economical coastal cruiser

Attached gives an introduction to some of the optimisation techniques that engineers are trained in. If you want to read more then there is part of a book here:
http://books.google.com.au/books?id...resnum=4&ved=0CBQQ6AEwAw#v=onepage&q=&f=false


Rick W

 

Yes... and your point is...?

 

I added a new variable, being the method of construction, so the shape had to be consistent with the new method of build, to arrive at the overall optimum that included flat panel construction. Within these new constraints I sought the best overall performance. I did not just accept that it was "not worth the effort" to do a complete redesign of the hull to optimise with the new constraints. I put in the effort to arrive a completely different shape based on the additional constraint.

This is very different to saying that it is not worth the effort to consider an alternative propeller to the current crop of outboard props. What basis do you have for this wisdom?

Rick W

 

Engineering optimisation is a concept understood by most engineers. I get the impression it is not part of the training that naval architects receive.

Rick W

 

What does the attached mean.

Attached suggests an exacting outlook. It does not mean the mathematical optimum that I am seeking but it does set expectations about the boats.

It is clear that some here are happy with "she'l be right mate - no need to look into that - it is not worth the effort - just compromise." That is not the way to get the best outcome. You have to ask what is the difference? Is there a benefit? How good could it be?

Rick W

 

Seems some of the designers of the America's Cup yachts understand design optimisation:
http://www.esteco.com/case_studies/marine2.jsp

Anyone putting a lot of effort into the design of an efficient coastal cruiser who simply accepted the current available outboard propellers as an acceptable compromise without question is likely missing the most easily won gains.

Rick W

 

No - the attached is a discussion about optimisation. Which has no direct relationship to the discussion about compromise. I've always regarded you as a fairly intelligent, if somewhat fanatical, sort of bloke Rick - but I gotta say, your testing my faith here!
Just a few posts back, you effectively stated yourself that you compromised the pursuit of efficiency for the sake of construction simplicity. And in the quote that I posted, you said exactly that. I fail to see how you can't comprehend that you are deriding the very same process that you have used yourself....
And once again, you are misinterpereting my posts..... I didn't say 'she'll be right mate...no need to look into that... it's not worth the effort". What I said is that the diameter of an outboards prop is severely restricted, and so is compromised from the start. And that any gains you may achieve by custom building an outboard prop might not be worth the effort. I never said you shouldn't look into it.
You go right ahead and design these guys a custom made outboard that can swing its custom made prop for the custom made boat that it's going to push. I'm pretty sure that when they get the bill, they'll pretty quickly teach you a thing or two about compromise.
It's now been 38 hours since I slept. To put it bluntly, your stupidity astounds me... I'm going to bed.

 

Just a reminder of what I responded to initially.

Why do props have to be a compromise? Do you know what the most efficient prop would look like? What is being thrown away by accepting what an outboard has? The props on these have changed very little in the last 40 years. The title of this thread is indicative of the changing requirements. Are props suited to planing boats 40 years ago still suited to the current requirements?

What would an uncompromised prop look like - that is one where there was complete design freedom without any constraints other than the thrust required at a certain speed?

How will the optimum prop change as the design space is constrained by material limitations, space limitations, draft limitations and so forth?

I disagree that all things in design are a compromise. (Maybe in your own little world of yacht design but not in the engineering field in general). A design should aim to optimise within the design space. A good designer will test the constraints and have a very good undertstanding of the trade-offs between the various constraints. The constraints should be tested as well to see how strict they are. It is not unusual during a design spiral certain constraints become obsolete because of new understanding.

Compromise is a sloppy approach without clear understanding of the constraints and how they impact on design objectives. It is not good enough to offer "may' and "might' as wisdom.

Rick W

 

I like to think of optimization as being associated with the word "Open Checkbook" and compromize being associated with the word "Budget"

The minute you hesitate on cost you have just compromised, not optimized.

 

What Pierre said.

 

Yes, but the design space does include the following:

1) Make a living for the draftman/designer if professsional.
2) Time and resources allowed for the design unless the design is to be never finished.
3) Buildable within planned resources and price unless the design is never built.
4) Final paying customer expectations unless you do not want to sell it.

 

I totally agree with FCFC.

A spacecraft is certainly highly optimized. The engineering challenges across the vehicle are very, very tough. Yet one can never just say "Let's do subsystem X using technology Y." One must do trade studies that examine values across the entire mission. Cost is usually tightly constrained. Schedule is usually absolute (launch windows refer to proper alignment of planets -- not easy to push planets around to accept a month schedule slip!). Mass is extremely fixed. Power. Volume. Center of gravity. And on and on.

So the problem is optimizing across a very large number of dimensions, not just one. So an "optimum propellor" may well be optimized to achieve some combination of qualities, the combination being optimized. That might mean, say, top speed is below the absolute that may be obtained.

In this discussion, I think the function we are trying to optimize includes the following factors, in no particular order or significance:

1) Cost
2) Ease of supply (off the shelf being best, hand built by the Dwaves of Narnia being perhaps not so good)
3) Propulsion efficiency
4) Diameter
5) Pitch (limited reduction in outboard gearboxes)
6) Some specific speed range, not maximum speed
7) Sufficient thrust, not maximum thrust

 

As noted earlier on the design space I most often work with is return on investment. That means bang for buck. The way to achieve this is through optimisation of the design.

Rick

 

This is exactly what needs to be done. The designer needs to know the trade-offs between objectives. Not simply dismiss the possibilities of something because it "may" or "might" not be worth the effort.

I can guarantee that the design of an economical cruiser to do 12kts would benefit from a thorough understanding of the cost, in terms of performance, certain prop constraints impose. Very sloppy design to dismiss it without this effort. Test the constraints and establish the design space to optimise within. Not make the bland compromise that it is not worth the effort.

Rick W

 

Rick, would you be so kind as to explain what you mean by this statement

Trade-offs...? No...surely you don't mean COMPROMISE....

And, whilst you're at it, could you please explain where exactly I said that one should simply "dismiss the possibilities of something because it "may" or "might" not be worth the effort"? As usual you are intentionally misinterpreting my statements. I simply said that building a custom prop to fit an outboard, which by its very nature imposes significant constraints may or may not be worth the effort.... not that the idea shouldn't be explored.

Lastly - as this all started based around prop design - you quoted me as saying that "like all things in yacht design, props are a compromise". I stand by the statement. For example, let me quote from one of the well known prop texts
"In other words, at low to moderate speeds, for a given horsepower, the slower the shaft RPM and the larger the diameter, the more efficient the propellor will be".
With this in mind, could you please explain how one might go about designing and building a prop that imposes no compromise? And how is it that you came to decide upon the diameter for the prop on your pedal boat?