Air Dump ?

[Don Templeton]

I asked this question on another thread but didnt get a reply.

Does anybody use an Air Dump on thier riggers? ( angled up rear tob bottom to reduce lift at the transom and keep some props from blowing out.

Any thoughts on the subject?

Don

 

[Terry Keeley]

I did that on mine but the transom narrows and the bottom corners are rounded so I don't think there's that much effect. Probably be more effective with a square rear and sponsons making more of a "tunnel" back there.

Pretty sure the Crapshooters and Roadrunners use that, what about the Eagles?

 

[Ron Jefferson]

Crapshooters are perfectly flat on the bottom at the rear for the tub.

RJ.

 

[Joe_Knesek]

Road Runners have been built that way for years.

 

[Don Templeton]

Crapshooters are perfectly flat on the bottom at the rear for the tub.
RJ.

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Crapshooters were made that way for many years but it must have changed some time after Stu took over. I guess because of the change to flex drives.

I was just thinking that with rear sponsons or center sponson helping to launch large props combined with the air dump at speed to keep the prop nailed it might not be a bad way to go.

Don :huh:

 

[Joe_Knesek]

Crapshooters are perfectly flat on the bottom at the rear for the tub.
RJ.

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Crapshooters were made that way for many years but it must have changed some time after Stu took over. I guess because of the change to flex drives.

I was just thinking that with rear sponsons or center sponson helping to launch large props combined with the air dump at speed to keep the prop nailed it might not be a bad way to go.

Don

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Don, I ran my Extreme with 3 sponsons with the middle sponson an 1/8 inch shallow of the outside sponsons. This allowed it to ride out of the water once the boat got on plane.

 

[Preston_Hall]

3 sponsons on a RR will killyou, too much lift. Crapshooters are flat and they launch extremely well.

 

[Don Templeton]

That's interesting Joe. On my older shooters with straight shaft the center sponson, which was also the strut bottom, was deeper than the outside rears by a good little bit.

"Go Figure!" :huh:

 

[Grimracer]

I asked this question on another thread but didnt get a reply.
Does anybody use an Air Dump on thier riggers? ( angled up rear tob bottom to reduce lift at the transom and keep some props from blowing out.

Any thoughts on the subject?

Don 

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Im not so sure that is whats happening with this... is it?

When you think about it having the back raise up it might slow the air down as the boat operates, but, this would only equalize things. If what we know is true and that is that air speeds up as its forced to pass through a smaller area,…. then if we follow the idea listed here good old atmospheric pressure is having its way with you..

i think i have just confused myself again. :blink:

Grim

 

[Joe_Knesek]

I asked this question on another thread but didnt get a reply.
Does anybody use an Air Dump on thier riggers? ( angled up rear tob bottom to reduce lift at the transom and keep some props from blowing out.

Any thoughts on the subject?

Don 

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Im not so sure that is whats happening with this... is it?

When you think about it having the back raise up it might slow the air down as the boat operates, but, this would only equalize things. If what we know is true and that is that air speeds up as its forced to pass through a smaller area,…. then if we follow the idea listed here good old atmospheric pressure is having its way with you..

i think i have just confused myself again. :blink:

Grim

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So what you and Preston are saying is that the flat bottom speeds up the air producing more downforce? This doesn't sound right. I think what causes the flat bottom tub to launch easier is that it doesn't allow the back of the boat to drop down as far at the launch. At the launch, the air speed is practically non existant. It's more to do with hydrodynamics than air at that point.

Preston is abolutely correct about 3 sponsons once the boat is at speed. If the 3rd sponson is in the water, the boat is so hard to drive it's unraceable. You are better of living with a tough launch than to add the 3rd sponson and suffer through 6 laps of holding your breath.

 

[Don Templeton]

OK, lets just call that center sponson a strut because that's exactly what it is. At speed it does not run on or in the water anymore than your metal strut or rear sponsons will if set up correctly. The prop is going to lift all of this out of the water.

I don't understand why you would call this set up unracesble. I have never seen or experianced this problem. Not in a 67, not in a 12! I'm talking about rears that set even with the centerline of the shaft and a center strut that is 1/32 to 1/16" max below the prop hub.

I'm not trying to argue, just asking. In my case the shaft is not adjustable so set ups are done with fronts and prop work.

Don :huh:

 

[Preston_Hall]

So what you and Preston are saying is that the flat bottom speeds up the air producing more downforce? This doesn't sound right. I think what causes the flat bottom tub to launch easier is that it doesn't allow the back of the boat to drop down as far at the launch. At the launch, the air speed is practically non existant. It's more to do with hydrodynamics than air at that point.

I only said that my Crapshooter launches very well and it has a flat bottom. But while we are talking about it I think the CS will slow to speeds that other boats don't dare and come back on the pipe almost instantly while producing some of the fastest heat racing speeds. I also think any boat will launch easily if set up correclty. If only the characteristics of a CS could be combined with the heat racing ability of a RR. Then again if you want to go fast then learn how to drive. I have watched John Finch drive F Hydro, E Hydro, and D Hydro into the record books. The big deal is that he took over 1 second of the existing records. The F Hydro was exceptionally fast but the E & D just seemed fast.

OK, lets just call that center sponson a strut because that's exactly what it is. At speed it does not run on or in the water anymore than your metal strut or rear sponsons will if set up correctly. The prop is going to lift all of this out of the water.

Not really because none of the well running boats run on the rear sponsons and outside sponsons aren't struts.

It seems funny how 3 sponsons (or the addition of sponsons) adversely affect some boats but yet the Eagle started with two, then one, and now three. What's the difference? The biggest difference I see is the tub is more narrow than the rest.

 

[Don Templeton]

Preston,

Re-read what I said. I believe it is exactly what you replied! :lol:

Anyway, thanks for your thoughts and I've got one more question.

How wide is the inside of the tub on a Crapshooter 60 boat?

Thanks,

Don

 

[Preston_Hall]

Preston,
Re-read what I said. I believe it is exactly what you replied! :lol:

Anyway, thanks for your thoughts and I've got one more question.

How wide is the inside of the tub on a Crapshooter 60 boat?

Thanks,

Don 

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Don,

That is exactly what you said. :huh: I wonder where I was?

I have a 40 CS.

 

[Terry Keeley]

I asked this question on another thread but didnt get a reply.
Does anybody use an Air Dump on thier riggers? ( angled up rear tob bottom to reduce lift at the transom and keep some props from blowing out.

Any thoughts on the subject?

Don 

84918[/snapback]

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Im not so sure that is whats happening with this... is it?

When you think about it having the back raise up it might slow the air down as the boat operates, but, this would only equalize things. If what we know is true and that is that air speeds up as its forced to pass through a smaller area,…. then if we follow the idea listed here good old atmospheric pressure is having its way with you..

i think i have just confused myself again. :blink:

Grim

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I think Don's referring to the "ramping" of the rear part of the tub instead of remaining flat. If the air does tend to adhere to the surface it would speed up causing a lower pressure just like a venturi, or negative lift (downforce) on the transom.

My old hard shaft Crapshooters were like this, looks like Stu made a switch to a flat bottom somewhere along the way.

I wonder if the shape and angle of the rear sponsons or winglets have a greater effect on keeping the transom planted or not? If you look at the rears on most successful commercial boats they look like they would cause downforce at the transom. I've mounted my rears (flat top) at 0-1-2-3 degrees (nose down) to the hull top, guess what runs the best?

How about tunnel hulls? Do they generally have a flat bottom towards the transom or is there some "rocker"?

 

[Terry Keeley]

How 'bout the Eagles, are they ramped or flat?

 

[Joe_Knesek]

I think Don's referring to the "ramping" of the rear part of the tub instead of remaining flat. If the air does tend to adhere to the surface it would speed up causing a lower pressure just like a venturi, or negative lift (downforce) on the transom.

My old hard shaft Crapshooters were like this, looks like Stu made a switch to a flat bottom somewhere along the way.

I wonder if the shape and angle of the rear sponsons or winglets have a greater effect on keeping the transom planted or not? If you look at the rears on most successful commercial boats they look like they would cause downforce at the transom. I've mounted my rears (flat top) at 0-1-2-3 degrees (nose down) to the hull top, guess what runs the best?

How about tunnel hulls? Do they generally have a flat bottom towards the transom or is there some "rocker"?

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Terry, E.J.'s Lynx has no rocker at all. The tunnel has treatment that seems to control the airflow but I'm not sure if that's Carl's intention or not.

 

[Don Ferrette]

How 'bout the Eagles, are they ramped or flat?

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Ramped.

 

[John Knight]

Preston,
Re-read what I said. I believe it is exactly what you replied! :lol:

Anyway, thanks for your thoughts and I've got one more question.

How wide is the inside of the tub on a Crapshooter 60 boat?

Thanks,

Don 

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Don,

If I have my measurements correct, the overall width of the 60 CS is 4.5". Using 1/4" side rails gives 4" inside the tub.

John

 

[Flyin Rat Sass]

it seems to me that any extra d-force (more than you need) is wasting energy. and for my other thousand words....