Hull displacement support versus planing support at speed

[swbca]

Another OffCourse owner came up with an ingenious way to make quick adjustments to the 2 Off Course markers when boat occupancy changes from the starting calibration.

 

He fastened a digital level to each of the markers and then zeroed out the electronic levels with the boat standing still to save each marker angle set during the initial calibration.

 

Summary Information

Initial Calibration is done at skiing speed setting the marker impact distance at 1/2 the course width.

Then The marker angle was recorded with the boat standing still.

 

The next day an observer was added to the boat.

The skier adjusted the two markers so the electronic levels were at "0" with the boat standing still.

 

The Question

Does adding an observer to the boat change the boat's roll attitude the same amount at skiing speed as when standing still ?

 

Bottom line; will adjusting the gun angles with the boat stopped to accommodate a passenger give you the correct gun angles at skiing speed ?

[Mastercrafter]

I think it all goes out the window with prop torque. A level boat sitting still does not necessarily stay level at speed

[Horton]

certainly a level boat at idle is not necessarily a level boat at speed.

[jpwhit]

If the boat is truly level sitting still, it'll be at the same tilt at speed. But this is only true if it's actually level. If it's level, then the weight distribution is even. And an evenly weighted boat will be at the same level of tilt at speed. That's because any tilt at speed due to hull hydrodynamics and prop torque, will always be the same for an evenly weighted boat. This does assume the mass of the boat doesn't change significantly. So assuming you not going from a couple of people to 10 people in the boat.

[swbca]

THIS SCENARIO HAS BEEN MISUNDERSTOOD BY ALL RESPONDENTS ABOVE

 

Yes, a boat's roll attitude is different at speed than when its stopped, because of prop torque and the prop's lateral crawl forces on the hull. I have tested that. But that's not the question in this scenario.

 

THE QUESTION IS . . .

Does the amount of roll caused by adding a passenger to a boat differ when measured at speed versus measured standing still.

 

(disregard any flattening of the roll differential measured with a level that is based on gravity caused by the changes in the boat's front-back attitude at speed.)

[Mastercrafter]

Oh, now I get it. Let’s say adding an observer sitting still tilts the boat to the left 5°. At speed, will it still be 5°, more, or less? “Level” doesn’t really matter.

 

At least with Mastercraft and Malibu using left hand props, I think the tilt delta with added observer side weight will still be smaller at speed, because of the prop torque.

 

Take away prop torque (like high school physics assuming everything is in a vacuum) and maybe it’ll be the same?

 

 

[jpwhit]

I understood the question, but since it's asked in the context of keeping OffCourse in calibration as you change passengers in the boat, it's not really the most important question. The answer to the question is the angle will change between sitting still and moving. But that amount of change will be most consistent if the boat is level and balanced when sitting still. Consistency is what matters for OffCourse. If the boat was level and balanced during the initial OffCourse calibration, then that initial calibration includes any dynamic tilt at speed. Then as long as there is consistency in the tilt, OffCourse will stay accurate. The amount of change in tilt at speed will potentially be speed dependent, but I don't think it'll change enough with speed to matter very much.

[swbca]

@jpwhit you are still talking about if there is a change in roll with speed. That doesn't matter if you have the correct method to adjust OffCourse when you add a passenger, short of starting over with the manufacturer's initial calibration instructions. We are only interested in the affect of a passenger at speed and if static measurements of hull attitude caused by a passenger are sufficiently accurate to predict the role of the occupancy change at speed.

 

@Mastercrafter addressed his view of the question asked.

 

All of this could be answered by measuring the boat's roll attitude at speed, but low cost electronic levels usually don't have electronic vibration dampening, so the reading may flutter +/- 1 degree.

 

In my view, the higher the speed of the boat, the affect of a passenger will be reduced because the lift potential on the hull cause by speed overwhelms the 150 pounds of adding a passenger in the love seat. The faster you go the flatter the ride regardless of occupancy . . . just theory that should be measured.

[UWSkier]

There's an easy solution to this... Use weight in the boat to simulate a spotter if you don't have one. When you do, remove the weight and leave it on the dock. That's how we do it and it works like a charm.

[swbca]

@UWSkier I already have 150lbs stacked against the side of the hull under the love seat and forward to get the boat level with just the driver. Not to easy to get it in an out. I'll probably still use the 2 levels on the two guns to set "with" and "without " passenger angles for the 2 guns.

[DW]

The other changing variable between static and dynamic is wetted surface area along with lift force of water flow across the lifting chines. After consulting numerous differential equation solutions, filling a notebook with CFD analysis the answer is … run the experiment ???