# evinrude 15 and 1236 how to get a lil more mph?



## erictetterton (Aug 23, 2013)

My setup:






88 15hp evinrude on a 1236 alumacraft with pods. I have 4 gallons of fuel, safety gear and a battery. With a few fishing rods and tackle, the boat runs 23mph with just me. That's decent for a 12' but I like to go fast if I need to out run bad weather or running late for a date lol. Is there anything simple I could do to gain a few mph? My prop is a 9&3/8" factory prop... Its in pretty decent shape


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## Johny25 (Aug 24, 2013)

Get a tachometer and find out what RPM you are running. Also 9 and 3/8 is only the diameter of the prop......what pitch are you running? You can move some weight around to try and get a little more out of it but 23 is pretty darn good for a 15hp. But if you can get a good RPM reading at WOT to us with the GPS reading we can maybe get you another mph or 2 possibly by changing props


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## erictetterton (Aug 24, 2013)

sorry I thought I included the pitch in the original post, its a 9. Yes I was thinking about getting the tiny tach to go on the motor and go with a higher pitch prop. What's the upper limit of the recommended rpm range for this motor?


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## Johny25 (Aug 25, 2013)

The RPM range is 5500-7000 for the 15hp in 88' peak HP being put out at 6250rpm and unless you are already turning around 7200 rpm with that 9 pitch you are not doing 23 mph #-o So either you are over spinning your motor or your speed reading is off.


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## erictetterton (Aug 25, 2013)

It states on the hub of the prop 93X9. I am using a lowrance mark 4 as a GPS so the speed reading should be pretty accurate. I have ordered a tinytach and it should be here next week


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## erictetterton (Aug 25, 2013)

I decided to go with the TTO trail tech tachometer because the reviews were just as good and it was $15 cheaper. It has a setting where u can put on 1 spark per revolution, 2 sparks per rev or .5 sparks per rev. I'm guessing since my motor is a 2 stroke I would go with the 1 spark per rev setting


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## Johny25 (Aug 26, 2013)

Correct. Get a tach reading and we can see where the numbers may be off


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## erictetterton (Aug 29, 2013)

Well I got the tach mounted today and took it out for a spin. At WOT I'm doing an average of 6550rpm and 22.5mph by GPS. The highest rpm reading was 6700 and that was 23mph. A little $32 tach is a great investment. I think I'm propped right


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## Johny25 (Aug 29, 2013)

Ok well here is what your numbers come out as....

You have a 9 pitch prop being spun by a 2.41:1 gear case at 6,550 rpm at 22.5mph which comes out to a 3% slip ratio. Now I have never seen a little motor like this have less than 10% slip ratio, fact is 3% slip ratio is better than anything I have ever seen on any motor and setup....including racing boats. If you understand prop slip then you know 0% slip is impossible because the prop is not traveling through solid matter. 

I do not believe you are lying about GPS and I can see your tach reading so my only other guess would be the prop is mis-stamped and it is really a 10 pitch. This would calculate to a 13% slip which is what is expected to see on this motor and setup, and 13% is a very good number at that for this setup. 

On a final note, you could go up another inch in pitch and get another mph or so out of her if you wanted. I would wanted the motor turning 6250-6500rpm tops


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## erictetterton (Aug 29, 2013)

Here's proof of the GPS reading at WOT. Do u think my prop could be bent a little and that would give it more pitch? Just trying to figure out where the #s are off


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## erictetterton (Aug 29, 2013)

also, if i went with a higher pitch prop would that increase or decrease the RPMs at WOT. I dont want to increase the RPMs anymore than they already are because im at the upper limit.


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## Johny25 (Aug 29, 2013)

The prop could be closer to a 10 pitch than a 9 pitch, could be mis-stamped or the tach and GPS could be both off just a little? I doubt your prop was bent or it would likely vibrate or shudder real bad. 

Hey is that a stainless prop? If it is it could have been worked on by a prop shop and could have had a pitch change done to it. They can change pitch on SS props about 1 step or one inch in pitch. It is a possibility....

Also going up in pitch will decrease RPM. So if you went up one inch in pitch you will typically drop about 200 rpm

Theoretically...you have a 9" pitch which you run at roughly 6600 rpm so a 10" would put you at about 6400....... 11" 6200 rpm and so on. Pitch is the theoretical distance a prop will travel through a solid mass per revolution. A 9" pitch will travel 9" in one revolution if it traveled through a solid mass with 0% slip.


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## erictetterton (Aug 30, 2013)

Very good explanation. I think its aluminum but I'm not 100% sure. It doesn't feel abnormally heavy like my SS prop for my 50hp. Is there any way I tell if its stainless?


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## erictetterton (Aug 30, 2013)

I tested the prop with a magnet and it stuck... so I have a stainless prop. That makes my day lol. So its likely been repitched to a 10


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## Johny25 (Aug 30, 2013)

Could have been? I won't put my life on it but it is a possibility. There is a formula to check pitch on a piece of paper, I have done it before to check pitch on a prop sold to me by Cabela's as it did not run anywhere near the RPM range I was expecting. Sure enough I did the test and it confirmed the prop was 2 pitch lower than it was stamped/sold to me as. My motor ran over 400rpm higher than expected and I was out of RPM range, I was pissed to say the least and will never buy props from Cabela's again even though they reimbursed me. Should of seen the look on the ladies face behind the counter when I showed up with my pitch test sheet, speed square and protractor and tried explaining to her why I wanted my money back :lol: 

Here watch this video and look below for the website for the sheet to copy https://www.youtube.com/watch?v=Uzv_m_Gm1es


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## shawnfish (Aug 31, 2013)

im way off base here but seeing the magnet stick to that prop surprised me because I always thought SS props were made of austenitic SS like 304 or 316 for corrosion resistance. not that other grades don't resist corrosion I just figured they 300 grade...


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## Johny25 (Aug 31, 2013)

I have an SS prop that magnets will not stick too.....looks like a mirror finish prop with plating on it. But I also have an OMC prop that is SS but came with factory black paint on it and it is magnetic. They make various grades of stainless steel....some magnetic and some not I guess.


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## erictetterton (Sep 1, 2013)

What do y'all think about putting a 20or 25hp on my setup? She's rated for a 15 and its got the flotation pods. Do u think it would be too much?


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## Johny25 (Sep 1, 2013)

You know I went back and looked at your boat in the pic and it looks like you have a long shaft motor? And your boat looks like it is made for a short shaft? If this is the case a jack plate would give you some more speed also.

Now to the 20/25hp.........You are going to obviously be over coast guard regs by putting anything over a 15hp on it and liability wise I would not advise doing so. But now that I have said that.........I would go with the 25 or even 30hp if I were to do it. Most 20hp weigh as much as 30hp and are usually made from the same block. You are at minimum looking at adding 40-50lbs onto your transom by stepping up to a 20-25-30hp motor. Would have to go with a 2 stroke to keep weight down also. 

Not saying to do it but that little boat would FLY! :mrgreen: should be able to attain speeds of 32-36mph possibly more depending on HP and setup. That is scary fast in a 12' tin with tiller control trust me :shock:


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## erictetterton (Sep 1, 2013)

I'm 100% sure its a short shaft motor. The cavitation plate may be a hair below the bottom of the boat, but not much. Yea I was thinking the 20s were the same motor at the 25 and 30s, just a detuned carb. If I'm gonna upgrade, I might as well go for the 30hp. I don't have to run it at WOT


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## shawnfish (Sep 2, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=327988#p327988 said:


> shawnfish » Yesterday, 11:13[/url]"]im way off base here but seeing the magnet stick to that prop surprised me because I always thought SS props were made of austenitic SS like 304 or 316 for corrosion resistance. not that other grades don't resist corrosion I just figured they used the highest grade.


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## shawnfish (Sep 2, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=328091#p328091 said:


> erictetterton » Today, 15:35[/url]"]What do y'all think about putting a 20or 25hp on my setup? She's rated for a 15 and its got the flotation pods. Do u think it would be too much?




I agree with johnny25 100%.... 

now I have this to add....the flotation pods provide enough square footage of foam to keep your boat(even if filled with water) floating provided you do not have more total weight than the max weight capacity listed on the coast guard tag on your boat

as long as your transom is solid and the brace or braces are also solid I say go as big as you want as long as you feel safe by doing so...that said remember if you have people with you in the boat when you use it you have their safety to consider also.....


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