Re-propping early 80's Johnson 25hp 2 stroke

[steelespike]

If it was planing with a 13 a 9" is probably way too small I would say nothing smaller than a 10".
I think an 11" would do really well.Your prop guy seems to think that as well.
If I remember right we don't have rpm or gps speed.
It would seem that $50 for a induction tach and a lightly loaded test run with your prop cleaned up,
rpm and gps speed would help narrow down prop selection,possibly to one prop.
I think your wot rpm range is 4500 to 5500 I would think we could get you within that with one prop.
You might be able to get us your full load rpm and speed.

 

[KC8QVO]

Well heres where I am at with it...

The boat/motor is 1000 miles away. I was up there last weekend so I was able to get the above mentioned specs - motor model # and prop. Right now our lake is 3 feet above flood stage - about 18" over top our main dock. As such - the maintenance crew flooded the boat on the dock to hold the dock in place. While I was up there all we had was the pontoon. This coming weekend I am going to get some other dock weights (probably rocks, as much work as that will be to pile them) and pump out the lund so I can put it on the water again. That will be my first opportunity to run it this season, and also the only week I am going to be there to work on the prop.

The tach sounds like a good idea, but if I buy a tack and go up to put it on and test it I am another year in the process because I'd have to drive up there with the tach, get some numbers loaded/unloaded and then waste the rest of the week on the prop I have then come back, get a prop, and wait all fall, winter, and spring to go up to put it on - unless I can get back this fall. That all totally defeats the purpose of getting a prop on there that we can use because the only time we can use it is when we're there and that comes and goes very quick.

I talked to one of the marinas up there on Thursday and let them know what I was up to/wanted to do with the props. They have not got back with me with any more info, but the typical issue there is they do not stock much in the way of motor parts/props because the range of equipment up there is so wide it would not make sense to stock everything under the sun. They order in as needed, and that can be a 2 week process - I'll be long gone by then. The best hope I have with the marinas is to get expedited service on the motor (gear oil change, tune up) within a few days, otherwise they are booked for a week to two weeks on normal service.

Just a quick rough guess on the speed - If I recall correctly - unloaded is about 25mph and loaded is about 18-20.

 

[KC8QVO]

Just thought of this... I do have a digital storage oscilloscope. If all the induction tachs do is sense the pulsing through the spark plug wires I can do the same thing on the oscilloscope then calculate the RPM's from there. the probe would be the same as the tach method - wire wrap on a spark plug wire. All that matters is the pulse count, not the strength of it.

 

[steelespike]

Sounds promising just bear in mind some motors have wasted spark ignition so you may have to calculate the results.
Also need to be aware of pulses for other wires.

 

[KC8QVO]

How are the induction tachs set up/dialed in with installs that are "tricky"? What adjustments are there (just amount of wire wraps and location of cable from induction point to tach)?

 

[KC8QVO]

Well I ordered a 10.5x11 prop. That will give me a couple to compare - what is on the motor and the new one. I will start there and see where it goes.

 

[steelespike]

While I have never installed a induction tach.Ypour 2 cylinder could;don't be much simpler.
Usually use the bottom wire Wrap the "antenna wire" around the plug wire avoid other
wires, get the antenna outside the cowl as soon as possible.
From reading here the most common problems are stray pulses and motors with wasted spark ignition.
Assuming no stray pulses my feeling is that the tach isn't likely to be off a few hundred rpm if set wrong
more likely would be off by thousands.
Having a good gps speed can help figure if the numbers make sense.

 

[KC8QVO]

I have a couple options for the probe. One is coax to the pick up wraps on the spark plug wire. The other is a standard 10x probe to the pick up coil. The 10x probe would be a safer route than the other, but may not work size-wise. I could put a resistor across the pick up coil with the coax method. Both, really, use coax - that is what is with a standard 10x probe - alligator clip ground and either pin center or clip attachment to clamp to signal line. The coax should cut down on interference/noise from other sources, more so than a non-shielded wire.

 

[KC8QVO]

I should be able to get some numbers here in the next few days. I am hoping to crank on the tach project by Monday. I have to tackle some set up tomorrow so chances are I won't have much tinker time. The boat is in the water (or, I should say, the water is out of IT, its off the dock and floating). I had an issue with the shipment of the prop I ordered. I am hoping to have that in the first half of the week. So until then I will try and get some numbers off the stock set up - as it sits now loaded/unloaded and once I clean up the prop run the same - loaded/unloaded.

I am trying to find the weight of the boat. So far I have not been able to download the old catalog off the website (lund site - archives). I am guessing it is the equivalent of a current model WC or A, 16ft and rated to 25hp, and ballparking the vintage of the boat as an '83 model. The rest of the weights - motor, fuel, gear, etc I can weigh and get a fairly accurate measurement. The boat is the only outliar.

I do see that the current model WC-16 is 325-335lbs. I would venture to guess the one we have is a good bit heavier. The floor boards are wood. The crew didn't pull them out when they flooded it on the dock, so the boards have been sitting in water for a few months. They have been sitting out for a couple days, but I'd say they're still fairly waterlogged. Just a guess - maybe a boat weight of 550lbs right now, no gear/motor.

 

[KC8QVO]

I haven't had an Internet connection in a few days so this is the first chance I've had to post back.

The oscilloscope method didn't net very good results. If I spent more time with it I could have done better I'm sure, but I didn't have the time or resources up there to work on it. So I pretty much just put the new prop on and left it.

I did notice that the speed, at the time I swapped the props, went from ~18mph to ~22mph with the new prop. On my trip this morning I was cruising down the bay at 27mph with a decent load - about 8 gallons of fuel, a spare 9.9 motor, and a bunch of fishing gear, etc. I had a decent tail wind, though, so the speed doesn't carry much validity. Though, at the same time, I could still make 22mph up wind so I'd say that is pretty respectable, considering.

*All speeds gathered with a mapping app on an iPhone (GPS), I'm not real sure how accurate it is.

I have some figures on fuel usage as well. Just a few rough numbers - a 6 gallon tank got me 37 miles today before needing to refuel. So at 22mph avg that is 6.1666mpg and .28 gal/hr.

 

[steelespike]

Guessing the slip at 13% the 11" prop at 22 mph is about 5219 rpm. Your 25 will use about 2.5 gph at wot
regardless if fully loaded or empty.I think your .28 gph is probably 2.8 gph. an acceptable number for a 30 year old motor.
At 27 mph guessing 10% slip the rpm is about 6192 You may want to throttle back to about 22 mph in those circumstances.

 

[KC8QVO]

After I posted that last night I realized I goofed up my calculation. 22mph and 37mi/6gal comes out to 3.57 gal/hr. Another calculation I ran came out to 3.78 gal/hr, and another 3.09 gal/hr. So that is all in the same range. From what you say steelspike - the fuel consumption there still seems a bit high, even for the age of the motor?

When I was at the marina the other day I had a nice conversation with the main tech and he seemed to think the 11 pitch was just right. He also commented about that vintage of Johnson motor in that they were known for smoking a lot. Maybe that is a contributing factor to the fuel consumption? After this season maybe we should just get it in to him for a good go-through/tune-up before next season.

 

[jauguston]

FWIW the factory installed prop for that motor is a 10d-13p. I have a 30 that is the same family and it came with a 10d-15p.

Jim

 

[Sea Rider]

A 2 stroke new engine consumes 12-13% of its stated HP per hour at wot, that's at near its full wot rpm numbers on flat waters. 10% is old fashioned and conservative, really don't know on wich basis is constantly reffered to. Is it a read it at the forums, word of mouth passed from boater to boater, stated on an owner's manual ? A older 2 stroker probably consumes bit more....

If you want to play with prop pitch must install an induction tach, go for a wot spin with you alone, with engine well trimmed and ballanced boat on flat calm no windy water cond and check your wot numbers, You must tach your engine to know your actual wot numbers with current prop and compare them to its factory stated wot numbers, then can go from there maximizing prop if in need to do so to take the best gloss out of that horse. That engine should run min 5, max 6 K ?

Happy Boating

 

[steelespike]

10% is simply an easy way to to figure typical wot consumption. Certainly easier to multiply in your head.
And accurate enough. I think a blanket 12-13% is just plain wrong. Checking Etecs 25,30,40 and 60.
25 at 3.09 gph,30 @ 2.65,40 @ 4.09,3.80,4.20, 60@ 5.29,5.31,5.20.

 

[Sea Rider]

12-13% is for plain 2 stroke, 2 cilinder Tohatsu, Nissan and rebadged Mariner, Merc up to 40 HP engines, Etecs are different animals, are injected, not plain carbed engines. All engine brands should have therir factory fuel consumption stated somewhere. 10% is simple math that could left you stranded in the middle of the ocean, oops, why did I ran out of fuel so fast..

Passing a Evi 25 to 35 HP is not a simple jet swap as one may think. Must change carb, which has wider fuel passages compared to 25 carb + reed valve plate assy which have slight larger ports to let more fuel in. Those 2 works hand in hand to deliver more power on same CC block.

You could maximize your prop performance to plane your boat with 4 passengers, will need a full bag of props to try different pitches along an induction tach till one gives near wot factory stated numbers.

Happy Boating

 

[steelespike]

You referred to"New"2 stroke engines I'll agree a carbed 2 stroke will use more fuel than a direct injected.
I think a 5 year old direct injected 2 stroke will use more fuel as well.In planning extended cruising you must calculate for worst circumstances
I don't think 1% will skew those numbers.
I think that" New" legal 2 strokes are all direct injected. I will agree that 50s and 60s 2 strokes are hogs but beginning in the 70s these numbers improved.
I don't think a 1 or 2% difference is going to strand anyone who has properly planned fuel reserves.
Averaging 8 new 50 hp Yamaha carbed 2 strokes The average gph was 5.6 a little over 11%.The worst motor was under 12%
I will grant you smaller carbed motors 25 will use more than 12% but I don't think extended cruise should be calculated with minimum input.

 

[jauguston]

A just FWIW having just bought a 1991 30 hp Evinrude I have learned the 30 hp and 35 hp engines are the same exact engine. Some where along the way they went from rating the power at the power head then changed to rating them at the prop shaft. I have the factory service manual for 1991 9.9 thru 30 hp. No mention of 35 hp.

Jim