I thought that might get your interest It is actually an airboat question, but airboats are somewhat related to gyros.... On an airboat forum I am also on, the question was asked "how much can I lower my prop inside the boat?" as in, how far below can a prop tip be compared to the transom?

http://www.southernairboat.com/phpBB3/v ... 5ad4fdecf7

My comment, based on my experience when I bolted a damaged gyro into a flat bottom punt years ago and used to blast around in the rivers of Canberra..... I just bolted the frame with no wheels and the bent mast sawn off into the boat, right on the floor.... the prop was down about 6" below the top of the transom... it ran ok, but the wind coming forward under the seat (bouncing off the transom) was cold and wet and anything that found its way into the boat, found its way to the back and kept getting blown forward, so there was a constant stream of crud/leaves/twigs cycling forward past my ankles and back again to be blown forward (annoying) ..... Anyway, I unbolted the gyro frame and lifted it so that the tip of the prop was level (maybe an inch below) the transom top and re bolted it.... not only was the excessive wind around my ankles gone, the boat went 10 times better...... it was like a different boat


this was my comment:


Online
Southern Airboat Member
User avatar

Joined: Tue May 26, 2009 10:28 am
Posts: 1052
Location: Landsborough, Queensland, Australia

The lower the prop goes below the back, the more anything down there gets blown into the boat, not out the back.... if you get some water over the back the prop can keep hitting the water which bounces back at the prop for another hit.... can damage prop...

How far above or below is it now? or is it level?

Anyway, another poster laughed and said:


Offline
Southern Airboat Member

Joined: Mon Aug 24, 2009 8:16 am
Posts: 3646
Location: SCRAMBLETOWN, FL


CactusJack wrote:

The lower the prop goes below the back, the more anything down there gets blown into the boat, not out the back.... if you get some water over the back the prop can keep hitting the water which bounces back at the prop for another hit.... can damage prop...

How far above or below is it now? or is it level?

:lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: really????
my c/m boat 2 blade 10 inches in
my a/c boat 8 blade saber 8 inches in
it does the exact opposite, throws the water out...

_________________
JUDGE NOT, THAT YE BE NOT JUDGED..........

And another comment:

Deano

Post subject: Re: how much can I lower my prop inside the boat

PostPosted: Sun Mar 08, 2015 7:50 am



Offline
Site Supporter - V
Site Supporter - V
User avatar

Joined: Fri Jan 07, 2005 2:16 am
Posts: 2726
Location: Inverness, FL

The answer to your original question would be NO, six inches wouldn't be to far.

A general rule of thumb is a beer can forward and a beer can down from the top of the transom.

The bulk of your thrust is centered closer to the hub, not at the blade tips. Some folks intentionally put the prop far enough in the hull to make them self bailing. Not that I necessarily recommend that, but it can and has been done. Six inches in the boat would be a complete non-issue, and is actually closer to standard than what you have now.

To me, it is obvious that the transom blocking (in my case about 6") 8" or 10" of the tip of the prop in this guys airboats and prolly 1/7th of the prop disk is covered by the transom which would be totally disrupting the propwash, the air compressing between the prop and the transom would be throwing junk forward under the motor, making the prop ark at the bottom pushing the blade away (forward) when it compresses air at the transom part of the ark (stressing the prop cyclicly) and basically wasting thrust/Hp/fuel and the drumming of the air pulsing on the alloy transom must be irritating? Remember, the driver sits like 3 stories up on an airboat, so they wouldn't feel the 'wind around the ankles' like I copped, but it would be there for sure.....

Anyway, the last guys comment: The bulk of your thrust is centered closer to the hub, not at the blade tips. to me seems completely wrong? With a prop (pusher) there is less thrust at the hub, heaps of thrust from about 60% out from center and a lot of thrust at the tips? So I believe that the tip being 10" below the top of the transom would be having a devastating effect on reducing raw thrust, as well as buffeting and discomfort (sound and pressure) not to mention the added stress on the prop/crank or gearbox/transom.... to me the transom will be cancelling over 20% of the props wash/thrust production?

I know that many folks have been driving airboats that way since grandpa was in nappies, however, I think they could gain benefits of free thrust, less fuel useage and more comfortable ride (the air pressure in an airboat is why they wear earmuffs, as well as the sound..... I would like to say more to this guy in relation to this prop blanketing, but I would like to hear comments from you guys if you think I am wrong or right before I continue with him about it. It is not important to me, but as you know, I like to help people and I can see performance gains of unbelievable amounts by getting the prop up to the 'beer can below and beer can from' the transom, so the prop blows over the transom, not right at it for a lot of the disk area.

I know trying to 'argue' my point with the posters is a waste of time, I am more interested in other readers who haven't 'always done it like this' who are building a boat, wont make the mistake of burying the prop below the top of the transom.... so I know it wont affect these guys posting, but it might help newbies..... or those who might want to experiment, instead of living in the past

Another reason I believe that the prop pushes more from the tips (right where the transom is) is because a friend in Canberra built a hovercraft, but it only had one engine for lift and drive.... not a separate lift engine.... the prop sat in a well about 8" deep with a slot/hole not much bigger than a letter box opening, the air from the tips pushing thru this slot was ample to lift the hovercraft, even at low throttle, so I know, having the transom blanketing the prop (with no slot) is robbing so much power from these guys....They are always hotting up the motors and chasing Hp, buying expensive props.... then costing themselves any Hp gain by blanketing the prop .

One thing I find amazing, is the reason they want the motor lower in the boat, which ends up with the prop well below the transom, is because they want a lower center of gravity.... Then they put the drivers seat and the 300# driver above the engine you cant get a worse high center of gravity than that if you tried They would be far better off raising the motor 6" and lowering the drivers seat 6" and the CofG would be about the same, with double the thrust (well 20% more for free)

I would appreciate any of your thoughts

I don't know anything about air boats...BUT the majority of the thrust is at the tips, that begs the question for gyros, is the horizontal more effective in the center of the propwash? Or slightly above or below where it gets more airflow from the prop over the entire surface?

Back to the airboat thing, anytning that blocks propeller thrust is going to compromise performance and efficiency.

And.... they like the prop tips down there because the prop 'smashes the water out of the bottom of the boat' like an 'auto bailer'

How good is that for the prop???? NOT simple rain eats away ALUMINIUM props on spam cans..... what does it do to a carbon fiber prop Maybe like in the pic?

Yeah, the tips are where the action happens, as far as I know a prop is a prop, whether it is on a plane, gyro or boat.... in pusher configuration.... the motor and platform before the prop can interfere with thrust production, and I am damn sure that the prop blowing directly at the transom cant assist real well with making thrust.... put a shoe box in front of a fan covering the bottom 20% of the fan.... the reduction of air movement is horrendous

If you look in the pic of the broken prop above, you will see they have made an effort to drop the transom.... many of the newer boats have the transom aerodynamically rounded for smooth airflow, so some manufacturers do know that the prop shouldn't be too far below the transom.... I would have thought that was a no brainer also, remember, for every action, there is an equal and opposite reaction' so therefore, the blast that is hitting the transom, fair on, will be in effect blowing the transom away from the prop (opposite to thrust) so any 'hidden prop' will be just counteracted.... like I said, they buy special oils, special spark plugs, special coils and engine chips, they buy $1,200 super dooper props.... all in an effort to gain some Hp.... then they cover 20% of the prop disk with transom.... and use the super doper prop to belt water out of the boat

Yep, got me on that one, using the thrust from the prop to bail the boat does not make sense to me, especially not only the bottom of the prop covered up but the turbulant dirty air that it kicks back is reducing thrust,,,

You can see the conundrum for them, that boat with the broken prop, they have beautifully dropped the transom.... a great job, but unfortunately I reckon a stern wave has caught up with them as they stopped and because of the low transom, a wall of water has come over the back and gone into the prop.... the prop could only take so much and that was prolly over what it could take I reckon they need a low transom and a flap with a spring up against the bottom of the cage.... when you power on, the blast blows the flap horizontal and not blocking the blast, but when you back off and stop, the flap goes back up to cover the bottom of the prop and water can hit this without reaching the prop t damage it, nor sink the boat

Those blades are about $350 each

I think those blades are probably more than that, that airboat stuff is pricey!!
Why don't gyros use big ol fat props like that? I know tha Bo Collins has one on his gyro but I don't know if He has flown it yet.

The airboats us monster props like that because even tho they can run fast (the boat) on skinny water with little throttle, when they come to running the hill, driving up banks/boat ramps or across the parking lot and up onto the trailer, the need the wide chord for mega thrust and a narrow chord prop cant push that hard.... it would be fine on water , just not on land.... this video is a 4 cylinder aero engine with 11 (eleven) people on it running the hill.... to get that much weight to move on land takes the right prop on a great engine Some of the big block boys have 700+ Hp so they need massive prop chords..... Tims airboat has a fine blade prop like a gyro prop, I think the seller took the good prop off and gave him a shit one.... it can just move on the hill with only the driver.... he needs a better prop to get the Hp into the air.... A gyro, on the other hand, would be fine with a smaller chord prop, because it never has to push that hard.... not to mention, a wide heavy prop like that would be terrible in the air, it is designed to push a boat over water or land, not to fly/cruise thru the air.... can you imagine the weight of one of those props with 5 or 6 blades

The airstream behind a propeller or powered rotor narrows down to about 70% of the.diameter. Doug Riley excellently demonstrated this at Bensen days with an ASI attached to a stick that was held in the prop blast of a gyro. He moved it around to various distances from the prop center and made a graph showing the results.

Basically it showed that any HS outside the inner 70% of the prop did not receive hardly any more airflow.

Precisely Stan! Thank you
The cone of reduction airflow in diameter is the rate of the in/revolution setting of the prop.

So Stan, does that mean that you can put 15% of the prop down in behind the transom and there will be no penalty of thrust? What about the wind buffeting off the transom and blowing stuff up the middle of the boat that I experienced, as well as the extra 100 rpm I got once the prop tips were raised out from behind the transom?

How does a hovercraft lift itself thru a slot near the tips? Not just pump out the skirt, I mean hover.... 1" off the ground... skirt as tight as anything? and that is from about 1/3rd throttle

What's a transom?

The ass end of a boat.... where an outboard clips on.... the flat upright bit at the back

Notice on this one, they have lowered the middle of the transom, so the prop isn't bashing the air against it? Many of the newer boats have reduced transoms so the prop can blow more air out the back, not into the boat? Why would they do this if they don't need to? If the tips do nothing, then why not trim them off? With ducted fan, why do they bother having the shield/surround so close to the tips? If the tips do nothing, then surely the outlet on the blow side of the prop can reduce to 70% of the diameter?

If you dip the prop 15% below the transom you are blocking airflow! There is going to be a reduction of course. But not because of the exiting airflow. That will make no difference but the incoming airflow.
Same as helicopters do while going through translational lift by hitting clean undisturbed air.
Blocking inflow is the same as having disturbed air and it reduces the performance.

Oh I see! Cool thanks

In that example the distance of those HUGE blades are waaaaaayyyyy too close to the transom wall.
There is going to be a direct bouncing effect. The cone size depends on the pitch of course.
Tails are not that close to props!

ANother thing I see with it that on the bottom there is nowhere to go for the air.
Horizontals are not bottoming out. The air flows around them!

I am not talking about inflow from the front of the prop, I am talking about the blast out the back.... like if you get your bench fan you did your experiments with and cover the bottom 1/4 of the fan with some duct tape, will it reduce the thrust/airflow coming up and out where it can get out? I think it definitely will reduce the thrust of an airboat to have the transome blocking the bottom 10" of the diameter of the prop It can't not

That's my point Gabs, there is nowhere for the air to go, except bounce off the transom and go forward under the seat into the boat, not over the back and produce thrust.... here is a pic of a boat being built.... the guy is holding the transom ready to weld it in. The transom he is holding is low in the center to let air over. The older boats I an talking about, the transome is straight across and 10" of the prop is below the top of the transom....

Here is a drawing of the back of an airboat.... the bottom 10" of the prop is directly in front of the transom, so that airflow/thrust is blocked.... does that rob a percentage of thrust? (and waste Hp?)

This drawing shows the motor higher, the transom stepped down in the middle and only 1-2" of prop tip right behind transom.... I believe this will gain a few # of thrust because hardly any air is coming back off the prop and being stopped by the transom