Is there a sensible maximum for the size/displacement ratio of yachts

[aluijten]

Comparable to the other question however different in nature.

I once sailed a yacht that was 64 foot and had a displacement of 60 tonnes.
Sailing this yacht on the sea was not great as is was too heavy for it's length to my opinion.
What happened is that the bow would dive into every wave because the weight caused so much inertia that the bow was too slow to raise because of the oncoming wave.

Any opinions out there?

 

[Salty John]

From a strictly LWL:displacement ratio point of view that 64' boat seems quite moderate - about 250 or so. Perhaps it had too little buoyancy in the bow, maybe too fine an entry or possibly carrying too much weight up front.

 

[TimBennet]

You don't give the waterline length which is the determining factor. There's no meaningful ratios using LOA.

However, assuming the LWL is not less than about 54 feet, its displacement length ratio is entirely within normal limits and the explanation of too fine entry or inappropriate loading would be reasonable explanations.

The characteristics of the various LWL/Displacement ratios are well known and will be in any boat design book. Try 'The Nature of Boats' by Dave Gerr.

 

[AntarcticPilot]

Well, I'd assume that displacement should go up as the cube of the length. My Moody 31 has a displacement of 5-6 tons, so I'd expect a 60 footer to displace about 8 times as much - about 40-50 tons. 60 tons does seem a bit high, but not vastly so.

 

[TimBennet]

Luckily there's a bit more to Naval Architecture than 'assumptions' and 'feelings'.

Using the LWL/Displacement Ratio as a non-dimensional comparison of different yachts of various lengths has been well established for over 100 years.

Your Moody has DLR of a little over 300 which may or may not be comparable with this boat. We can't tell precisely as no LWL has been given, but it's not far out.

 

[Tranona]

Well, I'd assume that displacement should go up as the cube of the length. My Moody 31 has a displacement of 5-6 tons, so I'd expect a 60 footer to displace about 8 times as much - about 40-50 tons. 60 tons does seem a bit high, but not vastly so.

Lot more complex than that. You are assuming all boats have the same length/displacement ratio. The whole point of the ratio is that it indicates whether the boat is light, medium or heavy displacement and the range is large.

As already suggested the boat in question is probably not extreme (although heavy compared with say an Open 60 of similar length!).

 

[aluijten]

Well, I'd assume that displacement should go up as the cube of the length. My Moody 31 has a displacement of 5-6 tons, so I'd expect a 60 footer to displace about 8 times as much - about 40-50 tons. 60 tons does seem a bit high, but not vastly so.

I remember the design to be with a straight (and quite massive) bow and a round stern So this supports the theory that the bow was too heavy for the good of the boat. It also created an annoying motion as if you were in some kind of roller coaster.

 

[TimBennet]

So this supports the theory that the bow was too heavy for the good of the boat.

What theory? Behaviour in waves is highly complex and can't be predicted by an approximate weight and a recall about vague shapes.

There are many who advocate plenty of displacement (as this would appear to have) as the means to achieve a sea kindly motion, but without more information about the hull lines, waterline length, beam, rig, draft, ballast ratio, ballast disposition, loading, etc, it would be wrong make sweeping assumptions about this boat.

Even with a full analysis, there is still 'personal preference'. I think any 65 ft boat can ideally make do with about half that displacement, but Bristol Channel Pilot cutters have fairly plumb bows, rounded sterns, and are highly thought of as sea boats by many, and these carry much more displacement on any given length, than your example.

 

[wotayottie]

" Is there a sensible maximum for the size/displacement ratio of yachts?"

I would have thought not. Uffa Fox said "the only place where weight improves performance is in a steam roller" and the performance of modern light weight racing yachts proves him right.

Is there a minimum for size / displacement. Yes. As the boat gets smaller for a given displacement it finally gets to the same density as water and sinks.