Draft on plane

I have always been curious... Does a boat on plane draft more or less water?

Less

Yep. Less on plane. But hell to pay if you bump and slow down!!!

Yes, less.
My small boat can run on plane in water that I can't float in if I stop.

How much less?

You just have to run is shallow water and see when you hit bottom...

Yes, but.

A large boat on plane can create a suction in shallow water that will pull the stern deeper than if the boat were in displacement mode. Simply, you can strike bottom with a boat on plane in a depth of water that the boat could transit safely in displacement mode. Have seen it happen with a sportfish in the back channels going to the Hatteras, NC inlet. One minute the boat is on plane, then the stern starts getting sucked down, then the smack as thousands of dollars in running gear damage occurs, engines stop, and the boat glides forward in displacement.

Ted

Tom.B said:

How much less?

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Depends. Inboard, outboard, keel, no keel, boat speed, waves. No way to know for sure how much without some very risky testing.

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

High Wire said:

Depends. Inboard, outboard, keel, no keel, boat speed, waves. No way to know for sure how much without some very risky testing.

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I would agree with that for sure.

Easting said:

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

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Damn, that IS ballsy!!!!!


Cheers, Bill

The bottom is not flat. Have your prop shop and seatow on speed dial.

Here in South Florida we do a lot of skinny water fishing. We typically run on plane to cross shallow water. The water is so shallow that the fish backs and tails come out of the water.

I've run my boat a good bit in the lower (fla) keys. There are areas that I know well, about a foot deeper than my draw. If I am running slow there, boat gets "squatty" and steering gets weird. If I try to power up on plane, boat will suck down and likely hit.

But if I am already up on step (like 16-22kts), I can feel the boat lift, speed pick up a touch and the wake gets flattened out. Feels good but I sure am on my tippy-toes!!

But this is a light planing hull. A big heavy Hatteras that is hardly planed out will most likely suck itself down into a bigger hole than usual and that will not be a good day.

Oc Diver is exactly right. Bigger displacement boats will suck down in shallow water. speed slows, attitude changes and wake increases.

Interesting. Like I said, just curious. We are 42' with a nearly flat bottom at the stern. A lot like a sportfish hull. The props are, by far, the lowest point on the boat and totally exposed. When we are planing with full trimtabs, she does get up on her toes, so to speak, but I do feel like her ass is in a hole. So really, it's impossible to tell. Maybe it's the same. Id sure like to test it some day with some sort of controlled test.

In Vietnam I ran jet drive PBRs that were based on a 31' Uniflight hull. At draft about 24" and on plane 8". I routinely passed over shallows at speed.

Parks has it right, at least when it comes to outboards built to run shallow. Jack plate up, motor trimmed part way up, tabs down, my skiff will run scary shallow on plane. Like 6 inches. Of course the consequences of a mistake there are nothing remotely close to running a big boat shallow.

Though I have spent quite a few hours waiting on the ride to come in.

I honestly don't think the draft is affected that much. Look at the hole that is created when the boat is on plane. Technically the hull is at water line, but the waterline is adjusted at the transome to be lower than the surrounded water surface. The stern squats, even when on plane. Any lift gained looks like it's offset by the squatting of the stern.

Dougcole said:

Parks has it right, at least when it comes to outboards built to run shallow. Jack plate up, motor trimmed part way up, tabs down, my skiff will run scary shallow on plane. Like 6 inches. Of course the consequences of a mistake there are nothing remotely close to running a big boat shallow.

Though I have spent quite a few hours waiting on the ride to come in.

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Once I was fishing with my buddy Darrell down by Turkey Point in Biscayne Bay. Darrell dropped something overboard so he jumped in after it. His cell phone was in his pocket. A while later we got caught on a flat by the falling tide. I started rationing food and water. Went into full survival mode.The wind picked up and it got cold. Darrell and I tried to get out of the wind but no place to hide on this boat. To make it worse we were both soaked from trying to push the boat off the very soft mud bottom. Six hours later we floated enough to push it to deep enough water. That was six miserable hours.

Easting said:

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

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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

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]

it depends on the boat....

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.

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...

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).

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.

ProMaritime said:

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.

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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.

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.

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.