Design vs actual displacement

Very pleased to see that you haven't let the B******s grind you down Jimi /forums/images/icons/smile.gif.

I would go with Snowleopard's 1 tonne but in most cases I think one would find that one has to add the water and fuel to the builder's claim if a stock boat. In our own case water and fuel add another approx 3/4 tonne in a 40 foot boat.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
As mentioned already by others lightship displacement is the one almost always mentioned by builders/manufacturers,and depending on what type of sailing one does and the type of boat the effect can be pretty dramatic.
For blue water traditionalists like myself with a 12t 37footer the effect of four persons gear and cruising equipment will be substantially less than a 6t 37ft production cruiser /racer. Our waterline is 4" higher than design wl ! Fuel and water are the two heaviest items and are easily quantified.But the main penalty is performance so either get a heavy displacement boat or powdered food and a watermaker!!

<hr width=100% size=1>
 
Ours is a bit smaller but on a 28ft Stag, design weight 3400Kg, when lifted this year (fully tanked) weighed 3950Kg according to the crane driver.


<hr width=100% size=1><A target="_blank" HREF=http://www.troppo.co.uk> Follow the Tightwad Sailor</A>
 
Re: Weight of teak

Seems that the max extra load advised from the designers is 1250kg. This includes people and stores, but not tanks. So I'll be bashing against the maximum but probably not too far over. But they seem a bit quiet about whether teak decks are included or not!

<hr width=100% size=1>
 
My Nic 38

has a design displacement of 15,500 lbs.

At the last weigh, when full of cruising gear and fuel was 20,750 lbs. An increase of about 33%.

I may be a little overweight but she still sails well even if a bit down on the marks.

<hr width=100% size=1>
 
If the bumph actually does say somthing along the lines of "designed operating displacement" then I would read the figure as the weight of the boat including reasonable cruising kit. The designer / manufacturer would be able to tell you what payload had been included in that figure.

If on the other hand you are reading off the displacement figure given on an old boat test / brochure, then the displacement given is likely to be the weight of the boat ex factory in my experience. The boats we have at our club, when lifted out by crane and weighed on the crane load cell (not the most accurate way of weighing) seem to average between 15 and 25 percent more than the magazine boat test displacements.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
 
Re: My Nic 38

My Moody 33 is quoted as 4.69 tons dry. When she was lifted in at the beginning of the season, the crane driver asked me how much she weighted, so i said probably about 5 to 5.25 tons. After the lift he told me that it showed 6.5 tons on his gizmo

<hr width=100% size=1>
 
Moo 33

Completely stripped out - i.e., no sails, fuel, water, safety gear, sprayhood... absolutely nothing on board - for IRC rating measurement, mine weighs 5335 kg. That's for a Mk 2 33 more or less as built with no heavy extras added. At that displacement, she sits on her design water-line, exactly, at the bow, and about 30 mm deep by the stern; that's still much higher than the other 33s around the marinas. I've done a rough estimate according to her lines in fully crewed-up cruising mode, and you don't get much change out of 7 tons. I'd like to know where the extra 12 cwt over Moody's advert value crept in.

<hr width=100% size=1>
 
Re: My Nic 38

It seems that most boats displace more than the manufacturers quote. Now i assume that the RCD group is based on a number of things, including hydrostatics, but based on a the "design" displacement. So a boat might leave the factory as RCD class C say, but will this still be valid when the owner loads her up with kit and crew?

<hr width=100% size=1>
 
Re: My Nic 38

I wonder what the impact of these differences is on stability figures? Depends how much is above the waterline I spose.

<hr width=100% size=1>
 
Re: My Nic 38

I would imagine it is significant. The CofG is likely to be higher for a start and the angle of vanishing stability will change too. How significant I don't know, but could it be enough to make a mockery of RCD catorgories?



<hr width=100% size=1>
 
Re: My Nic 38

Most of my extra are below the waterline - fuel, water, spares, tools, booze etc. The only things that I can think of aboeve are some items in the cockpit locker. My 33 does not seem to be any deeper in the water than others that I see.

<hr width=100% size=1>
 
Re: Stuff above the waterline

No
Light one
Yes
No
Minimal
Yes but not that heavy
Yes
Point taken but the total would appear to be a small percentage of the 2 tons + that I appear to have added to the makers' specs!

<hr width=100% size=1>
 
Re: My Nic 38

But for mono's have to keep in mind that resistance to inversion and rerighting is alot more complex than just AVS as they are always rotated to inversion by wave action, not the wind. Usually that wave will be breaking so the stability of the boat will be very much different to the flat water model used in calculating AVS. As a simple example of that, forgetting the case of breaking waves, a yacht's (or any boat's stability) will be different for each of flat water, or for waves and hogging, or for waves and sagging.

So quite alot of aspects of the boat come into account if one is considering resistance to inversion, But generally, a heavier boat of the same type as another lighter one will require more wave energy to invert it because of its higher mass moment of inertia. With mono's one has to remember that except in most unusual circumstances the wave energy that inverts it is also available from following waves to reright it. So there is a bit of a trade off, if it takes a bigger wave to invert the same boat but heavier, so there are biggish waves around to reright it, even though the AVS may be less than the same boat of less displacement.

The moment of inertia is of less importance for righting as the righting accelerations are lower and the greatest resistance is the rig in the water, should it still be erect. If the rig is not erect, virtually any mono will reright immediately (but become more susceptible to inversion than before, due to the lowered moment of inertia with no rig).

Is obviously of some importance where the weight is, of course.

I am personally unaware of any cruising mono that has not rerighted after inversion - am not saying there have not been any, just that all the losses I know of have been due to structural failure or other loss of watertight integrity, fire or grounding. I would be genuinely interested in any information on that. Of course, building on what Richard Woods has said in another current thread, the crews may not have gotten back to tell the tale.

John

<hr width=100% size=1>I am the cat but I am only 6.
 
You must log in or register to reply here.

Other threads that may be of interest

I
Turkey vs Greece for cheap boat storage on handstand + refit – advice needed!
Replies
13
Views
1K
Wansworth
W

Share this page

Top