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ContinuousWave: Whaler Performance
Effect of Planing Angle on Efficiency
|Author||Topic: Effect of Planing Angle on Efficiency|
posted 10-23-2009 11:47 AM ET (US)
In an interesting professional paper presented to the Society of Naval Architects and Marine Engineers (SNAME), ON THE SUBJECT OF HIGH-SPEED MONOHULLS, by Daniel Savitsky, I came across this assessment of the influence of a planing hull's trim angle (what we non-naval architects might call the running angle when on plane) on the effort needed to propel the hull:
"...the performance of a planing hull is very dependent upon the longitudinal location of the center-of-gravity that controls the trim angle of the craft when planning. The trim angle in turn has a major effect upon the resistance/weight ratio. Typically, the resistance of a planing hull is a minimum at trim angles between 3-4 degrees and increases for both higher and lower values of trim. If the center of gravity for a given hull cannot be varied, then transom flaps or transom interceptors [i.e., trim tabs] can be used to change the trim. Analytical methods are available for evaluating the effect of center-of-gravity position or transom flap design on the equilibrium running trim angle."
Savitsky seems to be saying, and quite clearly, that our Boston Whaler boats ought to be running on plane with a trim of about 3- to 4-degrees bow-high.
In another discussion (see link below), many photographs were analyzed to deduce the trim angle of a Boston Whaler boat while running on plane. See
Running Angle On Plane
Analysis of the pictures showed many Boston Whaler boats running at angles greater than 4-degrees, however there was some (valid) criticism of the method, as it used the gunwale line as a reference.
To me there seems to be two questions:
--Is there agreement with the SNAME paper's suggestion that 3- to 4-degrees should be a target for optimum? I realize that replies to this are going to based mainly on opinion, but that opinion should be based on real experience, and thus be valuable.
--Can we more accurately measure the trim angle of our boats using other methods? One method that I propose to try is to use a modern electronic inclinometer, such as can be constructed using an Apple iPod Touch or Apple iPhone and the application iHandy Level. Perhaps someone will undertake the collection of some data by this method while also taking some photographic images so that the readings can be compared.
posted 10-23-2009 11:58 AM ET (US)
Interesting info. One thing to keep in mind is that the hull efficiency is part of the equation, but not the entire equation. For example, the speed that gives the proper angle for highest efficiency (lowest drag) for the hull doesn't necessairily put the engine at its most efficient speed. Such as big boat and small motor; the most efficient hull speed/angle might put the motor at full throttle, etc.. But hull drag is a very significant part of the equation so definitely cool info.
posted 10-23-2009 03:18 PM ET (US)
Yes we can measure the trim angle using a modern electronic inclinometer and should be relatively easy and a step in finding a 3-4 degree angle to determine if it really would be optimum. I will take my AngleStar out one of these days see how it works. http://www.meas-spec.com/downloads/AngleStar_digi_protractor.pdf
What is the reference for zero degrees? Page 27, figure 2c seems to indicate we would use the keel.
Found a definition that could be helpful, "EVEN KEEL - When a boat is floating on its designed waterline, it is said to be floating on an even keel."
Due to the typical aft CG of the majority of outboard boats it's probably going to be difficult to obtain the optimum angle to reduce the resistance of the hull and maintain a comfortable ride unless in fairly calm conditions.
posted 10-23-2009 08:14 PM ET (US)
Based on the quoted article the discussion is on hull trim angle. Not on deck trim angle nor on gunwale trim angle. On our Boston Whaler boats that is probably the keel angle.
In order to make correct measurements using an inclinometer it will first be necessary to establish the static hull bottom (keel) angle and adjust the inclinometer to indicate zero at that point of trim.
This is all very interesting but as our Boston Whaler boats may well have a large variety of static trim angles it will require a significant number of participants to provide a meaningful measurement.
I believe that the optimum trim angle of a Boston Whaler boat can be best determined by an inclinometer measuring zero relative to the deck angle at rest using the old method of determining the trim angle at which maximum engine RPM is obtained at a set throttle setting while on a comfortable plane.
I've found that with my boat that best angle is with the trim gauge indicating the engine is trimmed out half way plus or minus a couple of degrees. It would be interesting to see how that relates to an inclinometer measurement. I would be surprised if the incliometer reading of hull trim angle would be much off the three to four degrees in the paper.
posted 11-01-2009 11:11 PM ET (US)
In POWER AND MOTOR YACHT magazine for September 2009, on page 46, while reporting on a sea trial of a new design for an 85-foot motor yacht, reviewer Bill Pike notes that the hull reached plane with a bow rise (or trim) of "just four degrees, an optimum angle of attack for a big planing-type vessel...."
This is another mention of 4-degrees as being an optimum angle for a planing hull, albeit one quite a bit larger than a Boston Whaler boat.
|R T M||
posted 11-02-2009 07:40 AM ET (US)
The running angle of a planing hull is dependent on the degree of deadrise of the bottom. A deep vee boat even with lifting strakes, will generate less stern lift than a flat or flatter bottom planing hull, and will ride deeper in the water. Therefore the fulcrum point will be further forward, with the result that more of the hull will be wet, and the running angle will be greater than a flatter bottom boat.
As everyone knows the running angle can be adjusted by changing the angle of the outboard motor. Generally the deeper the V the more kickout of the outboard is required to reach the optimum running angle. A flat bottom race boat reqires very little or no kickout, as the flat bottom provides enough lift to maintain a very low running angle with the boat running on a very small portion of the bottom near the stern.
Also, I don`t think measuring the angle of the gunwale in relation to the horizon in a photo will give you an accurate running angle, as in most boats the gunwale is not parallel to the keel.
posted 11-02-2009 01:19 PM ET (US)
Did the article in POWER AND MOTOR YACHT magazine mention how the angle was measured and the reference used for zero degrees?
posted 11-02-2009 04:19 PM ET (US)
As I recall, it did not.
posted 11-02-2009 05:19 PM ET (US)
The angle would be different depending on what the CG is and what the speed is. On my 40 Tollycraft I adjust the tabs for the highest RPM on my digital tachometers. This method won’t work with a diesel powered boat because the governor tries to maintain RPM. On my Montauk I adjust the same way, best RPM without proposing equals best speed and MPG, at that throttle setting.
posted 11-03-2009 02:09 PM ET (US)
Jim - The less the planing angle (of the keel), the higher the effeciency - because the form drag is dependent on the frontal area projection. The greater the angle of the keel, the greater the drag - which decreases effeciency. --- Jerry/Idaho
|R T M||
posted 11-04-2009 06:52 AM ET (US)
I would tend to agree with you, however, the lower the planing angle, the more area of wetted bottom surface, which would more than overcome the benefit of a lower frontal projection. The way to overcome this is with a design that will lift the hull with enough stern support that even though the boat is running flat, most of the bottom surface is clear of the water, as in the below pic. Most pleasure type boats can not do this, with the exception of boats with padded bottoms, such as bass boats, which are tricky to drive.
posted 11-04-2009 09:54 AM ET (US)
The universally recommended 3 to 4 degree planing angle provides the minimum water drag from the hull as cited by Savitsky who is one of the formost if not the most formost expert in the subject of planing hulls. Aerodynamic drag is certainly a factor as mentioned by Jerry but is much much lower at typical speeds that Whalers run at.
As your boat goes faster the hull lift increases by the square of speed which reduces the area of the hull in the water if you can maintain the the planing angle. With most boats as we go faster and the planing area is reduced the center of lift of the hull moves aft making it difficult to maintain the hull angle at the desired 3 to 4 degrees.
Very fast monohull boats like bass boats get results by having the cg very far aft which lets them get the center of lift and cg trimmed at high speed with the center of lift very far back. The undesireable effect is operation at low speed is not as frendly as it might be.
To test this fact for yourself go out with several passengers and see what max speed you can make with all of them in the front of the boat versus all in the back.
posted 11-04-2009 10:10 PM ET (US)
There must be some relative angle between the keel and the water - to provide the lift - but, in general, the faster the boat is moving, the smaller the necessary angle to provide the lift. That us why there has to be some dependency of weight and speed to the frequently quoted 3 - 4 degrees. One size does not fit all. And yes, the lift will vary as the speed squared - which comes from the forces being provided by the velocity pressure.
Now, I have not read the Savitsky report or publication - but will.
Form drag is a controlling factor - and is lower at typical speeds that Whalers run at - but simply because form drag also varies as the square of the speed. Controlling? - yes, as with, in the limit, of no drag or resistance, there would not be a limiting speed. That is, the speed of our boats is at the point where the resistance is balanced by the input power. ---- Jerry/Idaho
posted 11-05-2009 10:25 AM ET (US)
The planing hull is similar to an aircraft wing. There are two sources of drag from the hull. Drag due to lift and drag due to skin friction. The lift of a planing hull plus the buoyancy of the immersed portion of the hull is equal to the weight being supported. As speed picks up on plane the buoyancy portion shrinks and the boat "flys" on lift. The planing lift is a function of the lifting area and the angle of attack or trim. The lowest total drag for the hull happens when the drag due to lift and the friction drag are equal. This occurs for most planing hulls in the 3 to 4 degree range. When you run your boat at WOT with the drive trimmed in you maximise the lifting area and minimise the trim angle. As you trim the drive out you increase the lift due to trim angle and since the total lift required is constant and equal to the weight of the boat you reduce the hull area in contact with the water and thus the friction drag. If you trim too far you will actually increase total drag and lose speed. In many boats porpoising will begin before you can reach the optimum trim angle.
posted 11-05-2009 07:39 PM ET (US)
Dick - Almost. Many refer to the similarity of a boat and the wing of a plane - which provides the lift from the differential pressure (decreased pressure on the top surface and increased pressure on the bottom surface - via the contour of the wing) acting on the area of the wing.
Whereas a boat gets the lift from the "velocity" pressure, which varies as the square of the speed, acting on the underside surface. The lift component of this pressure is the sine of the angle of the bottom of the hull with the water - and acting on the wetted bottom area. Note that this lift force on the bow is larger - until the bow lifts from the water. The process of getting up on plane is a dynamic process where the speed and the angle are changing quite rapidly.
There is no drag due to lift (that I am aware of) - but you are right in that there are two sources of resistance - form drag and skin friction. Form drag involves the projected area of the moving object.
As mentioned above, the process of getting up on plane is a dynamic process where all forces vary with time - at rest, the boat is supported by it's buoyancy - and when on plane, by the lift forces.
I seriously doubt, but don't know otherwise, that the "... lowest total drag for the hull happens when the..." two drag/resistance forces "... are equal ..." And as I mentioned above, I don't know what the lift drag is - though you may be referring to the form drag.
Porpoising is a separate and much more complicated subject - but it involves the oscillating dynamics of the moving boat and the location of those forces relative to the CG. But the dynamics of the porpoising action is dependent on the speed, weight and the trim angle. ---Jerry/Idaho
posted 11-05-2009 09:03 PM ET (US)
"... lowest total drag for the hull happens when the..." two drag/resistance forces "... are equal ..."
I think the term equal maybe missleading drag/resistance are just 2 components that affect the efficency.
I'm not sure why you should think they should be "equal" at the hulls most efficent point.
Hence stepped hulls that change the laminar drag when at a certain speed/angle etc
Jerry agreed porpoising is a separate and much more complicated subject where a critical point is reached and "oscillating dynamics" is encountered
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