175 merc outboard prop question.

[NESAR]

Re: 175 merc outboard prop question.

okay. So I put the boat in the water the RPM's are way down with the 23 pitch, but I still cant get WOT that prop is spun also. Max speed was 20 MPH on the GPS. The seas was a good 4' so I didn't want to play to much. I'm confused I went to the boat shop to get some grease told him my story and he said 19 was my correct pitch for the boat. Now I'm off to a prop shop they told me to bring in the 2 props and he will make one of them work for the boat. I think this is the best way to go.

 

[cajunmerc]

Re: 175 merc outboard prop question.

bud,unless you are pushing an oil tanker around,a 2.0 liter 175 v6 merc.,should blow up(over rev) with a 19 pitch prop,a 23 is a good starting point,get a prop with a good center,and go from there.

 

[Chris1956]

Re: 175 merc outboard prop question.

19" pitch is close to the correct pitch. 23" pitch is likely too tall.

 

[fisherman81654]

Re: 175 merc outboard prop question.

Okay measuring the transom it is 20'' and measuring from the A/V plate it is 21'' so I am 1'' to long will that make a big difference in how the boat prop is running?
I can not lower the motor anymore then where it is sitting ,I'm on the top hole. I can bring the motor up as much as I want but can't bring it down . I have a hill I can pull the bot to so I can lower the motor trim all the way down. The prop may not be the correct prop for this boat as I can not get WOT to see where my RPM's are at.
I will be taking my cousin out Sat. he has been boating all his life. He should be able to tell me if I am setting the trim correct. I do think I need to prop the motor for this boat, but like I said I need to get WOT first. I should run 6000 RPM at WOT with the correct prop? I should have my tach in today to hook it up.

I just have to add this to the post this boat has a bunch of weight sitting at the bow of the boat as it is a cabin cruiser it is very light in the back of the boat. here is a picture of the boat under repair.

View attachment 118138

I know this boat is not made to be a speed boat but I think 18MPH on the GPS is still a bit slow for it and only can get 1/4 out of the throttle.

Maybe a dumb question ,but are your carbs open all the way at full throttle?
The reason I ask that ,you said you only can get 1/4 throttle.

 

[NESAR]

Re: 175 merc outboard prop question.

Now can you all see what I mean. Some say 19 pitch even on here. The RPM's are way up with a 19 but I can't get WOT (in gear) with a spun prop. My carbs are fine and working 100%. I can get WOT, but not with a spun prop it reves to high My max speed was 18 mph (GPS) with the 19 Pitch. and 20 mph with the 23 pitch. What I was saying is I can't get WOT is because of the spun props The power is there just can't get the speed because of the props. Once I get a good prop I can take it to WOT with no problem. The motor wants to go but the prop is saying no!!!

 

[fisherman81654]

Re: 175 merc outboard prop question.

Now can you all see what I mean. Some say 19 pitch even on here. The RPM's are way up with a 19 but I can't get WOT (in gear) with a spun prop. My carbs are fine and working 100%. I can get WOT, but not with a spun prop it reves to high My max speed was 18 mph (GPS) with the 19 Pitch. and 20 mph with the 23 pitch. What I was saying is I can't get WOT is because of the spun props The power is there just can't get the speed because of the props. Once I get a good prop I can take it to WOT with no problem. The motor wants to go but the prop is saying no!!!

Its hard to believe that you had two props that had bad hubs,maybe so.
I also think the 19 pitch is too small,23 pitch or something close to that,I would say is about right or even a 21 pitch.
Have you tried triming in the engine all the way in,when trying to get on plane and you do have the thrust washer installed before the prop goes on dont you ?

 

[CharlieB]

Re: 175 merc outboard prop question.

Do both props have pressed in hubs such that the only thing common are the thrust washer and nut?

Or are they the late model replaceable inner plastic and you are using the same 'hub' with both props?

 

[NESAR]

Re: 175 merc outboard prop question.

None of the props are bad they are both good per the prop shop. He said that the boat prop has a cavitation problem. He said to put the 23 pitch on it and give him the rpms at as fast as I can go before cavitation. He said I may have to go with a 21 pitch 4 blade prop but he cant say until I give him the rpms. I hooked up the tech yesterday so I will have the rpms for him Saturday.
Anyone with this kind of problem here are things to check per the prop shop. (1) Make sure the prop is sitting properly. (2) Make sure you don't have a second thrust washer stuck in the shaft. These are the first questions he asked me after looking at the prop. My prop was sitting correctly with only 1 thrust washer that was on the prop.

 

[CharlieB]

Re: 175 merc outboard prop question.

I am beginning to suspect that you are trimming up too far which would allow the prop to 'blow out'.

Start out with the motor trimmed all the way down, just 'bump' the trim button to raise the motor a couple of degrees. See how it goes, 'bump' the trim again, repeat and see high far you can get the throttle open without trimming all the way up.

 

[NESAR]

Re: 175 merc outboard prop question.

Yes he did say the same thing, I was doing it the wrong way I started out with the trim all the way up and worked back. After berrying the bow into the water I got a bit scared. I also think it will come with time for the trimming, I don't know the motor nor the boat yet.
So I will try all the way back and trim it a little at a time to keep the bow up.

 

[fisherman81654]

Re: 175 merc outboard prop question.

Re: 175 merc outboard prop question.
Originally Posted by NESAR
Now can you all see what I mean. Some say 19 pitch even on here. The RPM's are way up with a 19 but I can't get WOT (in gear) with a spun prop. My carbs are fine and working 100%. I can get WOT, but not with a spun prop it reves to high My max speed was 18 mph (GPS) with the 19 Pitch. and 20 mph with the 23 pitch. What I was saying is I can't get WOT is because of the spun props The power is there just can't get the speed because of the props. Once I get a good prop I can take it to WOT with no problem. The motor wants to go but the prop is saying no!!!
Its hard to believe that you had two props that had bad hubs,maybe so.
I also think the 19 pitch is too small,23 pitch or something close to that,I would say is about right or even a 21 pitch.
Have you tried triming in the engine all the way in,when trying to get on plane and you do have the thrust washer installed before the prop goes on dont you ?

 

[NESAR]

Re: 175 merc outboard prop question.

Okay here it is, 23 pitch prop, my max speed was 22 MPH (GPS) my rpm's was at 4800 not WOT because it still has a cavitation problem. I tried trimming it all the way down and going up very slow it pushes the bow under. If I go to far up the cavitation happens to fast and don't get any speed. The water was like glass coming back so I tried pushing it just to find out the bow will drop to low into the water and it gets hard to control. I get a great fish tail at 22 MPH. and seams to run great at that speed.
I have so much more throttle to go so I don't no if I should go with a 4 blade prop. It seams like the motor wants to go but the bot don't like it. I can't get the bow up enough to get more speed out of it so it don't get the cavitation. As far as trim there is no happy medium so now I know it's not the trim because I have tried everything I can as far as trimming it.
If I go with a 19 pitch the RPM's will be way to high. There is no way that prop will work for this boat. I'm not sure how to figure out the math to match up the prop if I cant get WOT. So all I can say is 23 pitch 22 MPH at 4800 RPM's. Any help would be great. I will also run this by the prop shop.

 

[fisherman81654]

Re: 175 merc outboard prop question.

I'm new to boating so please excuse me. I have a 20' starcraft and a 175 merc black max. I put the boat in the water today for the first time after rebuilding the boat. Here is the question when I put it WOT the prop spins out of the water and if I trim it down to keep the prop in the water it drives the bow under the water. I'm confused on what this could be, the motor is sitting in the correct place on the transom and the prop is in the right area as the mechanic looked at it before we put it in the water. I can't find a happy medium in the trim. When I go WOT the bow comes up nice but after it trims out that is when it happens. I can't keep it open and my max speed was 18 mph at trim. I don't know my prop pitch. I also can't find any numbers or anything on the prop.

I know you said you are new to boating and that you REBUILT the boat.
My question now is ,Are you sure the transom in not waterloged?
In one of your pictures,it looked like the transom had blisters which are caused by water inside.

 

[NESAR]

Re: 175 merc outboard prop question.

This boat is fiberglas The wood on the transom is PT and in real good shape. The blisters you see is years of paint build up that was not sanded down. We didn't get so detailed with the transom as we did with the rest of the boat.

 

[NESAR]

Just wondering how to do the math for correct prop;-)

Just wondering how to do the math for correct prop;-)

I have a post up because I'm having a problem with my prop. I didn't have a tach for it and now I do. I was wondering if there is a formula to find out the correct pitch prop. I am having a problem reaching WOT do to the bow berrying it self into the water. If I trim it up to much to bring the bow up but it is sucking air. I have a 23 pitch on a 175 merc,on a 20' starcraft cudy. All I can get out of it with this prop is 22 MPH at 4800 RPM's anything more puts the bow into the water. It is a 3 blade aluminum. I also tried a 19 pitch 3 blade aluminum witch brings the RPM's way up and still does the same thing.
I was thinking to try out a 4 blade 21 pitch, but I would like to get the formula you use.

Edit: This is from a thread started in the Prop section. Moved it to here. We need this all in one place to get this solved.

 

[fisherman81654]

Re: 175 merc outboard prop question.

This boat is fiberglas The wood on the transom is PT and in real good shape. The blisters you see is years of paint build up that was not sanded down. We didn't get so detailed with the transom as we did with the rest of the boat.

Ok
So you say the transom is fine,no water inside.
When you trim the motor up while on trialer,take note of the angle of the engine,the way its slanted in,when i say slanted in I am talking about the top of the engine goining in towards the boat,the prop will be going further away from the boat in this view.If you have a trim guage just note on the trim guage where the neddle is at full trim,full trim is when the two bottom cylinders are fully extended.
Once in the water,start the trim all the way in,give throttle a little till boat trys to get on plane,then trim the motor up some with more thorttle,if trim does not get to were you saw it was while on the trialer,then you have a problem with the trim/tilt system.
If you can trim all the way at more throttle then trim sysem is ok.
Take note of where the trim angle is when you experence blow out or cavatation .
One more thing ,I assume you have right hand rotation on the motor,A left hand rotation and a right hand prop would be a problem.

 

[QC]

Re: 175 merc outboard prop question.

This thread is a total mess. From what I can tell based on another post in the prop section, we have a boat with a very heavy bow AND he has a flippin' hydrofoil on it which is driving the bow down worse. When he tries to trim up the dang thing blows out.

I am going to move the post in the prop section over here. We have too many things going on and too many conflicting ideas, and a lot of conflicting data. Need to reel this one in . . .

1) Accuracy of tach must be confirmed

2) Hydrofoil needs to come off (CharlieB spotted in post #7)

3) As much weight as possible needs to be shifted aft. Batteries, fuel, tools, ANYTHING. For data alone the cuddy needs everything pulled out.

4) New data must be supplied and then let's start over.

 

[NESAR]

Re: 175 merc outboard prop question.

If you would like you can close this down, I can start a new one with the info I have. As far as moving everything AFT I have nothing sitting up front. fuel, bat,ect is all AFT. only thing sitting at the bow is the safety gear. and a head that is small with less then 15 lbs on the port side, some life jackets and dry gear. there is not much weight at the bow. I may have a total of 20 lbs or less in the bow.

 

[QC]

Re: 175 merc outboard prop question.

I don't think we need to close it, just rewind. Are you able to test this boat easily? Need that hydrofoil off first. I also think the tach is off. I'm getting 52% slip right now. It is NOT time to select a prop yet. Still have to figure out the setup. I agree with Chris1956 in that the 19 was probably close to right in the first place. 23 is typically for a faster boat.

Do you know the year and complete model of your OB?

 

[NESAR]

Re: 175 merc outboard prop question.

I will have it in a few I have all the info in my truck I'll go out and get it. First thing so I know all the slang, slip is what? Also I have tested a 19 pitch prop and it don't get as good performance as the 23. When you say hydrofoil off, you talking about the plastic wing on the motor?