Understanding the off set on my depth finder?

brewster16

brewster16

Well-Known Member
Aug 9, 2009
1,452
Long Beach Island, N.J.
Boat Info
'07 44 Sundancer
Engines
Twin Cummins QSC 8.3
FWebster posted a note on the depth finder offset that got me thinking. I really don't know how to accurately set up my depth sounder so that the numbers I read are dead on for how far my lowest point is from the bottom. Could we discuss this important topic? Please chime in for guidance on doing it accurately and properly. Thanks.
 
The actual how to depends on the specific hardware. The concept is simple, the location of the transducer face is zero. The distance from the face to the waterline is a positive offset (distance measured plus distance to waterline), the distance to the lowest point on the hull is a negative offset (measured minus distance to lower point).

Henry


Sent from my iPad using Tapatalk HD
 
ziekman@cox.net said:
I also would like some input. Something is not right when my draft is 44", yet my sounder says 2.5'.
Click to expand...

I believe your depth finder/sounder is reading the depth from the bottom of your boat where the transducer is mounted. It is not the true depth of the water from top to bottom. Your draft is the distance from the top of the water (your water line) to the bottom of your boat or outdrive. So, let's say the bottom of your boat, where the transponder is, is about 34" below your water line and the outdrive extendes about 10" further down. That would mean you would have about 20" clearance from your outdrive to the water's bottom.
 
I am not sure if there is a more correct way but I set mine up at my slip by taking a pole and putting it down into the water till it hit bottom. I then measured that with a tape measure. It was about 6' 4" so I then set up the offset on my vessel view so that my depth read out 6' 4". I don't remember what the offset is set at to achieve this. That being said, I was looking to get an accurate depth from the surface to the bottom and not from the bottom of my keel where the transducer is to the bottom. I am not sure which one of these choices is the "standard" but I guess this is where there could be some personal preference as to whether you want the depth guage to read actual depth from surface to bottom or depth from the bottom of your keel to the bottom.
 
I would imagine that the thing we are interested in is the bottom up to the bottom of the keel. After all that is the clearance we need to read on the sounder so we don't strike????
 
Jeremygavin said:
I am not sure if there is a more correct way but I set mine up at my slip by taking a pole and putting it down into the water till it hit bottom. I then measured that with a tape measure. It was about 6' 4" so I then set up the offset on my vessel view so that my depth read out 6' 4". I don't remember what the offset is set at to achieve this. That being said, I was looking to get an accurate depth from the surface to the bottom and not from the bottom of my keel where the transducer is to the bottom. I am not sure which one of these choices is the "standard" but I guess this is where there could be some personal preference as to whether you want the depth guage to read actual depth from surface to bottom or depth from the bottom of your keel to the bottom.
Click to expand...

+1,

I measured 8' 6" in my slip and played with the offset to read 8' this gives me a little room for error
 
And as you can see, some people prefer to set it so that it measures clearance (how much water is there between the lowest point of my boat and the bottom), while others set it so that it measures absolute depth, regardless of the boat's draft.

Either setting, or no offset at all, is fine just as long as you know what a given depth means for YOUR boat.
 
Chart readings are MLW (water depth at low tide). Therefore if your watching your depthfinder it should be set for the depth of the water not the distance between the bottom and the lowest point on your boat??
 
I like to know bottom of boat to bottom of water depth. I don't want to have to do a calculation when my attention is needed to drive the boat.
 
I think it comes down to a general change in technology. Before GPS became prevalent people used charts more actively, and knowing the absolute depth helped In the location process.

Those of us who boat in skinny water tend to want to know as Scott points out when to worry about shopping for new props.

Henry


Sent from my iPad using Tapatalk HD
 
islandhopper00 said:
I like to know bottom of boat to bottom of water depth. I don't want to have to do a calculation when my attention is needed to drive the boat.
Click to expand...

Your running gear is still below the bottom of the boat - so you still need to do some calculations.

I would say

1. If you leave offset at 0 - then you measure from the transducer - no matter what you need to do math unless you are in a jet boat and transducer is keel mounted.

2. You can set your offset to real water level (which I prefer) - and remember when you are at 3-4' your prop's are either getting a sand blasting or a rock casting.

3. Or you can adjust the offset the other way to compensate for your running gears max depth. Then you don't need to do math. But it still ain't fun sailing around in 1' of water.

But running with 0 offset would be confusing for me in @ any direction @ :) especially since I have boat depth finder at the "keel" and fish finder slightly higher mounted. (Plus a structure scan in between - which CANT be offset so I ignore the depth reading from that)
 
src said:
And as you can see, some people prefer to set it so that it measures clearance (how much water is there between the lowest point of my boat and the bottom), while others set it so that it measures absolute depth, regardless of the boat's draft.

Either setting, or no offset at all, is fine just as long as you know what a given depth means for YOUR boat.
Click to expand...

+1, as that's what I've been doing. There are differences of opinion here, but no matter what, the intent is the same.
 
This is why I always had 2 depth sounders on my boats. The 1 that came with the boat and 1 from the GPS/fishfinder.
 
I just upgraded my chartplotter to a Garmin 740S, and really like the ability to set the offset to show the depth of water below the running gear. The amount of water I draw varies depending upon the attitude of the boat and the speed (on plane or not). Also makes setting the shallow-water alarm much more straightforward.
 
I prefer to know depth below the xdcr; that makes is simple for me. Alarm is set to 3.0 feet below the xdcr which is a good clearance before the props start to kick up mud and grass at 2.5 feet below the instrument. On the 400DA the engine cooling water inlets are behind the props so that is why I'm more interested in the kicking up mud and trash.
 
I've tried both ways, like both the same. Currently reading from the boat bottom. It's funny sometimes when passengers ask how deep it is...I might say 3' and they think they're going to slide off into waist deep water...:smt043
 
Woody said:
It's funny sometimes when passengers ask how deep it is...I might say 3' and they think they're going to slide off into waist deep water...:smt043
Click to expand...

Just be sure to add "... from the bottom of the boat" just as they are about to go under. That way they KNOW you did it on purpose.
 
As you can see, it's all a matter of preference. For me, I set mine to the outdrive when it's in the full down position. On my previous 280DA, I anchored in shallow water and measured the distance from the transducer to the bottom of the skeg and made the appropriate offset in the unit. I'm close to what Scott said, if mine reads 1' I slowly start backing up and/or raise my drive a bit....or calculate the cost to get my props fixed. I know my lake well so this has not been an issue but it's nice if you venture into a bay that you're not familiar with or approach a dock that you are not familiar with.
 
You must log in or register to reply here.

Forum statistics

Threads
113,154
Messages
1,427,386
Members
61,060
Latest member
Danileo
Back
Top