1986 H34 Depth Offset

Nov 13, 2020
89
Hunter H34 Chesapeake Bay
The PO installed a B&G transducer and I'm validating the depth readings on my Zeus plotter. It typically reads about 1.5 feet more than the factory installed unit. If it's set to read from the waterline, is that the correct offset? Is anyone using readings from the bottom of the keel or transducer installation point (hull)? What is the depth of the keel from the hull?

Thanks,
John
 
Feb 21, 2013
4,638
Hunter 46 Point Richmond, CA
The depth meter sends the signal from the transducer at the through-hull fitting to the bottom of the ocean floor. The transducer counts how long the signal takes to go there and bounce back, does some fancy math, and displays the depth. There are different ways to measure depth:

1. Depth from the top of the water to the sea floor. To get that depth, you would have to add the depth from the transducer to the waterline.
2. You can have a reading from the transducer to the bottom of the seafloor. This requires no adjustment. The device comes out the box reading from it to wherever its signal bounces.
3. You could have the instrument read what is actually between the very bottom of your keel and the sea floor. This would require you to subtract the distance between the bottom of the keel to the transducer in order to get a “clearance only“ read out. This is the way my depth gauge is calibrated.
 
Aug 10, 2020
29
Hunter 34 Austin, TX
Why would you have depth information on your display other than the distance of keel to the sea floor? I want to know how much room I have before potentially running aground. My depth sounder is installed in the shower pan and from there to the bottom of the keel is 48" on my H34. Easy to measure when your boat is out of the water :)
 
Jan 22, 2008
1,654
Hunter 34 Alameda CA
I know to add 18" (1-1/2 feet) to my digital reading for the true depth from the waters surface. From experience I touch bottom when the number reaches 4 foot of depth.
 
Nov 13, 2020
89
Hunter H34 Chesapeake Bay
I know to add 18" (1-1/2 feet) to my digital reading for the true depth from the waters surface. From experience I touch bottom when the number reaches 4 foot of depth.
Thanks Allan. I forgot to mention that I have a fin keel which gives me a 5.3 draft. I believe the shoal draft keel has a 4.x draft.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Setting the depth offset is a personal choice.

The transducer, like @sail sfbay explains only knows the depth of the bottom to the face of the Transducer. Everything else is an "offset" calculation. Some like to add the 1.5ft of to adjust the display so that they know to match up with the tidal surface to bottom reference. Then they do their own math to adjust the "keel draft depth". If the display says 5 ft and the tide record says 5 ft they know they are going t be bumping ground.

Another set of sailors say... From the bottom of my keel how much free space is there. As the display scrolls to ZERO their anxiety level rises.

Others are somewhere in between. What you choose is what you can remember and are willing to accept.

Regarding grounding of your boat, if it has never happened to you, you are not sailing enough.
 
Nov 13, 2020
89
Hunter H34 Chesapeake Bay
Setting the depth offset is a personal choice.

The transducer, like @sail sfbay explains only knows the depth of the bottom to the face of the Transducer. Everything else is an "offset" calculation. Some like to add the 1.5ft of to adjust the display so that they know to match up with the tidal surface to bottom reference. Then they do their own math to adjust the "keel draft depth". If the display says 5 ft and the tide record says 5 ft they know they are going t be bumping ground.

Another set of sailors say... From the bottom of my keel how much free space is there. As the display scrolls to ZERO their anxiety level rises.

Others are somewhere in between. What you choose is what you can remember and are willing to accept.

Regarding grounding of your boat, if it has never happened to you, you are not sailing enough.
Thanks John. I'm familiar with how depth transducers work and the offset. What I do not know is the depth of my keel from the hull and various logic others may use. If there was only one way to use it, there would not be options in the software to configure it differently (grin). I like to think that my logic is good (most of the time), but I know I don't know everything, and like to get others' thoughts on the subject. :). Sometimes I ask questions to simply provoke the conversation and get ideas.

BTW, I ran aground twice in the first three times of owning my own boat. Both times I was praised for keeping calm and not freaking out. I dealt with it. Knowing how to quickly, under stress, make a bridle and tie a bowline offsets any embarrassment one might feel from running aground.

John
 
  • Like
Likes: jssailem
May 7, 2012
1,338
Hunter e33 Maple Bay, BC
I just went through a similar exercise when I was recently offered a new to me slip in shallow water, as in maybe less than a foot below the keel on a zero tide. My specs show a keel depth of 5’ 6”. Is the draft of 5’ 6” for light load, half load or full load. I also didn’t know from where this was measured, the top of the bottom paint (dealer applied), the bottom of the boot stripe (manufacturer applied) or somewhere in between. So I wasn’t able to determine exactly how much water the boat was currently drawing.
What I did do was measure the water depth with a lead line. I calibrated my Raymarine ST60+ sounder offset to read from the water surface which in my case I now know is +1.6’.
Knowing the depth and time I took the measurement and with my Navionics chart card (or any tide predictor) allowed me to determine what the depth of the water my slip will be at zero tide (I calculate 6.5’). For now, until I next haul the boat, I will assume the draft is 5’ 6”. And even during the 4 minus tides a year (max -.3’ or -4”) I should be okay pending wind or heavy wake from a large power boat. The bottom is silt so risk of damage is minimal.
 
Feb 21, 2013
4,638
Hunter 46 Point Richmond, CA
...............Another set of sailors say.... From the bottom of my keel how much free space is there. As the display scrolls to ZERO their anxiety level rises............Regarding grounding of your boat, if it has never happened to you, you are not sailing enough.
I am in the camp of wanting to know how much clearance between my keel and bottom. I confess that I have run aground a few times on my previous 2 sailboats because I was not paying attention to the chart plotter and depth gauge and since learned not to do that again on my latest sailboat.
 
Last edited:
Jan 7, 2011
4,727
Oday 322 East Chicago, IN
:waycool::waycool:
I have been sailing for about 10 years (on 2 different boats)...I am blessed to be on Lake Michigan, and am in 30+ feet of water as I pull out of my marina. No bar to cross, nothing but wide open (deep) water....and I can say that I have not in 10 years run aground.

Came close one time when I anchored as close to shore as I felt comfortable, and a wind shift overnight blew me closer to the beach..l woke up and could see them ripples in the sand below me. My boat only draws 4 1/2 feet, but I was close to sitting on the sand.

I prefer my depth to read the depth under the keel...as @jssailem said...my anxiety level goes up as the numbers approach zero.:waycool:

Greg
 
  • Like
Likes: jssailem
Nov 13, 2020
89
Hunter H34 Chesapeake Bay
While at the boat today in the slip, I decided to set the offset to -4.0 to read from the bottom of the keel. Then I set an alarm to sound at 2. The alarm sounded immediately. I only had 1.6 below the keel at that moment and it wasn't even low tide yet. I have a water issue in my slip. I think I've been on the keel a few times since I've been there four weeks.
 
Jan 22, 2008
1,654
Hunter 34 Alameda CA
Thanks Allan. I forgot to mention that I have a fin keel which gives me a 5.3 draft. I believe the shoal draft keel has a 4.x draft.
Yes, I have the fin keel as well. My total draft is 5'6". My transducer is under the wooden grate in the shower. That puts it 18" below the waterline. The keel bottom extends another 4' below that. Hence bottoming out at an indicated 4'.
 
Dec 25, 2000
5,703
Hunter Passage 42 Shelter Bay, WA
Shoal draft here that draws 4' 11". Our offset is five feet. When my depth meter reads 0.5 feet, I'm touching the bottom. Shallow alarm set at 8'. Have touched bottom a number of times, because I tend to seek out shallow anchorages; just mud and sand around these precincts, so no harm.

On the other hand I tend to get a bit nervous around rocks and reefs. Just keep a keen eye on tide state and chart information, which helps keep the boat safe.
 
  • Like
Likes: WasabiRush
Nov 6, 2017
76
Catalina 30 5611 Stratford, Ct
If you don't know where the water line is compared to the depth sounder then here is an easy way to get that number. This is easier with two people. First, shut any through-hull and remove the hose from whatever it is attached to. I used the engine cooling because it was the most accessible. Hold the hose attached to the through-hull up as high as you can and slowly open the through-hull enough to allow water to flow into the hose. Next, lower the hose until water appears at the end of the hose. Holding the hose in that position use a level to place a piece of tape or some other mark on a vertical surface for reference. That mark is the waterline of the boat as loaded at the time you marked it. It is not a bad idea to know where the waterline is inside the boat so you may want to make a more permanent mark. Using a level you can now measure from the mark using a level to determine the offset for your depth sounder. Be sure and add the thickness of the hull to the number.
A more direct way to do this would be to get some clear vinyl hose and an adapter to fit it onto your through-hull and use it as a water level and take a direct measurement right at the depth sounder, again adding the hull thickness.
My preference for depth sounder offset is to set it to the actual water level. By doing that I can then set the low water shadowing on the chart plotter to at least 1 - 2 feet deeper than the draft on my boat. When I am approaching any shadowed area on the plotter, I know that I am getting into shallow water and need to keep a close eye on the depth. You can also set alarms to warn you when you are getting into water that is shallow. I have mine set to 7.5 feet and my boat draws 5.5 feet. Setting the alarm a couple of feet deeper than your draft allows for an early enough warning to make course changes to avoid grounding. I have ground my boat twice before setting an alarm. Both times I grounded her I was not paying attention or was distracted. An alarm can help with that.