Are there any Mathamagicians in the house ???

[Bondo]

I've been Mathematically Challenged All My Life......<br />I passed Algebra,+ Geometry in High School.... But Just Barely... <br />I think if calculators were more common then I might have retained more of it......... <br /><br />Anyway,<br />After a post up on the outdrive board, I got to thinking.......<br />Just How Much Pitch does it take to make up the differences in the gear ratios ?????<br />The ratios in question are,<br /> 1.50/1<br /> 1.65/1 <br /> 1.84/1<br /> 1.98/1<br />How much is the difference in each, as Inches in Prop Pitch ?????

 

[ob]

Re: Are there any Mathamagicians in the house ???

Not sure I understand the correlation of your question.Gear ratio on a given outboard or sterndrive has to do with how a particular engine makes its power.Prop pitch itself is not a figure set in stone as each pitch with each application will have its own theroetical forward thrust in inches per revolution with a percentage of slip.<br /><br />If I had to guess.If the original application of a given gearcases ratio was 2 to 1 and it was running 6000 rpms at wot as propped,I'd say if the same engines ratio was changed to 1.5 to 1 ,it would be necessary to reduce prop pitch to do the same 6000 rpms.Unfortunately ,I can't tell you what that pitch reduction would need to be.My guess would be 3 inches in prop pitch reduction.Pure speculation.<br /><br />What is you're trying to do? Change a setup,or just picking the fly poop out of the pepper so to speak?No offense intended.Just wondering.

 

[rolmops]

Re: Are there any Mathamagicians in the house ???

Suppose,all other things being equal, you take your 1.50:1 as you calculation base.<br />You are talking roughly a 10%,a 20% and a 25% increase in gear ratio. Your plan of neutralizing this with increased pitch can only work if your engine has enough spare power to ignore the pitch difference.If you would give the prop size that you would use with your basic setup,I can give you roughly the corresponding increase or decrease of pitch with the increased or decreased gear ratio.<br />Help me, I had too many black russians and everything is beautiful.

 

[Bondo]

Re: Are there any Mathamagicians in the house ???

Well,..... The correlation of my question is,<br />Theoretically,+ Mathematically, If my engine is turning 5000rpms with a 1.50/1 ratio outdrive spinning a 21"prop at whatever mph this computes to,(without the slip factor)........<br />What Pitch Prop would you need to achive the Same overall effect with a 1.98/1 ratio... or 1.84/1 or 1.65/1 ???????<br />In other words, If the engine rpms,+ the mph are the same,+ the ratio changes, How Much Does the Pitch Need to Change ??????<br /><br />I Know this is an Easy Mathematical Equation........<br />I Just Don't Remember It..............

 

[rolmops]

Re: Are there any Mathamagicians in the house ???

I don't need the basic width of your prop I don't care about the width because that remains equal!!! Give me the basic pitch already!!!Anyways,the bigger your gear ratio the smaller your pitch, in corresponding percentages.<br />See now you've done it,I had to correct at least 5 spelling mistakes in this one message.

 

[Andrew Leigh]

Re: Are there any Mathamagicians in the house ???

Ok Bondo,<br /><br />Assuming all things stay the same, which they can’t.<br /><br />You motor @ 5000 Rpm with a ratio of 1.5 times is turning the output shaft at 3333 Rpm (Engine Rpm / Ratio = Rpm at shaft).<br /><br />5000 / 1.5 = 3333 shaft rpm<br /><br />This configuration will move the boat forward 1944.4 yards in one minute with a 21” prop. (Rev’s at shaft x pitch) / 12 / 3. The 12 (inches) and 3 (feet) are to convert to yards.<br /><br />(3333 x 21) / 12 / 3 = 1944.4 yd’s<br /><br />Example @ 5000 with a ratio of 1.98.<br /><br />To get the formula the other way around to calculate the pitch then first calculate your Rpm at shaft.<br /><br />5000 / 1.98 = 2525 shaft rpm<br /><br />To get the pitch one must take the same distance traveled / the rev’s at shaft just calculated multiplied by 12 and then 3.<br /><br />(1944.4 / 2525) x 12 x 3 = 27.7 inch pitch<br /><br />Always calculate the brackets first else your answer will differ.<br /><br />If this is going where I think it is, you want to re-gear and or re-prop. The problem you will then have is that power, speed and torque are an intertwined and often a misunderstood concept. If you are considering re-gearing your output you will not necessarily get the same result as the entire speed vs. torque curve for the output of your engine will change. At WOT is normally where your maximum power and torque is on your engine, changing the gearing and or prop may well change this to where is get far less than the optimum performance.<br /><br />There are some very good threads on re-propping just do a search.<br /><br />Someone check my logic please.<br /><br /><br />Andrew

 

[Bondo]

Re: Are there any Mathamagicians in the house ???

The problem you will then have is that power, speed and torque are an intertwined and often a misunderstood concept.<br />There are some very good threads on re-propping just do a search.

I understand Perfectly , Andrew........... <br />And, Thank You..... I just needed the formula..... <br /><br />I'm usually 1 of the people Giving that advise......<br />After giving that same advise on a post up in outdrives, here <br />I got to Thinking......<br /><br />On My boat, I'm running a 1.65/1 Alpha... I've got a Torque-Shift Prop that's screwed down to about 20"s of pitch.... <br />I've also got several outdrives on the shelf, in ratios 1.50/1,+ 1.98/1... I Know I can't pull the 1.50/1 with my 4.3LX...<br /> <br />But, I'm wondering if I put on a 1.98/1, Maybe I can increase my Trolling rpms,+ Still be able to Open the prop Pitch enough to hold the WOT rpms down to a reasonable 4800, by turning out the adjusting screws in my prop.....<br /><br />It Might Work.... My Prop will adjust Out to 26" at it's Max..........<br /><br />The Torque Loads will be interesting........<br /><br />These Props have a Severe amount of Cupping on the blades...... <br />So much so, that when I Over-Trim it, I can Look like a Jet-drive, with a Really Big Tall Roostertail.....And, Still Not Ventilate..........<br />I mean, I can over-trim to the point that I can Here the u-joints Protesting,+ Still Not Ventilate..........<br /><br />I just Might have to Test this Theory in the Spring.........<br /><br />Thanks Again Andrew......<br /><br />rolmops,<br />I Think that by your 11pm post, I don't Care What Color that Russian is............<br />It's Kicked Your A$$.........................

 

[BoatBuoy]

Re: Are there any Mathamagicians in the house ???

OK here goes. Andrew, I believe you are correct. I worked through this a little different, but essentially got same results. <br /><br />Irregardless of engine rpm, if you gear ratio is 1.5:1 and pitch is 21, that means for every engine revolution, the propshaft is rotating 1/1.5, or .667. <br /><br />Now with a 21" prop, that prop will move .667 revolutions which equates to 21*.667 (zero slippage), or 14" forward. <br /><br />Now, with gear ratio of 1.98, to achieve the same 14" forward movement, 1/1.98=.505. 14"/.505=27". So, to change from 1.5:1 to 1.98:1, the theoretical equivalent prop pitch would be 27".7<br /> <br />((1/current ratio)/(1/new ratio))= <br />(1/1.5)/(1/1.98)=<br />14/.505 = 27.7<br /><br />No consideration here for slip, cup, horsepower, etc. This is just theoritical equivalents.

 

[Mr.Ladyfish]

Re: Are there any Mathamagicians in the house ???

You guys are giving me a headache!

 

[TilliamWe]

Re: Are there any Mathamagicians in the house ???

Bondo, even though you confused a few people and maybe drove some to drink, I knew what you were getting at! Boatbout & Andrew, thanks for the straight math. Yeah, all things can't be equal, but you put it in straight math terms. I may even print this out for my future use...<br />I have a 2:0 to 1 Bravo 3, that someday I might replace with a Bravo 1. Wit your math, I may get even closer to guessing which prop to start with! But probably not! ha ha

 

[Stratosfied]

Re: Are there any Mathamagicians in the house ???

Ah, Ah yeah, yeah, what Andrew said.

 

[achris]

Re: Are there any Mathamagicians in the house ???

Bondo,<br /><br />Here's the one I use that really easy. Bear in mind it doesn't take into account the differences in the effiencies of the different size props. This is just a starting point.<br /><br />New pitch = Old pitch/old ratio x new ratio<br /><br />So for your example (21" on 1.5:1) it would be....<br /><br />for 1.65... pitch would be 21/1,5*1.65=23.1 (use 23)<br />for 1.84... pitch would be 21/1.5*1.84=25.76 (use 25, maybe)<br />for 1.98... pitch would be 21/1.5*1.98=27.72 (Do they make props that big?)<br /><br /><br />Hope this helps....<br /><br />Chris............<br /><br />P.S. It also works going the other way.... higher reductions to lower.... ie, needing a smaller prop.