Designing hulls for efficient slow motoring

[JonathanCole]

Is there any information out there about hyper-efficient hull forms? I want to build a slow motoring boat that is extremely fuel efficient. I know long slim cat hulls and perhaps SWATH or semi SWATH with a not too small waterplane have been discussed on this site. What if you wanted to make a world record in efficient motoring. What are all the elements, both conventional wisdom and more obscure for designing for motoring efficiency?

[Raggi_Thor]

By slow you mean speed/length ratio close to 1 ?
If so, go for a cp~0.5 and minimum wet surface.

[marshmat]

I'd add long and thin to that (which will contribute to minimum wetted area). Clean entry and smooth flow around the stern. Perhaps wavepiercing or bulbous-bow- any thoughts on that, folks?

[BillyDoc]

Brian Eiland posted a very interesting paper by Nils Lucander (http://boatdesign.net/forums/showthr...5&page=2&pp=15) that talks about ways of reducing wave-making (which he calls the "Pump Effect), and thereby resistance. Basically, Lucander suggests positioning the largest hull sections about 65% aft on the waterline to minimize resistance.

[JonathanCole]

Definition of cp? I am just getting up to speed on some of this terminology.

That article by Eilander is good, actually is very intuitive if you think about it. By putting the largest part of the hull form well aft, there would be a lack of suddenness to the change in flow and gravity then assists because the tendency to push a wave up, given time, would be neutralized by gravity. If the water never goes vertical, no work is done.

So we have long and thin, minimizing wetted area (by maximizing cross-sectional curvature?), wide part of hull 65% aft...
Fine entry? Bulbous entry? Wavepiercing? What about the stern? Should it be fine as well? How about surface treatments? Are there textures or coatings that reduce frictional losses at the laminar flow level?

I still am drawn to the potentials of small waterplane hulls with wider bouyancy producing struts. This is a compromise to reduce wavemaking while allowing some loading flexibility. Malcolm Tennant mentioned this is papers posted on the web (Thanks Yipster)

I also wonder whether you could have a submerged hull and very fine struts if the hull displacement is sufficient to float the hulls when water is blown out of ballast tanks (pipes) with compressed air. I can imagine some very simple arrangements that would work and be dummyproof and failsafe. That way you could float the hulls for maintenance, reduced draft, access to equipment or compartments, etc. If you are going to have submarine hulls, then why shouldn't they surface like other subs?

Another point of interest for fuel efficiency is this pulling prop arrangement that Volvo recently announced with great fanfare. Supposed to use 20% less fuel. If this is so, why doesn't everyone do it?

[Skippy]

Just a few random thoughts:

Cp is the prismatic coefficient. Submerged volume divided by ( length at the waterline times max submerged section ).
A semicircular section has the minimum wetted surface.
A tall narrow section (e.g. cat or small waterplane) has more ws but less wave drag. The tradeoff depends on your speed range.
I've seen a solar-powered RC model that was a trimaran. L/B of the main hull was 14.
Sailing cats do half decent speeds even compared to slower motorboats, so you might want to look at kayaks and canoes depending on your target speeds.

[JonathanCole]

Cp is the prismatic coefficient. Submerged volume divided by ( length at the waterline times max submerged section ).

Is length at the waterline combination of both sides or just one side? Is it straight line length or the length that actually follows the form at the waterline? Is the maximum submerged section measured as an area? If so then the prismatic coefficient would be a ratio between two volumes, is that correct? Is the maximum submerged section the section that has the greatest submerged area? Or, is it the section that is most submerged?

[FAST FRED]

The question is how fast you want to go.

A half submerged ball has the least wetted surface per weight , but malkes a lousey boat,
Define the range , speed and endurance required. Also the sea state the vessel needs to operate Ocean? Lake?

There was a fantastic thread a while ago about "Long Skinney boats"

That may answer 99% of your questions.

FAST FRED

[Raggi_Thor]

LWL is the straight length.

You started by "slow speed", but all you discuss is how to go faster.
At slow speeds a short fat boat will have less resistance than a long slender boat, a cat or tri or swat and a bulbous bow etc etc.
All the "extras" is made for increased speed and this means more power.

[CDBarry]

Efficiency has to be defined in terms of what you have to "pay" divided by what you want to get. This means that any sort of efficiency has to be defined by the minimum constraints as well, because otherwise the most efficient boat is none.

What do you want to carry, how big is it ....?

[JonathanCole]

Actually, the boat I would like to build is a live-aboard solar powered boat made for inland waterways - canals, rivers, lakes - with occasional transits of near-shore ocean when seas are less than maybe 3 feet. I have been thinking about a 15-16 meter length and 6 meter width. I am a photovoltaic systems designer and so have been interested in powering the vessel with electric drives. Numerous vessels of this size and power modality have been built. I want to design and build the most advanced vessel of this sort that has thus far been created. I am attaching several photos of some of the sun powered electric vessels that have been built. Other photos are attached to the s.w.a.t.h. thread. They are all multihulls, mostly catamarans. Many are used for shuttles and ferries.

Over a period of several years I lived on boats in the Virgin Islands and really loved it. I particularly enjoyed the spaciousness of multihulls. Later as a photovoltaic system designer I realized that a wide beam boat would allow for a large roof of solar-electric panels. You could have the advantages of space with none of the rigging of sailboats and extremely low operating costs.

Having lived in a solar powered house for fifteen years I found out how cost effective and low maintenance these photovoltaic systems can be when constructed properly. Now I want to build this solar powered boat that I have been thinking about for a decade.

Many people are into boats for speed and a spirit of competition. I just love living on the water and being mobile.

In order to get the most out of the craft, I am looking to maximize the speed per unit of energy expended. Of course it must be able to go faster that any opposing currents encountered. Probably at least 6 or 7 MPH. However, maximum speed may be important as well, if the best miles per gallon are achieved at a higher speed. This is why utilizing the most hydrodynamically slippery hull shape with the most advantageous sort of drives are in order. In other words there must be a balance of design features that give the most practical, comfortable, maintainable and affordable vessel possible.

[BillyDoc]

Coincidentally, I have been thinking along similar lines.

I’m just starting out on a design for an offshore cruiser of roughly 12 meters and have been thinking very seriously of ditching the diesel completely. Instead, I would install two retractable props port and starboard on vertical structures similar to the bottom end of a typical outboard (the shaft would be vertical, driving 45 degree gears to bend the torque 90 degrees). Each shaft would be powered with a DC brushless motor with independent PWM control of speed and direction. I envision a means of rotating the props about their vertical axis as well, so when docking I should be able to “parallel park” directly sideways if necessary. Or spin the boat on it’s mast if I want to.

DC brushless motors utilize strong (rare earth) magnets in their rotors, so when sailing with more wind than hull speed I can lower the props and use them as generators to recharge the batteries. Photovoltaics can charge the batteries when in port and at sea.

For 12 meters (40 feet) I’m guessing that two 15 hp motors should be way more than enough, which works out to a power drain at full load of a little more than 22 KW. To get any range will require a lot of batteries, but all that lead makes fine ballast. I haven’t got around to checking battery capacity and weights yet.

I also suspect that a couple of DC brushless motors, with their very high low-speed torque will translate into a much lower horsepower requirement than if the power is from a diesel and I won’t need to run them at full power all that often.

I would be very interested in knowing more about your thoughts and design!

Bill Cushman

[JonathanCole]

Hi Bill,
FYI, There are several electric outboard manufacturers making 5-10 HP electric drives which claim to have as much thrust as internal combustion drives with 3 or 4 times the horsepower. This makes sense since electric drives have full torque as soon as they start turning while ICE drives have to develop high RPMs to get their rated torque. So you can probably get away with much less battery power than you might think.
Check these electric outboards:
http://www.qis.net/~jmgraham/resrun.htm
http://www.rayeo.com/motors.htm
http://www.electriclaunch.com/outboards.htm

Currently I plan to have 8000 watts of peak photovoltaic power and 12000 watts of battery storage. I may also incorporate several small (500watt) wind generators, for night charging. I would look for a range of about 50 miles a day at 6-7 MPH.

I have also been thinking about using two electric outboards mounted in such a way as to have 180 degree turning plus instantaneous reverse (which is an advantage of electric drives). I am thinking that fore and aft mounting would give the greatest maneuverability but I am still thinking about that aspect. I want to be able to negotiate tight spaces for entering locks and slips. I have also considered whether to use a GPS positioning system to be able to maintain station without anchoring.

I think it will be a bit more difficult to design an offshore cruiser with this technology, just because there are conditions that can arise where you may need greater power than you can store in your batteries. But that can situation is easily met with a backup fuel generator on board.

An electric boat needs to be very efficient, since energy storage is heavier and takes much more volume. That is why I am interested in figuring out how to wring every drop of efficiency out through an intelligent design process.

Jon

[Skippy]

Most people think catamaran first when asked about multihulls. But unless the boat is very heavy, the lower ws of the tri will give it an advantage. I've always thought the tri's wider beam would be ideal for laying out lots of solar cells. If you want reliable transpo and living facilities, I would also include a sail rig and wind turbine capable of getting you around and generating energy on cloudy but windy days.

[JonathanCole]

These Students from Australia agreed with the tri configuration when setting out to break a world record. In their case (re)movable outer hulls were an advantage as well. Take a look at
http://www.cmsb.com.my/tpis/sunboat/

As far as the sail option, what do you think about this Australian solar sailor?
http://www.solarsailor.com.au/
These guys aren't fooling around. But the photovoltaics mounted to a vertical surface have limited output so I am not sure how much energy they provide. This ship is actually a hybrid that includes the use of an ICE generator.

Here's another interesting project along these lines trying to get off the ground.
http://www.solarnavigator.net/catamaran_hull.htm

This solar powered boat idea has been developing for 15 years and is getting a lot of interest. Germans, (Kopfdesign), British, Australians and a few Americans are pushing forward in developing this technology.