Depth Sounder Keel Offset

[Three Amigos]

I just installed a New Depthsounder on my 1979 Catalina 27. The transducer is about 4" above where the keel meets the hull, and is laterally within 5" of this point. What should the keel offset be if, I believe, I need at least 4 1/2 feet of water depth. Confusing? Yes. As is the instruction manual.

I guessimated that the offset would be 3 feet. I should have done all this measuring while on the hard, but forgot.

Jim Huhta, Sr Sturgeon Bay, WI.

 

[Stu Jackson]

It's a philosophical thing as to how you set it. You can do either of three things: 1) set it to waterline; 2) set it to keel depth or 3) don't bother and run up on a slowly sloping soft shoreline and see what it reads.

The C27 brochure will tell you your keep depth, unless you know that already. Go look at a C27 brochure on the C27 Association website where there are line drawings of the boat and do some estimating from there.

 

[Chris Burti]

If you have a reasonably shallow slip, take a tall wooden pole and dip it in the water next to your slip without splashing and gently touch bottom, remove it and measure the wet length. This a reasonably accurate reading of the water depth at your boat's location. When you run your calibration, adjust the keel offset so that the instrument reads that depth exactly and you will know the water depth precisely.

If you wish to use the keel offset info to set the instrument to read the depth of the water under your keel, you can do the math by subtracting the draft from measured depth, but that number will prove a bit confusing in having to convert all the time if you are comparing your depth readings to your charts as a nav aid. Here where 25' is considered deep water and most of my sailing is in water less than 10', I have found it much more useful to know the actual depth precisely.

 

[Stu Jackson]

The reason #3 is an option is you never have to do any math. For instance, when our depthsounder hits 4.4 we hit. No figuring involved. Since the depth of our keels, in general, is not that much compared to tidal changes, it rarely gets involved in figuring differences from charted depths at MLLW.

Chris' ideas are very good, too.

Just let other helmsmen know how you've done it!

 

[Three Amigos]

Skippers.....Lots of food for thought from you-all. I will decide which to use as the most convenient, as here in Sturgeon Bay it is 45 minutes down a marked channel to get to either Green Bay or Lake Michigan waters, and the water gets thin very close to the channel in several spots; fortunately only a "wind tide" (small) is involved when the wind moves directly down the channel NW or SE. Thanx, Jim Huhta

 

[Three Amigos]

Helmsmen, Stu J, and Chris, et all: I measured depth with a wood pole at my dock and it is 8.5 feet to the surface from the bottom. My offset I guesstimated at 3 feet; My depthsounder shows 5 feet, so I am within .5 feet accurate. I tried this method today and was satisfied knowing the depth beneath the keel; however, navigating by depth....dunno yet, but will try this weekend. Thanx, and hope this helps skippers. Jim Huhta