gearing for fuel economy

[Waddie]

this question probably belongs on an automotive thread, but a lot of you boat people are engineers and/or mechanical types so you might have some good input regarding this question;

Why can't we achieve way better gas mileage in our vehicles, including power boats, by improving gear ratios, adding more gears in a wider ratio, or utilizing devices like a Marlin Crawler ( which gives you a low/high gearbox behind your regular tranny, thereby doubling your gearing) ?

18 wheelers have always used gearing to produce the power to the wheels that they require. Without all those gears they couldn't pull a tenth of what they do. How about a 2 cyl engine in a Honda Civic with a 15 speed gearbox? What kind of mileage could we get?

just wondering,

Waddie

[PAR]

It's been tried and you have a diminishing set of returns as you add gear ratio combinations. You're right in that if you discount frictional and other energy loses, a very small engine can "spool up" a substantial amount of power. I can use a cordless drill to operate my 6,000 pound, twin sling gantry. It's a 12 VDC cordless drill!

Ultimately it boils down to a few key issues, Weight, frictional loses, complication and of course additional cost. In an 18 wheeler, you have to have a huge gear box, because it can't do it's job without it, so the only cost effective method is with lots of gears. In a car, you can get by with just two, so it's hard to justify the additional complication, weight and expense of additional gearing. Now this said and people being the finicky dolts they are, we have 6 speed transmissions, when a 3 speed will do fine. On a boat, the gear box is much less useful, as it is on an airplane, though you can employ them in special applications (read costly high preformance, likely to break applications).

In short, sure, you can use the idea, but in reality, it'll cost more then necessary or that practicality will suggest is reasonable.

[CDK]

Originally Posted by Waddie

Why can't we achieve way better gas mileage in our vehicles, including power boats, by improving gear ratios, adding more gears in a wider ratio, or utilizing devices like a Marlin Crawler ( which gives you a low/high gearbox behind your regular tranny, thereby doubling your gearing) ?

Actually we have something far superior to any number of gears, the continuously variable prop. It enables you to get optimum performance at any load condition by keeping the engine at the point of best economy.

But it does cost a bit more than a fixed prop, making it too expensive for boats that are only occasionally used.

[Frosty]

What you are talking about is keeping the torque on the button.

Auto trannies do this with in the confines of the ratios available.

The modern motorcycle uses a variomatic transmission like the Dutch Daff car in the 70's.

Its all been done, yet Hummers remain popular,--go figure.

Mind you there are two very differnt ecomony expectations that of European and of American iron push rod V8s of the last era that still survive as dinasours as far as Europeans are concerned.

Economy sells, I think we have just about got it cornered, but in the US it seems of little concern as long as you can afford it --burn it.

[yipster]

http://www.boattest.com/boats/boat_video.aspx?ID=1956
http://www.boattest.com/boats/boat_video.aspx?ID=2416

Months ago we tested the Meridian 441 with straight-shaft twin 425-hp diesel inboards and noted her performance numbers. Some among us, including Capt. Steve, thought that the boat was slightly underpowered, but that is sometimes the case with brands that are trying to come in with as low a price-point as possible. Instead of simply putting a higher horsepower engine in the 441, Meridian went one better and put in a 480-hp Zeus pod drive propulsion package. When we found out about the upgrade, we asked Meridian when and where we could get aboard for a new performance test. Here is the comparison of the two different drive systems.

Meridian offers three power choices for the 441 Sedan. Standard is a pair of 380 QSB Cummins MerCruiser Diesel (CMD) Electronic diesels. Two upgrade options are available. First is our previous test power of 425-hp QSB CMD engines with straight shafts. The second option is a pair of 480-hp QSB CMDs. A further option with both of the larger engines is a Zeus pod drive propulsion system. Our test boat had the 480s driving through the Zeus pods.

Pods in general have a reputation for adding about 30% to the mpg performance of a boat at best cruise. Most of this improvement in efficiency is due to the fact that their props are working horizontally rather than angled down, and have two counter-rotating props. As a result all of the energy is directly transferred to driving the boat forward, rather than down and forward.

[Waddie]

It would seem that friction and excessive weight are the enemies of developing more complex transmissions whether in boats or autos, though there is quite a lot of experimentation going on in this field.

here is a link to one effort;

www.orbitaltraction.com/marine.html

As engines become more efficient perhaps transmissions will, too.

I looked up variable speed props and saw that they use engine rpm to change the prop pitch, but several sites recommend them only for boats that are somewhat underpowered, claiming they aren't as efficient as fixed blade designs properly matched to the boat's power and weight.

Does anyone make a folding prop for outboards ? Especially in the 6 to 10 hp range ? Or is this not practical for some reason ?

thanks,

Waddie

[PAR]

A folding frop for an outboard might rival the cost of the outboard. This is also the case for huge gear reduction systems, they cost so darn much you can learn to live with less.

[Waddie]

Hello Paul,

I would like to get away from having to raise the 6hp outboard when under sail.

On my 19' Mariner it doesn't seem to produce all that much drag if I leave it in the water, but I have no way of accurately measuring speed so I can't say for sure. I don't race, just cruising around. If a folding prop existed, and wasn't priced too high, I would try one.

I wonder if there is a market for such an item in the sailing community if the costs could be controlled? What would be a marketable price point for an outboard folding prop ?

regards,

Waddie

[CDK]

Originally Posted by Waddie

I looked up variable speed props and saw that they use engine rpm to change the prop pitch, but several sites recommend them only for boats that are somewhat underpowered, claiming they aren't as efficient as fixed blade designs properly matched to the boat's power and weight.

There is more to it than just that. Several Scandinavian companies offer systems allowing the pitch to be changed by the pilot over a wide range including vane position (neutral) and reverse, eliminating gear box control.

Here is an overview:
http://www.nauticexpo.com/boat-manuf...ller-1966.html

"I have no way of accurately measuring speed" - Can you possibly borrow a watch and a piece of string for this?

[Waddie]

OK, I'll bite.

I actually own a watch and a piece of string.

How does it work for measuring speed on the water.

How precise is this method?

Waddie

[wardd]

you throw someone overboard to hold the string and time how long they can tread water

[Frosty]

What they did in the olde days was throw or place a beer can in the sea at the front of the boat then they would get it out again after a certain time and mark the position is was recovered. This distance from the front to the back was measures with a polypropelene rope with Knots in it.

The measurement would be in knots. They would then take this information to the capatain who would check it against the GPS.

[Waddie]

the watch/string is to sailing as snipe hunting is to, well, hunting.

I get it now.....you're actually measuring how gullible a person can be..

thanks,

Waddie

[rxcomposite]

Originally Posted by Frosty

The measurement would be in knots. They would then take this information to the capatain who would check it against the GPS.

Rofl.