Ski tow pole leverage

[Gingerbread Man]

Hey.

My friend has a Haynes Hunter (style) board with a two stroke 115hp outboard on the back.

She uses it to water ski, wake board and knee board behind. They use a bridle to tow, but the owner fancies a tow pole due to the higher tow point.

When driving, a big skier can really pull the boat off kilter a bit. Come turning at the end of the run, it can with the right person behind, make the boat feel a bit dangerous.

With a tow pole (higher tow point), would this just get worse as the skier would have more leverage on pulling the boat over?

I can see it behind a better point to be towed from when behind the boat as the rope will be higher, but it's more when the skirt comes further around to the side of the boat.

Thoughts and experiences?

 

[gonzo]

Yes it would. Torque is Force x Length. This means that if you double the length of the pole, the torque will double too.

 

[jehardiman]

Yes, girding is real and very dangerous. The vessel needs to be designed to take the higher tow point.

Girding danger under the spotlight https://www.rivieramm.com/news-content-hub/news-content-hub/girding-danger-under-the-spotlight-56417

 

[fastwave]

I would be surprised if a boat able to handle a 115hp outboard has such an issue. The tow post just needs to be above the engine cover and not more.
Also the force is max 30degrees either side on a typical slalom.
Bridles are a pain in the ***

 

[fastwave]

Mind you,
There are plenty of overpowered or crazy boats out there. Perhaps a few photos might help.
The comments above are ofcoutse all valid.

 

[Gingerbread Man]

Some pictures attached. Not always an issue, more if a bigger stronger person being towed.

Wake or dedicated ski boats have a flatter hull design? Lower in the water.

Thanks for the comments so far.

 

[fastwave]

The boat looks nothing unusual.
I struggle to understand how you ski in a turn to create an issue. Even during a slingshot, not sure if that is the proper term, it should be fine.
The feel on the wheel changes and maybe that’s unnerving.

we are not speaking about a bridge wakeboard style tow which sits 2m above deck.
The best are the ones slowness above but in the photo the post is unnecessarily long. You only need to clear the engine by 2-3 inches when the skier is standing and the engine down.
Also the engine trim makes a huge difference to the behavior or the boat. It usually is trimmed a little bow down for skiing compared to optimum running

 

[fastwave]

Ps: don’t use the post to tow boats etc for reasons mentioned by the other members. Especially at high power.

 

[BlueBell]

Yes, girding is real and very dangerous. The vessel needs to be designed to take the higher tow point.

As does the driver!

 

[fastwave]

Ok guys, let’s do some rough numbers. A big male pro skier can have ca 300-400kg line load. Old tow lines used to be rated at 450-500kg and used to break when old.
Assuming normal slalom sking of max 45 deg tow direction you get a moment of 350kg x sin 45 of ca 250kg force at let’s say 1m tow post height.
That is equivalent to 2 big guys sitting on the same side on the boat we see above.
I think that is manageable since the boat does not have any strange features to make it directionally unstable etc. however you know your boat best.
Secondly the above is for a bloody good skier. If you can ski that well, great!!!
I am pretty good but only weight 70kg so will never reach those loads.
I think the comment about the driver is a valid one. You definitely feel a good skier on these type of boats. I never felt concerned since I grew up with it but also never felt the urge to do something completely stupid when steering.
Give an idiot any Maschine and he will find a way to kill himself.

 

[Gingerbread Man]

90% of the time it's fine. Maybe that 10% was a bad time.

Will look into the trim when up and running for the skier.

Thanks.

 

[gonzo]

@fastwave: You need to account for the heeling generated from the boat turning and add it.

 

[fastwave]

It’s not a complete analysis for sure.
In anycase a boat will lean into the turn, which is the opposite.
If the boat does not lean into the turn then it’s a bad design to start with and can even be very dangerous.

 

[gonzo]

In anycase a boat will lean into the turn, which is the opposite.

It is not the opposite. When a boat turns, the skier is turning into a tighter turn which heels the boat inwards.

 

[fastwave]

A skier will not produce the forces mentioned above on the insider of a turn. He can barely ski if he is so far inside