Draft on plane - Page 2

OldDan1943 · 08-08-2018, 06:41 PM

Quote:

Originally Posted by Easting

Cruise ship increases speed to get under bridge.
Surly not on a plane but ”ballsey” just the same.

Cruise ship barely squeezes under bridge - CNN.com

How many people were onboard?

One the submarine, with a new dive officer, we used to mess with his mind by shifting the off duty crew, fore and aft. LOL. I wonder if they shifted the passengers fore to aft to clear the bridge. SMIRK

diver dave · 08-08-2018, 06:45 PM

I learned one skinny water trick at deep water cay. Works only in single engine outboard boats. If u are faced with crossing a sand bar of known length, lay the boat over at full planing speed. I did this in a mako 25 with the owner onboard. We crossed sand with about a prop diameters of water above it at 25 knts. Yes u r in a fast turn so takes a bit of planning. [emoji847]

Benthic2 · 08-08-2018, 07:19 PM

it depends on the boat....

psneeld · 08-08-2018, 07:34 PM

Every planing boat I have driven when well up on plane draws less and those with significant vee hulls even less in a gradual turn.

At the transition to plane they draw much more but gradually rise significantly when on plane.

Even smaller vessels like Shamrock keel drives will squat when getting into shallow water....if you don't know which way to turn fast....hold tight and don't reduce throttle if you think you are going to make it to the channel.

Jeff F · 08-08-2018, 09:31 PM

I was on a whale watching trip off the south coast of NSW in Australia a few years ago on a catamaran maybe 80 feet long. To get in and out of the port we had to cross a shallow bar. Interesting experience. All passengers had to don lifejackets and they went at top speed across the bar, probably ~20 knots.

I asked the skipper about this afterwards and he said that not only did it reduce draft, but if they hit bottom they would cut the throttle suddenly and the overtaking stern wave would lift the boat and carry it over the shallow spot. They must have had well protected running gear...

ssobol · 08-13-2018, 02:28 PM

The thing with a displacement hull sinking down (increasing draft) only works in relatively shallow water. It is the venturi effect or Bernoulli's principle. The boat moving displaces the water around the hull. If the bottom is close, the water must move faster through the space between the hull and the bottom. This reduces the water pressure and effectively some buoyancy is lost.

In open water this will not work.

A boat that is truly planing will have less draft than one that is not (pretty much by definition). A boat halfway up will usually draw more (the way a lot of express cruisers seem to drive their boats).

ProMaritime · 08-13-2018, 03:09 PM

A planning hull will displace considerably less water than a motionless hull in full displacement mode but the aft portion of a hull while planning may be below the full displacement depth as well. If there is a lower leg of an outboard motor or even a prop, shaft or rudder on or aft fo the transom that extends below the bottom of the lowest part of the hull these attachments will determine the true draft for avoiding obstacles. the power required to produce a plane usually draws the aft end of a boat lower in the water that its full displacement draft.

ssobol · 08-13-2018, 03:37 PM

Quote:

Originally Posted by ProMaritime

A planning hull will displace considerably less water than a motionless hull in full displacement mode but the aft portion of a hull while planning may be below the full displacement depth as well. If there is a lower leg of an outboard motor or even a prop, shaft or rudder on or aft fo the transom that extends below the bottom of the lowest part of the hull these attachments will determine the true draft for avoiding obstacles. the power required to produce a plane usually draws the aft end of a boat lower in the water that its full displacement draft.

Yes, the lowest part of my boat is the bottom of the motor skeg. However, my boat draws a lot less water at planing speed than hull speed. The motor rises up with the rest of the boat when it gets on plane.

rgano · 08-13-2018, 05:17 PM

Looking at the photo of my boat on plane, you cannot really tell. Is it really up on step? Well, the bow goes up in the air when powering up to fast cruise, and then it come down some. So maybe no "pancaking" along like a hydroplane, but planing. I'd say the lowest part of this particular boat is lower than when she's flat on the water at hull speed.

psneeld · 08-13-2018, 05:28 PM

From the angle, she could use more power (speed) or needs trim tabs.

She is running like a semi displacement boat in my eye.

Truly on plane for a planing boat is .....after the bow comes down after the initial rise.

AKDoug · 08-13-2018, 05:36 PM

I think the difference is "planing vs semi-planing" and displacement just plain sits deeper the faster the boat goes. My exhaust pulls under at 7 knots and sits above the water at 5.5 knots, no speed would make it plane out...

I could imagine a semi-planing vessel would sit "down at the stern" as the bow lifted. JMO