3 point theory

[MitjaJ]

I used Solidworks.

The difference between moments of inertia is around 2 (I've checked it again.)

Old = 80175179 g/mm2

New = 38178657 g/mm2

Some pictures:

 

[moparbarn]

moment of inertia can be huge! on many hulls, more weight,closer to the cg has worked better for me than less weight, out farther. not for planting the hull, but to keep from getting a pendulum effect. weight at the edges - front,rear or sides can be nasty......

 

[Alexander King]

moment of inertia can be huge! on many hulls, more weight,closer to the cg has worked better for me than less weight, out farther. not for planting the hull, but to keep from getting a pendulum effect. weight at the edges - front,rear or sides can be nasty......

Great looking build MitjaJ! I have to try working in Solid works when I get a chance.

On to the subject of inertia & CG... there was a post here about Japanese riggers running their motor to the rear. In some cases the motor was way back it seems on a few of their designs :huh: no one really had a answer to how and why they work with the Moment of inertia being moved way back. I guess that will remain a mystery.

 

[MitjaJ]

Moment of inertia is not something that you can move foward/back - that can be done with CG. Moment of inertia just tells you the "arrangement" of mass in a volume regarding to a fixed origin (for us this is the CG).

I've seen those Japanise riggers with the motor way back - one would have to make the same boat to see the results.

I'm sure that they had a reason for moving the motor to the end of the boat.

 

[Scott Schneider]

Moment of inertia is not something that you can move foward/back - that can be done with CG. Moment of inertia just tells you the "arrangement" of mass in a volume regarding to a fixed origin (for us this is the CG).

I've seen those Japanise riggers with the motor way back - one would have to make the same boat to see the results.

I'm sure that they had a reason for moving the motor to the end of the boat.

Shorter drive cables/wire drives, less whip, less overall drag perhaps. What weight hull has and due to OAL so great perhaps biasing more weight on the prop so it stays hooked up and not blowing out as easily ?

Only guessing here ....... :blink:

 

[Alexander King]

Moment of inertia is not something that you can move foward/back - that can be done with CG. Moment of inertia just tells you the "arrangement" of mass in a volume regarding to a fixed origin (for us this is the CG).

I've seen those Japanise riggers with the motor way back - one would have to make the same boat to see the results.

I'm sure that they had a reason for moving the motor to the end of the boat.

I stand corrected I had the moment of inertia on my mind I ment CG .... :lol: had to edit my reply

Yes! I agree that whip and short drives seem like good reasons to move the motor back. :unsure:

 

[Ub Hauled]

Speed always increased for us as we increased the sponson angle. We've run up to 10 degrees. The problem is stability. As the angle increases the sponsons can move in and out of the water a greater distance for the same correcting force. Sponson walk gets worse with higher AOA. Air trap sponsons are one answer, and shingles are another.

Lohring Miller

Interesting ideas you are proposing Lohring...

I just have to mention that Joerg M. has no air trap sponsons nor shingles on his boats and it seems to have worked out well (140+mph), he was riding on a "cushion of air". Wouldn't the air traps create too much pressure, thus lift, to the sponsons and how exactly do shingles work? Is it the "micro vortex" principal?

BTW, I don't mean to put any ideas down, I am just curious as how they work.

 

[lohring]

All our SAW boats after the first 100 mph gas boat ran air trap sponsons. These range from a tiny 3.5cc outboard hydro to a 35cc gas hydro. Air traps provide great height control. They lose almost all their lift at very low heights and are very resistant to being depressed. They also track very well. We made a 100 mph gas sport hydro pass when Legg Lake was so rough no one else could run.

They aren't the only answer. I think that stability rather than low drag is their best feature. There are other ways to solve the same problems. Joerg's boats are very carefully thought out. If you look at the videos, his sponsons seldom touch the water. He gets stability in other ways as well. Because he has a propeller/power plant combination that delivers more power from the prop at high speed than most, he can get great speeds.

Shingles give a moderate angle planing surface a high angle contact area. This should lower the planing drag without decreasing the resistance to sponson depression. When the sponson is depressed, more shingles come in contact, increasing the lift. Sometimes two rows of shingles are used to give an air trap effect as well. Again, I think stability is as important as low drag.

Lohring Miller

 

[dschigoda]

Hmmm... for an 1/8 scale hydro (1980's type pickle fork), do the air traps that run the length of the afterplane work the same way or provide any benefit to the stability or drag of the boat? Or are they too high off the water, and in too much turbulent air to do any good? (Sorry if this is sort of hijacking the thread). I recall a "Pinkert" rigger that used "ground effects" on the afterplane back in the 1980's.

 

[lohring]

Air traps give a huge increase in the lift of any body running close to the water. Most of the lift comes from pressure increases on the bottom. The air traps seal in the air. The problem starts as a wing rises and falls. The center of lift moves forward as height above the water increases leading to the classic blowover. Sponsons are short so the effect is small. If the wing is the whole boat, it is a problem. You can have too much lift in the wrong place. We added 25 mph to our gas sport hydro with big spoilers on the wing section. Otherwise the forward lift blew the boat off.

As I keep saying, stability is very important. Full size hydros run big rear wings as do the Russian WIG vehicles. Twin wings are another approach. Active stability control with driver adjustable canards is also done, though fixed canards are destabilizing. Model boats usually run very little lift on riggers because we have lots of power and can't handle the stability problems. With scale and sport hydros it takes careful adjustment of the CG and air trap length to get good results. I haven't found that scale size wings do much. I'm building a twin wing scale boat to see how that works in models.

Lohring Miller

 

[Neil LaGarry]

Can we now say that a rigger is a Hydro HJ

I like your theory by the way A rigger is a striped down hydro.IMO

Like Ron said a{ hydro in it's simplest form }.