Planing boat water displacement

BartW

BartW

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scuse for the geekyness, but his pic got my attention:




A planing boat makes a dent in the water
This dent is visible behind the boat, speed and weight depending
Further away behind the boat the water flows back in the dent. (we all know the phenomen)

If we go faster, the visible dent is longer (?) but less deep, less resistance, …

He guy’s with the drones,
Would love to see that image with bigger planing boats 50..80ft, at different planing speeds, …
 
It would appear that speed is king to get the effect you are after

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BartW said:
Would love to see that image
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LOL, you don't really want to see that, B.
What that pic really shows (together with the others posted by BK) is how most of the fuel is burnt just to move around tons of water... :rolleyes:
 
Well, since among other toys I owned a 70+ mph Fountain for a while, I can't blame you for thinking that I am. :cool:
But the above was actually more spoken like someone with almost 2 decades of cruising in the 8 to 9 kts range, sipping just 15 to 20 lph of the Planet oil reserves. Not as eco-friendly as yotties of course, but not too far either.
Just look at the wake shot, and compare it with the previous ones...
qw5jmDXE_o.jpg
 
Well spotted B, that's indeed the Kornati archipelago.
Old pic - as you can see the boat didn't have the foldable HT on the flybridge yet.
Can't remember the exact spot, but probably we were on our way to Otok Levrnaka.
One of our favorite spots for home made brodet, at the fisherman's restaurant at the bottom of the bay.
Can't wait to try it again, but I'm not holding my breath.
Several folks told me that HR is not as nice as it used to be, and more expensive too! :ambivalence:
 
BartW said:
that was exactly what I was thinking when I typed that post :ambivalence:
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Do you actually move the water though, it being a wave? Look at how much water the bow rider supposedly displaces. It must be the equivalent of several tons per second if not more. You cant tell me a 130 or whatever hp outboard can physically shift that amount. It must be mostly wave
 
MapisM said:
Can't wait to try it again, but I'm not holding my breath.
Several folks told me that HR is not as nice as it used to be, and more expensive too! :ambivalence:
Click to expand...

HR will alway's remain charming imo,
if one thing, the croats are very conservative in keeping their heritage nice and un touched.
but indeed the hot spots sa Dubrovnik, Trogir, ... are totally and unpleasantly overcrowded in high season
same for the famous anchorages..

I read on the liveaboard forum that there is a new, fairly expensive tax for foreign boats passing the Croatian waters.
more info here: http://total-croatia-sailing.com/ne...ourn-tax-for-boat-owners-to-increase-in-2018/

about prices, well isn't that the result of the economic principle of offering and demand ;-)
prices all over europe have gone up the last few years,

except in Italy, I'm surprised the region around and south of Naples how cheap that is, ...
yesterday a nice dinner for two, 2 courses and whine, at 70 euro in total (and a grappa / limoncello for free )
that was in Agropoli !
 
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BartW said:
I read on the liveaboard forum that there is a new, fairly expensive tax for foreign boats passing the Croatian waters.
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Since when?
Last year, I paid the yearly permit in July, and this summer we're confident to pass along the HR coast before our current permit expires...
 
BruceK said:
Do you actually move the water though, it being a wave?
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Yes of course, at least to a large extent.
There's a flat surface in front of the boat, and a large hole behind her. That can't happen without physically shifting water.
Of course what you see well behind the boat (also in the pic I posted) is just the residual wave effect, but that's created by displacing water, to start with.
 
MapisM said:
Yes of course, at least to a large extent.
There's a flat surface in front of the boat, and a large hole behind her. That can't happen without physically shifting water.
Of course what you see well behind the boat (also in the pic I posted) is just the residual wave effect, but that's created by displacing water, to start with.
Click to expand...

I know, it makes sense, but just doesn't seem credible. The weight of that amount of water vs the available hp. It's as if I am missing another dynamic as to what is happening.
 
BruceK said:
I know, it makes sense, but just doesn't seem credible. The weight of that amount of water vs the available hp. It's as if I am missing another dynamic as to what is happening.
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well obviously its all exactly in ballance:

the horizondag thrust of the screw pushing a wedge over the water,
and the gravity - vertical force - weight of the boat.
the result is a continiuos "dent" in the water surface,
causing a wave of water flowing back in that dent...
 
That much is understood. I am thinking more in lines of how much water has actually been moved. Lets say in the instance of the bow rider is at that speed is displacing 500kg of water. But it's travelling at 30knts or 15ms. So every second it displaces 7.5 tons of water. This is where me and maths become really shaky but that seems a lot for a ~130hp motor unless something else is occurring.
 
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In addition the depth of the hole behind the boat appears to exceed the draft and the total amount of water displaced over a given timeframe not including the actual water displaced by the ensuing wake waves. I'm not sure it can be explained so simplistically.
 
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