New Hobie Design?

Hello,

I have a client wanting me to produce a hobie-cat with a very different hull design. Take a look and tell me what you think.
He is a boat designer but, I have never seen this before.



He designed the bottom of the hull after the sailfish. One the he built in 1958 "never lost a race".
The designer is Dough Harrison

 

I guess you mean "beach cat" not Hobie cat??

Well - if you are a boatbuilder then take the money and build it. But you won't build a second one.

if you are the customer/owner/sailor then no, forget it

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard called it perfectly. If you are the builder then build it, but be sure that you get all your money first. This "design" has the potential for causing you untold grief. It will be the builders fault when the boat turns out to be a dog.

 

The shape isn't defined by the drawing, but it looks like a flat bottom with waves ? I wonder what the rationale behind it is.

 

JHF, I never saw a waved bottom like this. But the coke bottle form is well known in aerodynamics to make a better flow at the root of an aeroplanes (Jet) wing. Harmen Hielkema used it on one side of his proa: http://harmenhielkema.blogspot.de/2009_04_01_archive.html and http://harmenhielkema.blogspot.de/2010/09/answer-to-good-question.html and here another Proa http://www.proafile.com/archive/article/a_proa_for_ariadne. And please have a look at: http://www.boatdesign.net/forums/mu...new-multi-50-trimaran-46659-2.html#post622246. Would be interesting to do some tank tests, to find the lift and sucction areas and might be the benefits or the nonesense.

It must be a newer design than 1958 because the plan shows infos like: "Chopped carbon and epoxy". Interesting asymmetric boards.
Any more infos about the design of Douglas Harrisons catamaran ???

 

Zodicat

Thanks for the quick replies.
It is interesting that the fighter jet comparison came up because Doug Harrison worked for the air-force or navy (not sure) and he talked to me about that same problem they had. The answer was the sailfish shape. It allowed for more speed and cured many other problems.
Doug is now retired and not doing so well but, he always wanted to see this boat built. He made one in 1958 but, I think it was a mono-hull. The hulls on this version are hard-chined but, he thinks that is just fine.
If the potential for increased speed is there and the principals are sound, then maybe a prototype should be made. I would love to tank test a model but, I have no facilities near me.
The sail calculations and rudder etc. can be worked out and I could build a round hull design in strip fashion.
As far as the carbon fiber suggestion on the plans, he put that in there to increase strength but, I have a feeling that he really does not know of any other ways because of his limited building experience.

"unlimited speed" in a southern drawl ....that is what he sounds like.

Jim
Check out my site
www.innerbayboats.com

 

I have this other drawing from Doug. It has some pics and some ideas.
What do you think?



This was the last boat that I finished. We had to beaf up the centre-board and rudder to handle the sail area but, she goes like stink. I also move the cb slightly forward.
Good prototype but, next time, I will make her with more beam..
The original hull was a rowboat that was converted to sail..

 

Submarines, like fish, travel underwater so don't make waves. Norman Cross used the "coke bottle" hull shape in some of his trimarans. He was also an aircraft engineer. But I thought it was all to do with the large wing area/volume and fairing out what would be a SAC if it were a boat hull.

Apart from the hull shape I'd worry about the central daggerboard, the massively raked rudders and the weight

It would be interesting to know what boats he raced against

Richard Woods

 

The coke bottle shape thing is the "area rule" and used in very high speed aircraft to help manage the transition zone at the sub/supersonic barrier. The area rule has been in place since its discovery in the 50's on high speed aircraft, but is of dubious value in considerably more viscous fluids and much slower speeds. If well shaped (the big key) at S/L ratios of 1.5 to 2.5, it can have some usefulness, but a well designed beach cat can easily exceed these speeds with more conventional shapes, so why bother with the wavy bottom.

 

I know a boat builder who was approached years ago to build a boat based on a sailfish design. He turned it down because it was way outside of his usual construction methods.

 

the "area rule" used in supersonic aircraft is not translatable to boat hulls, water is not a compressiible fluid like air, and as pointed out, a hull travels on the surface.

It is a complicated issue to explain why "coke bottle" aircraft fuselages have less drag at super sonic speeds, due to the compressablity effects of air, and it does not translate to a boat hull.

It is a really bad idea in a boat hull. I worked for many years in the past as an aerospace engineer doing aerodynamics on military aircraft. Please take my word for it, it would be a waste of time and money to try it on a boat hull.

 

Jim, thank you for the interesting picture.

First of all, I´m not an expert as you and Richard, Petros, PAR and many others here. I am only an amateur in hydro- and aerodynamics and boat design, construction and bionics.
I googled a lot for Sailfisch, sail fish,.... and I did not find pictures of sailfish with clear coke bottle bodies as above and I do not know how the body is shaped when moving fast in turbulent water. So I do not know enough about the benefits of the hollow in the middle and of all the fins and their action. None of my books of Bionics and Ökophysik (German) can tell me anything about the benefits of the dimple in the middle of the Sailfish. Sorry

On the other hand there are the interesting thoughts of Harmen Hielkema who is used to fit his proas with a slight dimple on the leeside.The so called Kiribati dimple is a feature of some of the variants of proa characteristics found around the Pacific past and present and is most pronounced in the proas of Kiribati.

http://i28.photobucket.com/albums/c...c-4a7e-47d6-911d-05a4d35664e7_zps2c20e1c9.jpg


I think the benefits of the soft dimple (waist) on the leeside of a proa are worth to discuss the question wether it is useful to make a model with the hollow at the bottom of a catamaran hull.

 

I wonder whether the sailfish even is, as claimed, the fastest swimming fish, it is more anecdotal evidence than anything else. Either way, this boat would only achieve a fraction of the claimed speeds of that species. The tunas and fish such as wahoo are incredibly fast, but don't have undulations in their body contours.

 

Not going to progress anytime soon..

Well, by the looks of things, I will be working on the many other jobs in the shop before taking on a boat that is untested and quirky at best. Thanks for the info everyone. You confirmed my suspicion about this boat. It just didn't add up to me either.

 

Jim, is it possible for you to get another Photo or sketch of the "First Hydro Curve Boat" from 1958 with which Doug never lost a race ? Best regards, Manfred
From hydrodynamics it is interesting to have a boat with a bottom (two areas of low pressure) where the flow is accelerated two times.