Depth / Speed problems

Bucksribster · 03 September 2012, 22:13

I'm quite new to RIBs, so I'm not sure if this is normal. I have a Raytheon ST40 speed and depth unit fitted to my Avon Adventure 560. It had the sensors mounted on a bracket off the transom, just about level with the bottom of the boat. Hopefully the photo will upload

My problem is that both speed and depth work OK off the plane (although the speed is a little erratic), but as soon as the boat is on the plane, there is no speed or depth indication.

Any thoughts?

jyasaki · 04 September 2012, 00:44

Can't really tell from the pic, but looks mounted way low to me.

Transducer centerline (the horizontal plane of the lower bulb part) should be about even with the hull; the paddlewheel fins (if it is a paddlewheel) should extend just under.

I suspect you're getting a vortex back over the top of the paddlewheel that's stopping it at speed. If it's a pitot type, then no idea.

Any chance you can post a more detailed pic(s)?

jky

wellhouse · 04 September 2012, 05:35

i agree, too low. get a ruler or a block of wood and place that along the bottom of the boat so that it overhangs the back of the boat next to the transducer to act as a guide to the boat bottom. Adjust the transducer upwards so that the bottom of the transducer is only slightly lower than the bottom of the boat. On mine i put the front edge level and the back edge slightly lower (about) 2-5mm) you might have to use wedges (which are normally supplied with the transducer by raymarine) to get the right angle. (other point is if it's too low it's asking to get hit by debris in the water as well as the vortex problem mentioned by jyasaki). The back edge of the transducer needs to be slightly lower than the front to give the paddle wheel a clean flow of water although for depth finding it needs to be pointing straight down otherwise the return signal can get confused/erratic. I've got the raymarine instructions if you need them but they are available on raymarine website) Make sure the paddlewheel is spinning freely as well (not gunked up) They do say on the instructions to avoid mounting next to chines. I can't see on the picture but if your bottom is like my avon, it looks like yours might be mounted directly behind one?

Bucksribster · 04 September 2012, 19:14

Thanks for the advice.

I attach a couple if other photos. You guys are right- if I run a ruler out from the hull, the unit is below the level of the hull, even at the left side ( i.e. where the hull is at the lowest point).

It is a paddle wheel type, and the wheel is running freely.

So I assume the unit has to be horizontal for the depth sounder. If so, where should the unit be level with the hull? Left edge, centre or right edge? The unit seems to be held in place by three screws in slots, so the unit can be adjusted. Just not sure if it can be adjusted enough!

Thanks again for the help!

wellhouse · 04 September 2012, 19:54

try adjusting it up as far as the adjustment allows (looks like you have about 10mm to play with on the slot) if that doesn't work you may have to screw higher up but try that to start with.

Into The Blue · 04 September 2012, 21:33

Not sure which transducer you've got but try looking on the Airmar technology website for detailed instructions on location

This is the link for the P66 which is typical of the units Raymarine supply

P66 Transom Mount - Airmar Marine Transducers

Bucksribster · 05 September 2012, 22:12

Thanks for this guys. I undid the mounting screws today, and found a wedge behind the unit which was mounted with the thick side down. This might have been put in to compensate for the slope on the transom, but bearing in mind your comments, and the most useful web link instructions, I have taken the wedge out, and moved the unit up about as far as it will go. This looks to be about right according to the mounting instructions. Pointing slightly down, and the left edge level with the bottom of the boat.

So assuming this sunshine holds, I shall tow it down to Camber dock on Sunday and give it a try. I'll let you know how I get on.

Thanks again for the advice!

wellhouse · 06 September 2012, 05:34

glad it was of help, hope it sorts it for you.

Bucksribster · 10 September 2012, 22:11

Just a quick update. Had the boat out in the Solent on Sunday, and it works! Got speed all the time, and depth most if the time. So I now know I had the boat up to 40 knots before I lost my nerve. The scary bit that caused me to back off the throttle was the expression on my wife's face lol. Can't wait for a flat water day to go flat out :-)

Thanks again for the tips.

wellhouse · 11 September 2012, 05:46

did you launch/recover at saxon wharf on saturday?

Pikey Dave · 11 September 2012, 08:44

Quote:

Originally Posted by Bucksribster

Just a quick update. Had the boat out in the Solent on Sunday, and it works! Got speed all the time, and depth most if the time. So I now know I had the boat up to 40 knots before I lost my nerve. The scary bit that caused me to back off the throttle was the expression on my wife's face lol. Can't wait for a flat water day to go flat out :-)

Thanks again for the tips.

I don't want to pi55 on your parade, but I would take any paddle wheel speed reading with a large pinch of salt. Do you have GPS or can you borrow a handheld from somewhere?

Crusher · 11 September 2012, 09:50

Just a bit of further experience.... I found the one I had (on my previous boat) worked far more reliably and accurately when it was fixed to the near level section on the transom. In your pictures immediately bellow the drain trunk, not sure if you have room or cable enough to reach, but IMO worth looking to see if you can move it.......

Bucksribster · 11 September 2012, 22:07

I do not have a GPS on the boat, but I do have the very wonderful Navionics app on my iPhone that gives speed, as well as the chart plotter. Only problem - left the phone in the car on Sunday :-( Of course you need to allow for the tide with GPS. I come from rag-and-stick boats where the tide makes a big difference.

If I find a big difference between the GPS and paddle wheel speeds I may try moving the sensors to by the strake as you suggest.

So if paddle wheels are not accurate, what technology is, aside from GPS?

Pikey Dave · 12 September 2012, 04:19

Quote:

Originally Posted by Bucksribster

Of course you need to allow for the tide with GPS. I come from rag-and-stick boats where the tide makes a big difference. ?

the paddle wheel will be giving you speed through water, not speed over ground (SOG), so it's actually the other way around. With the paddle wheel, you need to allow for tide, not the GPS which gives very accurate SOG (generally)

Quote:

Originally Posted by Bucksribster

So if paddle wheels are not accurate, what technology is, aside from GPS ?

Can't think of anything else as accurate or affordable as GPS TBH. GPS is incredible value for money when you sit down & think about it.

wellhouse · 12 September 2012, 05:42

once you get the raymarine working properly (and it sounds like you have) the manual takes you through a procedure to calibrate it to make it more accurate. Involves speed runs over a known distance etc. Whether you want to be bothered with this feature i'm not sure, it might not be available on your model but generally it is. What model is the transducer connected to? (At best the speed on it is a guide, mine's about 2.5/3 mph out at 30, i found the engine speed readout is very similar to the gps sog on mine and tend to use that)

Bucksribster · 12 September 2012, 22:49

It's an ST40, and I think there is something in the manual about calibration. I'm not too bothered about a few percent difference, but it is nice to know...

By the way, you asked if I launched from Saxon on Saturday - not me. I went from Camber dock on Sunday. Too many of us Avon owners around I guess ;-)