Log - proximity to skin fittings

mullet

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What does the brains trust think about locating a paddlewheel log relatively close to existing skin fittings - with regards to concerns about log accuracy, not hull integrity.?

Between 2 seacocks, roughly 30cm from each, in the path of flow between the 2 (ie directly downstream from the first, upstream from the second). It's not the speediest boat on the water. The transducer documentation states to avoid positioning it near existing skin fittings due to turbulent water flow and possible cavitation around them - but I wondered if this is more relevant to a higher speed boat.

Context: transducer is an Airmar DST810; boat is a Twister (long keel with cutaway forefoot), kept on a mooring which gets terrible fouling so I'm expecting to want/need to remove the transducer anytime the boat is left unused. The transducer is a bit of a pig to get in/out of the through hull (at least it is when trying it dry and not lubricated, with the through hull in my hand). That makes me want to put it somewhere relatively accessible, and where nothing is stowed (which would either need removing when the transducer is taken in/out, or would get wet). The most straightforward position for this is beneath the head, between its (forward) inlet and (aft) outlet - both Blakes seacocks. Further forward becomes less accessible, further aft likewise (and I'd prefer to have a transducer forward of the deepest part of the keel so I can edge into shallow water and know it is getting shallow before the boat actually goes aground).
 
Every location has compromise somewhere in its range ...

Question is : Do the skin fittings have actual raised parts outside the hull - or are they just flanges ? Of course any 'hole' in the hull will cause some cavitation - but at sail cruiser speeds - unlikely to cause too much error.
 
Turbulence has a devastating effect on depth transducers. I once had a boat in which a previous owner had installed the transducer aft of the keel. It worked when the boat was not moving but otherwise was totally useless. If you have ever sailed across the wake of a ship you will have noticed that the depth goes haywire for a surprisingly long distance.
 
How many transducers do you see in this photo of my 38 ?

RhH8QYdl.jpg


There are 5 ..... two speed paddlewheels (pulled at this time) then two ultrasonics inboard - with a central sounder ahead centre ...

Look closely !

Inside hull there is #6 ... a Sonar ahead of all to stbd ..

UjSjWwcl.jpg


Not my 'design' - all installed by previous owners ...
 
I would urge the OP to not fit paddle wheel log transducer. The fouling will drive him mad. Either fit an ultrasonic log not common or just rely on GPS.
From one who removed his paddle wheel log many years ago just because it was always fouled when I needed it. Yes I could pull it and clean it but I job I could do without prior to each outing. ol'will
 
It really doesn't matter. The flow is being disturbed by a much larger obstacle - the hull - so you need to calibrate the log anyway.

And I'm with William (partly). Try a NASA EML instead of a paddlewheel. Time between cleans is much increased.
 
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