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AI: Has anybody tried moving their weight aft in heavy air?
http://www.hobie.com/au/en/forums/viewtopic.php?f=71&t=61476		 Page 1 of 1
Author:  PeteCress [ Wed Sep 06, 2017 11:38 am ]
Post subject:  AI: Has anybody tried moving their weight aft in heavy air?
I'm thinking remove the seat and replace it with a sort of motorcycle seat arrangement where one can slide their butt backwards a foot or two in the interest of getting the hull up on some sort of plane.

I'm on the verge of trying it, but nothing seems to turn out to be as trivial as it sounds and if somebody else has already been there.....
Author:  mmiller [ Wed Sep 06, 2017 1:40 pm ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
I've laid in the cargo area... works!
Author:  stringy [ Wed Sep 06, 2017 2:22 pm ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
A level boat is fastest.
To get the hull to plane you not only need to get aft but out as well, so that the ama is not submerged and dragging. You can only really achieve this by adding haka or a quarter deck.
When I had my AI I came closest to planing when hiked out opposite the cargo well on my cantilevered haka.
Author:  bluelaser2 [ Wed Sep 06, 2017 5:28 pm ]
Post subject:  The guru of planing sailcraft
Would be a fella called Frank Bethwaite. He literally wrote the book on the subject.

From my understanding, the TI's beam to length ratio is too low to ever really plane. The foreparts are too deeply vee'd, the aft parts are too rounded, and there is not enough wetted surface to weight ratio either to generate enough lift. Finally, the sailpower (righting moment) to weight ratio is likely well below 10%, which is about the minimum for true planing.

A TI will surge and surf, somewhat, but if you want to really plane on a sailboat you need a flat planing hull and a high righting moment to weight ratio.

https://www.amazon.com/High-Performance ... TRSG3YWZ06
Author:  WAVERIDER [ Wed Sep 06, 2017 8:13 pm ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
Increasing seat recline moves body mass backwards, helps lifting front bar on vantage seat models to increase recline range and add weight to tankwell (Eg waterbags etc so you can fill and empty to suit), this should help trim boat backwards. trimming backwards helps control a kayak in strong following seas, dont know how it affects an Island though.

Raising nose is usually slower though than nose down, so its only done to dampen the effects of weather, and prevent nose diving.
Author:  walt [ Thu Sep 07, 2017 6:39 am ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
Displacement sailboats can get up in the vicinity of their theoretical hull speeds https://en.wikipedia.org/wiki/Hull_speed Getting a few percent over in a displacement hull is fairly hard to achieve.

Using the equation in the link above (which is the standard equation typically used for THS), the Hobie AI at 16.583 ft has a theoretical hull speed of 5.45 knots.

But Pete just said in another thread that he had a peak of 11.2 knots. That is just a hair over TWICE the theoretical hull speed. The only way that happens is if the main hull is planning. If the main hull does not plane, your speeds would be limited to about 5.5 knots..

I have been up to speeds near what Pete got in my TI (deep reach sitting in the rear seat) and the main hull was definitely planning.
Author:  bluelaser2 [ Thu Sep 07, 2017 10:27 am ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
The usual formula for displacement hulls is 1.34 x the square of the waterline length.

However, as Bethwaite points out in his book, long narrow hulls (like catamarans and tandem kayaks) easily surpass that ratio while remaining in semi-displacement mode. A TI can be pushed through the water at 12 knots yet not be supported by dynamic water pressure on the lower hull surface (i.e. planing). That's why catamarans like a Hobie 16 are so fast in medium-air, but the fastest hull-borne boats are monohulls like the Australian 18-foot skiffs.

One of these days I'll get some video of a TI hull at speed :P
Author:  walt [ Thu Sep 07, 2017 11:10 am ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
Quote:
yet not be supported by dynamic water pressure on the lower hull surface (i.e. planing)


The main hull of the TI has a lot of fairly flat area throughout its length and I think does get "lift" from dynamic water pressure similar to a planning hull at speed. The Ama's are more like the cat hulls. The TI is not a "high aspect" very efficient planning hull like the formula windsurfing boards but it does get lift. Ive owned a H14 and H16, have a C15 (planning dingy) and spent years on planning windsurfers and my opinion is that the TI main hull occasionally planes. But it is of course just an opinion since what exactly is planning doesnt really have a firm definition.

Edit.. Pete's speed in the other thread was in MPH and converting it to knots is 9.74. That is still a HUGE 78 percent over the theoretical hull speed of 5.45 knots.
Author:  AccessAdventure [ Thu Sep 07, 2017 11:26 am ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
Quarterdeck is the way to go IMHO. I fashioned one out of a 3' wide plastic "step" (the stackable exercise kind" that I found. Cut some bits out to make it fit over the ama brace and hold out arms, and WOW did that boat start to fly! Having the weigh aft, but also able to quickly shift from port to starboard on tacks to level out the Island made a big difference in speed...and fun! It was nice to get a higher perspective on the waters ahead. Now if I could just figure out about reaching the peddle drive to push thru coming about...
Author:  PeteCress [ Thu Sep 07, 2017 4:46 pm ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
bluelaser2 wrote:
The usual formula for displacement hulls is 1.34 x the square of the waterline length.
My understanding is that 1.34 represents the hull's "K Factor".

Different hulls have different K factors, but 1.34 is a commonly-used general-purpose approximation.

Reducing it to the ridiculous, think about the K factor of a telephone pole:

  • Going through the water long and skinney like most would expect, it would have one K-factor.
    .
  • Going through the water sideways, it would have another K-factor.

Lightning sailboats one K factor, Hobie 16's have another, higher, K factor.
Author:  PeteCress [ Thu Sep 07, 2017 4:53 pm ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
AccessAdventure wrote:
Now if I could just figure out about reaching the peddle drive to push thru coming about...
From somewhere in the mid teens to somewhere in the twenties, I stow the Mirage Drive because the boat really comes alive without it in those winds.

For when I need to come about instead of gybe, I use this bad boy, http://www.foxworxpaddle.com/holopuni_classic.html, which doubles as my get-me-back-to-the-beach steering if/when the rudder fails.

It's nothing anybody would want to paddle any distance with, but it has a good, solid bite when needed for a few quick come-around strokes and once one gets the knack, it steers the AI quite well going anything up to 8 MPH. Above that, I just reduce sail to make the paddle work easier.

When the wind gets into the high 20's, I put the Mirage Drive back in because it's such a beast if/when I have go go directly into the wind - as in launching off of an onshore beach.
Author:  AccessAdventure [ Fri Sep 08, 2017 11:41 am ]
Post subject:  Re: AI: Has anybody tried moving their weight aft in heavy a
Thanks for the advice on stowing the drive. I'll try to post pics of my improvised quarterdeck.
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