FOND DU LAC, Wis. (July 23, 2015) – Mercury Marine this week demonstrated to the media its new Active Trim system, which automatically delivers instant, effortless trimming of boat engines.
Designed to suit entry-level and expert boaters alike, Active Trim simplifies boat operation, while improving engine performance and decreasing fuel costs. New... read more
Any Vee hull, and particularly pad-vee hulls, can experience chine walk. Hull inconsistencies or design issues, incorrect hull setup, water/wake conditions, even wind conditions or driver inexperience can initiate chine walking. Veteran performance boat drivers have all experienced chine walk in their boats at one time – and have learned how (hopefully not the hard way) to deal with it for their hull.
If left unchecked, the boat will rock from side to side with increasing drama. This is because the boat is now “inherently unstable” – meaning that if left alone (no driver input), the “imbalance” of the hull is more likely to get worse on its own, not better (in other words, the worse it gets - the worse it gets!). If it gets out of control, you can get into real trouble!
Note that the notion that you can “drive through” chine walking without making adjustments is a dangerous approach. Do not try to merely drive a boat through a chine walk. Some drivers hope that if they keep trimming and/or adding power, the boat will somehow stabilize itself. This kind of thinking will almost always get you into more severe trouble. Make the proper setup changes first. With experience, you can then learn to make the subtle steering, trim or throttle adjustments that might control the condition – but don’t try to just blast through the onset of chine walking, as this usually ends up to be a very wet conclusion!
Chine walking is usually observed more often in vee-hulls with high-lift running pads, or narrow running pads, high-deadrise hulls (deep Vees) and straight vee hulls with no pad. Hulls with a Veed pad (instead of a flat pad) are also less likely to avoid chine walking. All of these bottom designs are just naturally harder to balance at high speed. Vee-pad hulls all have a tendency to fall off the “pad" under some conditions, which can lead to chine walking. Step-notches that are too wide, or a planing pad/beam which is too narrow, can tend to permit chine walking at high speed. Hull designs with reverse-chine configurations can experience “lifting” or “falling” (chine walking) through a turn.
Another contribution to chine walking can be seen at higher speeds from "propeller slap" (usually observed more with propellers that have fewer blades). As a prop turns, each blade enters the water and another exits the water. This irregular in/out of every blade changes the dynamic forces at the location of the prop shaft, creating an imbalance to a hull that is already trying desperately to balance on its vee or pad, and ultimately initiating chine walking.
Cleanup and setup of your hull and driver “seat-time” are the best solutions to the problem. Hull design changes can also be made to alleviate chine-walking symptoms.
Here are some established steps toward minimizing chine walking:
1. Check and adjust steering - There should be no play in the steering mechanism. Dual-cable or dual-hydraulic setup should be used for high performance hull setups. For cable setups, be sure that you adjust the cable system to remove all slack. Reduction of backlash helps minimize engine flutter and the resulting handling instability, such as chine-walk. This is a must to provide good control and reduce boat instability - especially on vee-bottom hulls. Same story for hydraulic steering - ensure the system is adjusted so that there is no play in the wheel. Bleed the lines to remove any air from all parts of the system.
2. Use solid engine mounts - Stock rubber motor mounts can allow too much slack movement between steering wheel and the engine, and can reduce control at high speed. Fixed/solid mounts can have the rigidity needed to assure precise steering at high speeds and can reduce ‘propeller fishtailing’, chine walk and ‘sponson-walking’. Replacing the OEM mounts with solid motor mounts adds boat control and driving ease, especially with jacked-up engines in rough water. Solid (fixed) mounts can usually be purchased in steel, aluminum or composite materials.
3. Clean hull lines - make sure that any non-designed irregularities such as hook, rocker, bumps or other notches in the running surfaces are faired away. Use a long metal straightedge to look for surface variations, check for structural damage inside the hull and fill until the surfaces are completely smooth and true. Every ‘dip’, ‘hump’ and ‘valley’ should be faired out – until you have a perfectly straight and perfectly flat running surface. A “hook” section of the running surface curves ‘down’ towards the water instead of running ‘straight’ aft – this will produce an aft lift/bow down force on the boat. A “rocker” section of the running surface curves ‘up’ away from the water instead of running ‘straight’ aft – this will produce a bow lift force on the boat. Both of these conditions can be the cause of the onset of chine walk at higher speeds. Be aware that some boats are manufactured with a "hook" or "rocker". Contact the dealer or manufacturer to see if there are any designed features that you shouldn't disturb. Otherwise, rework your bottom running surfaces until they are very straight and very smooth. Although this can be a tedious operation, it can make a world of difference to the way that your boat handles – and can often be the trigger that initiates hull-instability and the onset of chine-walking.
4. Weight balance of hull - Improper weight balance is most often a contributor to dynamically unstable hull setup and the onset of chine-walking. This can however, be a tricky thing to optimize for all speeds - since the dynamic balance of a hull shifts significantly throughout the operating velocity range of the boat. Here is a tip for “homing-in” on the best balance for your boat. The onset of the chine-walking phenomenon usually occurs at a particular speed for each hull - so you can focus on correcting balance at that bothersome speed. The dynamic balance of a hull comes from the combination of 1) the deadweight of hull, engine, payloads, etc.; 2) hydrodynamic Lift and Drag forces generated by the hull shape at speed on the water; and 3), from aerodynamic forces (at higher speeds). Try to situate the movable payloads close to the static center of gravity (CofG) – both longitudinally and laterally. This can often be a trial-and-error experience, but you’ll see the results of weight balance changes immediately in the handling of the boat. Optimize portable equipment, batteries, oil tank and fuel tank positions. Also, situate passengers for the best weight balance. An evenly balanced passenger/driver load is best for stability, so if one (driver) seat is located more to one side than the other, you can add weight to the opposite (passenger) seat to equalize the load distribution. Or if you’re really serious, you can relocate the driver’s seat to the middle of the boat. A balanced boat setup will make your boat much easier to drive.
Engine height can play a key role in mitigating chine walking
5. Adjust motor height - First you should check to ensure that the engine is properly mounted in center and perfectly straight. You can adjust engine height to minimize the instability. A manual or hydraulic jack plate will make these engine adjustments much easier. A hydraulic jack allows you to adjust the engine height while in full flight – the best of all worlds! This is the safest feature of the hydraulic engine jack, because it eliminates the need to “compromise” your engine setup. [Remember that as you raise the engine height, a low water pickup may become necessary to ensure that the engine gets enough water pressure.] Test your rig at different speeds and water conditions to find the best height for each. You may find that different operating conditions will benefit from different engine heights, which is another good reason for using a hydraulic jack plate. Often, as the engine is raised on the transom, the reduced lower unit drag can have an improved effect on handling instabilities such as chine walking. Engine setback can also affect stability, although it is more difficult to test using the ‘trial and error” method.
6. Propeller selection - The right propeller design can change the balance of a hull as well as make or break its performance. Rake, diameter, pitch, cup and blade number, can all influence the Lift and Drag forces that are generated at the aft-end of the hull. Lower-pitched propellers (although higher RPM) can make the boat easier to drive and can ultimately contribute to a higher achieved top speed simply because you have less difficulty driving them with confidence at higher speeds. Similarly, larger diameter props with lesser rake can improve handling situations like chine-walk. And propellers with fewer blades can degrade handling (more prop-slap) to some extent, so you might try a prop with more blades – less vibration and less prop-slap – the smoother rotation is less likely to disturb the delicate balance of the hull at the speed of potential instability (chine walk). Propeller testing is time-consuming, but can really pay off in overall performance and stability.
7. Seat time (experience) - Chine walk on a vee hull can often be controlled by the driver as he gains more experience and skill with his setup. Unfortunately, there is just no substitute for experience!"Timing is everything" - When you sense the onset of chine walking, reduce engine trim and/or throttle. When the motion stops, increase trim and throttle smoothly as the hull drives through the previous chine-walk speed barrier. Steering adjustments need only be small, but should be made in a timely manner in the opposite direction of the hull bow movement. The key here is NOT to over-correct with your steering input. When the left bow drops or the bow moves left, steer slightly right. When the right bow drops or the bow moves right, steer left. This steering input is done swiftly and in short motions. With practice you will be able to make these small steering inputs "before" the motion actually occurs. Turning the steering wheel slightly into (opposite to) the rotation of the propeller as soon as you “sense” the onset of lateral imbalance (side-to-side rocking) will tend to interrupt the onset of chine walk before the rocking motion starts up. Drive your hull in different conditions at lower speeds until you are completely comfortable with your ability to “sense” and “correct” for motions of the hull to conditions and speed changes. Then gain more experience at a slightly faster speed, in the same way. Work you way up to higher velocities slowly, under good control. With familiarity, you will develop a sense to predict your hull’s motion and you’ll soon be able to react accordingly to correct it prior to it getting severe. The correct driver input to balance a vee-hull or a pad-vee hull at higher speeds is very minor if the adjustments are made quickly (but gently), immediately at the onset of motion (“timing is everything”).
8. Minimize trim angle - Use as little positive trim as possible. More trim (higher running angle of attack) causes the onset of dynamic instability to occur earlier and with more drama (more surprises). A high-flying attitude is also harder to balance. When chine walking starts, it is not likely that you can simply "drive through it" safely – so you’ll need to reduce trim slightly first. The best approach is to minimize the extravagant use of “positive trim” in the velocity region that you know to exhibit the onset of chine walking.
9. A safe view - Now, there is one more recommendation worth mentioning – and that is safety! In order to drive fast, and do it more than once, you need also drive safely.
If you are driving your boat fast enough to experience an annoying chine-walk, then you should also ensure that you have your boat rigged as safely as possible. Consider investing in a performance life jacket & helmet, engine kill switch (always connected to your life jacket), solid engine mounts, foot throttle, wheel-mounted trim switches, nosecone & low water pickup and engine temp/water pressure gages. Installation of key safety features will provide reliability, and stability to your hull setup - which will allow you to drive faster, more reliably, with confidence - and do it with safety.
About the author:
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, MI and University of Toronto, Canada and marine research at the NRC water channel laboratory in Ottawa, Canada. His published papers are highly acclaimed, and are specifically related to the aerodynamics and hydrodynamics of high performance catamarans, tunnel boats and vee-hulls. Russell has designed, built and raced tunnel boats, and consulted on design of many recreational, racing, commercial offshore and government hull designs. He has appeared on SpeedVision's Powerboat Television as a guest expert on 'Tunnel Hulls’, and as performance/design technical consultant on National Geographic's 'Thrill Zone-Extreme Powerboats' TV show.
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. Russell is the author of the "Secrets of Tunnel Boat Design", "History of Tunnel Boat Design" book, and the "Secrets of Propeller Design." books. His company 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, and performance