Depth loss at speed

[ROBMH]

Hi,
My friend has just taken delivery of a targa 27.1 equipped with the latest Raymarine chart plotter with a Chirp transducer.
The depth indication is totally lost around 24knots Is this normal?
I know depth measurement at this speed is largely irrelevant as any indication of depth is history, but we were wondering weather this may highlight other transducer problems.

The transducer needs quite a large fairing block so we expect turbulence, but 24knots seemed a bit low compared to previous boats.

Your comments would be appreciated.

Regards
Rob

 

[gravygraham]

What is displayed on the screen? Do you get any sort of reading again, say at wot? The obvious suggestion, as you suggest, is turbulence and air bubbles round the transducer head, possibly caused by something else under the hull? Did you (he) try speaking with a Botnia dealer?

 

[Nick_H]

My Windy always lost depth reading as soon as it was on the plane.

 

[jimmy_the_builder]

Usually means the transducer has been positioned somewhere where the water flow over the hull is too turbulent for a reading when travelling at speed. I had this on my Targa 40; I repositioned the transducer and the problem was solved.

 

[gjgm]

My Windy always lost depth reading as soon as it was on the plane.

Did it ?
OP.. when I changed plotter and so transducer, I went from a reading at high 30s to low 20s, even though I was using the same hole in the hull ! After experimenting with 5 more transducers, I am now up to high 20s. I concluded that the newer hardware must be moe sensitive in some way and keeps losing what it requires for a stable reading.
I never did resolve it, but then I am doing far better than NickH !

 

[Nick_H]

Did it ?

Yes

 

[MapisM]

The depth indication is totally lost around 24knots Is this normal?

Yes.
If anything, I'd rather call it a better than average, than a normal performance.
I've never seen a small boat where the sounder worked anywhere near 30 kts, and in the large majority the signal was lost even before 20.

 

[Nigelpickin]

Interesting thread, I find, (I think), that the sea state and general mess beneath the surface makes a big difference; would that be correct? Also when in close quarters it is not uncommon for the vectoring on my pods to churn up so much that the depth gauge starts having a fit...bit of a down side ...but I haven't run aground...yet.

 

[Deleted User YDKXO]

On every boat I've ever owned the depth sounder has stopped working at planing speeds. Its perfectly normal. The idea with motor boats is never to cruise anywhere at planing speed where you might be worried about the depth

 

[ROBMH]

Hi All,
Many thanks for you speedy responses to my questions Of course it is common sense not to use a boat at speeds where the depth is questionable.
We were wondering whether this this was a problem common to other boaters or was due to to an ill positioned transducer.
Best regards
rob

 

[Deleted User YDKXO]

FWIW, you can place the transducer inside the hull in a piece of plastic tube filled with castor oil http://www.force4.co.uk/450/Nasa-In-Hull-transducer-Kit-for-Fibreglass-Hulls.html?referrer=froogle&gclid=CK3QuPWu2LwCFdShtAodQhoAjA and then move it around until you find a position where it works at speed. I did try it once on a Turbo 36 I used to own and did find a position where it worked some of the time at speed but it was a big faff and I haven't bothered to do it on any boat since. Basically you need to find a position where the hull is still constantly in contact with the water at speed (ie well aft) but at the same time not affected by turbulence from the props and thats a bit tricky

 

[MapisM]

Basically you need to find a position where the hull is still constantly in contact with the water at speed (ie well aft) but at the same time not affected by turbulence from the props and thats a bit tricky

That's actually tricky only on shaft(s) boats. The Targa 27 has an outdrive, so prop interference is not an issue.
Agreed re. the best placement, anyhow.
"Solid" water is what allows the transducer to work effectively. In this respect, as close as possible to the keel is as important as placing it well aft, if not more.

 

[Deleted User YDKXO]

That's actually tricky only on shaft(s) boats. The Targa 27 has an outdrive, so prop interference is not an issue.
Agreed re. the best placement, anyhow.
"Solid" water is what allows the transducer to work effectively. In this respect, as close as possible to the keel is as important as placing it well aft, if not more.

True to some extent but I've had the same issues on outdrive powered boats too. Maybe there is also turbulence ahead of the outdrive props as the water is drawn towards the props?

 

[vas]

as close as possible to the keel is as important as placing it well aft, if not more.

MM, care to elaborate why?
is it only due to more water and less likely to be airborne or is there any other reason?

Asking since I've still not drilled the hole for the transducer (actually triducer) and I'm interested.
Also remember someone mentioning that port side is better but cannot remember the reason (if any) Mine has to be port as strbrd is full of holes

V.

 

[NoviceRod]

I have had this depth-loss problem too with a Simrad system.

It only happens on the plane, and when I trim the bow down, so I have guessed it may be due to turbulence. It also developed part way through the season, so I am wondering if it might be exacerbated by fouling. Will investigate at annual liftout.

 

[MapisM]

Maybe there is also turbulence ahead of the outdrive props as the water is drawn towards the props?

I don't think that's the real reason. Well, I've never heard or experienced that, at least.
Thing is, when at speed, a xducer installed "too much" astern has a lot of hull bottom in front of it.
And with its drag, the hull "pulls" the first layer of water forward, together with the boat, making it more turbulent and less "solid".
That's the reason why the typical recommendation, in outdrive and o/b speedboats, is around half of the w/l, and as close as possible to the keel.
Not more forward, for the reason you mentioned, but also not much more astern, for this very different reason.

 

[WF36]

Also had this on all RIB's I have owned, be it with Garmin or Simrad plotters.

 

[MapisM]

MM, care to elaborate why?
is it only due to more water and less likely to be airborne or is there any other reason?

Yes, that's definitely one thing V, but there's also another reason.
If you consider what I just replied to M., I could have said in other words that you actually want the xducer placed as much forward as possible, rather than astern - though in a P hull, obviously you can't place it as fwd as you could in a D hull.
Now, the footprint of a P hull on the water while on the plane is an irregular pentagon (or even just a triangle, at very high speed, but that's not relevant in this context).
The most forward corner is always exactly under the keel, obviously.
Its position along the hull length depends a lot on each hull and its speed, but for most "normal" boats it's around 1/3 of the LWL.
Moving toward the stern, the next two corners of this pentagon are along the chines, somewhere around 2/3 of the LWL.
The last two corners of course are exactly where the chines meet the transom, but that doesn't actually matter.
It's the "V" shape of this pentagon, towards the bow, that is interesting for the xducer placement, because that's what allows a xducer to be placed more forward, when nearer to the keel.
The limits being (roughly) between 1/3 and 2/3 of the LWL, for a placement along the keel or along the chines respectively.
Another thing to consider is that whatever position is good for any given hull and speed in flat water, is bound to change - possibly a lot - in the rough stuff.
But the general principle still stands, particularly if you're interested in having the sounder working at the highest possible speed.

Re. port or stbd, I can't see how that can make any difference.
The only other thing I can think of is that the a/p electronic compass sensor is often placed along the boat keel, and around the LWL center - so, potentially not far from a xducer.
If you've got one, you'd better check its min distance from other electronic stuff, before drilling any hole...

 

[MapisM]

I am wondering if it might be exacerbated by fouling

It sure can - see my previous post #16: higher hull drag in front of the transducer=more water turbulence.

 

[Matt341]

I have the same issue with our Raymarine plotter. At displacement & semi displacement speeds the depth reading is shown but anything over 20 knots and the display just shows --:--m. It will sometimes work if the bow is trimmed up so that the stern is lower in the water but this does no favours to the fuel consumption and comfort of the crew! As previously mentioned in this thread, it's a good reminder not to plane at speed anywhere where the depth is unknown.