Is boat design a guessing game?

[aquafiend65]

Let me qualify that question. I have spent the past two months trying to find answers to design questions on this forum and others. (see my threads on wave anatomy and hawaiian outrigger). the funny thing is i have more questions now than when i started and i am unsure of the things i used to be sure of. my original question was simple. how do i make a canoe that holds six athletes go faster and surf better? but it seems that it is all subjective. is it all relative? or are there design "absolutes"?

[aquafiend65]

Oh yeah, i forgot to ask y'all. some one told me that if my human powered boat travels at say 10 knots then 1/3 of the speed is because of the entry/bow and 2/3 is because of the exit/release of the water off the stern. is that true?
he also said that if you have a wide flat stern it must have lots of rocker or the water will hold it like a vacuum. is that true?

[duluthboats]

Aq65,

The more you learn the more questions you have, I know the feeling. Are there any absolutes in hull design? There are a few and you find good explanations of them in Dave Gerr’s The Nature of Boats, or Ted Brewer’s Understanding Boat Design. Ted also has some good stuff on his web site. http://www.tedbrewer.com/yachtdesign.html Where it all starts getting fuzzy is when you mix in materials, use, individual preferences, and appearance.

I’m just an interested amateur but will offer my $.02 without any firm answers. The history is long for the outrigger canoe; it was developed for very specific needs. If you improve on the design, it will likely be from refinements related to your area of use.

Keep asking questions.

Gary

[SailDesign]

Harrrummmpppphhhh........!!
Boat design is certainly not a guessing game! It's more like..... say..... Poker. You know some things absolutely, but the way small changes work together can be deceiving, sometimes disastrously so. Using computers to "tweak" some parts of a design can also be embarrassing (but I'm not going there...) as the computer cannot calculate the effects of pitch and yaw on things like foil sections. Experience has to be used to allow for this, which brings us back to poker, kinda.
Gary is right - advances tend to be made from small refinements, which can turn into larger ones if you're lucky.
Steve "still guessing"

[CDBarry]

"On the Internet nobody knows you're a dog" (New Yorker "Dog Cartoons" calendar).

On every forum, medical, animals or whatever, there are any number of people ranging from very qualified to not at all. There is no good way to tell the difference unless you are one of those who know at least somewhat.

There is a great deal of non-guesswork about hydrodynamics, including human powered vehicles, and the interaction of hulls with waves and so on.

Unfortunately, it is a difficult question, and requires real analysis, not rules of thumb. Usually a given feature does two things, one good and one bad, and you are looking at the difference, so both have to be calculated accurately.

There are hull form optimization systems that work very well for what you are interested in, see Noblesse's papers in 2002 (or so) SNAME Transactions. Nelson and Marek did a lot of good work on the effect of surge added mass on rowing shells and designed some very good ones. At a simple level, Saunders Hydrodynamics in Ship Design always good advice.

[SailDesign]

Are you calling me a dog, Barry?

Yes, hydrodynamics has some very hard-and-fast rules, but the effect of waves on smaller boats can be very complex. You only have to look at the different shapes of indigenous boats, say US East Cost vs. English Channel types, to see that the local wave shapes and heights and frequencies can "define" what's best for a region very markedly.
Rowing shells are a classic example of a boat that can benefit from extensive theory, since they rarely have to deal with large waves.
Steve

True with regard to large stern for displacement hulls because the stern surfaces are not "pressurized" by the water as it doesnt stay attached to the hull at aft exit. Dimpled golf balls work on the same princiiple. The dimples create a turbulent boundry layer that attaches better on the backside of the ball and pressurizing the back side better than a smooth ball. Pressure front and back= no drag. Pressure front and not on back =drag.
For a hobiecat type hull, look at the volume and loading and aspect ratio and then size for your expected weight as a first thought. planing is probably out of the question. I would look at the hull type you are already on to.

WRT the surfing, sounds like empirical design evolution is the way to go. Would be interesting to look at the crossections of a hawiian canoe.