What is Making This Frame Shake?

Yeah, I think the whole contraption is designed so it's going to vibrate.

This is just my gut feel from a quick glance... bear in mind I'm a cat so I know zip.
I can't really see how it's tied into the front of the boat, looks like wire ropes*.
I'd say, ideally, a more rigid attachment from the outer end of the outrigger to the prow (tube rather than wire) whould give much better stability. The inboard end of the outrigger wants a big soft rubber mount to the hull, and maybe should be slightly ahead of the actual outrigger, like a car suspension? (So everything is trailing)
Round sections rather than square for any tube in the water. The main horizontal top member of the outrigger seems to be flexing a lot at it's centre, maybe this needs bracing to something else or to be made of two tube cross braced along it's length. Or simply triangulated from a short central stub back to the two ends.
Other than that it's fine.
Hope this gives you some ideas... feel free to shoot me down all you mech engs out there.
Del
*Wire ropes are going to much more stretchy than tube and will also vibrate more. Just look at guitar strings

like a car suspension? (So everything is trailing)

Yes, put it in tension, just make sure you have a solid enough foundations

Maybe they could try slipping some PVC pipe over the square frame pieces to see if there is any improvement...You could e-mail the drawing to somebody who could then post it....maybe

This appears to be a similar setup with the net mounted closer to the frame mount...

Same rig...these are the 8x6 frames....the guys tweaked the others when they migrated to 12 x7.

Videos are all the same. we are the only ones foolish enough to venture out this far outside the box. ha ha

The new Plateena nets with the Dyneema netting are super fast.

I am waitig for a report tonight.

I'm thinking like you.

If there were some way to turn the square tube 45° it wouldn't dump water off one side, then the other. It would split the flow equally around each side.

Or round PVC over that.

Does the amplitude of the shakes vary with boat speed? If so, how? - i.e. does it just get worse and worse as it goes faster?

[Edit] - ... and does the frequency change?

It looks like it may be at or near a resonance at the speed shown in the video.

The frequency seemed fairly constant and I wonder if there is vortex shedding there, this might be too complex to analyse with four, or two pairs (inner pair and outer pair), also, the two inner verticals are in the most turbulent part of the boat's wake. I think that the wake is normally somewhat uneven with the maximum switching from side to side in a fairly regular pattern when travelling at at constant speed.

You might try moving the rig forward to get out of the wake. If it is vortex shedding, you should be able to change the Reynolds number by winding a rope tightly around the verticals with a few inches between windings.

Can not go too far but yes, it could be moved. The larger rigs we use down here are located just ahead of mid ship by about 15% or so....

'vertical legs are pushing too much water'

The vertical legs, should have a profile of an arrow head, to reduce resistance. Think of the behaviour of a rectangular barge, ploughing through water. Perhaps, a suitable MS angle, welded on the forward side, will help.

Thank you Joshi.

we tried this with other frames and it worked very well for a while....

There was one problem ( as my old friends at Garware Wall in Wai would say, ..." modi chukala" ... spelling ?) ha ha .....If the frame work "racked" or twisted later, the water hit the " arrow heads" at a different angle and made a serious wake and disturbance. With round pipe it made little difference.

These rigs are meant to be flexible because they WILL hit something and rack the whole thing out of line. It is seldom that thes frames make even one week without slamming into something under water.

Right now these guys have made this out of round pipe but reinforced it on both sides with angle steel, making it square and hitting the water as you see here.

The top horizontal of the frame continues along to meet the vertical pole mounted on the side of the boat rather high.
Wouldn't it be better if the horizontal section from frame to pole was lower and met the vertical at its bottom, near the mounting point?
With the trailing rubber mount idea from #1.
If it's not practical to replace cables with tubes, maybe just some fishing floats or chunks of rubber might dampen out vibration in them.
(or pehaps a sack of squirrels?)
Del

I noticed they had that higher than normal.

I will make a note of that when we speak tomorrow.

Thank you.

excellent analogy. I never heard about 'hunting' before.

Thank you.

I'm agreeing with mike. Was thinking the same thing, but you put it much more eloquently than i could have.

Just heard from the research crew. Both Top and Bottom pipes caved in and warped.

Plan is to now go with the more reinforced frames that we started with back in Nov. of last year. Hopefully, no more tweaking of the framework. Stick with what works.

Thank you all for the input. it seems the law of physics solved our dilema over guessing.

The "how" of the "cave in" would be interesting and might tell us a great deal. Can you get photos?

Supposed to be a video coming. I always ask for video and pictures...however I seldom get what i need....more like pictures of just fish and people.. ha

Passingtogreen has the right idea.Vortex shedding. We get the same effect only smaller when a taut wire suspends a heavy weight such as a roll damper on a fishing boat.

I had a sailboat with a swing keel held by a wire hoist. when the swing keel (700 pounds) hung on the wire at hull speed about same as your survey boat the wire would vibrate and hum.

The square tubing aggrevates the problem because a low pressure area is developed behind the square surface.

Round tubes would definitely reduce the shaking but not totally eliminate it. Rotating the square tubing might reduce the vortex.

You see the same situation on hydrofoil supports, which is why hydrofoil boats always has a naca shape on the support struts.

Incidentally we also see a bit of vibration in rubbers on sailboats if the rudder is not shaped correctly. This happens for the same reason. Vortex generation on the trailing edge. Same solution. Shapes using a naca profile to reduce vortex generation and shedding from trailing edge.

The question becomes how much $$$ must you expend to reduce vibration to tolerable levels. Square tubing is definitely the worst case.

Vortex Shedding. It is a word i will use in our meeting next month with these folks to explain what is happening to their "tweaked" rigging.

The original rig we used down here was made of all round pipe and so much stronger.

They experiemnted for a lighter, more easily transportable rig and it just didn't work out. Thankfully they will go back to the original, more durable specs.

Thank you for the input.

Hi netmaker, few ideas.

To obtain (experiment with) better tubing you could press brake sections into various
profiles for the least effects of vortex etc. (Also see online research labs.)

Experiment with fins or attach different spoilers to clear the water effect?

To avoid collision damage use a slipping clutch or shear pin on the boat side post.

Hope these help.

jt

I've just heard the sad news that the Arabian inventor of the bullet-proof vehicle died today.

R.I.P. Ahmed Carr.

Thank you. We had not thought of a shear pin on this, but that might work as a sfety feature.

The frame design is causing the problem. Any protrusion into the water will cause this type of turbulence and cause vibration. Redesign the square tubing to a design that has a fairly sharp airfoil on the front or leading edge and then change the angle of the verticle tubing to a 45 degree angle pointing toward the rear of the boat and the vibration will be greatly subdued but not entirely eliminated.

Thank you for the input.

Just out of curiosity have you not thought of using a profiled tubing almost like a rain drop we use them on race car suspensions to keep the under car aerodynamics clean and not create lift. Formula one and Indy cars both use this profile and it is readily available in a lot of race shops and it isn't all that expensive but cost is relative to how many times you want to rebuild it.

No..... what does it look like and would you have a picture of it?

sounds like something we might try.

Thank you.

Look up NACA curves. This is a family of curves defined by a common formula and used extensively by hydrodynamicists and aeronautical wing designers for foil shapes. We used this for designing the rudder shapes on trawlers in the design office I worked in. I worked as an electrical designer but the naval architect sitting next to me explained what he was doing and why.

We may be getting into confusion over semantics. The original issue was vibration in something passing through water. The steel cables from the outriggers to the 'torpedoes with wings' vibrate due to vortex shedding along the taut steel cables. Similar effects have been noticed elsewhere. Fairings and foils have been used for both supports in water and air to reduce vibration due to the passage of a fluid past a support. The effect is more noticable in water because water is much more dense than air but aerodynamic test in wind tunnel have demonstrated the same effect for sailboat masts etc.

Many sailboat masts are now oval to minimize turbulence from mast affecting air flow past the sail. It stands to reason a square shape not streamlined is likely to cause the strongest turbulence.

vortex releasing or cavitation is what i would be addressing here

it can be eliminated , look at race cars that use small teardrop difusers on the trailing edge or on the surface skin of racecars.

you need to difuse the effect into smaller waves , one gentleman suggested wrapping rope around the struts but i think look to aerodynamics on racecars for some clues

Thank you....someone else suggested the same. I am looking into it now.

The only thing I can tell you is to look up contoured tubing and find one that fits a profile you think might work to eliminate the vibration and cut a clean line through the water like a hydrofoil. I don't have any way to create a picture I can only say it looks like a raindrop with the rounded edge leading in to the water and the pointed edge trailing to eliminate the turbulence you are seeing.

I have someone searching now...............By the way.....YOU ALL have these same carp also.

thank you.

Thank you too.

Everythng is going in my notebook for our meeting.

Thank you again.

URGENT............

I am going to need some help here. Now almost every time I try to start a new reply OR post even a jpeg, the CR4 screen gets a dull gray color over 1/2 the screen and freezes up.

At this rate I will not be able to do anything but read...

I have no issues with any of the AUTOCAD forums or the scientific forums I go on.

Any ideas??? I would hate to not be able to come here and ask these questions.

I think somebody else reported a similar problem.Apparently, I don't have that problem.

Solar Eagle, do you notice the boom in your image is a triangular network not a straight piece of aluminum? That's why it doesn't have problems, it self dampens, the harmonic frequency of that boom is very high, it may have even been tensioned when welded to be loaded slightly for damping effect if it was done well.

The one the poster made is (no offense intended) very crude. It's functionally complete but from an engineering standpoint it's missing structural components.

I think that ANY tube in the water should be OVAL shaped, and mounted edge-on to the direction on flow .

A tear drop form would have a better laminar flow than oval, but an oval shape would still be batter than a square tubing or an angle material if it was the only material you could get

Correct on that. Right now we are going step for step as I can not redress any of this at a later date. There are now three other prototype gears that do not require any frame work that are left to be field tested. if WE SEE BETTER SUCCESS, I will recommend we step up to the tear drop shape.

Got to prove the entire system is feasible first.

So far, its looking better than was expected by many.