92 Johnson, 120 HP re-prop

gatorsteel

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Sep 2, 2011
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I notice a lot of people have issue with props, or prop questions; Here is my information

I have a PERFECT running 120 HP Johnson long shft, mounted on the transom of a 92 Glastron 20' Cuddy Cabin. Its a heavy boat with Deep Vee.

I had been running a 13.25 x 17 pitch aluminum, but was only able to obtain 4700 RPM, with the perfect trim, 4900 RPM WOT. MAX MPH was 32 MPH at WOT. That was on a glassy day, perfect conditions. The most common was about 28 MPH, 4700 RPM, WOT.

I went to a 13.25 x 15 prop, aluminum prop. Put it on last night after work, dropped it in the water and was hoping for the best! It was choppy and the wind was against me. However, I was able to obtain 5600 RPM, with a top speed of 37 MPH per the same GPS. Half tank of gas, and three other people on board.

I was surprised, but very happy with the results. It stayed on plain better, and the engine did not seem to be working near as hard. Before with the old prop, it sounded like it was a 6 cylinder not struggling, but deeper throaty sound. With the new prop, it sounds like a 4 cylinder that is perfectly humming in tune.

Maybe this will help someone that was working on the prop issues.
 

gatorsteel

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Sep 2, 2011
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Re: 92 Johnson, 120 HP re-prop

I forgot to advise, I have a dolphin fin only. I do not have trim tabs. Also, the hole shot was even better, not spectacular, but better.
 

steelespike

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Apr 26, 2002
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19,069
Re: 92 Johnson, 120 HP re-prop

I would say those were excellent results.And exactly what should be expected
though very difficult to predict accurately.
You may also notice slightly less fuel use.
 

gatorsteel

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Sep 2, 2011
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Re: 92 Johnson, 120 HP re-prop

Do you think a SS prop would really make that much difference?
 

Animal Chin

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Sep 1, 2011
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Re: 92 Johnson, 120 HP re-prop

You gained approx 900 rpm from 2 inches less pitch? Thats 450 rpms difference per 1" of pitch. A far cry from what I've been reading and researching before getting my prop. From what I read you could expect 150-250 rpms per inch of pitch change.

I just ordered a prop myself and now I'm getting worried I may have went down to much pitch lol. I'll report back here when I get the new prop on because I think all these prop testimonials really helps people make a good decision when trying to work out their prop issues.

I have a 18 foot Sunbird Neptune dc with a 90 hp Evinrude and a 14x17 aluminum prop. I'm getting 32 mph on my best days and around 28 mph average. I'm only getting around 4200-4500 rpm wot depending on the load.

I just found a used 13 3/8 x 12 stainless steel prop on Ebay in great shape and at a good price ($70) and I was hoping it's just what I need.

Now going with the math I've been reading from all the prop matching articles every inch of pitch results in 150-200 rpm gain/loss. So 5 degrees of pitch loss should equal 750-1000 rpm gain at wot. I know the prop is a bit smaller diameter but I also understand that the stainless steel should be stronger and therefore result in less flexing and therefore less rpm gain.

I'm looking for better hole shot to give my skiers a better chance of getting out of the water faster. I just hope the smaller prop doesn't lead to cavitation or over revving.

Concerning stainless props, from everything I read people say to expect a better hole shot and 2-3 mph gain going from aluminum to ss. Makes sense because the ss fins are thinner therefore less drag and 5 times stronger so less flex. I hope it's true but we'll see!
 

gatorsteel

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Sep 2, 2011
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Re: 92 Johnson, 120 HP re-prop

Well, with the perfect trim, glass water I had reached 4900 RPM WOT. Now, we live in Panama City Beach, there is a storm approaching. When I took it out the other day, it was very very choppy and windy. I did reach 5600 RPM and gained speed too WOT. This was of course with a aluminum prop. I bought the aluminum to experiment with because their only about 120.00 vs what, 400.00 plus? I am hoping this storm will pass today, so I can go out tomorrow and test on some good weather.

I am thinking that your 13 pitch is going to be a little to big of jump from a 17, but you never know. The only way is to experiement, everyone can do their calculations etc, but in the end, no body knows what exactly all is involved with your particular boat. Your just a couple of hundred RPM under what mine was, but I am serioulsy thinking 15 pitch would have been more than sufficient. My real test will be on the day I can go out and test it when its the same water conditions. I will bet I will go over 5600 RPM, with even better speed. If that is the case, I will go ahead and buy that stainless prop and keep the aluminum ones for back up.

I have just recently destroyed one prop, I bought a H2 to pull it with and did not get a drop down hitch, when I hooked it up to the straight reese, it lowered the rear end of the trailer. I had trimmed up just like I did when pulling it with the ford expedition, but didnt compensate for that rear end being lower. so if anyone buys a new truck, make sure your ball is the same height!
 

Animal Chin

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Sep 1, 2011
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Re: 92 Johnson, 120 HP re-prop

Update*

Well it turns out the prop I ordered on ebay wasn't a 13 3/8 x 12 but a 13 3/8 x 17. That means, supposedly, the only difference between my old prop and the new one is 5/8 inch diameter.

Well I thought I would give it a go anyhow just to get an idea and wow, much better hole shot and gained 500 rpms. I also picked up about 2-3 mph, achieved 34 mph on the gps in rough waters. I think I'm going to stick with this prop for a while. Still have a bit of room for improvement but for a $70 ebay stainless steel prop, I'm pretty happy.
 
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