Prop question on evinrude 175

[gwgautney]

I posted a while back trying to figure out what prop i needed. Just got boat back in the water today with working tach which i did not have before. Work a light load i was a touch over 600 at idle and at wot i was at 5000 running about 42mph. It's an older 20 foot pro line center console. Prop on the engine is a 14 3/4 X 19. I think I'm supposed to be running around 5600 or so i thought. Any suggestions? ? I have a 15 X 17 also would that help me our hurt me?

 

[gwgautney]

It is a stainless steel viper prop. Boat specs show it weighing 1650 dry according to the old spec sheet i saw for it.

 

[SkiDad]

i won't hurt it at all. It might not raise your rpm much if the prop is pretty aggressive. Sometimes those aggressive props run like the next size up. But I think you will have better acceleration. if both props are the same brand than your rpm will probably jump to around 5400 and you speed might drop 1 mph.

 

[steelespike]

At 1600 lbs my feeling is the motor should do better on a 19" ss prop. I found a new evinrude test 1780 lb dry+ 900 people,gear and fuel.+ motor
21"ss prop 5325 rpm @ 50 Of course it's a new motor and boat.
Your prop calculation works out pretty good at a about 13% slip.I do think the 19" should get a little more rpm.Which may lower the slip a little.
Is the bottom clean and smooth? Is the motor in excellent tune.
The 17" prop should improve low and mid range performance but may sacrifice some top end.
You should definitely give the 17" a try and get back to us with the numbers. Speed by gps is best.

 

[gwgautney]

The boat had fresh paint on bottom and its not bottom paint. I would say motor is tuned for the most part in going to rebuild carbs this week have a little hesitation. I'm going to get it back out next weekend with the 17 and take the 19 with me. and will post results. I just want motor to be most efficient and run as its supposed to. Is 5000 rpm at wot going to hurt anything. Also its a14 1/2 X 19 instead of a 14 3/4 don't know if that matters much

 

[steelespike]

5000 is a little low If you haul extra weight a lot it could cause problems.If you usually run light I think it will mostly cause poor performance
My info indicates 5250 as the low end of rpm.
as it stands the motor will be happier with the 17" prop.
Diameter is a secondary consideration. Usually as pitch goes down diameter may go up.

 

[gwgautney]

k thanks im going to do a little towing with the kids this summer too so I think the 17 may be the best alternative 42 in a 20 footer is plenty fast enough I just want to make sure the motor getting what it needs

 

[Chris1956]

You might see if you can raise the motor up on the transom. Maybe that will give you less drag and better performance. I have a 150HPJohnny on a real heavy 21 footer, and get about 40MPH. I would think your boat would get 45MPH or more.

 

[gwgautney]

It's a 25 inch shaft I'm going to try and upload a pic and maybe you can tell me if its to low don't have a straight on picture will have to try and get one tomorrow if you cant tell anything by this one chris

 

[steelespike]

Usually the anti vent plate (just above the prop) should be about even with the bottom.
While not a high performance hull it does appear it could make good speed.
The side view looks like it could be ok but rear view not so much.
Can you get the motor more vertical? Perhaps put a straight edge on the anti vent plate.
If you don'r experience any ventilation(cavitation)issues your motor may be too low.

 

[Chris1956]

That motor should have several adjustment holes and slots. I would take some time trials and then raise it up as far as you can with the existing holes and do some more time trials. If she ventilates (esp. in corners), drop her down a hole, and retest. When you max speed, and ventilation is not a problem, that should be max performance.

BTW - I did this on my 150HPV6 Johnny, and it made no difference, but my hull is a dog. Hopefully you will have better luck.

 

[gwgautney]

ok got her back in the water with the 15x17. im getting more rpm now at wot right around 5300 to 5400 but I think I may have stale or water in fuel because she's not idling right and wants to bog when im at wot. If it's not one thing it's another

 

[steelespike]

What was the speed at 5400?Have someone squeeze the primer bulb when at speed.Perhaps the fuel pump is weak.

 

[gwgautney]

speed was about 41 then it would bog I will try the bulb thing next when I go out next time

 

[gwgautney]

the only thing that gets me is it didn't do this before I took her down to restore it. It ran great except some idle issues and coughing at idle