Fishfinder to see Bottom

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AndrewPalmer

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I have just bought a bilge keel Moody 35 and am planning on exploring creeks and out of the way places and drying out from time to time. I am thinking of fitting a fishfinder to see the shape of the bottom so that I don't settle on any steep slopes or obstructions. Does anyone know if this will work or have any recommendations? I am concerned that in shallow muddy water the bottom shape might not show up very well anyway?
 
The bottom 'shape' only shows up on a fish finder as you travel over it, so to get a true picture of the bottom you need to travel over your chosen spot in two passes at 90deg to each other. You can't just stop and look at your finder because it will show you a level bottom at whatever is the local depth. With practice you can identify weedy/muddy bottoms from the look of the picture on the screen. You can't normally work out how soft/deep mud may be.
 
I would be wary relying on anything electronic (unless things have moved on a lot) to tell you where to dry out. Some places it is obviousbthat you will be ok such as gently shelving beaches but in harbours it is best to ask or even better to look at low water. Do not kniw your rudder configuration but in our Westerly Pentland it was always the rudder that gave the most cause for concern as when dried out flat there was only a few inches of clearance. You will find that most drying harbours keep their bottoms clean !

Envy you your voyage of discovery of drying out - really nice feeling as the last of the water goes and your keels touch :)
 
It'll work great. You can see the bottom and a growth of weed here in 28.5 feet of water. If it is very weedy you may get a false bottom reading. The weed on Windermere grows about 4 ft tall and I have seen "minus" 2.5 ft on the fish finder.


Fish.jpg
 
Storyline said:
I would be wary relying on anything electronic (unless things have moved on a lot) to tell you where to dry out. Some places it is obviousbthat you will be ok such as gently shelving beaches but in harbours it is best to ask or even better to look at low water. Do not kniw your rudder configuration but in our Westerly Pentland it was always the rudder that gave the most cause for concern as when dried out flat there was only a few inches of clearance. You will find that most drying harbours keep their bottoms clean !

Envy you your voyage of discovery of drying out - really nice feeling as the last of the water goes and your keels touch :)
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Thanks, I have never done it before having only ever had fin keel boats, I'm really excited about exploring places on the East Coast where I have never dared to venture before.
 
alahol2 said:
The bottom 'shape' only shows up on a fish finder as you travel over it, so to get a true picture of the bottom you need to travel over your chosen spot in two passes at 90deg to each other. You can't just stop and look at your finder because it will show you a level bottom at whatever is the local depth. With practice you can identify weedy/muddy bottoms from the look of the picture on the screen. You can't normally work out how soft/deep mud may be.
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Is that correct; I am ready to be corrected but I always thought that the display on a fishfinder showed the bottom of the cone which may be larger or smaller depending on the depth and whether a 4 degree transducer or 7 degree or whatever is in use. If I am anchored and totally still as confirmed by the GPS I can see a linear picture of the bottom in front and behind the boat with variations in depth as expected from the depth contours. Yes the depth reading is a spot figure presumably directly below but you normally see a decreasing or increasing depth picture apart from this and whether you are on the edge of a ledge.
The OP is right to be very carefulm when drying out. I have dried out hundreds of times nearly all without problems but was lucky to get away with it on a couple of occasions (before I used a fishfinder). Once I was right on the edge of a steep sided v shaped gully and 3 ft to the left could have seen us topple over into it as we dried, another time nearly dried onto a wooden stump that would have pierced the hull I think. Another aspect to drying out is what the current will do on the ebb and flood. For example we dried out by the beach at Porthmerrion and there was a strange stream that ran along parralel to the beach on the ebb and was so strong that one hull was buried in sand to within 6 inches of the toe rail and would have given a small boat a real problem. I once anchored by Spike Island up the Mersey having been a bit late to make it up to Fiddlers Ferry.All was fine until the flood started and then being sat with the stern presented to the flood I had an anxious hour hanging onto the boat at about 45 degree of heel as the flood tide tried to turn the boat over.
I have also seen people anchored in the gully at East head when they intended to be up the beach-they did not seem to be enjoying the experience. So yes dry out but be careful!
 
As a regular bilge keeler sailor I would say a Fishfinder is one of the biggest advances in drying out aids of recent years. But an AID, not the final oracle. As suggested, several passes over the chosen spot are needed to ensure the bottom is reasonably level, and no obstructions or gulleys. You will soon learn to 'read' the Fishfinder trace and identify what sortn of bottom is below you - mud, weed, sand and rock all give distinctive traces even on a cheap machine. The best option is the expensive 3d Fishfinder which gives a relief map picture of the seabed below and a little ahead of the boat, but they can be four or five times the cost of the single beam version!.

With a bilge keeler, creek crawling is much safer, as running aground is simply a time issue and rarely a threat to safety or comfort. But if you run 'on' at much more than 2 or 3 kts, you are there until the tide lifts you off unless you have a big engine. With a bilge keeler you can do little to decrease the draft, except to move everyone aft to see if you can reduce draft enough to pull her off backwards.

Things to watch out for are , as above, gullies, or obstructions that can tip the boat up, or even right over. Drying out alongside you need to be sure there are no sills, ledges or drains you can come down on. Surprisingly often there is a projecting foot at the base of a harbour wall. If your keel lands on that you will have an uncomfortable time. Likewise as happened to me once in Lyme Regis, i was given a berth normally occupied by a large fishing boat that had dug a hole for itself. By the time I realised it was too late, and we had a mighty uncomfortable six hours tipped at 25 - 30 degrees. Its pretty impossible to do anything, move around safely aboard, or even brew a cuppa, with the boat over like that! If the boat does start to fall over on the falling tide, there is nothing you can do that I know of to help her.

Finally, avoid doing as I did once, tacking near HW and coming unaware over saltings, I managed to sail with one keel nearly flying up a a gulley. As soon as I went to come about the upwind keel plonked itself firmly onto the salting, and I was stopped there for nearly 12 hours!
 
catalac08 said:
Is that correct; I am ready to be corrected but I always thought that the display on a fishfinder showed the bottom of the cone which may be larger or smaller depending on the depth and whether a 4 degree transducer or 7 degree or whatever is in use. If I am anchored and totally still as confirmed by the GPS I can see a linear picture of the bottom in front and behind the boat with variations in depth as expected from the depth contours. Yes the depth reading is a spot figure presumably directly below but you normally see a decreasing or increasing depth picture apart from this and whether you are on the edge of a ledge.
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My fishfinder is maybe 12 years old or more, so modern ones may be different. The horizontal axis on the display is time (not distance) so it can only be an indication of what is immediately below the boat at that instant. If the boat is immobile relative to the bottom then the trace will be horizontal irrespective of the slope or bottom features.
 
alahol2 said:
My fishfinder is maybe 12 years old or more, so modern ones may be different. The horizontal axis on the display is time (not distance) so it can only be an indication of what is immediately below the boat at that instant. If the boat is immobile relative to the bottom then the trace will be horizontal irrespective of the slope or bottom features.
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Yes perhaps modern ones are different. When my boat is immobile the horizontal axis shows whatever gradient there is above or behind the boat. My transducer is a shallow water one I think with a cone angle of 20 degrees. According to my estimated this will show only about 10 ft in front and behind the transducer (in 20ft) so I think on reflection that your recommendation to pass back and forwards over the drying area is sound advice as the boat will never settle where you start to anchor.
 
alahol2 said:
My fishfinder is maybe 12 years old or more, so modern ones may be different. The horizontal axis on the display is time (not distance) so it can only be an indication of what is immediately below the boat at that instant. If the boat is immobile relative to the bottom then the trace will be horizontal irrespective of the slope or bottom features.
Click to expand...
Modern single transducer Fishfinders are the same: the horizontal axis is time, so although the line continues to move, it only indicates what is directly below the boat at that moment. But if there is much change as the boat swings on its anchor, then you need to think again because the seabed is uneven and may be unsafe to dry out on. The trace will show that up very quickly, particular if you us the bottom magnifying setting.
 
They are a brilliant bit of kit. The view is a bit like looking sideways through an aquarium. With practice you can tell the composition of the bottom by interpreting the trace. Basically with the "whiteline" feature switched, the thicker the white line, the harder the bottom. Lumpy rocks, weed, oh, and maybe your lunch all show up. Can't understand why anyone would fit a regular echosounder.
 
Now I know this is not a sensible choice for a yacht but I was doing a bit of work on a small trawler recently and whilst out testing the engine he flashed up the Olex plotter. This thing runs off a PC and takes feeds from the gps and the echo sounder and correlates that info with a database back in Norway to produce a 3d picture on the screen. The system checks in with base in Norway to update the database so every boat fitted with Olex automatically updates the system with real information such as sea bed density and echo readings ect. When I went up to the wheel house after the engine checked out ok I was so fascinated I wasn't even looking out of the window.
http://www.echomastermarine.co.uk/products/olex/olex-3d-chartplotter/

On a sensible note, I hear a lot of guys praising the Raymarine Dragonfly kit.
 
Having had fish finders on all previous boats and a friend who is very accomplished at drying out without mishap I can pass on his routine in unknown drying areas. He has two bouys on two separate lines terminating with a weight on each.

Pick a spot on the water you wish to dry out on.
With fish finder see what the bottom profile looks like doing several passes.
If it looks suitable drop one bouy at one end of chosen sitte, then motor to the other end and drop the second bouy.
He will the anchor between the surveyed bouys, which sounds easier said then done. He has mastered it very well with no surprises in 50years.
 
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