Anybody in here mess with getting better FE out of Boats?
 

Boats are pretty thirsty an inefficient. I'm looking into ideas to get my 1538 1970s fisher water rover with a 1989 yamaha 30hp outboard to get better mpg. What ideas have you guys tried?

There are really only two areas you could look at.
 

The first is engine efficiency.

The second is your hydro-dynamic drag.

Both are complex.

I prefer sailboats but if you want to optimize on the water fuel economy, you have to design from the water up.

Foils that lift the hull out of the water reduce hydro-dynamic drag tremendously. Then, with reduced drag, you can outline an engine design that minimizes fuel use at that reduced drag.

If speeds get high enough, then you can start down the road of reducing your aerodynamic drag.

Water drag is much worse than aero drag and you have both with the boat. I assume with just 30hp it doesn't come up on a plane but I could be wrong. If it does plane you want to get it on a plane and then back off some to save gas but maintain the plane. If it isn't planing but the hull is designed to you need more power to make it happen or lighten things up and hope. Make sure its operating in the motors designed range as well, if it over revs you need more pitch on the prop and if it never revs all the way you need less pitch.

I took the Jetski out on a new-to-me lake last summer, got to the end, and turned around to head back. 5 minutes into the ride back, I had to switch over to reserve. Thinking I would conserve fuel I backed off to about half speed. 15 minutes later I was out. 3 hours later I had swam the boat back to dock.

Someone told me 2-stroke motors want to run at near full-throttle to get the most power out of the fuel. I'm not sure what is most efficient, and haven't seen a BSFC for 2-strokes.

All that said, I think reducing weight is probably most important if you can't change the hull shape.

Diesel engine?

Bingo!
 

Fuel efficiency in a hot pocket pouch.

https://newatlas.com/yanmar-dtorque-...utboard/51675/

Its great but $25,000+ for 50hp buys an endless supply of gas.

Some years back I found I intriguing a 30’ turbodiesel with Anderson drive prop a “high” mpg commuter. Hawaiian Islands, IIRC. Both of those were new enough (fir a small package).

One has to start from scratch. And optimize steady state cruise. It’s start and stop that kills boat FE.

Operating at the interface between air and water, you want a hull that optimizes for both.

https://www.brighthubengineering.com...ull-technolgy/

Apparently typical bsfc is on plane 5000rpm at throttle valve fully open (volumetric efficiency) with a mpg prop... big prop that wont let the motor rev up. So currently I run 100% throttle, 5000 rpm (max hp is 5500... max rpm is 6000 rpm). And.... I get a whopping 9 mpg lol. 15' flat bottom 25 mph. 90s yamaha 2s 30hp.

Have you looked at checking proper engine mounting height and tried different props?

You're a little underpowered probably. I have a '96 Boston Whaler Dauntless 15 with a Honda BF60 4-stroke and get 8-9mpg. I think my boat is bigger, has a sharper bow and more deadrise (less efficient). Mine tops out at 35mph, planes at a little over 10mph and cruises comfortably at 20mph/4ish KRPM.

If you can find something with a bit more umph but not much heavier, that may change your ride significantly. What is the hull rated for?

I was actually reading up on this subject earlier, and chances are boat efficiency is very closely related to airplane efficiency. One big thing I found is that jet drives are not as efficient as props. Just like planes, you will gain efficiency from large diameter 2 blade props, more blades=more turbulence.

No idea on hull design and water interference yet, haven’t looked into that enough.


That is really disappointing. I believe the BF60 engine is closely based on the 1st gen insight engine, and comes with lean burn.

I would say this closer to the ecomodder solution:

https://http2.mlstatic.com/cabo-mano...5_042015-F.jpg

here is a little calculator to play with. It really depends on the type of hull. You can underpower (or throttle down) and efficient displacement hull and get great results. Just like slowing down a car because of aero drag but it's much more drastic with water. A 30 foot displacement hull has a theoretical top speed of 9.2 knots if you put 81 horsepower to it, but to go 8.3 knots on the same boat you only need 38 HP. If 5.5 knots was enough it only takes 8 HP.

Now on a 20' planing hull it takes 210 HP to reach 35 knots. Dropping down to 75 HP will go 15.2 knots. Down below 40 HP it turns into a poor displacement hull design and goes slower on the same power as the bigger heavier 30' displacement hull.

once you know the horsepower required for what speed you can figure engine efficiency and get a MPG.

Boat Speed Calculator

I also plugged in my ship the USS Carl Vinson for fun. The US Navy always says 30+ knots, but if you put enough HP to that hull it is capable of 65 knots. That would require almost 1.8 million HP. Well it doesn't have that much but it does has an advertised 260,000 HP which would be good for 37 knots (which is pushing 43 mph). I can't confirm or deny that number but physics is physics.

I just wonder why boats are always cut off flat in the back rather than achieving a teardrop shape? Shouldn't the pointy end face aft? Liquid fluid dynamics must not directly mimic gas fluid dynamics.

Displacement hulls do have rounded backs.

For a planing hull the design objective is to get as much of the hull out of the water as possible to reduce drag.

Can anybody explain how to use that calc for a 15' flat bottom? lol

What's a boat?

A 1969 Deuce-and-a-quarter.

It's easy to get great fuel economy from a boat. You just need the right boat, like a canoe, kayak, or a sailboat.

Use the planing hull and you need the weight, it should be pretty close. A flat bottom with enough power is the most planing hull there is, the v-hulls plane but are designed to keep a cut in the water for stability on waves and turning. A semi-displacement hull is the most inefficient trying to do both a smooth ride and good speed, think James Bond yachts. Who cares how much gas costs because money is no object. There are some exceptions but speed is always a consideration. They will make claims like most efficient design over 15 knots. Displacement hull are traditional ships and sailboats. They are the most efficient hulls speed not a factor. Yet with enough length they can also be pretty fast. The French built a 434 ft long destroyer that ran 52 mph. I would love to have seen that flank speed, boilers at battleshort pressures, blow by.

If you do a little research, you'll find out that 8mpg is really good for a boat. Although my boat is lighter than that Insight that you're somehow comparing it to, the drag and single speed make this an apples to freesbees comparison.

Try using a kite to pull the boat.

https://encrypted-tbn1.gstatic.com/i...7us_jHxzLnEF0g

That's quite common in Amazon.

In Vietnam they stick the long tail down a pipe and use it to pump water in the rice paddies. In Thailand they race.

https://duckduckgo.com/?q=long+tail+...deos&ia=videos

What does surprise me about the usage of those long-tails in Thailand is they fit it to nearly every Diesel engine regardless of size.

They use them here as well. Well, we call them mud motors. Basically makes it to where the boat can go anywhere hunting wise. The Game and Fish comission about bans them every single year so not something I wanna sink money into and then not be allowed on WMA land. Mostly, they are noisy and everyone hates them.

In Amazon they're used for general transportation. Even in big cities such as Manaus we see a lot of them in the "voadeiras". It's used there to prevent the propeller to get stuck on tree stubs below the water while crossing an igapó.


Well, soundproofing is not really a top-tier priority on those, but honestly it didn't bother me too much.

I think the most interesting use is to make one an Archimedes screw (sorta), to pump water out of rice paddies.

The boat I use most often gets about 150 MPGe on 8 210 amp hour 6 volt golf cart batteries 3 phase permanent magnet dc motor with 2 belts driving a prop shaft out the keel.
Cruise speed is 5 mph Flank speed is 6 mph, realistic range is 70 miles. It’s a 1982 Duffy electric packet.

Edit: here we are on our local river. https://youtu.be/rm2FhL8X4yU

I just sold my pedal boat projects, but they were winning races right out of the box. For me, the key was propeller efficiency. While studying the possibilities, I decided to try out a model aircraft propeller, and stayed with them. For motor power, you'd probably want to copy one in metal or carbon fiber, but if you are going for a dragon tail, there's room to gear it right.
I was recently appalled to learn that marine architects are still using the Betz limit in propeller calculations. This is usually quoted regarding windmill efficiency. It applies to tugboats, and pulling contests against stern wheelers, but once a vessel is under way, we need the Froude numbers, which take into account the steady supply of undisturbed water. By pushing gently on a large mass of water, we do much better than with a smaller diameter prop, which is always pushing on water that is running away. The situation is even worse with jet drives, but both are selected for their ability to run in shoal conditions. A dragon tail is easy to lift, or even operate surface-piercing at need.

Wouldn't changing to a 4-stroke outboard produce a substantial increase in MPG? Not to mention less stink.

Or turn off the gas engine and use an electric trolling motor. :-)

Got a picture ?
Google turns up some really squarish shallow draft boats


A newer engine will likely get you better mileage

Clean the hull
Paint & polish it really smooth & glossy

Add "stern plates"
Dunno what they are really called, but they are plates added aft of the stern, along the underside of the hull, and extend the apparent length of the hull

https://upload.wikimedia.org/wikiped...a%2C_Japan.jpg


Increase the fineness ratio: make it longer, or narrower

For displacement hulls , long boats go faster i.e. better MPG .
But since you are stuck with the boat you have - fussing with the prop and how you 'manage' your ride makes all the difference.
Virtually no 2 boats are the same - load, engine, prop etc etc.
So what works for you probably won't for the next man.

Trim tabs are handy for trim adjustments, and are a cheap way to lengthen a hull in plane. However, width is much better than length for planing, for the same reason that gliders are wider than fighters.
Displacement hulls have the unique problem of producing waves, which gives rise to a given hull speed often quoted as 1.37 X the square root of the waterline length in feet, to get the speed in knots beyond which great increases in power barely return any more speed, until the boat planes over it's bow wave, if possible.
Hydrofoils are quite feasible as add-ons, and increase both efficiency and available speed in a given sea condition. A main wing can be almost flat, carrying over 90% of the load, with a small canard having it's angle of attack controlled by a little spoon follower on the surface, provide a simple, stable and efficient configuration.

Clean and smooth the bottom. Wax it to make it slicker. A friend's Father had a 3/4 cabin Larsin with a v-8. We cleaned and waxed the bottom. End result was 7 mph faster.

The only thing I've tried that might apply is putting a bellmouth on the air intake. :confused:

https://www.physicsforums.com/thread...r-work.678202/

Depending on the hull architecture and performance envelope maybe a surface-piercing propellor? You might have to raise the transom.

I like the M-hull. Maybe you could get some benefit by strapping surfboards vertically on either side of the bow (sort of a trimaran) to kill the bow wave.

Free: Try raising motor height, experiment with weight placement in the boat, and if you have power trim, try trimming out just a bit. Figure out your most efficient cruising speed and try and operate at that speed as much as possible.

$$: get a propeller that brings gets you to max rpm @WOT

$$$$: buy a newer motor, consider the evinrude DFI 2-stroke E-TEC line, they are lighter than 4-strokes and get equivalent, if not better FE. From a performance perspective they blow 4-strokes out of the water.

I have a 14’ alumicraft and Yamaha 9.9 4stroke. 2.5 gallons seems to last for days. I think weight is your biggest issue.

Add a sail!!! Duh!!!