How much to add to displacement?

[mick]

If a 29ft yacht has a displacement of 3800kg, what should it's approximate total cruising weight be, given the usual sort of stuff that is normally taken? I'm aware of questions involving string and length but I'm only looking for a rough guide.

 

[Saguday]

With my family add about another 2000kg

 

[wotayottie]

I have at the back of my mind an idea that the normal quoted displacement is with quite a lot on board inc people.. But I doubt that many boat builders ever put a boat on a weightbridge which is why you often get different weights quoted at different times for the same design.

Maybe half a tonne to answer you question but its a "how long is a piece of string?". A pals 39 footer has so much junk on board that it goes from 8 tonnes official to 11 on the club crane

 

[DownWest]

If you can get at the designers info, then light weight verses displacement might give an idea. My current build is 1100kg to 1450kg . So the designer is expecting 350kg for the 'useful bits'. Sounds a bit optamistic for a 4 berth trailer sailer. But, if 3x75kg for peeps, leaves 125 for kit. ( I am not sure if I want 4 on a cruise...) I may be lying about my 75kg).
A

 

[phildowney]

extra weight

hi as well as the usual stuff it might also be worth bearing in mind ;

mast sections are specified using the designers gz (righting moment)
if there is more displacement it will increase the gz (at least initialy) and thus the mast and rigging has a reduced safety margin

for best speed and stability concentrate the weight amidships and low down and keep the weight out of the ends

weight in the ends gives a slower pitching time but isnt as fast
light ends shorten pitch duration and is faster as the boat responds to waves ans wont crash into waves out of phase

some designs carry load better than others of the same displacement due to hull shape , a traditional fin and skeg racer cruiser from the 70s wil probably be as quick with another ton aboard. a modern high volume beneteau wil slow down and could become less stable due tio raising the c of g

racers sometimes consider designers weights somewhat optimistic and boats get heavier with age to the extent that they hire load cells and weigh them to get a true weight which can result in a lower rating! so treat brochure weights with caution when working out light ship weights also rorc rating office dosent accept weights from cranes and travel hoists, they have been known to be a long way out for our purposes.

have fun , phil

 

[rivonia]

overweight

Our cats displacement is 5 tons. We live aboard and have approx 2 tons added. We raised the water line 6 inches.

 

[santeana]

In old money, ft, ins, pounds an approximation of safe payload may be found as follows:

Calculate water plane area = approx LWL x BeamWL x 0.7 (BeamWL approx 0.9 x B max)

Calculate pounds per inch immersion = WPA x 5.33

Calculate allowable safe sinkage = 0.1 x LWL

Calculate approx payload = safe sinkage x ppi immersion


OR alternatively,

safe payload approximates = 0.2 X Light Ship Weight (displacement less sailing payload. Which may or may not be the published displacement of your boat)

In boats I've used these calcs for, method 2 usually gives a somewhat smaller value.

Sources - Nigel Calder/Dave Gerr

 

[mick]

Thanks, all.