Yacht Aerodynamics

[monrosm@shrewsb]

I have a university project based on Yacht Aerodynamics, and would appreciate any knowledge anyone might have on the subject.

The project is only looking at the components ABOVE the water line so no hydrodynamics involved however as far as I am aware very little work/research has been done on components above the waterline as they offer comparatively less resistance than any wetted surfaces.

The project applies to yachts of 50ft and above predominantly motor Yachts but don't hesitate if your knowledge applies to Sailing Yachts im sure it's useful. As more and more companies are thinking about fuel economy and performance in greater detail I think this is an area of Yacht Design that might come under some scrutiny in years to come.

I am aware that aerodynamics applies to anything travelling through air (not just Yachts) but am in need for any specialist knowledge or specific factors that apply to Yachts.

Thank you

 

[gonzo]

If you do some research, you will find many studies on the subject. This may be a start for you: http://ittc.sname.org/facilities.htm

 

[yipster]

if into Wikipediaism: http://en.wikipedia.org/wiki/Drag_coefficient
and top of the list S.F. Hoerner a popular writen book worth reading in full twice

 

[Yellowjacket]

First of all there are two aspects of aerodynamic drag that predominate in this case and that is areas of where the air is stagnated or separated.

Stagnation occurs on blunt forward facing surfaces, and separation of flow around the rear surfaces results results in low pressure on the aft facing surfaces which results in drag. With a yacht, there might be some areas of stagnation, but most of the drag results from separation on the aft facing surfaces. Unless you are willing to make the aft vertical surfaces taper to eliminate the separation you aren't going reduce the aero drag by much.

Once you get over 50 ft, you aren't likey to be going all that fast (like somewhere around 30 knots maybe, and likely less for crusing) and hydrodynamic drag will likely be several orders of magnitude greater than aero drag anyway. Since aero drag is a speed squared function and power is a speed cubed, going faster starts to cost big power, but only if you are going a lot faster than a typical yacht.

So while improving the aerodynamics will save you some fuel, you are spending probably 10 times as much for the fuel to overcome hydrodynamic drag and there are bigger payoffs that can be had by reducing that aspect of the problem.

If you really want to save fuel, get agressive on reducing the weight of the craft and that will provide lower hydrodyanmic drag.

If you are talking about going real fast (greater than 50 knots), then shaping of the aft body (think of a rear body shaped like a fastback) could be done to provide lift as well as reduce drag and that might be worth something, but that's a whole different subject and something very different than a 25 knot 50 foot sport fisherman.

 

[Petros]

you might benefit from looking at research of reducing drag for land vehicles. Same principles apply since the air only sees what is above the waterline. Some of the "ideal" shapes are Reynold's number dependent, so the size and speed of your model may have some influence.

Although your design project is related to a motor yacht, I think there are very significant benefits to be gained for a sailing vessel because there is so many cables, lines, rigging and crap out in the breeze, most of it in a place to affect the flow over the sails.

 

[jehardiman]

It is important to remember that it is not only speed of advance but any local wind also. Aircraft carriers often have 50-60 knts across the deck and a sailing yacht under bare poles may see 60-70 knts in a violent storm (Force 10).

As Gonzo said, there are many studies by SNAME, RINA, and others as this was an important topic in the pre-jet ocean liner days (many liners had enclosed promenade decks for this very reason). Saunders covers it well in HSD as a part of overall ship design.

 

[MechaNik]

It seems that a lot of fast big boats don't bother about aerodynamics whereas sail rig designers dedicate a lot to this area. So is wind resistance really a factor for yacht design.
However some friends and I always wondered if superyachts like Ermis 2 and Sussurro would have been more efficient/faster with less wind resistance?

 

[Tim B]

Styling usually has more to do with topsides on a motor-yacht than anything else. The whole point of having a big boat is to get noticed! If I remember correctly, Sussuro was fast because it came out slightly below the design weight. And this is key in addition to the Aero considerations. If you can't build it light you'll lose any advantage that you gained.

You can gain a bit by streamlining the front, but the stern will always be pretty bluff, and this is what causes the problems. For some strange reason people want to put decks, saloons, flybridges and all sorts of rubbish at the stern.

For pretty optimal deck forms look at the Class1 offshore powerboats. you won't get much slicker than those.

Tim B.