Proper Prop Pitch

[Traceyh]

Say that three times fast... but I digress...

I have a Lowe 24' Windcruiser with a 90HP Mariner on it.
It has a 14x13P prop on it. I feel that maybe it needs a little larger pitch prop on it. I am totally unqualified to make that statement however and am looking for advise on proper pitch. My gut feeling is something like a 13x19P or maybe a
13x21P, but I am really clueless. I have done "some" research, but am not finding any solid answers. I know there are some "pros" on here that can/will help out I'm sure.
Thanks ahead of time.

 

[VaGent]

Re: Proper Prop Pitch

I have a Johnson 70 on my 24 ft pontoon boat & have been told by several people I needed to use a 14X13 or a 14X11 prop on mine. I now have a 13.75X15 which everyone says to too big for my engine. Like you I don't have much experience with props so I had to ask around to find out. Hope this helps in your decision.

 

[EGlideRider]

Re: Proper Prop Pitch

You are probably ok but you certainly won't need a 19 or 21. Do a search here and you'll probably answer your question. If not, report back with your load and max RPM at WOT and you'll get lot's of responses.

 

[Silvertip]

Re: Proper Prop Pitch

Not knowing how your current prop performs (wide open throttle rpm) with an average load, there is no way we can suggest a prop. What makes you think you need more pitch? I suspect you want more speed and think sticking any prop on the motor with more pitch automatically equates to more speed. That is not true and in fact can cost you speed since the engine simply cannot twist a prop with that much pitch on a pontoon. On a planing hull boat, yes. On a pontoon -- no. Give some rpm numbers to work with. Wide open throttle rpm as observed on the tach with what you consider an average load is what we need. I, like the others, assure you that you do not need a 19 or 21P prop.

 

[EricJRW]

Re: Proper Prop Pitch

I recently went from a 13x11 to a 13x13. Manged to drop RPMs a little and squeak out a couple more MPHs. One thing I did, at the suggestion of someone here on the forums, is call the manufacturer and ask them what is supposed to be on the boat. As it turns out a 13x13 is the recommended prop. In my situation going higher than a 13 pitch would probably not have worked out (based on input from several sources).

 

[Traceyh]

Re: Proper Prop Pitch

Not knowing how your current prop performs (wide open throttle rpm) with an average load, there is no way we can suggest a prop. What makes you think you need more pitch? I suspect you want more speed and think sticking any prop on the motor with more pitch automatically equates to more speed. That is not true and in fact can cost you speed since the engine simply cannot twist a prop with that much pitch on a pontoon. On a planing hull boat, yes. On a pontoon -- no. Give some rpm numbers to work with. Wide open throttle rpm as observed on the tach with what you consider an average load is what we need. I, like the others, assure you that you do not need a 19 or 21P prop.

Ok, my bad, I didnt supply the needed info. Currently I have a 14x13P Blackmax prop, and WOT is about 5300 RPM with normal load. Now here is my personal strange "thing". I have never, nor will I ever run any engine at WOT for any extended period of time, I never have and never will. What I am looking for is not top end speed, but maximum mid range speed. I have done the WOT test just to see what it achieved. I normally run around 4000RPM.
I currently get about 15 MPH at 4000 RPM. This was clocked by a boat running nest to me, I will get GPS speeds today, as I am headed out to the water early this morning. I am also used to working with 4stroke engines, as I am a Honda technician on motorcycles and ATV's, and a 35+ year veteran mechanic on all things motorized.
I have worked on the 2 strokes mainly as maintenance and repair but not performance. I assume, which is bad to begin with, that like 4 strokes, they produce maximum torque in a specific RPM range, that is not WOT, but I could be wrong. That is why I am asking here. Is there a taller prop that will give me better midrange performance. Im not interested in WOT performance.

 

[EGlideRider]

Re: Proper Prop Pitch

Your R's are little low. I would go with an pontoon specific 11p. Your engine will not be under such a load, but you'll probably not see an increase in speed. As my mechanics like to say, "How would you like to pull a 10K lb. trailer behind your 1/2 ton pickup everywhere you go?"

 

[Silvertip]

Re: Proper Prop Pitch

You misunderstand why a boat must be propped so the engine CAN operate at or very near the upper end of the manfacturers WOT rpm band. Boats don't have transmissions like every other motor vehicle so the prop must get you moving, cruise efficiently and provide optimum WOT performance. Manufacturers WANT it propped that way and the engine operates best that way. If the engine is propped wrong at WOT it is wrong across the entire band. How can that be??? You are simply lugging the engine continuously that's why. Just because you operate at part throttle does not mean the engine is not lugging. Take away all but one of the gears on a vehicle with a transmission and you will will quickly understand why a boat needs to be correctly propped. If you refuse to operate a boat at WOT thats your choice, but deliberately propping it for what amounts to a lugging condition is not doing the engine a favor.

 

[EricJRW]

Re: Proper Prop Pitch

Good explanation Silvertip. Marine motors are very different beasts and I'm still learning to appreciate the differences.

 

[EGlideRider]

Re: Proper Prop Pitch

Lots of wisdom and expertise in Silvertips post.

 

[Jeep Man]

Re: Proper Prop Pitch

I have basically the same lower end as yours . The gear ratio on Bigfoots are the same as the 75 and 90 hp I believe. Mine has a 14x 11 prop. I have tried a 9p and it WOT rpm goes to about 6200. Higher number causes the motor to bog down. 11p with a WOT rpm of 5800 is just right.

 

[Traceyh]

Re: Proper Prop Pitch

I am beginning to understand a bit better. But I dont feel I need to go to an 11P as I dont "feel" like the engine is lugging any, and it does have an acceptable speed, I was just looking for more midrange performance. On the vehicle with only one gear analogy, I would still be looking for one that performed best at the RPM range that I intended to run, rather than WOT throttle performance. But then again, that is based on engines that provide peak power somewhere in the middle, not WOT, as the torque curve falls off. Now I expect someone to reply, "the manufacturer says" But that isnt always the best advice, they give advise based on average usage, but some people are outside the "average" and want something different performance wise. The manufacturer gives their "advise" assuming you will normally run your engine at WOT, but Im outside that realm. Just like racing drivers, they dont shift at WOT to change gear ratios, they set their shift lights so they can change when the torque curve begins to fall, so that they can keep their engines pulling in the "power band".
I have never seen a dyno power curve on an outboard.
But if they are like other engines, it would be able to stand a taller prop in the midrange, then it would on WOT, based on what RPM peak volumetric efficiency occurs. Is there somewhere that shows some dyno charts so that I could see more about how an outboard performs? I have worked with 2 stroke dirt bikes, weed whackers, chainsaws, and none of them perform as well at WOT as they do somewhere just below, I just assumed that an outboard would be similar, however, I could be wrong.
I hope this clears up my "thoughtline" about why I was asking about proper prop pitch in the first place, and I really really appreciate everyones input so far.

 

[Boss Hawg]

Re: Proper Prop Pitch

I hope this clears up my "thoughtline" about why I was asking about proper prop pitch in the first place, and I really really appreciate everyones input so far.

I am in NO way qualifed to answer your question
All i can add is i totally understand your reason for asking as i run my motor in the same manner you do ( have to in my favorite fishing spots due to stumps & such- I do my best to run it at WOT at least once each trip tho )
Anyway i have a 20' Bass Buggy with a 110 Johnson & after trying a 13, 15, 17 & 19 I find the 17 gives me the best results for my way of boating ( 13P gave me best speed tho )
Hope you find the answer your looking for

 

[Cheaptoon]

Re: Proper Prop Pitch

If you prop it best at wot the rest will will fix itself.Outboards are a whole different animal than any other combustion engine known to man.

 

[Traceyh]

Re: Proper Prop Pitch

If you prop it best at wot the rest will will fix itself.

Can you qualify that statement? I dont mean to be rude, but I dont buy into "just because they say so", thats why I am trying to learn a little more about them. I am engine savoy, just not outboard savoy. But before its over with, I will have a much deeper understanding. For the most part, your statement is inherently incorrect, as an internal combustion is for the most part just an internal combustion engine, it is how it is used that sets it apart from other engines. For instance, the chainsaw that you might use to cut firewood, has the same engine that competition lumberjacks use in hot saw races, but the modification made to their saws make them unsuitable for everyday use. I.E the giant expansion chamber exhausts, carb mods, fuel mods, intake mods, and any other engine mods. But the heart of the beast is still the same. The outboard IS a simple 2 stroke engine at best. But how it is tuned and used is what sets it apart. The basic information I am trying to find, is what RPM range does it pull the hardest, if that happens to be WOT, then so be it, but "most" engines including 2 strokes, do NOT pull the hardest at WOT.
I am looking to use the peak torque, not the peak horsepower.

 

[EGlideRider]

Re: Proper Prop Pitch

You will have to obtain the actual torque curves from the manufacturer, but most two-strokes develop maximum torque between about 2500 and 3500 RPM's and maximum HP at WOT. As you know, torque pushes us through the water. We can't measure HP, only torque. HP is calculated from torque.

In reality, your engine and current prop will give you good service during normal usage of maybe 100 hours per year at less than WOT. However, you are definitely lugging the engine at all RPM's and the life of the engine WILL be reduced. I'll refer to my earlier analogy; You and your neighbor buy a new pickup. You tow a 10K lb. trailer everywere you go. Your neighbor does not.
Obviously, your engine won't last as long as your neighbor's regardless of your average speed. But if you only travel 5,000 miles per year, then you'll get several years of service.

I have owned many engine powered toys. Like most here, I like to pamper my toys in order to maximize the enjoyment I expect in return.

 

[Cheaptoon]

Re: Proper Prop Pitch

Can you qualify that statement? I dont mean to be rude, but I dont buy into "just because they say so", thats why I am trying to learn a little more about them. I am engine savoy, just not outboard savoy. But before its over with, I will have a much deeper understanding. For the most part, your statement is inherently incorrect, as an internal combustion is for the most part just an internal combustion engine, it is how it is used that sets it apart from other engines. For instance, the chainsaw that you might use to cut firewood, has the same engine that competition lumberjacks use in hot saw races, but the modification made to their saws make them unsuitable for everyday use. I.E the giant expansion chamber exhausts, carb mods, fuel mods, intake mods, and any other engine mods. But the heart of the beast is still the same. The outboard IS a simple 2 stroke engine at best. But how it is tuned and used is what sets it apart. The basic information I am trying to find, is what RPM range does it pull the hardest, if that happens to be WOT, then so be it, but "most" engines including 2 strokes, do NOT pull the hardest at WOT.
I am looking to use the peak torque, not the peak horsepower.

Well Not to be rude back....but......If you dont want to buy into it just because they say so, Why get on here and ask?There are several people on this particular post with Tons of knowledge to offer you ,Any 2 stroke I have ever owned generally works best at wot,including chainsaws and weedeaters. It is where there power band is at.

 

[Silvertip]

Re: Proper Prop Pitch

OK lets try this another way. I'll ask YOU why the manufacturers want their engines propped at or near their WOT rpm? While you ponder that and formulate an answer I'll move on.

Consider your car, bike, moped, yard tractor, or whatever as having just that one gear to get you moving, cruising, and well - ignore WOT since you aren't interested. So since you have one gear only, how would you leave a traffic light or start on a hill? You would race the crap out of the motor to get on the HP curve and dump the clutch would you not?. Are you old enough to remember what an automatic transmission was in a 1950 Chevy. If you are you would recognize the name Power Glide. Actually it wasn't automatic at all since in DRIVE it had only one forward gear. It had LOW but that was for pulling stumps. So how did they get away with that. It was called a torque converter. These marvelous devices provide somewhere between 2 and 3:1 torque multiplication until revs get up a bit and that multiplication effect goes away. So you see, your boat is the same way, If you elect to just putz away at a snails pace you can probably overprop by a bunch. However, water resistance is like wind resistance on an a land-based vehicle. While it may take only 20 HP to maintain 50 MPH (no wind/hills) in your car, it take many times more than 20 HP to double that speed. Same on the boat. The power required from being overpropped means you need more throttle opening (not only to get moving but to maintain a reasonable speed). As a result this does strain the engine even though it is not running as fast -- you are essentially dragging an anchor (only its your prop). Back to your car. If it were a stick shift, you might be cruising comfortably at 55 MPH in 5th but come to a rather steep hill. What happens to the gas pedal if you are running with cruise control on? Yup -- the throttle opens farther but speed does not increase does it? Overpropping does the same thing to a boat only it suffers at every speed -- meaning you need more throttle at any speed than you would need with the proper prop. To what degree that is would only be determined by testing. On a car it is easy to check the effect of gear ratio. Simply add a vacuum gauge. Higher the vacuum, the better the economy and less engine stress. Remember the rumpity-rump cams in the big and small block classics. Remember how some of those radical (for the time) engines needed vacuum canisters to store up enough vacuum for the power brake system. There was so much overlap in the cams these things had practically no vacuum once you opened the throttle even a little bit. That was also evidenced in old cars with vacuum operated windshield wipers. Jump on the gas you couldn't see because the wipers quit.

Being the land based engine guy you are, you know that the same basic engine can be set up for low end grunt (as for a truck), mid range power (for the family truckster), or high end power (for your go fast vehicle). However, economy takes a different set of cams, compression and carburetion/EFI parameters. There are all sorts of compromises that can be done by having a transmission. You don't have that flexibility with a boat. OK -- I've yakked enough about this and am outa here. I have the feeling that like politics, there is no way I can change your mindset about this so you need to just try what you feel you need to do. I'm a skeptic about certain things and I've been wrong before -- once I think!
Peace -- have a good week ahead.

 

[Traceyh]

Re: Proper Prop Pitch

Yes, I am old enough to remember the "powerglide", and vacuum accumulators.
I guess Im gonna have to live with current prop specs, as it does provide acceptable performance at my desired RPM range, I was just looking for "better".
And yes, I understand that there is a lot of knowledge on this site, and thats why I asked in the first place. I will not however just "take" anyones advise without an explanation of some sort, just because "they say so" isnt good enough. I have worked for manufacturers, and their advises are for the average consumer, I or you may not be within the average ideal of the manufacturer, so their advise may not pertain to the way we use their product. I also use a truck forum, as I drive a 1 ton Ford, and there are tons of people there that are constantly changing gear ratios of their final drives, bases on how the truck gets used, so basically what "cheaptoon" is saying is that the owners should not change their final drive ratios because Ford would only put the correct one in.
I feel the same analogy "might" apply to boats also, I was asking about changing prop pitch based on how "I" use the boat, not how the engine was designed.
I dont mean to agitate people on here by my questions, and I am doing my best to help others in the Mercury/Mariner forum with knowledge that I have to offer in the electronic and ignition and mechanical field. I am "NOT" knowledgeable on prop issues, so what better place to ask, but I need more than "because they say so".

Thanks again.... sorry for the ramble.

 

[Silvertip]

Re: Proper Prop Pitch

You aren't agitating people by any means. And your analogy about the Truck forum and about how they continually change axle ratios to better match the use of the vehicle is exactly my point. Land based vehicles have transmissions, differentials, transfer cases, and many optional tire sizes that can be played with to optimize performance for any given type of service. If one truck is towing a load of feathers it can be geared very differently than one that is towing a load of hogs. And by the way, bad selection of axle ratio on a land based vehicle is one of the biggest mistakes people make when buying a tow vehicle. It takes a lot of power or costs a bunch in increased fuel consumption (depending on which way the error was made) to make up for that poor selection. With a boat, you have no tires, you have no transmission and you have no transfer case. You do have a differential (the lower unit gears) which must contain a set of gears that have a very specific ratio range because propellers need to operate in a specific rpm range to be effective. Spin them too fast and they lose effectiveness. Spin them too slow and you go nowhere and burn lots of fuel. Overprop for the power of the engine and you lug the engine regardless what rpm you run it. Compromise in the middle you screw up both the top and bottom end of the performance spectrum. Prop at WOT you have the best it can be.