boats that are designed with car aerodynamic method?

Wooooah, what a website!
That Rumpler Tropfenwagen at the end of the page is a little piece of artwork.

I don't wear a hat, so would rather choose that Delahaye 135 MS Torpedo Roadster: http://twistedsifter.com/2010/07/gallery-of-art-deco-vehicles/

 

It's one of the better compilations around because they have omitted a lot of
the junk.
And there's plenty there for anyone interested in strange planes, boats and
cars.

"Metropolis" was a very stylish movie for its day, so it's no surprise Lang used the most futuristic cars of the time.

 

Good engineering (by both people with degrees and people without degrees, but willing to "do the math") is frequently applied to understanding boat airflow and making good choices. The post 86 boats all look very smooth from max cross section forward.

This does matter some, but the real opportunity for drag reduction would be require more.

The unfortunate nature of boating is the a smooth front (the easy part) with true "fast back" design (to use the late 1960s car term) would drop drag a lot, but would be DOA in terms of sales. Boats of this category will not sell without the "open air" move around deck space and/or seating to the rear.

Super areo is actually easy for a boat. Think football topside up front, and teardrop profile to the rear.

Go ahead and get an NA to draw up this smooth boat with an enclosed cabin that sweeps down to near the water line on a narrow width rear transom. Understand that there will be little or no provisions for moving around anywhere on top with the boat out on the water.

With the above you may get a 5% - 10% overall drag reduction (water drag is huge compared to air drag). The "useful" index of the boat will probably be reduced by oh say 80%.

Plan B would be to take the above goal but design in the ability to retract, fold or or otherwise eliminate the rear half of the cabin on demand. Now you start getting back some usefulness, but complexity, maintenance concerns, weight and cost increase a lot. You are still less useful than "normal" and you start sacrificing efficiency due to weight and less clean exterior surfaces that go with the required compromises.

Plan C - Just get one of "better than average" designs similar to those shown in the previous post.

 

One of the aero issues boats face, is their exposure constantly varies, depending on trim angle and the wave they just flew off of. I can see a well shaped form, pitch up and double it's Cd, then fall off a wave and double it's static Cd again. Operating in anything but dead calm waters would drive any aero engineer nuts. High speed ocean racers do put a fair bit of work on aero, respective of it's value over all, in the resistance department (which as everyone except Tunnels knows, is miniscule in comparison to the hydro concerns). Again for pleasure craft which typically operate in a much smaller aero envelope (further decreasing aero's importance on the design), these considerations take a back seat to the other concerns, like a market nitch to fill or market demands, unless you listen to Frosty, where the hair brain designers, just take a guess at it and hope for the best.

The whole problem with this thread isn't that the original poster's questions haven't been addressed, they have, but a few who have demonstrated they don't have an understand of either aerodynamics, industry marketing or the way a design comes to market, stubbornly have clung to erroneous, misguided preconceptions, of physics or industry procedures.

 

Time to let go

Ignorance is a human right!
Why aren't you guys going out on the water. We have, at least here, such a beautiful fall. Go sailing, canoeing, angling - anything is better than this thread. Life is so short, don't spoil it with sour feelings.
Or wear a hat and follow Leo's post, it'll make you happy.
Uli

 

A number of posts in this thread argue that air drag is fairly insignificant compared to water drag and this argument seems to be made primarily with reference to fast power boats. However, in the third post in the thread the aerodynamisist Tom Speer provides a link to an article by John Shuttleworth which draws a very different conclusion, albeit with respect to a cruising multihull sailboat rather than a power boat.

The article by John Shuttleworth includes a diagram of force vectors for a fairly typical 50foot LOA cruiser/racer type catamaran sailing to windward with 19 knots true wind. This diagram shows that the total air drag (rig plus above water hull) significantly exceeds the total water drag (underwater hull plus keel). Furthermore, the air drag of the above water hull without rig exceeds the water drag of the hull without keel. So, if this diagram is even anywhere close to the truth, air drag of hull and superstructure will certainly have a significant effect on performance. OK, this is for a catamaran sailing to windward. If it were sailing downwind then obviously air drag of the hull would actually be slightly beneficial rather than a major hindrance, but my view is that windward performance is all that really matters with a sailing boat (I do admit to a bias here - I cruise in an engineless sailing boat!).

I have to say that I would have liked to have seen more detailed references to sources of data than is provided by John Shuttleworth in this article. John Shuttleworth makes only vague reference to some tank tests he commissioned at the Wolfson unit, also to a couple of his other promotional writings and to a couple of books by CA Marchae, but without detail of exactly what data was taken from these books. Having said that, I think the vector diagrams and table presented in this article are about the clearest and simplest summary I have seen for the main forces acting on a sailing boat going to windward.

I note that John Shuttleworths diagrams are for a boat making pretty good progress to windward, the keel is working at a favourable angle of attack and presumably the boat is about optimally canvased. A more critical situation for a sailing boat is beating to windward reefed down in very strong winds and steep headseas. The air drag of the above water hull and superstructure then becomes even more important relative to water drag since apparant wind is increased and speed through the water is almost certainly reduced.

I am not sure that I can entirely agree with John Shuttleworths proposals for reducing the air drag of the hull. He proposes a 'lightbulb' hull cross section. He says that 60% of the airflow towards the hull is deflected over the hull (good), 40% along the length of the hull then round the stern (not so good). I would have thought that if he had avoided the outward flare that the 'lightbulb' section has above the waterline then he could have reduced that 40% somewhat, also he would have more hull centreline separation for a given craft overal beam, but of course he also would have less internal width in the hull at ellbow level. I imagine that internal accomodation requirements are the real reason for the 'lightbulb' section. Other features of John Shuttleworths design do seem to make sense aerodynamically - no raised bridgedeck superstructure, minimum freeboard, topsides smoothly radiused into decks. Getting rid of all guard lines, stanchions and trampoline nets might help too!

John

 

Tried to tell you so !!

At last !!
I hope after all this time some people are taking note maybe hes not so silly after all !!!!

Over the years i have worked with some really bright designers and some awful ones as well .
Some are down right clever and there is not box to think outside of ,while other are not only in a box with high sides but have the lid sealled shut ! tight !!
Everyone has good and bad ideas some are workable and just need advice and a helping hand to make them work, but others are just plain not workable at all ,usually because of closed mind thinking . When you try to point out why it wont work they get in a huff and become abusive ( dont they !!).
We can read and study and learn all kind of things but there is a point where you need to lay down your books and pickup what you have learned and do you own thing and build upon . Taking whats written as the absolute gosple truth and cant be changed you are in for a big surprise because if your not prepaired some one some time will kick the stool out from under you and you will crash and hit the ground with a big thud , and if you think you know it all your are an idiot !! your not alone mind you !! eneough said !!
I wasnt going to revistit this sight till monday !

 

Thank you John, but the primary discussion was pleasure powerboats in their typical operating speed ranges. There's no doubt that vessels propelled by mother natures breath, designed to be efficient, particularly upwind, will have considerably higher aero considerations, especially in light of it's propulsion. If we take a 25' power power cruiser, traveling along a 30 - 40 knots and total up all the resistance, the figures for aero stand unsurprisingly short in comparison to the rest of the package. If we take a preformance oriented cruising cat, as in your example (okay Shuttleworth's), propelled by wind, naturally they're be a significant difference. One has a power to weight ratio of 10 pounds to each HP, while the 70' cat, say at 31k pounds (Blue Pegasus, for example) will have a considerably lower power to weight ratio, 155:1 under power and sail I estimate would be in the 80 HP range with her area (387:1), which justifies the argument, for maximizing the aero aspect portions of the design. Naturally, using a cat such as the one I've described, which is 2/3's to 1/2 the weight of comparable craft, really isn't a fair comparison, which just hammers the power to weight ratio further and decreasing the value of aero considerations in the over all resistance figures.

 

I refer you back to Leo's post #22, but add that I think it would be higher than his graphs indicate for a light boat with medium freeboard, a medium weight boat with a lot of freeboard, or a boat fighting a headwind. If you're going 20 knots into a 20 knot headwind the apparent wind of 40 knots will exert four times the force exerted by an apparent wind of 20 knots.

To Leo: I think the Aptera automobile is the result of a serious CFD driven design program. Before you mock - and if you are genuinely critical I will take you seriously - at least listen to this radio piece about it: http://www.studio360.org/2010/dec/31/aptera-car-of-the-future/. Unfortunately the company is defunct unless they get a new investor interested in reviving it.

 

It doesn't necessarily require major alteration of the design to achieve significant drag reduction. Whether we're talking about power boats or sailboats, reducing windage is still worthwhile.

I find the attached figure from Hoerner's Fluid Dynamic Drag to be really interesting. It shows that for a rectangular cylinder, it only takes a corner radius of around 10% to 15% of the cylinder's frontal thickness to significantly reduce the drag - by as much as 80% in some cases. Making it more rounded than that does not cut the drag by much more. If you were to apply this to, say, a cabin house, it means you could still have a flat surface over 70% to 80% of the house and get most of the drag benefits of a more rounded form. That's not such a big compromise in internal space, etc.

Of course, it requires a rounded corner on the lee side as well as the windward side, and that goes counter to what one typically does for a cockpit, because most people like to take shelter in the cleanly separated wake. So there are lots of practical considerations that lead people to not take steps that might reduce the windage.

 

OTOH, the Reynolds numbers for small boats will be lower, and therefore
skin-friction will be a higher fraction of the total to start with.
Also, those graphs are for vessels with fairly large, blocky above-water
portions, so the air drag fraction is probably a bit larger than for most vessels.

 

At last !! well done people , its taken a while !!

The effects of air movement and water flow over shapes is all having and effect on performance of what ever it happens to be and at last i think a few people are starting to take a little notice .
Shapes of everything is important . simply just rounds are not the answer tear drop and foils are much better .
Be they in water or air if it moves it has resistance , even stationary objects the wind and water currents affect everything . Sure small effects but add all those small things together and all of a sudden where did that figure come from??.
I dont have to think outside the box because there hasnt been a box for long time and i dont wear blinkers .
Yes i understand principles of boat design both on the water and above . I have worked on race boats both sail and power and learned heaps, non of the things i learned can you find in books so when you confront any seasoned designer and say this is not going to work very well you need to do that they really dont like being questioned at all , How dare you question my abilities , how dare you !!who do you think you are ??
How many times do you see this throughout so many of these threads ,all of a sudden theres a shadow appears and they stamp there foot !.

 

I had read about the CFD work on the Aptera several years ago.
I was being a bit sarcastic about the appearance of the Aptera, but I do
(seriously) think that the front wheels sticking out like that are silly and
easily damaged at low speed.

 

A bit off the original topic, but since it's there I take the chance to comment:

- I wonder how Shuttleworth has estimated the air drag - it does seem large to me. Did he allow for wind gradient & twist/shear? Much less wind at the freeboard-height level, and more directly from the bow (smaller AWA).

- When comparing air drag to water resistance, you should take the component of the air drag in the direction of the motion of the boat, in this case cos(27) x air drag. Hence my suggestion to discuss drive and heel instead of drag & lift for sailboats.

- In case of the Star as presented in ultimate validation, the air drag of the hull is nil - varying between slightly positive to slightly negative when the bow pitches up or down the wave. For boats with a higher freeboard, the drag is positive, but small - the drag of the crew sitting on the rail can be twice as large. When a catamaran or trimaran is flying its hull high in the air, the drag could be more appreciable.

 

And that's precisely the whole point of the discussion.