boats that are designed with car aerodynamic method?

You are avoiding showing calculations or experimental evidence to support your argument.
I want more than anecdotes.

 

Tunnels, I just can't figure out how do you arrive to compare cars to boats, when it comes to the importance of the aerodynamics?

1) Cars driving at a highway speed: around 65-70% of the axle power works against aerodynamic drag.
2) Boats running at common high speeds (35-50 kts) in calm water: around 5-10% of the shaft power works against the aerodynamic drag.

It gives a 10:1 ratio of importance of aerodynamic drag between cars and boats. So how do you arrive to state that boats' aerodynamics should be given the same importance it is given in the cars design?
Racing boats are, of course, other kind of animals, but you weren't referring to them anyways. At least not in your post #16.

 

You live in a world of mumbo jumbo and figures and calculations and computer displays that at the end of the day to an ordinary person are absolutly meaning less . so sorry to disapoint you !!
unless you make models and do windtunnel displays of what and how differant shapes work with smoke trails silk tell tails and what happens as you shift differant parts and change angles and the like and see turbulance over deck edges and behind square ends and shart edges edges then you get a really good understanding of what shapes work and what dont . add all the defects together and you have a small disaster but thats what we take as normal in our every day boats because designers dont know any better and they sure as hell dont want to change things and be seen as a outsider !!.

 

So tell me why do tunnel hulls perform and go fast and catamarans dont ?? that seems a pretty simple question !!!
im talking 25 foot power boats not yachts !! :?:

 

That's why I want to see drag comparisons based on calculations OR
experiments. I agree that it is an enormously complicated flow to be handled
by computer models if you want great detail.

 

Show us two equivalent vessels and I might be able to answer the question.
Otherwise you might not be comparing apples with apples.

Why do foil-borne craft go faster on-foil, than when off-foil. After all, the
air drag is much less for the latter.

 

No you draw the boats you want to evaluater one a tunnel and one a cat the same length with etc and do your majic on the pair of them !!

Last small boat (12 foot ) was a tunnel race boat we built a small model 600 mm long and used a small wind tunnel and a video camera and was able to start with our basic shape and then added to it with modelers clay to get shapes and rounds and curves locations and where they were to be located etc etc !. was a very interesting subject to work on !! and the reall boat when it was built went absolutly beautiful !! even on a windy day at over 80 mph it was predictable and did nothing dangerous at all went round righ angled corners like it was on rails and never dropped any speed and that was an enormous plus !!
While i been away to here in China all Has been packed and in storage so could be back there in nz wee while and hope i can draag it out and get it all working again !

So will a jet fighter plane stay stationary on the run way with just its brakes on and the jet motor running at full power !! or will it simply slid and pick up speed rather quickly and take off !!
whats your answer > yes it will take off or no it wont ??
Theres only one answer ! any one want to have a shot !!

 

tunnels,

can you show us a picture of a tunnel boat?

 

Tunnels, then you have to decide if you want to discuss 35-50 mph boats, or 80 mph ones. The importance of aerodynamic component of boat's resistance strongly depends on that factor. You have started this discussion with 35 mph example, hence the replies so far.

 

What ??

Speed is no important be it 20 ,30, 40 ,50 or 60 mph or even higher speeds !!you are completely missing the point !! shape make things move through the water and through the air much easyer and with very little effort . dont you get it ?? what is the problem ?? is it all to simple to understand !! Whats the problem ??

 

I am sorry, you are the one who is missing the point of the replies you've been given so far, starting from PAR's post #10 till the recent ones - like Leo's #22.

The point is - you should not consider the absolute value of aerodynamic resistance but the relative one. Relative to what? Relative to the hydrodynamic resistance.
Your example with the hand out the car window - let's say you're not in the car but in the boat. Stick your hand above the windshield at, say, 30 kts speed. You can clearly feel the aerodynamic force pushing on your hand, just like you said.
Now try to dip your hand in the water, at the same speed and do the same movements you did in the air. Your hand will be like pulled by an invisible giant! The hydrodynamic force will be much, much bigger than the aerodynamic one at the same speed.

That's the point you're failing to grasp - the relativity of it all.

So if you know that by far the biggest force to overcome is due to hydrodynamic resistance, and that the most subtle aerodynamic fairing wouldn't give you more than a couple of kts of max speed difference, how much effort would you put into that fairing? And how much would it pay-off in terms of boats' market value, when compared to a more boxy one but which gives berthing to 6+2, with all the whistles and amenities for comfortable living aboard?

To not talk about the seaworthiness, which you are completely dismissing in your argumentation...

Oh, and let's not compare tunnel hulls for racing, which weigh 400 kg for 20' of length, and powerboats for usual leisure use, which are around 1000-1200 kg for the same length. Being in the world of racing boats, you should know well how much that kind of weight difference count from the powering point of view, and the difference is ALL due to increased hydrodynamic resistance of heavier boats! To me it looks like you have a very particular kind of boats in mind, when discussing these issues. But if you think more generally, you'll realize that the requirements which a commercial or a live-aboard vessel must satisfy will go much beyond the quest for a sheer speed and fuel efficiency. Whether it is a fortunate or unfortunate fact is left to your personal point of view about boating.

Cheers

 

Air resistance is logarithmic, so the speed matters a lot. Also, it will change dramatically in proportion to wave/skin resistance once the vessel is in planing mode. Many tunnel hulls ride on the compressed air between the hulls. There is a minimum speed for the effect to start and a maximum when the boat starts flying out of control.

 

Many tunnel hulls ride on the compressed air between the hulls. Yes i totally agree but what is one of the contributing reasons they flip over backwards ?? hydro and aero dynamics with tunnels both go hand in hand and they need both to work in harmony together !! they need to fly !!!!

Hydro dynamics ! i have spent hours and hours with yacht designers and powerboat designers discussing there theories and understanding what they wanted and why
With boats upside down filling fairing and introducing speed bumps in places and sanding to get the desired shapes that when pointed out all make sense . Yachts in the area from behind the keel to the back edge of the transom

Tunnel boats with multipal steps in the bottom and changing the angles and adding a slight hook in places to deflect water flow to help minimize skin friction and airate the water !!
Two completely differant types of boat one doing just a few knots and hardly moving in light breeze and the other high speed in the 100mph plus not only circut racers but also off shore boats . trick and things learned over long periods of time from past exsperiances . things not written but handed on from one person to the next.

Have spent weeks sanding filling ,fairing changing shapes and adding to places all in the search for that extra one or two knots !! fast boats or slow always searching .

 

Good for you, but what does it have to do with your previous comments about aerodynamics, cars and airplanes?

 

Oh yes the fairing the sanding went on ,on the upper surfaces as well but mainly on the power boats not so much with yachts ! Its more channeling spray and water off the decks as quickly as possible !!
The smaller race boats yes we used firm but flexable batterns to get smooth flows from the front to the back no unessasary bumps and humps or things jutting out where there is no need ,small upstands to channel airflow down the side of a deck rather than letting spill over the side . laping joins all with flush fitted speed clips , moulded perspex canopy covers reinforced with multi layers of carbon fibre to form a frame , small push out air scoops to bring fresh air into the cockpit .