Inches vs pitch vs ????

[fishhawk16]

I undertand that at WOT, for every inch reduction in pitch (19 to 18) the RPMs will increase by 200 RPM.<br /><br />But I can't find anywhere that tells me what effect diameter will have on RPM at WOT. <br /><br />For example if I went from a 14x19 to a 13 3/8x19.<br /><br />My 90Hp Evirude reves about 5200 at WOT. I'm thinking of getting a Stainless 13 1/4 x 17. What should I expect for RPM's? (approximately of course)

 

[Dhadley]

Re: Inches vs pitch vs ????

Theres a lot to it but very generally speaking the more diameter you have the higher you can mount the motor. Besides the increase in blade area it, in theory, puts the tips in "cleaner" water. At speed the tip is what does most of the work.

 

[fishhawk16]

Re: Inches vs pitch vs ????

Thanks Dhadley,<br /><br />What effect does it have on RPM. Given the above can I run a 13x17?<br /><br />BTW - your the reason for the question . A while back you replied to a post where you said that my 1996 90HP Looper should rev 5500 or higher. You said that these motors can handle it. I'm curious as to why? Should I be concerned with my 5200RPM?

 

[jim dozier]

Re: Inches vs pitch vs ????

The calculation for pitch by a propeller manufacturer includes the variation in diameter. If two propellers had the exact same blade twist and angle but one had a larger diameter, the one with the larger diameter would end up with a higher calculated pitch.

 

[Dhadley]

Re: Inches vs pitch vs ????

5200 max rpm will shorten the life of the motor considerably. Even if you only run in the mid range its still under a lot of strain.

 

[fishhawk16]

Re: Inches vs pitch vs ????

Jimd<br />If I'm following the logic in your statment correctly - your saying that a 13x17 would provide approximately the same RPM as a 14x17?<br /><br />Dhadley<br />Would 5800 be to much for my motor (model # E90ELEDR) 1996 90HP.

 

[Dhadley]

Re: Inches vs pitch vs ????

For a 60* V4 5800 is about as low as you want to set it up for. I shoot for 6000 with an average load. <br /><br />Its a 2 stroke, needle & roller bearing motor. There is no reason to be afraid of rpm. You should be afraid of lugging it. Pistons dont like that. <br /><br />The rev limiter is at 6200. The limiter is the only thing that stops the motor from being capable of higher rpms. Remove the limiter and the reeds are good thru about 6500-6600. Maybe a tad more. Then they start to flutter and the motor wont go any higher (cant breathe). Replace the stock steel reeds with Boyesens and it'll pull right on thru 7500 or so. <br /><br />At about that range the stock stator cant keep up. So then you call CDI. With their stuff that motor is good right thru 8000+.<br /><br />All of this is assuming youre still using stock porting. Youll get more with a ported block.

 

[Moody Blue]

Re: Inches vs pitch vs ????

Does the gear ratio not play a factor in determining diam and pitch ? What is the affect of changing for example a 10"x15 pitch to a 12"x13 pitch ? When do you choose a larger diam over the smaller diam props ?

 

[ttp]

Re: Inches vs pitch vs ????

I have the Mercury 2003 book in front of me. Mercury 90 hp RPM range is 5000-5500. This is for the classic carbureted 2 stroke. The 90 hp fourstroke RPM range is 5000-6000. I would assume Evirnrude RPM range would be very similar. Now, the 2 stroke classic Mercury 125, 115 and 75 hp have a range of 4750 to 5200. I think you are set up about the best you can be. A stainless steel prop will perform better than an aluminum prop of the same size and pitch. They better work better, they cost three and a half times as much. $229.00<br /><br />ttp

 

[Dhadley]

Re: Inches vs pitch vs ????

Gear ratio does play a part but since ratio is not easily changed we dont worry about that too much. As long as we know what the ratio is we can calculate slip. <br /><br />As gearcases get larger the prop diameter will increase. <br /><br />In some prop lines like OMC, as the pitch gets bigger the diameter gets smaller. Their theory is that youre not going to change the X dimension and therefore the rpm will remain constant as far as change goes. Some lines like Turbo maintain the diameter thru the style line. <br /><br />Diameter does have an effect on rpm. By theory the more diameter, the more bite so therefore the less rpm. Also more blade area has the effect of, theoritically, slowing down the rpm. <br /><br />Another theory is that too much diameter will decrease the efficiency because of too great a tip speed. <br /><br />When choosing a prop you have to look at a lot of things. X dimension, jackplate or not, setback, hull design, gearcase shape, intended use etc. Personaly I prefer a larger diameter for my boats. But we are willing to change the set up to conform to the prop. Most folks want the prop to conform to their set up.

 

[StanMan]

Re: Inches vs pitch vs ????

Originally posted by fishhawk165:<br /> I undertand that at WOT, for every inch reduction in pitch (19 to 18) the RPMs will increase by 200 RPM.<br /><br />But I can't find anywhere that tells me what effect diameter will have on RPM at WOT. <br /><br />For example if I went from a 14x19 to a 13 3/8x19.<br /><br />My 90Hp Evirude reves about 5200 at WOT. I'm thinking of getting a Stainless 13 1/4 x 17. What should I expect for RPM's? (approximately of course)

 

[umblecumbuz]

Re: Inches vs pitch vs ????

Now this thread really is an education. <br /><br />I reckon Dhadley and Walleyehead should run a FAQ between them all about props.<br /><br />There'd be a big heap of interest.<br /><br />Who seconds that?

 

[markvan]

Re: Inches vs pitch vs ????

I think there are a pile of prop questions and tons of info provided by both Dhadley and walleyehed about props but it would be nice to see all together in an faq.........oh I second that

 

[walleyehed]

Re: Inches vs pitch vs ????

If you'll check the FAQ's you'll find one on props that was wriiten by me, edited some by JB and posted in that section....since that time, I have spent more time testing and changing set-ups, the whole 9 yards...I only wish I would have consulted Dhadley for the rough draft before we went with it.<br />I tried to hit the "Basics" on props and their design, but as with everything else, there are times when the rules of thumb don't apply or must be bent. Dhadley would be the one to "Fine-tune" what I had wrote, and if JB would be game, I'd love to have Dhadley edit and "Fine-tune" what I have posted in the FAQ.