175 merc outboard prop question.

[NESAR]

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.

 

[Chris1956]

Re: 175 merc outboard prop question.

The first question is how high above the keel is the motor's antiventilation plate? Are both the boat transom hright and motor shaft length the same?

Sometimes straight deep vee hulls rise up enough to cause ventilation. There are not many of them around. What year and type of hull do you have?

 

[NESAR]

Re: 175 merc outboard prop question.

I dont know the year of the boat but,I did snap a picture to show you what i'm working with.
hope the pic works this time;-)
sorry i don't no what a antiventilation plate is.

 

[Chris1956]

Re: 175 merc outboard prop question.

The A-V plate is the horizontal plate above the prop. It should be about even with the bottom of the hull when the motor is trimmed down. Measure from the A-V plate to the top of the transom clamp, where it rests on the transom. It will be 20" (most likely) or 25". Now measure the height of the transom. It should measure nearly the same.

Post back the results.

 

[ufm82]

Re: 175 merc outboard prop question.

Here's a question for you. What is your prop like? I see the edge in your picture- it appears to be aluminum. Is it in good shape? When you trim up is the prop cavitating and then losing grip? Even with the engine trimmed down it should do better than 18 if the prop is in good condition. I have a 150 on my 19 footer and 40 is about where I get to before starting to trim up. You should at least be in the 30's before you start trimming up. I'm guessing a bad prop with no bow lift is causing your problem. Just a guess though- there are a myriad of other things at play here. WOT? Engine running properly? Weight distribution not way out of whack? You don't have your 300lb mother-in-law in the bow do you?

 

[NESAR]

Re: 175 merc outboard prop question.

I'm thinking after reading a page on google, I think the motor needs to go up a hole I never measured it I just put it on the boat. By looking at the boat and motor it looks to be in the correct place I will measure it and get back to you on what I come up with.

 

[CharlieB]

Re: 175 merc outboard prop question.

Hard to tell from your picture. you may have to block the trailer up so you can drop the trim all the way and take another picture.

The A/V plate is the one that your plastic fin is screwed on to. Imagine s line drawn straight back from the boat bottom intersecting the drive unit, the A/V plate should be no more than 1 inch above this line, on a flat pad/bottom. If your boat has much of a Vee at the rear, the drive may need be slightly lower.

Picture the water path from the rear of the boat to the drive, if it gets too shallow the prop can suck all the water away and start sucking air and you loose traction, the prop 'blows out'. Lowering mounting height will help regain traction.

 

[NESAR]

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.



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.

 

[NESAR]

Re: 175 merc outboard prop question.

Is 1'' a big difference in where it sits in the water?

 

[cajunmerc]

Re: 175 merc outboard prop question.

yes,the motor is way too low,raise it up 2 holes,and it will be night and day,you really need to find out what prop you have,that boat should fly,w/a 175,is that a starcraft holiday ? you should have a 24 ish pitch stainless prop on that thing.

 

[NESAR]

Re: 175 merc outboard prop question.

Here is a picture of it lowered. hope this helps. it was taken at night so its not the best picture.

It looks to be correct but like I said it is off 1'' by the measurements.

 

[NESAR]

Re: 175 merc outboard prop question.

yes,the motor is way too low,raise it up 2 holes,and it will be night and day,you really need to find out what prop you have,that boat should fly,w/a 175,is that a starcraft holiday ? you should have a 24 ish pitch stainless prop on that thing.

If you can tell me how to get the pitch I will tell you . There is no numbers on it that I can find anywhere. Yes it is a starcraft but not sure if it's a holiday.

 

[NESAR]

Re: 175 merc outboard prop question.

So I found out what the problem was on the prop. My cousin and I went out today to show me how to trim the boat. The motor is sitting correct. The prop is the problem it is spun, it is a 19 pitch we have a new 23 pitch we are going to put on it. Thanks for all the help. Off to the prop section now for more questions. Thank You all again.

 

[NESAR]

Can some tell me how a prop spins?

Can some tell me how a prop spins?

I have lots of post up on this boat we built.
My first post was this: 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. So I took my cousin out he said the prop is spun. Put on a new prop the old one was 19 pitch I replaced it with a 23 pitch.
when looking over the old prop all the insides look fine to me.
So my question is if it's not the prop witch I can not confirm that yet because I haven't put it in the water to test it. What could be spinning on the motor other than the prop can you spin a drive shaft? It pee's fine so I know the water pump is working good. When it was going in the water it would work fine if I was going slow, but when I go to give it a good amount of thrust it would sound like it is just spinning. If I back it down it works fine but cant get any speed out of it. Is there some kind of clutch on the motor that would make it do that?
It is a 175hp merc black max 1986 on a 20' starcraft holiday.

I was looking at other post some of them said the shift cable. I hooked up the cable at the engine side I moved the adjustment screws on it can that also be it? If you think so how can I get the correct adjustment?

 

[Texasmark]

Re: Can some tell me how a prop spins?

Re: Can some tell me how a prop spins?

Either you didn't say how the 23p responded or I missed it. Your heigh looks good to me. What else you said makes sense to me that you have a spun hub on the old prop.

Mark

 

[NESAR]

Re: Can some tell me how a prop spins?

Re: Can some tell me how a prop spins?

Mark, I haven't put it in the water yet looking on doing it tomorrow. I need to get some grease for it before I put it in.

 

[Chris1956]

Re: Can some tell me how a prop spins?

Re: Can some tell me how a prop spins?

Going from the 19" pitch prop to the 23" pitch prop is a pretty big difference. That motor may not have enough power to spin the bigger prop. Check out max RPM with the new prop and make sure it gets to 5200RPM at minimum, with average load.

 

[cajunmerc]

Re: 175 merc outboard prop question.

That is your spinning problem, a 19p is waaaay to small,is it even a big case prop? not sure if you have a 2 or 2.4 chrome bore 175,but the 23 will be a good start,most 2 liter 150's like a 23p of one form or another , if the 23p over revs a little then you will need to play with pitches, boat and load weight will come into effect, i still think the motor needs to come up to the 3rd hole. It will trim out better and give you more control, and will still holeshot.

 

[cajunmerc]

Re: 175 merc outboard prop question.

That motor will run at 6k all day long, see where the 23 lets it run to, then go from there.

 

[Texasmark]

Re: 175 merc outboard prop question.

That motor will run at 6k all day long,

and, if like my Mercs "just love it".