Pressure forces working on a prop strut?

[sdowney717]

I have been rebuilding the bottom of my Eggharbor and was wondering about this. Which direction is the force directed, towards the hull? Or Away from the hull, is the strut being forced up or down or sideways?
How much force is involved or is most of the pressure pushing into the transmission hub
It seems to me some force is directed downward, if the bow is pushed up and the stern drops when the engine is throttled up. That would mean the planks would be pulled away from the floors? It makes more sense structurally from what I see under there that there is upwards force directed under the hull.
Thanks for any ideas.

[carioca1232001]

Your undersides overhaul is being a source of food for thought !

A prop(airplane/boat)generates thrust (force)by pushing fluid (air/water)in a given direction, depending on blade angle, RH/LH rotation etc.

Newton´s Law has it that "reaction force = acting force", and this results in a propulsive thrust(force)pushing the airplane/boat in the opposite direction.

In pleasure boats, surface drives notwithstanding, the prop shaft subtends an angle of some 8-to-13 degrees with the water surface.

Resolving the propulsive thrust (boat case) into vertical and horizontal components - a simple exercise in geometry - will reveal:

- a vertical force through the prop strut pushing the latter (and the hull attached to it )upwards

- a horizontal force pushing the boat forward (or backward).

This is what - in essence - is observed in VTOL aircraft as they adjsut their engine-thrust diverters when getting airborne / landing.

[carioca1232001]

sdowney717 wrote:


It seems to me some force is directed downward, if the bow is pushed up and the stern drops when the engine is throttled up . That would mean the planks would be pulled away from the floors? It makes more sense structurally from what I see under there that there is upwards force directed under the hull The explanation for the highlighted bit is rooted in aero/hydrodynamics.

"Lift" is generated by virtue of motion in the fluid, dependent on hull (wing) shape.

Now weight distribution, C-of-G, center of bouyancy etc, causes the stern to squat.

Switch on your trim tabs (airlerons in airplanes !) and the bow(nose) may be lowered again(or nearly so).

Forumites such as "mmd", Bruce Hooke etc. can supply the finer details.

[carioca1232001]

In motion, the prop shaft flange end has the following components of torque:

1. prop-load torque
2. cutless-bearing friction-load torque, assumed for simplicity to be uniform over the bearing circumference.

N° 2 is in the opposite direction to prop shaft rotation (Newton´s Law).

A RH rotating prop shaft generates a LH tangential torque over the bearing circumference (and vice-versa).

This is equivalent to a force applied in the "hard-left" direction (and vice-versa) - and at right angles (90 °) - to the the prop strut where it is welded to the cutless bearing

Hope this helps. (second order effects, out of balance props, off-centre shafts etc.etc. ignored for simplicity´s sake).

[JimConlin]

Do consider what happens when you run aground.

[carioca1232001]

Jim Conlin wrote:


Do consider what happens when you run aground. At least a breathalyser test conducted on the skipper by the Coast Guard ? :D

[mmd]

The forces on a strut are pretty much global in direction. Torque imparted to the strut through friction between the shaft and the bearing will be resolved into a force in the direction of the strut's weakest axis, which is ninety degrees from the vertical axis. If there is possibility of the shaft moving fore-and-aft, which is conceivable in a system where the shaft is hard-mounted to the engine but the engine is on anti-vibration mounts, frictional loads on the strut bearing will result in a stress on the strut that is longitudinal. If the shaft is at an extreme angle compared to the LWL there will be a component of the prop thrust that will act upwards on the strut. Gravity acting on the prop, shaft, and strut will impart a downwards force on the strut.

But all of the above-mentioned forces are "steady-state" forces - and pretty small, as such things go - of a boat assumed to be moving smoothly in a straight line in flat water at constant speed. We all know that those conditions are pretty rare, and that the real big forces on a boat happen when you are trying to manoever in rough seas. Then things get really exciting. As the boat pitches and heaves, the prop & shaft get dragged through differing water pressures (related to depth of immersion) that will dial up some significant up-and down forces in the strut. Surging speeds as the boat traverses up one side of a wave and surfs down the other will place longitudinal loads on the strut from both propeller thrust and frictional drag on the strut itself. But by far the greatest forces on a strut occur when it is forced through the water sideways. This occurs every time the vessel turns - the boat turns on its centre of lateral resistance, which is located some distance forward of the strut, so the strut has to move sideways through the water until the turn is completed. To that sideways force add the force created by the boat rolling from side to side - the roll swings the strut through the water sideways, exposing the strut's broadest face to the pressure of the water. Then add the forces created by the whole boat moving sideways through the water, such as when you come down the face of a big wave at an angle and the hull "slides" sideways down the face into the trough below. Finally, add the vector force derived from the forward motion of the boat as it moves through these big waves.

The total forces acting on the strut can be quite large, and the largest forces transmitted to the hull are the lateral (sideways) forces. As the greatest forces are lateral, the result is that the strut tries to rock from side to side on its base, alternatly lifting and compressing the lateral outboard edges of its base flange. This acts on the bolts holding it to the hull like rocking a claw hammer on a nail, alternately pulling at the bolt and crushing the timber around it. As the bolt stretches and the wood compresses, the strut becomes looser, and the lateral freedom of motion increases. At some point, without due attention, the motion will become extreme enough that something reaches the point of failure and you have a very big problem.

If you want to see an example of what your strut is doing in the water, hold the very tip of a ruler between your clenched thumb and index finger, then slightly shake your hand like you were shivering from the cold. If you shake the ruler in the direction of it's width, the end of the ruler moves a bit, but not much. But if you shake it "on the flat", the ruler tip moves a great deal more in this orientation than in the vertical mode, and requires a much firmer grip to maintain a good hold on it. Your strut does the same dance underwater, just at a much slower frequency and with monsterously greater energy.

So, now that I've frightened you, you'll be careful to check your strut bolts at every haul-out, right? :D
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EDIT TO ADD: The bow-up attitude that your boat takes when accelerating is from the pressure of the bow wave acting on the hull, not from prop thrust. In fact, the force of the prop will try to lift the stern up and force the bow down, and the greater the down-angle of the shaft the greater the upward force on the stern of the boat.

[ 10-27-2004, 02:11 AM: Message edited by: mmd ]

[carioca1232001]

mmd wrote:


...But all of the above-mentioned forces are "steady-state" forces - and pretty small, as such things go - of a boat assumed to be moving smoothly in a straight line in flat water at constant speed. We all know that those conditions are pretty rare, and that the real big forces on a boat happen when you are trying to manoever in rough seas. Then things get really exciting You have correctly identified the dynamic forces as the most pertinent in real-life boating.

[Dan McCosh]

The above comments are good. The strut mainly takes a sideways load from the prop torque--forward thrust is on the engine mounts. Might note, however, an issue which I have encountered. That is if you catch a secured line in the prop. It winds around the shaft as if it is a high-powered winch with significant force. In my case, it snapped a 1/2 ins. dock line, but in a similar situation locally, it ended up tearing out a large section of the hull and sunk a 50 ft. schooner. The strut is a significant structural element.