Why the diameter decrease as the pitch increases?

[Nos4r2]

Just been looking at a new prop for my 50 merc/mariner blueband type thing... I'm looking at a 14" pitch one of these :-<br /><br /> http://www.magicmarine.co.uk/props/mercury/mercury2.html <br /><br />but I can't work out why the prop diameter decreases as the pitch increases. Anyone got any ideas? Will it make any difference and could it be because the separate blades for all the pitches are all made from the same basic blank?<br /><br />BTW this is a UK site and as such is not in competition with Iboats unless you've started shipping to the UK(for the mods)

 

[Texasmark]

Re: Why the diameter decrease as the pitch increases?

I've wondered the same thing. I just went from a 17" pitch to a 21 (same mfgr and same brand) and the dia dropped slightly.<br /><br />Looking at both props side by side, makes me think, about as you do, that you have the same blade area, but as the blade rakes back with the increased pitch, it forces the blade closer to the hub. Obviously, the blade occupies more of the hub length which supports the previous assumption.<br /><br />I doubt it's performance related; i.e. done specifically for performance......not that big a change.<br /><br />My 2c,<br /><br />Mark

 

[Silvertip]

Re: Why the diameter decrease as the pitch increases?

Texasmark has it figured out!. The slight decrease or increase in diameter is you decrease or increase pitch has an insignificant affect on performance. It's the pitch change that makes the performance difference. Diameters will also not be the same between manufacturers. It's all in the engineering.

 

[Silvertip]

Re: Why the diameter decrease as the pitch increases?

Texasmark has it figured out!. The slight decrease or increase in diameter is you decrease or increase pitch has an insignificant affect on performance. It's the pitch change that makes the performance difference. Diameters will also not be the same between manufacturers. It's all in the engineering.

 

[--GQ--]

Re: Why the diameter decrease as the pitch increases?

Originally posted by Upinsmoke:<br />[QB] Texasmark has it figured out!. The slight decrease or increase in diameter is you decrease or increase pitch has an insignificant affect on performance. It's the pitch change that makes the performance difference. Diameters will also not be the same between manufacturers. It's all in the engineering.<br /><br />Hmmmm...let see if i can clear my head on this. Upinsmoke are you saying that props of the same manufacture, same materials, same pitch will give the same performance even though one is an inch bigger in diameter?

 

[imported_JD__]

Re: Why the diameter decrease as the pitch increases?

I don't want to stir up a hornet's nest here but I think diameter will affect performance. I got into this with my Johnson/Suzuki 4-stroke which has a 2.38:1 gear ratio. I tried running props that were designed for 2-strokes like the 115-125 Mercs which had a 2:1 ratio. Top speed was ok but it would literally spin the prop coming out of the hole. Because of the lower gear ratio the 4-banger needed a 14" diameter prop rather than the 13-1/2" diameter 2-stroke prop. Plain and simply, the larger prop moves more water.<br /><br />I would guess that the increased pitch/reduced diameter is a compromise, allowing you to go faster without sacrificing too much hole shot?

 

[--GQ--]

Re: Why the diameter decrease as the pitch increases?

I don't know anything about boats much less props. but physic will only allow so much. We live in a world of naturally restricted elements. Anything that moves will/must aquire three things; the ability to move; the force to maintain movement under load; the resistance acting on the load.<br /><br />A prop pitch is the ability to move. Increasing the pitch will propel the prop further per revolution. The diameter (cross section) is the ability to move under load. Increasing the cross section will increase moving ability.<br /><br />Disregarding engine factors, a prop at set diameter / pitch, at a set rpm will move a boat at certain speed. If 1500 lbs of weight added onto the boat, the balance is tipped. The prop will have to move a higher volume of water to compensate. Increasing the prop diameter will do just that. <br /><br />When the pitch of prop is increased, the cross section is inversely decreased. The higher the pitch, the smaller the surface area. Furthuremore increasing the sideway thrust pushing perpendicular to the prop movement. <br /><br /> Contradicting most statements made on this forum, I believe if you increase the pitch, you have to increase the diameter aswell unless poor holeshot is what you're after. I just see it the other way around. I may be alone on this.

 

[Nos4r2]

Re: Why the diameter decrease as the pitch increases?

Originally posted by --GQ--:<br /> Contradicting most statements made on this forum, I believe if you increase the pitch, you have to increase the diameter aswell unless poor holeshot is what you're after. I just see it the other way around. I may be alone on this.

Yes-but you're forgetting a few things.<br />1)There's only a limited amount of room between the anticavitation plate and the propshaft-I don't think I can actually run anything over a 10 1/2" diameter prop anyway.<br />2)We're only talking 1/8" diameter difference.<br />3)To make a different size blank for each prop pressing would cost more.<br />4)if mine and texasmark's observations are correct across the board then all non custom built props will go down in diameter as the pitch is increased-simply due to the same blank being used each time for a range of pitches.<br /><br />To be honest all I really needed to know was "is this normal and will it be ok"-from what I've read I think there's no other option. In the ideal world things it would only be constrained by physics and not accountants

 

[gatorred]

Re: Why the diameter decrease as the pitch increases?

jd if you up horse power like 35hp to a normal 150 engine do the same like 2/1 i suffer the same thing, sound's like my problem

 

[Silvertip]

Re: Why the diameter decrease as the pitch increases?

GQ -- The original question dealt with the 1/4-inch difference (up or down) when going up or down in pitch. A few posts later we are discussing one-inch changes in diameter. Of course a one-inch change in diameter will have an affect on performance. Unfortunately, one-inch change in diameter means that prop will not generally fit on the engine. I stand by my comment that the average boater will not be able to distinguish a performance change between a 13-1/4 x 17 prop and and 13 x 17 prop of the same basic design.

 

[Texasmark]

Re: Why the diameter decrease as the pitch increases?

Hey guys.<br /><br />Take me for instance. I needed lots more pitch, like 4 inches. The percent change of the pitch was much more than the percent change of the dia and since I didn't have 2 equal pitch props with slight dia changes to test under the same conditions, I didn't know any different; but baby I noticed the 4 inches.<br /><br />I like discussions, as long as the participants stay focused and keep it friendly. Makes you think. Keeps the mind active. Sometimes uncovers a lingering problem you had but blew off or just didn't know the answer.<br /><br />Have a great holiday.<br /><br />Mark

 

[--GQ--]

Re: Why the diameter decrease as the pitch increases?

Originally posted by Texasmark:<br />[QB] Hey guys.<br /><br />I like discussions, as long as the participants stay focused and keep it friendly. Makes you think. Keeps the mind active. Sometimes uncovers a lingering problem you had but blew off or just didn't know the answer.<br />Have a great holiday.<br /><br /><br />I totally agree, this is not a win loose or draw debate rather a meeting of the mind. Appearantly mind is smaller so I have to be more assertive........<br /><br /><br />Upinsmoke, I'm not doubting your assestment. I was just trying to get you to elaborate further since my understanding is limited. I used the " one inch" to support my findings and nothing more. Again i'm not an expert in anything. I just see things from a mathematical point of view. If you can explain it with math, maybe it doesn't exist and or can not be true. <br /><br />There are a few "Prop Doctors" on this board. The name Walleyhed is the one i'm familiar with. Perhaps he'll see this thread and fill my glass to full.<br /><br />HAPPY BOATING.........

 

[WillyBWright]

Re: Why the diameter decrease as the pitch increases?

They try to keep the amount of metal in ceratin classes of propeller about the same. (Anything to reduce production costs. ) It makes for a more uniform cost accross the lineup of different pitches, and fewer machinery adjustments. And they can cast many pitches from a single pour.<br /><br />Consequently, it has a fair correlation to total surface area of the prop. Total surface area is also important to performance. As the angle of pitch increases, more metal would be needed if the diameter were to remain the same. Surface area would also increase making for an even higher load on the engine.

 

[walleyehed]

Re: Why the diameter decrease as the pitch increases?

There are a few "Prop Doctors" on this board. The name Walleyhed is the one i'm familiar with. Perhaps he'll see this thread and fill my glass to full.<br />

I'd be happy if you'd just refer to me as my posted or first name...<br />As for the thoughts at hand here, it's not rocket science, AND, each prop/diameter has a different purpose..you can't put'em all in the same box. What works on my boat more than likely, won't work on yours(anyone's) even with the same HP.<br />Diameter of a prop on a slow boat is helpful to push a load...diameter on a fast boat is helpful for higher running heigths and harnessing true HP equating to lower slip ratios.<br />In general boating (general public) a small diameter prop will be faster on lighter boats...in Racing, this tends to go the other way, BUT, again, as you add more blades, the diameter "tends" to go down.<br />And to confuse even farther, each application react differently, especially when you start comparing gear ratios...then pitch/diameter ratios change as well.<br />You guys already know there is no std rule that applies to "all" boats.

 

[Silvertip]

Re: Why the diameter decrease as the pitch increases?

This discussion could go on forever but let's take another stab at it. Let's assume we have two engines (Merc 50 2S and a JohnyRude 50 2S). The JohnyRude has a 2.3:1 gear set and the Merc has a 1.83:1. The Merc runs a smaller diameter prop with less pitch than the JohnyRude. The JohnyRude runs a larger diameter prop (about 1-inch larger) and it also has more pitch yet both engines on similar boats will perform very close to the same. The Merc will have higher prop RPM at any given engine RPM than the JohnyRude due the gear selection. Conversely the JohnyRude would have lower prop speed at the same RPM. The difference in this scenario is in the pitch. Consider that one Rev of the JohnyRude prop theoretically moves 15 inches. In reality there is a certain amount of slip so its really less than that. The Merc on the other hand although it has less pitch (moves less distance each rev) but the prop speed is higher so it makes up for the difference in pitch. As Walleyhed pointed out, diameter in prop design has to take into account gear set selection and engine HP. A small engine can't efficiently turn a large diameter prop. It can however turn a relatively large diameter prop with very little pitch. These designs however are used on barges and pontoon boats. Performance boats need a different set of parameters. Go to iBoats prop selector and look at the range of props for each engine. There will usually be anywhere from three to about five pitch selections. You will notice about 1/4 inch difference up or down from the center of that range. Most engines cannot accept a prop larger that what appears in that range due to interference. Happy New Year -- now if we could just get the ice off the lakes !

 

[Nos4r2]

Re: Why the diameter decrease as the pitch increases?

While we're at it here.... (yes, I'm hijacking my own thread... )<br /><br />Whqat difference is having a smaller hub than the standard merury one likely to make to my engine? From what I've seen marine 2-strokes seem to lack finesse in exhaust tuning but is this just that it's tuned a different way?<br /><br />If you lose backpressure on a 2-stroke bike exhaust it melts pistons, runs hotter and performance is severely affected-see where I'm coming from?

 

[walleyehed]

Re: Why the diameter decrease as the pitch increases?

As with a bike, we can run tuned pipes, or tuned exhaust on the outboards. Along with this there is usually jet/carb/timing changes, and in some cases, porting is changed too.<br />But, to my knowledge, the mid-section is built in a manner to allow a certain amount of constriction even without the stock megaphone installed.

 

[Nos4r2]

Re: Why the diameter decrease as the pitch increases?

Cool, shouldn't make a difference then. Cheers Kenny

 

[imported_bjs]

Re: Why the diameter decrease as the pitch increases?

everything you need to know about props is<br /> <br /> http://www.mercurymarine.com/everything_you_need_to_know_about_propllers5 <br /><br />i hope this works

 

[imported_bjs]

Re: Why the diameter decrease as the pitch increases?

Originally posted by racer 15:<br /> everything you need to know about props is<br /> <br /> http://www.mercuryoutboards.com/everything_you_need_to_know_about_propllers6 <br /><br />click on propllers then cilck everything you need to know about propllers.i hope this helps maybe this will give a better adivce on props.