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Author Topic:   SPORT 15 with Evinrude 48 SPL
Oldslowandugly posted 06-01-2015 11:17 PM ET (US)   Profile for Oldslowandugly  
I have a 1985 Boston Whaler 15 foot Sport with a 1993 Evinrude 48spl. The motor came with an OMC 390850 12.25 x 15 SST prop. On this excellent hull that prop allows the motor to easily over-rev, right up to 6100 rpm causing the rev-limiter to kick in, and top speed is sad. I went to the dealer and asked for the next higher pitch prop, in aluminum only, just in case I didn't like it. They sold me the 386841 11.75 x 17 aluminum STD prop which gave a huge improvement in top speed, and it planed well, loafing along at 4000 rpm. My concern is that with this prop I can only turn 5000-5100 rpm at WOT. The motor's range is 4500-5500, and I am right smack in the middle. Is that OK or do I need to run at 5500 rpm? To get up to the max rpm of 5500 I figured I needed a prop with 16 inches of pitch but no one offers such a prop except PowerTech. I thought that would be perfect but they describe that prop as having 20 degrees rake and is that OK on a 15 foot Whaler? I was advised against such a "high rake" on a stock boat yet eyeballing my OMC SST it appears it too is about 20 degrees rake checked with a protractor. Somewhere on here I read that PowerTech's props are cupped and act like they have 1 inch more pitch than advertised. That would put me right back where I am now with my 17 inch prop. I would buy the 16 inch PowerTech if it would raise my rpm's, and I prefer stainless anyway. But should I even care if I'm turning 5000 rpm now with no apparent problems? It is usually just myself and my Grandson, with fishing gear, for 10 years now without any mechanical issues.
tedious posted 06-02-2015 08:19 AM ET (US)     Profile for tedious  Send Email to tedious     
OSU, it all depends on how much you want to optimize things. Your current prop is doing fine - you're in the proper RPM range. That said, if you want to take the time and trouble, you can optimize your performance by going to modern stainless steel prop with a pitch that will put your RPMs up at the top of the RPM range. You could even go a bit over - as you found out, the motor does not grenade if you go over 5500.

A modern, cupped stainless prop will grip the water better and you'll get better performance and mileage. The cupping will make it act as if it has more pitch; I'm mostly familiar with Stiletto props, which act like they have 2" more pitch, or even more, but checking Dan's Discount Props, I do not see a Stiletto in an appropriate size.

You might want to try the PowerTech SRG (link: I have no experience with PowerTech, so maybe others will have a different opinion. I'd probably start with a 15-pitch, but you may actually even end up with a 13 - it's trial and error.


tedious posted 06-02-2015 08:51 AM ET (US)     Profile for tedious  Send Email to tedious     
OSU, re-reading my response, I left out a couple of points:

1) don't overthink about the rake - most of the props you'll be looking at are designed for recreational boats, and will work fine unless you have problem you're looking to correct.

2) propping so you are up at the top of the RPM range will help ensure the motor is adequately cooled, and will improve acceleration and overall performance when you are heavily loaded, should the occasion arise when that matters. It also will typically help you hold a plane at a lower RPM - just a little, but that matters in a 15.


Oldslowandugly posted 06-02-2015 09:36 AM ET (US)     Profile for Oldslowandugly    
Tim, thank's for the quick response. That was my concern, if I take more people out, a bit more bite would be nice just to be safe. I saw that the Stiletto Triad 2 came with 16 inches pitch, but no one has it. The PowerTech looks like the best bet, in fact the only bet. They say it compares to the OMC SST. I had contacted them but they danced around the question and said I could return the prop for another size pitch if I needed to. I'd rather get it right the first time. I feel like Goldilocks- this prop is too hot and that prop is too cool- but which one is just right? As for cooling my temp gauge at WOT is always 120ยบ, in fact the motor eats thermostats and seems to run cold mostly.
tedious posted 06-02-2015 11:17 AM ET (US)     Profile for tedious  Send Email to tedious     
You know, I did not think to check the Triad - for some reason I was thinking the 48SPL was an intermediate gearcase motor, but going by the Dan's Discount Props site, I see that the Triad will work. And it's available in a 15 and 17 pitch.

Based on my past experience with Stilettos the Triad 11.75-15 might be a very good one to try. The price is pretty decent too:


Tom W Clark posted 06-02-2015 11:56 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The Stiletto Triad is not offered in 16" pitch, and if it was, it would be too much pitch for this boat/motor. You want the 11-3/4" x 15", part # C802415

There is a special version of the Triad for the OMC motors with more blade area. Do not confuse it with the other 3.5 Triad model.

The Stiletto Star will also work very well, again, 15" and the OMC version, part # C802215

In either case, you need a C-012 hub kit for use on the Evinrude 48 SPL

Performance if the boat will be greatly enhanced if the motor is raised on the transom from its lowest setting, which is almost certainly where it was mounted back in 1985

jimh posted 06-02-2015 01:15 PM ET (US)     Profile for jimh  Send Email to jimh     
Re where in the range of full-throttle engine speed one ought to be with a particular boat, engine, and propeller combination: there is a general trend to recommend choosing the propeller so the engine can accelerate to a speed in the upper half or upper third or even upper quarter of the recommended full-throttle engine speed range.

The range of engines speeds recommended by the manufacturer are those engine speeds when the engine will be producing its rated power or very close to its rated power. The exact relationship between the peak of power produced by the engine and its speed is not clearly defined by this range. Some manufacturers define a narrower range of engine speeds to be an optimum range. The inference I make is that this narrower range likely represents the range in which the peak horsepower is produced, which may be greater than the rated power

To get a feel for how the recommended full throttle engine speed range and the optimum range (when recommended as well) can compare, look at the ratings given by one engine manufacturer in my article at

To illustrate the various speeds, let me use my own engine as an example, a 2010 E-TEC 225. The manufacturer data is as follows:

Recommended Operating Range = 4,500 to 5,800-RPM

Optimum RPM Range = 5,500 to 5,700

Rated HP RPM = 5,150

Based on this data, a propeller would be chosen so the engine speed at full throttle would be in the range of 5,500 to 5,700-RPM for the optimum set-up, but propellers that permitted engine speeds of 4,500 to 5,800-RPM would also be acceptable.

The range of 5,500 to 5,700 is in the upper quarter of the wider range of engine speeds recommended. The propeller I have chosen usually allows the engine to accelerate to those speeds under normal to heavy load conditions. Under very heavy loads, the maximum engine speed may be just below that optimum range, and, accordingly, under light loads the engines speed will be higher. On that basis, I feel that the propeller selection has been made with good agreement with the manufacturer's recommendations for engine speed at full throttle.

Oldslowandugly posted 06-02-2015 11:56 PM ET (US)     Profile for Oldslowandugly    
So, yes I should be operating at a higher rpm. That said, how should I do that? If OMC offered a 16 inch pitch prop I would be right there. But they don't so now I have to either find a 16 incher or use a 15 incher that ACTS like a 16 incher, right? Tom, why would the Stiletto 15 inch work when my OMC 15 inch SST over revved? Cupping? Isn't the SST cupped? The blade faces are concave, and have some rake. I think it should have acted better. Then I read that the Powertech props are also cupped, and act one or even two pitches bigger. Yet the Powertech site says their SRG3 prop is "factory equivalent to SST". But these manufacturers don't advertise if they are cupped, double cupped, or not at all, just that they are "faster", "better", etc. This is what's eating me- the supposedly cupped OMC 15 inch SST prop revved too high, yet the plain jane aluminum 17 inch prop under revved. So either the Powertech 16 inch should be spot on, or I need a 15 incher so cupped that it acts like a 16 incher. Am I on the right track?
tedious posted 06-03-2015 07:45 AM ET (US)     Profile for tedious  Send Email to tedious     
OSU, you are mostly on the right track. Remember also that there is more to props than pitch - blade area matters too. So that's why when you originally compared the 15-pitch SST to the 17-pitch STD aluminum, your revs changed more than can be accounted for with a 2" change in pitch.

The SST is slightly cupped, but nowhere near what the Stiletto is. The additional cupping acts like about 2 inches more pitch, or maybe even a little more, not 1. Now if you were just worrying about pitch numbers, that would concern you, based on your experience with the 17-pitch aluminum, but there's more to it than that.

As you were hitting 6100 with the 15-pitch SST, it's likely the Triad that both Tom and I suggested will get you right around 5500. And even if you're hitting a little more than that, I would not be concerned - that motor will take it fine, and as you saw, even 6100 does no harm in the short term.

Tom is also quite correct about raising the motor, if you currently have it all the way down. Once you install the Triad you'll be able to lift it to one hole down from the top - that's where I run on my 15, with a Stiletto Advantage.

Seriously, get the Triad - you'll be very happy.


tedious posted 06-03-2015 07:49 AM ET (US)     Profile for tedious  Send Email to tedious     
Sorry OSU, just after re-reading your last post, your impression that the SST is cupped is the source of your confusion - the SST is cupped, but just barely. I went from a 19-pitch SST to a 17-pitch Stiletto Advantage when I was running a Johnson 70 on my 15, and my WOT revs actually dropped a little.


Oldslowandugly posted 06-03-2015 11:12 AM ET (US)     Profile for Oldslowandugly    
My head is hurting but I am learning all I can from you guys. Thank you for taking the time to educate me. I think before I make any moves I will take my friend with his GPS out for some test trials and record exactly what gives. This way I will know speed vs rpm, planing speed, etc. Thank's!
jimh posted 06-03-2015 12:14 PM ET (US)     Profile for jimh  Send Email to jimh     
Another Rule of Thumb suggests that propellers with cupped blades be considered as having more pitch than their rated pitch. In my instruction for use of my PROPELLER CALCULATOR, I recommend:

Add one-inch to the PITCH for any propeller with cupped blades.
Tom W Clark posted 06-03-2015 07:53 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
All modern outboard propellers are cupped for outboard motors this size and up and have been for decades. Do not add one inch of pitch when using a propeller calculator. That is bad advice.
Tom W Clark posted 06-03-2015 07:53 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
There are dozens of different models of the stainless steel propellers that can be used on a 1993 Evinrude 48 SPL. The SST is just one of them, and not the best in my opinion.

PowerTech! alone makes no less than 15 different models of stainless steel propellers to fit this motor. I would not choose a PowerTech! model that simply tries to be like an SST.

All of those different models of propellers will behave a little differently on your boat, even if they are the same pitch. That is why one much always consider the model of the propeller when selecting the pitch.

dg22 posted 06-03-2015 09:31 PM ET (US)     Profile for dg22  Send Email to dg22     
The add 1 inch to the pitch for SS (cupped blade) props is a rule of thumb that I've seen noted in the past when choosing a Stainless Steel propeller over an aluminum propeller.
Oldslowandugly posted 06-03-2015 11:30 PM ET (US)     Profile for Oldslowandugly    
So today I went fishing and made some observations. This Whaler 15 will hold on plane at 3000 rpm, but is best to cruise at 4000 or so. At 4500 it's going pretty fast, and at 5000 I don't let my Grandson go that fast. At that point the motor is maxed out and thankfully so. At that speed, wind, waves, current, nothing slows this baby down. At 4000 rpm and above the bow wave is at the 2/3 point of the hull length. At 3000 it is at the mid point. No GPS yet, maybe tomorrow, then I can get some honest speed numbers. I know in a little boat like this with such little free-board any speed seems exaggerated, but other guys in boats have commented "man that thing moves!". Maybe I'm just getting old, but I'm curious to know for sure.
jimh posted 06-09-2015 11:57 PM ET (US)     Profile for jimh  Send Email to jimh     
If a propeller calculator like the one I have provided is used with a propeller with cupped blades, and no allowance is made for the cup, you often get results with negative values for the SLIP. Adding one-inch to the pitch for a propeller with cupped blades when using my Propeller Calculator will tend to avoid results like negative SLIP values. But feel free to use my Propeller Calculator any way you like. If negative values for SLIP don't bother you, and all the propellers you compare have cupped blades, it really makes no difference. The Propeller Calculator is not an oracle; it just crunches some simple algebraic relationships to solve for the missing variable.
Oldslowandugly posted 06-10-2015 09:24 PM ET (US)     Profile for Oldslowandugly    
Tested with GPS today. Average of 2 passes both with and against wind/current. 28 mph with 2 big butts and lots of fishing gear. 30 mph just myself and my gear. Wind/current made a 2 mph difference. The 2 big butts made for a big effort to plane, but once up, she ran great. What a nice running little boat, but I could really use less pitch.
Tom W Clark posted 06-10-2015 09:29 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
What was the engine speed at 30 MPH?
dgoodhue posted 06-10-2015 10:08 PM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
28-30mph seem a little slow. I would check the motor height. It very well could be mounted with the blind holes in the engine bracket and is in the lowest poisition. Classic 15' whalers should be mounted in either 2 or 3 holes up position depending on the prop.
dgoodhue posted 06-10-2015 10:10 PM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
If it's mounted in the lowest position currently, when moving up to the 2 or 3 hole position the boat will plane off easier and the bow won't go as high on the transition to plane.
jimh posted 06-11-2015 09:58 AM ET (US)     Profile for jimh  Send Email to jimh     
We can use my other calculator of boat performance, my Crouch's Calculator, to estimate the weight of the "big butts and lot of fishing gear."

First we consider the boat went 28-MPH with "two big butts." Than we compare the speed with just one "big butt" and one set of gear, 30-MPH. This means the weight of one big butt caused the speed to decrease to 28 from 30-MPH.

First pass we use these inputs


The calculator predicts the weight would be 2210.2-lbs

Then we use these inputs


The calculator predicts the weight would be = 1925.3-lbs. This suggests that the weight of one unit of "big butt" and "fishing gear" is

2210.2 - 1925.3 = 284.9-lbs

Oldslowandugly posted 06-12-2015 09:47 PM ET (US)     Profile for Oldslowandugly    
Well my butt is an even 200, and the other almost the same. RPM was 5000, 5100 with the current. Does the calculator figure with a bare hull or loaded with stuff? When I add up everything, gas, console, seats, etc, and get 1000 lbs of stuff, 400 for butts, bare hull 500/550, and it totals almost 2000 lbs. So methinks your calculator is very accurate. About raising the motor- my ventilation plate is even with the bottom of the boat, and indeed is on the lowest hole. Raising the motor would have to wait until this fishing season ends or before next season begins for some test and tune. I have Dave Gerr's "Propeller Handbook" and with it I deduced a 17 inch pitch would get me 30 mph. But I probably have a lot of stuff on this boat, holding it back, since it is foremost a serious fishing boat.
Tom W Clark posted 06-12-2015 10:58 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Given the loads this boat will be carrying, my recommendation of the Stiletto Star is reinforced.

Given the discontinuation the Stiletto brand, I checked with the factory and there are only seven C802215 Stars left. Get 'me while you can.

This is a very good prop. I have used one personally on an OMC two cylinder 40 (slightly detuned version of this motor) and it was good for 36 MPH at 5500 RPM on a Sport 13.

Oldslowandugly posted 06-13-2015 06:47 PM ET (US)     Profile for Oldslowandugly    
Tom if I go with that prop then a 15 inch pitch would be best? If it very cupped would it act like a 16 or 17 incher? The cupping would also allow me to raise the motor, which reduces drag, and also resists ventilation, right? I know what cupping is, but could you explain how cupping works? I know at speed it acts bigger but at low rpm's does the prop act like it's rated pitch or is the affect across the board? If that is the case why say it is a 15 inch pitch when it acts like a 17?
Oldslowandugly posted 06-13-2015 07:10 PM ET (US)     Profile for Oldslowandugly    
Just so you understand the amount of stuff I squeezed into this craft, here are a few pics. The flip up mahogany bench at the stern covers two six gallon gas tanks and two watertight boxes of tools, parts, emergency fishing tackle. I have since removed the seat at the stern and use type-IV cushions. A lot of stuff, but all necessary.
Freddy posted 06-13-2015 07:47 PM ET (US)     Profile for Freddy  Send Email to Freddy
Oldslowandugly posted 06-14-2015 10:29 AM ET (US)     Profile for Oldslowandugly    
Thank's Freddy, I don't know how to get the link highlighted like that.
Tom W Clark posted 06-14-2015 02:56 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
All modern outboard motor propeller are cupped, and have been for decades, so it is academic to talk about cupped vs. uncupped propellers.

Cupping increases the water pressure on the trailing edge of the pressure side of the blade thus increasing its loading at that point. Think of you driving your car on level ground and then the road abrupty starts going uphill. When you start to go uphill, you feel compressed down into your seat.

Small tube Stiletto propellers have typical trailing edge cupping but are also concave across the entire face of the blade to a greater extent than most other propellers. This is referred to as progressive cupping and it also creates more of a foil shape to the blade.

Like airplane wings, propeller blades are foils too and create pull ("lift") in addition to thrust created by the angle of attack (pitch)

Tom W Clark posted 06-14-2015 02:57 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I am confident the 15" Star is appropriate for your boat. I know this in part by process of elimination. There is no Star with greater pitch and the next lower pitch is 13" and I am sure that will not be enough.
Peter posted 06-14-2015 07:45 PM ET (US)     Profile for Peter  Send Email to Peter     
I had a Super Sport 15 with a Johnson 48 SPL mounted as low as it could go spinning an aluminum 11 3/4 x 17 OMC propeller. With just me and a full 6 gallon fuel tank the Super Sport 15 would hit 37 MPH as indicated by GPS in a two way run. I have no idea how fast the motor was spinning because the boat didn't have a tachometer.
Oldslowandugly posted 06-15-2015 08:49 AM ET (US)     Profile for Oldslowandugly    
Peter, when I first tested this boat it was also stripped down bare with the same prop. Even into a stiff wind with a medium chop it flew like a bottle rocket. The tach still read 5100 but it felt much faster than it does now, no doubt due to the weight difference. Tom, assuming I get the 15 inch prop, how would it act at 2000 rpm? Right now 2000 gets me a smooth fast idle and the 5 mph which I need to do in no-wake zones. The OMC 15 inch prop was hard to hold down because it wanted to rev and lift the bow.
jimh posted 06-15-2015 12:51 PM ET (US)     Profile for jimh  Send Email to jimh     
Propellers are often reworked by propeller shops. The best propeller I had for my old engine was an Evinrude SST that a propeller shop reworked to add more cup. The pitch stamped on the propeller was "15" but it acted more like a 16 or 17-pitch.
jimh posted 06-15-2015 12:59 PM ET (US)     Profile for jimh  Send Email to jimh     
All modern propellers tend to have progressive pitch and cup, so it is not particularly useful to talk about pitch as if it were just one measurement of a propeller. The rated pitch given by the manufacturer is an index into their line of propellers with the pitch number being more of an effective pitch or a collective effect of pitch rather than some particular unique physical parameter. Exactly how a manufacturer assigns a pitch number to a particular propeller they make is not completely uniform or consistent across the entire propeller industry, across all models, across all manufacturers, or even across the same brand and model due to inconsistencies in manufacturing. Pitch is a nice concept and useful for comparing propellers, but it is difficult to measure. For that reason, I prefer to not say a propeller is a "17-inch pitch" but rather to say a "17 pitch."

One "17" pitch propeller might let an engine accelerate to 5,500-RPM and another model from another maker might only accelerate to 4,800-RPM. Are the two the same pitch? Who knows? About all you can know is, within a particular manufacturer and in a particular line of propellers, a 17 pitch will have more pitch than a 15 pitch model and less pitch than a 19 pitch model.

Oldslowandugly posted 07-02-2015 03:47 PM ET (US)     Profile for Oldslowandugly    
OK, since I was suffering from information overload, I went and found a prop shop in my area for help. I took the 15 inch SST prop and asked if they could change the pitch to 16 inches. They said no, they don't do that on outboard props, but they could cup it. I explained my situation and they said the cupping would make it act like a 16 incher. Fair enough. I picked it up today, installed it, and tested it. My max rpm-5600-awesome! It planes easier, feels like it goes faster, and the motor sounds quieter. I will get the GPS speed later and know for sure. Many thanks to all who contributed info, I learned a lot. For anyone on Long Island, Lawrence Johnson Machine Works in Freeport did the work and they are super nice to deal with- real professionals.

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