how low can the end of prop be in the boat and still be ok

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
dick2256
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how low can the end of prop be in the boat and still be ok

Postby dick2256 » Sun Jan 29, 2017 9:37 am

I am going to lower my motor

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Deano
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Re: how low can the end of prop be in the boat and still be ok

Postby Deano » Sun Jan 29, 2017 9:57 am

The realistic answer really depends on what power/prop you are asking about.

Typically, for a DD with a 72" prop, the rule of thumb has historically been
a beer can down and forward from the top of the transom.

At the other end of the spectrum, I've seen a big block with BIG prop down near
the stringers literally making it self bailing, and with no deleterious effect on thrust.

More info please . . .
What do have, where is it at now, and where are you wanting to move it to?
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

dick2256
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Location: Carson Iowa

Re: how low can the end of prop be in the boat and still be ok

Postby dick2256 » Sun Jan 29, 2017 10:06 am

DD 60" prop on that corvair Right now prop is about 1-1/2"s above the top of the boat

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Deano
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Re: how low can the end of prop be in the boat and still be ok

Postby Deano » Sun Jan 29, 2017 10:26 am

I wouldn't worry at all about dropping it 6 inches. That would put you 4.5" below the transom.

If you figure 6" on a 72" prop (which is known to be fine), that represents 1/12th or about 8.3 %.
The same difference in your case would be 60" x 8.3 % = 4.8" below.

The bulk of the air coming off the prop moves down the blade and then goes back.
There is very little thrust from the tip of the blade. If 4 1/2 - 5" below will accomplish what
you're after, I wouldn't worry about it at all.

Probably, a little more may be ok, but to much more may become the subject of justifiable debate.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

dick2256
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Location: Carson Iowa

Re: how low can the end of prop be in the boat and still be ok

Postby dick2256 » Sun Jan 29, 2017 11:43 am

Ok Deano
I'll go 4-1/2 in the boat
it has always been an easy roller and that should help
I am pulling the engine to put in a few helicoils in some pan gasket bolts holes and 0-rings in the oil return tubes
and it just seems like a good chance to drop it
may be my last year but hope not

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Deano
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Re: how low can the end of prop be in the boat and still be ok

Postby Deano » Mon Jan 30, 2017 10:00 am

Not sure why you would say that, but I will agree,

and also say that I hope and pray that it is not either. :)
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

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Bonefish
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Re: how low can the end of prop be in the boat and still be ok

Postby Bonefish » Mon Jan 30, 2017 7:52 pm

"Typically, for a DD with a 72" prop, the rule of thumb has historically been
a beer can down and forward from the top of the transom. "

Now that's some genuine engineering right there :cheers: :toothy7:

dick2256
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Posts: 305
Joined: Sun Jun 05, 2011 12:25 pm
Location: Carson Iowa

Re: how low can the end of prop be in the boat and still be ok

Postby dick2256 » Tue Jan 31, 2017 8:53 am

Deano wrote:Not sure why you would say that, but I will agree,

and also say that I hope and pray that it is not either. :)

may be my last year of air boating not much traffic on the river I run on and at my age
it is some times a log walk if I would have engine problems
I even bought a new carburetor for that thing so looking forward to a few more years.
This johnboat has ribs and I tried 1/4" poly on it but could never keep it from leaking
Thinking about finding some used 1/2" poly and ripping it 2 " wide and putting a piece 2"s wide in between the ribs and 12 feet long
any idea how much weight I would be adding would be 14 pieces 2"s x 12"
it would be great protection against the larger rocks we have in this river
would like to find some older poly someone has removed


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