Effect of propeller shaft angle

[chuck]

During WWII, the propeller shafts on many destroyers seem to angle downward more steeply than is required for the propeller blades to clear bottom of the hull. While the shafts on larger ships seem to be almost perfectly horizontal. Is there any reason why it is desirable to have destroyer propellers angle downwards?

[Werner]

Interesting observation when you consider 2100 ton USN destroyers seemed to "dig in" aft when at speed. Some strange combination of dynamics, or unrevealed fault?

[haloeight]

Perhaps the "pushing up" effect that the high angle shaft created assisted the ship in going over waves rather than through them.

For a capital ship im sure the preference would have been to try to remain as a stable gun platform at all speeds.

Just a theory.

[Werner]

Except at speed, and especially when under acceleration, the Fletchers and later ships tended to push their sterns down, often to the point of being awash

[Sean Hert]

Perhaps the pitch/rotation of the blades overcomes the forward force?

I can tell you, with my R/C ships; which can have some pretty serious angles on the shafts due to motor placement, that I have dragged the stern of my 1/144 Mogami underwater while in reverse.

[Laurent]

Hi Guys ,

the main problem they had in that era was that the evolution of ships was based on trials & errors . If those destroyers had such propellers , it was to attain a designed speed , but to attain that same speed , the hull had to be long , very thin with almost no draft . As the boiler rooms were most of the time in the middle of the ship , the turbines had to be set in the after third of the hull , this is why the propeller shafts go at a great angle below . It was not to raise the stern at speed .

Later on , I have in mind the french "Mogador" class in mind , they put from bow to stern : one boiler room , a turbine room , another boiler room and then a second turbine room , to avoid a immobilisation of the ship when torpedoed in a boiler or turbine room . When you look at the propeller angles of the Mogador shafts , the angles differ , there is a very long shaft , going almost to the middle of the ship , the other shaft went a at very sharp angle into the hull . I even think some US DD's got the same feature ...

There again , it was a matter of design . there were compromises that had to be taken , they couldn't take another way at that time ...

Regards ,

Laurent

[Andy G]

The amount of pure forward thrust varies with the cosine of the angle that the shaft makes with the horizontal.

Given how slowly the cosine curve decays, you can be nine degrees out before you lose 1% of the thrust. (Nine degrees is a very droopy-looking shaft angle).

I suspect the angle depends mostly on getting the machinery in - harder in a destroyer hull than a fat-beamed ship.

Andy

[gerinc]

I don't know why the propeller shafts were angled downwards on destroyers, but the reason that ships' sterns settle in the water when moving forward at high speeds is a phenomenon called 'squat.'

As a ship moves, water is displaced around the vessel, mostly pushed to the sides, but some goes under the keel. This flowing water underneath creates an area of low pressure and causes the ship's stern to settle. It happens any time the ship is moving, but it's a navigational consideration and potential hazard when in shallow waters, as it will reduce your under keel clearance.

It's proportional to the speed of the vessel, so the faster a ship is moving, the more pronounced the squat. It is calculated by trials and a table is carried in the wheelhouse and on a ship's pilot card so that the officer on watch will know about how much he can expect his stern to settle at a given speed.

[Werner]

If that's the case, then it must be extremely sensitive to design, what type of stern (cruiser, transom, etc.) and the amount and location of deadwood.

[Walt]

I don't know why the propeller shafts were angled downwards on destroyers, but the reason that ships' sterns settle in the water when moving forward at high speeds is a phenomenon called 'squat.'

As a ship moves, water is displaced around the vessel, mostly pushed to the sides, but some goes under the keel. This flowing water underneath creates an area of low pressure and causes the ship's stern to settle. It happens any time the ship is moving, but it's a navigational consideration and potential hazard when in shallow waters, as it will reduce your under keel clearance.

It's proportional to the speed of the vessel, so the faster a ship is moving, the more pronounced the squat. It is calculated by trials and a table is carried in the wheelhouse and on a ship's pilot card so that the officer on watch will know about how much he can expect his stern to settle at a given speed.

This plus the fact most large ships have displacement hulls, The Trimming is aquired by the location of the screws relivent to the hulls displacement. This is why huge warships screws are under the hulls in a more or less straight line to acquire the most torque trust from the ships powersourse. The ship's weight beam and length would have a different effect on the screws because speed vs displacement is much lower hence the screws are more effective under the hull in a thrust line. Triming of the ship could be aquired on some warships by ballast transfer from stern to bow and visa versa.. This is the same idea used an Ice breakers trim proceedures. Also bow bulbs will effect the ships trim especially at speed almost all modern hi speed ships have bow bulbs.
Most destroyer hulls are semi displacement or semi planing hulls depending on vessel. The screws must be deeper in the water set off from the hull so as not to cavitate or "free wheel" under high speeds when the "squat is so pronounced that a trench in the water at the fantail could uncover the screws if they were closer to the surface.. This is why modern subs with the cigar hulls have limited surface speeds.. The trench made by the bulbous hull causes a huge trench as the speeds increase.. I have witnessed surface cavitation or Broaching the screw on a few occaisions. Also the fact that narrow long hulled vessels like destroyers and light crusiers that don't displace that much water have a tendacy to porpose over waves rather than to plow through them like bigger ships do.. The porpose effect would bare the screws on occaision especially in high state seas...

[chuck]

I don't think any destroyer, certainly any WWII destroyer, has a semi-planning hull. All destroyers I know of have displacement hulls.

[gerinc]

I agree with chuck here. Warships the size of destroyers are displacement hulls.

Werner: Squat is common to all stern types and probably will vary with type. I wouldn't say it's "extremely sensitive," though. I personally have experienced squat on a ship with a cruiser stern (T/S Empire State VI, a modified Cape J class steam ship), a transom stern (M/T Overseas Puget Sound, product tanker), and even a 19th Century baltimore clipper with a counter stern (S/V Amistad). The extent of the squat is simply a matter of vessel speed. I think that the fletchers being so low to water when accelerating had more to do with their low freeboard than the type of stern. Also, the only opposing force to squat is the buoyancy of the hull, so perhaps since a fletcher has very little volume beneath the waterline at her stern, she will settle a bit lower than a ship with deadwood farther aft, or a hull which is more full in shape. How much a ship squats at a given speed is unique to the vessel, but the faster she goes, the more she will settle.

[Werner]

I agree with chuck here. Warships the size of destroyers are displacement hulls.

Werner: Squat is common to all stern types and probably will vary with type.

Do aids like the Burke stern flap change this characteristic? What about the flow around the transom? Is there any way to "trick" the flow into reversing the tendency to squat?

[Walt]

I don't think any destroyer, certainly any WWII destroyer, has a semi-planning hull. All destroyers I know of have displacement hulls.

True about most WW2 destroyers especially the flush decks but many modern designs are quasi semi planing hulls as made by the bulbous bows at keel level deep V bows and a tapered keel line as well as the hull strakes set off to beam ends on the keels.. I was not comparing planing hulls to the hulls of power boats or other traditional planing hull designs like those of PT Boats, what I was refering to was the many attempts by design to get the hulls higher up in the water while at speed by inducing a planing effect from bow bulbs and hull strakes as well as screw placement relivent to the keel, thus reducing drag and increasing effecency and speed if not ride. on larger displacement hulls like BBs and Carriers who have so much more drag to deal with the problem is addressed by bow shear and beam width.. as well as horsepower. In other words plowing less water to get to and maintain speed easier. Drag like on airplanes and autos is a huge factor on a ships effeciency. A problem ship designers have struggled with since oar boats. I have always thought it strange how rudder designs lagged so far behind on early ship builder's design boards. Another interesting screw placement vs hull design is with ths German "S" boats, Here you have lower reving diesel motors pushing 3 very radically pitched and broad blade almost tug boat style torque screws pushing a true semi planeing hull with good results. Of course their narrow beams and long hulls aided the effect of reducing drag. But I'll wager they did not ship well in high seas much like their DD cousins.
Geez I thought I had enough problems already finding the correct pitch/Dia. on my sportboat's wheels after I re-engined..thank goodness for computers and smart and experienced yardbirds etc. I can only imagine the problems the engineers of early steelhull fast ships faced with same issues..
The albacore hull is the best design for low drag.. No how can that be applied to a carrier????

Now want a topic to drive us all crazy .. let's try screw pitch/diameters for most effecient operation on warships.. Truely a MIT grads worse nightmare not to mention Submarine screws.. Nuke Boats in particular.. 4 vs 5 vs 7 blades etc.

[Andy G]

Is squat about water pressure under the hull, slurping the aft end down?

I thought it was more due to the fact that at speed the hull is attempting to climb the bow wave. I know from sailing planing hulls it takes some effort (and angle) to transition from mere displacement, up and over the bow wave, to high-speed plane.

Andy

[Lesforan]

This "squat effect" is the very reason you can't use an increase in speed to get a boat over a shallow water area, like a sandbar. As many a recreational boater has found to their sorrow.

[chuck]

I would guess and at least some of the squatting effect has to do with water pressure reduction ahead of the propeller. At high power outputs the propeller would excavated water out from underneath the stern at high rate, and discharge them rearwards to propel the ship. I would guess this would create a substantial dynamic pressure reduction underneath the stern just ahead of the prop, and act to suck the stern downwards. This pressure reduction ahead of the propeller would be matched with a major dynamic pressure increase behind the prop. But on WWII destroyers the props are usually outboard of the portion of the stern behind the prop plane. So there would be very little hull area behind the propeller for the increased pressure to act on, so the pressure increase behind the props would not be able to counter the squatting effect of the pressure reduction ahead of the props. But I will guess the increased dynamic water pressure behind the prop is partially responsible for the "roaster tail" spout of water in the wake of WWII destroyer traveling at high speed.

May be some one with hydrodynamic experience can say whether the above has any basis in fact.

[gerinc]

I agree with chuck here. Warships the size of destroyers are displacement hulls.

Werner: Squat is common to all stern types and probably will vary with type.

Do aids like the Burke stern flap change this characteristic? What about the flow around the transom? Is there any way to "trick" the flow into reversing the tendency to squat?

I don't know the answer to that, but all ships squat, as far as I am aware. I think it's likely that the hull features and shape may be able to influence how much. Squat is usually noticed most in shallow waters because it becomes an issue with clearance.... but it simply has to do with the flow of water beneath the ship, the same as airflow over the top of a wing creates lift. Disrupting the flow in either case causes drag, which is not something you want.

You do see examples of this same thing happen in a lateral way when a ship is in a canal or when two ships pass near one another. In the case of an UNREP, for example, a lower pressure exists between the two ships and they have a tendency to come together, especially at the stern, since the pressure wave from the bows holds the forward ends apart. It's all the same principle of the flow creating the lower pressure, just acting on different dimensions of the ship.

I would guess and at least some of the squatting effect has to do with water pressure reduction ahead of the propeller. At high power outputs the propeller would excavated water out from underneath the stern at high rate, and discharge them rearwards to propel the ship.

The term squat applies only to the increase in draft resulting from the flow of water under the ship from her own motion and it has everything to do with speed. Even sailing vessels will squat.

Someone else mentioned something about squat being the result of a vessel attempting to plane and jump her own bow wave- Displacement hulls do not do this of their own accord. It is very common today for merchant ships to trim by the stern, (have greater draft aft), called "drag," to prevent the bow from digging in and creating more resistance; even ships with bulbous bows do this, as it helps fuel efficiency. But, they are in no way attempting to plane. In order to even consider planing, a vessel must reach her theoretical hull speed and with most large ships this is not possible with their power plant. In any event, it requires the trim of the ship to be adjusted and this is a measurable and (fairly) constant change in the deep draft of the ship. I say fairly because things like fuel and store consumption will necessarily lighten a vessel on a voyage, but these weights are tracked and the true draft of the vessel can be calculated at any point.

[Lesforan]

If I am understanding the Coast Guard's explanation correctly, the "squat" effect is the result of water being pushed away from the sides of the hull as it moves through the water. It is quite independent from propellor effects. If one would observe the form of the bow wave as it passes the sides of the hull, the amount of water displaced increases with the length of the hull. With a displacement-type hull, the size of this bow wave will increase with speed, thus increasing the squat effect. So this will happen with both sail and powered vessels. I would think the only circumstance where this would not occur would be a vessel underway in a current that exceeds the speed of the vessel, in which case the vessel could not make steerage and be uncontrollable.

The situation described about the sterns of vessels running parallel in close proximity is a variation on that is referred to as "bank effect". This is more commonly encountered in narrow channels, where sterns of vessels passing close to banks will be drawn toward them due to low-pressure areas formed between the hull and the sides of the channel. It can be very trickly when meeting another boat in a narrow channel: bank effect may be drawing the sterns of both vessels to the outside, while low pressure caused by the meeting of the hulls in close proximity can draw the vessels together. This, too, is a hydrodynamic effect independent of propellor action.

Hull squat in vessels on plane is not so much a displacement effect as it is a result of the hull "riding up" on the water. Passing over a shallow-water area would still draw the stern down as the relative lack of water to displace would still create a low-pressure area. Trim becomes very important to a vessel on plane, as it is sensitive to weight distribution in the hull and trim adjustment to motor outdrives.

[gerinc]

Squat is a vertical movement of the ship, so it has nothing to do with the amount of water pushed aside, but with the water which passes underneath. As any fluid's (gas or liquid) velocity increases, it loses pressure. Since the gravity pushing the hull of the ship down does not change, the result is the vessel 'squatting' lower in the water.

Bank suction and bank cushion are the two effects that you described, though they apply to ships passing one another, as well. Bank suctions works precisely the same way as squat, with the fast moving fluid passing between the ship and the other object exerting less pressure than the water moving more slowing on the other side. In a narrow space, the water sluices through at a greater velocity than where there is more room.

The cushion effect is a result of the bow wave pushing the bow of a vessel away from a bank or even another vessel. In narrow areas like the Kiel Canal, for example, when two ships must pass, pilots will steer directly for the oncoming vessel and will only apply rudder to keep the sterns of the two ships apart, as the bow wave pushes their heads away from one another.

All of this is, as you said, independent of propeller action.

If I am understanding the Coast Guard's explanation correctly, the "squat" effect is the result of water being pushed away from the sides of the hull as it moves through the water. It is quite independent from propellor effects. If one would observe the form of the bow wave as it passes the sides of the hull, the amount of water displaced increases with the length of the hull. With a displacement-type hull, the size of this bow wave will increase with speed, thus increasing the squat effect. So this will happen with both sail and powered vessels. I would think the only circumstance where this would not occur would be a vessel underway in a current that exceeds the speed of the vessel, in which case the vessel could not make steerage and be uncontrollable.

The situation described about the sterns of vessels running parallel in close proximity is a variation on that is referred to as "bank effect". This is more commonly encountered in narrow channels, where sterns of vessels passing close to banks will be drawn toward them due to low-pressure areas formed between the hull and the sides of the channel. It can be very trickly when meeting another boat in a narrow channel: bank effect may be drawing the sterns of both vessels to the outside, while low pressure caused by the meeting of the hulls in close proximity can draw the vessels together. This, too, is a hydrodynamic effect independent of propeller action.

Hull squat in vessels on plane is not so much a displacement effect as it is a result of the hull "riding up" on the water. Passing over a shallow-water area would still draw the stern down as the relative lack of water to displace would still create a low-pressure area. Trim becomes very important to a vessel on plane, as it is sensitive to weight distribution in the hull and trim adjustment to motor outdrives.