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Twin fin sailboards
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beaglebuddy



Joined: 10 Feb 2012
Posts: 688

PostPosted: Sat Sep 14, 2013 6:51 pm    Post subject: Reply with quote

On some companies website I was reading about this ratio you are referring to for multi fins over a single fin. In their illustrations they were indicating how the fins needed more area when heeled over like in a jibe or a wave, is this what you are referring to?
My main concern if getting enough lift to plane in flat water while mowing the lawn, do those little side bites actually contribute some lift?
I always assumed a thruster setup was for waves and the side bites were for traction. Also the toe in confuses me.
Adding two side bites and using a weed fin would certainly be the easiest way to go.
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U2U2U2



Joined: 06 Jul 2001
Posts: 3056
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Sat Sep 14, 2013 8:56 pm    Post subject: Reply with quote

beaglebuddy wrote:
On some companies website I was reading about this ratio you are referring to for multi fins over a single fin. In their illustrations they were indicating how the fins needed more area when heeled over like in a jibe or a wave, is this what you are referring to?
My main concern if getting enough lift to plane in flat water while mowing the lawn, do those little side bites actually contribute some lift?
I always assumed a thruster setup was for waves and the side bites were for traction. Also the toe in confuses me.
Adding two side bites and using a weed fin would certainly be the easiest way to go.

Maui Ultra Fin, MUF have a chart on their site, with a percentage needed to go from a single, to twin, 3 fins or quad arrangement. Other fin companies, quote the size of say 3 fins with the total volume, Maui Fin Co.

Black Project fins, BPF, scroll down have this chart::
http://www.blackprojectfins.com/products/windsurf-fins/wave-quad/#

these are guides and should be treated as such , so not written in stone.
You are not going to find a fin size or area posted for your board, noone makes it.

Of course the sides fins make lift . They do quite a lot, because the fin area is increased, you have more to push against, and can go upwind better, because the fin depth is shorter the board will turn much easier, this in part because all the area//fins are not involved in the turn as a long single is, the outside fin is almost neutral.

Toe, is being added to production 3 fined boards and quads now, a few years back it was considered BS/hyle and a unproven theory, not so anymore.

see this for a simple version of TOE:
http://www.tactics.com/info/guide-to-surfboard-fins

K4 , a UK composite plastic fin company now have versions of fins with either 1 or 2 degree of toe, these are also asymmetrical, so the inside foil is different that the outside, its either a flat or shaped VS the foil that you see on most windsurf fins.

Windsurf board in 3 or 4 fined versions have been done with no toe, and seem to work well, all mine have 1.5--2.0 degrees, part of the thinking is what fins you may use, and then which box will accommodate them.

The addition of sidebites on your board to make a 2+1 setup( small sides and larger rear, VS thruster type where all 3 are similar or the same size) would be simple. The 2+1 would be the way to go, IMO, the tailwidth and bottom shape would come into the equation.

_________________
K4 fins
4Boards....May the fours be with you

http://www.k4fins.com/fins.html
http://4boards.co.uk/
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isobars



Joined: 12 Dec 1999
Posts: 14050

PostPosted: Sat Sep 14, 2013 9:08 pm    Post subject: Reply with quote

This whole thread presumes your hull is always ≤ X inches below the surface and the reef is always ≥ Y inches below the surface. Are you willing to bet your fin ... and your face ... that either is true?

Mike \m/
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beaglebuddy



Joined: 10 Feb 2012
Posts: 688

PostPosted: Sat Sep 14, 2013 9:22 pm    Post subject: Reply with quote

isobars wrote:
This whole thread presumes your hull is always ≤ X inches below the surface and the reef is always ≥ Y inches below the surface. Are you willing to bet your fin ... and your face ... that either is true?

Mike \m/

Well I snorkel and swim the whole area frequently so i'm pretty sure of the depth and aware of the tide, i'm mostly clear except for a few coral heads that pop up when you aren't paying attention i just need some more clearance for comfort so yeah i'm willing to bet on it.
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beaglebuddy



Joined: 10 Feb 2012
Posts: 688

PostPosted: Sat Sep 14, 2013 9:39 pm    Post subject: Reply with quote

U2, "Toe-in causes the water to pressure the outside of the fins, which in turn makes the board more responsive to rider input"
So for mowing the lawn on this 198L free formula type board I'll assume toe is is not necessary or desirable.
And zero cant as well?
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beaglebuddy



Joined: 10 Feb 2012
Posts: 688

PostPosted: Sun Sep 15, 2013 4:02 am    Post subject: Reply with quote

slinky wrote:
I'd go for the thruster mod as U2 suggests. Works great on my windsup. Makes a 10' board turn like an 8'. Not very fast, but lots of fun

A 10' Exocet windsup? Did you use the Probox system? What size fins and sail are you using?
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beaglebuddy



Joined: 10 Feb 2012
Posts: 688

PostPosted: Sun Sep 15, 2013 4:51 am    Post subject: Reply with quote

I was checking it out today, the tides vary from about 0 to 2', the tide was about 6" and there was about 22" depth over the reef.
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slinky



Joined: 24 Aug 2007
Posts: 412
Location: Old Saybrook Ct.

PostPosted: Sun Sep 15, 2013 6:57 am    Post subject: Reply with quote

beaglebuddy, My board is a Starboard windsup, 10' x 34" [86 cm.]. It has an a box, and two small 3" long slot boxes, not sure what they are called, for the " thruster" fins, which about 5" long. I sail it with a 10" Flat Island weed fin mostly in 5-15 mph winds. The board will plane in about 12mph wind using a 7.5 meter sail.

I have another board, a 145L Tabou rocket that is 245 cm. X 76 cm. I sail this with a 15.25"True Ames Shallow water weed. This works great for me with either a 7.5 or a 9.0. I'm a lot lighter than you at 160 lbs.

You probably would be fine with just a good weed fin on there. Try that first. The 15" shallow water weed should on be good with an 8.0 on your board. This is 10" less length than the stock 65 cm. fin
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U2U2U2



Joined: 06 Jul 2001
Posts: 3056
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Sun Sep 15, 2013 9:24 am    Post subject: Reply with quote

beaglebuddy wrote:
U2, "Toe-in causes the water to pressure the outside of the fins, which in turn makes the board more responsive to rider input"
So for mowing the lawn on this 198L free formula type board I'll assume toe is is not necessary or desirable.
And zero cant as well?


All my conversions , whether quad, tri , trailor tri fin have toe on the new boxes. With the toe the board turns better, you don't constantly sail in a straight line do you ? I am not familiar with the larger SUP or Hybrid SUP, having a mast track in regards to IF they have toe, I would suspect they do. My Witchcraft tri fin has side fins set with toe, a benchmark in multifin design.

Having said that I wound not hesitate to use NO toe, and toe being just one of the factors to consider, fin placement is , cant, I use none, but adjust the box on installation to set 90degrees with the bottom deck, so not canted either way, if it has a deep concave or vee, allow some cant to be level

If you install side fin boxed with NO toe, you can purchase K4 fins NOW,
that have 1 or 2 degrees built in, so you can try both ways, for a fin price which is very reasonable ( cost to USA less VAT about 17%, so price in UK pounds is about the delivery cost) These are not yet available in all boxes.

http://4boards.co.uk/windsurf/fins


You could very well try a shallow water fin and or weed fin and get the depth clearance you desire, or not

_________________
K4 fins
4Boards....May the fours be with you

http://www.k4fins.com/fins.html
http://4boards.co.uk/
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dsgrntlxmply



Joined: 16 Jun 2010
Posts: 227

PostPosted: Sun Sep 15, 2013 12:01 pm    Post subject: Reply with quote

I sail two boards: an RRD Fireride 155L twin fin, and a JP Xcite Ride 134L single fin.

Following another local sailor's lead from my former sailing site, I have twin Unifiber/Lessacher Antiweed fins, 28cm, on the RRD. (I believe he was using 24cm fins on the 135L version of this board.) The twin 28's work well through my range of sail sizes: 4.5-7.3. They are not quite as good at shedding eel grass as I'd hope, but they are better than non-weed fins. The twin configuration does have an annoying tendency when going through rafts of detached grass, to pick up festoons of grass between the fins, which make the fins useless until cleared. I do not have the skill level to comment very usefully on comparative turning performance.

The JP has a single 36cm straight fin, which works better for me than the stock 44cm fin. I was sailing this board earlier in the season in higher winds, before the eel grass was as much of a factor.

Both my former and my current sites go very shallow, so the twin fins continue to be especially useful in allowing more sailing time. I have heard claims that the RRD Fireride twin was built first for Lac Bay at Bonaire, which is shallow, with coral.
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