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Discussion Starter · #1 ·
How you all doing tonight? I bought a boat last summer and I'm worried about it b/c it will 7k at WOT. Its a 305 mercruiser with an alpha 1 outdrive. I have only ran it to 7 once trying to see if the prop had the right pitch. Well obviously its doesn't!!! Max rpm should be around 5400, i normally cruise around 3200. My question is which way do I need to go with the prop to get the rpm to come down and how will that affect the performace of the boat. I mean it gets on plane pretty good. Its a 22ft thompson carrera, cuddy cabin. What are your opinions on this?
 

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SUNDAY-FUNDAY (3/15/2010)higher pitch will bring your rpms down, as well as giving you much more top end speed. being 2 grand over MAX i'd say you need at least 3 degrees more pitch.
Typically one degree in pitch will change the rpm's by 150-200. So he would need a lot more than 3 degrees of pitch. My guess is the RPM gauge is not working if it is showing that much over the max RPM and you have not blown the motor yet.
 

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Discussion Starter · #4 ·
Telum Pisces (3/15/2010)
SUNDAY-FUNDAY (3/15/2010)higher pitch will bring your rpms down, as well as giving you much more top end speed. being 2 grand over MAX i'd say you need at least 3 degrees more pitch.
Typically one degree in pitch will change the rpm's by 100. So he would need a lot more than 3 degrees of pitch. My guess is the RPM gauge is not working if it is showing that much over the max RPM and you have not blown the motor yet.
I would agree with the blowing of the motor except for thefact that I havent ran over 4k because, being and automotive tech and my motor is basically a car motor with some modifications, i know what kind of damage comes can come from over reving. By the way the rpm gauge is pretty much on I tested it with another one.
 

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Telum Pisces (3/15/2010)
SUNDAY-FUNDAY (3/15/2010)higher pitch will bring your rpms down, as well as giving you much more top end speed. being 2 grand over MAX i'd say you need at least 3 degrees more pitch.


Typically one degree in pitch will change the rpm's by 100. So he would need a lot more than 3 degrees of pitch. My guess is the RPM gauge is not working if it is showing that much over the max RPM and you have not blown the motor yet.




i was thinking the same thing. but, didn't want to say "you need 15 degrees more prop." what pitch, and how many blades, are you running now?
 

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You want to go just past your max RPM (by 100-150 rpm)with no load and a light gas tank in the boat. You said what type of motor, but what type of boat is it? And what type of pitch is the prop right now? Then we can give you a better guestimate of what you need. But if you want to be sure without buying all kinds of props to test it out, go down to either Accu Prop or the Prop Shop. They'll be able to hook it up right the first time. And if not, they will swap it out with no problem till it gets right.
 

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Discussion Starter · #9 ·
Its a 22ft thompson carerra, its in the pic under my name. I dont know what the pitch is at right now, but its 3blade and at 3200-3400 i cruise between 27mph and 30mph just depending on the water. I know the speed is pretty close b/c thats on my gps. I wish i knew the pitch now. But this is something I would like to get fixed pretty quick..
 

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Curtisimo81 (3/15/2010)Its a 22ft thompson carerra, its in the pic under my name. I dont know what the pitch is at right now, but its 3blade and at 3200-3400 i cruise between 27mph and 30mph just depending on the water. I know the speed is pretty close b/c thats on my gps. I wish i knew the pitch now. But this is something I would like to get fixed pretty quick..
It should be stamped either on the barrel of the prop or on inside portion barrel maybe by the splines. It's stamped somewhere on there.

I just do not know how that boat could be turning 7K RPM's unless there is a john boat propon there????????????? And if that was the case, it would not be going faster than about 10mph. Are you sure that your max RPM's are hitting 7000?

On a side note. If that's a 22' boat, that's a lot of boat for a single axle trailer.
 

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Discussion Starter · #12 ·
Telum Pisces (3/15/2010)
Curtisimo81 (3/15/2010)Its a 22ft thompson carerra, its in the pic under my name. I dont know what the pitch is at right now, but its 3blade and at 3200-3400 i cruise between 27mph and 30mph just depending on the water. I know the speed is pretty close b/c thats on my gps. I wish i knew the pitch now. But this is something I would like to get fixed pretty quick..
It should be stamped either on the barrel of the prop or on inside portion barrel maybe by the splines. It's stamped somewhere on there.

I just do not know how that boat could be turning 7K RPM's unless there is a john boat propon there????????????? And if that was the case, it would not be going faster than about 10mph. Are you sure that your max RPM's are hitting 7000?

On a side note. If that's a 22' boat, that's a lot of boat for a single axle trailer.
Well I say 7k but i have actually only got 6800 before idling down, that was high enough for my blood..lol I dont know why it will turn that many rpms, I have only done it once because i dont want to blow my motor. I mean i guess i could take it out this weekend and just run WOT utill it stops and see where it is, the motor was just rebuilt when I bought the boat. the motor wasnt even in the boat I had to install it. Yes I agree 100% with you on the trailer, I have asked everyone but that the original trailer. But it seems like it should be a dual axle in my opion. I was wondering if there was a place that could add another axle to it, but it would probably be worth just buying one.
 

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7 grand is way too much for a Mercruiser engine! Prop diameter and pitch are keys to getting your boat running the most fuel efficient and best speed. Seems to me like your under-proped.
 

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Discussion Starter · #14 ·
bonita dan (3/15/2010)7 grand is way too much for a Mercruiser engine! Prop diameter and pitch are keys to getting your boat running the most fuel efficient and best speed. Seems to me like your under-proped.
I know... I guess i just need to take the prop off and bring it to the prop shop. I can napa to pick it up for me and take it over there. What do you all think it will run to have it the pitch adjusticed on it?
 

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Call the boat manufacturer and ask them. Then take the prop to Accu prop DON'T rely on what is stamped on the prop have them check and advise you before you waste money on trying different props.
 

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Curtisimo81 (3/15/2010)
bonita dan (3/15/2010)7 grand is way too much for a Mercruiser engine! Prop diameter and pitch are keys to getting your boat running the most fuel efficient and best speed. Seems to me like your under-proped.
I know... I guess i just need to take the prop off and bring it to the prop shop. I can napa to pick it up for me and take it over there. What do you all think it will run to have it the pitch adjusticed on it?
This is just a guess. And it's all a guess untill you take it to an expert and let them tell you what you need.

But I just ran some info through a prop calculator using your boat and motor combo and it spits out a 14-1/4 x 21 prop if you load it down a good bit. Or a 14-1/4 x 23 for top end speed. These are both 3 blad stainless props. If you want a little more hole shot and low end pulling power, drop the pitch by one degree in order to go to a 4 blade prop.
 

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I posted a reply on your other post. Did not see this till later. Your engine max should be about 4800 rpm. From what you are saying with the current speed and performance, it sounds like your tachometer is not adjusted properly. I would double check the tach read with a shop tach /handheld tach. Some timing lights have a built in tach. If you were turning that high of rpm, you would not have any speed and not really be able to get on plane.
 

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Discussion Starter · #19 ·
Thats just it, I did test with another tach, and I tested it today with the shops tach reader and its testing ok. I'm plaining out just fine. Just if I put it wide open It'll run upto about 6700-6800 rpm, but I have only done that once because thats too high. I didnt have my gps hooked up at the time so i have no idea how fast i was going. But it still pulls hard so I know the prop isnt slipping.
 
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