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Old 23 June 2008, 21:32   #1
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Country: USA
Town: Negaunee, Michigan
Make: Avon SR6
Length: 6m +
Engine: 150hp Evinrude
Join Date: Oct 2007
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21 pitch to 17 question

My boat is at the shop and they want to switch me from a 21 to a 17.

What will that do for me?

Jeff
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Old 23 June 2008, 22:10   #2
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More revs and more holeshot. Your prop must have been way out before - that's a huge difference!!!
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Old 24 June 2008, 10:41   #3
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Uhhh, did they run the boat? Kind of hard to decide on what prop to go to if you don't know where it is to begin with.

21 to 17 is 4 inches; unless you were somewhere around 1000 rpm low at WOT, they're not thinking right.

In addition, that big a jump usually throws things like the general rules for prop changes out the window, and you have to resort to trial and error again.

But, Cod's right: You should hop up out of the hole quicker, probably lose a bit of top speed, and attain a much higher WOT rpm.

jky
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Old 25 June 2008, 09:08   #4
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Here is the scoop.

4.5 meter Achilles with a Johnson 50hp VRO.

I purchased the boat late last fall and was unable to put it in the water before winter set in. I at least was able to start it to fog the engine for winter and it started and ran pretty decent.

This spring I got it out replaced the plugs and ran it a few times before I started having difficulty at low idle. It ran as if it had a miss and was stalling at low idle. I was reaching top speed of 34mph per the gps though.

took it into the marina the first time. They said I had good spark and good timing but my oil looked contaminated. I only flushed the fuel tank when I got it, didn't consider the oil tank. After they changed oil they ran it and found that head temperature we only 82F. They found and replaced a stuck open thermostat. They explained that the operating temp should be 120F and that with the low temps combustion was not adequate. They charged me $36usd for a gallon of oil when I could have purchased it anywhere else for $8usd. Oh well. $130usd for that visit.

I put it in the water last Saturday and it was no better than before.

Took it back in Monday morning. Stopped in at lunch and saw it in the water with motor cover off. Stopped in after work and was told that I needed a new prop. The shop tach was only showing 2400rpm and is should be 5000rpm. I had a 21 pitch and they wanted to go to a 17 pitch. I told them I would stop in after work on Tuesday.

Tuesday afternoon I stopped in and they handed me my old prop and suggested that I only use it as a spare. They put different plugs in, reset timing and reinstalled some throttle linkage that they said someone had removed and put back together incorrectly. $200usd $109usd of that was the prop.

Ok so what happened? I asked. The shop said that the motor identification tags were missing, which I can confirm. So they based the year on the decals on the motor cover. So when I had it in before they did everything for the older motor specs. This time they set everything to the newer model. Apparently the previous service person made the same mistake because I only replaced the plugs with the same that was in it. I was warned to now Hang On!

Last night my wife and I went for a ride. Whoo Baby! What a difference! I could not get out to where I could open it up full because my wife does not like the rough stuff. But the low idle problem is gone and I did a couple of hole shots like Cod said and he was right!

At the moment I am happy. You know I wish they would have done an in water test in the first place. I can't imagine dealing with an inland marina. Auto shops regularly test drive. I was frustrated with one return trip. I wonder how an inland would have handled it?

I still have to post some pics. Maybe this weekend!

Jeff
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Old 25 June 2008, 11:16   #5
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Not hard to swap the prop back to try that out:

Remove the Cotter pin, use a piece of 2x4 to block the prop, undo the prop nut, taking care to note what order stuff comes off in so it goes on correctly. Change prop, making sure to get the thrust washer on correctly first; replace all te mounting stuff, torque as appropriate, and replace cotter pin.

Sounds involved, but shouldn't take more than about 5 minutes assuming you run into trouble. About 3 minutes if you don't.

You really should run it at WOT to check whether you're propped correctly or not. Eventually, you're going to hit flat water and want to open the thing up.

It sounded as if you didn't have a tachometer on the boat? If not, I'd suggest getting one, even if it's just for diagnostic purposes. Tiny Tach makes a little clip on jobbie that sits under the cowling (though I'm not sure how easy it would be to run the boat and check that thing...)

Anyway, hope all runs well from here.

jky
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Old 25 June 2008, 16:48   #6
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Yeah, 21" pitch is way too big for that set. 17" pitch is more like it. I used to run a 17" pitch on a smaller rib with 40hp (and about the same gear ratio as your 50hp), so with your engine and boat combination 17" pitch sounds not too bad.

Rule of thumb: 1" increase in pitch is 200rpm less. 1" decrease in pitch is 200rpm more.

This rule only applies for situations where te prop is not TOO big or TOO small for the engine... Otherwise it hasn't got te power to get up in the revs enough.
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