Calculating keel offset

[catlotion]

I want to add the keel offset to my ST60+ Tridata on my Bavaria 30 Cruiser but not sure on the figure.

What's the best way to calculate keel offset accurately? Or does anyone know it for this boat please?

 

[scruff]

Measure the depth with a lead line / long tape measure then adjust according to your draft to either show depth of water or under your keel, whichever you prefer.

 

[Catalina36]

One of the problems of using keel offset is trying to remember years later whether or not you set it up and whether the depth you're being shown is from the bottom of your keel or from the surface. I used to use it, now I just opt for it to show me the true depth. Simpler.

 

[PabloPicasso]

You could use the keel offset function to accommodate the depth beneath the waterline that the sensor is if that is significant I think that would be a minus figure

Surely you know your boat's draft? According to https://sailboatdata.com/sailboat/bavaria-cruiser-30. It is 1.4 meters. But it may have been available in deep And shallow draft versions, or even bilge keel. It would depend on the model and year I expect

ST60 tri data manuals are likely available online if you don't have them, Google will likely give you a link to them

 

[LadyInBed]

Lead line to get depth to surface.
Subtract boat draft. If you don't know it tie two lines to the ends of a negative buoyancy beam and pass it under the keel.
Adjust the echo sounder until it reads your calculated depth below the keel.

 

[pvb]

I've never bothered with keel offset. If you're very close to going aground, I don't think 0.5m or so is important -just head for deeper water!

 

[LadyInBed]

I've never bothered with keel offset. If you're very close to going aground, I don't think 0.5m or so is important -just head for deeper water!

You don't navigate Wareham channel! ?

 

[pandos]

I calculated mine last week... first time ever...

Measured from underside of deck to sounder inside the boat.

measured from waterline to deck standing on the deck..

measured depth of water at the sounder with a string and weight

I know my draft is about 1.4m ish

Discovered my sounder is .7 under the water and .7 from the bottom of keel... makes things easier... I just add .7 to the water depth shown on the sounder and am aware that .75 i am about to hit the bottom. (2" to spare)

sounder shows error frequently but in anything less than 10m it seems spot on... I intend to leave it as it is so that there is no ambiguity in my mind especially as it seems to be a convenient mid point

It is worth knowing as .5m under the keel is fine in purely sandy places but .5 under the sounder is 8 inches too little no matter where....

 

[Lucky Duck]

I've never bothered with keel offset. If you're very close to going aground, I don't think 0.5m or so is important -just head for deeper water!

You don't navigate Wareham channel! ?

On the East Coast much more than 0.5 under the keel and I start to suffer from vertigo

 

[vyv_cox]

When I sailed in tidal waters I used to run aground gently on the sandbank at Foel Ferry on a rising tide and set the depth at zero. I still prefer to know depth below the keel rather than actual depth.

 

[Lucky Duck]

One possible advantage of either no offset or showing total depth that it can show you just how far aground the boat is!

 

[prv]

You could use the keel offset function to accommodate the depth beneath the waterline that the sensor is if that is significant

Errm, yes, that's what the thread is about...

Surely you know your boat's draft?

Probably, but he doesn't know the vertical position of the transducer face, a figure that is less commonly published. Hence asking here.

I've never bothered with keel offset. If you're very close to going aground, I don't think 0.5m or so is important -just head for deeper water!

Depends where you sail!

One possible advantage of either no offset or showing total depth that it can show you just how far aground the boat is!

Not exactly, since the transducer gives the distance to the nearest surface within its sonar cone. If things are close enough that you're aground, the nearest surface might not be the same distance away as what's under your keel. It could be a rising bank beneath the bow which the keel hasn't reached yet, or a hummock off to one side.

When I sailed in tidal waters I used to run aground gently on the sandbank at Foel Ferry on a rising tide and set the depth at zero.

I just took a tape measure to it in the yard when we first bought the boat . Simple vertical distance from the transducer down to the plank the keel was sitting on in the cradle - both boat and plank were level enough for the purpose.

Your method did give you an annual recalibration though - be interested whether it ever changed?

Pete

 

[Lucky Duck]

Not exactly, since the transducer gives the distance to the nearest surface within its sonar cone. If things are close enough that you're aground, the nearest surface might not be the same distance away as what's under your keel. It could be a rising bank beneath the bow which the keel hasn't reached yet, or a hummock off to one side.

Pete

I find this most useful when sat in a marina near low water that isn't as deep as it claims to be

 

[vyv_cox]

I just took a tape measure to it in the yard when we first bought the boat . Simple vertical distance from the transducer down to the plank the keel was sitting on in the cradle - both boat and plank were level enough for the purpose.

Your method did give you an annual recalibration though - be interested whether it ever changed?

Pete

Not in the time I was in tidal waters, about 8 years. Since then I have changed out the complete instrument system and used a similar method to yours. I also made up a plumb bob that allowed me to check occasionally but had something of a scare when measurements seemed quite different. It turned out the string I had used for the plumb bob had shrunk after wetting.

 

[[70521]]

You don't navigate Wareham channel! ?

Or the Exe!

 

[Spirit (of Glenans)]

Post deleted due to not reading whole thread.?

 

[pvb]

Or the Exe!

And I thought the East Coast was shallow!