Lowest Aerodynamic Drag Foredeck/Hull Shapes

[Bill Roberts]

Foredeck shapes?
Has anybody seen any test results, real data, numbers, that show or prove there is a boat speed advantage associated with downward sloping foredecks. I've heard talk that there is less aerodynamic drag. Most of these deck downward sloping foredecks that I have seen have an outstanding ridge down the center to split the water when the reduced volume forward hull shapes are pushed underwater. This ridge might reduce water drag with a bow/foredeck pushed underwater but what about aerodynamic drag for example sailing to windward, hulls are on their load waterline?
Where I sail and am sailing to windward, there is always at least a 6 ins to 1 ft chop. More normally it is 1 ft to 2 ft chop. This is the lumpy/bumpy surface that the wind is blowing over. Therefore the airflow for the first few feet above the water is not nice airflow made up of nice straight smooth streamlines. It is lumpy and bumpy flow reflecting the shape of the surface of the water.
Streamlining a body: Let's think about the relative wind blowing across a foredeck at about 30 degrees, windward hull and bow just touching the water. What is the cleanest aerodynamic shape to put on the deck/hull? Is it a house top type shape with a center ridge and rounded corners/edges where the deck and hull meet at the shear line? When we think about that center ridge and the airflow coming across it at about 30 degrees, that shape is pretty draggy. The airflow is not going to be attached to the downwind side side of the roof shape ridge and so it is going to be low pressure separated flow and draggy. It is a lot like the spoiler plates on a NASCAR car. In this case the spoiler plate runs along the hull/foredeck centerline with the relative wind coming across it at 30 degrees.
Question of the day: What is the foredeck/hull shape that would have the lowest aerodynamic drag coefficient with the airflow coming across the shape at 30 degrees? I use drag coefficient because that takes area of the side of the hull and hull height out of the question. I' just looking for the lowest drag aerodynamic shape for the deck/hull in front of the main beam.
Remember that the 30 degree off hull centerline airflow is blowing across the top of both hulls, two sources of aerodynamic drag.

[Bill Roberts]

HEY, WHAT'S UP?
Where are the comments?
Here's what I have heard from A cat World Class Sailors. They see no difference in speed sailing to windward. Downwind the boat is much harder to sail----- prevent/avoid pitchpole.
Aerodynamics: Suppose we had a shape of 1 cubic foot, 1 ft by 1 ft by 1 ft with sharp corners and edges. This thing is really draggy in the wind tunnel. Now we are asked to reduce the drag. What do you do and keep the basic shape of 1 ft by 1 ft by 1 ft? Well we could round all of the edges and corners with a 1 ins radius. WE go to test and drag reduced, yea. Now we are told to reduce the drag even more. What do we do? Increase the corner and edge radius to 2 ins. Go to test and drag reduced, yea. Now we are told to reduce the drag even more. What do we do? Increase the corner and edge radius to 3 ins.
WE do this and go to test. Drag is reduced even more, yea. Now we are told to reduce the drag even more. What do we do? Increase the corner and edge radius to 4 ins. WE test again and the drag is reduced. What are you going to do now? Oh I know, go to a 5 ins radius. Measured drag is down again. What do we do next? Go to a 6 ins radius. Test, and the drag is the lowest tested so far. What is the shape of our rounded edges cube? IT IS A BALL; IT IS A SPHERE and its diameter is 1 ft and the aerodynamic drag is as low as it will go!
Well, what is the shape of the cross section topside of any Supercat product? It is an ellipse, very very close to a circle. The deck and the side of the hull come together so smoothly you can't even find the line. The ellipse shape deck on SCs is a circle that is slightly taller, a small fraction of an inch taller, than it is wide. The ellipse shape will split the water better than a circle at verge of pitchpole. The difference in aerodynamic drag betwwen a circle and a SC ellipse with the relative wind crossing the hull at about 30 degrees is so small that it can't be measured in the airflow field crossing the forward hull section at about 30 degrees.
On SCs we have a Playstation version II style front end, tall with lots of volume, on our boats. Slope the foredeck about 1 ft downhill by the time it gets to the bow and you have a pitchpole prone forward hull shape with a lower top speed. WHICH ONE DO YOU WANT??????

[T Peterson]

Hi Bill. I am on vacation and got bored (go figure) and decided to check in on this forum. I saw the post, gave it a day of thought, and got up this morning to post a reply and you beat me to it! To my everlasting credit, I had postulated a round hull shape to be the most aerodynamic but less help in a pitch pole and less help when going to windward (I have a 17 with no boards) so since most designs are some sort of compromise I decided what I had on the 17 was best. A slight puzzler to me however is that the 27 and 30 foot versions of Supercats seem to have flat top decks. I am going to guess that with the weight and power of those boats that aerodynamics are a smaller factor and with the hull length pitchpoling is less of a problem or maybe it is a construction element and you know what, I have no idea. What gives on the larger boats?

[Bill Roberts]

Hi TP',
Thanks for your comments. The 27 and 30 are totally different boats from the 15 thru 22. They are aimed at a totally different market. These two boats have two large tramps; lots of room. Therefore they can carry several people at one time.
These two boats are excellent day sailing boats. They can carry 6 to 8 people, no problem. Go day sailing with friends and a cooler of drinks and snacks and have a fun time. There is a Bimini top available for shade, 9ft long by 14 ft wide.
For overnighting get a pop up tent at an outdoor store for $100.00 . These tents are great. They have 7 ft headroom with a 8 ft by 8 ft base. They have a waterproof top and misquotes netting sides or vinyl sides. They come with a waterproof floor. Folded up these tents are the size of a large bag of potato chips. Sail to your destination and stay on the boat or set the tent up on the beach. Take a long walk on the beach; have a small campfire. Have fun on this day sailing boat. Use it as a swim platform. Children love this boat.
Now why the flat decks? This boat is large enough that many people, passengers, will likely be stepping onto this boat from a dock. Flat deck is needed for safety and ease of boarding.
More on flat decks and pitchpoling on the 27 and 30. The deck on these boats in front of the front beam is very close to an elliptical shape. So no problem pushing the bow underwater but it is time to back off the main sheet a little. On any boat that has a front beam, when the majority of the length of the front beam, 50% or more, hits solid water, the boat will pitchpole. The drag on the front beam is large enough when pushed under water that the boat will slow down quickly and pitchpole. Therefore flat decks behind the front beam are no compromise to pitchpole.
Now, this daysailing boat, this cruising boat, can also be raced. The RC 27 and 30 are truely cruiser/racer boats.

More early thoughts on the RC27. It could have been built with no front beam and tramp. The hulls would have been built with a horizontal bulkhead at mid hull height bow to stern to make them strong enough to handle the hull side loads, forestay bridle etc. The topside decks would have been elliptical bow to stern. Now the boat would be a three man racer and that's all. So you can see the decision on the front beam was a BIG DEAL. It greatly increased the usage and applications of the RC27.

You know, beach cats don't lend themselves very much as yacht club boats. They are not a very social boat. Why???
You can't take extra passengers with you to go out and socialize and sip and talk and enjoy the day and sunshine etc. There isn't room. The RC27 and 30 break that barrier. Sit or lay or recline anywhere on the boat you want to; it doesn't matter. Just sail along at 5 to 10 mph and talk with your friends and have fun; let the kids steer the boat, they love it.

[J Drew]

I've always liked the nose on my 20, one of many reasons I picked it over a tornado or an A cat
The bows on the 27/30 are so narrow and sleek it's hard to think they would dig much, but it would be an interesting situation to see.
I'm sure a 27 would accommodate a group of people, but i don't know that many people I want to go sailing with all at the same time.
Based on the thought of weight, I'm more inclined to wrangle up a babydoll that knows how to sail and leave the pack behind.
Well, one step at a time

[Bill Roberts]

Drew, The most commonly used sales point for selling sailboats is the "RACER CRUISER". My point is that the 27 and 30 can do both jobs very well. The 27 and 30 are excellent Racer Cruisers. You don't have to fill the tramps up with people every time you go out, just take one or two couples with you when you want to . The boat can be used to party one weekend a month and race three weekends per month or maybe you prefer to party three weekends per month and race one week end per month. The point is the flexibility of use of the boat. I hear people say about my 30 is "that's a lot of money to spend on a boat that is just for racing". They don't understand. The sailing public will go out and spend twice the price of a 30 on a small keel boat because they can take one or two couples with them squeezed together in the aft cockpit. If they look forward, they are looking at the aft bulkhead of the cabin. If they want to see where they are going, they have to stand up and look forward. On race day they have to take big strong guys with them to turn the winches. Supercats and ARC boats don't have winches because they are not needed. The only tactic used in racing the slow keel boat is " if you are headed TACK, if you get a lift, DON'T TACK and this all happens at 5 knots max speed and by the way, if you tack, it doesn't cost you any distance lost relative to the other keel boats in the race. This is "let's pretend racing".

[J Drew]

Bill,
I agree, the 27 has plenty of room for versatility and I'm sure it's a lot of fun, but I'd l lean towards it's racing side, given the opportunity

While I'm sure that slow boat "racing" is fun for some, I know why I have the one I do

All in good time