Boat Acceleration and Turn Radius?

[lantley]

Does anyone know how fast a tournament ski boat gets up to speed and how many feet/yards it would take? Also, why can ski boats turn so quickly, or is it just an illusion? I ski behind an old runabout that seems to turns slowly compared to ski boats. For reference, while pulling a skier, what is the minimum turning radius/diameter of a ski boat?

[LeonL]

Boats have to pass a performance test to be approved by USAWS. I dot know the acceleration minimum, but it's readily available I'm sure. These tests are somewhat questionable as some bad handling boat were approved about 15 or so years ago and some poor accelerating boats just 3 years ago. That puts the whole process to question.

I can't think of any situation where a extreme tight turn radius, with a skier, would be necessary or advisable.

[DW]

@lantley : from some old published data prior to the latest series of hulls and more powerful engines, 0-36 mph times were in the upper 5 second range and distance was 150-180 feet. Lots of variables in engine power, hull drag and propeller choice (and not in any order of impact on performance).

 

The boat turn so quick due to tracking fins which counter the typical runabout center sliding action due to no tracking fins. Creates kind of a pivot point in the middle of the boat which allows the transom to slide out or around.

[lantley]

@LeonL , I agree that tight turning is not the smartest thing to do :grin: , but we had to do a dogleg in the small pond I used to ski in and I felt like I was going to get flung out the passenger seat when I would ride (which makes me wonder exactly how fast we were turning).

 

@DW , thanks for the numbers and explaining how the tracking fins cause the tight turns. Our old runabout isn't nearly as fast and agile as that! 5 seconds to 36 MPH is faster than my mom's little VW... :D

[Chef23]

We ski a tournament site that we basically do a pinwheel slide at one end to set the boat down. It is necessary because there are rocks all over the place. Modern ski boats are pretty stable but you still need to pay attention.

[lantley]

@Chef23 I'm not sure what a pinwheel slide is but it sounds like fun! Could you give the location on Google Maps so I could check out this site?

 

I have been wanting to "ski around the boat" but definitely will not attempt it behind the big ol' Galaxy; there is a show team nearby so I might join it and learn how.

[Jordan]

You asked why ski boats can turn so tightly....

 

We'll. ..the engine is in the middle, great center of gravity. Secondly there are fins/skegs in front of the engine..

Finally, the rudder is at the back. The net effect is that the boat can just about spin.

[Lars]

another thing that hasn't been mentioned is that a comp boat is essentially flat on the bottom, so it can "slide" in a turn where as a runabout will just "lean" on one side of the V hull. my terminology is probably poor.

[Edbrazil]

Back when I was involved with boat tests, the specification was 250 feet or less for the

boat to achieve 36 mph from a dead stop. And, this was WITH a skier doing a 1-ski

deepwater start. Initially, using a real human; later on Ron Tanis designed and built the

"drone" to simulate a skier When the 'Stalker' radar system came out, the technical aspects

of measuring this became a lot easier.

[Orlando76]

https://goo.gl/lHXchP

 

I hope that long link shortening process worked, I don't want to get panda'd.

 

Anyways, USWS has a section describing the prerequisites for an approved towboat.

[lantley]

Thanks @Orlando76! Lots of information there.

 

So as an overview of what I saw...

 

For approved tournament boats

0-36 MPH (deepwater start)

250 ft max (normal tournaments)

230 ft max (record capability)

Turning

75 ft diameter circles as Figure 8 (17 and 26 MPH)

150 ft diameter 180 degree (36 MPH)

 

For approved ski boats

0-36 MPH (deepwater start)

300 ft max

Turning

100 ft diameter 180 degree (17 and 26 MPH)

150 ft diameter 180 degree (36 MPH)

 

Another question... On page 4, it mentions accelerating from 35.4 to 38.2 MPH. When would this be necessary?

[keithh2oskier]

Jump maybe? I think when pulling longer distances the boat can run faster on the second segment than the set speed to keep up with the skier.

 

But I'm not a jumper or a jump driver...

[Chef23]

@Mark_Matis competition boats most certainly do slide. At one site we ski tournaments on we do basically a power slide at one end to set down to keep from getting in to rocks. At the other end we have to do a tight turn in to the course after picking up the skier. As the driver for 34 and 36 mph skiers you have to get the boat as fast as you can without the rear end sliding out to try to have as much speed as possible coming out of the turn.

 

The beauty of the competition boats in this situation is that a comp boat stays flat in these situations where you could never turn most outboards in the way you need to at this particular site.

 

 

[wtrskior]

I was always under the impression the fins were for tracking. There is an obvious effect on turning but the intent is for better tracking while pulling a skier.

[wskier]

@LeonL You have not skied the California Delta. You need a quick turning boat just to protect your skier. Its so narrow you just drop at each end. Off shore racers and jet ski idiots. Miles of glass when you know where to go!!!

[Jody_Seal]

Directional stability.