Towfish vs. Downrigger

[Novak5001]

Hello,

Although this doesn't exactly pertain to shipwrecks, I hope you all can give me some advice.

I am working on an underwater mapping project (mapping locations of inundated towns). The location I am currently looking at is under about 70-80 feet (20-25 meters) of freshwater. I will be using a Lowrance HDS12, but if I want to get any imagery worth looking at, I was advised to use a towfish and run the transducer about 20-30 feet (5-10 meters) off of the bottom. I will be primarily mapping an area about 1 square mile in size (1.6 square km). There will be a lot of stopping and turning.

Since I will only be using this once, I am looking for a cheap and quick solution. I found some good ideas for design on the Humminbird Side Scan forums. My biggest challenge will be to find a reasonably priced transducer and cable. The Humminbird forums are not as active as here and I noticed some of the same people on both boards, that's why I am posting here.

Since GPS accuracy is critical to this project, I am considering using a simple downrigger setup: stick the transducer on a heavy pipe, lower it strait down over the side, make slow passes over the target area. Has anyone used something like this? Does it work? Is it a waste of time?

I know most of you are Humminbird folks, but does anyone have experience with the Lowrance units? What kind of imagry should I expect at those depths? Any suggestions on what to use for range?

Thanks!

 

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[ropesfish]

I just set up a Humminbird Solix 15 hd SI CHIRP unit for bottom mapping in shallow saltwater <less than 300'>. I would strongly disagree with the idea that you need to run that close to the bottom with a modern sonar unit to map submerged buildings.
My system uses a Z-Wing downrigger with the Humminbird transducer. We ran about 15 miles of lines with the unit today and got great images. On a flat sand bottom in 45' we were showing every ripple in the sandy bottom and on the limestone ledges every known artifact we scanned (cannons) were easily visible. Granted, cannons on the Treasure Coast are ofter partially or completely buried in sand, but you could tell it was man-made and a target worth diving on.
The biggest reason I rigged mine this way was to get away from having the boat's motion/surface action directly affect the transducer. I will probably at some point rig a surge compensation device, but first I'll have to figure out how.
If it were me, I'd try the Z-wing (less than $100) and put it down 8 or 10 feet. We survey at about 4.5 kts and this really gave us good images.
Trial and error will be your friend!
Good luck!

 

[Jolly Mon]

I just set up a Humminbird Solix 15 hd SI CHIRP unit for bottom mapping in shallow saltwater <less than 300'>. I would strongly disagree with the idea that you need to run that close to the bottom with a modern sonar unit to map submerged buildings.
My system uses a Z-Wing downrigger with the Humminbird transducer. We ran about 15 miles of lines with the unit today and got great images. On a flat sand bottom in 45' we were showing every ripple in the sandy bottom and on the limestone ledges every known artifact we scanned (cannons) were easily visible. Granted, cannons on the Treasure Coast are ofter partially or completely buried in sand, but you could tell it was man-made and a target worth diving on.
The biggest reason I rigged mine this way was to get away from having the boat's motion/surface action directly affect the transducer. I will probably at some point rig a surge compensation device, but first I'll have to figure out how.
If it were me, I'd try the Z-wing (less than $100) and put it down 8 or 10 feet. We survey at about 4.5 kts and this really gave us good images.
Trial and error will be your friend!
Good luck!

Thanks for turning us on to the Z-wing! I like it much better than a cumbersome and potentially suspicious looking towfish. Any tips on mounting the transducer to the Z-wing for a stable flight? Did you just epoxy the transducer directly to the Z-wing?

 

[ropesfish]

I mounted it just like you would on the transom with the original bracket as you need to adjust it to be parallel to the sea floor when you are under way. Here's a few pictures from this morning. If you right click on the picture (in Chrome) and click "Open image in new tab". Then you can blow it up to whatever level of excruciating detail you like.

 

[Salvor6]

Novak here is my simple towfish that I made from PVC to tow my Humminbird 1198 transducer. It cost less than $15. The block at the bottom is aluminum that I machined to fit the transducer. It helps to have a full blown machine shop at your disposal. It is small and inconspicuous.

 

[ropesfish]

Nice, Pete.
I may have to give that a try too.
I like the downrigger wing, but it does pull with some strain. I have 3 or 4 feet of 2" PVC rod that I could use....hmmmm.
Right now I get to go pull 3 days of mag and sidescan results off the gear and do a bit of mapping.

 

[Salvor6]

Bill, what does that "Z" downrigger cost?

 

[ropesfish]

Bill, what does that "Z" downrigger cost?

[FONT=&quot][h=1]ZWING 350 BLACK[/h]$69.00

https://zwingdownrigger.com/
[/FONT]​

 

[Salvor6]

OK, that's not bad. I thought it would be another "marine" ripoff where they charge hundreds of dollars for a chunk of plastic.

But still, having worked in the Injection Mold industry for the last 40 years, they can make these for about $2 to $3 each.

 

[Rüdiger]

Hi ropesfish

Had the idea to mount the transducher directly to a downrigger, nice to see that it works.
Which towing cable do you use?

mvh
Rüdiger

 

[ropesfish]

Hi ropesfish

Had the idea to mount the transducher directly to a downrigger, nice to see that it works.
Which towing cable do you use?

mvh
Rüdiger

I just zip tied 3/8" yellow polypropylene rope to the Humminbird extension cables, which are greased and sealed with silicone inside the good adhesive lined heat shrink. If I were in an area with obstructions instead of the open ocean I would consider stainless cable zip tied to the transducer cable.

 

[ropesfish]

OK, that's not bad. I thought it would be another "marine" ripoff where they charge hundreds of dollars for a chunk of plastic.

But still, having worked in the Injection Mold industry for the last 40 years, they can make these for about $2 to $3 each.

It does have some structure inside to anchor the SS mounting and it appears to have a good bit of lead inside. It probably weighs 5-8 lbs

 

[Rüdiger]

I just zip tied 3/8" yellow polypropylene rope to the Humminbird extension cables, which are greased and sealed with silicone inside the good adhesive lined heat shrink. If I were in an area with obstructions instead of the open ocean I would consider stainless cable zip tied to the transducer cable.

How many extension cables do you use?

mvh
Rüdiger

 

[FISHEYE]

Novack,keep in mind that a lot of those flooded inundated towns had tall trees and most are still standing and you dont want your transducer getting tangled up in them otherwise you will have to go dive for it or worse,your line could break.

 

[ropesfish]

How many extension cables do you use?

mvh
Rüdiger

I have two 30' extensions for a total cable length of about 80'. I have one more 30' extension in reserve in case I need it.

 

[ropesfish]

Novack,keep in mind that a lot of those flooded inundated towns had tall trees and most are still standing and you dont want your transducer getting tangled up in them otherwise you will have to go dive for it or worse,your line could break.

and take note - if you snag something immovable with your transducer fish, you better have some way to either stop the boat pretty quickly or a weak link that will separate the cable from the head unit with no damage to either.

 

[seekerGH]

Novak,

Instead of stopping and turning, run a continuous overlapping square pattern... increasing by the scan width each time
hope this makes sense...

 

[Novak5001]

Thank you everyone for the great information and suggestions. I am glad I asked. I was under the assumption that I needed to be close to the bottom to get any sort of usable imagery, but if I only need to go down 10 feet or so, that makes my life easier!

Instead of posting a bunch of replies, I’ll just list my comments below.

Ropesfish – Thanks for letting me know about the Z-wing. I have not heard of it before. It appears you drilled some holes in it for your mount, is that correct? Dumb question, do you drop the Z-wing alongside the boat or do you pull it behind?

Salvor – I like your towfish. What did you use for transducer cable?

Fisheye – Thanks for the advice about the trees. I should have thought about that, but I didn’t. Thanks in advance for saving me from potential disaster.

Seeker – I think I will run that pattern. What width in between passes do you recommend?

 

[ropesfish]

Novak5001 - yes, drilled holes and mounted with stainless bolts and lock washers. We have been tying it off the port stern corner and trailing it on about 30 feet of line which has given it a depth of 6 feet at 4.5 kts. That is subject to change, dependent on what boat we are using it behind, speed, etc. This WILL give you a layback error to compensate for. The distance of the offset between your GPS antenna and the location of the fish is going to introduce some variation between the real world and your map.

 

[stevemc]

Years ago I have seen Humminbird towfish made like Pete’s yellow PVC one. They were made with PVC pipe about 2” OD and about 3’ Long. Pointed plastic cone. They had lead shot mixed with epoxy inside just in the belly. And 3 or 4 fins on the rear. They used different tow hook ups, mainly just a stainless piece about 1/3 the way back and mounted so it would not rotate. And rigged like ropefish with a cable. They made a strain relief and motion shock absorber by making a big loop, and having 2 places that were securely attached to the cable, and a thick rubber piece like a rubber bungee hooked in there to flex.