CGs

[Hunter]

For those of you who read every post, ignore this. I had placed it in a category of responses that probably got it lost. At least, I'm assuming more folks might take a whack at this.



Just curious on something. I haven't seen any discussion on determining CGs. Maybe I'm not reading carefully enough; if that's the case, ignore this. Otherwise....

Pilots are very concerned about CGs. It kind of ruins your day when you're doing 120 or so with 3000' remaining on the runway and when you go to rotate, it turns out the nose won't come up because you've incorrectly loaded the plane and don't have the elevator authority.

Just how much of this effects the propensity of a boat to porpoise at moderate speeds until the aerodynamic forces generate a stabilizing effect? Where the heck is my CG anyway and what's a good point to be at?

Being stuck in Korea without my boat, there's little I can do but were I home....

I'd take my trailered boat to the local scale and weigh the entire trailer, the weight on both axles, and the caster wheel weight. Based of the principle of WEIGHT X ARM(in inches) = MOMENT, we can multiply the caster wheel weight by the number of inches from the hitch point (the datum point as I'd choose to establish it) and the distance from the trailer axle to the hitchpoint in inches times that weight and add those numbers (moments) together. Then, by dividing by the total trailered weight, one can determine how many inches from the hitchpoint the CG is located.

Hope that point isn't behind the axle! Anyway, by popping your boat off the trailer (I store on the municipal pond but a local boat dealer might let you drop on an empty trailer of theirs for a small fee) and performing the same weight measurements on the trailer alone and then extracting the trailer weight from the total weight you can not only determine your boats weight but the CG.

From this point, mark and measure the boat. Unless you've managed to somehow get the bow exactly over the hitch, you'll have to re-establish a datum point for the boat alone. Really, you can define the datum point from anywhere relative to the boat but the math's easier using the bow and since most of us don't have anything hanging off the bow but do have stuff hanging off the stern, the bow's my choice. It gets interesting from here.

As pointed out previously, anything removable and consumable can be placed at the CG point and not effect the static CG.

Here's where I go from idiot to total idiot. It would seem to me that you would want your CG to fall somewhere between your hull's contact patch with the water while planing. I assume that having the CG forward of the contact patch would be the breeding ground for porpoising as the boat struggls to keep picking up the bow. Still, I'd think you'd want the CG on the forward end of the contact patch to generate a dynamic stability.

Maybe I'm missing something on that. I do know that before I buy another prop or make another assumption on setback, I'm going to pursue the above thoughts, put a masking tape stripe on the side of my boat (along with a horizontal tapeline on the engine- convoluted, curved thing that it is) and make some video taped passes while varying the CG within reasonably usable limits.

Anyone have any other ideas?

[Gerben]

When you increase the setback on a boat, porpoising starts, or existing porpoise gets worse. So with a more rearward Cg porpoising gets worse.
It's all a matter of stability. A plane with a rearward CG gets a lower margin of stability. Until its far enough back to get neutral. And then it gets real interesting with negative stability. At this point a plane is out of control.

I guess the "point of lift" for a boat at speed consists of hydrodynamic lift at the pad, aerodynamic lift of the hull and lift of the prop. If the combined point of lift (or whatever the correct term is) gets close to the CG, the boat will porpoise. However the closer these two are, the less power is wasted at flying the hull (f.i. need to trim out too far) (and assuming the cg is far back enough to fly the hull), the faster a boat will be.

Thats my view on how this works. (With my limited knowledge of boats and planes. that is)
Greetings,
Gerben

[Hunter]

Thanks for coming back to this with your perspective.

Again, I think we're in agreement. The reason I brought it up again is there's a lot of talk about bow lift, setback, props. Sure, it's all inter-related but has anyone gone out and:

identified their actual CG
experimented with percentile shifts and it's effects on trim
measured trim variances as off CG fuel burnoff occurred

See, I pulled some heavy crap (a bizarrely designed and shockingly heavy unit mounted aft of the CG) out of the back and went with a smaller lighter fuel tank which surely moved my CG forward. At the same time, I established 11 inches of setback, running the prop from one inch above 1.5 inches above to 3.5 below the transom where it's generally run. I want to maintain as much engagement on the Stainless Marine jackplate as possible to provide max stability for the engine but apparently this is too shallow for the 26 Trophy Plus, allowing tremendous ventilation. Ballasting back up isn't what performance boating is about. Actually, I pulled the crap out so I could move the rear bench seat back and maybe putting passengers further back will help enough to make it a dead issue.

Yeah, I can lower my manual jackplate until I get the prop fully hooked up and I'll have to plug the thing so it doesn't scoop water up at the engine. That has to happen as a baseline but I'm looking for maxims that people might be using. In an absence of that, I'll do my own. It'll be a great excuse to go solo the boat...once I get back.

[Techno]

Here is another plane thought. The tail plane produces a down force. The unusual designs, wing or canard don't. So depending on the style the CG is located in different spots.
On the usual style The CG is actually located too far aft with the tail apposing this unbalance. The canard has the CG better located as there isn't a surface producing a negative force, both are lifting.
The same would apply to a boat. The CG is located for a total lift with nothing suppling a down force, other than maybe the deck at high speeds, which is on the other end from a plane.

Then to confuse it more it changes form hydro lift to aero lift.

The reason I thought the CG would be useful is for someone else using it on their own boat.
If you knew what the CG is of XX model yy boat then it shouldn't matter what the slight differences are. You should be able to shoot for this same CG and be close to the same performance.

Something that really confuses me is a lot of the STVs are using mild to huge setbacks. But they are supposed to run best without any(exept Euros). This is a huge change not a small one that you would think.

[captcarb]

Different hulls react in different ways. Adding a passenger to the front seat of my tunnel increases the tendency to porpoise in the transitions speeds and reduces the top speed. Moving the passenger to the rear seat makes everything better. The CG is as far aft as I can get it without major changes to the boat.

CC

[Hunter]

Yep, that's my issue - porpoise at transitional speeds. You say it's better with pax aft; true with my boat as well. It's really only an issue as the hull transitions from low planing speeds (where the contact patch is large and the CG is more centrally located) to when there's little contact patch (and the CG is less relevant due to aerodynamic stability. At this time, the CG may be so far forward of the contact patch that stability isn't the issue - it's the prop's inability to consistently provide sufficient bowlift effectively stalling.

Sadly, the only immediate solution is for me to diet or to find fat people to ride in the back...when I'm back.

Regarding applicability of aircraft control surfaces, loading, and boats - I don't know how applicable it is. Efficiency occurs when the CG is controlled to allow both the wing and horizontal stab to be producing lift: aft with a conventional config; forward with a canard and the benefit of the canard being you get stability and effeciency from the same CG delta.

[captcarb]

Bow up trim will prevent the midrange porpoise (on my boat). It takes more with a passenger in the front seat. It is necessary to trim down when the hull starts to fly or it will slowly pitch up.

CC