble there are other people who will gladly help you.

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 maximum 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. manufacturer 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 then do the same thing on the motor and turn the motor until the tape lines up with each other on the prop, 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, use a board under the keel and stiking out to the anti ventilation plate for a referene
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. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
RPM ___________ Speed ___________ No. of people ____________ Gas ________
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
 

LIGHTFOOT

Seaman
Joined
Oct 22, 2008
Messages
50
Re: Prop questions.

Too technical for me! lol My question is, is a 13 1/4 X 17 prop the correct size for a 1975 75 HP Johnson, and how do I tell if my gearcase is large or small? Thanks for any input.
 

LIGHTFOOT

Seaman
Joined
Oct 22, 2008
Messages
50
Re: Prop questions.

Just an FYI to anyone out there looking to buy a New prop. I ordered my new prop here on iboats.com on Tuesday night, it arrived at my home on Thursday! Now thats fast service!!!! And, it was $50.00 less than at my local marina! Thank you iboats!!
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Prop questions.

If you have a 1975 75 with a 13 1/4" prop your motor is a 20" model. That's the "larger" gearcase. The 1975 75 15" model Stinger has the "smaller" gearcase.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop questions.

1975 brochure says .414 gearcase came with a 13 X 19 prop,
.536 came with an 11 3/4 X 17 I assume the .536 is on the short shaft
or small gearcase??
Without a tach or rpm and speed no way to know what prop is right
for your setup.
 
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