Prop question on 85hp evinrude

[gunlocators]

Ok I have a 24' Intec Marine Pontoon 88 model I put a 1978 model 85 HP evinrude off a 78 glastron bass boat to repower it. Just had the motor checked and carbs rebuilt and left the prop it had when it was on the 16' bass boat which is a 13 1/4x17 prop The issue today when I ran it out the motor sounded great however RPMs never got over 4400-4500 running wide open. My gps showed me running average of 19.2mph. I was told I need to be running wide open at around 5500rpms how can I acheive this is it possible its a prop issue.

If so I also have a 13 1/2 x 17 as best i can read on it its pretty wore also have a new 13 3/4x15 or do i have bigger issues than the prop here

 

[GeneH]

Re: Prop question on 85hp evinrude

Your pontoon is much harder to push than the previous boat. If your tach is accurate, you need to drop about 5 inches off your present pitch which would be about a 12". This will be trial and error so it would be nice to borrow a prop and try it out rather than buying a new one and finding out its not the right one. 1" equals about a 200 RPM change. You are dcorrect in that you want to be about 5500 RPM at WOT and you are presently overloading the engine.

 

[hwsiii]

Re: Prop question on 85hp evinrude

gunlocators, at this point I did some preliminary investigating and I found your motor should turn 5,500 max RPM at WOT and your gear ratio is 2.29:1, please verify this.

I also show you have a prop slip of 40%, which tells me you definitely have the wrong type of prop on your boat, as well as the wrong pitch, if you would like my help please fill this form out.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

Welcome to the forum, I will try to help you as much as I 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 the best you can and I will help you as much as humanly possible. My expectations are that you are willing to expend as much time and energy as I will to find a better prop for your boat.

Just copy this form to your computer and paste it into a word processing program and fill it out, then paste it into your post or just use the numbers for reference in your reply and type in your information.

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 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, manufacturer and model of motor

7. HP and gear ratio of outdrive IMPORTANT
If you don?t know the gear ratio, you can pull the plugs out of the motor 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 flywheel 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 the bottom of the transom of boat if it is an outboard in inches, use a straight edge or a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

10. Is it a bass boat or does it have a pad bottom and is it on a bracket, if so how far does the bracket extend from the stern.

11. Does it have a hydrafoil, dolefin, whale tail or trim tabs

12. Manufacturer, model, diameter, pitch, number of blades and whether SS or aluminum props.
IMPORTANT
1.
2.

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 less than $ 50
RPM ___________ Speed (GPS)___________ No. of people ____________ Gal. 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 by only using the control on the console, checked compression, looked at the plugs and checked spark, is the bottom of the boat clean and barnacle free, and have you checked the Tachometer against a mechanics tach, all of the foregoing could be the reason your prop is not attaining full RPM, or too many 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 or feel.

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 prop I recommend 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