Low Displacement Length Powerboats

Discussion in 'Boat Design' started by Willallison, Jul 23, 2009.

  1. Willallison
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    Willallison Senior Member

    The mood for change seems to be with us - at least from the designers perspective. The days of the overweight, short, fat, floating condo are over! Hooray!! :D
    Err... well, maybe not.... the reality is that for most people - those who rarely venture much beyond the end of their pier, these kind of boats make sense. Dare I say it - they could even be regarded as good design...:eek:

    But there is definitely an awakening in the marketplace... a yearning for a return to 'sensible' boats. The current catch-cry that the media has grabbed hold of (and jolly good too, I say) is the Low Displacement Length (or LDL) motoryacht. Whitness the attention that Nigel Irens is (deservedly) getting as proponent for just such a style of vessel. He's not alone of course, and nor is he blazing any new trails... perhaps just mowing the grass on a few overgrown ones.

    But a design in the August / September issue of Professional Boatbuilder led me to question just how far this quest for lower displacement length could go. The vessel in question was 78' (24m) long, with a loaded displacement of 51,700 lbs (23,500 kg) , 32780 lbs (14,900 kg) light.
    That gives a loaded DL of 48, and just 31 in light trim....

    My 1st reaction was to question whether you could physically build a well-spec'ed motoryacht that light....
    The afforementioned Mr. Irens quotes DL's of around 80 for his boats, which I would have thought is about as low as you could expect to manage - short of building a mastless equivalent to a stripped out racing yacht
    Tom Fexas's original Midnight Lace was probably the forerunner to this current trend towards longer, lighter and narrower cruisers, but that had a DL of 110.

    So - what do we think.... has Proboat let its guard down and published a flight of fancy, or has this (quite experienced, from what I can gather) designer simply taken the LDL idea and taken it to its logical conclusion...?
     
  2. tom28571
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    tom28571 Senior Member

    Will,

    I wondered about some other aspects of Paul Bury's boat as well. Of course, he is Australian so that may have something to do with it:D

    As you already know, I am a believer in light powerboats but a D/L of 50 seems too low. My boats generally come in at 75 to 100 or so and they have hard chines. I would not like having to trust a dynamic gyroscopic stabilizing system on a boat with Bury's displacement and sectional shape. The flat aft sections will gain some stability but the underwater sections still look a lot like the hull of one of my previous racing sloops, a Grand Slam 7.9M. If the gyro goes walkabout, the bridge might be a wee bit uncomfortable.

    Edited to add: In his defense, D/L does not tell everything about how a boat will respond to sea conditions.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    In my filed, commercial design, this length displacement ratio (just over 8, metric) is quite normal...well, for applications that require it of course

    Bog standard hulls around 6~7 L/D ratio, metric.

    I am talking about catamarans of course. This slender L/D would be impractical for a mono, since it would not pass the stability requirements.
     
  4. apex1

    apex1 Guest

    Has anyone around here the data of Windhorse? I would´nt feel comfortable going much further than Dashew did.
     
  5. tom28571
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    tom28571 Senior Member

    A fixation on D/L is not going to mean very much to vessels of greatly different size or other configuration. In boats of otherwise similar characteristics, yes. What Dashew does on his big boats does not mean a lot on boats that we normally deal with here.

    Very high or low numbers might cause us to look at other things more closely though.
     
  6. apex1

    apex1 Guest

    No one here (I guess) is fixed on that issue, but interested in new developments and their possibly problematic sides.
    What Dashew did (you might be impressed) has become a market trend! So at least I am dealing with that topic since 3 years.
     
  7. Willallison
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    Willallison Senior Member

    I couldn't quickly track down the specs for Windhorse, but Dashew's FPB 64 shows a (loaded) DL of about 130. Whilst these boats bear a superficial resemblance to the design in question, they are built to travel offshore and are really a whole different kettle of fish IMHO.
     
  8. apex1

    apex1 Guest

    Will, I thought the design you mentioned was a open water (coastal, offshore) vessel too? At least I could´nt find any contrary comment.
    And as far as I can recall the 83ft Windhorse that was above 100 too, but I´m not shure.

    Regards
    Richard
     
  9. Willallison
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    Willallison Senior Member

    You're right - it's difficult to ascertain for sure Richard - I got the impression that it was intended as a 'coastal cruiser' as opposed to an offshore passagemaker. Either way... the question remains.... is it possible to build a comfortably appointed boat this light, that is strong enough to withstand normal day to day use...?
    I agree - there are other questions about the practicality of the design (like the reliance on stabilisers to provide adequate initial stability), but if you can't physically build it, they are somewhat moot...
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    willallison

    The easy answer to this would be:

    Take all the 'things' that you need for your design to satisfy daily use and the daily rigours of being bashed etc. Lets say the boat size ends up being 25m. If you then take that very same "space envelope" do not touch it, but now just make the boat say 35m longer, with everything else being maintained. The L/D ratio will be improved, but the function remains.

    However, this does distract form the notion that just making the boat longer is that simple. Side loads, berthing etc etc, all play a part....the only real way to answer it i suppose is to establish an SOR and see what can be designed in various lengths.
     
  11. Willallison
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    Willallison Senior Member

    I think you may be misunderstanding the question I've posed - which of course isn't really a question, as I know the answer...;)
    Of course I know how to reduce the DL - in the simplest of terms, as you suggest, you simply make the boat longer.
    In reality, I'm questioning the validity of a design proposal that I don't believe can be built. I don't want to call into question the designer himself - We haven't beed presented with sufficient information to completely dismiss the thinking behind it.

    I was reminded of the Open 60, Holger Danske, which managed to achieve a DL of 40, with a beam of only 9' 7".
    In the designer's own words, "to say this boat was stripped out would be an understatement".
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    ok noted.
    Well, in my filed, commercial...there are endless "claims"...of which almost everyone knows them to be either stretching the truth or near blatant lies. But in the world of marketing, it is what is wanted, an "edge", on the competitor to sell boats. To sell your boats not theirs!

    Nowt we can do about it...until the boat is in the water..then the gloves come off
     
  13. apex1

    apex1 Guest

    to 1) I would say yes! With one restriction though: the owner should apply for permanent connection to grid, water and sewage!
    to 2) Yes again......moot.

    I found the Windhorse data:
    LOA 83
    Beam 18
    Draught 5
    Displ. 41 tonnes
    Fuel 13.000ltrs.

    see:
    http://dashewoffshore.com/pdfs/mby-fpb-72dpi.pdf
     
  14. Willallison
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    Willallison Senior Member

    I found a little bit more about Windhorse. Her fully loaded displacement is quoted as 45,462 kg (did well to measure her that accurately...;) ) So a DL of around 87. Interestingly, significantly less than her 64' little sister... which just goes to confirm Ad Hoc's statement about the easiest way to lower DL!
     

  15. apex1

    apex1 Guest

    Well, the info I posted was just the first I found. Your source seams to be better than just a magazin article. I collected some data in the past but thats on my home computer. Seems I did not recall the right figure (above 100).
    Ad Hocs statement did´nt really need a proof, right?:D
     
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