beam windage

[Samnz]

Originally Posted by bruceb

I was involved with determining the difference in drag of a small wooden block on wheels compared to a streamlined shape. To make it short, at 8 mph, the shape drag was not much, at 12 mph, it became significant, and the plain block was drag limited at about 15 mph, while the streamlined shape could pass 20mph. A flat front beam looks the same as the un-improved block. I am sure the up-wind hull has a lot of drag as well as the beams, but it seems that any thing that can be done to lessen drag, should be. Rounding the outer deck edges of the floats and streamlining the beam fronts doesn't seem to weigh much and should result in some increase in speed- more as the boat size and speed increases. B

this is true but only if the beams are exposed to the apparent wind, which im not convinced at this stage they will. I will temporarily put some windex's on the beams to see what they register, if they spin around I will know that its sheltered?

[Gary Baigent]

Sam, it all depends on the overall beam of the multihull; an older, narrow tri will have the beams blanketed but a 6.5 metre wide by 8.5 m length tri or cat ... you'll find plenty of beam out there in the wind ... and your boat has quite high arched beams as well. Also a close winded, modern multihull carries the wind well forward; you're not talking TWA but apparent wind angle here. I'll give you one guess what your telltales will do.

[nordvindcrew]

I row in open water races in an open boat. How much would it help to close off as much of the cockpit as possible with a fabric cover to prevent a wind on the beam ( 15, 35 MPH ) working on both the outside of the hull and the inside of the hull? No specifics needed, just; none, a little, or a lot would help

[Gary Baigent]

High performance, open water, rowing skiffs should have rounded coamings leading on to as near completely covered deck as is practical (with opening for the sliding seat). Also the bow should be as low as possible too, yet still not bury into waves. And the rower should sit as low as possible, yet still be able to extract power from the oars. Here is my "knockabout to high end performance" 2 person skiff. A stretched version of this would really move in open water.

[Samnz]

Originally Posted by Gary Baigent

Sam, it all depends on the overall beam of the multihull; an older, narrow tri will have the beams blanketed but a 6.5 metre wide by 8.5 m length tri or cat ... you'll find plenty of beam out there in the wind ... and your boat has quite high arched beams as well. Also a close winded, modern multihull carries the wind well forward; you're not talking TWA but apparent wind angle here. I'll give you one guess what your telltales will do.

mnn im hearing you Gary, but would you consider 6.5m wide and 8.5m long to be a wide tri? Also how to calculate these apparent wind angle roughly from the sisterships speeds/angles, (guessing a bit here only had speedo no other instruments) im sure I could do it but dont have time flat out finishing the boat.

25knots TWS, 15 knots BS, Tacking thru 90
20knots TWS, 14 knots BS, tacking thru 100
15 knots TWS, 12 knots BS, tacking thru 100
10 knots TWS, 10 knots BS, tacking thru 100
5 knots TWS, 6 knots BS, tacking thru 110

I do know 20 degrees heel is about ideal upwind to have the main hull just kissing the water

[Samnz]

Vector Addition, Two Vectors... 2/3 of the way down this page...
http://hyperphysics.phy-astr.gsu.edu/hbase/vect.html

as you can see im to lazy to draw a diagram in Cad!!!!

says apparent wind would be 37 knots at 28 degrees in the 1st option I gave above, thats a lot of wind drag!

does anyone know what say a decent farrier apparent wind angle would sail at upwind?

does 28 degrees apparent sound good or bad for a modern Multihull upwind?

[captainsideburn]

"does anyone know what say a decent farrier apparent wind angle would sail at upwind?"

No idea.
However I just came in from riding my bike in a fair headwind and let me tell you even on something so small, the wind drag is phenomenal !!!
Surely as mentioned above, any fairing you do will have significant pay off.

[oldsailor7]

Originally Posted by captainsideburn

"does anyone know what say a decent farrier apparent wind angle would sail at upwind?"

No idea.
However I just came in from riding my bike in a fair headwind and let me tell you even on something so small, the wind drag is phenomenal !!!
Surely as mentioned above, any fairing you do will have significant pay off.

+1.

[Samnz]

Originally Posted by captainsideburn

"does anyone know what say a decent farrier apparent wind angle would sail at upwind?"

No idea.
However I just came in from riding my bike in a fair headwind and let me tell you even on something so small, the wind drag is phenomenal !!!
Surely as mentioned above, any fairing you do will have significant pay off.

what sort of bike you got? iv got a 2008 GSX650. Always amazes me how quickly it slows down from the air resistance on the motorway off ramps compared to my Pajero which has so much more air resistance but the weight of it keeps it rolling much faster.

Guess its similar comparison to a multihull and monohull, the heavier solower monohull will not notice the wind pressure so much.

However if the wind doesnt "see" the beams will a fairing make a difference?

the question is I guess whether the wind hits the main hull and veers off creating a high pressure zone where the beams exits the main hull?

[oldsailor7]

I am certain the apparant wind can "See" the beams, especially the windward one.
Flat fronts are draggy. (and daggy).

[captainsideburn]

"what sort of bike you got? iv got a 2008 GSX650. Always amazes me how quickly it slows down from the air resistance on the motorway off ramps compared to my Pajero which has so much more air resistance but the weight of it keeps it rolling much faster. "

Heh, well the bike I was talking about was my mountain bike, you really notice the wind when its your own body providing the energy

However my motorbike is a 2004 BMW f650CS, possibly the best ever commuting bike in my opinion

[fng]

Sam your frontal area drag is a given, what you need to do is reduce your pressure drag or separated flow.
By adding a nose cone of sorts you will be able to get the laminar flow to get to near on the rear of the beam, more cleanly than without.
Assuming that your tramps run off the bottom of the beams you are half way there. The trick then is to induce flow over the top of the front beam back towards the tramps, this could be as simple as sized rope bags or modifying the aft radius of the fwd beam.
I wouldn't be concered with trying anything with the aft beam at your scale, as it will be mostly separated flow by then.
Gliders, even slow ones will stall at near on 30 knots, most 40 knots, so you would need to be in the AC scale of things to use even the older ones as a yard stick for what your looking for
GP