Why do boats squat?

[Polaris4937]

While power boats are "level" in the water when not moving, why does the stern sink - and the bow rise - when underway? Planning hulls skim over the water and displacement hulls push through the water. Semi-displacement hulls seem to have the water hitting the hull about half way back. Is it caused by something happening with the bow or by something happening at the stern?

Usually the angle of the drive shaft is downward so the prop is actually pushing up. So, again, why does the stern squat? Enquiring minds want to know.

 

[wkearney99]

I'm sure there's more accurate terminology to describe it, but a starting point to think about it is simple lever action. You're forcing the boat forward and the hull shape wants to help angle it upward on top of the water. But that levering action has to deal with the sheer weight of the boat. There's not enough power being applied to get that much weight pivoted up and over onto plane (and keep it there). This is also a point where's the water at the stern is under lower pressure than the bow and this makes the stern squat. Pressure is also how planes fly using 'lift'.

STFW would likely enlighten further.

 

[SteveK]

WAG. The hull of the boat below water level has displaced a lot of water. The faster the boat moves causes the bow into a wall of water so it rises while at the stern drops as more water displaced has not had the time to fill in the stern hole.

 

[Bacchus]

Just an educated guess...
At slower speeds the boat still displaces the same volume but as others mentioned the bow rises so the stern has to sink to maintain the same displacement.

 

[mvweebles]

Boats squat when they are transitioning from displacement to planing, meaning the hull is supported by water plus horsepower. Usually the bow is lighter than the stern so lifts first giving a squat appearance. Large motoryachts with engines mid-ship appear to squat much less.

I was on a friend's AB 15-foot with center console last week. With the center console far aft and my fat arse even further aft, the bow could shoot the sun out of the sky until she got on-step.

My 30k lb displacement Willard with full keel and round bottom will not plane with a pair of Pratt & Whitney's on her. The only way she'd exceed 8.5 knots is if the slings on a gantry crane parted while unloading her as deck freight.

Peter

 

[Jeff F]

As you speed up a trough is created behind the bow wave. At hull speed the trough is under the stern.

The stern doesn't sit lower in the water, rather the water at the stern is lower than the water at the bow.

 

[Benthic2]

Because you are applying a force below the center of gravity. If you accelerated with only your port engine, your boat would turn to starboard because you are applying an off center force. Its the same thing in the vertical plane. I have never been on one, but I bet those swamp boats with big airplane propellers have some "dive" to them when they start off for the same reason. Application of off-center force, in this case, above the center of gravity.

 

[RT Firefly]

Greetings,
Welcome aboard.

 

[fgarriso]

Welcome Aboard !

 

[Comodave]

Welcome aboard.

 

[Shrew]

While power boats are "level" in the water when not moving, why does the stern sink - and the bow rise - when underway? Planning hulls skim over the water and displacement hulls push through the water. Semi-displacement hulls seem to have the water hitting the hull about half way back. Is it caused by something happening with the bow or by something happening at the stern?

All boats do this when they begin to exceed hull speed. Displacement boats don't typically exceed hull speed. Planing boats climb out of the hole and get on plane. semi displacement boats exceed hull speed, but never crawl out of the hole and get on plane.

It's not that the stern digs in. It's that the bow rises and the stern has nowhere left to go but down.

 

[Nomad Willy]

Yes you’re right.
There’s not much mystery about why bows rise. Push something up against a Hill and it will tend to rise up and over.

But the stern is different .. but not exactly a mystery.
Water strongly tends to follow a curved surface. That’s the essence of what’s going on. And water is heavy. So when water flows aft under a boat hull that has rocker (curved) it tends to be pulled up by the advancing curved hull bottom at the stern. Water also tends to continue to go where it has started to move in some direction. So when water is following the curved surface of the hull bottom it’s tendency is to adhere itself to the bottom. Then a great force develops changing the direction of water flow. The hull is trying to pull the water up and for every action there is an opposite and equal reaction …. the stern gets pulled down. The forces are great so the stern gets pulled way down and the bow is still trying to climb over the bow wave it created trying to get up and over the water ahead ….. serious SQUAT.

Hence the stern gets pulled down and the bow gets pushed up .. SQUAT.

But w/o the curved after end of the hull bottom (planing hull) the stern does not get pulled down. The bow continues to climb up increasing the angle of attack of the hull and when the angle of attack of the hull is sufficient the boat will rise until most of the hull is out of the water and planing takes place. Now the speed of the hull is limited mostly by the friction .. hull to water below. The boat squats excessively only at slow speeds (the “hump”) and after that the hull is free to go faster depending on her fwd thrust.
The angle of attack of the straight hull bottom must be maintained to continue to push the hull up (mostly above) above the water’s natural surface level. The equal and opposite reaction now is manifested by the following hump of rising water aft of the boat. How far aft depending on the boats speed.

 

[wkearney99]

As you speed up a trough is created behind the bow wave. At hull speed the trough is under the stern.

The stern doesn't sit lower in the water, rather the water at the stern is lower than the water at the bow.

Agreed. Though this means the stern now sits lower than it would at rest/displacement speed. The water's been pushed away and that means the stern now rests at a lower point than before. Which is why it's best not to go through shallow areas on plane.

It'd be interesting to know just how much clearance above the bottom is lost in this manner. I suppose depth sensor placement comes into play too. Mine's probably farther forward than ideal for knowing the 'true' amount of water under the props when up on plane. Mine draws 4' and I make it a rule to avoid going through anywhere charted less than 7' when up on plane.

 

[rslifkin]

Agreed. Though this means the stern now sits lower than it would at rest/displacement speed. The water's been pushed away and that means the stern now rests at a lower point than before. Which is why it's best not to go through shallow areas on plane.

It'd be interesting to know just how much clearance above the bottom is lost in this manner. I suppose depth sensor placement comes into play too. Mine's probably farther forward than ideal for knowing the 'true' amount of water under the props when up on plane. Mine draws 4' and I make it a rule to avoid going through anywhere charted less than 7' when up on plane.

If you're actually up on plane, your draft should be less than at rest, not more. It's in the intermediate stage where you're moving close to (a bit above or below) hull speed where you're likely to gain a little draft from squatting. At the same time, running through questionably shallow areas on plane isn't a good idea anyway, as you don't really want to hit something at 20 kts.

 

[Hippocampus]

All boats do at any speed below full planing . Sailboats, power boats and even row boats. At extremely low speeds it may not be noticeable but at higher speeds it is.

Physics are simple up to 1.34 x sq of dynamic lwl (lwl changes on most hulls as speed increases) there’s a wave formed along side and under the boat. For most hulls it’s roughly 1 1/2 x the lwl. So at any speed below planing the hull has to climb that bow wave or slice through it. Very narrow long boats with very acute half angles mostly slice through it particularly if they have wave piercing bows. So boats attempt to change the dynamics of the water passing around the bow using an attached bulb. Some by using reverse bows. But even in those cases there’s some rise to the bow.

Boats are a rigid structure. So when there’s a force raising the bow there’s a force pushing the stern down. Boats handle this differently but basically the more reserve buoyancy and the farther aft it is the more force there is to resist that downward force on the stern.

Another thing to think about is total displacement and where that weight is as this will affect localized reserve buoyancy. A boat with little weight in the ends (particularly aft) will have more reserve buoyancy at its ends. In other words gyradius affects this behavior. A lighter boat with good gyradius will swat less. The opposite is true which it why outboards are in just the wrong spot until you’re on plane.

Air weighs less than water. Any upwards force will decrease swatting. Air also produces less friction than water. So various designs try to get air under the boat or try to increase lift aft. All of these techniques are mostly aimed at when the hull is on plane but to varying degree also help at the transition from displacement to plane. They tend to be more effective with light boats as they don’t develop as much force as the effects from displacement (weight).

So sailboats or motor boats with “balanced” hulls swat a lot. Particularly sailboats under power. Under sail sailboats are sucked forward by the negative pressure in front of the sails not pushed from behind by an engine. The exception is DDW. With a kite up some sailboats swat. Boat with wide sterns and not much weight back there don’t swat as bad. Ultralight sailboats and LDL power boats don’t as well. It’s rare the amount of swat is dangerous and designer may have allowed for a certain amount of swat as a trade off to improve other behavior.

BTW on full plane draft both fore and aft is lower for the canoe hull. The at risk appendage is the prop(s) and/or running gear. But even that is less in the water. Boat should be level at plane although angle can be adjusted with trim tabs/flaps/interceptors or flaps/tunnels on the outboard itself. Generally speaking on plane you do want the bow up a bit when going into the waves. Helps from burying the bow unless your speed and the period of the waves allows you to skip on top of them.

 

[Judy at JWY]

Great responses chock-full of good information. However, let's make sure that the OP is referring to the "normal" state of squatting. Unexpected or unusual squatting can occur in shallow water - I believe this is called the venturi effect, which I believe is still considered "normal." However, I know of a full displacement boat that squats in many different situations and I have considered it to be a flaw, either in manufacturer or more likely in design. I disclosed this when I sold the boat. Not meaning to be a pessimist here, but just making sure the OP is talking about the same physics and "normal" situations as the responses above.

 

[CaptTom]

...running through questionably shallow areas on plane isn't a good idea anyway...

Ahh, reminds me of my misspent youth. Of course, you have to know where all the rocks are if you're gonna try this.

FWIW, I first learned the definition of "squat" to be what happens when a large hull moves at speed through shallow water. I guess you could relate it to the venturi effect. The way I understood it was the hull pushes water away in all directions, but if there's not much room to move down, then it'll get squeezed out and basically leave less water under the hull. I remember reading (and apparently, not fully retaining) the technical details when it was first discovered. I think it happened to some well-known ship like the QEII or something and at first they couldn't figure out why she'd touch bottom when there was technically enough water for her draft.

 

[wkearney99]

Great responses chock-full of good information. However, let's make sure that the OP is referring to the "normal" state of squatting. Unexpected or unusual squatting can occur in shallow water - I believe this is called the venturi effect, which I believe is still considered "normal." However, I know of a full displacement boat that squats in many different situations and I have considered it to be a flaw, either in manufacturer or more likely in design. I disclosed this when I sold the boat. Not meaning to be a pessimist here, but just making sure the OP is talking about the same physics and "normal" situations as the responses above.

There's also the suction and bank effects of passing too close to the other vessels or a bank/shore wall. Water flow being constricted often has effects most folks wouldn't consider without knowing of them ahead of time.

Closest I've ever come to unexpected water effects is the propwash from a water taxi using forward thrust to maintain contact with sea wall. Getting pushed aside, when passing astern across that wash, was a sudden surprise. (and, yeah, afterward it was like, duh, of course that would happen). Likewise left-over whorls of current/propwash from ferries on the East River. Nothing like what's discussed here though, just a reminded that the water isn't just a 'still' thing like a road.

 

[wkearney99]

stfw brings up a 1911 article from the Navy on these sorts of things: https://www.usni.org/magazines/proc...action-between-passing-vessels-called-suction

 

[PhilPB]

Boats squat when they are transitioning from displacement to planing, meaning the hull is supported by water plus horsepower. Usually the bow is lighter than the stern so lifts first giving a squat appearance. Large motoryachts with engines mid-ship appear to squat much less.

I was on a friend's AB 15-foot with center console last week. With the center console far aft and my fat arse even further aft, the bow could shoot the sun out of the sky until she got on-step.

My 30k lb displacement Willard with full keel and round bottom will not plane with a pair of Pratt & Whitney's on her. The only way she'd exceed 8.5 knots is if the slings on a gantry crane parted while unloading her as deck freight.

Peter

Just one PW J58 would get her flying. That is the absolutely loudest engine I've heard, except maybe an RL10

 

[wkearney99]

I was on a friend's AB 15-foot with center console last week. With the center console far aft and my fat arse even further aft, the bow could shoot the sun out of the sky until she got on-step.

heh, our dinghy is like that if you don't apply FULL POWAH when you want to get up on plane. Applying power gradually just points the bow to the sky and digs in the aft ends of the sponsons... until it suddenly falls forward... and at that moment is VERY twitchy to control. But GUN the engine and it'll rise a bit... and then settle right on plane with no trouble at all.

 

[Benthic2]

Keeping in mind that there are no stupid questions.... I have a question. Considering the fact that lift is created because the air going over a plane's wing is thinner (because it covers more distance ) than the air under the wing.... is there any of that at play here ? I know a liquid can't be compressed, but can it be decompressed ? Is it possible that water goes under the hull at miship and then spreads out as it passes under the stern providing less bouyancy ?

 

[Hippocampus]

Great article but unfortunately couldn’t find the diagrams.
Boats displace water. After the boat passes by the water rushes in to fill that hole. When on passage we wanted miles between us when passing in front of a ship. But also miles behind us when passing behind. With ships now being >1000’ it isn’t the wake that’s most troubling as you can usually see it and know to stay away and or line up your little thing to let it pass by.
Same thing happens in shallow and narrow bodies of water but here you get the additional effect of accelerated flow leading to a Venturi effect and Bernoulli. Water isn’t compressible so to get the same volume of water through a limited area it must move faster creates these local effects. Several time the author mentions Hell Gate. Have learned to look at local traffic on AIS as we approach. Look at the L.I. Sound side and the East River side. Not much goes through Harlem River. Try to pick a spot where we won’t be close to anything big during transit. Much to the brides consternation will go to WOT or slow idle to avoid any spot two big tows or ships are crossing. I tell her her why the sudden change. She says “oh” and then “carry on “ with a smile.

 

[Hippocampus]

B unlike sail where the boat is sucked along by the negative pressure in front of the sails boats under power are pushed along by the positive pressure on the backside of the prop(s). That means the speed of the water coming off the prop is faster than the speed of the water going around the boats hull elsewhere. So yes you are on to something. That does produce a force that sucks the stern down.

 

[Insequent]

Yes you’re right.
There’s not much mystery about why bows rise. Push something up against a Hill and it will tend to rise up and over.

But the stern is different .. but not exactly a mystery.
Water strongly tends to follow a curved surface. That’s the essence of what’s going on. And water is heavy. So when water flows aft under a boat hull that has rocker (curved) it tends to be pulled up by the advancing curved hull bottom at the stern. Water also tends to continue to go where it has started to move in some direction. So when water is following the curved surface of the hull bottom it’s tendency is to adhere itself to the bottom. Then a great force develops changing the direction of water flow. The hull is trying to pull the water up and for every action there is an opposite and equal reaction …. the stern gets pulled down. The forces are great so the stern gets pulled way down and the bow is still trying to climb over the bow wave it created trying to get up and over the water ahead ….. serious SQUAT.

Hence the stern gets pulled down and the bow gets pushed up .. SQUAT.

But w/o the curved after end of the hull bottom (planing hull) the stern does not get pulled down. The bow continues to climb up increasing the angle of attack of the hull and when the angle of attack of the hull is sufficient the boat will rise until most of the hull is out of the water and planing takes place. Now the speed of the hull is limited mostly by the friction .. hull to water below. The boat squats excessively only at slow speeds (the “hump”) and after that the hull is free to go faster depending on her fwd thrust.
The angle of attack of the straight hull bottom must be maintained to continue to push the hull up (mostly above) above the water’s natural surface level. The equal and opposite reaction now is manifested by the following hump of rising water aft of the boat. How far aft depending on the boats speed.

Eric, I'm disappointed! Here was your opportunity to explicitly espouse the consequences of Quarter Beam Buttock Angle!

After all, that angle is a key parameter in determining whether a hull can plane, or not.

 

[markpierce]

If a displacement boat, stay below hull speed.

 

[BruceK]

All getting very complicated. Maybe they squat for the same reason dogs squat.

 

[mvweebles]

heh, our dinghy is like that if you don't apply FULL POWAH when you want to get up on plane. Applying power gradually just points the bow to the sky and digs in the aft ends of the sponsons... until it suddenly falls forward... and at that moment is VERY twitchy to control. But GUN the engine and it'll rise a bit... and then settle right on plane with no trouble at all.

Problem is no wake zones. I know, I know, no-wake means no wake. But on the ICW, mostly means 6-8 kts is acceptable. At that speed, the bow is very high. Better with a passenger you can perch forward. No wake zones often stretch of a couple miles (as an aside, part of my reasoning that a displacement trawler makes sense in go-fast Florida).

I just don't understand the attraction of center console dinghies. They are expensive, heavy, fragile, cluttered, and the weight distribution sucks. Lot of compromise just to sit like you're driving a car.

Peter

 

[Hippocampus]

Peter
Have a Rigid dinghy. Although it kind of looks like a RIB it’s all fiberglass so very heavy. It has 40hp (don’t see tillers on that range of HP too much) on the back and a steering wheel.

Advantages are its truly a daily driver. Due to weight even on plane will handle chop and modest waves with ease. So doing a 20 or even 30m round trip isn’t frowned upon. I fish more from the dinghy than the boat. The dinghy has a bunch of rod holders installed. I can reach two without letting go of the wheel. With a tiller I never figured out how to troll. With the wheel I can steer with my knees or an elbow and have both hands for a rod. With a tiller I’d have to be twisted and have one hand on the tiller. With the wheel I’m sitting comfortably and can look around easily to both sides. My back isn’t turned always to one side creating a blind spot which takes effort to reposition myself to survey. No after decades of tillers on outboards will put up with the lost space from a wheel. Wheels are safer, more comfortable and allow multitasking.

Never understood some low speed zones and no wake zones. Just like many SD hulls I produce much more wake at some speeds than others. I can speed up and produce less wake but be above the speed limit or slow down and produce a large wake. Or slow way down and crawl along then rock around with every pass if the SeaKeeper isn’t on. As regards the AICW I produce less wake at 10kts than 7-8. Been through stretches with no one around and no marinas nor little boats for hours. Don’t want to disturb the shore line. Don’t understand why I shouldn’t be able to pick a speed (either well below hull speed or well above) that produces the least wake and lets me make my bridges. Think it’s the speeds around hull speed that give the most wake but even in that short range a change of just a 1/10 of a knot may markly decrease wake.

 

[mvweebles]

Hippocampus - sounds like a decent sized dink. There is a point where center-console steering makes sense. But on smaller dinks, say under 420, perhaps 400 size range (13-feet or so), there are a lot of compromises. RIBs don't have a ton of interior space to begin with. My first boat - a Uniflite 42 - came with a CC RIB and I had a blast with it. But for all intents, they are not beachable in the way a lightweigh RIB with 15hp tiller-steer is. To each their own.

I can tell you from firsthand experience that a 21-foot CC fishing boat doing 40-kts does not throw much of a wake. Especially the skinny-water sleds with low deadrise that seem capable of running on wet grass. But the realities of the ICW means politics. Turns out million dollar homeowners don't want wakes. Recognizing they are unlikely to garner sympathy on their own, they gladly serve-up the lumbering manatee as their proxy. So it's not just a matter of wakes, it's the prop-scarred back of the oddly loveable manatee (BTW - one of their distinctions is they have the lowest brain to overall size ratio in the mammal world).

Apologies for the thread drift. CC Dink vs Tiller-steer dink would be an interesting thread in itself.

Peter