Do you calibrate you depth sensor to water level or keel?

[Lodesman]

Originally Posted by s/v Jedi

No need for evidence because this is how it is taught in naval academy as well as all navigation courses

You've taken "all" navigation courses? Whose naval academy? It's true that most warships influenced by the RN set by the waterline, but submarines, dock-ships and others with large variability in the W/L set by the keel. Many surface warships have little variability in the W/L depth, but frequently have variable draughts to consider - retractable sonar domes, towed acoustic devices, etc - it makes sense to for them to fix to the W/L as effective keel-depth changes. "Naval academies", I'm certain, teach either/or as dependent on the type of ship.
I was a professional navigator in the navy and I set my own boat by the keel. I don't keep a 6-minute fixing routine with a navigation team, I don't lay sonobuoy fields, I don't stream a towed array, my draught doesn't change much. I use the echosounder to see how much room I have below my keel. It's easy enough to relate that to charted depth or HOT if necessary. It makes gunkholing easier, as all I need is tide range showing on the E/S to know I'll keep off the bottom, without doing math gymnastics in my head (even though I'm quite capable of doing so).
There are arguments for either method - it comes down to how it's being used, and the characteristics of the vessel. So you're right that there's no need for evidence - parkstone bay is obviously incorrect.

[Dsanduril]

Originally Posted by Training Wheels

This thread reminds me of the old Archie Bunker/Meathead skit of how to put on your shoes and socks! [emoji23]

And to keep it up, I know it’s not the Naval Academy, but the USCG Boat Crew Handbook - Navigation and Piloting has this to say:

Offset is a function available in most depth sounders. Its purpose is to compensate for the display of available depth from the sensor location on the hull to the actual lowest point of the vessel (Figure 3-32). When the offset value is reading correctly, zero ft beneath the keel means the lowest part of the vessel is touching the bottom, even though the actual transducer location may be several feet from the bottom

source page 3-46

And just for more humor:

This Handbook provides the safest, most efficient methods, techniques, and informational guidance possible to navigate boats in maritime environments and situations.

[Perfect Ride]

Originally Posted by Training Wheels

This thread reminds me of the old Archie Bunker/Meathead skit of how to put on your shoes and socks! [emoji23]

To your point, I have both readings on the helm. ;-)

When we get close to that depth we normally have someone on the bow watching the action. Bottom type is an important input too. I have chosen to slug though a little light silt/mud on occasion to make a river landfall, in this case water depth holds most value.

Water below keel is best for hard surfaces. We only draw 1.3 meters so the difference is slight. Crew has instructions to wake me if depths ever hit a minimum I set prior to any passage and alarms are set.

[s/v Jedi]

Originally Posted by Perfect Ride

To your point, I have both readings on the helm. ;-)

When we get close to that depth we normally have someone on the bow watching the action. Bottom type is an important input too. I have chosen to slug though a little light silt/mud on occasion to make a river landfall, in this case water depth holds most value.

Water below keel is best for hard surfaces. We only draw 1.3 meters so the difference is slight. Crew has instructions to wake me if depths ever hit a minimum I set prior to any passage and alarms are set.

Dragging through mud is done often. I wonder what will happen when the offset is at bottom of keel... will it show negative numbers? If not, it’s meaningless to see 0 and not know if it gets shallower or deeper.

[Lodesman]

Originally Posted by s/v Jedi

Dragging through mud is done often. I wonder what will happen when the offset is at bottom of keel... will it show negative numbers? If not, it’s meaningless to see 0 and not know if it gets shallower or deeper.

Post # 136:

Originally Posted by Lodesman

The offset doesn't change where the transducer is mounted - while the transducer has a minimum, the sounder likely doesn't. My transducer is about 2 ft below the waterline, or about 4 ft above the bottom of the keel. I have it offset to read from the keel, and not only will it read down to zero, it will give negative readings, which is very useful when driving through the soft mud/silt that is sometimes necessary in Florida.

[RaymondR]

I have noticed that generally when I get into a situation where I need to be constantly conscious of how much water is left under the boat there tends to be other demands on my consciousness and it would be impracticable to have to be constantly doing the mental math, even though simple, to keep from grounding.

Consequently I will continue to have the sounder set up so that I know instantly at a glance whether or not my draft is about to exceed the water depth and leave the maths to those times when I passage plan on a chart and have the leisure of zero consequences (stranding wise) if in error or inattentive.

[Clivevon]

RaymondR - I like that. That is exactly what & why I do what I do (calibrate depth under keel) I just hadn't realised quite why I do it that way until I read your post. I just trusted my instincts. Thanks. Always good to failsafe.

[Tonali99]

Originally Posted by s/v Jedi

Dragging through mud is done often. I wonder what will happen when the offset is at bottom of keel... will it show negative numbers? If not, it’s meaningless to see 0 and not know if it gets shallower or deeper.

I think the 'mud' part is interesting. My finder will occaisionaly show two depths. The real depth and another and the signal penetrates the mud and rebounds off something deeper. Always go with the smallere one.
I did add and extra fishfinder type near the bow to see what I was heading towards. But it gets bubble disturbance and lost signals.
Anybody else have/do that?

[s/v Jedi]

Originally Posted by Tonali99

I think the 'mud' part is interesting. My finder will occaisionaly show two depths. The real depth and another and the signal penetrates the mud and rebounds off something deeper. Always go with the smallere one.
I did add and extra fishfinder type near the bow to see what I was heading towards. But it gets bubble disturbance and lost signals.
Anybody else have/do that?

We have a single Airmar DST800 sensor far forward where our draft is 1'. This works fine, but it is at least 6' aft from the bow.

We often see readings from water layers (temperature), schools of fish and indeed bottom soup vs solid seabed.

We did have the problem of lost readings but that turned out to be a worn out transducer.

Also, when you have multiple transducers, make sure they are on different frequencies!

[wingssail]

Originally Posted by s/v Jedi

We have a single Airmar DST800 sensor far forward where our draft is 1'. This works fine, but it is at least 6' aft from the bow.

We often see readings from water layers (temperature), schools of fish and indeed bottom soup vs solid seabed.

We did have the problem of lost readings but that turned out to be a worn out transducer.

Also, when you have multiple transducers, make sure they are on different frequencies!

50khz, 192khz, 200khz

On the fishfinder display we often see multiple "bottoms" where the signal coming back from the bottom bounces off the hull, goes back down and returns again. You see bottom at 100 & 200 ft, or 10' & 20'. The digital sounders reduce the stength of the output to reduce multiple returns but sometimes they get confused and do not provide a reading. One of the others usually does.

[Cynara]

Keel. No mental calculations, especially when things get tight

[JeanF73]

Did not read the whole posts.

But

What should I do with a retractable keel?

[fxykty]

Originally Posted by JeanF73

Did not read the whole posts.



But



What should I do with a retractable keel?

Same as with daggerboards: set offset to deepest possible draft, or maximum hull draft, or waterline.

It doesn’t matter - pick one and get used to it.

[Dsanduril]

Well, it took me a while to find some Naval Academy training materials, but I finally found The American Practical Navigator - 2017 Edition - Volume 1 (lovingly referred to as "Bowditch"). Bowditch makes a distinction between running echo sounding equipment at sea and when transitioning to piloting. The procedure when changing to piloting:

• Fathometer Operator: Run the fathometer continuously and station an operator to monitor it. Do not rely on audible alarms to key your attention to this critically important piloting tool. The fathometer operator must know the warning and danger soundings for the area the vessel is transiting. Most fathometers can display either total depth of water or depth under the keel. Set the fathometer to display depth under the keel. The navigator must check the sounding at each fix and compare that value to the charted sounding. A discrepancy between these values is cause for immediate action to take another fix and check the ship’s position.

emphasis added - pp168

If we want to go back in time to older navy training (possibly before an offset was even available), A Navigation Compendium, Revised Edition, Naval Training Command, Pensacola, FL - 1972:

The navigator must remember that the fathometer sends the signal from the keel and therefore recorded or indicated depths are depths under the keel. Actual depth is equal to the sum of (1) depth under the keel and (2) draft of the ship. More importantly, the navigator must know the depth under the lowest projection, usually the sonar dome in naval ships.

[s/v Jedi]

Originally Posted by Dsanduril

Well, it took me a while to find some Naval Academy training materials, but I finally found The American Practical Navigator - 2017 Edition - Volume 1 (lovingly referred to as "Bowditch"). Bowditch makes a distinction between running echo sounding equipment at sea and when transitioning to piloting. The procedure when changing to piloting:



If we want to go back in time to older navy training (possibly before an offset was even available), A Navigation Compendium, Revised Edition, Naval Training Command, Pensacola, FL - 1972:

Well.... it clearly says that the -sounding- must be compared to the charted sounding. This means total water depth. Every log entry, every calculation for tidal state, is done with water depth.

The reason for setting an offset, if possible, to depth below keel, is that this is the number as it always was before offsets were possible. For ships, the DBT is the same as depth under keel.
Ships have both numbers available. Their draft is also available on scales painted onto the outside of the hull.