CG Calculations, Trim, Bouncing, Driver Weights etc.

[smiley]

I' am getting back into boat racing after a respite since 1963 with the grandkids. These six kids come in all sizes and weights thus CG becomes a consideration in trimming. Recent experiences with aviation taught me how to calculate CG for various components and from that experience I make the following assumptions:

Once a boat (runabout) is properly trimmed for smooth water you calculate the CG for that condition. Then you trim a boat for rough water and calculate that CG.

Assumption Rough water optimum CG is forward of smooth water CG

Assumption, regardless of driver weight etc. That CG point goes with the particular boat, prop and setup configuration. This is the optimum trim setup for the boat.

Thus, if you are going to have multiple drivers of varying weights in a boat; you will have a wide envelope range of CGs for the drivers. Performing multiple calcuations, I have discovered that lighter drivers with multiple weight distribution anchor points on the boat have the widest fixed CG range.

Larger drivers have a narrower fixed CG range but a wider CG range in live action. They can move their weight around inside the boat and affect performance to a greater degree in real action.
Lighter drivers need to have their boat more closely trimmed for the existing conditions.

I have seen this play out to heavier drivers who are 30-40 lbs overweight in A class handling the boat with authority in varying water conditions while the lighter drivers need to throttle back in rough water.

From the above I surmise that a more closely optimally balanced runabout is more beneficial to the lighter drivers.

Also given an optimum CG point for each boat and water condition. You can calculate weight distribution on paper (ie computer) for each driver and not need to real time test each setup for each driver and still get the same results.

What are your thoughts on these assumptions and am I missing anything here in this thought process?

[Mark75H]

That would assume all props are actually identical and boats likewise


Knowing that these 2 variables are not controllable, you are still obligated to test, test and test. Assume nothing.

[smiley]

You are correct.

I assumed the same boat, prop, setup, envirionmental conditions. When you change drivers you need to adjust weights to maintain the same CG to repeat the same trimmed ride. That was my premise. That can be mathamaticly calculated thus eliminating the need to test each driver for the same conditions.

Now you change prop, setup, weather conditions etc. and it is test, test, test. I don't know of any way compensate for that. If you properly docucument all these tests and know the optimum CG for each condition, you can then develop a body of knowledge to proactively adjust the CG for anticipated conditions.

Smiley

[Mark75H]

In J you do not get to use your own props, you use APBA provided props. Doesn't that put you back at square one?

[smiley]

You are right again with respect to J. In fact everyone has the same disadvantage in that everyone must use a prop provided by the racing club sponsoring the race. According to Craig Dewald, he can make a J copy from his J pattern that we can use for testing but of course we all know that it is near impossible to make an identical prop with the same characteristics. Hence everyone has that problem. I was told that there ia about a 2 mph variance in the club props so it is the luck of the draw.

Now with AXS and A that is a different story. We have control of our props and setups.

In private aviation, as a pilot, we are required to obtain all necessary information for a flight. One of those items is CG calculations for fuel, baggage, pilot and passengers to insure we are not over gross and within the operating CG envelope. For optimum speed, you want a rear CG loading. This gives you the fastest speed but also the least stability and the most difficulty in landing with adequate control at slow speeds. Everything comes at a cost.

Forward CG loadings require down force on the horizontal stabalizer which increases drag because you must now lift that downforce plus the weight of the plane and the increased angle of attack on the wing increases drag.
This also becomes a dangerous situation when landing at low speeds. There may not be enough downforce on the stab and elevator to prevent a nose wheel first landing which can be very dangerous. I assume we should have our boats properly trimmed as well for similar reasons.

Smiley

[Skoontz]

Don't get too confident with J props and duplications. We have always had difficultied planing using J props from the club boxes, and never had an issue using our own J prop. 99% of this problem was using an AX/A boat to race J, when a decicated J boat needs to be used to run optimally in J class.

The inset transom of the A/AX boats is the largest issue, also the dedicated J boats are a tad shorter, like 8" or something.