Why Don't Boats Have Gears?

[Turtle]

so pretty much as the title says, particularly in regard to propeller driven small boats. beyond having forward-neutral-reverse, why is there no multiple forward gears as in a car? let's for the sake of argument take a comparison of a 2200 pound automobile and a 20 foot boat with a planing hull of equal weight. wouldn't having at least a second gear in the boat let you shift up at speed and reduce engine rpm and fuel use? :steering: :shrug: discuss. :cup:

[CraigD]

Boat's don't (with a few quirky and more-or-less incidental exceptions) have multiple forward gears (most of them have a clutch and a reverse gear) because they don't drive tractor wheels like cars do, but (in most modern cases) turn propellers like aircraft do. Propellers are more-or-less jet pumps, expelling fluid at a greater speed than the vehicle travels through it.

 

Like propeller driven aircraft, boats do have a need to optimize their propeller's efficiencies under different speed and force conditions, so do have a need for something functionally similar to a car's gears. Like aircraft propellers, one of the best solution for boat propellers is a variable pitch propeller.

 

The quirky exceptions I've seen to the boats don't have gears rule have been boats there a car engine with attached transmission was just dropped into a boat, with a non-steerable prop simply attached via a marine bearing to the end of the car's (usually shortened) drive shaft, where the differential would be on the rear-wheel drive car it came from. Mostly, you just left it in third (of four) gears, or reverse to reverse, but first gear was handy when you wanted to move slow and not worry about surging if you pushed the throttle open too far (eg: dock). As most of the boats had souped-up engines in the 250-500 HP range in around 20 feet of light wooden hull, and often balky throttle cables, this made them a good bit less scarey (though people accustom to stern drive and outboard motors tend to find prop and rudder inboards intrinsically terrifying).

 

I can't recall the name, but there WW1 fighter airplane that had a standard auto transmission with a reverse and multiple forward gears, because it was made by an Italian (if I recall correctly) car maker that simply mounted the engine and transmission backwards on the nose of a biplane. It was an awful plane, overweight and underpowered, but I read that its pilots, likely more in desperate bids for survival than victory strategies, would shift gears in flight to produce weird braking effects to shake superior planes off their tails.

[Turtle]

good stuff. :thumbs_up i've seen some video of boats with car engines on the amazon & such in travel-logs on rivers. looks damn scary all right! :omg: wouldn't want a sleeve or long hair or a necklace dropping onto that shaft; no sir! i've seen car engines driving irrigation pumps too, and now in thinking of it i have to wonder what gear they use. :QuestionM i suppose it's not a fair comparison though as the load remains constant.

 

but still, i don't see variable pitch props on small boats, rather the manufacturers & dealers seem fit to pinch you for buying a selection of props depending on what you're doing. that's also a rather bit of inconvenience having to change them. is there a legitimate reason not to give me 1 prop and a shiftable transmission?

 

mind you that i am fully aware that a boat is just a hole in the water that you throw money into. this is all just armchair speculations and i do not really have a 500cc 4-stroke motorcyle engine and i am not really planning on putting it sideways in a 12 foot skiff hooked to an outboard prop with a welded on u-joint. that would be koo-koo.

[Moontanman]

Interesting link CraigD, I was unaware of Variable pitch props in boats, I have seen them in aircraft. In most small boats it is both easier and cheaper to put a propeller of a size and pitch matched to the motor speed and power and it's use than gears or a variable pitch propeller. I once had a shrimp boat with gears, it had an old power-glide transmission and a straight 6 engine (I installed both myself) it was a pig and slow as the itch but it would pull the net because of it's rather large propeller, the two gears were useless and I ran it in low gear most of the time.

[Knothead]

Interesting topic. I'll be interested in what all the high IQers have to say.

In my boat I have a very small one cylinder ten horse diesel and an over sized three blade prop. I am able to exceed hull speed and still achieve full RPM.

I don't see how a transmission would help in my case as the engine would lug down in a higher gear. I suppose that with planing hulls however it would be a completely different situation.

[C1ay]

I don't see how a transmission would help in my case as the engine would lug down in a higher gear.

 

In the marine world the variable pitch prop is effectively a variable transmission. By implementing one in a configuration like yours you could keep the engine at an optimal speed while varying the prop. This would allow you to maximize the efficiency of energy throughput.

[Knothead]

That's the thing though. It seems to me that the engine needs to rev out to get the boat up to hull speed. I don't really think that by varying the pitch of the prop I would be able to run the engine at lower RPM and still achieve the same speed. But what do I know?

I remember that with my last engine, a one cylinder eight horse, I would start to get black smoke when I tried to get max RPM. The pitch of the prop was just a little too much. Had great torque though.

I have a three blade feathering prop that I am hoping to try sometime soon. Should help reduce drag for sailing.

 

Maybe it's just a matter of cost. Feathering and folding props are extremely expensive compared to regular fixed props.

[C1ay]

I don't really think that by varying the pitch of the prop I would be able to run the engine at lower RPM and still achieve the same speed...

 

That's not the point of having a variable pitch prop or variable transmission in line with the prop. They allow you to control the rate at which the engine sees the load so that you can maintain the engine's RPM in the range where it is most efficient. From a standstill you can start with the prop at minimal displacement and stroke it to increase the load on the engine without lugging it down. At top end you can have a higher prop displacement than you could otherwise have in a fixed prop because the fixed prop is limited to a displacement that doesn't overload the engine when taking off from a standstill. You get a prop that makes takeoffs more efficient while increasing the top end as well.

[Turtle]

Interesting topic. I'll be interested in what all the high IQers have to say.

In my boat I have a very small one cylinder ten horse diesel and an over sized three blade prop. I am able to exceed hull speed and still achieve full RPM.

I don't see how a transmission would help in my case as the engine would lug down in a higher gear. I suppose that with planing hulls however it would be a completely different situation.

 

the more i thinked about the replies the more i expected something about drag, and hull speed & prop size seem to fit the bill. the problem with hull speed is that it's seldom a boat's maximum speed and it's drag is a matter of constructive wave interferance and not about the shape or surface area of the hull or prop.

 

thinkling more on my comparison to a car, it seems now that it's an unfair comparison. while a car coasts during a gear change a boat & its prop slow down fast when the the power is cut, and with a planing hull falls off-plane. i think knotty is right that the engine would lug on the upshift then.

 

that's all i got.

[C1ay]

i think knotty is right that the engine would lug on the upshift then.

 

 

This is what torque-shift propellers are good at. As the drive torque increases the prop varies to a lower pitch automatically. As the craft picks up speed the prop adjusts automatically to a higher pitch. An ideally selected prop will allow the engine to run at the peak of it's torque curve.