Yet another lost soul looking for some prop advice

[mrainey889]

Question about props... I am currently running a 20pitch 4 blade mercruiser prop I purchased last year from a local boat shop. My boat is a 1990 Rinker, 18.5' runabout, with open bow and a 4.3 175hp (2-barrel carb) engine, Alpha I outdrive (1.81:1). I bought this boat used 3 years ago this month. I used the prop that came with it up until last May. The original prop (definitely not the manufactures by the way) would net me a top speed of about 36mph on the GPS. The motor definitely sounded like it was tacking out...however I did not know for certain because my tachometer does not work. I figured this was not good, so I took it easy on the boat (avoided WOT) and the next spring I purchased a 20p 4-blade at the recommendation of my local shop and went out for a test run. My top speed moved up to 46-47mph GPS indicated and the motor no longer sounded like it was going to fly apart. My hole shot as also more than adequate for my usage. I was very happy with this situation and drove the boat all summer in this condition. Towards the end of the summer I got the itch for a little more speed (and economy...rationale, 30mph at x rpms vs 30mph at a lower rpm would be better I would think...) I hooked up a dwell tach to my engine and went for a ride. I found that at WOT I was turning 4600 to 4700rpms. Since my engines spec'd max range was 4400 to 4800rpms, I felt I could move up a little in pitch, especially if I dropped from a 4 blade to a 3 blade.

So, I went to my local dealer and told them I wanted the next pitch up in a 3 blade. They told me that would be a 23pitch, and that my engine could not swing that prop. In other words, they refused to sell me the prop I wanted unless I was willing to sign off on not being eligible for a refund/exchange.

Here is my delima. I typically travel lightly loaded. I have my 20p 4-blade if I ever want to pull a tuber or skier. Based on what I have read here I believe I can run a 3 blade 23p no problem for most days. Should I ignore my local shop and go ahead and buy a 23p 3 blade prop on the internet? I ask this because the prices on the internet websites I have found beat the $200 price tag my local shop is asking.

Is anyone else running a boat like mine with that setup? If so, what has it gotten you?

While looking at props online I have come to another realization. What diameter should I purchase...there are several choices.

Any help/thoughts/critiques would be greatly appreciated.

Thanks,
Mike

 

[Bondo]

Re: Yet another lost soul looking for some prop advice

Ayuh,.....

I Agree with your Shop,....

You're turning 4700rpms Now,....
Jumping to the 23" will put you down to about 4300rpms which is too Low.....

 

[mrmushroomman]

Re: Yet another lost soul looking for some prop advice

I bought 95 sea ray with 4.3 alpha1 genll and 4 barrl carb. THe guy had just put a 23 pitch 3 blade prop on it. When i did my sea trial the boat felt very slugglish coming out of the hole. I had it up to 38mph in about a 200yrd run. I did not pay to much attention to what the rpm's were doing but i think i'm going to go with a 21 pitch which is what the manufacture suggest using. Now i'm trying to get rid of the 23 pitch prop since it is brand new but not too many people around where i live use that pitch prop so it's hard to get rid of. When we finally get some good wheather i'm going to see exactly how the prop performs with the 23 pitch at wot and i will let you know what i find out. I would say that the 23 pitch is a little big for our boat and motor.

 

[TilliamWe]

Re: Yet another lost soul looking for some prop advice

Why don't you buy a 21p three blade that has a little cup to it. It should pull your rpms down a little or keep them the same, which is were you want it. Turbo props come in even numbered pitch 3 blades. I'll bet their 22p would be spot on for what you are trying to do.
A 23p three blade will most likely drop your WOT RPMS too low. The shop is right.

 

[mrainey889]

Re: Yet another lost soul looking for some prop advice

Thanks to everyone for their responses. I suppose I should introduce myself, my name is Mike and I live in central Indiana. I have been a lurker here for a couple of years and finally got around to signing up so I could actually post.

I will have to give this prop situation some more thought. My tach reading last summer came from an old dwell/tach with an analog style needle...which was bouncing around a great deal during my measurement. The needle often flared above 5k, but appeared (to my eyes anyway) to mostly stay in the range I mentioned earlier, 4600-4700rpms. I have heard the local Autozone will loan me something that should be more precise. Once I have a firmer number on my max rpm, I will decide rather I want to go higher in prop pitch or not. Perhaps some day I will get around to replacing that tach...

It has crossed my mind to look in to a fuel flow rate gauge so I can keep tabs on how many gallons per hour I am burning and adjust my throttle/trim to eek out the best mpg I can. Any recommendations on an affordable one? Most I have looked at on the web are in the $300-$400 range, which would take me at least 2-3 summers to recoupe that cost in saved fuel...if ever. If fuel cost were a MAJOR concern for me, I suppose I would not own a boat, however I often find myself running low on fuel quickly since my boat only has a 17 gallon tank...most of my buddies have at least 42 gallon tanks, so I am trying to do the best I can.

Thanks again for the responses,
Mike