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Old 28-07-2013, 03:15   #1
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Fishfinder on Dinghy Transom

I want to install one of the cheap fishfinders on my tender (for depth display rather than fishing). I often anchor in unrecognised anchorages that are tight.

Motoring around the anchorage in the dinghy would be a good way to check the depths in the swinging circle. The water is nearly always clear enough to see rocks etc, but the clarity makes it difficult to judge the depth from appearance alone.
There are handheld depth sounders, but the continuous reading of a Fishfinder appeals.


The problem is I have an aluminium RIB, so the only transducer option is a stern mount. Given the small size of the dingy transom it puts the transducer very close to the outboard motor. As the outboard will sit lower than the transducer I fear that the corner of the acoustic cone will be in, or very close to some disturbed water from the prop-wash, even if I angle it slightly away from the vertical.

Has anyone tried this? Will it work with the outboard running?
Any cheap Fishfinder models / brands to buy or avoid?
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Old 28-07-2013, 04:18   #2
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Re: Fishfinder on Dinghy Transom

Have tried that exact scenario but I did install a transom mount transducer on my ski boat that has an I/O. Due to the design of the transom I had to mount the transducer pretty close to the drive and have had few problems. Even works when I'm on plane.

I do get some spurious reading in certain situations: when first planing off or coming off plane, very shallow water (2-3') where the prop wash off the bottom creates a lot of turbulence, and when reversing. Otherwise seems to work just fine. But as they always say on the internet, YMMV.
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Old 28-07-2013, 04:33   #3
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Re: Fishfinder on Dinghy Transom

Didn't Eagle/Lowrance have a transom suction cup transducer? something like this then:

LOWRANCE or EAGLE Suction Cup Transducer Mounting Kit | eBay

Or buy a complete kit on one go:

Lowrance X-4 Portable Fish Finder - Only 124.95 - Force 4 Chandlery

I have run larger ribs with sounders close to the engine without problems, also the engine will be further back than the transducer. Never had a problem with the Lowrance / Eagle stuff although Garmin have stolen alot of their market share in the last decade.

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Old 28-07-2013, 04:58   #4
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Re: Fishfinder on Dinghy Transom

According to the Hummingbird Instructions:

... On outboard or inboard/outboard boats, it is best to locate the transducer at least 15" to the side of the propeller(s)...

I believe they are assuming high(er) speed operation, where the prop' will create more disturbance.

http://www.humminbird.com/pdf/transd...somInstall.pdf
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Old 28-07-2013, 10:40   #5
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Re: Fishfinder on Dinghy Transom

I've epoxyed my hummingbird transducer to the inside of my fiberglass RIB and mounted the display under the seat for easy viewing from the steering position.

I'm not sure how well this wouuld work on Aluminum but works great on the RIB I have.

Easy enough to test. I tested by pushing the transducer against the bottom of the boat when there was a little water in it so as to make "full contact".

Regards

PS - It does work through Al. Will degrade performance and max depth. From Airmar:


Airmar does not recommend it. The transducer's sensitivity will be reduced transmitting through the metal hull, so you will not be able to see as deeply into the water. However installation in an aluminum hull is acceptable if the thickness is 3.8 mm (0.15",3/16") or less. The transducer must be epoxied directly to the aluminum hull. Use Airmar models P72, P79, or M260.
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Old 09-05-2014, 03:18   #6
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Re: Fishfinder on Dinghy Transom

I finally got around to doing this.
I made a bracket out of some scrap timber that just drops onto the dingy transom so it can be removed easily.

Garmin make a fishfinder with a built in clamp designed for mounting like this if you don't want to make your own, but it is a bit plasticky.
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Old 09-05-2014, 04:03   #7
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Re: Fishfinder on Dinghy Transom

The main use of this fish finder is just as depth display to allow some scouting of the anchorage to check the depth around the swing circle. It will also answer questions like "is there enough depth to move and anchor in front of the red boat"

Maybe I can even anchor in front of some cats

Surprisingly for a cheap fish finder it picks up the bottom detail quite well. I am wondering if it also has some application locating the anchor. On the depth sounder display the anchor itself is difficult to distinguish from a bit of weed etc on the bottom. On some occasions the anchor will be completely buried so this would not work at all (although with the big roll bar on the Mantus that will take some doing)

However I have a small float permanently attached to the anchor. The float sticks up a meter and half , but remains underwater. The idea is to make the anchor easy to find if the anchor is completely buried or covered by tall weed.

The float shows up on the fish finder display and being away from the clutter on the bottom it is easier distinguish than the anchor itself. I am wondering if it possible to choose a float that is particularly reflective to the depth sounder acoustic beam. A very strong return would help distinguish the anchor float from small fish etc. To motor the dingy around and quickly find the anchors location with just the fish finder would be useful.

I gather the fish finder picks up fish mainly from the return from their air bladder so presumably an air filled float would give a better signal. (The current float is a foam filled fishing net float). Andrew Troup had the great idea of using a strong metal float that could be snagged with a grapnel and used to pull the anchor out backwards if it was caught. The two ideas would work well together. If the anchor float could be easy found with the fish finder it would be easy to catch with the grapnel.

So what type of float would show up best on the fish finder? Does anyone know?

You can see the existing float on the right of the picture attached to the roll bar of the Mantus anchor.
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