Structure Scan: Fwd Looking Sonar ?

[surfmachine]

Ahoy Mike, I bought a refurbished interphase forward looking sonar, probe, from interphase in Calif, great service and backup from Corey there!

As a navigational tool, very limited, does give warning of floating logs, and bombies, but only if they are very close. I think that it more of a tool for finding schools of fish?

Biggest limitation is, it only works in 6X the water depth, so in 4m of water, 24m ahead or 2 boat lengths, you have to be going very slow. In 20m you get 120m ahead, but you don't really need it there, although, you can see it shoaling and coral walls show up clearly where they rise abruptly from the ocean floor, but in shallow waters, very hard to use!!! I use it when coming in to anchor next to coral islands, to check out the bottom before anchoring. Also, very useful when slowly making your way into shallow areas in rain, night, any time !!!! in conjunction with radar and a forward lookout. It does give one more layer of protection, and I need all the help I can get!!

Doesn't work in over 70m of depth unless you tune it to just look in the top 10m of water, for floating hazards. You can steer the beam to scan just a portion of the 90 degrees ahead, also gives a running display of the bottom contour underneath the yacht, and a depth reading, very glary at night!

Can see the anchor chain, but no anchor!

Cost $900 usd landed in Malaysia, they replaced it when it had a fault on installation.

All the best from Keith.

[downunder]

Originally Posted by Don1500

I am thinking along the same lines. I, too, have an HDS8 that would only need the Structure Scan module and a transducer (~$600 total with std transducer, $750 with thru-hull). But I am thinking that it would work fine set up normally. Knowing you are centered in the channel NOW will help keep you there. This is not meant to be something that is your be all and end all of navigating in tight places, just a reference. And it finds fish.

I think that is a realistic assessment. Simply not a foward looking sonar.

[Don1500]

I can see it being very useful on long voyages through channeled waters (ICW) Trying to stay centered thru the Rock Pile or the Alligator-Pongo Canal is very stressful on your own. And it would be worse in fog or at night. Knowing that the channel walls are ten feet away would be helpful.

[Kettlewell]

If it could look far enough ahead to be useful, at a reasonable power consumption, it could be very useful. I have a poor man's version of this with one depth transducer (a fishfinder) located up near the bow and one depth transducer ( just depth) near the stern. I find it very useful when in tight harbor situations as it will give me a warning of the trend of the depths, and can give me an idea of which direction to go to get back towards deeper water. Obviously, you have to go slow for this to be of much use, but I have found it helpful getting off when aground and when surveying harbors. I sometimes "paint" a depth picture of the bottom by swinging my bow deliberately close to shallow areas, while keeping an eye on the aft depthsounder to try to prevent actually going aground. I have a longish fin keel, so the bow can float in a lot less water than further aft.

[prof_mariner]

I would just like to know how far ahead does the Side scan (Structured scan) look. even 20-30 yards would let me know that my course would intersect the side of the channel.

Side scanning sonar doesn't look ahead at all (at least not when it is mounted "normally"). I have a Humminbird SI unit here that I use to create custom charts and wreck surveys. The Humminbird has a very narrow sonar beam in the fore/aft directions, but is about 95 degrees wide from port to starboard. Max depth varies, but contacts in less than 50 fsw usually are very clearly defined.

Another scan you may want to study that Humminbird and Lowrance units are capable of is what they call "Down Imaging". A down imaging scan is essentially the side scan beam rotated 90 degrees so it projects 95 degrees fore / aft and directly below the vessel.

A lot of what has been suggested has been tried before with limited success. Give the Humminbird forums a good search ( Humminbird Side Imaging Forums ) and you'll find a number of threads on the topic. Unfortunately, Lowrance doesn't seem to have as big a following as Humminbird, but a lot of what has been done with the Humminbird units can also be done with a Lowrance.

Good luck!

[Katiusha]

Originally Posted by Dockhead

... I have an Echo Pilot forward looking sonar set on my boat which was installed by the PO. It's great as a redundant independent source of depth data, but I'll be damned if I can glean any useful information out the display concerning depths ahead.

Try contacting the Echo Pilot guys - they are very good (and patient) at explaining the imaging, just remember that they're in UK.

[prof_mariner]

To correct something I posted earlier: "down scan" doesn't scan forward, as I incorrectly posted above, but rather directly down. The image on the screen displays the bottom in a fore / aft projection. Here's an example of a down scan image:



This type of image would be very helpful while navigating, except that the display is -historical-. In other words, what is displayed is what has already been passed, not what is up ahead.

[prof_mariner]

And here's a direct link (at bottom of post) to the video of a Humminbird Side Imaging sonar being used with DrDepth software to provide a continuous 360 degree sweep of the bottom.

Assuming the video there was captured in real-time, it takes about 30 seconds for a complete 360 degree sweep. 7 knots is roughly 12 ft/sec, so in 30 seconds you would have traveled 360 feet. At 3 knots you'd travel 5 ft/sec, so in 30 seconds you'd travel 150 feet. In 20 feet of water my best guess is that the 95 degree sonar beam would only project about 20 or so feet ahead of the boat, so you'd have to go considerably less than 3 knots in order to use this for navigation.

Another way, would be to simply tilt the transducer 45 degrees forward (instead of plumb). I think the difficulty then would be interpreting the displayed image, although it might be worth giving a try.