Displacement ratios. Should I be scared?

[john_travers]

My current boat has displacement ratio of 310 on a water line length of 22.5 feet and feels controllable and safe in bad conditions.
I am thinking of buying a boat with a displacement ratio of 175 on a water line length of 27 feet.
Will it scare me to death?
Does a longer keel with more weight alter the displacement ratio?

 

[billmacfarlane]

You can use the displacement ratio, which is simply a ratio of water line length compared to the total displacement of the boat as a rough guide to how the boat will perform. It's only a rough guide and there are other indicators as to how the boat will perform. A ratio of 300 and above is usually an indication of a stoutish cruiser boat. Boats like Hallberg Rassy and Naiad are in this range. Coming down to 200 you're more in the performance cruiser range BavJeanBen area and below 200 you're into the lighter displacement performance orientated boats. An SAisp of 175 isn't horrendous and the other factors to take into account are the Sail Areaisp ratio and the ballast ratio. Modern continental cruisers tend to have a fairly low ballast ratio relying on form stability i.e beam for stiffness. The advantage being more accommodation. A more traditional design will have a higher ballast ratio relying on the ballast for stiffness and a more slim and better balanced hull form. There are other things to take into account but if you're still reading this a DispRatio of 175 is an indication of a sprightly boat but that's about it. More weight in the keel will affect both the ballast ratio and the Disp:WL ratio. Hope this helps and I haven't bored you.

 

[Guest]

Sailing - A recreational drug ?

Give it a go - It's the only way to find out.

Ellen's Kingfisher probably comes out at around 175 and that went round the world.

Providing you're under 50 and not going through a middle age crisis you should be ok.

 

[Guest]

Also I forgot...........

MECHANICAL SIMILITUDE !

The bigger the boat the better the stability .

If you doubled the size of your boat i.e. x2 weird things start to happen :-

You will need 4 times the sail area.

Hull volume goes up by 8 times.

And what may interest you for your bigger but much lower DL Ratio boat is that stability goes up by the power of 4 - SIXTEEN times that is.

Providing the scantlings are up to scratch and not like most racing maniacs ( If it fails to break it's too heavy for me) you will probably come out safer.

Happy Sailing

 

[vyv_cox]

My first boat was a Challenger Europe, very lightly built French 27 footer, with a 2 ft sugarscoop added on the stern, so it called itself a 29. Displacement ratio was somewhere around 175, deep and short aft-swept fin keel, spade rudder. The previous owner sailed her extensively on the west coast, from the Scillies to the north of Scotland. He then sailed to the Med, non stop UK NW coast to Gibraltar, then back through the Canal du Midi.

In our first three years of cruising we took her half way around Ireland, N of Scotland, Cornwall, and many times to IOM and E. Ireland (from N. Wales).

We sold her on to two young guys, who also sailed to the Med via Gibraltar, lived on board for a year and sailed back.

The boat was lively, fast and loads of fun, but we never felt her to be excessively frisky. We certainly sailed her in some heavy conditions, and no doubt the other owners did also. Maybe not the ideal cruising boat but I would argue that she has done more actual cruising than many boats with twice the displacement ratio.

 

[Guest]

John, 5 years ago I moved from a heavy displacement 27' to a light displacement 28'. Although I loved the old boat and had it eight years, it was too heavy to perform well in normal conditions.. that is when the majority of us go sailing. She would look after the family well in F7 and over, but that is not the weather that you take the family out in!

With the current boat we cruise further, faster, see more, are more relaxed after the sail and really get a buzz surfing her at 12knots +. The extra space from the extra 2'6" beam is useful too!! It's like sailing a dinghy again.

Go for it. KCA

 

[PeteMcK]

175 is certainly at the light end of the spectrum but you MUST consider other factors, as mentioned in other posts. In the case of my own boat, its D/L is only 205 (which is also regarded as low) on a 28.5 ft LWL. On the other hand, its Stability and Safety Screening number is 43 with 40 being the RORC's lower threshold for "ocean races of up to 1000 miles in all weather conditions", and the cut-off for the Fastnet being only 30. It's built of simple, bombproof, solid laminate, deck and hull.
Moody 33? Light displacement? It's true - sort of ...

 

[hugh_nightingale]

Do you have a method of calculating the Stability and Safety Screening number or is it something for which you have to apply? To the RORC?