FISHSTICKER wrote:So I can run my prop as deep as 14" down in the boat. Would any of you consider actually doing that?
I see in the post above someone was running 12" in the boat
IMO, in your specific case the answer would be a definitive NO, I wouldn't do that.
As is true in many different cases, more of a good thing doesn't always remain a good thing if it becomes excessive.
It is worth pointing out, in that particular instance, the test subject was an 84" four blade prop, not a 76" three blade.
It's important to remember that:
Deano wrote: . . . this percentage of loss also gets smaller as the prop diameter and blade count get larger.
Realize that what is allowable or works well on a 20' boat w/an 84" four blade prop, will not necessarily be desirable on a 12' boat w/ a 76" three blade. Although the variables do remain the same, they are quite simply 2 completely different orders of magnitude.
FISHSTICKER wrote:Since thrust center line being close to the bottom of the boat is important should I run a shorter 4 blade?
Again, it is my opinion that I would not expect that would be the preferable option; although I don't exactly know the parameters under which you are operating (more on that toward the end). This question bears on different variables than do the previously mentioned ones.
Fundamentally, diameter trumps pitch where generating thrust is concerned. Consequently, it's the first variable that should be considered.
In our case . . . the boat and (then subsequently,) the cage width define this constraint for us. (Whoever doubts this premise need only spend some time trying to find a large helicopter running less than an 86" diameter prop.
Where blade count is concerned, adding another blade will introduce two potentially negative aspects: First, there will be more ongoing parasitic drag due to increased total blade area which creates more 'overhead' before the desired increase in thrust can be realized or seen. Secondarily, it will increase the 'total mass' of the propeller which will primarily effect the 'snap' due to the additional loading put on the engine when it is attempting to quickly accelerate.
If any given blade count (of any type blade set) will hold the engine back to the desired engine rpm, then adding an additional blade will have deleterious effects on performance and will be moving backwards where propeller efficiency is concerned. Conversely, If the given blade count will NOT govern the engine at the desired rpm, then adding a blade WILL improve the propeller's efficiency and adding another blade would logistically be justified.
I am quite familiar with the Maximus blades sets (and consider them to be a fine choice), but lack enough experience with your EcoTech and your gearing to comfortably predict the point at which you would justifiably need to add a fourth blade to govern it at your intended rpm. In this regard, OneBFC, is most likely your best resource as he is both familiar and experienced with those specific parameters.