| Author |
Message |
    
Mike Cox
Member
Username: mixin
Post Number: 11
Registered: 06-2009
| | Posted on Thursday, July 09, 2009 - 10:05 pm: | 
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Hello,
I am wondering if the prop my boat came with is the right size and pitch for it.
I currently have a 9x9 prop on my 1954 QD-15 10hp. The boat is a 12' shallow v aluminum that probably weighs <150lbs. I usually have 2 adults and my 3yr old son (350-400lb) of people and gear.
The boat gets up on plane well and runs out nice with this weight in it- but I've never had another prop on it, and I don't know what it came with originally.
So, I was wondering if this prop is the best one for me. Any recommendations?
Thanks!
Mike |
david real
Advanced Member
Username: david_r
Post Number: 945
Registered: 07-2008
| | Posted on Friday, July 10, 2009 - 06:16 pm: |
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you need to install a tachometer.
i believe you can get a cheap one for a 2cyl at tinytach.com.......... or if you have a timing light with a tach made into it that would work.
if you want to max your performance that is............ of course that usually goes hand in hand with better fuel mileage believe it or not.
what you are looking for is max rpms. i dont know your eng specs ,and am not on my own computer, but lets say for instance your supposed to reach 5500 and you only reach 4200 you need to go down in pitch. if your eng doesnt reach full rpms and you run it wide open all the time it wont last as long.
ill see if i can find the max rpms for your eng. then you can go from there......... and we will help you along the way.
lots of factors also to consider are
weight distribution
gross weight
hull design
max hp
current
wind
stainless or aluminum
2,3,or 4 blades |
Graham Lamb
Senior Member
Username: galamb
Post Number: 5604
Registered: 05-2007
| | Posted on Friday, July 10, 2009 - 08:27 pm: |
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A 9 pitch prop is fairly standard kit on 8, 9.9 (10) and 15 horse portable outboards.
You can tweak it a little, but in general, unless you severely overload the boat, a 9 pitch will keep you within your optimal wide open rpm range... |
Mike Cox
Member
Username: mixin
Post Number: 12
Registered: 06-2009
| | Posted on Monday, July 13, 2009 - 07:44 am: |
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I have a tach/dwell meter - maybe I could make that work even if I have to do a little math (4cyl setting). I think my Hp rating is at 4,400 rpm, is that my max RPM that I need to shoot for? I imagine the RPM that the motor operates at varies a little depending on how much weight is in the boat. How much RPMs above or below your max RPMs is ok? |
Graham Lamb
Senior Member
Username: galamb
Post Number: 5628
Registered: 05-2007
| | Posted on Monday, July 13, 2009 - 10:35 am: |
|
Each motor will have a Wide Open Throttle (WOT) range. On an outboard it's normally a range that extends between 500-1000 rpms leading up to the max.
Running above max rpms is potentially dangerous to the motor. In general, running about 200-250 rpms below max is considered "optimal".
That gives you a little wiggle room if you do run light occassionally (will not over rev), or run a little heavier than normal (would not take you all the way out of it's WOT range which can cause detonation an all kinds of nasty things to happen to the motor).
All of the above applies to more modern outboards, but believe it will still hold true for the earlier models.
I dug out my "Vintage" Johnson manuals and it shows the max rpm for your model to be 4000 rpms (doesn't give a range).
A couple other tidbits from the manual - it is a 16.6 cubic inch powerhead, weighs 60 lbs (dry) and they originally sold for $297.00 brand new (which was a 22 dollar increase from the QD-14 models produced the year earlier - inflation I guess)  |
Mike Cox
Member
Username: mixin
Post Number: 13
Registered: 06-2009
| | Posted on Monday, July 13, 2009 - 11:44 am: |
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Thanks! I will try to get my diagnostic tach hooked up and see where I'm at now. I'd feel a lot better knowing I'm in the "optimal" range.
Thanks for the tidbits! My dad was amazed that the motor only weighed 60lbs - his 4 stroke 9.9 is near 100!
The 22 dollar increase was probably from the redesign from 53 to 54. 54 was the first model year that resembled the 54-58 editions. Although my 54 lacks the sealed belly pan and rubber motor mounts of the 55 and up models. Maybe I'll get a 55 someday, but it's kinda hard to sell a motor that I have time into, that runs so good now. Thanks again for all the help! |
