Low Displacement Length Powerboats

As a type that I've been advocating for over 10 years, I applaud the work Mr. Bury has done in presenting his 78' design. It's great to see more alternatives to the average piggy Sea Ray being offered. Will any of the major builders actually take this concept to heart? Doubtful in our present world, but one remains hopeful.

In my opinion various technical aspects of the Bury 78' are questionable, but that's why we're designers....everybody gets to air their concepts. Can the boat be built as light as indicated? Probably...but the results may (IMO) only be suitable to a select few. I can't find any indication of what this boat is intended to be used for? I would guess day trips with the occasional overnight stop? Apparently she's aimed at people who have $3m+ to spend on a custom day boat.....

Once again I will mention that published weights and reality are very different things. If the Bury 78 comes in at 75,000 pounds, who is to know or care? She will float 5.5" deeper and just look better, the top speed will not be achieved....but that's normal anyway......

A quick look indicates she might just reach 20 knots with the installed power at lightship weight....which wouldn't give much range at that speed! The 15 knot cruising speed is possible at full load with the 335 HP, but full (20 knts) speed at full load will require almost twice the installed power.

 

The longer the length the lower the vertical accelerations too.

If one is clever, one can also "manipulate" the the weights to a degree that the radius of gyration is reduced, which again affects the vertical accelerations.

It seems to me that many aspects of hull design/hydrodynamics, as noted on a another thread here, which has been hijacked by button pushers, are not actually "well known" as i would have expected. I suspect some of the reasons is because many yacht designers do not have the resources to perform extensive tank testing (especially sensitivity analysis) and as such keep their "data" confidential, to maintain some edge against their competitor.

The only reason for this is that those making such statements (like why would L/D make a difference or even what is L?D etc) do not design or design frequently or design a wide variety of vessels with a wide variety of SORs.

Length displacement ratios, increasing LWL, changing hull separation etc etc...all these "form controllers" are nothing new. But this information does not exist on a forum website like this and most certain is not included in a computer program. Not even in the manual does it suggest as such...that is if those button pushers can be bothered to read the caveats and understand them. Consequently button pushers play with their playstation blissfully unaware of such..until they chnage one parameters and Eureka...wow...as if they have found the holy grail or suddenly understand the complexities of hydrodynamics. (When in reality the chnage is minor or the new form is beyond the limitations of the program and hence highly questionable).

There are endless technical papers and very expensive tech books out in the "academic" and "professional" field that give very good indicators of what happens when "X" or "Y" is done. Some not so easy to simply digest though.

But, the problem when someone is unable to separate the issues and focus upon the the heart of the debate, say L/D ratio or increasing length, everyone says...oooh, oooh..what about this boat, she carries much more fuel or this one has showers too...or the structure is lighter on this one..etc etc. Non of these aspects are part of the debate, these are secondary, since the hull shape and form is simply that. What one selectively does with it, that is the difference. This simple fact is lost on many. Not in patronising sense, but simply because their day to day involvement in design and form factors and their effects are not known. As such they choose to remain in their "comfort zone" and debate away from an indefensible position, simply to remain in the dark or ignorant about what changes what and how.

As noted above by TAD...every design is different, and generally in this context "design" it should be read as...what it is carrying and its day to day function. Since this is the main focus of everyone's arguments rather than, what affects the hydrodynamics, all things being equal.

 

Tad, I agree that the notion of presenting low DL boats as a responsible move forward is one that we should all be applauding. And for that, I certainly support this designer's efforts.
I do, however, take exception to the idea that it's ok to present a concept which is undeliverable and package it as a viable design.
Who cares if the boat comes out weighing 75000 lbs? If it was me who'd just shelled out the 3m to build it, I sure as bloody hell would!!
I agree that it's difficult to know the intended purpose - or SOR as Ad Hoc is so fond of saying - for this vessel. But the boat is shown with three staterooms and extensive living spaces, suggesting that it should be capable of accomodating half a dozen people and all their gear, along with all the associated 'stuff' that goes with them. There's no point in quoting a loaded displacement of 23.5 tons if it can't be delivered. (Having said that - bravo to the man for giving us lightship and loaded displacement's as well as a few other form coefficients! )
Is it possible to build a 78' boat that weighs 23500kg loaded? Almost certainly. Is it possible to build this one... that is the question. As such, I think it is valid to compare the boat to others that have been built. It's also valid to note the DL is not the be-all and end-all - the design as a whole must be considered.
I would also like to make it clear, that the intention of my initial post was in no way meant to call into question the abilities of Mr. Bury. I think he - along with a number of others (myself included) - is promoting the kind of boat that should be a lot more common.

 

"...I do, however, take exception to the idea that it's ok to present a concept which is undeliverable and package it as a viable design..."

Hear hear...but very very few will jump in on this. The trouble is is when charlatans take over the selling or prompting the design. The designer is not going to say...oh don't say that....even though they know it to be slightly or majorly incorrect. If it sells, who cares...

Take the wonderful very fast super yachts of the 1990s....when i was working downunder next door to a yard making a very fast superyacht..the press where lauding it as a wonderful bit of engineering and style fusion etc etc...doing some 50 knots if memory serves.

What they failed to tell you...yes it did and does do 50 knots. But every time she did this, she had to be dry docked and repaired for all the leaks and misalignment etc. The structure was so light is was near paper thin (the WTBs didnt fair very well at speed!)....in the public sector one only hears about the positives, for very obvious reasons...in the technical/academic sector...one hears both sides, but rarely these days.

SOR=statement of requirements......it is basically the specification and general arrangement. It "defines" the objective of the "design". I don't know how else to say it..been using this term for 20 years.

 

Sorry - I wasn't having a go at you about the use of the term SOR....

 

Will

I realise that, I'm not childish and insecure as others who don't like being questioned. I can comprehend what you're saying...only takes two brain cells

What similar term do you use, if any?

How would be best to present data of other boats too?..you mention L/D...but the trouble this as with all the data that comes out....is only really piecemeal and can easily be 50 or 60 and no one knows which is correct, so no chnage in data being provided, L/D or speed or displacement (full or light). We can of course use very simple naval architecture to reverse engineer...doesn't mean it wont stop the mis-information. That sadly, will continue until the sun stops spinning

 

You must be a bit slow then... most only need one brain cell to comprehend what I'm saying!!

I generally use the the phrase "Mission Statement", but hey - they all work for me...

As for the data I would like to see presented, it obviously varies with vessel type - power / sail, displacement / planing etc. It also depends where they are being presented to some extent, I guess. There's not much point in telling the average Jo the block and prismatic coefficients, for instance in a consumer magazine. Then again, I read those mags too....
Displacement is the one that annoy's me most. It's VERY rare for the condition to be given, so no sensible conclusion can be drawn about the real numbers.
Waterline length and beam are also basic stats that should be provided.
In some cases righting moment, moment to trim and kg to immerse would also be good - though you have to draw the line somewhere....

 

Agreed displacement is the biggest one of all.

In my field, commercial, the same is true. Hints and suggestions of what it may be are given, but no more. Why?...because most with 2 brain cells to rub together can reverse engineer the values, since most of the hydrodynamic principles are know for such boats. Then we can establish ourselves whether any porky pies are being told. Those who look at the images, well ....just look at the images!

I've complained to RINA about their magazine in the past too, on this subject. But as always adverts pay their publishing etc..so they refrain from being totally independent as such. sadly. Magazines like Nature and other magazines do, so why cant these?....well, as you noted. I too read those other magazines...not always easy bed time reading....need strong coffee to stay awake sometimes!

 

Well, I´m proud to say that I personally ran two yards into bankrupcy due to completely misestimating their abilities (and resulting vessels performance), building my personal boats. It is just a matter of evolution, the able survive, the crap has to leave us.
It is not acceptable that a designer or builder, claiming to be able, presents a concept which cannot stand the test of time (market, performance etc). I do not assume that was done in the given example, I just state a general line of view.

But being a builder myself I have to stay calm in terms of argueing, too often we (read I..) fail, when it comes to the very last millimeter, hp, ton or knot. It is by no means a field of proven data and exactly predictable properties when we construct (I mean the so named design included) such a complex structure as a seagoing vessel.
Reducing a complex structure as the mentioned, to one or few "theoretical" values is never a valid way of comparison (as we all know).
So, I would like to give the vessel in question a shot, but as sure as I was with my first statement about grid connection, as sure I am still, I could destroy that structure way before the classification rules fail (and months before I feel uncomfortable) in just a shakedown run. (shakedown I said)

A well known Italian based (but internationally operating, and widely unknown, building in Turkey) GRP builder recently lost a bet against my "non performers" as he liked to name them (wood Epoxy). I nearly destroyed his foam cored vessel in a 3hrs sea trial at wind 8 sea state 4-5 in the Marmara sea, almost all time full throttle. 78 structural failures had been reported, 2 could not stand a classification survey after repair. My "outdated" wooden junk had lost the Radar arch (to my greatest shame), that was it.
I am proud to say that my boat was a mere 1kn slower in almost all conditions!
A boat, as light as possible, is for sure a better performer than a boat as "good" as possible, but average joe has a right to be protected by international standards to buy a vessel getting him home as safe as design could provide.
Adventourous claims and designs are contradictive to the idea of a "recreational" vessel, and have no place in our market.

Yes, we are conservative, yes, the 21st century is present and most of us (builders) ignore it.
Why? Most of us have just one precious *** to risk.

Nobody would in all seriousness claim to market a formula one car for everyday use! But hundreds of so named "Yacht Designers" do exactly that!!!!!!! And average Joe does´nt know that all the claims are bare lies, all the bubbles are exploding at the very first gale.

We all know that this junk will never come out at the given weight, so what are we discussing?
Marketing hype

in all seriousness
Regards
Richard

 

"..I am proud to say that my boat was a mere 1kn slower in almost all conditions!...A boat, as light as possible, is for sure a better performer than a boat as "good" as possible, but average joe has a right to be protected by international standards to buy a vessel getting him home as safe as design could provide. Adventourous claims and designs are contradictive to the idea of a "recreational" vessel, and have no place in our market..."

Fully concur.

"..Nobody would in all seriousness claim to market a formula one car for everyday use! But hundreds of so named "Yacht Designers" do exactly that!!!!!!! And average Joe does´nt know that all the claims are bare lies, all the bubbles are exploding at the very first gale..."

Again, fully agreed.

So what does joe public do, gets himself a a bunch of magazines and program to try and understand those wonderful claims....what happens, more misinformation to the point that they do not know what they are arguing about!

Just because you can run, doesn't make you Usain Bolt, Just becauseyou can swim, doesn't make you Micheal Phelps....so just because you have a computer program doesn't make you a designer...yet all these are ignored by those hungry for information yet throw the baby out with the bath water when being questioned on what they understand!

It boils down to fitness for purpose, cost and performance....doesn't sell well going 1knots slower...but i know which i would buy and be happy to plant my bum on in a gale.

 

Tad - may I ask what methods you used to establish this?

 

Will...

I note Mr. Bury worked for some time at Wally....examine the interiors of those boats (I know you have). To my mind they represent extreme minimalism...no stacks of books or papers, assorted junk, souvenirs, miscellaneous bronze parts, months of groceries and provisions, etc.

This is what I was trying to say, we all want something different in a "cruising" boat. To me a real cruiser is not only self sufficient, but able to render assistance to others. Thus one has all his tools aboard, a portable generator, several small boats, 5-7 anchors, paint and consumables, spare line and lumber, etc. There is little space in the Bury 78 weight budget for this stuff. I don't think the designer intends that the owner stores his winter clothes aboard!

Of course you are sensitive to the ultimate weight of your vessel, this is due to your experience and knowledge. I would be willing to bet the average stock boat owner (and many custom boat owner's) have no idea of the actual weight of their boat at any particular time. Yes, they know the published weight, some even know the Travellift weight (usually mistaken). Few have any concept of the consequences.

Personally....I do not believe any published weights....they are too inaccurate to be useful. They can, perhaps....be used as a general guide....but do not believe them!!! This is the difference between noobs and knowledgeable designers. You have designed and built some boats, put them in the water, measured the flotation, and found out what they really weigh. Now you have a proprietary knowledge base that is sound and salable. Knowing what a boat really weighs, rather than what you guess it might weigh, is perhaps the crux of a successful practice.

Mat...
For a quick study I often use Wyman's Formula, an adaptation of Kieth's Formula published in Skene's Elements. Google will find a calculator. I also note a mistake in my previous post....quick looks ....20 knots at full load will require about 450 HP for the bare hull....missed the changing constant.

 

Yes - we are clearly all agreed that this is not intended as a long-term live-aboard offshore cruising boat. And I have no qualms with the idea that in order to perform as designed the owner must be frugal in terms of the 'stuff' that is brought aboard.... it is simply one of the compromises to be accepted as a result of striving for extreme low DL.

You mention Wally, so I looked at a couple of their boats. As you say, similarly not intended to carry all the gear that would make them self-sufficient. Wally have always been at the forefront of 'progressive' design and their boats are high-tech.

1st I looked at the 73' Power. Obviously a very different boat with a top speed of 34 knots and over 3000hp on tap. However, the level of accomodation is similar... 1/2 load displacement is quoted as 46 tons yielding a DL of 170. I abandoned any comparison at this point....

Next I looked at the Wally 80, a performance sailboat, but certainly closer to the Unity 24 in every other way. Her keel-less displacement (yes, I know it's not as simple as that) is 25 tons and yields a DL of 70. Clearly there are a multitude of differences between the two boats - not least the mast and the structure required to support it - but could we build a practical vessel and shave an additional 10 tons from the displacement...?
It does, however, demonstrate that a lightship DL of less than 80, and probably < 70 is certainly doable, whilst still maintaining a comfortable level of accomodation. It's still a long way from a DL of less than 40 though....

 

Looking thru the specs of one of my favourite superyachts, this thread came to mind. Proof if we needed it that low displacement length is a more simple proposition the longer the boat is, Silver has a WL length of 221.5' and a displacement of 600t (condition not specified..) ... giving a DL of 54

http://www.hanseaticmarine.com/index.php?id=27

 

You'll find that the "SOR" of most private superyatchs, in general, have a finite SOR. As such the envelope of space required to fit this remains. As the vessel gets longer, the main elements remain ostensibly the same. It is the "mine is bigger" mentality. Having one small state room compared to one large state room..the only difference is the weight of the outfitting. The space varies with a cubic function, the displacement also. The amount of weight added, from a small to a large stateroom, will not be greater pro-rata, generally speaking. Hence the L/D ratio is slowly coming down the longer the boat is made to find the "same/similar" requirements.

Which is why small "superyatchs" require loads of power and produce loads of wash...trying to fit the "SOR" space envelope into a hull that really shouldn't be done! Longer is always better, generally! (In some cases not, hence the broad statement).

Which goes to the statement i made on another thread about L/D ratios, but appears the non-naval architects simply fail to grasp because their computer/PlayStation's required numbers rather than ideas or creativity and trend analysis, not to mention comprehension and education.