Boat tails
 

Ok, so I got to thinking, funny name, boat tail, then I had a strange question. Why don't boats use them? I think I have seen something similar on racing yachts and catamarans but nothing on boats. Anyone know why?

boat tails
 

Think canoe,kayak.And your correct,America's Cup yachts use an International Canoe Hull.Catamarans,bingo! Also Yale racing sculls,sweeps.Torpedoes,submarines.The last 15% of a hull can be chopped off without alot of form drag increase and the skin-friction drag reduction is significant,since water is over 800 times more dense than air and much more viscous.Remove the tail from a wing and drag goes up 900 %.

So if I understand you correctly, by chopping off the back end of a pleasure cruiser (Like most are), you lose nothing? Except weight or additional mass in the water?

I've never been on a pleasure cruiser that goes fast enough to create serious drag.

I've been looking into electric boats and I've learned a lot about hull shape and speed/drag. Boats like canoes and sailboats are "displacement" hulls - they literally push the water around the hull to move the boat forward. A smooth transition at the rear becomes more important for drag reduction. Most recreational power boats are "planing" hulls with the boat riding on top of the water. The less hull in the water the less the drag.

Cars are definitely displacement hulls. ;) :)

chopping
 

It's not that you don't loose,it's just that it's an acceptable tradeoff.In my installment on aft-body/boat tails I mentioned an "embryonic transom hull"attempted on a racing yacht.It was basically a truncated boat tail,as Kamm recommends for cars,but the design blew up in the boat-builders face.By chopping the tail off where they did,they were attempting to "prune the wetted area",the most significant aspect of a boats drag(friction drag).But the cut off created so much turbulence behind the transom,the rudder was rendered useless.They couldn't steer the boat! So it appears that for boats, there is a limit as to how much of the tail can be removed,before diminishing returns.Bear in mind that this was a racing yacht,built for the America's Cup,built for high-performance,with no pretense as to practicality if used as a pleasure yacht.

Boattails
 

The name came from attempts by early body-builders of expensive cars to produce and aerodynamic product. Examples are the speedsters by Dusenberg, Cord, Auburn and others in the 1920's and 1930's. These cars had rear ends shaped like the rear hulls of fast boats. The expression eventually came to mean a gentle closing of the rear body to reduce drag by achieving a pressure recovery that increased base pressure and reduced the base area; both of which lowered drag. A good example 9is found in modern high-velocity bullets and artillery shells that have a tapered rear profile to a small blunt base. Interestingly, the optimum 1/2-closure angle is 15 degrees, which is the best angle for the boattail designs beginning to be seen on van trailers.

Kevin,Thanks for your injections of current data.Do you have an opinion of Mair's taking the boattail's convergent angle all the way to 22-degrees? All of the pioneering work of the 1920s and 1930s arrived at less aggressive angles for aftbodies,however,many of the Cds developed in those early investigations failed to hold up in more modern tunnels,and it makes one ponder,whether or not more can be done with less.I'd be very much interested in your insights.Thanks,Phil.

Yes, your average out-board boat has a truncated tail, but at speed, think about how much of the hull is actually submerged. A great part of the boat comes out of the water and "skis" along the surface. What water would otherwise create vorteces around the tail is already turbulated by the propeller and its effects significantly negated.

*edit* missed this post \/

A 22 degree angle will have separated flow. In the syudy on front and rear shaping the drag reduction was maximum at 15 degrees and had declined to by 20 degrees.