Low Displacement Length Powerboats

No - not really!
I doubt my info is any better than your though - it was from a mag too!

http://www.rocna.com/press/press_0509_boatingnz_dashew.pdf

 

What I have read in magazines about "boats and their claims" are often so far from the truth it is a joke. Having been in the private/confidential side and and public side of the same boat at the same time when details have been published...one begins to understand the PR world side much more. Engineering goes out the window...just fancy words and colour plots etc.

Never trust a magazine article...it is just free PR, as such, you'll never read anything negative...just wonderful claims. If something seems too good to be true..it often is.

 

Too true Ad Hoc... often frustrating to have specifications quoted that bear little resemblance to the real thing - displacement is usually the prime culprit too, so our calculated DL's must be understood to be based on published figures... right or wrong....

 

The FPB 83 should be something like 30 tonnes empty and 45 tonnes loaded. Just for fuel, she has 11 tonnes of tankage (13 000 liters) .

If you want to know how light you can go, just substract ballast weight from displacement on this boat : http://andrewsyacht.com/andrws77.htm

 

Thanks fcfc - extraordinary! A 77 foot yacht that weighs 6 tonnes (exluding ballast)....
There's nothing inside it, of course, and one wonders in what state of trim (lightship or 'real') those figures are.... but either way a DL of 20 (ballast excluded) is amazing...

 

And now Will, we have a look at page 19 of the recent issue of "Proboat" and we learn that a 37,8 meter power yacht (nearly 60kn) of 12.000hp has a "structural" weight of about 20 tonnes!!??!!
I do´nt say anything more, except: maybe the boat we were discussing must not apply for permanent grid, water and sewage?

Regards
Richard

 

Yes - but there's no point considering only the weight of the structure - I guess that's really the point I'm trying to make. It may indeed be possible to build a 'shell' that results in a very low DL. But to do so in cruising trim - which is the only configuration that is worthy of note.... well... as I've said... I have my doubts.
It's also obvious that the longer the boat, the easier it is to achieve a low DL: provided as we've detailed already, that you don't simply fill it up with 'stuff'.
Ermis 2, to which you refer is a good example. With 50% fuel, her DL is about 65, which, as the article details, comes as a result of some very careful engineering. (Her displacement fully laden is around 130 tons).
At the other end of the spectrum (no offense intended! ) Tom's Bluejacket range manage between 75 and 100, with equal care and attention paid to weight saving.

 

Will, these two statements (both true) point out the shortcoming of reliance on D/L to tell much about a boat without know anything else about it. The lower figure mentioned for my boats is the one with greater beam and heavier scantlings. To get a lower D/L, it only needed to be longer.

So, my question is: lacking other details, what do these two numbers tell about the relative performance of the two boats? I don't have a clue.

 

I agree - the DL ratio of itself is only a part of the picture - as are the many other form coefficients. It stands to reason, however, that for vessels of otherwise similar form - and size - a lower DL should produce a more easily driven vessel.

 

The truth comes out

 

What about dock-fee to fuel-price ratio?

A fatter boat burns more fuel, but has more room per linear foot.

So a narrow boat might be what you what if you run it for good distance often, but then again a stubby, tubby boat will be able to be used in more weather windows.

 

:?:
Sorry... but how do you come to that conclusion?
Certainly, the case for short, fat, tall boats can be made - as I've already said - but why would they get used more often?

 

Willallison, I figured a more 'trawler' like boat could at least

go out in weather that might be dangerous for a narrower boat. I wasn't thinking 'taller'.

 

Narrow doesn't necessarily imply less safe. It is often the case than narrower boats have lower initial stability, but ultimate stability is another matter.
Consider a catamaran, which by virtue of it's greater beam, has enormous initial stability. That same form stability means that it is just as stable upside down... hardly a desirable trait for a sea boat! Conversely, self-righting lifeboats are often quite narrow.
The snappy motion induced by broad beam can also be dangerous at times.
As is usually the case - there's more than just one aspect to consider...

 

The opposite is true! Well, in general.