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Old 06-09-2014, 16:47   #16
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Re: Sonar for Determining Bottom Type

If you want to penetrate the bottom you need to get down in frequency to maybe 5-10 kHz and get the power up! To get the best detailes you also need a small lobe which is hard when you use low frequencies unless you combine two high frequences into one low frequency, thats callad parametric sub bottom profiler.

If you happens to own a lot of money the Topas PS40 from Kongsberg will give you the best possible picture of the sediments, we use that one on the research ship I work on when we do geological surveys.

/hakan
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Old 06-09-2014, 17:04   #17
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Re: Sonar for Determining Bottom Type

A trained eye can tell a great deal about bottom composition when using the high-end sonar units. Look for something with a large color screen (not one of the 5" units) that transmits on multiple (>3) frequencies. Don't expect to get the type of information you want for something selling less than 500.
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Old 06-09-2014, 18:45   #18
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Re: Sonar for Determining Bottom Type

I have used a cheap Kingfisher fish finder for the last 12 years or so and find it much better to use than just the depth sounder I used to have.

Being able to just watch the bottom display for trending deeper or shallower water rather than interpreting the numbers is worth the fish finder if nothing else.

I generally circle around a couple of times and watch for whether the bottom is rough or smooth if rough I go elsewhere if smooth drop the pick. I have not lost an anchor in 3000nm per year of coastal cruising for the last 12 years so am tending to the opinion that the fish finder chosen anchorages works for me.
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Old 07-09-2014, 06:08   #19
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Re: Sonar for Determining Bottom Type

Seriously ...

The same things I didn't like about those cheap fish finders years ago ... when I was surveying, are EXACTLY what I'd want now for laying an anchor.

Just enough information to give me a general idea of what the bottom is like, which is much more than a simple depth finder will tell.

I don't need to know exactly how soft or exactly how hard. I don't need to know exactly how deep either ...
just a general idea ..... which is, again, more than a simple depth finder will tell me.

I'm not a geologist and I'm not doing hydrologic surveying ... I just want to know which anchor "might", have a better chance of holding on the bottom ... just another help full tool.
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Old 07-09-2014, 08:08   #20
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Re: Sonar for Determining Bottom Type

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Originally Posted by Dockhead View Post
I wouldn't really worry about someone getting tangled in the trip line (my main reason for never using them). Really good idea.
Just be careful that you cannot get tangled in the trip line. It happens more often than you think. You don't need any current with a simple wind change the boat often travels over the top of the anchor.

A tripped and dragging anchor together with a rope disappearing under the boat is not what you want to see.

Don't ask how I know

An easy solution is a weak link in the trip line that will break before the anchor pulls out (I use a cable tie). Arrange things so the portion of the trip line prior to the weak link can still can reach the surface and the line can still be used for its intended purpose. Most often it the float that gets caught, usually on the rudder. A weak link a 2m down is usually effective.
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Old 07-09-2014, 10:17   #21
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Re: Sonar for Determining Bottom Type

Wow you're getting a lot of advice from people who've never used a sonar...

I have a Garmin 740s with a 600-watt shoot-through transducer.

If you have a cored hull, you'll have to pour some epoxy to avoid new holes. It's worth it - external transducers are a pain. (Worth it if you need them, but you don't.) A transom mount (the "three screws") might be OK if you have a place for it, but I suspect if you get this thing you'll come to enjoy using it, and that a transom mount won't work for much more than puttering around an anchorage. (Transducers are VERY sensitive to bubbles of any kind.) If you're really OK with nothing but puttering around an anchorage, a stand-alone unit ($100) probably makes the most sense.

A patchy bottom will look like patches, and seagrass will look a lot like seagrass on a decent sonar. Finding those things are WHY people buy sonar units, not some fatal error to operation.

My transducer DOES have limited power - if you're anchoring in ~100 feet or more (and presumably have NO idea what might be at the bottom of the big rocky cliff), you might want to go to 1000w. I'm not sure where I lose resolution, but at ~40' I can usually see a couple feet into mud, and lose the bottom altogether somewhere around 2000'. I suspect you can about always see where your anchor will end up.

(Since someone asked, the 1KW have more "antenna" in there - they have more power AND more resolution. If you're fishing daytime swordfish or grouper in deep water you NEED this - otherwise, maybe not so much. I'd love one, I just don't want the big hole, in my boat OR my checkbook....)
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Old 07-09-2014, 10:33   #22
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Re: Sonar for Determining Bottom Type

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Wow you're getting a lot of advice from people who've never used a sonar...
Odd, you have posted almost exactly what I said last week.

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Old 07-09-2014, 10:36   #23
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Re: Sonar for Determining Bottom Type

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If you have a cored hull, you'll have to pour some epoxy to avoid new holes. It's worth it - external transducers are a pain. (Worth it if you need them, but you don't.)
Just to clarify it is the core that clauses the problem. You therefore need to cut the inner skin remove the core and replace this with solid epoxy.

Sometimes external transducers can be a better option.

However, most yachts have a solid section near the centreline with the core removed anyway.
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Old 07-09-2014, 10:42   #24
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Re: Sonar for Determining Bottom Type

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Odd, you have posted almost exactly what I said last week.

Pete
I did not mean to imply that ALL of the advice in this thread is misplaced. Yours certainly is not. Please excuse the awkward phrasing.
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Old 07-09-2014, 10:47   #25
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Re: Sonar for Determining Bottom Type

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Just to clarify it is the core that clauses the problem. You therefore need to cut the inner skin remove the core and replace this with solid epoxy.

Sometimes external transducers can be a better option.

However, most yachts have a solid section near the centreline with the core removed anyway.
And to clarify the clarification, solid means SOLID - air bubbles anywhere between the transducer and the bottom, including in whatever you're shooting through, will affect the results - make sure you don't make any! You can test a hull by filling a plastic bag with water, placing it firmly between the transducer and the hull, and moving things around to see what happens. I've never met a hull that didn't have a few bubbles - moving a couple inches one way or another can make a big difference.
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