175 merc outboard prop question.

[NESAR]

Re: 175 merc outboard prop question.

The model # is 6619602, also the tech is new it sits at 0 and goes up to 1200 rpm in neutral.(in the water) Not sure how to test if it off. it is a teleflex amega tachometer 7000 rpm. It is not easy for me to test the boat it is 22 miles to the ramp.

 

[fisherman81654]

Re: 175 merc outboard prop question.

Take the( hydrofoil )wing off and leave it off.
http://www.mercuryracing.com/propellers/propslipcalculator.php

 

[QC]

Re: 175 merc outboard prop question.

I think the Tach is waaaay off. Check for a dial in the back of the tach. I believe it should be set to 6P. Need confirmation form a Mercury head . . .

 

[QC]

Re: 175 merc outboard prop question.

I don't think that model number is correct on the OB . . .

Slip is the difference between theoretical travel of a prop (23 inches for one revolution) and actual. If you spun your prop through a cork like a cork screw it would go 23 inches if it is a 23" pitch in one revolution. It can;t go that far in water as it is pushing your boat and the weight pushes back against the water. Typically we see 10 - 15% slip if things are right. I don't believe the 52% or so that I posted, so I think there is some bad data. That is possible with ventilation or a spun hub, but you would be complaining of the RPM going way too high though and not going much of anywhere. None of it is valid if the data is bad, so we need to confirm the gear ratio, and the tach.

The "plastic wing" is indeed the hydrofoil and may be why your bow is getting pushed down. Very high horsepower OB for one of those. Need to try it removed.

 

[carholme]

Re: 175 merc outboard prop question.

I believe that 6619602 is his s/n and the model number is 1175725.

Gerry

 

[QC]

Re: 175 merc outboard prop question.

Gear ratio? I was using 1.87 . . .

 

[NESAR]

Re: 175 merc outboard prop question.

I Had a chance to speak with the prop shop today. He told me the same thing that I needed to take off the fins on the motor. He told me to do a new test run and report back. The tach is set at 6p and I'm not sure what my gear ratio is. I don't know where to find my model number let me know and I can report back to you on it.
is there way to post a short video on here you can see the motor running at 22mph in the water? I don't use youtube.

So after looking at the ( Prop Slip Calculator ) I should be doing 2100 RPM's with Prop pitch being 23, at 22 MPH if my gear ratio is 1.87.
Now I know why you are saying something is way off.
So I should use less pitch like a 19 and use a 4 blade I'm thinking. But I know the 19 pitch I had on there the rpm's was a lot higher.

 

[fisherman81654]

Re: 175 merc outboard prop question.

I Had a chance to speak with the prop shop today. He told me the same thing that I needed to take off the fins on the motor. He told me to do a new test run and report back. The tach is set at 6p and I'm not sure what my gear ratio is. I don't know where to find my model number let me know and I can report back to you on it.
is there way to post a short video on here you can see the motor running at 22mph in the water? I don't use youtube.

So after looking at the ( Prop Slip Calculator ) I should be doing 2100 RPM's with Prop pitch being 23, at 22 MPH if my gear ratio is 1.87.
Now I know why you are saying something is way off.
So I should use less pitch like a 19 and use a 4 blade I'm thinking. But I know the 19 pitch I had on there the rpm's was a lot higher.

I would try the 23 pitch prop you have on there now ,with the fin off.Sounds like the prop guy suggested the same thing.
Here is where your serial number is located.What you are trying to achieve is somewhere close to 5500 rpms at full throttle with as little slip as possible.

 

[Chris1956]

Re: 175 merc outboard prop question.

Unless that boat is real light, a 23" pitch prop will be too tall. I am running a Merc 135HPV6 on a 16 foot low profile speedboat(600# weight). I cannot make 5500RPM with a 23" pitch prop. I would doub't your motor will achieve that either, with a 23" pitch prop.

 

[QC]

Re: 175 merc outboard prop question.

I agree ^^^^ even with 20% slip that's 51 MPH and I don't think she'll pull it. I think the 19 was close to right from the beginning.

 

[NESAR]

Re: 175 merc outboard prop question.

We are going to have to put this off till next year. The temp is going to be in the low 30's and there is also a small craft advisory in effect for this weekend. We will pick up where we left off in the spring.We don't want to hurt the motor, time to winterize it.
I did take off the fin and had it ready but it is not safe. Also I dont think this boat can get over 30MPh don't think it is made to go over that speed. I realy think i'm over powered.

 

[Chris1956]

Re: 175 merc outboard prop question.

You are very likely not overpowered. My 21 footer is rated for a 200HP motor. That 20 footer is likely rated for 175Hp, or maybe a bit less.

 

[Chris1956]

Re: 175 merc outboard prop question.

I think a 19" pitch prop will give you 40-45MPH top speed, which is about what you would expect from the average setup.