WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

[SaltInMyVeins]

Hey this is my first post but I have been lurking for years and have found lots of useful information on this great site that has helped me work on my boat. I have over 35 years of shadetree boat and engine work under my belt. Glad to finally be participating.

I recently had my 1983 Evinrude 25 hp powerhead rebuilt (blueprinted, balanced, polished, intakes matched, exhaust ported, pistons ported, 35 hp rebuilt carb installed and rejetted, etc.), new water pump, new seals in foot, etc. Rebuilder estimates new power at 40hp. It runs like a pure dream. Idles great and cranks every time and doesn't cut off until I shut it off. The boat is an aluminum flatbottom 14' jon boat that is used for saltwater fishing and carries relatively higher loads.

I purchased a brand new (old stock) 12" (actually measures 11.5") pitch OMC SST cupped stainless steel propeller for my new ride (9.25" diameter, non-thru-hub exhaust). This prop turns 4500 rpm at WOT with a top speed of 31.5 mph. As a comparison, I tried my old, beat up, nicked up, Michigan Wheel aluminum propeller which is a 10.5" (measures 10.5") pitch. It spins up to 5280 rpms with a top speed of 28.5 mph. The sound of the motor at wot with the aluminum prop is harsh and sounds like it will come apart if its run for any length of time. The stainless prop sounds smooth and nice at wot. Speed measured with depthfinder and paddlewheel type sensor. RPM measured with temporary, simple digital tach with sensor wire that wraps around spark plug wire. Both tests performed same day with same load and no other changes. This is my second set of measurements that confirms earlier tests. The name plate on the engine states an operating range of 4500 to 5500 rpm. On a seperate day i carried a normal but heavier load with the 11.5" stainless prop and the rpms only got up to 4400rpm.

The rule of thumb according to Evinrude, is that a change of 1 pitch size, or 1" of pitch for this motor, equals 200-300 rpm change. Cupping can reduce rpm another 200-300 rpm. In that case, the rpm spread between the two props should be 400-600 rpm vs 780 rpm that I recorded. In addition, a stainless prop should turn up a little higher than an aluminum prop of the same pitch.

Question: Can I run with the 12" (11.5") stainless at around 4500 without damaging the motor? I like the way it sounds and accelerates. Also, would the reduced rpms extend the engine life? Would the extra hp achieved during the rebuild prevent damage or lugging? Or is the lower limit of the operating range sacred?

If not, how much should I change the pitch? My prop man tells me that these smaller props are difficult to alter without cracking.

 

[jestor68]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Your motor's rated horsepower occurs at 5000 rpm. Less than 5000 rpm and you're not developing full power.

An inch of pitch is 150-200 rpm on average when dealing with props of similar design and same material.

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Thanks for the reply. What explains the rpm and pitch results that I obtained above? They certainly do not correlate with your formula. Also: If I am not developing full power, is that so bad? The boat runs well and it may extend the life of the engine. Opinions?

 

[jestor68]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Thanks for the reply. What explains the rpm and pitch results that I obtained above? They certainly do not correlate with your formula. Also: If I am not developing full power, is that so bad? The boat runs well and it may extend the life of the engine. Opinions?

You are trying to compare the performance of a beat up aluminum prop with a good condition SS prop. An inch of pitch might account for 200 rpm. Changing from al to SS could account for another 200 rpm. The fact that the beat up al prop had lost a bunch of efficiency could account for additional rpm increase; the amount dependent upon how bad it was damaged. Was the AL prop cupped? If not, that would result in additional rpm change.

When comparing the performance of identical size AL and SS props, the SS prop will normally run less rpm(and more speed) due to it's greater efficiency; it grabs and holds onto more water per revolution, resulting in more work for the motor; hence the drop in rpm. The rpm can usually be gained back by raising the motor a hole.

The bottom line here is that you are only attaining the bare minimum(4500) of your recommended WOT rpm range. The sweet spot is 5000-5300 rpm.

If you're running an SST, the only accurate outcome is to compare it to another SST. If you put a Viper on there, it will affect the outcome because it's a different blade design.

That's what makes prop selection so much fun. It's not "my" formula. It's been the industry formula for the past 50 or so years.

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

That all makes sense. The old AL prop is most certainly less efficient. It has thick, dull blades and lots of nicks and gouges. The blades are reasonably consistent in pitch, which is amazing considering all the stuff I have hit.

Finding Stainless props for this motor is pretty tough. I will have to base my actions of whatever I can glean from the stainless 12" (11.5") prop.

So do you think I will damage the motor at 4500rpm at WOT? Or should I attempt to re-pitch the stainless prop?

Thanks

 

[Texasmark]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

+1. Additionally, the torque curve of the engine plays into the performance too. Where was it at the orig rpm and where is it at the new? Other thing is the hydrodymanics of the hull to the water. What changes when you get more speed? Do your losses decrease as expected, or does something else (torque) change to offset that and on and on.

Only way you know anything is to run it. Everything else is pure speculation based upon a person's experience and information obtained from reliable data sources.

My 2c,
Mark

 

[steelespike]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

I agree throw out the ratty props performance.keep it for a spare Technology wise not worth a repair but may serve to get to shore.
A good prop shop could work wonders with a ss prop. The can remove pitch,remove or modify cup with excellent results.
Its safe to do some testing but it appears you need to sell or modify the ss prop.
Your antivent plate (just above the prop needs to be about even with to about an inch above the bottom of the transom.
This may be limited by the non through hub gear case.A 2" increase in height could add up to 2 mph. and some rpm.

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

OK, it sounds like my next step is to raise the motor again. I raised it once, which made the plate almost dead even with the bottom. I may come up another 1", I guess. The boat is in my shop getting a coat of deck paint this week, so the first chance I will have to run it will be the weekend. I will report back my findings.

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Update: I am still painting because it always takes longer than you think. My theory is that I would rather have the boat out of commission for these 2 weeks early in the year rather than later in the middle of fishing season. Also, I decided to paint the motor as well. Pics to come.
Anyhoo - How much should I raise my motor? The reason I ask is that I need to drill new mounting holes everytime I raise it. I dont mind this, because I will go back and fill the unused holes with structural epoxy filler. BUT - I need to determine the spacing. The last time I raised it, it came up 3/4". This brought the cavitation (ventilation) plate even with the bottom. Any help is appreciated. BTW: The transom is aluminum skinned plywood and is in pretty good shape, not rotted.

 

[Texasmark]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

3/4" is the normal hole spacing. Realizing that 1" is not that dimension you have to decide which it will be. Either is/should be better than what you have.

Mark

 

[Sea Rider]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Don't have tech specs for that model, know that 9.9 & 15 share same CC, 9.9 is just reed valve and carb restricted. If 25 & 30 share same CC how can you rebuider state the engine has 40 HP punch if you haven't change pistons ? The block from the 25/30 should be completely different as of 40 HP, so impossible to place bigger pistons. Probably you have just a 30 HP in mint & top runing conditions. Being an old non through hub prop, will have market availability limitations to pick the best one for your set up.

Happy Boating

 

[steelespike]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

I believe the 83 has a 1.75 gear ratio the calculator says you can't do 31.5 at 4500.
Using 10% slip 31.5 mph indicates about 5600 rpm. a more reasonable number.
If thats accurate that may be an indication why the motor sounded like it was coming apart with the old prop.

 

[steelespike]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

I believe the 83 has a 1.75 gear ratio the calculator says you can't do 31.5 at 4500.
Using 10% slip 31.5 mph indicates about 5600 rpm. a more reasonable number.
If thats accurate that may be an indication why the motor sounded like it was coming apart with the old prop.

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Thanks for all the input.

1) I will raise the motor 3/4"
2) I suspect my paddlewheel speedo is a little off. I will take a GPS next time and calibrate it.
3) The 40 hp estimate was figured this way: A) In 1983, the 25 and the 35 shared pistons, rods and crank. The only difference was the carburator. So the rebuilder installed a rejetted 35 hp carb. B) The other 5 hp came from the matched flows in the intake manifold, the balancing, the opening of the exhaust ports and drilling 2 ports in the exhaust side of the piston skirt. Its and estimate that makes some sense to me, but I would like to hear your thoughts.
4) I spent some time refurbing the aluminum 10.5" pitch prop. I filed down the leading edges, removed nicks and sanded out surface gouges. I also put a high angle (semi-sharp) edge on the 12" SST. Gonna run both on the next sea trial.

Pics for reference:
Topsides painted w non-skid on horiz surfaces.


Stern view


Mounting bracket. Actual shaft length is like 17.5".


Ventilation Plate and Hull Bottom. Motor is being sanded for painting.


Refurbished Alum and SST Prop

 

[steelespike]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Looks good hope the results are as good.

 

[torbjorn]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Operating range with 50:1 is 5000-6000 RPM, you need 5500-6000 to develope 35 hp (40 hp as
estimate is a pipe dream). Best props for that motor are Stiletto (designed by Jim Booe), you need
a pitch low enough to get the right RPM. The motors hold together and run well but hit a
wall at about 7000 RPM because the 1951-style flat plate reed system. Above 6000 RPM, run 25:1.
Oil. Stick to Evinrude, Mercury, or Yamaha oils to be safe. Cheap oil may lead to carbon buildup and poor
lubrication. The cleanest burning oils I've used are Evinrude XD-100 synthetic (in OMC 15 hp, 35 hp, 60 and 75 hp
racing motors) and Red Line synthetic racing oil.

Hey this is my first post but I have been lurking for years and have found lots of useful information on this great site that has helped me work on my boat. I have over 35 years of shadetree boat and engine work under my belt. Glad to finally be participating.

I recently had my 1983 Evinrude 25 hp powerhead rebuilt (blueprinted, balanced, polished, intakes matched, exhaust ported, pistons ported, 35 hp rebuilt carb installed and rejetted, etc.), new water pump, new seals in foot, etc. Rebuilder estimates new power at 40hp. It runs like a pure dream. Idles great and cranks every time and doesn't cut off until I shut it off. The boat is an aluminum flatbottom 14' jon boat that is used for saltwater fishing and carries relatively higher loads.

I purchased a brand new (old stock) 12" (actually measures 11.5") pitch OMC SST cupped stainless steel propeller for my new ride (9.25" diameter, non-thru-hub exhaust). This prop turns 4500 rpm at WOT with a top speed of 31.5 mph. As a comparison, I tried my old, beat up, nicked up, Michigan Wheel aluminum propeller which is a 10.5" (measures 10.5") pitch. It spins up to 5280 rpms with a top speed of 28.5 mph. The sound of the motor at wot with the aluminum prop is harsh and sounds like it will come apart if its run for any length of time. The stainless prop sounds smooth and nice at wot. Speed measured with depthfinder and paddlewheel type sensor. RPM measured with temporary, simple digital tach with sensor wire that wraps around spark plug wire. Both tests performed same day with same load and no other changes. This is my second set of measurements that confirms earlier tests. The name plate on the engine states an operating range of 4500 to 5500 rpm. On a seperate day i carried a normal but heavier load with the 11.5" stainless prop and the rpms only got up to 4400rpm.

The rule of thumb according to Evinrude, is that a change of 1 pitch size, or 1" of pitch for this motor, equals 200-300 rpm change. Cupping can reduce rpm another 200-300 rpm. In that case, the rpm spread between the two props should be 400-600 rpm vs 780 rpm that I recorded. In addition, a stainless prop should turn up a little higher than an aluminum prop of the same pitch.

Question: Can I run with the 12" (11.5") stainless at around 4500 without damaging the motor? I like the way it sounds and accelerates. Also, would the reduced rpms extend the engine life? Would the extra hp achieved during the rebuild prevent damage or lugging? Or is the lower limit of the operating range sacred?

If not, how much should I change the pitch? My prop man tells me that these smaller props are difficult to alter without cracking.

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Thanks torbjorn. Would you recommend Boyesen reeds?

The plate on the 25 hp engine says 4500 - 5500rpm operating range. Not sure what the 35 hp plate says.

This engine has the split gearcase with non-thru-hub exhaust with splined shaft. Stilletto website does not show any in SS or Al. Sounds like you may be thinking of a different year. Or maybe Stilleto has a seperate custom prop website?

Although I want to go fast, I need to balance that with longevity and toughness. It will be a workhorse motor on a fishing boat. So I would probably not run it much over 5000 unless it sounded real good. At 5280 rpm with the old alum 10.5 prop it felt like it was screamin.

 

[Sea Rider]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

If both 25 & 35 HP models have same CC, usually the converted 35 HP runs slightly with -150-/200 less top rpm compared to the 25 HP. Try to get factory specs to wot on the max rpm safe parameters. Probably runs 4300-5300 rpm ? You should get 35 HP near max stated rpm at wot. Boysessen Reeds are used mostly for racing at high speeds, if just doing recreational boating standard reeds will perform well.

Test your boat firstly with engine sitting straight on transom, place a enough heavy mate at wheel, if with mechanical trim, trim engine to be prependicular to water level once boat is floating, you could seat near transom, ask driver to full throttle, once on plane pull your head out transom, check water level passing through lower tail. By visually inpsecting and not blind guessing, will know if in need to raise engine and by how much. Try different wooden shim heights untill this height position is perfectly matched.

Your ideal water flow height for your boat/engine, should be slightly under upper deflector water plate, check pic and video : Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Happy Boating

 

[SaltInMyVeins]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

Thanks Sea Rider.

The ventilation plate is completely submerged and not visible when running. Also, I have added a vent plate fin (Sport Marine Technologies) to try to raise the stern. This too, was submerged and invisible when running. It sounds like I need to raise is quite a bit.

Instead of drilling a bunch of holes in my transom, I am considering a manual jack plate. I have seen some really simple ones made from 1/4" 3x3 aluminum angle. The set back would be about 4". Would this help?

 

[Sea Rider]

Re: WOT Engine RPMs and Prop Pitch for Rebuilt 25 hp Evinrude

The lower anticavitation plate must always run submerged and ride paralell to water level when boat is on plane, if exposed will experiment prop aireation at close turns or at choppy waters, but perfect for top end speed at flat straight water course.

If anticav plate rides at an angle will experiment tail drag, much worst drag if using a large doel fin. Probably not big deal with large & punchy engines, but noticeable on smaller ones. Did you mange to visually check the tail height where water flow is passing through ?

Happy Boating