Topic: Razorfish II => length = 20 inches / weight = 7 pounds.

[Drifter]

Razorfish II => length = 20 inches (.51m) / weight = 7 pounds (3,2kg).

AKA: The Limbatus Towfish

Click to enlarge =>

My goal was to get decent side scan images and recordings from a small boat in the choppy shallow waters off of the Florida gulf coast using a 997c SI unit. Mostly it would be used while trolling for grouper and looking for new bottom structure.  I wanted to build a towfish that was easy to stow, easy to deploy and ran true on a short tether (so it wouldn't get in the way while trolling with up to 4 lines out at a time). I built 3 prototypes before I came up with the working model I am using now. In rougher water it needs to be deployed close to the stern.  Replacing the dive plane with a 4 pound belly weight allows the fish to run true at speeds up to 6 mph and helps to overcome the drag produced by the cables (so it gets deeper with less cable out). Hanging it over the side of the boat about 6 feet in front of the transom with 8 feet of cable out, it runs about 3 feet below the bottom of the hull and about 2 feet in front of the transom.  It takes less than 30 seconds to fold out the fins, drop it over the side, toggle the TS2-W switch from the transom transducer to the towfish and then start a recording.

Link to a brief writeup with more pictures ====>

I am also working on a smaller, lighter version to be used from a canoe with a 798ci SI unit.

Click to enlarge =>

Is anyone else out there using something other than the cylinder shape or using 3 fins instead of 4 for their DIY towfish?



(Edit: Moved pictures in post to new location and updated links.)

[Jolly Roger]

Wow Drifter,
I'm impressed! This design is brilliant! Congratulations!
In fact I like the folding wings very much, because it makes the fish so small when not in use. One of the issues I have with my fish is the size besides the weight. My fish is 150 cm (approx. 4.5 feet) long. There's no problem on the boat though, but getting it into my small VW Polo race bun isn't easy. Usually I have fold down the rear seats.

I have three fins on my fish. The idea behind my design is that the fish sits stable when I lay it down and the transducer is safe as well.


Nice work Drifter!!

Regards / Harry

[Humminbird_Greg]

I'm with Harry on this: brilliant design!
I along with others will want to know more specifics on how to build one. 

[Jolly Roger]

Drifter,
I'm even more impressed that you have taken your time and even wrote a description of how you built it. Thanks for sharing the information and keep us updated!

Best regards from Germany / Harry

[Drifter]

Harry,

Thanks, but it just came down working with materials I'm more familiar with and that I had on hand.

==========================================================================================

Greg,

I figured you might have come across different types of towfish in your "travels".

The construction is as basic as it looks. If I was starting from scratch, I'd skip the "pontoon" weights and just use a belly weight. If I was trying to run deep, I would have tried to setup a "break away" tow cable attachment like they use for anchors ...

Breakaway anchor rig

Tools: Hacksaw, drill, screw driver and some vice grips.

Materials: Aluminum flat bar 1/8" x 2", aluminum angle bar 1/8 x 1-1/2, stainless steel Phillips machine screws 10-24 x 1 , stainless steel nylon insert lock nuts 10-24, stainless steel vinyl coated cable and a downrigger weight

Instructions: Cut up the aluminum bar stock similar to what it looks like in the pictures. Sandwich the 2 inch flat pieces between the 2 angle pieces and clamp the 2 angle pieces together with the vice grips. Drill the holes and bolt it all together.


The pictures really do a better job of showing how it fits together than I can string the words together to describe the process.


==========================================================================================


I liked the looks of the Starfish rig that Kimi linked to in "A fish that (finally) works" and I'd really like to hear about different designs.

[Kimi]

A totally new design!

The folding tailwings is a very good point. I assume that they are made to make the storage of the towfish easier.

I'm going to steel this invention to my first towfish project. If you don't mind? 

Kimi..

[sonar2000]

This design looks like it may track cleanly thru the water. It should not bounce or sway.  The fish should perform well at deeper depths giving a great side scan image.
We will be interested in the pictures that follow from recorded runs. 
Chuck (Sonar2000)

[Drifter]

Kimi,

If you're going to use "steel", it should probably be stainless. ;~)

I used two 1/2 inch stainless nuts for spacers on either side of a piece of the 2 inch flat bar for the transducer mount.

=>

==========================================================================================

Chuck,

It should be able to run deep, but there isn't deep water in my area until you get at least 30 miles offshore. I'd think if you scaled the design up some and used a 10 pound downrigger weight it would run deep enough.  From what I read though, the quality of the cable and connections determine the signal / image quality. I've just been trying for something practical in size for a small boat and something that could overcome the effect of hull movement.

There are screen prints of several recordings in the links above. Here are a few shots of the Nauset and some from the initial run in the canoe.

The Nauset (saltwater) ...

=> =>

=> =>

==========================================================================================

Canoe test run (freshwater) ...

=>

=> =>

=> =>

==========================================================================================

Overhead shots from the canoe ;~) ...

=>

=>

==========================================================================================

(Edit: Moved pictures in post to new location and updated links.)

[Kimi]

If you're going to use "steel", it should probably be stainless. ;~)

Sorry... I meant... "steal" your invention.... 

Kimi..

[sonar2000]

The length of the cable has everything to do with signal. I think in electronic terms it is called impedence. HB designs the unit for the standard cable but we have found that we can get longer cables to work. Some with better resolution than others. It depends a lot on the length and impedence characteristics of the particular wire.
Chuck (sonar2000)