RPM question

[foots]

Hey guys,

I have a 1988 2 stroke Johnson 40hp engine, runs like a top. Recommended rpm is 4500 to 5500. It currently runs at 5440 rpm (properly trimmed) with a 10.125 x 13 Turning Point prop. This pushes my 1988 Traveler aluminum boat @ 31 mph. I am happy with this configuration. I also have a Solas 12.5 x 15 that came with the boat, but have no data on this prop. I changed it before I put the tach on... but based on current info, this would drop rpm drastically.

Now, because the rpm rating is a 1000 rpm area, is it better for the engine to bring the rpm closer to 5000, which falls in the middle of the rated rpm, or does it really matter... just as long as it stays in the rated area? This boat is fairly new to me and I am wanting to get it right.

Thanks for any info /advice.

foots

 

[steelespike]

Your numbers don't add up.I can almost guarantee you can't make 31 mph.
Is the speed by gps?

 

[SkiDad]

those turning point props run like the next size up - so your 13 is really running like a normal 15.

That is what I noticed on my boat with a 15 pitch TP - it would run 38 mph @ 5300 (2.0 gear box) which is negative slip. I compared it to a 17p i had and rpms/speed were almost identical across the board.

 

[foots]

Your numbers don't add up.I can almost guarantee you can't make 31 mph.
Is the speed by gps?

31 mph according to the gps on my Helix 5. That is with 3 deep cell batteries (I keep one spare in the boat... just in case) both live wells full, and me and my wife... about 400lbs between the both of us.

I was really surprised when, in preparation for my first post as I got the numbers off the props this morning, the Solas was numerically much bigger than the Turning Point. The Solas came with the boat when I bought it last year. When I ordered the Turning Point I followed the recommendations from this website. It was early last year so I don't remember the specifics.

But my question is not about comparing the two props to each other. All this is subjective and changes from boat to boat. I am looking for the optimal rpm. Is anywhere between 4500 and 5500 rpm ok for an older 2 stroke or is there a preferable rpm that it will be happy at and help to promote longevity?

SkiDad, thanks for your input.

 

[SkiDad]

i think most people believe the top 1/2 of the range is best. i would leave it b/c it gives you some room to add weight.

 

[Sea Rider]

Was 5440 wot rpm achieved with a 13 pitch while lightly, medium or heavily loaded ? It's better to run at max wot revs when lightly loaded, as soon you start to add weight wot rpm will drop accordingly. Only add more weight to my max wot revs so it doesn't fall below medium wot range as to maintain fast hole shot for that added weight.

Happy Boating

 

[foots]

@ Sea Rider, that was pretty much the most weight it will have in it without fish in the live wells... in fact, if I take out the third deep cycle battery (spare) it will lighten the load about 60 lbs.

 

[steelespike]

My info says 40 and 50 hp motors 1975 - 2005, are 2.42 gear ratio.?Another chart says E/J 40 and 50 2 stroke carbed. is 2.42.
Rpm 4500-5500.
Based on the 2.42 ratio and 12% slip 14'" turning point prop (1" added for cupped prop). rpm is 6400 at 31 mph.?
If we use 5440 with the same numbers we get about 27 mph.?

 

[Sea Rider]

Are you happy witht the combo performance up tp now, have good hole shot and nice top speed ? How about wot revs keeping extra bat and both live wells filled with water and fish. If wot rpm drops around middle or less wot revs could go for one less pitch to bump lost rpm, that's if like running at top wot rpm which is great doing so. If with slight over rev just throttle less, anyway whatever future weight you add in terms of another boater, more fuel , will compensate over revving at wot.

Happy Boating

 

[foots]

@ steelespike, assuming my tach is correct (it is a tiny tach on one cylinder that I multiply by 2) the engine idles between 400 and 450 rpm and WOT shows 2720. Gear reduction is 2.42 as you stated. I forgot to factor in the weight of the fuel, it is a 12 gallon tank and was approx 3/4 full at the time of the run. Am unfamiliar with the formulas/charts for figuring rpm ? prop size = speed, I am just stating what rpm the tach was showing, the speed the gps was showing, and the present prop on the engine. The how and why is above my pay grade. The tach accurately reflects the idle speed so I am assuming it is doing the same at WOT. It actually shows 2700 till trimmed properly. Maybe the amount of surface area/friction is less due the size of the boat itself? I do not think it i quite as wide as the newer 17.5 ft Bass tracker/Lowe style aluminum hull boats at only 70 inches wide and has a very slight V botttom. Other than that, I am at a loss to explain the speed/rpm discrepancy.
@ Sea Rider the WOT rpm of 5440 was fully loaded, except the 12 gallon tank was only 3/4 full and no fish in the live wells. Hole shot is "ok", top end is apparently above average. I usually pull it back just a little off WOT for fuel conservation.

I am now at work and will be away from home for a minimum of 28 days but I am thinking that when I get back home I will put the Solas prop on and compare the rpm/speed info.

Thanks for the advice and help guys. I appreciate your time.

 

[Sea Rider]

Is that a Tiny Tach with programmable firing, if so adjust to 2P or at a higher setting that can read current revs so as not to multiply half rpm numbers by two. 4.5 gal of fuel, won't add that much weight to your fully loaded run, full live well will certainly add more.

Happy Boating

 

[foots]

@ Sea Rider, I will check that out, thanks for the tip.

 

[Silvertip]

Having owned a couple boats with that exact engine I can assure you the prop you have is not what typically belongs on that engine. The "go to" prop is a 12-1/4 x 15P (cupped). 17P uncupped will give a tad bit more speed lightly loaded but is a detriment when heavily loaded. I also used that motor on a 17 Alumacraft Navigator with a 13P cupped stainless and suffered severe blowout. The engine was later replaced with a 75 HP Merc on the Navigator as the 40 was at the bottom of HP recommendation.

 

[foots]

@ Silvertip, the Solas prop I have is 12.5 x 15 uncupped. On my next set of days off I will compare the two and go from there. Thanks for your "real world" experience/info with an identical engine. I will find, through trial and error, what works best in my application. Should be fun, and a good learning experience. Appreciate your comment.

 

[foots]

Ok, it is apparently time for new glasses. When I pulled the Turning Point prop off the engine what I initially saw as a 3 turned out to be a 5. So the Turning point prop is in fact a 15" pitch prop, which explains why the rpm/mph numbers didn't "add up". I put the Solas 12.5 x 15 on and hopefully will get to take the boat out tomorrow to compare performance numbers. I am curious to see how it compares in both engine rpm and mph, to the Turning Point. I also ordered a 17" pitch today to see where that drops the rpm to with the boat loaded, as the engine reving just shy of the upper end of the power band when loaded right now with the Turning Point 15" pitch prop. I will post results between the two 15" pitch prop when I have them if anyone is interested.

 

[foots]

So I got the boat out today and tested both props under identical conditions... a full 12 gallon fuel tank, 3 Everstart 27DC-6 deep cycle batteries, both live wells full of water, trolling motor on the bow, 2 - 18lb anchors, ice chest, bimini top, fish finders, 6 rods, assorted tackle boxes and me at 175 lbs. There was a light chop on the water and several runs with both props were made. They were all identical. This is how the props performed on my boat, your mileage may vary.

The Solas propeller 12-1/4 x 15 (2311-123-15) has a really good hole shot, better than the Turning Point, and gets on plane quickly. With the engine properly trimmed it had my engine over reving by 180 rpm at 5680 rpm. Top speed at this rpm was 30 mph. The boat also sat deeper in the water when on plane and I had to use more throttle when cruising to keep it on plane. When I adjusted the rpm to 5500 the speed dropped to 26 mph.

The Turning Point propeller 12.25 x 15 (21311510) has a slower hole shot but a slightly higher top speed at 31 mph at 5480 rpm when trimmed. This prop also held the boat higher out of the water when on plane and would stay on plane at speeds where the Solas prop wanted to settle back in the water. With the exception of the hole shot, my boat handles better with this prop.

That's all I got. When the 17" gets here I will post the results if anyone wants to know.

 

[Sea Rider]

A 17 pitch should worsen hole shot and make OB to rev much less at wot that both tested 15 pitch props, just for the record post results.

Happy Boating

 

[foots]

Will do.

 

[foots]

Quick run down of test numbers and props. The 17" had a slower hole shot and a faster top speed and dropped rpm much closer to the 5000 recommended in the owners manual. The operating range is 4500 to 5500 but my original manual for the engine recommends 5000.

Turning point 12.25 x 15 = 5480rpm. 2264 prop rpm. 31 mph gps.

Solas 12.25 x 15 = 5720 rpm. 2347 prop rpm. 30 mph gps.

Turning point 11.75 x 17 = 5140 rpm
2124 prop rpm. 33 mph gps. Did see 34 a couple of times but mostly 33.

This results were on my boat with an 88 Johnson 40 hp engine.

 

[Sea Rider]

Seems that each OB manufacturer has their own "best all around performance" achieved from middle to max wot revs for current load. Preffer to run at full wot revs, can throttle say 3/4 and ride at middle of my wot rpm and at max wot revs full hammer down. Have more throttle/rpm range to play with as just achieving middle wot rpm at full hammer dowm..

Interesing test and the time for doing them to illustrate properly how props plays. So, which prop will you be staying for your type of boating ?

Happy Boating