Proa questions...

I think one can accurately make both point, either for high or low volume hulls. In fact, Cold Coast, made both kinds of boats whether the ferry or their "regular" cats. The big difference is that the low volume bows were found on boats with far greater ratio of WLL to displacement.

"" Originally Posted by Inquisitor View Post
2) It seems to me that the required double ended nature of the beast makes it behave like a Wharram catamaran. Is hobby horsing an issue (or why not)?
Traditional proas and copies have very Wharram like deep V hulls and hobby horse in a chop. Harryproas have rounded hulls, with extremely high prismatic coefficients and no rocker. This makes them much easier to build, much less prone to hobby-horsing and nose diving and gives a potentially higher top speed."

This is an interesting feature of your boats. Would seem to be applicable to other designs, if they didn't mind slow tacking. Not a dis. If a boat was designed to haul it with low resistance trans-at, would it be a bad thing for it to have a similarly designed hull?

 

Straight sided hulls give more room - over more length. Deep v = better sea keeping? So sort the ends.... all boats are a compromise - speed and space vs bad handling? I can live with that......

 

But then, if you don't believe a math and physics-based rational method, how can you expect that rules of thumb and personal feelings will give you more accurate and reliable numerical results?
Surprises me a bit to hear that from a person coming from the aerospace field, where EVERYTHING is based on science, method and measurement, not on impressions and opinions.

This is the true point:

The math of that second-degree equation given by Ad Hoc, particularily with the simplifications in his second post (#25) offers you a deep and very elegant insight into which parameters influence the longitudinal dynamic stability of a boat.
Yes, you often can't define the numerical values exactly, because ships operate in a very variable and random environment. But you can't do it neither by CFD nor by rules of thumb.
What you CAN is to have an overview of what is important and what is not.
In cases where reliable numerical values for various coefficients are not readily and freely available (as usually happens), the said equations will give you the qualitative indications about directions to undertake, parameters to modify in order to minimize pitch amplitude and frequency. Parameters, not just a parameter.

And the equations which AH gave you in the post #25 shows the most important parameters involved in the analysis. The essential minimum, I would say.

A fundamental parameter in any longitudinal dynamic analysis is the natural pitch frequency (or pitch period) of the hull. The equation in the post #25 tells you that it depends on 3 important parameters, not just one. Continue with the damping and you will add more and more parameters, due to both underwater hull and to the superstructure or sails.
Where you will stop depends on how important the project is.

I believe that if you want an unbiased view of the facts, as you claimed in the first post, than it is wrong choice to hang to just a pair of thumbs. But I am also well aware that - at the end of the story - it is your boat, your money and your free choice.

Cheers!

(Edit)
P.S.:
I understand that we basically all do agree about the importance of a parametric analysis, in order to understand the problem. It is important to know that we all know what is important, I'd say.
The rest is probably just a problem of the form, due to interposition of the PC screen between the faces. Hope none of the previous contributors will leave this constructive discussion.

 

The only advantage of deep V is that it can sit on rocks with less chance of damage . Not a big deal if you carry fenders (old tyres, for example, which can double as a drogue) strong enough to support the boat's weight.

Compared to a straight sided, rounded bottom hull with oversize rudders, no rocker and high prismatic a deep V pitches more, is slower in the light, does not go upwind as well, has less internal space, much less headroom,weighs and costs more and is harder to build.

rob

 

It is certainly seat of the pants in that I am pushing a hull through the water under my own power and have intimate feel for what is happening like few others will ever experience. My understanding of physics and fundamental theory goes well beyond those who make arguments here based on what they have been taught or read in ancient books.

If a theory on hull performance covering pitching in a seaway does not cater for the buoyancy of the hull above the waterline then it will not be applicable to anything that is going to happen in the real world on the lee hull of a harryproa. I know good theories cater for the buoyancy in the bow.

Rick

 

Rob
I have attached the michlet comparison for both hulls displacing 1 tonne. I have given speed range from 12 to 30kts as I know you are aiming for higher than 20kts.

The round section gives a slightly better result in this comparison. For 20kts GODZILLA also produced close to round section as I noted above so this is not surprising.

What I need to do is the comparison of these two hulls using Flotilla. This gives the trim and squat changes. The results correspond well with what I see on my V11 and V14 hulls. The V14 trims bow up and lifts at speed whereas the V11 with semi-circular section does not change much. Attached video gives an idea of the lift at speed on V14. The bow drafts 20mm in static trim and I can get it above the surface around 16kph.

I would be aiming to design the flat bottom hull on a proa to give sufficient lift in the bow to balance the sail forces so trim remains level as the force and speed increase. At your target speed even a slender flat bottom hull will produce dynamic lift.

For the windward hull I would not use a hull with a flat bottom. I would want to look at the lowest drag shape of various windward hulls in conjunction with the lee hull based on weight transfer between the hulls with sail loads. My expectation is that the waterline length would change quite a lot with different load conditions and it would have a significantly deeper draft than the lee hull with a good deal of rocker and possibly V section.

It takes quite a lot of fiddling to set up to run Flotilla. It is not as user friendly as michlet - I am certainly not as familiar with it yet. If someone was seriously considering a flat bottom lee hull then I might make the effort.

One aspect of your new hull I have an interest in is what is does down a wave. I hope to be able to drive hard down waves with the V15 hull without it diving.

Rick

 

Rick,

I'm curious...

are your human powered efforts, pet projects for your fun or are you on a research path toward a goal? Say some deep military speical-ops insertion program? Stupid question - if you answered you'd have to kill me.

How does your 17kph speeds compare with the typical 1-man racing shells? Leg power, versus... I'm guessing those racing shells, use some kind of sliding thing so you use legs, back and arms. Yours, I'd imagine, would more easily sustainable for long distances.

 

Boating is a lifetime hobby. I stopped sailing when my kids started organised sport. When I had more time I started thinking about boating again with the aim of building a yacht in my retirement. I built a few 1m scale sailing boats and then decided I wanted to do something on the water again that enabled me to test ideas on hulls and propulsion. This has evolved into probably a solar powered boat but I have a lot of fun with the pedal boats as they are good for general health and lots of interesting engineering. I have an industrial background in research and development. (I have since developed a keen interest in human powered vehicles - a friend is currently doing a full circuit of Australia on his recumbent bike being on target to do the 15,000km in under 50 days. Like me he is not far off retirement age. At Easter this year I officiated during the setting of the current 24 hour land distance record of 1107km.)

An Olympic single sculler does 2000m in 400 seconds or 18kph average. A sprint cyclist on an optimised pedal boat could average around 19kph on a displacement hull and maybe mid 20s on a foiler. The fastest single person pedal boat did 37kph over 100m.

I design for long distance and my V11A design holds the world distance record over 24 hours - 245km. Many thanks to Leo Lazauskas and his GODZILLA hull optimising software.

My sustainable power output is between 120 - 130W. An Olympic sprint cyclist can hold over 600W for 5 minutes. So a boat designed for 2000m would be different to what I am building. At that power level a foiler would be faster than displacement. The break even power for my weight is about 220W, which equates to 14kph.

I believe proas are the least developed configuration for a sailing boat and believe they offer the best bang for money for long range cruising. I am a keen follower of what Rob Denney is up to.

Rick W

 

Rick what hull speed would you give a proa with a 25 meter waterline? I too have aspirations for a world cruising proa. I plan on cruising the pacific northwest for a bit then I would start heading your way after everything has been worked out.

 

I've downloaded Michlet, and later I tried Flotilla and neither zip file will decompress. I've tried it on Windows 7 (says its empty) and XP says its corrupted. I also tried it with WinRar and still no joy. Any ideas?

 

Hull speed has a defined meaning related solely to the phase speed of gravity waves, which really only applies to beamy hulls.

Michlet is well suited to determining the drag curve for a slender hull. This is what you want to know to get an idea of the hull's speed potential.

GODZILLA is an optimising routine that is incorporated with Michlet so it can search out the lowest drag hull for specified hull parameters and constraints. The key inputs are the speed you are aiming for and the displacement. Sometimes the results are surprising but they always make sense.

If you nominate the target displacement and target speed I can give you an idea of the lowest drag hull for those conditions.

Rick W

 

There is a version of Michlet posted here:
http://www.boatdesign.net/forums/design-software/michlet-godzilla-availability-27241.html
that should be OK. I do not know if Leo currently has it on his web site. You should be able to open the version on the thread.

I suggest you spend time to get a grasp of Michlet before you step into Flotilla.

Post #25 on this thread:
http://www.boatdesign.net/forums/design-software/racing-canoe-19110-2.html
gives a quick guide to get started with Michlet/GODZILLA.

Another neat bit of free software is Delftship. It can import michlet offsets and export michlet in.mlt files. They work well together. There is a pro version of Delftship that has a bit more functionality and allows you to show your appreciation to the developer, who has also made life more fun for me.

Rick W

 

Thanks for the comparison. near enough the same, thought I might have to turn my hull upside down and use the flat deck as the wet bit!

Flotilla results will be very interesting, particularly what happens to the rounded chines/flat bottom when they are at an angle to the water flow.

On the plus side of flat bottomed windward hulls, no floor is required, thus lower topsides and a little less weight. Why would you put rocker and a V in the ww hull?

You are not the only one with an interest in the down wave behaviour! I suspect the flat deck won't do me any favours, but I have a metre long piece of bow which is all foam so I can play with the shape and see what works best. The flat deck is also easy to build an inverted V on if it is slow to come up, but the boat is so light and the hull so long that I suspect it will be pretty rare to get the decks wet, much less seriously immersed. If it was not for structural reasons, I would have made it half as wide and half as high.

rob

 

Hmmmm - V hull........
But why lots of rocker?

 

Rob
I might spend some time with Flotilla on the weekend if time permits. It is time intensive for me to set up the hull offsets for analysis. A bit more complicated than Michlet.

My thought on the V and the rocker on the windward hull is that it could be optimised with the main hull based on displacement transfer with sail loads. Having a deeper draft with changing waterplane with immersion would make it more progressive toward lift off. It would also make for gentler landing if you do lift off. A small flat section might be tolerable.

How would the displacement be shared between the hulls under ideal sailing conditions for cruising? What is a safe margin to avoid constant attention.

With a flat bottom lee hull I would rotate it slightly about the longitudinal axis so it sits flat when the windward hull is just flying - the same as they do with the amas on the big tris.

How high above water level will you set the sail centre of pressure. If you give me a figure I will look at pitch angle at say 20knots on both hull types without any compensating ballast?

My flat decks have not been good at lifting when pushed into a wave. The longer round section is worse than the shorter flat panel hull. It is quite surprising how far I can drive the long hull into the back of waves just using my own power. I back-off when my feet are pedalling through water.

Rick