changed prop with no gains?

[Srooney89]

hey guys,i have a 2000 18v90 kenner with a 2000 90hp mercury 2 stroke 3 cylinder on it. i was getting 36mph with a stainless 3 blade 13.25 inch 19 pitch prop at 6000 rpms. i know the working range of the motor is 5000-5500 rpms so i went with a 13.25 aluminum 3 blade 21 pitch. i am still getting 6000 rpm wot at 36 mph. all on like days calm water at same timeframe for tide. all on gps. what is going on? why am i seeing same rpms with a higher pitch prop and same speeds when i should see around 40-43 mph wot? also tests were done with same 2 people on boat with same gear.

 

[jimmbo]

Are both props made by the same manufacturer? Switching from SS to AL adds another variable to the equation. A 19 in SS may have more rake and cup than a 21 in AL. The AL might be ventilating or cavitatiing causing more slip than the SS. There is also the possibility the hub is slipping on the AL. Or one of the props was incorrectly labeled.

 

[Srooney89]

the stainless is a Michigan prop and the aluminum is a turning point hustler. i was wondering if the flex of the 21 aluminum makes it more of a 19 pitch with slip and flex. im wondering if a 21 ss would actually give me the numbers i am seeing on paper. i just didn't think the aluminum fresh prop was going to actually lose about 1-2mph and under perform the chewed edge of the ss prop. by chewed i mean it has a slight rough edge as in not perfectly polished and just a tad sharp/rough. could the 21 aluminum under load cause it to flex and axt as a 19 stainless? performance wise?

 

[jimmbo]

The past few years it seems people have really wrapped their heads around Aluminum props' 'Blade Flexing'. It may have occurred in the 1920s and 1930s when racers were trying to get better performance by having very thin blades. Improvements in Aluminum alloys have resulted in stronger denser blades. Grab a blade and try bending it. It is not very flexible. On a 20 inch pitch I doubt any 'flex' would be more than 0.02mm or 0.0009842 inches. Spun rubber hubs will slip, raising rpm but not delivering the higher rpms to the prop Can you provide pictures of the props, one looking at the rear of the props, the other from the side. Because the props are not the same lineage, things like blade area, rake, cupping, and even pitch measurement can vary considerably among different brands.

 

[Sea Rider]

Expect rpm, performance issues when changing a OEM prop for other non OEM one or when switching between 2 different prop brands as well.. Which prop material do you like most SS or Alum ? Can send the one you like to a prop shop and have it repitched + 2 sizes to lower wot rpm to around 5500 max wot range, assuming OB will achieve + 250 wot revs going per one more pitch which isn't known. Only a wot test as currently loaded will tell...

Happy Boating

 

[gm280]

I realize some folks think both Aluminum and Stainless Steel materials will basically run the same. But take one of each from the same manufacture and cup and pitch, and the SS will perform better in almost all situations. And that is because it is a stronger material with less flex and blade thickness. A thinner blade allows it to spin easier in the water because of reduced friction. Same effect when you raise your OB on the transom. Less water friction. Otherwise aluminum props would rule the speed boat industry. Also, hit something with a SS prop and the same with an aluminum prop and see which one has the most damage. I bet it will be the aluminum every time. There are just different types of metals with different abilities. JMHO