3 blade or 4 blade prop?

[Dave T]

I am trying to find some info and opinions on which is better a larger diameter 3 blade prop or a smaller diameter 4 blade prop. I have built an outboard using an 11 horse Briggs [see my thread 11 horse Briggs outboard conversion under do it yourself marinizing]. I ran some rough calculations with the prop calc program from Castle marine and I will need either a larger diameter 3 blade prop or an 8 1/2 diameter 4 blade. To use the 3 blade required I'll need to cut off the cavitation plate and raise it about an inch this I can do if needed or I could go with a 4 blade 8 1/2" that would clear the plate. Is there any advantage to either solution?

Dave T

[Richard Woods]

I assume you are looking for high thrust, not high speed?

I asked a Honda dealer that question just last week

He said go for the 4 blader. But then Honda is one of the few outboards that come with a 4 blade option.

So I hope someone else answers your question to give an unbiased view

Richard Woods of Woods Designs

www.sailingcatamarans.com

[tuantom]

You're using an 11hp low rpm engine - without getting lost in the details, and if costs are equal, I'd think not having to cut off or modify anything with the 4 blade prop has some weighty advantages right away.

[Submarine Tom]

For the degrees of magnitude involved here (small) go with the four blade and skip the modification.

-Tom

[gonzo]

If the diameter is not restricted, three blades will be more efficient.

[tunnels]

Originally Posted by gonzo

If the diameter is not restricted, three blades will be more efficient.

But 4 blades will be smoother running and has a smaller wake !!!

Personally having a outboard and used both 3 and 4 bladed props the 4 blades was the better option and just kept the fully reconditioned 3 bladed prop for a spare under the seat !!

[Mr Efficiency]

Given that 95% of outboards carry 3-bladed propellors, they must have an inherent advantage. There are 2,4,and 5 bladed options in some cases, but it seems experience has led to 3 being the norm, except in specialty applications.

[gonzo]

Outboards have a limited space so four blades often work well. The anti-ventilation plate is the limiting factor.

[sabahcat]

Originally Posted by Mr Efficiency

Given that 95% of outboards carry 3-bladed propellors, they must have an inherent advantage.

cheaper to make than 4 or 5 bladers?

[gonzo]

They are cheaper and more efficient. A two blade would be even more efficient, but have a bigger diameter and not as smooth running.

[Ad Hoc]

A 4 bladed prop is only useful where there is insufficient total blade area when sizing a 3 bladed to avoid cavitation. The disadvantage of a 4 bladed is that they are slightly less efficient, you would experience slightly lower rev's at full rack. 3 bladed props also help to avoid resonant frequencies.

A rule of thumb is when changing from 3 to 4 bladed take about 1 inch off the size of pitch + diameter.

[Dave T]

Thanks for the replies guys. It will probably be about a month before I can get back to my motor projects I want to get my boat and trailer finished first and then I'll run some better calculations.

Dave T