How is Depth Calculated

[DDD]

Novice question....
How is the depth to the bottom measured on the depth finder?
Let's say my draught is 3 feet...
Does that actually mean I have 3 feet before my bottom hits? And if so, it measured from the lowest part, i.e. running gear?

And, the depth I see on the depth finder... is that the depth from the surface of the water? Or the depth from a certain part of the bottom of the boat?

Trying to get a better feeling of depth, as I had to call for a tow this past week.
Very shallow in Tampa Bay.

 

[electricaldoctor]

I think that you will find that it is measured from your transducer. You can go into the settings for your depth sounder and adjust the "keel offset" to give you the comfort that you are looking for. When mine reads zero, I actuall have 5' below my running gear.

 

[DDD]

I think that you will find that it is measured from your transducer. You can go into the settings for your depth sounder and adjust the "keel offset" to give you the comfort that you are looking for. When mine reads zero, I actuall have 5' below my running gear.

So you have yours set to "zero feet" even below your running gear? That gives you a safety zone even when you are reading zero feet?

 

[electricaldoctor]

Go to a shallow sandy beach and take a depth reading. Jump in the water and measure the depth below your running gear to the bottom and adjust your keel offset accordingly to your comfort zone. e.g. If after you adjust your keel offset your depth sounder reads zero and you actually have 3' below your running gear, you now have a safety margin of 3'.

 

[Sea Gull]

The specification for the draft of your boat is the measurement from the designed waterline to the lowest point on the boat (likely your props). The actual depth of your boat below the surface of the water depends loading of the boat, running speed and angle, and even on salt vs fresh water. The boat draws the least when planing, but when you go from dead slow to planing speed the boat will squat just before planing, so you'll need more water at that point. Also, in shallow water a sandy or muddy bottom can be churned up by your props and sand/silt can be sucked into your raw water strainers.

The two most common settings for your depth sounder are to measure from the transducer ( factory default) or to set an offset that accounts for the depth of the transducer and shows you the depth from the water's surface. Again, there is margine of error in setting the offset, because that number changes as described above.

Either way, give yourself lots of margin for error.

 

[Tonka Boater]

Everything above is perfect advice. From a practical standpoint, on my 280DA, I measured the distance from my transducer to the lowest point of my outdrive (don't remember, but let's say it was 18"). I then added to 2' and used that as my keel offset -- in other words, I had my keel offset set at 3 1/2'.

I had 2 transducers....the factory one connected to the Smartcraft dash display and an aftermarket one for my GPS/fish finder unit. I had the GPS unit set to the actual water depth and the Smartcraft gauge with the 3 1/2' keel offset. That way, when I anchored or fished I new the real water depth with the GPS but for cruising, when the Smartcraft read 1' or less I knew I was getting too shallow and had time to make a change. The extra 2' on the offset also allowed for margin of error with a loaded boat and for the hole shot when I would gun it from a dead stop.

 

[Woody]

Novice question....
How is the depth to the bottom measured on the depth finder?
Let's say my draught is 3 feet...
Does that actually mean I have 3 feet before my bottom hits? And if so, it measured from the lowest part, i.e. running gear?

And, the depth I see on the depth finder... is that the depth from the surface of the water? Or the depth from a certain part of the bottom of the boat?

Trying to get a better feeling of depth, as I had to call for a tow this past week.
Very shallow in Tampa Bay.

Read up on your depth sounder capabilities. I don't know what you have but my 280DA allows me to set an alarm depth. When that alarm goes off I then keep a close eye on the depth gauge. I have my depth gauge reading from boat bottom, gauge shows actual true depth. If the alarm goes off I then know to pay attention to depth. I personally see no point in having the depth gauge show anything other than the actual depth.

 

[Madifonzo]

I set mine for the published draft of the boat. So when mine reads zero I'm really in 3' of water.

 

[Woody]

I set mine for the published draft of the boat. So when mine reads zero I'm really in 3' of water.

Do you mean zero like you on the bottom or you still have 3' under the boat?

 

[Henry Boyd]

The actual depth measured by the depth gauge is the distance from the face of the transducer to the bottom. This distance is "averaged" in the sense that solitary high points may ignored.

Other than the least expensive depth gauges will come with an adjustment in the display called an offset. The off set can be either the distance from transducer face to the lowest point on the boat/running gear, or to the waterline.

We boat in tidal water that can get very shallow fast, so I use the boat bottom as the reference. Many people prefer using the waterline because that can be matched to charts if you have the correct adjustment for the time of year. In the time before GPS depth was often used as part of the data set in determining location.


Henry

Sent from my iPad using Tapatalk HD

 

[Henry Boyd]

I set mine for the published draft of the boat. So when mine reads zero I'm really in 3' of water.

Technically that's not correct unless your transducer is mounted on skeg of the out Drive!

Henry


Sent from my iPad using Tapatalk HD

 

[JeffM]

I have my Garmin 740s set up with an offset equal to our draft and the alarm set at 3'. That way the gauge shows how much water I really have beneath the out drive, while the alarm wakes me up if I'm not as attentive as I should be.

 

[Madifonzo]

I met a plus 3' offset? Or is it a minus 3' ft offset? I can't recall how I have it set. On the smarcraft guages I have it set with the offset. On my ray arime system I have it set to the actual depth from the transducer.

 

[Woody]

I'd rather leave the offset alone and have the depth from the bottom of my keel. On my boat, I know that at 1.3 feet, my props are making contact with the bottom. By not messing with the offset, I don't have to remember what I may or may not have set the offset to.

I don't use the alarm function, as I am not boating in areas where I need to worry about making bottom contact by accident.

That's the way I like it too. I try not to go under 4' but on occasion will go near 3' if I know it's a uniform bottom like at some beaches. Draft on my 420DA is almost the same as my 280DA, 40" vs 39", so nothing new to get used to. I don't like sucking up the bottom so I try to keep some water under the props.

 

[islandhopper00]

Whew, to much to have to remember. I set mine to read actual depth. If the water is 3ft deep that's what my sounder shows. I want to know actual depth. I know what the draft of my boat is.
Haha, some friends set theirs as most of y'all describe, When we were anchoring in shallow water near the beach, I looked at the sounder it showed 3ft, I jumped in the water expecting my feet to hit bottom...before my head went under, nope. I fully immersed to my surprise.

I guess it's kind of like people that set their clocks 5 mins fast. That never works for me because I know it's set fast, so.....

If the op is boating in Clearwater or Tampa waters and sets theirs like y'all describe, they will never be able to go anywhere. The sound and bay is shallow.

 

[sandydlc]

With Zeus pods we draw almost 4' so you can bet that we added an offset to our transducer setting. I would rather think I'm in shallower water than I really am than to have the actual setting and end up in water too shallow or possibly even get too close to rocks. Personally I never like to be in water below 12 feet or so. We hit bottom once in our first boat, an 18' runabout, and it's not a situation that I'd like to repeat. We got lucky that time but polished our prop in the sandy bottom where we ended up.

I tend to be more conservative so I would recommend using an offset and know that the reading is the depth of the water below your gear. Anything else really doesn't matter, does it?