What is the difference...

[Alessandro Garabaghi]

Can someone explain the difference besides numbers between

2 blade 36mm 1.9

and

2 blade 38mm 1.8

they both give 68.4 but one is bigger with less pitch?

[Jamey]

Actually both have the same pitch since that is the 68.4 mm number you have listed. What is different is the DIAMETER 36 vs 38 and the pitch RATIO 1.9 vs 1.8 .
Pitch = Diameter X Pitch Ratio

[Alessandro Garabaghi]

so what characteristics does ratio give?
same with diameter... i assumer the large the diameter the more load, the higher the ratio the more load?

[Jamey]

It is complicated to say the least. Different set ups will respond differently. Personally I start with a low ratio like 1.4 and find the appropriate diameter range for the hull. Maximum diameter without affecting handling and keeping slip in check will likely be the most efficient. Obviously larger blades will have more water resistance so there will be a point of diminishing returns as well. At that point if I need more speed I may experiment with higher pitch ratios in the range of the diameters I previously found appropriate. This maybe totally wrong advice for your boat so it will take some research on your part to see what people are running and their results as well as testing on your end. Diameter increase does increase load but it reduces slip. Going from a 2-blade to a 3-blade can have the same affect. Increasing the ratio will increase load as well but usually increases calculated slip. Pitch ratio can have an affect on transom lift as well but I won't get into that

So in your example above.... theoretically those props will go the same speed. Potentially the smaller diameter prop will need slightly less power to turn due to less water to blade resistance. Sounds like an easy choice? But that all depends on the boat and setup so it is very important to test. It may actually be that the smaller diameter prop doesn't have enough blade surface area to push the hull without slipping like crazy and giving unexpected results.

I only touched on the subject but hopefully that helps a little. Read test read test read test is the best advise I can give.

[Alessandro Garabaghi]

yea ive found the graupner plastic lines which will let me check various diameter inexpensively... here goes!

In the setup posted the boat with the large prop lower pitch is about 10mph faster

(same boat same electronics but different brand batteries not that i think that would make a difference of 10mph)