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my new prop...

The majority of airboaters are skilled fabricators and can build something like this. However I personally believe the limiting factor to anybody building, designing and testing a duct or shroud will be cost. I can't run one because of how often I hit trees, branches and various other obstacles plus the fact my boat is a floating R-n-D platform that I change my prop configuration on a weekly basis.
 
Yup, even the guys who run the big blocks have to look at overall weight. Adding struture to tie the engine into a ducted prop could get stupid, and expensive, fast.

olf
 
Leo, question.....

We have for years gone to pusher prop arrangements because they appear to be safer. But a tractor arrangement might be quieter (?) for the occupants of the boat (engine sound behind the prop), and more efficient. And ducting an arrangement like that might be easier too.

What are your thoughts?
 
Still thinking ...

Where the hang-up is with a pusher and ridgidly mounted cage is that the engine/prop are able to move around inside the cage because of the need for motor mounts to absorb some of the shaking and torque roll that nearly any engine of any type will do. That appears to preclude a ducted prop because of prop tip clearance.

So what if the engine/prop stand and duct were all ridgidly welded together as one structure, and the flex mounts for all of this were at the attachment points of this structure at the hull stringers ?

Conventional construction welds the forward structure (seat stand, etc.) on to the engine stand for additional strength, but in this case the seat stand would need to be a free standing, solidly mounted structure independent of the engine stand to allow that structure some movement.

That should not cost a lot of money or additional weight to do.

olf
 
No, for simplicity and rigidity I'm talking about making the engine/prop/shround one unit, BUT vibration damping that whole assembly at the base of the engine stand, where it attaches at the hull stringers.

There may be some resonance set up there because of the height of the thrust line vs. attachment point, and the stand would have to very strong to support added thrust without benefit of having the seat stand welded to the front of it, but the engine/prop/shroud could be unbolted and lifted out of the boat as one assembly.
 
It may be too high speed, but what are your thoughts on hydrostatic drive between the engine and the redrive?

There is also a flex coupling thats rubber bushed used in normal inboard marine applications. It may not have enough isolation, just don't know enough about them.

Scotty
 
The big tug boats use hydraulic motors to turn their props, Scotty. It sure adds flexibility. They can rotate on their own length or back up without ever stopping the prop.

I've thought about that before ..... setting the engine in the hull of a boat, driving a pump, with a hydraulic motor up top turning the prop. I discounted the idea because it all started lookin' pretty heavy. A lot more than a long belt.

A flex coupling might work, but then you'd have the engine alignment critical to drive when it torqued.

Thinkin out loud here. Tell me I'm wrong. It makes me think harder :) .

olf
 
What about putting bearing on the tip of the prop to run in a track grove design in the inside of the duct. With some clearance for movement up and down shock. The bearings can absorb the shock, thus making a working platform.
 
Olf,
What I think you are talking about is an airboat with an engine,engine stand, ducted fan/cage that would pivot as to steer? And the seats would be clear of this by mounting them farther forward.

Guess What,
That was patented back in the 70's by Mr. David Wine, Inventor of the first hollow carbon- fiber airboat prop known as Powershift.

Oh, the boat was called Water Whisper.



Bob
 
Swamp, thank God for you guys that know this stuff. No, I'd never seen that design. I was still thinking that the boat would have to have conventional rudders ..... that it would be easier to do it that way.

I'm still kinda hung up on the idea of a tractor prop. Leo doesn't think it would be quieter, but I think he's basing that on the overall sound level.
I'm talking about the sound from the cockpit, and I think the prop sound (which we have anyway) would mask the sound of the engine. And an A/C motor would have to run cooler because it's directly in the prop blast.

See, I want one of those windshields and a big steering wheel like those pictures that Cowboy and Canefan post ..... :lol: .

olf
 
yep it was and I understand that Dave at alumitech was the one who finally got it to run half a$#. There was a lot to that baby. I don't beleive it was meant to run anywhere but in the water.

I have done a lot of research on the shrouds and on the ducts. they hold a lot of promise and all I am going to say for now is that the DEVIL is in the details and so far ya'll aint even close to the problem. It isn't that the motor moves, you can have a fair amount of clearance depending on your prop diameter especially if all you are trying to do is quieten down the boat.

Study on guys it isn't that hard to find and I don't want to take the fun out of it for leo and whitebear and olfart.

The ending will surprise you in more ways than one. You will want one real bad. I mean real real bad, maybe even triple real bad.

White bear I am surprised you havent found it, leo I am surprised you don't know it. And olfart clear your head the answer is right in front of you. When ya'll find it and figure it out you will know why I have not mentioned it on here in any kind of detail ... please understand it before you post it, it is not an area for the careless to go ... bad things will happen to the foolish and you may even find examples of those who have dared enter although most of them have no idea what happened or maybe why it happened would be better, one little slip in fabrication spells disaster, one bad calculation yep disaster again, But the hunt was fun and I will try it one day that is why you may notice the large cage on my current boat. The engine cone/cowling is already being worked on and there is no flat surfaces in my stand and the radiator was supposed to be somewhere else and it will be someday lots of little things add up on this one guys.
 
But I'll bet that you didn't have the prop tips in a trough/groove/indention in the duct that would allow for some movement ...... ?

By the way. You're up early.

olf
 
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