If a snuffer bag has that much windage that it causes a cat to run deeper then one without, what about the higher bows of the Viper versus the Blade. Would a Blade sail higher because the bows have less windage?

Gilo, I beileve that is part of the "concept" behind all the new cut back bows we are seeing on the newer A cats, F16's and F18's. Some call them wave piercing, but in fact, every bow is wave piercing, right? So it must be to reduce windage while out of the water.

Not only the reduced height but also the egdes of the hulls as the wind pass over them. Sharp egdes give more drag than well rounded egdes.

Welcome to the world of boat design. Every feature and descision is compromise. Big volume and high freeboard provide more wave drag and windage. Lower of these can cause beam slap and increaded wetted surface.

The list of compromises is imense. Most boats have gradualy drifted into a minor range varience where the whole thing works pretty well. Changes to stuff like this is really nit picky and of such marginal difference between plaforms as to be ridiculous. I doubt there are more than a couple sailors in the world who could actually tell a difference.

It all goes back to the ballance thing as Rolf said. Nothing you can do or change on the boat will equalize things exactly. Get it close and go sail.

If you worry about it, then it is slow no matter what. If you think it is fast then it is.

I definately agree 100% on that!

Rolf - You get your ticket to come over for the Global Challenge yet? We will find you a spot to rack up.

Lottery company have not called me and confirmed yet

(it is a money issue, and nothing else)

Rolf, I have never seen sharp edges on the bow of any boat/cat. They are blunt enough not to stall and generate lift when sailing upwind. So cutting them away for aerdynamic reasons is not as clever as some people think. You will find a chapter in Marchaj's book about some of the effects involved.

I have a furling genaker on my boat. I sailed the boat with and without it, never realised any difference, so I don`t care even if it is an easy exercise to calculate it. I wouldn't care about the drag of a snuffer as well, maybe about the blocked airstream to the trampoline. But only if I am bored and want to invent problems
Maybe somebody else has tested his boat with and without snuffer and can us tell the difference?

Cheers,

Klaus

I have the same Promlem.

Would love to come; Dosh precludes.....

Since everyone is racing with basically the same setup, does it really matter at all? Fugedaboutit and sail fast!

No, got to do the catering circuit for a year and get the internet business going. Hopefully next season. Being boatless is KILLING me.

Doug

So who died and named you spokesmen for the class?...

What a tetchy useless piece of drival, why is it that people can't see the lighter side of the internet and its forums, this is supposed to be a good natured humour filled piece of reading not a bun fight because someone is all pissed about dissing their particular class of boat.

Go back to the F18 Forum and play there before we all start dissing the F18 for bit of good wind up banter.

There is some references to pratical experience on sailing without spi gear earlier in the thread and on the F16 class forum. I think we can agree that there is a difference and that the difference is due to increased drag.(AAGH! I got to stop connecting drag with The Adventures of Priscilla, Queen of the Desert)

About bows and drag. You should know..
John Shuttleworth had an article about how he loocked at minimizing drag from wind passing over the hull in his Tektron and Dogstar designs. He reported that the optimalization done was worth the effort as far as I remember. The articles I read about the design and build phase of Cogito said similar things. They looked at minimizing drag from the platform.
Minimizing exposed area and rounding off surfaces sounds like a logical thing to do to reduce drag? Please do share your thoughts. Interesting topic and something that we can adjust as compared to drag from our bodies while trapeezing.
Is this drag tiny compared to going the wrong way or not knowing how to trim the sails.. Indeed

Doug,
hope you succeed and get finally get that F16

Built one successful before. I will be back up again...thanks Rolf. It is killing me not to sail...Although Ike took away our local place to sail. 1 1/2 hrs drive now

Doug

Rolf,

I am always very careful if people tell me something about minimising drag. I don't want to have a boat minimised for drag. I rather want to have a boat optimised for speed or efficicncy or whatever. To make it clear, a boat optimised for drag is a boat without a sail, or even better: no boat at all. And it should be clear that this should not be the target for any trade.

Yes, bows generate drag, and no bows generate no drag, however bows generate also some lift (like a sail) if sailing upwind, and as with sails, bows with higher aspect ratio are more efficient than bows with low aspect ratio. Apart from that hulls accelerate the flow at the low part from the sail by displacing a volume of air and reduce the tip vortex (at the boom) around the sail. As mentioned this is explained in Marchaj's book.
Last but not least, the hulls of a beach catamaran help to make the flow around the trampolin more efficient, hence it generates more lift and less drag (this effect might be offset with a spinnaker kit). The lift of the trampolin lifts the boat (slightly) and reduces hydrodynamic drag.

All of these effects are of secondary order. But it shows that optimising a single component (the bows) of a boat for a single reason (aerodynamic drag) doesn't guarantee an improvement of the whole system (which includes much more than just aerodynamics).

Regarding the drag of the sailor traping out: The same sailor would have about the same drag while sitting on board, but he interfers less with the sail when trapping out (although I don't know if this is good or bad). Again this is a second order effect.


A bit off-topic: When building your rudders and boards, you shoudn't round the tips. Maybe you know that already.

Cheers,

Klaus

Klaus,
Can you explain your comment on not rounding the boards and rudders. My blade boards are cut straight at the bottom with the same cord from top to bottom. I think the falcon's boards taper a little from the trailing edge at the bottom. The infusion's boards round from the trailing edge down to a straight leading edge and the wildcat tapers from both the leading edge and the training edge. I am just trying to get an idea of what each design is trying to achieve hydrodynamic-ally. Thanks.

Hi Chris,

I meant rounding the tip edge of the rudder, not the planform. The planform is what you see from your rudder, if you lay it flat on the ground. The wing tip edge is best seen if you look from the leading to the trailing edge (e.g. front view). If you make it round, you help the tip vortex, which is always there, to be stronger and hence more dragy. If you make it sharp, you hamper the vortex a bit. Hence rountig or fairing the tip edge is not good. A simple cut is better, just have a look at modern aerobatic aircraft (which have a symmetrical foil as our rudders).

In terms of planform you have some degrees of freedom from an aerodynamic point of view, many shapes are possible. Some people "believe" in elliptical shapes, however a simple taper ratio of 0.4 is as "good" (depends largly on your figure of merit, which is not only drag) as an elliptical shape, but much simpler to fabricate. There is a small "improvement" possible if you have rounded leading edge on the last 5% of span, as you can see on many airliners when not equipped with wingtip devices. Again second order effects, most important parameters are area, span and the section shape(s).
It is difficult to judge these rudders unless you know what are the most limiting factors (stalling at low speeds, cavitating at high speeds, structural strength), what are the design targets (optimising lift/drag) and what is design philosophy (rotating or sliding kinematics, designed to be loaded or unloaded when sailing straight) , etc.

When we broke the rudder of my friends T, we decided to keep the same area and span, but went for staright leading and trailing edge and designed a new section shape, which should have less cavitation, slightly less drag and better structural stability than a conventional NACA 4 digit section.

Cheers,

Klaus