By Jim Russell, P.Eng
Wouldn’t it be nice to be able to know why your boat has a tendency to porpoise? And nice to know how you can improve or eliminate a porpoising problem? The causes of porpoising and how to correct it are frequently mystifying accounts. Here is an explanation of porpoising, how to design to mitigate the potential for porpoising and how to fix it when it’s a problem.
What is Porpoising?
Porpoising is the cyclic oscillation of a boat in pitch and heave, of sustained or increasing amplitude, occurring while planing on smooth water. Picture a porpoise leaping through the water and you get the image of what it’s like in a powerboat. Severity can range from an uncomfortable ride, to aggravating or even dangerous.
|Porpoising occurs when lift is generated at a sufficiently high trim angle or sufficiently low deadrise so as to cause a dynamically unstable loading on the lifting surfaces.|
Here’s some examples of porpoising complaints that some readers have expressed:
►"My ModVP style sport tunnel with center-pod has a "hopping" motion at about 70 mph. It's like porpoising, but it goes away at about 80mph. What is wrong with my setup?"; OR...
►"I have a 24 ft Vee-pad hull that porpoises at one speed and won't stop - why is this?
Why does a boat porpoise?
Any Vee hull or tunnel hull can be susceptible to porpoising depending on design and setup, but some hull designs are more prone to porpoising than others. For example:
- Flatter bottom Vee or sponson surfaces are more prone to porpoising than steeper deadrise hulls (higher deadrise is better to avoid porpoising)
- Higher trim angles are more likely to initiate porpoising than lower trim angles (lower trim angles are better to avoid porpoising).
|Porpoising is the cyclic oscillation in pitch and heave, of sustained or increasing amplitude|
- Is the design predisposed to porpoising? - Some hull designs are more susceptible (e.g.: lower deadrise, higher trim angles) to porpoising than others. There is a way to determine whether the design is likely to experience porpoising or not. It's good to know this at design-time, or when you're buying a new boat.
- A trigger to initiate porpoising - When a hull experiences a significant change in Dynamic balance of Forces, it will encounter some longitudinal instability, and if the hull is one that is already susceptible to porpoising at this velocity, then the dynamic instability usually triggers the onset of porpoising.
Design Compromises - is the hull prone to porpoising?
For hull designs that will have a tendency to experience serious porpoising, there is a particular combination of speed, hydrodynamic Lift coefficient (efficiency), hull shape, hull deadrise and trim angle that will define the limits of porpoising.
As explained above, there are two main factors that influence whether a design is prone to porpoise. Higher deadrise is better to avoid porpoising; lower trim angles are better to avoid porpoising.
|A design configuration and setup can be analyzed to predict the velocity where the hull is 'Stable' or in 'Porpoise Regime.'|
So, even though a "higher deadrise" vee hull should have LESS tendency to porpoise than a low-deadrise bottom - since the high deadrise needs to operate at higher trim angles this makes it MORE susceptible to porpoise. So it's just not a simple rule-of-thumb to design for avoidance of porpoising!
The apparent contradiction makes the design of porpoise-resistant hull designs a challenging one. It’s also why we often see well-know hull manufacturers that have boats that are surprisingly susceptible to annoying porpoising. [There is an engineering method to assess whether a design is predisposed to porpoising at any velocity - we use the “Tunnel Boat/Vee Hull Design Program (TBDP©/VBDP©)" performance software].
Onset of porpoising - when will it start?
For a boat design/setup that is prone to "bouncing" or porpoising, the onset is usually triggered by a rapid change in the location of the center of all the lift and drag forces (Dynamic CofG) as the boat accelerates. This relocation of net forces comes from the ever-changing balance of hull surfaces that provide the lift (and drag) for your boat throughout the operating speed range. The "Dynamic CofG changes at every velocity and is different for every boat design/setup. It's not until this "trigger" occurs, that we'll usually see the onset of porpoising.
|The 'dynamic center of gravity is the combination of all lift and drag forces acting on a performance boat, and its location changes through the operating velocity range.|
For a tunnel hull, the "hump" or "transition zone" represents the speed at which the amount of lift aerodynamic (air lift from tunnel and aerofoil) lift becomes significant compared to hydrodynamic (water lift from sponsons). On a performance Vee-Pad hull, the "hump zone" is the speed at which the amount of lift changes from mostly the Vee surfaces to mostly lift from the pad surfaces. Every boat and setup has its own “hump zone” transition velocity.
At the speed that the transition (hump) occurs, the hull will always experience some longitudinal instability - and often triggers the onset of porpoising. The hull experiences a dynamic CofG shift through the "hump" zone. When driving your performance boat, you can usually "feel" the change when your hull enters the "hump zone" velocity. The transition velocity can be accurately determined (by analysis) for any given hull design and setup, and can be altered by hull design, weight distribution, propeller selection and engine/hull setup. The best performance hull designs minimize the "hump zone" to one that is somewhat mild and occurs through a narrow velocity range. This balance can be optimized when designing the hull.
So, what can I do about it?
Porpoising is a function of the lift generated by your hull, the deadrise of your running surfaces, and the trim angle that is needed to get that lift. If the hull design/setup is 'prone to porpoising', the onset of porpoising will start at a velocity that triggers a change in dynamic center of gravity (often the hump zone).
The resolution to a porpoising problem with a hull design is most always addressed by causing the boat to run with less trim.
If a boat is porpoising at a given speed and load, lowering the trim angle will reduce or eliminate the porpoising. There are several ways to get there, but the bottom line is to reduce the trim angle at the velocity of porpoising onset. Even if the hull design is operating in the "Porpoising Regime" through a full range of velocities, reducing trim in some way will improve or resolve the problem.
Resolutions to an existing porpoising problem:
- Reduce Trim Angle
- Change Static Weight Locations
- Change Dynamic Forces location
- Clean up Hull bottom condition
- Optimize Propeller Selection
- Design with Higher Deadrise (bottom surfaces)
- Trim Tabs
Reduce trim angle – If your boat tends to begin porpoising at a given speed and setup, lowering the trim angle (using the engine trim buttons) will usually reduce or eliminate the porpoising. While this might seem like a unattractive method to correct the performance of the hull, it will, at least, get you and your passengers through the uncomfortable ride experience – even if it’s only a temporary ‘fix’.
Changing "trim" angle while driving through the "hump zone" even if less efficient, will also provide a better ride experience, and when well controlled, can "close up" the range of "hump zone" substantially – eliminating porpoising.
Change static weight locations - The onset of porpoising is, in part, influenced by the weight balance in the boat. Altering the dead weights in your boat (fuel, battery, payloads, etc) can affect the speed and trim angle at which porpoising will initiate.
Weight distribution changes can also have a positive effect on "where" the "hump zone" will occur.
Change dynamic forces location – Changing motor height, engine setback or propeller selection can all change the dynamic balance of the hull – often reducing the porpoising effects. Raising the prop shaft higher will shift the Dynamic CofG forward and reduce trim angle. Changing the setback of the engine can often change the dynamic balance of the hull at the porpoising velocity, so as to reduce the effect of porpoising or change the onset velocity to a speed where it’s less annoying. Note that more HP also reduces trim angle.
Clean up Hull Bottom Condition - Sometimes unplanned 'hook' or 'rocker' in the hull's bottom surfaces can exaggerate the performance effects thru the "hump zone". These variances in the hull bottom can sometimes be the "trigger" that initiates porpoising for a boat that is predisposed to porpoising. Cleaning up the bottom lines can often solve the porpoising problem.
Optimize propeller selection - Propeller selection can often change the dynamic balance of the hull/setup. For example, a change to a prop that provides more aft-Lift can alter the dynamic balance of the hull, and similarly change the speed and range of the "hump zone" – often eliminating porpoising.
Design with higher deadrise - Higher deadrise hulls are less susceptible to porpoising. Wider planing surfaces (vee hull, vee-pad, sponson pads or center-pod surfaces) can also reduce tendency for porpoising.
Trim tabs – Adding trim tabs (or more extreme whale fins) will also improve a porpoising problem, but these will also affect overall performance of the hull. Adding transom wedges can often help too, since they will allow for more negative trim travel, if it's required.
The bottom line - Porpoising will happen when your boat has 2 conditions: first, if the design predisposed to porpoising; and second, when the hull experiences a significant 'trigger', like a change in longitudinal instability. If the hull is one that is already susceptible to porpoising at this velocity, then the dynamic instability usually triggers the onset of porpoising.
The good thing is that we can calculate if a design is prone to porpoising, and we can analyze performance to predict the velocity at which we are likely to see the onset of porpoising.
There are several operating, setup and design actions that we can take to minimize or prevent porpoising. Reducing your trim angle in some way will always help a porpoising problem.
The complex engineering analysis of "porpoising" is addressed in more detail here: "Porpoise Regime Analysis"
AeroMarine Research has developed an analysis tool in the TBDP©/VBDP© software that helps predict a hull's inherent instabilities leading to porpoising. The technique is a prediction of the critical porpoise trim angle (CPA) and shows the velocity at which your hull configuration will experience instability in Porpoise Regime and has a susceptibility to porpoising.
About AeroMarine Research:
Jim Russell is a professional engineer with a mechanical and aeronautics background. Currently living in Canada, he has done extensive aerodynamic research at University of Michigan,OH and University of Toronto, Canada and marine research at the NRC water channel laboratory in Ottawa, Canada.
His published works and papers are highly acclaimed, and are specifically related to the aerodynamics and hydrodynamics of high performance catamarans and tunnel boats, vee and vee-pad hulls. Russell has designed and built many tunnel and performance boats. As a professional race driver, he piloted tunnel boats to Canadian and North American championships.
He has written power boating articles for many worldwide performance magazines and has covered UIM and APBA powerboat races. He has appeared on SpeedVision's 'Powerboat Television' as a guest expert on 'Tunnel Hulls', was performance/design technical consultant on National Geographic's 'Thrill Zone' TV show, and editorial consultant on Discovery Channel's 'What Happened Next' TV show. Russell is the author of the "Secrets of Tunnel Boat Design" book, and the "Secrets of Propeller Design" book. His company, AeroMarine Research, has designed and published the well known powerboat design software, "Tunnel Boat Design Program" and "Vee Boat Design Program" software, specifically for the design and performance analysis of tunnel boats, powered catamarans, performance Vee and Vee-Pad hulls.
Jim Russell, P.Eng,