MH40 - Depth Transducer Offset
- Thread starter Jim Beck
- Start date
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 23,015
Jim. There is no hard set rule for the offset.
It is your boat and your choice. That way you control how you want to operate your boat.
It is like anchor chain. No hard rules. Have chain don't have chain.
There are lots of opinions.
It is your boat and your choice. That way you control how you want to operate your boat.
It is like anchor chain. No hard rules. Have chain don't have chain.
There are lots of opinions.
- Your offset should be from the water line to the bottom of your keel.
- Your offset should be from the transducer to the bottom of your keel
- You should choose an offset that is 1ft greater than boat draft.
- you should choose the offset based on your published boat draft.
Thanks for your reply. I clearly understand the choices involved and know that I prefer for my sounder to tell me the depth of the water from my waterline. The offset I want is the distance the transducer is below waterline . That distance is what i am asking and thought other MH 40 owners might know. My old transducer got removed without the offset being recorded so I have lost that information. I guess my best option is to get use of a hand held sounder to measure water depth and back into the offset.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 23,015
I did that to identify my need. Did not have a sounder. I used a simple weight and line. Dropped it to the bottom then measured the line below the waterline. I later confirmed my depth when a friend fired up his depth sounder. We compared number over a beer.
You are only talking 4.8 inches here. I think the measurement with a hand line over the side when anchored would be a better way to establish the offset to whatever you want to use. Even then, just a small bump or depression on the bottom where you lower the handline could make a difference. Just be aware that there are variables like the overall load impacting where your actual waterline is, your fore and aft load (squat) and other factors that can easily account for 4.8 inches you seem worried about. I set mine so that the charted depth "should" correspond to the indicated depth as read on the depth sounder. I set my alarm at 5 feet deeper than my draft and then "edge in" if I have to transit a shallow spot or get to a good anchorage depth. Even then there are possible factors as mentioned that make the number to imprecise to really count on the reading of a depth guage. Me thinks you are trying to measure with a micrometer, mark it with a piece of chalk and cut it with an axe. Just my 2 cents and worth about that.
All good points. I do have good reason for my approach. We have negligible tide changes in our area, but meaningful water level variation caused by winds. A sustained strong SW wind, for instance, results In low water in our marina and thin water in the channel out to the river. I need a fair amount of accuracy to know whether I can get out (or back in) the channel or need to stay tied up at the dock. A physical measurement of water depth in order to calibrate is not ideal due to a thick layer of silt on the bottom. As i noted in a previous post above, I think using a hand held depth sounder is the answer.
Your boat, your call, of course. From your response it would appear that you are trying to make a decision as to whether to get underway rather than "feeling your way" out of a skinny channel using a very precise and accurate depth sounder. Why not note the depth of the water by some mark on a piling you can see from your dock or some other fixed mark. If you know the depth at the channel that corresponds to the mark when you can't get out you have a fixed point of reference. I do it all the time to decide whether I can make it under a bridge. The silt in the channel can probably vary by several inches on any given time based on a lot of factors.
Jim:
A friend with a 2013 MH40 is currently out of the water in Deale, MD, and working on his boat. I could ask him to do the measurement and see what he says. I'll let you know. I'd like to know for my boat as well.
Tom
Make sure you know exactly where the depth transducer is located on your "sample" MH40 that you are taking the measurement from. I don't know if the depth transducer is "dealer installed" or the location is determined by the factory. Also, did PO of your boat or the owner/PO of the "sample" boat change the location? What fixed point on the boat are you taking the measurement from? Are you sure his "waterline" is the same as your's. Has your boat or the "sample" boat had the bottom painted? Was the painted waterline changed? There is a "Designed Waterline" and there is the paint line for the bottom paint which may or may not have been altered. Just make sure you know what your are measuring! I'd measure from a fixed known point like the bottom or top of the rubrail rather than some arbitray paint line on either boat.
For the level of precision you are seeking, you also need to know your load as compared to the designed load and adjust for the change in displacement and thus draft as a result of the difference between your actual load and the design load and where your load is placed. You may need to adjust for how much water and fuel you have on board, how many passengers you have on board and how much they weigh if you are planning on sneaking through on the cusp of "just deep enough." Good luck in getting them to reveal their weight. Somewhere there is a calc for your MH40 for change in draft compared to a change in load. Even then, location, location, location of the load is important too. Think of your boat as a see-saw rotating around a fore and aft center of bouyance. You need to know that exact point to do those calcs. Does a 1 inch squat at the stern due to your new big batteries and that new generator correspond to a 2 inch lift at the point of the depth transducer. See what I mean. We did these complex calcs everytime we came out of the drydock on a 425 foot submarine.
Your call but I you are still trying to make your depth transducer a "micrometer", your display mark with a piece of chalk, when your boat is an axe. Just saying.
For the level of precision you are seeking, you also need to know your load as compared to the designed load and adjust for the change in displacement and thus draft as a result of the difference between your actual load and the design load and where your load is placed. You may need to adjust for how much water and fuel you have on board, how many passengers you have on board and how much they weigh if you are planning on sneaking through on the cusp of "just deep enough." Good luck in getting them to reveal their weight. Somewhere there is a calc for your MH40 for change in draft compared to a change in load. Even then, location, location, location of the load is important too. Think of your boat as a see-saw rotating around a fore and aft center of bouyance. You need to know that exact point to do those calcs. Does a 1 inch squat at the stern due to your new big batteries and that new generator correspond to a 2 inch lift at the point of the depth transducer. See what I mean. We did these complex calcs everytime we came out of the drydock on a 425 foot submarine.
Your call but I you are still trying to make your depth transducer a "micrometer", your display mark with a piece of chalk, when your boat is an axe. Just saying.
If you are worried about silt when you are trying to do the handline check, just tie something flat and wide to the end of the line so it sits on the top of the silt. Even then, you are faced with the difference between the depth where you drop the line and the depth at the actual sensor (could be a few feet inboard of where you drop the line.) micrometer, chalk, axe.
We have a similar situation, where an extreme blow out tide can make one spot on our marina fairway too shallow to pass, even if we’re floating in the slip. We draw 4’7”, and have not bothered setting an offset for our transducer. I just know it’s almost exactly 18” below the waterline. We’ve learned from experience that if the sounder reads less than 3.5’ at the slip we won’t be able to leave.
So I don't have your answer yet. My friend's boat is in for sand blasting and painting over the next few days. He did say he uses an offset of 2.2 from the waterline. If he can get the transducer to bottom of keel measurement I'll post it.
I wanted to know how far my tranducer is from the water line so while I was replacing the hose from my galley sink to the through-hull, I rigged up an adapter to connect some 3/8 I.D. vinyl hose to the through hull and opened the through-hull. The water level rose in the hose to match the water line and I measured that height above the cabin sole. Then I measured the distance from where the transducer goes through the hull to the cabin sole. The sum of those two figures is the approximate depth of the transducer below the water line.
Someone pointed out, in response to a post describing this on another forum, that with a long enough section of tubing I could have taken the tubing to the transducer location and measured the water level distance directly.