Need help w/ heavier loads - propeller change?

surfstar

Cadet
Joined
Oct 9, 2003
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18
I have an 11'2" Mercury inflatable with a 2003 9.8hp Nissan four stroke. The boat goes plenty fast with 2 people. Unfortunately with 4 it won't plane. I have lifters installed which are like smart tabs and they help, just not enough. (the boat can maintain a plane at 1/3 throttle w/ 2 people) The boat almost planes w/ 4, so I was hoping a propeller change would make the difference. <br /> First off I am unsure which propeller I have. The guide online http://www.nissanmarine.com/tech_talk/prop_chart2.html? says I have an 8.5 X 8.5 My owners manual says it comes with a 8.9" X 10". Measuring it, it seems closer to the 8.9"<br /> Whatever the case, I need to know what propeller size I should step down to, in order to get the boat planing with a heavy load. I guess I need a smaller pitch, right? What size would you guys recommend? <br /><br /> thanks - Matt
 

JB

Honorary Moderator Emeritus
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Mar 25, 2001
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45,907
Re: Need help w/ heavier loads - propeller change?

Howdy, Surfstar.<br /><br />First, you need to know where you are now. Place a ruler along the hub, parallel to the propshaft. measure the distance from the leading edge to the trailing edge of one blade of the prop, then multiply by 3 (or 2, if your prop is 2 bladed). That is your present approximate pitch.<br /><br />I would go to a prop with 2" less pitch for those days that you have the heavy loads, and keep your present prop for light loaded days.<br /><br />Good luck. :)
 

Boilermaker

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Sep 28, 2003
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388
Re: Need help w/ heavier loads - propeller change?

JB>>><br />I don't understand something , maybe you could explain it to me. Isn't the pitch, the distance the prop would travel forward for 1 revolution??? Wouldn't that be the same for a 2, 3, or more bladed prop of the same spec.??<br />Does the pitch of each blade change with the number of blades on a prop (for a given spec pitch)???>>>In other words, would the actual physical pitch of a blade on a 2 bladed 17" pitch prop be differant then the actual physical pitch of a 3 bladed 17"pitched prop??? I'm fairly new to this, and I'm just trying to understand this stuff better.<br />THANKS!
 

JB

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Re: Need help w/ heavier loads - propeller change?

Howdy, Boilermaker.<br /><br />Yes. The pitch of one blade for a 2 blade prop is 1/2 the total pitch for the prop, while one blade of a three blade prop is only 1/3 of the total pitch. So, on a 15" pitch prop one blade of three would be about 5", and one blade of 2 would be more like 7.5".<br /><br />This method is only approximate, because blade width, shape and cup all affect the true pitch, but it is a reliable way to estimate.<br /><br />Good luck. :)
 

surfstar

Cadet
Joined
Oct 9, 2003
Messages
18
Re: Need help w/ heavier loads - propeller change?

I determined that I have the 8.9x10 prop. Optional sizes are 8.9x8.3, 8.9x7, 8.5x8.5 or 8.5x7.5<br /> Should I stick to the same diameter? and what pitch size seems right for the job?<br /><br /> thanks in advance
 

walleyehed

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Jun 29, 2003
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6,767
Re: Need help w/ heavier loads - propeller change?

surfstar, I've got some fair experience with props on outboards a mite bit bigger, but taking what you say about "almost planning" with 4 onboard, I believe I would go with the 8.9X8.3 because I think you want to keep the diameter up for bite in load conditions, and the 8.3 pitch should give about 300-350RPM more on top-end thus allowing a better hole-shot.<br />This pitch would allow some room to re-pitch slightly if needed to fine-tune what you want.
 

backdraft

Petty Officer 1st Class
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Dec 29, 2002
Messages
266
Re: Need help w/ heavier loads - propeller change?

Surfstar, One rotation of the prop is the pitch no matter how many blades. I believe possibly JB meant the surface area was half that with a 2 blade as opposed to a one blade for a given pitch. For example, a four blade could have twice the blade area , suface area, as a 2 blade, depending on mfg. Thats why a prop with more blades has more push off the line and you have to lower the amount of pitch with more blades. This is just my understanding of the engineering of props.Not trying to start a war like on the Etec site. nevjb
 

Jdeagro

iboats.com Partner
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Jul 30, 2003
Messages
1,682
Re: Need help w/ heavier loads - propeller change?

Surfstar;<br /><br />I would agree with Walleyehed and go one step lower in pitch. As for the Lifters being similar to the Smart Tabs, I would disagree. Smart Tabs are preloaded with lift pressure and are angled down at slow speeds for more lift. Even when the boat is on plane the tabs are preloaded with lift presure. This insures that the boat is getting the appropriate lift for the conditions and speed. It is an active system similar to the suspension on a car.
 
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