Propeller dynamics?
 

I have a question that some of you aeronautical engineers or experienced pilots might be able to answer for me. My situation is as follows. I am building a twin engine seamaster and my props ended up being closer to the water than I wanted. I am turning 9x4 props with my magnum 25 engines. I know that there is a low pressure area in front of the props that will want to pick up water and “suck it into this area. So the question is this, will a large diameter lower pitch prop create a larger low pressure area than a shorter higher pitch prop at static or near static speeds. For example if I go with a 8x 7 prop instead of a 9x4, as I throttle up to full throttle for takeoff will there be less of a low pressure area in front of the prop with the higher pitched prop when I am just starting to speed up from a stop. I realize that the 8” will be ½” farther from the water and am thinking that the smaller prop disc and higher pitch will not create as much of a suction until it gets up to speed at which point the fuse will be up on step and farther from the water. Is there any science behind this or is it just a try it an see kind of thing.

RE: Propeller dynamics?
 

My gut tells me the smaller props would be less likely to have issues. Throttle up slowly to minimize the vortex. It will diminish as speed picks up.

A bit off topic.... I once watched the engine on a 747 suck the sand off the runway in Hawaii[X(]

Ken

RE: Propeller dynamics?
 

For those .25 engines you have few options:

http://masterairscrew.com/manualinst...ROPELLER2a.pdf

http://www.top-flite.com/accys/topq5000a.html

Go with the 8x6; it is still good, just a little less punch at take off's and climbing at low speeds (due to increased pitch and slip).

The low pressure ahead of the prop does not reach far from the blades.

RE: Propeller dynamics?
 

Also, NEVER use wood props if you are getting water spray in the props else they will soon be shredded.

RE: Propeller dynamics?
 

Thanks for the advise and comments guys. I don't necessarily have a problem yet. I am still building. I was thinking of raising the nacelles but am just going to proceed and fix it later if there is a problem.