Author Topic: Balance Point for Towfish  (Read 9707 times)

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Offline Kennyscott

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Balance Point for Towfish
« on: January 16, 2011, 03:21:25 PM »
P
On various posts I've heard that the balance point should be about 1/3 from the front of the fish. Slightly nose heavy. I've also heard that alot of people have added weight to the fish. I'm wondering if the balance point once achieved and slightly nose heavy if any weight at all would be needed. I'm still making mine with a removeable nose cone to add weight  but also could use a 2 lb clip-on weight on the tow line like a down rigger.
Any thoughts..Mine will be constructed from PVC  and only real weight will be the transducer so should be fairly light construction wise.

Kenny


Offline Rickard

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Re: Balance Point for Towfish
« Reply #1 on: January 16, 2011, 03:48:16 PM »
Kenny,

You are absolutely right, it is not necessary to place all mass in the towfish. There are systems where the towfish is near boyant and all the weight is in a downrigger on the wire. Such a system provides very stable movement of the towfish in heavy seas. I have not tried such an arrangement myself, but I suspect it is much harder to control than the simpler arrangement with a heavy towfish. But please try it, we will be eager to read about your results!

Rickard

Offline abraquelebout

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Re: Balance Point for Towfish
« Reply #2 on: January 16, 2011, 05:09:34 PM »
in begining, my first towfish have a litle mass on it and i use a second cable with 15 kg weight
to go on  max depht
i put the towfish on water with 55 m cable and i make a large bucle to take my heavy weight and slide on the towfish cable
i use an hydrolic winch for the 15 kg  cable
for my second towfish, there is 5,7 kg on nose and plugs to take some additive weight (2x 5 kg)
i don't know if it's good solution
the next scan on sea must say it, i'm waiting better condition

Offline Kennyscott

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Re: Balance Point for Towfish
« Reply #3 on: January 16, 2011, 11:29:47 PM »
I will be using it primarily only in fresh water so waves and surge/swells should not be a problem. With it being a bit nose heavy and being pulled it should track fairly level.  Biggest problem will be the drag from the tow cable causing it to lift.  The faster speed being towed the more drag the more lift.  Less speed the deeper it should tow. And since i would like the sharpest images (more pings/foot) slow is not a problem.
On another note: Any trouble playing back the recording from the card back on the PC?

Offline Jolly Roger

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Re: Balance Point for Towfish
« Reply #4 on: January 16, 2011, 11:30:34 PM »
Kenny,
you propably read my the thread about my fish here. I built it heavy (25 Kg / approx. 50 pounds) and I realized that I need a winch to handle it safely (and comfortably).
The fish works fine, but because I'm out alone most of the time, I will make my next fish as light as possible.

I had another thought about stability:
Just an OPEN tube without a nose cone. I think that the tube would stabilize itself by the water flowing THROUGHT it. The right horizontal angle for the transducer has to be found out by testing, but I think such a set up would work. Additional stabilizers at the rear end would provide extra stability and safety for the transducer for towing and transport. A depressor wing will give down force to reach more depth without too much weight.

Regards from Germany

Harry


EDIT: For reviewing the records on the PC I use Martin's Hummviewer. There is an option in the settings to set the distance of the fish behind the boat.
I haven't used it a lot, so I cannot tell you if it is giving precise positions for the waypoints you're creating when reviewing the records.
« Last Edit: January 17, 2011, 03:38:27 AM by Jolly Roger »
YES,......
WE SCAN!

Offline RGecy

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Re: Balance Point for Towfish
« Reply #5 on: January 17, 2011, 12:16:21 AM »
I had another thought about stability:
Just an OPEN tube without a nose cone. I think that the tube would stabilize itself by the water flowing THROUGHT it. The right horizontal angle for the transducer has to be found out by testing, but I think such a set up would work. Additional stabilizers at the rear end would provide extra stability and safety for the transducer for towing and transport. A depressor wing will give down force to reach more depth without too much weight.

Harry,

I dont know that I have ever posted the images of my towfish here, but its based on this exact principle.  I considered having the nose open but decided not to and have the weight in the nose.  Although, the open design may have been more stable and put the weight on the bottom.

What I did do is use this principle in the fin design.  I know it looks a little unorthodox, this towfish actually pulls deeper and straighter than any I have ever used.  The tubes beveled design actually seem to act like a thrust or kort nozzle and help reduce drag and push the towfish forward.

No need for a depressor wing.

Its a little different looking, but it works, and was very easy to make!

Robert

« Last Edit: January 17, 2011, 12:20:54 AM by RGecy »
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Offline Jolly Roger

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Re: Balance Point for Towfish
« Reply #6 on: January 17, 2011, 01:55:25 AM »
Wow Robert!

That's a step I haven't thought of and it makes perfect sense!
My thought behind the "open" fish is that it rides on its own internal "guide rod" produced by the water that flows through it.

Have you ever tried to tow the fish without the nose cap? I wonder what difference would show then.

Looks like I need do to some shopping for PVC tubes in the near future  ;).

Regards / Harry
« Last Edit: January 17, 2011, 02:19:36 AM by Jolly Roger »
YES,......
WE SCAN!

Offline Dale

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Re: Balance Point for Towfish
« Reply #7 on: January 19, 2011, 06:38:54 PM »
G'day all,

I'm just back from three days of towing our open nose PVC fish. I can report that the fish tows straight.  The fish has two 500g diving weights in lead bars that sit inside the PVC tube to add some displacement. We did over twelve hours of towing, at depths where the fish was visible from the surface.

I'll have some photos, more info and data in my PVC fish thread:

http://forums.sideimagingsoft.com/index.php?topic=1887.0

Offline Kimi

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Re: Balance Point for Towfish
« Reply #8 on: January 20, 2011, 01:44:08 PM »
Biggest problem will be the drag from the tow cable causing it to lift.  The faster speed being towed the more drag the more lift.

The problem whit shaking and dragging cable can in many cases be minimized if you put some small flags (2x2inch) on the cable. Distans betwen the flags can be about 2 feet from each other.
The flags can be made of tape etc..
Haven't tried this my self, but it should work?

Kimi
homepage: ( in Finish only) www.wreckdiving.yolasite.com

Offline Terschelling

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Re: Balance Point for Towfish
« Reply #9 on: January 21, 2011, 08:26:01 AM »
Hi. Kimi,

The flags is what I see by some comercial / navy users of tow fishes, when I asked about the purpose of it the answer was: to reduce fibration in the cable due to movement through the water.

To reduce drag of cable and fish, I made a fish according RICKARD's design (see picture on his personal page).  Its like a dart: a slim design, 32mm pipe >>> the transducer can be mounted just by a bolt/nut. (see photo on my personal page)  Stainless steel hose clips for solid but easy to adjust connections, dive belt weights at the tip.
Maybe the fins (stainless aswell) are a bit big but everything works fine !

Cable: standard 30'feet and 10' extention cable , for the towing I use 4mm polyester line.
Both cables put together with ty-wrap's. The transducer cable has some slack between 2 ty-wraps to compensate for eventual stretch of the polyester cable.
This little slack of transducer cable can act as "flagg" aswell.

We did some try-outs with fish and everything seemed O.K. , even with 10 knots boat speed.

Regards,

Frankila_renderedila_renderedila_rendered

Offline Rickard

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Re: Balance Point for Towfish
« Reply #10 on: January 21, 2011, 06:49:05 PM »
Frank,

A really authoritative towfish, I think. It looks a bit scary to me!  :) How much weight do you use?

Rickard

Offline Terschelling

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Re: Balance Point for Towfish
« Reply #11 on: January 23, 2011, 06:33:51 AM »
Hi, Rickard,

Some figures:
Lenght  1.50m
Tail fin in X configuration , made out of 2mm stainless steel plate  14 x 45 cm

Pipe 32mm diam. wallthickness 1mm
2 divebelt weights of 2kg at the front
total weight 8.5 kg , easy to handle with 4mm polyester wire and without gloves.

Objective: a tow fish capable  to be used with relative high boatspeeds to make "quick" scans of large area's. For this I need a fish with some weight / less drag or a depressor wing.
 A "big" wreck will be noticed, afterwards these positions will be run with reduced speed and range  to get detailed images.

Before modification she was equiped with plastic fins (to weak > deformation and un-balance and much less weight > towing point more to the forward as on previous picture)
Now the towing point is almost in centre (1 a 2 cm before centre of weight), the fins will take care for a stable course.

Regards,

Frank
ila_rendered

Offline abraquelebout

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Re: Balance Point for Towfish
« Reply #12 on: January 23, 2011, 08:57:04 AM »
i see 2 kg of diving lest on picture
you said take scaning at 10 knts
what's the picture at 10 knts ?
i hope that, at 10 knts the towfish goes at surface sea or near the surface
for recent wreck is good but for anciant ?
do you have picture take at 10 knts


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