Determining Mono CoB... Transom or Prop??

[Darin Jordan]

I should already know this by now, but apparently it hasn't sunk in yet...

When determining the CoB (Center of Balance, otherwise mistakenly referred to all the time as the CG... ), does one measure along the keel from the transom, or from the prop?

 

[Blackout]

Usually it's measured from the transom. I suppose because strut length can vary depending on the chosen hardware, so it would get confusing comparing different builds....

So why is "Center of Gravity" the wrong terminology? I see them as the same...

 

[Darin Jordan]

So why is "Center of Gravity" the wrong terminology? I see them as the same...

Most people do, but it's not "correct"... accepted, but technically, incorrect...

CG has a vertical and lateral component to it. It's defined by NASA to be:

"The center of gravity is the average location of the weight of an object." ( http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html )

It's measured on the X, Y, and Z to determine it's location.

 

[Blackout]

So, the center of balance is the Y axis of the center of gravity?

 

[Hammerhead]

I need a beer :blink:

 

[John Finch]

I normaly use the cg/ x/y/z/ center of balance to keep the boat flying level when it leaves the water, so the strut would have no effect. when the CG gets more forward than 32 percent as measured from the transom the bow usualy falls first when the boat becomes airborn. Of course boat design dictates a lot of the aerodynamic flight characteristic. I find 27 to 31 percent works on most of my boats so when they leave the water they come back down flat and keep on trucking. I use about 34 percent on SAW record trial boats. I think maybe the seaducer works well at 34 percent for heat racing but it usually falls bow first when it leaves the water. One way to prevent stuffing is to use a spoiler at the transom. Not a wing. A wing is not legal. If the transom does not leave the water the boat can not stuff! Just my experience. I am not the science guy but I do remember the XYZ in flight school when I was in the Navy.

 

[dwilfong]

I normaly use the cg/ x/y/z/ center of balance to keep the boat flying level when it leaves the water, so the strut would have no effect. when the CG gets more forward than 32 percent as measured from the transom the bow usualy falls first when the boat becomes airborn. Of course boat design dictates a lot of the aerodynamic flight characteristic. I find 29 to 32 percent works on most of my boats so when they leave the water they come back down flat and keep on trucking. I use about 34 percent on SAW record trial boats. I think maybe the seaducer works well at 34 percent for heat racing but it usually falls bow first when it leaves the water. One way to prevent stuffing is to use a spoiler at the transom. Not a wing. A wing is not legal. If the transom does not leave the water the boat can not stuff! Just my experience. I am not the science guy but I do remember the XYZ in flight school when I was in the Navy.

Good info John. The way the boat flyies in free air means allot to the whole equation. also how the bow is made. Many a boater knows what that means to reentry. Will it pop the front back up or head for the bottom. Have put my boat completely under and did not lose a position in the race.

then again my boat is possessed .

David

 

[DickJones]

I normaly use the cg/ x/y/z/ center of balance to keep the boat flying level when it leaves the water, so the strut would have no effect. when the CG gets more forward than 32 percent as measured from the transom the bow usualy falls first when the boat becomes airborn. Of course boat design dictates a lot of the aerodynamic flight characteristic. I find 27 to 31 percent works on most of my boats so when they leave the water they come back down flat and keep on trucking. I use about 34 percent on SAW record trial boats. I think maybe the seaducer works well at 34 percent for heat racing but it usually falls bow first when it leaves the water. One way to prevent stuffing is to use a spoiler at the transom. Not a wing. A wing is not legal. If the transom does not leave the water the boat can not stuff! Just my experience. I am not the science guy but I do remember the XYZ in flight school when I was in the Navy.

Sooooo, they actually had airplanes back then?

 

[Hydro Junkie]

I know Darin is asking about monos, but in the case of a sport or scale hydro, CG does apply more as the boat is designed to fly rather than plane. This means that balancing lengthwise is no more important than balancing across the boat's width. At the same time, you need to keep the CG as low as possible to the boat stable. Just a little FWI that some may debunk