Best prop for pocket tunnel

If, the cutout, it is as above, posted by Mr. E

Then the shape of the 'tunnel' cutout for the prop...is terrible.
The hard corners will not promote smooth clean axial flow into the prop.

 

That is the boat in question, as he posted on another thread.

 

Great information here. I plan to experiment with thru hull fittings to keep more or less a "vacuum" I guess you could say to keep the tunnel full.

Picture this. I will put a bronze scoop style fitting at the furthest part of the tunnel facing backwards toward the lower unit then, put 2 chrome thru hull fittings ahead of the bronze fitting. One at the leading edge then the other at the middle. Hopefully this will keep the tunnel full. Let me know what you think.

 

It is not a catamaran. The tunnel goes 6' towards the front the the bottom of the hull. Rule of thumb is 1" of rise to 1' of length, or so I've been told.

 

Proper venting negates a lot of that problem. But, this is the first pocket tunnel I build so it's trial and error. If I have to, I will add some radiuses in the corners. What do you think?

 

Ask yourself this question - what shape are the inlets on waterjets?
Square, square with radii in the corners, or just circular...

 

Waterjets are a whole different being. They are round, due to the impeller of course. I see your point though. Great point my friend. With all due respect sir, point out what is wrong, tell me how to fix it please. My ex father in law had a pocket tunnel just like this one. No vents, it did ok. Not perfect, but ok.

 

Also, the scoop is square.

 

I never said it wont work, it is just not dieal hyrdodynamically - that's all.

So, you're not using a prop - to move your boat?

If you are using a prop... is the prop round or square?
Then you have your answer...

 

Lol! I figured that's where you were going. It makes perfect sense. No it isn't ideal, but, I plan to experiment with a vent. I may even put a water pick up on the transom for extra protection. Will just have to see how it works is all. If I build another I will round her off. Thanks for the input AD HOC! It is GREATLY appreciated!

 

I think you are confusing ventilation and cavitation. A propeller will cavitate because it is overloaded and the forward face will have a decrease in pressure that make the water boil. In short, cavitation causes steam explosions resulting in vibration and erosion on the propeller blades. A tunnel needs to have little clearance between the hull and the blade tips. Otherwise, the propeller will probably ventilate, create a huge bubble around it and thrust will go to zero. The reason for the circular section of a tunnel and small clearance, is that the water should flow in it and fill the tunnel.

 

Gonzo, an outboard is going on this boat. I have been looking at surface piercing propellors. I found a vendor that is willing to let me try any prop I want. It will be a learning experience for sure.

 

Let me post a video from you tube of what style of boat this is.