Thruster streamlines?

dmilovich · Joined: 19 Jul 2009 Posts: 99

Got a strange thruster hydro-phenomenon I'd love some feedback on.

Background: Got a new stick, Fanatic Freewave STB 105. Like it a bunch. Really wanted tri-fin for shallow-water and shallow small wave riding, i.e, SPI and other places. Have now tried it with three different thruster setups, including stock fins as well as various weed fin/weed thruster setups. Ctr fins 20-21 cm, thrusters 11-15cm.

Sails well, but regardless of which fins, I always see what I'm calling a "streamline" of entrained air coming from the upwind thruster (only). Small thruster - small streamline, big thruster - big honking streamline that actually shoots almost a rooster tail back behind the board toward the upwind direction. Tried moving them forward and back, no change. Only goes away when I'm using a single fin.

Fins are all symmetric. Toe-in on board is .86 degrees per side (from K4 fin calculator checked with my rusty trig skills as well). I'd say this is just my board or setup, but when I watch the Fanatic 2017 Freewave promo video, I can see the same thing at brief moments.

Like I said, the board sails well, but this offends my aero-hydro sensibilities. Should I just deal or can someone give me some insights here to comfort me, or maybe make it go away?

PS. I assume it's occurring because the thruster angle of attack is negative compared to ctr fin and other thruster, and it's sucking air in under the side. Kind of interesting, suggests that overall board/ctr fin angle is maybe less than ~.5 degrees. I could try twisted K4 Ezzy asym thrusters and run them on the wrong sides to get no streamline and better upwind, but at the cost of loosing the thruster looseness.

skyking1231 · Joined: 10 Jul 2000 Posts: 280

it maybe that shape of tail causing it? If it sails well...than let it be.

my 2014 fanatic frewave TE seems to have what you are talking about...but maybe not as much. One thing i did notice, when i swtiched to makani fins thruster setup...vs the MFC...the makanis seemed to be much slicker through the water. It didn't feel like i was dragging three fins through the water...like the MFC. the makanis are thinner, and have a bit more flex at the tip. could try swapping out just the thrusters.

btw how do you like the 105 STBY ? what is the largest sail...or lightest winds you have gone out in ? how does it sail with a single fin...what size fin _

dmilovich · Joined: 19 Jul 2009 Posts: 99

The Makani thrusters, Walu mini, are one of the combos. Definitely slick, but not as much bite upwind because they are so much smaller than the stock (11 vs. 12, but MFC shape bigger). Used w/Makani NAIA 21 (nice fin so far!) to sail a 6.5, which is largest sail to date. (this on flat water, not waves). Combo not enough for 6.5, need more upwind performance,, but it's great for gliding through jibes and likely w/b great for waves. I can prob sail w/7.5, but haven't yet. I'm 175 lbs, 80kg.

105FWSTB great as a single fin board (6.1 w/Makani ONO 30 cm - nice fin as well) with filler plugs in the thruster boxes, still very loose and skatey. BTW, no streamlines with single fin, so thinking it's not the tail at all. Still new, only 2 days on it so far.

cgoudie1 · Posted: Wed May 17, 2017 11:04 am Post subject: Re: Thruster streamlines?

Hey Dimitri,

My OO does this occasionally as well, but the Thrusters on it are
canted and toed. Interestingly, it seems to clear up when I'm jibing.
I just stopped looking at the tail while sailing. ;*)

Sort of surprising that thrusters without toe would do that. It could mean
that your board is crabbing a little too far sideways and the incident angle
on the (small) thrusters is causing them to stall. What happens if you
use a much larger center skeg? Or, what about taking some #600
grit sand paper to the thrusters to get some water stuck to the surface.

.01

-Craig

dmilovich · Joined: 19 Jul 2009 Posts: 99

Hey Craig! Ah, they actually have almost a degree of toe-in on each side. I don't think the board is crabbing much at all (hence the less than 0.5 degree comment) because the thruster would be closer to 0 degree and neutral if it was. Also - note that I can see the streamline in the Fanatic video, so they are in the basic (MFC 20/ 2 x 12) setup as well my other fins. I will try larger center fin, but wanted the tri for shallow water conditions...

Maybe I need a fenced-weed-thruster? (kidding)

And yes, good tip, I should take my eyes off of the slipstream when jibing! Ha.

skyking1231 · Joined: 10 Jul 2000 Posts: 280

I was thinking the shape of the tail and the thrusters...is making 'rooster tail' ...vs a tradition tail more so.

dmilovich · Joined: 19 Jul 2009 Posts: 99

Thanks, Skyking, but it's actually like a fin spinning out (just a bit). The streamline is air coming off of windward thruster, quite clearly. Without thrusters, no streamline, thus it's not shape related, just fin related.

I'm puzzled.

U2U2U2 · Posted: Wed May 17, 2017 7:33 pm Post subject:

Try K4 Ezzy 2degree asymmetrical .

No one I know of has tried mounting asymmetrical fins the wrong way.

My Witchcraft has streams at times, I don't notice them so much, I usually look ahead.

You might contact Keith or Mike at CNC FINS , they are developing fins for quads and tri.

dmilovich · Joined: 19 Jul 2009 Posts: 99

Thanks, U2U2U2. Ordered some K4 fins just now and also dropped Keith at CNC fins a note.

U2U2U2 · Posted: Thu May 18, 2017 9:11 am Post subject:

Steve at K4 , would be a great source .

Bigyellow@k4fins.com

My only issues would be 2or3degree of toe, 2being a safe bet.

Mike at CNC , may suggest more toe, irregardless all parties I feel would suggest asymmetrical.

The new fins this season of free move// wave K4s, works well for me as the center in a tri fin setup.

I also use the SUP race fins as a weed Fin.