Low Displacement Length Powerboats

True - but how refreshing to find an owner willing to exercise restraint in the amount of stuff crammed into the given space - or more appropriately 'stretching' the given amount of stuff over a longer length.
...that is, of course , if you could ever call building a 240' foot yacht exercising restraint....

 

exactly....not on everyones budget!

But the larger the house, the larger the rooms, not more of them...usually too. Is this what they call economies of scale?? .

 

Just asking the question here, but could there be a "reverse death spiral effect" going on here?

In aircraft the designer often runs into the situation where you need to add more wing strength, because the aircraft is overweight, this results in more weight, which requires more wing, which requires more power, and that larger engine is heavier, which requires more strength, and so on.... It's commonly called the death spiral,.....

With the latest generation of materials, the hull can be a lot lighter. If you retain the beam in the lighter hull, the boat is going to ride like a cork and get bounced around a lot. So, to address that you it seems logical to reduce the beam.

The narrower boat is now even lighter, simply because there is less material.

This much lighter boat requires less power, which reduces the weight further, and engine manufacturers are reducing the weight of their engines (like the recently released MAN engines) for a given power, which also makes the boat lighter.

Now, the boat got even lower in terms of D/L.

So the question remains, could we be seeing a "reverse death spiral" effect here? Is the better hull material driving a paradigm shift in where previous notions of an acceptable range of D/L needs to be modified to fit the technology available?

 

yellowjacket

Interesting question.

Structurally, if we take a boat that has a very good L/D ratio, so it is incredibly light, what does this mean? It just means it is light. But it begs the questions, which has been raised above and the raison d'etre for this thread, does it do the job? For example, a grounding or a minor collisions or whatever, this is "fit for purpose". Assuming these Qs are ticked yes, then the weight issue doesn't really affect much else. The only influence it can have is in the draft or its overall displacement; this affects the added mass component in dynamic motions. The deeper the draft the more added mass and the greater the influence on said motions....but this leads and crosses over into hydrodynamics.

So, looking hydrodynamically things are different. As you have noted a narrow beam affects stability, but what do you mean by stability. This has many different meanings.

A L/B ration of say 15~20 is great in many applications, but as a mono, not so. The series 64 hulls proved that, (a potential new class of frigates for the US Navy in the 1950s). But place two side by side, eureka, perfect. No stability problem. Er...but what stability??....this is referring to roll stability and hence the ability to restore itself once disturbed by an external force. But what about motions, does she "feel" stable, ie stiff or tender, hence the roll period, which is influenced by the inertia of transverse waterplane area. Buy simply making the hull longer reduces vertical accelerations....and on it goes.

So you see, designing boats has many layers to it and many paths in which one can take, all different and all are working solutions.

Designing aircraft requires sufficient power and minimum drag to work effectively (notwhilstanding minium weight). Whilst this is a nice achievable goal for a naval architect, other factors often come into play and the low drag and high power often do not even come into the "equation" of the whole design.

There are far too many compromises that require to be satisfied. And what is satisfactory for one is not for another, despite both designs being successful.

So to go back to your point. If one were designing a boat with one objective and one objective only, then yes i would agree that there must exist some "death spiral". However, boats, most normal boats, have much more than one design brief and hence too many compromises to be made for a 'death spiral' to ever really influence the design as it does in aircraft design. Since just changing one attribute (keeping all others the same) can affect the whole deisgn for the better or worse...depending upon the path choosen.

 

I was thinking that the "reverse death spiral" effects would be more pronounced with a planing hull, and that is what I had in mind in posing the question.

Since a planing hull "flies" on the water, the analogy with the aircraft is much more appropriate, and with the greater payoffs, the designer may be more willing to go in that direction, since the lightness effects would snowball much more in a hull that is more weight sensitive than a displacement or semi-planing hull.

 

I generally don't design boats with planning hulls, but have done so in the past. Others that do design with planning hulls on a regular basis may chip in better. But once you're up on the plane, ie got over the main prismatic hump, addition weight will just slow you down some. So long as the boat can get up onto the plane. Which is related to the weight, the hull design and of course the power delivery.

The problem is that your death spiral is more related to pure racers and such...which generally have one design objective. Since even a "normal" planning boat, say to an owner, the hull is now lighter owing to material XXX, you can go faster...they'll do that, but also hoist in the "lighter" and start adding weight, such as extra people onboard, more fuel, or more outfitting etc...what you gain with one, the client takes away with the other.