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Old 10 August 2018, 08:41   #1
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Physics of max speed/chinewalking

I understand that once a planing hull is up on the tip of its V the prop torque will tip it over and then bounce back and forth.

So is there a max speed of a planing hull? Or if you add weight to sit lower in the water, can a more powerful engine then push you along at ultimately a higher speed?

I’m sure some of you guys are in the know!

I ask as our H 6.3 OP with 175hp opti will do 50 mph, trimming out it gets up to 53mph but then starts to chinewalk/very flighty!

With more power (boat rated to 200hp) “could” it theoretically push past 53mph before trimming right out and getting twitchy?
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Old 10 August 2018, 12:21   #2
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If your theoretical 200 weighed the same as the 175 then I think it would start to chine walk at the same speed/trim. If you add more weight then it will sit lower in the water at any given speed but accordingly will have more drag so may not actually go any faster.

If you want silly speeds - you may need a foil - I see someone in France is producing a foiled rib - how practical that is for the other 362.5 days a year I wouldn’t like to speculate. I’d say the same applies to your hull - it would be very rare someone would really want to take a Humber into the 50mph range...
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Old 10 August 2018, 13:11   #3
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Keeping the boat well balanced fore and aft will defer the onset of instability. If you get maximum speed only at maximum trim it indicates an issue elsewhere since at maximum trim you will be applying a fair amount of down force to the rear of the hull. With practise, it's possible to learn to control chine walking to some extent. I'm not suggesting a boat that is becoming unstable at 45knts can be driven at 55knt with care but it's certainly possible on a reasonable hull to learn keep a measure of stability at speeds above it beginning to feel edgy. Gently does it. Don't get lost in the rush of the moment.

With regard to your 175 being replaced by a 200, given everything else about the motor to be equal, you'll get only a minor gain in speed. Remember, engine power per boat follows the square rule, so to go twice as fast you'll need four times the power. An extra 25hp will therefore give you only a small gain in speed.
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Old 10 August 2018, 13:15   #4
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Physics of max speed/chinewalking

To be honest we only ever have a quick sprint to 50 for fun and with 4 on board it tops at high 40s and is well planted, not twitchy at all. It was a theoretical question.

The 200hp 3.4L should mean we can get to 35/40 4up with less revs/less fuel.
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Old 10 August 2018, 13:24   #5
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Originally Posted by HUMBER P4VWL View Post
To be honest we only ever have a quick sprint to 50 for fun and with 4 on board it tops at high 40s and is well planted, not twitchy at all. It was a theoretical question.

The 200hp 3.4L should mean we can get to 35/40 4up with less revs/less fuel.
Now you're suggesting something else. Is the 3.4l a bigger capacity engine than your 175? If so and it's not too heavy, you'll get much better midrange performance. I don't think you'll improve the fuel economy. You're powering a boat of X weight and Y drag and that takes Z amount of energy. Unless the replacement engine is more efficient at converting fuel into propulsive energy, it's going to burn a similar amount of fuel.
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Old 10 August 2018, 13:38   #6
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Physics of max speed/chinewalking

Yes, 175hp 2.5L V6 2stroke Optimax vs 200hp 3.4L V6 new Merc normally aspirated 4S

It will certainly use less fuel (though that’s not a real driver). It’s about 10kg heavier I believe so nothing too different.

There’s so many variables re 2S vs 4s, 200 vs 175, weight and hydrodynamic properties of the legs...
Either way it’ll be quieter and use less fuel! As long as performance is similar we’ll be happy. Time will tell!!!

I was just curious as to the theoretical max speed and chinewalking.

Need to do some holeshot times with a set weight, and say 20-40mph time’s on the plane, prior to changing.
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Old 10 August 2018, 16:49   #7
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When I had my ribquest the tubes were in contact with the water all the time no chine walk the boat before it a quicksilver fast fisher with planing pad was a mare move the slightest weight and it was all over the place trim tabs helped though down sizing to the sib the bow flat out use to wave from side to side as if the boat was Balanceing on the prop throwing weight forward sorted it out instantly not an expert but a hull has a point when it becomes unstable at certain speeds I find weight distribution very important as much as trim,tilt,speed etc if not more OMO
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