Fuel economy

Corrosion, Paint, Through Hulls, etc.
ddependo
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Re: Fuel economy

Post by ddependo »

I always thought B.O.A.T. stood for break out another thousand.
Wayne
1973 32 express fly bridge
Chattanooga
"Southern Lady"
garycarroll
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Re: Fuel economy

Post by garycarroll »

I heard it was "Broke, or about to." Functionally, this is po-ta-to/po-tah-to - they work out to the same thing.
1979 32' Sedan Cruiser Flybridge, "Extended Play"
Dual 350 Crusader, carbureted with closed freshwater cooling
Westerbeke gasoline marine generator
robalo220
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Re: Fuel economy

Post by robalo220 »

[quote="Fastjeff"]One more thing...The Germans (who else?) developed a surface drive propulsion system with a variable pitch propeller.

Of all the difficult prop pitch selection deals, the surface running drives are the worst. Half the prop is out of the water at full plane, yet it's all under the water at rest. That generates a massive over-pitched conditions at plane off, then an over reving condition at WOT. The German design allows a shallow pitch it get on plane, then adds pitch at the speed builds up--like an auto tranny would do.

Slick, eh?

Jeff

PS: One mire thing: A guy I met who runs an off shore catamaran race boat once told me he could not get her on plane with the prop pitch the motors needed for top speed (around 160 mph as I recall)--and his engines EACH put out 1,400 hp! The solution was to pipe exhaust to just before the props which made them cavitate enough to get the revs up--like slipping a clutch.[/quote]

Jeff, the theory behind the surface drive units are that a prop spinning in water only the bottom half of the prop actually has any "grab" in the water. Think about that for a second. A single engine boat pulls to the side the prop is spinning in reverse. So that means only the bottom portion of the prop is grabbing water so to speak. If you have ever run a single engine inboard M or other boat in reverse, you know that this is true. If its a right hand or left hand engine the boat will "pull" to the direction the prop is spinning in reverse.
Anyway the same works in forward. We just dont notice it because the thrust of the water off the prop hits the rudder and the rudder keeps the boat in the direction of the wheel/rudder (thrust of water against the rudder).
Now think about a surface drive setup. If the top portion of the prop isnt doing anything as far as thrust for the boat, if you have it spinning in air while the boat is on plane the thought is that there is less drag on the prop and the drive system and engine, then the prop can spin faster for less HP or even faster for more HP compared to a prop that is fully in water with more HP.
Look Jeff, I'm an old boater, been reading boating magazines for a long time. I'm not saying they are the best but I dont agree necessilary that they are the "worst" as you say.
Again, I"m just passing along the "science" behind surface drives. Agree or disagree really is ok with me. Just passing along the "why"
John

1991 41 Marquis Convertible "Running on Water"
454 Crusaders
robalo220
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Re: Fuel economy

Post by robalo220 »

[quote="javalin390"]To answer Coastie's question of why boats don't use muti-gear transmissions, like say a 3 speed automatic in an old Buick, you have to get into the study of water hydraulics. When any boat is being designed, multiple factors are figured in: weight (boat and crew), hull displacement/type, engine size and power output, prop pitch, etc... This is then factored against the RPM of the prop shaft. On Big M's, ours for the most parts use Velvet drives with a 1:1 ratio, 1:1.5, or 1:19. For the sake of simplicity, we'll look at the simple 1:1. It's basically a direct drive, meaning if you engine is spinning 2500 RPM, so is your prop shaft. With an certain diameter prop, with a certain pitch, and an engine with X horse power, the boat will perform as designed at an acceptable speed-to-fuel consumption ratio. One can change and experiment with different size/pitch props to achieve at one end, good fuel economy or at the other end, more top speed. With all those factors staying constant, adding a second or even third gear would not produce the same results like in a car, the prop will either bog the engine down (like a big truck trying to pull a mountain pass in high gear) or the prop would spin so fast that it would cavitate and the boat just won't go any faster, just make an awesome rooster tail. And that would be on a glass smooth inland lake. In choppy seas the trans would shift from gear to gear, wildly hunting for what gear it "thinks" it should be in. Also, in a car, you are instinctively and constantly adjusting your gas pedal to compensate for climbing hills or coasting down grades and keeping a consistent speed. Imagine trying to keep a boat with an auto trans happy, while constantly adjusting twin hand throttles ! Maybe I'm overthinking it but I don't believe it would work, at least on a large cabin cruiser.[/quote]

Jim, good analysis. You mentioned transmission ratios. The different gearing allows the shaft to spin at different speeds at the same engine RPM. So a 1:1 the shaft spins at the same RPM as the engine. 1:2 means the engine turns 2 revolutions and the shaft turns one revolution. This is called gear reduction. Obviously there are many different ratios. The science is that a small prop spinning fast is less efficient than a larger prop spinning slower. So the gear ratios help achieve this. Next is the prop size (diameter and pitch). So if you prop your boat to be able to achieve max engine RPM that is about the best you can do in an ideal situation. However if you overprop your boat you might plane out at 1500 RPMs but WOT (wide open throttle) may only be 2800 RPMs which isnt good if your gas engine should attain 4200-4400 RPM's. Likewise if you under prop your boat you may not plane out till 3500 RPMs but if WOT achieves 5000 RPM's you will blow an engine due to over revving.
Now think about a multi speed transmission. You've got the perfect setup for your boat in first gear. The boat planes out nicely at a reasonable RPM. WOT is 4300 RPM. Right in that sweet spot. Now you add a higher gear. Due to drag of the water, weight of the boat, etc, etc, your boat wont suddenly go faster at WOT in that second gear, the engine will just bog down and not achieve WOT just as if you have over propped (too much pitch or too large a diameter or both) and your boat will not speed up but the RPMs will drop and you will be lucky to maintain the same speed as with the first gear where you achieved WOT. If you have ever changed props to too large or to small and experienced over revving or under revving you know this is true.

Just my 2 Cents worth. Good conversation on this topic. Keep it going....
Cheers
John
John

1991 41 Marquis Convertible "Running on Water"
454 Crusaders
Fastjeff
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Re: Fuel economy

Post by Fastjeff »

Glad you mentioned multi-speed automatic transmissions...

Mercury has just come out with a 600 hp, V-12 outboard with a two speed automatic transmission! A game changer for the world of boating, for sure. A but pricey at (gulp!) $77,000 each, but...

Jeff
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington
garycarroll
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Re: Fuel economy

Post by garycarroll »

I was mulling over that excellent post on why multi-gear automatic transmissions have not been practical for boats. It occurs to me that with the advent of computer controlled integrated engines and navigational equipment this may not be true forever. Given inputs on engine loading, shaft speed, and true speed (via GPS) and knowing desired speed, it should be possible for a program to pick a gear to optimize using fly-by-wire control for throttle and transmission. The software would know not to "hunt" (change gear too freqently) and would not allow over revving. You'd see this first on new boats. The technology is possible now, but evidently isn't high on the priority list. I make no judgement on the marketability of such a system for private pleasure boats. It might be that we will go to electric motors driving the shafts before that comes out, and I such software would be even easier on electric drive pleasure boats.
1979 32' Sedan Cruiser Flybridge, "Extended Play"
Dual 350 Crusader, carbureted with closed freshwater cooling
Westerbeke gasoline marine generator
javalin390
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Re: Fuel economy

Post by javalin390 »

Interesting. There have been some serious advances in automotive electronics in the last 25 years, but most recently in the last 5 or so. Anyone one that's familiar with SAE advances especially, concerning the way automotive electronics communicate with one another. Working in the trucking repair industry, we are currently working with what is known data bus networks that allow the engine module communicate with the ABS module, transmission module, insterment panels, etc... The new top end trucks, like Peterbilts and Kenworths integrate touch screen GPS screens into this as well. The two most common in the truck business are known as SAE J1587 and the much faster J1939. Recently an even faster network protocol is being introduced that blows the others away, from what I've read. (there is endless reading material on the subject if you simply google SAE J1939) With a super-duper fast computer network on a boat that could also integrate possibly GPS, real time weather conditions, or even fishing hot spots, maybe it could be possible. But it would likely work better on a big cruiser if it had powerful diesels. Maybe even a variable ratio transmission (think like a clutch on a snowmobile). Honda offered a variable ratio transmission a few years back, I think in the Civic, but it didn't seem to pan out. But with the ever accelerating technology, who knows ....
Jim Elias
1974 37' SedanFlybridge
Twin 360 Chryslers.
Marblehead, Ohio
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