Prop diameter/pitch changes 1996 Evinrude 70hp

[ctf58]

Here is my situation.

I just got a 1996 16? Stratos 258 with a Evinrude 70hp motor. It has an aluminum 14x17 prop. WOT will go over 6000rpm!!! I have got to get the RPMS down. I just got what I hope to be a good deal, a 13x19 SS prop used but in excellent condition for under $125. I understand more pitch = less RPM, but will less diameter = more RPM.

Speed is not an issue, I'm not in a hurry to get anywhere. 5000 RPMs is about 26mph now and that is fast enough. I just want to make sure I or anyone else that might use the boat does not hurt the engine going to WOT and exceeding 6000RPM.

My question is below

Am I going to be in about the same place because 1" less diameter and more 2" more pitch will cancel each other out??

I understand 2" more pitch should reduce RPMs 200-400. I've heard smaller diameter increases RPMs and SS turn easier in the water than Aluminum and increase RPMs.


What is your thoughts? No one around here has a clue.
Boat loaded is around 2000lbs. That's boat890lbs, motor,gas, 2 people, gear, etc...

Thanks in advance
ctf58

 

[hwsiii]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

You are probably going to lose about 500 RPM.


H

 

[ctf58]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

Thanks for the reply. Losing 500 RPM would put me right in the 5500 range.

 

[hwsiii]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

You are very welome

 

[Dhadley]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

ct, a couple of things to keep in mind. The 200 rpm per inch rule only applies when you're comparing the same make and model props to each other. Also, larger diameter doesn't necessairly mean less or more rpm. It all depends on design. As for your motor, 5800 is a good target with an average load. Those motors like that.

 

[ctf58]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

When I get the prop, I'll post back the results and make and model of the new prop.
Thanks again hwsiii and Dhadley

 

[hwsiii]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

That sounds great, and Dhadley is 100% right about blade geometry, it
can and sometimes does matter more than pitch on a prop.


H

 

[ctf58]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

If needed I'll post a new thread, but, I seem to have some knowledgable people responding to this one. This boat came with a Sting Ray hydrofoil on it. I think the reason was because the previous owner and his son were both well over 275lbs each. I like the way I get on plane fast now, but, the boat leans to the left so bad on take off that it scares the crap out of my wife. When on plane it gets better as I trim up. When I get to the point of trimming up and it starts to porpous I'll trim down to calm things down but there is still a noticable lean to the left and the boat pulls to the right. If i allow the boat to turn to the right it levels out. I've really tried to make sure the boat is balanced. Same battery on each side, gas tank in center 2 people within 30 lbs one on each side. Could it be the hydrofoil? I measured it last night and it is dead cantered on the vent plate. The trim tab was move over so the rear of the tab was to the right a little soo I move it to center and the boat pulled more tot he right with the same lean issue.
The foil has a nick on the right tip and is missing about 1 inch along the trailing edge about 1/2 inch deep into the foil. I read about the smart tab but the money is not in my pocket right now and probably will not be until spring, plus i think my speedo and transducers are going to be in the spots for the tabs.
What do you guys think??

 

[hwsiii]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

When you change props you need to run it with the hydrafoil first, just in case it wants to porpoise and we are not able to get good WOT RPM and speed readings. Then pull the hydrafoil off and run the boat without it and lets see if the boat reacts differently and doesn't lean as much.

Also move the trim tab over to the right again, except more than it was before to correct for the pull you are experiening from prop walking. Please take a picture from the side and back of the boat of the motor and anti ventilation plate so we an see how high it is relative to the bottom of the boat.

As far as the boat leaning there are many possible reasons for this condition and we are going to check the easiest ones to see if it an be changed economically. Apparently this not that uncommon a problem with this particular boat as I found two other owners complaining of this issue.
1. The prop is possibly exacerbating the problem because of a bad hull design. By changing the prop possibly we will see a difference in how the boat handles.
2. The hydrafoil is making the problem worse, by taking it off after you get the test results we might possibly see a difference.
3. The last 3 feet of the bottom has a problem that an be cured, we will check this by taking a staright edge to the last 3 feet of the bottom, running it over the hull from chine to chine and from bow to stern, looking for any concavity or convexity over 1/8".
4. The motor was mounted wrong and it is not in the center or it is canted to one side. All we have to do is take measurements to see if this is the case. Points A should be the same length as well as Points B should be the same length.

Motor mounting

H

 

[ctf58]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

New prop made a world of difference! WOT rpm is about 5600 now. 5000rpm was 26mph with old prop, now it is 33mph. I took the hydrofoil off after running the boat with the new prop. This helped correct about 50% of the leaning issue, but the boat does still noticeably lean to the left. It takes a lot longer to get on plane and it takes a lot more speed to stay up on plane. I measured the foil to the ground and it is about 3/4" higher on the right side. I also measured the motor mount as suggested. It is pretty close to center and looks centered on the boat when looking at the keel line. Now my problem is torque from the motor. It pulls pretty hard to the right and I have moved the trim tab all the way to the right until I get it trimmed out. If the trim tab is in the neutral position (centered) the pull to the right is so bad that it takes two hands to go straight at any speed or trim angle. I'm going to tread up on motor torque but any suggestions are always welcome. I think I am going to put the foil back on for now and get some smart tabs in the spring.

 

[hwsiii]

Re: Prop diameter/pitch changes 1996 Evinrude 70hp

ct, the smart tabs should take care of the problem.


H