Low Displacement/length ratio boat designs

[Theotilus]

There was an article in the Jan/Feb Wooden Boat magazine on LDL boat designs
with Nigel Irens as the featured designer. His boat "Greta" being a great example of this concept. I really like this kind of design for its seaworthyness, fuel economy and speed. It seems that a lot of more recent sailboat designs I have looked at meet these parameters. I would like to see some lively discussion on this type of design. Pros and cons.

 

[JSL]

are power boats included or are you limiting this to sail.

 

[Theotilus]

I am definitely interested in power boat design

 

[JSL]

I will check the numbers on some designs I have done and if they qualify I will post them.

 

[Robjl]

LDLs

Here a a few links that I have found useful.
http://www.setsail.com/
http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf
http://www.artnautica.com/designs/long-range-cruiser-58

 

[Mr Efficiency]

Comfort could be the problem, unless in mainly flat water. A catamaran might get around that.

 

[PAR]

LDL designs aren't something new. Many older designs have these features, though without the advantages of modern building techniques and materials, they're not as light as they can be now. I think of these more as speed limited hull shapes, rather then a new, super efficient hull form.

This said, some new designs are taking a new look at much more efficient shapes decisions, especially with new build and material uses, with the given accommodations compromises these usually require. The market typically hasn't seen these designs in a very popular light, though a small market is growing in recent years.

It's a SOR specific approach, with significant changes to the some of the requirements, that aren't typical of the powerboat market in general. Most clients can't accept the lack of elbow room, these designs force, plus they often want more speed as well, but if you can live with the less than common compromises, you can have a fine power yacht. This usually requires more length on deck, for the accommodations you get, which is costly in terms of ownership. Simply put, you may need a 60' yacht, just to have the accommodations of a 35' - 40' yacht, which hits you hard when looking for a berth, travel lift and other ownership concerns.

 

[Rurudyne]

Yep. Cheaper to cruise in, (sometimes much) dearer to park.

 

[JSL]

Okay - see if this qualifies
The design I have was a 54' cruiser (FRP/sandwhich) launched in 1978. LWL = 48.5' and the displacement was 32,000 lb (14.28 tons) which would make the D/L 125.
Power was a single 260 hp diesel. Speed max was 17 knots and fuel consumption at 14 knot cruise was just under 7 g/h (32 l/h). Apparently at about 8 knots the fuel was about 1/2 that
After 2 years of living & cruising the owner added 11' (for the new family member) and the max speed dropped to 16+ knots.

 

[Rurudyne]

What would you guess the interior volume would be for a 65' (if I'm read you right) boat like yours relative to a 40' class A RV? I'm guessing upwards of twice. I recently read that a relatively new class A RV was getting something like 10 mpg trip ... which is better than my mom's 70s era Newport she used to have ( http://chrysler-parts.uneedapart.com/images/chrysler-newport-parts.jpg ... her's was powder blue with a 400ci big block) ... no telling about a 70s era RV.

4 nautical miles per gallon slowly moving around isn't all that bad if you consider how much space you've got. Of course parking an RV doesn't cost a lot....

 

[Theotilus]

Robjl, thanks for the links. Those boats are exactly what I am talking about. I remember reading about the Beowolf quite awhile ago and then read about the powerboat version thereafter. I realize alot of people would discount this type of vessel for various reasons, but for safe and economical cruising I really think this is the way to go. NOW, I need to translate that design into a 25 to 30' version, which may not be practical although Nigel Irens is working on it. I am really looking at a boat that would make a good weekender or perhaps a week long cruiser that I could cruise around the San Juan Islands and parts of the Inside Passage. Having been raised in a commercial fishing family I am well aware of the limitations of a boat like this and would not push a boat like this beyond its limits. Having fun is where I'm at.

I really like the Bluejacket Boats, but they don't quite fit the LDL idea, although they are
economical to operate. I really wish I had the education to be a competent designer as I would just design my own and go for it. The WHIO is another boat that I believe fits the LDL idea. I think that I enjoy the design process as much anything and appreciate the responses to date.

 

[Mr Efficiency]

How fast do you want to go ? (cruise speed)

 

[Alik]

Every boat design is a compromise of controversial qualities. By making slender hull one can reduce the drag and get better performance and seakeeping, but will loose load carrying capacity and accommodation comfort. Thus, look at priorities and consider all pros and cons.

Today, most of motor yachts below 24m in length are intended to sit 99.9% of time in marina. They are designed to fit maximum accommodation in minimum length, to reduce the marina fees. Marina fees give the primary expense of the owner, not the fuel burn. So the performance secondary issue, though in adversed materials it can claim as high performance boat, probably due to excess engine power, not to the hull parameters and shape.

I would say average motor yacht today is a fiberglass box designed to carry furniture on the hump of resistance.

And LDL is another extreme from the above.

 

[UNCIVILIZED]

A couple of perks of the concept are;
- Ease of towability, assuming that the boat's displacement is kept reasonable.
- Being able to ship it in a cargo container, if the design size is kept within certain parameters. Multihull (disassembleable & or take down design type), or Monohull.
- When designing/building, unless you use exotic materials/are trying for a super light boat. The cost of a vessel is determined to a great degree by her displacement, so that within reason, long & thin is close (cost wise) to average length & beam of the same weight.

As has been stated, if you're going to keep her at a marina most of the time, then a design like this doesn't make a lot of sense. But... if you intend to use her to go/see places, then there are a good number of benefits.

 

[nzboy]

The Midwest eco trawler 33 would probably fit the sor with a diesel stern drive, with the added benefit of just towing it home when your not using it