Changing Any Prop for Better Performance

Replaced

http://forums.iboats.com/showthread.php?t=369057



H

Re: Changing Any Prop for Better Performance

Thanks H, useful stuff...

Re: Changing Any Prop for Better Performance

Excellent post hwsiii. Highly informative.

I was especially interested in the formula you used to calculate RPM changes when changing prop pitch. I was under the impression that about a 200-300 RPM change could be expected for each inch of pitch.

Clearly not as with your example the 11p to 9p results in a change of 666 RPM per inch of pitch.

I have much left to learn. Thank you for sharing.

Cheers!

Re: Changing Any Prop for Better Performance

Thank y'all very much, I was hoping this information would help people have a better understanding of props and how they affect the performance of your boat when trying to get better performance for your applications and needs.


H

Re: Changing Any Prop for Better Performance

If i run your formula I get a slip of -11.6. I have a alpha 1 gen1 5.7l 260hp 15p ss three blade best info i can find is it is 1.5 gear. My top speed is 50mph(gps)at 4100rpm. What am i doing wrong. Thank you

Re: Changing Any Prop for Better Performance

century, I show a 1986 Mercruiser 5.7L to have a gear ratio of 1.32:1, please verify this. But whether it is 1.5 or 1.32 gear ratio with a 15" prop you cannot attain even a theoretical speed of 50 MPH much less an actual speed of 50 MPH at 4,100 RPM.

Either your GPS was wrong, the pitch of your prop is not 15" or your tach is showing the wrong RPM or possibly your gear ratio is wrong, or a combination of these elements. Tachs are notorious for being wrong, I would turn the switch on the back of the tach back and forth 8 or 10 times to clean any corrosion and try it again, just make sure it is in the right position after you do this.

Here are the numbers, I don't know how you came up with -11.9 % though.

4,100 Motor RPM / 1.5 gear Ratio = 2,733 Prop RPM

2,733 Prop RPM X 15" prop Pitch = 40,995 X 5 = 204,975 / 5280 =38.8 Theoretical MPH

38.8 Theoretical MPH - 50 Actual MPH = 11.2 / 38.8 = - 28.8%

Which is negative prop slip and that is impossible, and if I even use the 1.32 gear ratio it still is - 13% prop slip.

Prop Slip




H

Re: Changing Any Prop for Better Performance

Thank you and you are on it I just took a closer look it is a 19p. There is still somthing not right if you use the 1.32 gear it comes up with 0.10 possible but I don't think I am that lucky. Thank you again for the help

Re: Changing Any Prop for Better Performance

When I run the numbers using the 1.32:1 gear ratio and the 19" pitch prop I come up with a prop slip of 11%. In my opinion prop slip that is between 6% and 11% is fairly normal and any time it goes over 15% prop slip the prop that is on the boat is not suitable and needs to be changed. Your RPM are a little lower than where they should be, as the recommended RPM for your motor should be 4,200 to 4,600 and you could take the prop to a shop and ask them to remove 1" of pitch which should bring your RPM up to about 4,350 RPM, but if they don't have the correct pitch block for your prop or they just remove the cup on the trailing edge then they could actually increase your prop slip, so I would leave it like it is.


Prop Slip




H

Re: Changing Any Prop for Better Performance

I don't think I will mess with it. I don't need to go over 50 anyway. Thank you for your time and putting up with my stupidity

Re: Changing Any Prop for Better Performance

hwsiii said:

Replaced

http://forums.iboats.com/editpost.php?do=updatepost&p=2458311



H

Click to expand...

this post is for HWSiii
i dont know how to get up with you so i am trying to reply to this post. ok so i have researched you and your advice. i have heard alot of wind blowing from alot of lips aver the winter but by all means i have read all and i mean all your post. i have come to the conclution that u may be the only person on the face of the planet who can help me.
ok here goes my real email is EDIT
let me start by telling you what i have as a problem. ok ever try and pull a log out of water? that is kinda like my hole shot. not to to bad but noticable. next is the boat rpm. now eather my guage is really bad off or i am spinning a prop like mad. next is boat speed. at 3/4 throttle maybe a little better the boat is at 46mph (gps) then when i gun it it jumps from 5g to 5600but boat goes no faster.
now i have spoken to several ppl and they say no way my setup will spin a prop that fast must have a rpm guage off. now i can see 2 or 3 mayyyybe even 400 rpm but 6plus???????? dont sound right. pluss i have a pro street drag bike so i am use to hearing rpms and it sounds above the recomended 4800 i beleave. the motor was a 305 spun bearing and now has a 350 with 305 heads and thats all. no rev limiter. keep in mind i would like to stay with a alumin....prop as i am on a budgett at this point. new to boats.

now for the setup i will try and answer all your questions in order
1) 1990 regal valanti 200 bowrider (open bow)

2) 20 foot 8 foot beam 2400 pounds

2a) max hp is 300

3) 3ppl sometimes 4 (if son has a friend over) not sure on gal of gas but allways stay around 1/2 tank 25 gal

4) ride the water with family (tube and ski behind 2 jetskis)

5) it is a v but dont think it is a deep v

6) 350 block 1969 heads 305 1990 (was all i had) stock 2bbl carb

7) alpha 1 outdrive 1990 stock not sure of gear but if i am right there r only one for those older years 300hp max

8) 4800 wot max maybe 5g but i think it is 4800 u will know better than i

9)n/a

10)n/a

11) has a wale tail but will remove on your recomindation if needed but will leave on eather way. doesnt matter 2 me

12) michigan ballistic 23 pitch 3 blade is all i got came with boat

13)wot rpm as of saturday 5600 5500 me and wife 330punds 2 life vest 3/4 tank gas gps 46 mph

14) lake james nc elev 1200 i think

15) motor new

16) not sure how long it has been on boat i bought boat at end of season last year only used about 8 times then winter hit

17) prop looks great and if u suggest a new one i will sell this one cheep

18) i think the boat is a little sluggish @ hole shot and it has to many rpm (if tack i right ) also top speed is it right at 46mph? boat planes at just over 3/4 ish throttle at 4800 ish rpm then when u give it wot rpm rais but speed doesnt

19) shew kinda hard this one but it cruises at 38 to 3900 rpm at 20 to 24mph any lower rpm and the boat comes off plane maybe i can get 3600 befor breaks off plane. 44 4500 rpm is about 40 then wot 54 to 5600 46 mph

email me if u have any other questions
i am afraid i am gonna hurt the alpha 1 at those rpms but not sure also am thinking of a turning point legacy do to the vent holes to give me that hole shot just in case i ever wanted to pull up a sing ski not sure i ever would but @ least i think the holes would help me get that log of a boat out on plane. but i will listen to any prop name and type u sugest

thank you for any help
joey

Re: Changing Any Prop for Better Performance

Won't let me open the link

Re: Changing Any Prop for Better Performance

good luck with that.

Re: Changing Any Prop for Better Performance

I fixed the link, it works now.


H

Re: Changing Any Prop for Better Performance

thx!