Some things may not apply

[Papee]

After watching posts for information on some other sites I visit I felt the need for a "some things just don't apply" thread. These are things that may not apply when using common ideas about airboat setup. As we all know, on mini airboats everything you do to make out small power plants work better is a gain on a mini. We do things we wouldn't normally pay attention to on a full sized boat.

One of these is putting part of the prop below the transom, we all know about this and we do it to lower the center of gravity on a boat. Most times if you have the width for a prop you also have the vertical room needed to keep the prop above the transom, the only reason to lower it is because of the COG deal.

When someone asks me the old question" How far below the transom can I run my prop?" My answer is a question, why would you want to? Most of us are running a full width boat and lowering the prop a few inches is not going to make a huge difference but that little extra push you gain is small but still a gain when setting up a mini airboat.

As always this does have it's exceptions but in general, in my opinion it is best to keep all of the prop above the transom. This is the way the prop was designed, to have unrestricted flow to and from the prop. SO, to get the best performance from a prop my advice is to keep it above the transom if possible.

Disclaimer: These are only my opinions and do not reflect the opinions of most normal airboaters.

[buster]

Papee,

Just so we know, could you give us your definition of a "NORMAL" airboater


Last summer, I took a bunch af 12" long strings and taped one end of each to different spots around my rudders, and the rear of my cage. I just wanted to see where the propwash was the strongest. To say the least, I was very surprized. It was nothing like I figured it would be.That bottom few inches of the prop may not even do any pushing on some boats!

Thanks, Dave

[Duane Scarborough]

After watching posts for information on some other sites I visit I felt the need for a "some things just don't apply" thread. These are things that may not apply when using common ideas about airboat setup. As we all know, on mini airboats everything you do to make out small power plants work better is a gain on a mini. We do things we wouldn't normally pay attention to on a full sized boat.

One of these is putting part of the prop below the transom, we all know about this and we do it to lower the center of gravity on a boat. Most times if you have the width for a prop you also have the vertical room needed to keep the prop above the transom, the only reason to lower it is because of the COG deal.

When someone asks me the old question" How far below the transom can I run my prop?" My answer is a question, why would you want to? Most of us are running a full width boat and lowering the prop a few inches is not going to make a huge difference but that little extra push you gain is small but still a gain when setting up a mini airboat.

As always this does have it's exceptions but in general, in my opinion it is best to keep all of the prop above the transom. This is the way the prop was designed, to have unrestricted flow to and from the prop. SO, to get the best performance from a prop my advice is to keep it above the transom if possible.

Disclaimer: These are only my opinions and do not reflect the opinions of most normal airboaters.

Papee,

I agree. One should not start out a build with the intention of having the prop tip running below the transom. It just makes more sense to keep it above the transom.

In my case, I built my engine stand with the idea of using a 60" prop, but ended up using a 67" prop. So my prop tip will be running about 3 1/2" below the transom. Fortunately, I built the cage to fit the width of the hull. (approx 72" wide, and proportionally as tall)

In hindsight, I should have built the engine stand at least 3 1/2" taller. But I'm not going to start over. I "may" loose a little thrust because of it, but I'm not starting over. I'd change props first.

So, my recommendation to anyone starting a new build would be to build the cage as wide as the hull allows, and tall enough to accommodate a prop that long. And the engine stand should be tall enough to handle that prop. Then you can use the longest prop that will fit in your hull.

It's very easy to use a shorter prop with rigging that was built for a longer prop. The opposite is not true.

Duane

AKA "Abnormal"

[Duane Scarborough]

Papee,

Just so we know, could you give us your definition of a "NORMAL" airboater


Last summer, I took a bunch af 12" long strings and taped one end of each to different spots around my rudders, and the rear of my cage. I just wanted to see where the propwash was the strongest. To say the least, I was very surprized. It was nothing like I figured it would be.That bottom few inches of the prop may not even do any pushing on some boats!

Thanks, Dave

Buster,

I sure can't (and won't) argue with that. But I also wonder if the design of the Aircat's transom affected that some? The way I remember, the actual transom of your Aircat dips lower than the (whatever you call it) that is a little ahead of it. I wonder if maybe that is blocking some air flow.

Did you take any pictures of this test? It'd be interesting to see.

Thanks,

Duane

[buster]

Duane,

I did try to take pictures without sucess, I was alone and my arms wouldn't reach from the camera to the gas pedal That test should be easy for anyone to duplicate though, If anyone is interested.

I guess the Aircat might be different from the average boat. The top of the transom is now flat and about 14" front to rear. I changed that "recessed" part you may recall.

Dave

[Duane Scarborough]

Duane,

I did try to take pictures without sucess, I was alone and my arms wouldn't reach from the camera to the gas pedal That test should be easy for anyone to duplicate though, If anyone is interested.

I guess the Aircat might be different from the average boat. The top of the transom is now flat and about 14" front to rear. I changed that "recessed" part you may recall.

Dave

Buster,

You're 100% correct. I had forgotten the change that you made to the Aircat's transom profile. (Isn't CRS a wonderful thing?)

I also understand about short arms.

Thanks,

Duane

[Duane Scarborough]

Papee,

Just so we know, could you give us your definition of a "NORMAL" airboater


Last summer, I took a bunch af 12" long strings and taped one end of each to different spots around my rudders, and the rear of my cage. I just wanted to see where the propwash was the strongest. To say the least, I was very surprized. It was nothing like I figured it would be.That bottom few inches of the prop may not even do any pushing on some boats!

Thanks, Dave

Buster,

I just had another thought.

Someone (I don't remember who) told me that most of the make-up air from a prop does NOT come from directly in front of the prop (like we'd assume), but from a radial angle around and forward of the prop. At low airspeed, that's a severe angle. At high airspeed it becomes shallower. So, a prop moving through the air at 25 MPH would get more of it's air from the sides than one moving at 75 MPH.

If that's true (I ain't sayin' it is), then wouldn't ANY hull somewhat block the thrust provided by the lower 1/4 of the prop? I'm just saying that It ain't an airplane with open air all around.

Maybe that explains some of what you saw with your yarn wind indicators?

Just giving you something to think about...

Duane

[Papee]

OK, first of all I am not trying to start an argument about this subject, just pointing out differences. The bigger boats just throw a bigger motor at it and are not concerned about the little things. With that being said, if you're going to put part of the prop below the transom why not jut cut the prop down and use more pitch. I not sure the end doesn't do much, just what you are seeing is different than the other length of the prop. Sort of like the air flowing over the back of a pickup.

[buster]

I think Chuck might have been who mentioned that an airboat prop gets a good amount of it's air supply form around the outside circumfrence.

Thats why when some guys cover that area to reduce noise, they also lose a lot of the thrust.

Now that you guys got me thinking about that, I'm guessing I'm loseing some thrust at the bottom, since my prop tips swing by the top of the flat transom about halfway point, with about 1" clearance.

Dave

[Duane Scarborough]

OK, first of all I am not trying to start an argument about this subject, just pointing out differences. The bigger boats just throw a bigger motor at it and are not concerned about the little things. With that being said, if you're going to put part of the prop below the transom why not jut cut the prop down and use more pitch. I not sure the end doesn't do much, just what you are seeing is different than the other length of the prop. Sort of like the air flowing over the back of a pickup.

Papee,

I don't think that either me or Buster are trying to argue your point. And we (at least I) agree with what you've said.

I sure as heck ain't a aero physicist. But I do like to understand how things work. I think that's all me and Buster were doing, is discussing that.

Here's how I understand it. That does NOT mean it's right.

Air goes into a prop at an wide angle (big area going in), and leaves it in a narrow angle (smaller area of thrust). It's kinda' like a cone of air movement. Bigger in the direction of forward movement, and smaller behind you, in the direction of thrust.

The faster it's moving forward (airspeed) through the air, the shallower the incoming angle becomes, and the broader the leaving area becomes. At very high speed, it'd be almost the same. (almost a straight line) So it becomes less like a cone, and more like a tube.

Where are the edges of this imaginary cone? I don't know. But I suspect that it's a little out from the tip of the prop. Some claim that it's a little in from the tip of the prop.

I know I haven't explained it very well. But that's how I understand it.


Duane

[buster]

Papee,

What Duane said .

Dave

[Papee]

I knew you guys got my point of the thread, I was just adding it for future readers. I DO give you some credit Duane.

[Duane Scarborough]

I knew you guys got my point of the thread, I was just adding it for future readers. I DO give you some credit Duane.

O.K.

BTW, here's a link to something that sort'a supports my cone concept...

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

Don't get lost in the formulas.

Just study the illustration.

Duane

[Papee]

That may explain my thoughts a little better. I was trying to get across that if you disrupt the tips of the prop you may be messing with the cone pattern losing thrust.

[n4wsp]

To deep for me

Steve

[Flathead]

Judging by the the amount of trouble I'm having with my electrical system, I'm not the brightest bulb in the box (ha!), but it seems to me no matter how high the propeller is mounted in relation to the transon, the bottom of it is going to be 'starved' of air no matter what. A few inches above or below the transom is going to make little difference in the final amount of thrust. Of course, what do I know. Really, Seriously

Flathead

[buster]

See Duane,

I couldn'ta said it better myself

Dave

[Duane Scarborough]

Judging by the the amount of trouble I'm having with my electrical system, I'm not the brightest bulb in the box (ha!), but it seems to me no matter how high the propeller is mounted in relation to the transon, the bottom of it is going to be 'starved' of air no matter what. A few inches above or below the transom is going to make little difference in the final amount of thrust. Of course, what do I know. Really, Seriously

Flathead

Flathead,

I think that you deserve a GOLD star. You have, in very few words, summed up what I was TRYING to say.

Of course, I don't think that anyone should start out with the intent to run the prop tips below the transom. That HAS to have SOME negative effect.

But I also think that just having a hull (doesn't really matter what kind) forward of, and IN that incoming cone has got to affect the airflow that the bottom part of the prop gets.

The illustration is of a prop in free air, with no obstructions of any kind. DUH, we also have engine, rigging, etc. in that airflow.

Purely from a thrust perspective, we'd be better off to place the center-line of the prop about 20 feet above the water, in clean air. There's thousands of reasons why we don't do that, of course.

But a few inches below the transom won't be a big deal I hope not anyway.

Duane

[chuckitt]

When mounting the engine too high on a boat, the trust will make the boat run on the nose to a piont to where you can't drive the boat. Most high HP boats will have some amount of the prop tip below the top of the transom. We try not to exceed 3 inches. The shorter the boat, the more critical the center line of trust is. On mini airboats with short hulls we have put the tip of the prop level with the top of the transom using 60 inch props. I think with a 64 inch prop, we had about 1 inch below the top of the transom and the Orange Krush hauled but on dry ground. On a 16 ft boat with 40 to 60 HP, you might want to keep the tip of a 60 inch prop about 3 inches above the top of the transom so the leverage will help keep the bow down.
Thanks, Chuck

[Papee]

Ah the voice of reason.