Why do boats squat?

[rslifkin]

For the dingy bow rise / squat issue, trim tabs on mine made a huge difference. It planes at lower speeds and with a heavier load, and the bow doesn't come up nearly as high before getting on plane (and the wake never gets as big through the plowing range either).

I think a lot of dinghies suffer from too much weight aft, especially when operated with 1 person (and having the outboard hanging behind the end of the bottom surface). So adding some more running surface back there can help significantly.

 

[SIBERNUT]

The often overlooked simple answer is that many of the faster boat owners are full of it.

 

[Nomad Willy]

Brian (insequence) wrote;
“ 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.”

Brian many times l’ve not used the QBBL to explain things. Too many go crazy, make SA remarks and otherwise throw me over the rail. The’d rather make jokes of it.
I finally decided no one wants to talk about something they know nothing about.


But your’e right. QBBL plays a big role in the squat. This time I needed the QBBL but I used it as a (curve in the stern hull bottom) that pulls water up and the energy used to raise the water is big drag .. and pulls the stern down.

Somebody mentioned the Dole Finn. They cause a lot of drag and if you really need them you’ll need an angle of attack to get the job done. That (just like on an airplane) makes even more drag. And you have little chance of going faster.
Unless the DF is quite small but it may not lift the stern then.

Thanks for responding.

 

[Hippocampus]

QBBL is definitely a way to think about things. But then again you have the deep Vee which seems to have done well to present. The derivatives of that 70 year old C.Raymond Hunt hull are still in very active production and when done in Al to keep weight down the preferred hull for many law enforcement, SAR, pilot and supply hulls. Very fast for HP and surprisingly efficient with excellent seakeeping properties. Seems the hull of choice in the 50 to 80’ range for that purpose.
Hunt and his family were members of Duxbury Yacht club for many years where I lived for decades. Gladdings Kearns which use the Hunt hull in their Al boats is the major producer for those types of boats in this country. They are on the south coast of Massachusetts. All local history so warms my heart. Wonderful there’s still boat building in the USA.
Dead rise is very high but they plain just fine. For us recreational boaters the downfall is the motion at rest. Spend some time trying to bottom fish in a deep Vee center console with even very modest chop and you’ll see what I mean. Even at trolling speeds it can be uncomfortable . But at speed they’re rock solid.

 

[SteveK]

WE need a list to explain what initials stand for.

OK I'll ask, what is QBB?

 

[Nomad Willy]

For the dingy bow rise / squat issue, trim tabs on mine made a huge difference. It planes at lower speeds and with a heavier load, and the bow doesn't come up nearly as high before getting on plane (and the wake never gets as big through the plowing range either).

I think a lot of dinghies suffer from too much weight aft, especially when operated with 1 person (and having the outboard hanging behind the end of the bottom surface). So adding some more running surface back there can help significantly.

Also a huge part of the problem is aspect ratio. Dinghies are made short for obvious reasons and people don’t like “tippy” so you get a short/wide dinghy. Short/wide = maximum squat.

 

[rslifkin]

Also a huge part of the problem is aspect ratio. Dinghies are made short for obvious reasons and people don’t like “tippy” so you get a short/wide dinghy. Short/wide = maximum squat.

Yep, short fat hulls struggle through the drag rise and plowing range more. They're not as bad a compromise once you're up on plane and moving fast enough to get it up high enough in the water though.

 

[Nomad Willy]

WE need a list to explain what initials stand for.

OK I'll ask, what is QBB?

OK,
“Quarter beam buttock line”

It refers to a longitudinal line at the quarter beam point. A line (fore and aft) half way between keel and chine. It’s kind of an average of the fore and aft curve of the hull bottom. A very important aspect of the curve on the hull lines of the hull bottom is the degree of curve aft .. or well aft. If we were talking about all the fore and aft bottom lines it would be “hull bottom lines”. But taken at the quarter beam it’s a bit of an average of the hull lines (keel to chine) on the bottom. If it referred to all the bottom lines aft it would be QBL If it was re a gentle curve of the hull bottom bow to stern it would be often called “rocker”. Many small boats have a gentle bottom curve over the entire length of the hull bottom. Given enough rocker the resulting hull becomes full displacement .. less would be SD and flat or straight would be planing.

So the QBBL tells mostly if the hull is planing, SD pr FD. If the curve is aft and comes up to the WL (at the transom) it’s a FD. The old GB’s have nearly straight bottom lines aft. The CHB has bottom lines have a bit of curve aft. Given enough power the GB’s will plane. The CHB’s won’t.
So the QBBL (often or usually expressed as an angle) tells a great deal about the drag characteristics of our kind if boats.

 

[rslifkin]

You can also get a case where the bottom angles up a little bit aft, but it's a straight run. My own boat has a bit of angle there. The angle leads to less submerged transom and less drag at displacement speeds, but it also means the boat will run with a higher deck angle on plane.

 

[Insequent]

You can also get a case where the bottom angles up a little bit aft, but it's a straight run. My own boat has a bit of angle there. The angle leads to less submerged transom and less drag at displacement speeds, but it also means the boat will run with a higher deck angle on plane.

And squat.

 

[rslifkin]

And squat.

Not necessarily a significant amount. I've never noticed a perceptible amount of squat from our hull, but it doesn't need a ton of speed to plane, so it starts to build noticeable lift at fairly low speeds. If I'm moving along at about a knot under hull speed and pull the throttles back to idle suddenly, the bow drops a bit, but the stern doesn't come up. We don't have a lot of angle up towards the transom. It's slight, but definitely there.

 

[Nomad Willy]

A boat “needs a bit of squat” to get and/or stay up on top far enough for decent efficiency. Like an airplane needs angle of attack to lift the aircraft off the ground.

 

[rslifkin]

A boat “needs a bit of squat” to get and/or stay up on top far enough for decent efficiency. Like an airplane needs angle of attack to lift the aircraft off the ground.

You need some angle of attack, but depending on how the bow sections build lift, you don't necessarily need the stern to squat to get that angle. It can come from either the stern squatting down or the bow lifting up. My boat does the latter, as the bow sections will build lift with speed even at the at-rest angle. Then once the bow comes up the stern and trim tabs can build enough lift for the boat to come up out of the hole and settle into plane.

 

[Hippocampus]

A boat “needs a bit of squat” to get and/or stay up on top far enough for decent efficiency. Like an airplane needs angle of attack to lift the aircraft off the ground.

Multiple situations where this doesn’t hold. Most obvious is foiling boats. Then there are any boat with trim tabs. Or jets. Or even old school hydroplanes. Even tunnel drives on ultralights will not swat to any appreciable degree. As soon as the prop wash is angled to slightest degree down it lifts the stern. If either there’s sufficient reserve buoyancy aft or the prop wash is at a downward angle and the boat light enough or the boat rises out of the water as power is applied swat is negligible. End of day swat occurs when the bow rises on the bow wave so there’s a downward force on the stern. If either that doesn’t happen or there’s strong resistance to the stern depressing it doesn’t happen. Have you ever noticed when you go from displacement speed to planing speeds surfing down a wave front there’s no swat. Any bow wave you make is falling and your boat is angled bow down. Only when you apply huge power to go faster than the overcoming wave is falling will you get any appreciable amount of swat. Scary as you get more green water. You want swat when surfing. Same kind of thing as loss of any rudder control if you’re moving as fast as the wave when surfing. Scary.

 

[psneeld]

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.

Watched a couple airboats this weekend on the St Johns River...each one seemed to start bow rising like a normal boat.... they are usually sled like and don't accelerate too fast so the curving up hull in the bow probably is more force than the high up push from the prop.

Just like inboard boars with downward angles prop shafts still usually rise in the bow when starting.

 

[Hippocampus]

PS makes the point initial swat in large measure depends to the extent the boat floats on the water or in it. In other words displacement. All boats weigh something so some of the boat is displacing water when still. Even before the boat develops a bow wave that portion is in the water when moving. For ultralights it’s a small distance and forces involved are small. Hull geometry, bow configuration and other design techniques can minimize this swat.

PS is correct. He saw these light fan propelled boats rise bow first as they pushed the water they displaced at rest out of the way. Given these boats want to skim over wet lands they have flat bottoms with significant rocker near the bow to intentionally trap water under them and keep a thin layer of water under them even before plane as well as at plane when going over very skinny water or short stretches of no water. Still at rest airboats, although very light, have a fair amount of weight aft. Immersed hull at rest is usually a simple rectangle. So they’re a bit stern heavy at rest. As they start to move bow comes up in response to this tilt. Once moving water flow under the hull pushes the stern up.

This is a very different situation than a boat in deep water using a wave piercing bow or a bow with an acute half angle or bulb where the goal is to manipulate the hydrodynamics of how the bow wave is developed or passes by the hull. Question is whether the vessel will continue to swat at non planing speeds. Sailboats and other “balanced “ hulls do when under power typically. To the extent the designer and owner have kept weight out of the ends and preserved reserve buoyancy it will be minimized.

Angled prop shafts waste energy. Some of the shaft HP is an upward vector and some a useful forward vector. The vectors isn’t applied at the prop but rather where the shaft forces are coupled to the hull. The location of the upward force varies with design but commonly isn’t all the way aft.

 

[psneeld]

Many boats have flat surfaces underneath called lifting strakes...even many deep-vees....they do offer additional advantages....but lift is definitely an important one.

The more flat area the faster the boat planes, stays up and is more efficient fuel wise....at least in my understandings of boats.

 

[dutch-barge]

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.

And old trick barge captains used. the pilot house is in the back of the barge. going under a bridge, with just not enough clearance.....go slowly to the bridge and when the pilot house is close by, FULL throttle, the pilot house will sink a few inches, enough top clear the bridge.

 

[psneeld]

I saw something in tbe news last year where a large cruise ship did the same to clear a bridge.

 

[HopCar]

Boats squat so the poop deck doesn’t run down their keel.
Someone had to say it.

 

[SteveK]

Boats squat so the poop deck doesn’t run down their keel.
Someone had to say it.

Best one yet, and your avatar is showing it.

 

[Lepke]

Powerful boats and ships squat partially because of the suction of the props. In shallow water the squat can greatly increase at speed. Destroyers traveling up a fast river may have to make turns for 25 knots to make 10 against the current. The stern can draw a couple more feet when over a shallow area. They teach it in navy school.

 

[Hippocampus]

Powerful boats and ships squat partially because of the suction of the props. In shallow water the squat can greatly increase at speed. Destroyers traveling up a fast river may have to make turns for 25 knots to make 10 against the current. The stern can draw a couple more feet when over a shallow area. They teach it in navy school.

Yup. Yet another reason for swatting not previously mentioned. Good on ya. Like with prop walk varies within proximity to canoe body and geometry of the stern and running gear.

 

[greatwhite]

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.

You would think this is true but certainly not always. I fish a 38 foot Henriques sportfishermen and at a cruise speed of 26 knots the cockpit floor is at least 5 feet below the surface of the sea. The bow rides high even on plane which is what makes it one of the best 38 foot sport boats in a head sea. It is definately drawing alot more while on plane than at rest.

 

[psneeld]

You would think this is true but certainly not always. I fish a 38 foot Henriques sportfishermen and at a cruise speed of 26 knots the cockpit floor is at least 5 feet below the surface of the sea. The bow rides high even on plane which is what makes it one of the best 38 foot sport boats in a head sea. It is definately drawing alot more while on plane than at rest.

Are you looking at the water surface behind the stern for that 5 foot comparison?

 

[Hippocampus]

Know that Henrique well. Friend had one and I’d crew occasionally. If I recall she was a heavy deep Vee for the forward half of the hull and still a significantly angle at the stern. Strong .powerful and dry boat. A pleasure to take out to the canyons. But as you say was inclined a bit , maybe 10 to 15 degree, at speed. You could use the tabs to flatten out. Think strong heavy sportfish behave differently than lighter craft. Was always shocked at his gph.