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Old 10-08-2014, 11:23   #1
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Sonar for Determining Bottom Type

I am just coming to the end of my Baltic cruise, where one of the best things of all was the infinite number of viable anchorages. Anchoring out is a pure joy here -- I have only once during the whole summer ever shared an anchorage with even one other boat.

The only problem is that the charts don't show bottom type, and in many, maybe most places, the bottom is very rocky. I anchored in fear of losing my anchor all summer long. The way I tried to determine whether the bottom was rocky or not was to circle around a potential anchorage watching the 10 minutes depth history which the Triton display gives. You can see from the depth history -- if it jumps around a lot and traces a jagged pattern -- that the bottom is rocky. If it looks smooth, it seems more likely to be mud or sand. That was the theory, and it worked ok all summer, but I reckon luck was a big factor.

Next year I'm going to have an old-fashioned lead line with wax on the bottom to test the bottom conditions. But that won't necessarily prove that the bottom is not littered with boulders waiting to trap your anchor.

I wonder if it makes sense to use some kind of fishfinder sonar, like Structure Scan or Side Scan or whatever they're called -- my Zeus plotters have the sounder module built in. Do these have enormous bulky transponders you have to cut a special hole for? And which would drag down a sailboat? Are there other viable systems? Or will this not work?

As always, grateful for any suggestions.
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Old 10-08-2014, 11:38   #2
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Re: Sonar for Determining Bottom Type

I've always thought a fishfinder would give better information ... could never understand any advantage of the "feet only", readout.
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Old 10-08-2014, 13:45   #3
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Re: Sonar for Determining Bottom Type

Will not help much as the bottoms tend to be patchy. You will read mud then where you drop your hook there will be this one odd rock and you will get it. We would just carry a spare and if we can retrieve number one then that's fine, if not we can buy another in our next harbor.

This much said we have never lost an anchor in over 11 years of our sailing.

We do not buoy our hook either.

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Old 10-08-2014, 14:03   #4
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Re: Sonar for Determining Bottom Type

Your first problem is the cored hull so any transducers that shoot through the GRP which can be very effective aren't going to work. They have to be through hull.

Next if you want a really good picture then you need power. I am a little out of date now, but the two measurements are watts and RMS. Doesn't matter what they are you just need lots of them. Raymarine did have a 1kw kit which looked like a large yellow oblong box which then connected to the DSM 300 and onto a chart plotter. Since Arimar make most of the transducers you might find they have one for the Zeus.

Alternatively remove the Flirs echo sounder transducer and fit this at the wheel.

Raymarine Dragonfly | Sonar and GPS

You need to come sailing with me and see my little Garmin in action as we creep into Bembridge with a feeler gauge thickness to spare between the keels and the sand. Oh I ran around just off Haslar last month, shocked and totally my fault for being complacent. Had the chart plotter set to charts and not fish finder or dual screen mode.

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Old 10-08-2014, 14:18   #5
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Re: Sonar for Determining Bottom Type

That's the reason the Manson Supremes have that sliding channel shackle slot. There are also anchor retrieval set ups that allow you to pull the anchor out backwards should the need arise. The problem with some of those is they cost almost as much as the anchor.

Getting an anchor irretrievably stuck is usually not a rocky bottom but cable, chain, mines and other detritus that have been dumped over the decades. Old commercial/wartime anchorages are the worst for this but can happen anywhere you are unlucky to find the errant trap.

Some ideas:


Boat Anchor Retrieval Systems, Anchor Retrieval Systems | T-H Marine
Review of AnchorWitch Anchor Retrieval System
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Old 11-08-2014, 02:09   #6
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Re: Sonar for Determining Bottom Type

I've never used one of those, but lots of places here inthe Baltic, there is sea grass - I suspect you will get false readings from your fishfinder etc.

I suffered several "heart attacks" this summer in the Swedish archipelago mwhen my depth finder suddenly showed I was clsoe to running aground (I wasn't but there was a lot of grass).
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Old 11-08-2014, 04:31   #7
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Re: Sonar for Determining Bottom Type

Quote:
Originally Posted by Pete7 View Post
Your first problem is the cored hull so any transducers that shoot through the GRP which can be very effective aren't going to work. They have to be through hull.

Next if you want a really good picture then you need power. I am a little out of date now, but the two measurements are watts and RMS. Doesn't matter what they are you just need lots of them. Raymarine did have a 1kw kit which looked like a large yellow oblong box which then connected to the DSM 300 and onto a chart plotter. Since Arimar make most of the transducers you might find they have one for the Zeus.

Alternatively remove the Flirs echo sounder transducer and fit this at the wheel.

Raymarine Dragonfly | Sonar and GPS

You need to come sailing with me and see my little Garmin in action as we creep into Bembridge with a feeler gauge thickness to spare between the keels and the sand. Oh I ran around just off Haslar last month, shocked and totally my fault for being complacent. Had the chart plotter set to charts and not fish finder or dual screen mode.

Pete
Ah, so you're using a Garmin fishfinder for this very purpose? Can you describe how it works and does it suit this purpose well?

Navico make something like a bajillion different transducers for their Structure Scan system, the brains of which are built into my Zeus plotters. Some of them look like they could work. Some of the are mounted externally to the hull with just three small holes for retaining bolts and cable.
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Old 11-08-2014, 04:54   #8
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Re: Sonar for Determining Bottom Type

Just got a lowrence DSI down scan image type, much higher frequency is supposed to give a very good definition of the bottom at anchoring depths.

Hopefully it will work as advertised. If not, it was only $100 as boatshow special runout sale, and not having anything it will still be better than a leadline. Hopefully in month or so I will be able to report if it works.

But adding one of the newer hybrid imaging stand alone sounders might be much cheaper, gives redundancy and means your chartplotter can be full screen.
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Old 11-08-2014, 05:09   #9
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Re: Sonar for Determining Bottom Type

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Originally Posted by Snowpetrel View Post
Just got a lowrence DSI down scan image type, much higher frequency is supposed to give a very good definition of the bottom at anchoring depths.

Hopefully it will work as advertised. If not, it was only $100 as boatshow special runout sale, and not having anything it will still be better than a leadline. Hopefully in month or so I will be able to report if it works.

But adding one of the newer hybrid imaging stand alone sounders might be much cheaper, gives redundancy and means your chartplotter can be full screen.
I know sailors don't usually use sophisticated fishfinders much, so maybe this is the wrong forum for asking this question. But I wish someone would explain the basics of the different types, how they work, what would be suitable for this purpose, etc.

For example, if the more powerful and thus more expensive transducers are just for imaging greater depths (true?), this is overkill and totally wasted for this purpose, since we are interested in the bottom at anchoring depths (never more than 20 meters or so, and usually not more than 10 meters).

I have seen fabulous images showing amazing detail of the bottom, but I don't really know what kind of transducer is needed to produce a decent picture of the bottom at our depths.

Maybe I need to ask this question on a fishing forum.

Have really no sailors ever considered using this technology for this purpose?
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Old 11-08-2014, 05:17   #10
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Re: Sonar for Determining Bottom Type

Quote:
Originally Posted by Dockhead View Post
I am just coming to the end of my Baltic cruise, where one of the best things of all was the infinite number of viable anchorages. Anchoring out is a pure joy here -- I have only once during the whole summer ever shared an anchorage with even one other boat.

The only problem is that the charts don't show bottom type, and in many, maybe most places, the bottom is very rocky. I anchored in fear of losing my anchor all summer long. The way I tried to determine whether the bottom was rocky or not was to circle around a potential anchorage watching the 10 minutes depth history which the Triton display gives. You can see from the depth history -- if it jumps around a lot and traces a jagged pattern -- that the bottom is rocky. If it looks smooth, it seems more likely to be mud or sand. That was the theory, and it worked ok all summer, but I reckon luck was a big factor.

Next year I'm going to have an old-fashioned lead line with wax on the bottom to test the bottom conditions. But that won't necessarily prove that the bottom is not littered with boulders waiting to trap your anchor.

I wonder if it makes sense to use some kind of fishfinder sonar, like Structure Scan or Side Scan or whatever they're called -- my Zeus plotters have the sounder module built in. Do these have enormous bulky transponders you have to cut a special hole for? And which would drag down a sailboat? Are there other viable systems? Or will this not work?

As always, grateful for any suggestions.
Sounds like a great place Dockhead
As a rule of thumb, I tend to look at the shoreline characteristics and extend those features underwater.

However, if it is unmarked and generally rocky, why not just accept it and rig up an anchor trip-line and marker in case you get stuck?

Edit: On Superyachts...I have installed and used Sonars both for fishing and navigation.
Different types with 2 or 3 pods each coordinated for different purposes and sectors

For just bottom mapping, you don't need a lot of power as that develops side lobe problems, but you do need training and practice to make it worthwhile, if doing things other than searching for schools of fish

The reality, is the bottom can appear smooth to the sonar, but that is only because there is 10ft of soft slurry that is hidding the rocks underneath
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Old 11-08-2014, 05:24   #11
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Re: Sonar for Determining Bottom Type

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… But I wish someone would explain the basics of the different types, how they work, what would be suitable for this purpose, etc… (snip)

Have really no sailors ever considered using this technology for this purpose?
agree, I guess the technology is pretty recently dropped to a price range that makes it affordable. My DSI doesn't have much range probably 30-80 meters max. The slightly more expensive HSI combines the old tech 50,80 and 200 khz for range and the 400, and 800 dsi for detail. But they are low power units so won't get down through much substate to give a picture of what's under the sand and weed. Not sure how the broadband sounders improve things…
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Old 11-08-2014, 06:04   #12
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Re: Sonar for Determining Bottom Type

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As a rule of thumb, I tend to look at the shoreline characteristics and extend those features underwater.
That's what I used to do, also.

Then only problem is that around here, there is simply no shoreline like the bottom you would like to anchor in. It is all rocky as hell. What you look for is where the rocks have a flattish place where silt and mud or sand has accumulated. I have been guessing at what makes that kind of place based on the charts, and based on what I can see from my sounder -- an area of constant depth with smooth changes.

As I said, this has worked successfully so far all summer long, but I don't want to press my luck.


Very good suggestion about rigging a trip line. Since the anchorages are empty here, I wouldn't really worry about someone getting tangled in the trip line (my main reason for never using them). Really good idea.
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Old 11-08-2014, 06:44   #13
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Re: Sonar for Determining Bottom Type

If I really wanted to do it right, I'd make one pass through the area at some speed towing a small 500 kHz sidescan sonar towfish. Best bottom map image you will ever see.
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Old 13-08-2014, 15:37   #14
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Re: Sonar for Determining Bottom Type

Quote:
Originally Posted by Dockhead View Post
Ah, so you're using a Garmin fishfinder for this very purpose? Can you describe how it works and does it suit this purpose well?

Navico make something like a bajillion different transducers for their Structure Scan system, the brains of which are built into my Zeus plotters. Some of them look like they could work. Some of the are mounted externally to the hull with just three small holes for retaining bolts and cable.
Well, whilst power does help picking up features at depth, in shallow water it will help differentiate the bottom type. Rock will show with lots of edges obviously but also a very thin bottom line. Soft mud will show up as a thick layer as the signal is reflected at different depths. Wrecks show up as square blocks, fish as fish symbols or half moons depending on the setting used and finally divers show up swimming horizontally with legs kicking.

It does take time to get used to it and helps having dived the location to see what the sea bed actually looks like, but I would be lost without the fish finder on board having had one for 20 years.

Just been looking at the Raymarine ones, sexy bit of kit at a sensible price, the Raymarine C80 might be getting swopped out next spring.

The little Garmin unit has the sender unit glued to the inside of the hull and filled with oil. Despite firing through 3/4" of GRP you wouldn't know from the quality of the picture it wasn't externally mounded.

You mount a simple fish finder next to the Zeus as a stand alone unit. I am curious about this new down angle ideas from Garmin and Raymarine, if its marketing hype or does what it says on the tin.


http://www.ebay.co.uk/itm/GARMIN-Ech...item1e8ece7c7b
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Old 13-08-2014, 16:11   #15
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Re: Sonar for Determining Bottom Type

Sidescan sonar for yachts is not sophisticated enough yet to be able to tell bottom type. I have had commercial grade side scan (high frequency-high resolution) onboard which clearly tells bottom type. The lead line is probably your best indicator without spending a fortune.
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