Quote:
Originally Posted by ed_dash
how do you stop the chining? I used to have an SR4 with a 40 yammie 2 stroke on and started to chine walk at 28kts. Surely with more power your just going to chine walk more and more violently?
|
ed_dash - Here is some more info, specific to chine walking, that might help...
The chine walking experience refers to the situation occurring with high performance vee-hulls and also tunnel hulls. As the boat accelerates, lift increases and the running surfaces raise out of the water. As hull speed continues to increase, the wetted surfaces are sufficiently diminished on the vee-portion of the hull that it becomes challenging to "balance" the hull on its keel (either vee or pad). To counteract, additional driver steering input is necessary in order to maintain the hull in a balanced state.
If left unchecked, the boat will rock from side to side with increasing motion and drama. The boat is now "inherently unstable" – this means that if left alone, the "imbalance" of the hull is more likely to get worse on it’s own, not better (the worse it gets, the worse it gets). So the hull will now start to rock from port chine to starboard chine – back and forth - "chine-walk".
Chine walking is predominantly characteristic of vee-hulls, with deeper Vees (more deadrise), hulls with deep or narrow running pads and hulls with a Veed pad or hulls with no pad. These bottom designs are just more inherently difficult to balance at higher speeds. Another contribution to chine walking can be seen at higher speeds from "propeller slap" (usually seen more with props of fewer blades). As the 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 propshaft, repeatedly putting an imbalance on a hull that is trying to balance on it's vee or pad, and ultimately it creating chine walking.
Setup of your hull and driver "seat-time" are the best solutions to the problem.
The experience of chine walk can be only slightly bothersome on some hulls or can get to be out-and-out dangerous on others. The latter can ultimately result in catastrophic consequences if the condition is not corrected quickly. Usually an alteration to the hull setup and/or modification to your driving methods (read seat-time) will improve the problem.
Here are some established steps toward minimizing chine walking:
1. Check & adjust steering. There should be no play in the steering mechanism. And a dual-cable or dual-hydraulic setup should be used for high performance hull setups. For cable setups, be sure that you have all of the slack adjusted out of it, so the cables are slightly pre-loaded against each other. Same for hydraulic, ensure it's adjusted so there is no play in the wheel and carefully bleed the lines to remove all air from the entire system.
2. Use solid mounts - Stock rubber motor mounts allow for too much slack movement between steering wheel and engine. Solid mounts are much tighter and provide much better steering control at high performance speeds.
3. Clean Hull Lines – make sure that any non-designed irregularities such as hook, rocker, bumps or other notches in the running surfaces are removed or faired away. You can use a long straightedge to visually inspect your running surfaces and fair out the imperfections.
4. Weight balance of hull – Although this is 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), the onset of the chine-walking phenomenon usually occurs at a particular speed for each hull and you can focus on correcting balance at that bothersome speed. Try to situate 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 equally balanced passenger/driver load will help allot, so if the driver’s seat is positioned much to one side, add weight to the passenger seat to help balance the load and make learning to drive the boat much easier.
5. Motor height – You can adjust engine height to minimize the instability. This is easiest to do with a hydraulic jack plate. Remember that as you raise the engine height, a low water pickup may become necessary in order to ensure that the engine gets enough water pressure. Test your rig at different speeds, weight distributions and water conditions to find the best height for each. Often, as the engine is raised on the transom, the reduced lower unit drag can have an improved effect on instabilities such as chine walking. Engine setback can also affect stability, although it is more difficult to experiment with.
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 generated at the aft-end of the hull. Most high performance vee-hulls will handle well using medium-rake, large-diameter propellers. High-pitch propellers can make the boat more difficult to drive and ultimately contribute to slower achieved top speed simply because they are more challenging to drive. More blades will also usually improve handling. Propeller testing is also time-consuming, but can really pay off in overall performance and stability.
7. Seat time (experience) – Chine walk on a vee hull can usually be controlled by the driver as he gains more experience and skill with his setup. Unfortunately, there is just no substitute for experience! 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, immediately at the onset of motion ("timing is everything").
"Timing is everything" - When you sense the onset of chine walking, reduce engine trim and/or throttle. When the motion subsides, you can increase trim and throttle smoothly as the hull drives right 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. 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 (against) the torque of the propeller as soon as you "sense" the onset of lateral imbalance (side-to-side rocking), can help drive through the chine walk stage too.
8. Minimize Trim Angle - This was mentioned above, but worth saying again. Use as little positive trim as possible. More trim (higher running angle of attack) causes the onset of instability to occur earlier and with more drama. A high-flying attitude is harder to balance. When chine walking starts, it is not likely that you can simply "drive through it" without first reducing trim slightly.
Summary: Hull setup and driver "seat-time".
...Also check out the
full article on "Chine Walking".
__________________
Whether it was wise to recommend putting a 60HP engine on a SR4 to a guy who's still at school 
might be a different issue.
