Beam and efficiency

[rolomart3]

How does the beam of a boat with a full displacement hull affect efficiency when cruising at a speed/length ratio of 1? Do narrower boats have significantly greater efficiency or does it not matter much?

Ok let us be scientific about it. First, we clarify the question. We are wondering about the fuel efficiency.
When the boat moves one length, it moves the mass of water that it displaces to float. Ignoring additional weight to maintain stiffness, the shape of the hull does not impact the amount of water mass displaced. Not much of a change there.
However, how far the water needs to be displaced is an entire matter altogether. Here shape is paramount. Consider the fate of a little cube of water in front of an incoming catamaran and in front of an incoming monohull both with the same beam. On which hull most likely the cube of water will have to be moved further to the side?
All that additional distance for the motion of all those thousands of water cubes requires a lot more power to do when moving the boat.

Then we can safely assume that for the same type of hull in general the wider the beam the less fuel efficient the vessel becomes. We can also assume that the catamaran hull type is also more efficient.
One point to keep in mind is that as the length of the sponsons in a catamaran increases to allow
narrowing the beam an still displace the same amount of water mass, the wet surface subject to friction increases. In a laminar flow of a liquid it has been observed that friction causes drag proportional to the cube of the velocity. Consequently, a catamaran with very large length may have significantly larger surface to drag such that at certain higher velocity becomes less fuel efficient than the monohull.
Lastly, is the consideration for the forces acted upon the vessel. A catamaran is exposed to forces its sponsons a mono hull is not. To maintain integrity alignment and structural integrity of a catamaran, additional weight needs to be devoted to reinforcing structural members. Although this additional weight does not affect anything said above, it does reduce the payload of the vessel.

 

[Hippocampus]

Would note
Displacement = more wetted surface. Wetted surface = more parasitic drag. So regardless of hull shape there’s a relationship between displacement and drag as long as the hull is functioning in displacement mode.
Increased frontal plane = more resistance. More resistance = more energy required to move it. More energy = more fuel burn. So the better the prismatic coefficient and less the beam the less energy (fuel) required. Mono or multi.
Stability is a function of meta centric righting arm and form stability. The greater the beam the greater the form stability (mono or multi) and less likely to roll (in general) and the less ballast or other measures required to get the same initial stability. (Gz curves will be different however. More dependency on form stability the greater the area under null when inverted and poorer AVS).
At the same displacement long narrow hulls will be more efficient in general but have less propensity to hobby horse and more to roll. However the are multiple other factors involved so this is only a general statement.
Think in the sizes of most recreational trawlers useable space and payload are important so aside from docking expenses with increased LOA there’s a fundamental design problem. Corridors are a necessary evil to get from space to space unless you will accept walk through rooms. Compare a New England four on four cottage to a New Orleans shotgun. Both may have the same square footage but one provides much greater privacy. Once you’re talking about two or three stateroom boats this becomes an issue. Some narrow boats solve this by having staterooms feed off a common space. So you have wheel house (and possibly salon) near the center with stairs down to staterooms aft and forward of that space. Engine room is under the wheelhouse. Other way is to have a significant aft cockpit with engine(s) under but this is usually best reserved for larger boats unless you willing to put up with a stateroom above that heat, vibration and noise. Naval architecture is quite complex. Gotten even more so with computor modeling. Personally think my knowledge is very limited but do think the above grossly addresses some of the issues.

 

[Nomad Willy]

Anybody know yet what the boat “Idlewild” is?

 

[mvweebles]

Anybody know yet what the boat “Idlewild” is?

It's a Buehler design. Some farmer from Buffalo NY had it built to do the NW passage or something crazy like that.

http://northwestpassage2014.blogspot.com/2014/06/i-remember-mv-idlewild-2005nwp-on.html?m=1

 

[RCook]

This one?


Idlewild

 

[DDW]

The prismatic coefficient isn't a measure of frontal area, it is a measure of solidity, or how fine the ends are. Two boats can have exactly the same midships section (frontal area) and very different displacement and wavemaking characteristics. The difficulty a displacement boat has (compared to planing or a submarine) is operating at the interface of two different fluids. Wavemaking - not form or frictional resistance - is a very large percentage of the drag at higher speeds, and very complex to understand.

Also, meaningful comparison between hulls is fraught with difficulty, it is like a balloon: push in here, it pops out there. If you simply make the beam less, the displacement also shrinks, unless the draft or length change - then what are we comparing? There are comparisons in this thread of a 30 ft boat to a 57 ft boat, trying to draw conclusions about L/B ratios. How can you do so when virtually every other measurement and parameter are different?

To be meaningful, one must decide what the fixed variable is - LOA? beam? displacement? draft? accommodation volume? Change several of these at once and you have apples and oranges. And yet it is nearly impossible to change one without also changing others.

 

[Hippocampus]

Yup DDW that’s a key point. Still that picture showing it has less beam than the skiff to starboard is quite remarkable.

 

[Nomad Willy]

Oh yea I’ve seen that.

It’s light and narrow. Aspect ratio and weight. Two big boxes to check.

And the boat is huge compared to a W30 (Willard 30’).

But the thing that makes them incomparable I think is that they will be operating at a wildly different S/L (speed length ratio). That’s what they did in the old days (20’s-30’s). They didn’t have light and they didn’t have power either. All their boats were long and narrow.

Look at a late 40’s/early50’s CC. They didn’t look it because they had modern looking hulls and cabins (think bull nosed) but they were narrow. And all they had for power was low compression flathead .. mostly six cylinder ... and heavy. So all the boats needed to be narrow.

But I see the farmers boat has good rocker for efficiency .. mostly all in the stern. Good for bigger rowboats too for the same reason. Lots of old rowboats had a flat bottom that flat and mostly straight but turned up abruptly near the stern. Look at the picture. The drawings showing the interior ... you can see the chine (or QBBL) and the abrupt turn-up near the stern. Very efficient. My W30 has a different version of the same thing but w a “hooked” bottom aft in a rounded hull design. Not as easy to see as the chine boat.

My boat (W30) is efficient for carrying weight but not for attaining speed.