would boats be faster if outboards had gears+transmission?

[sstone]

Re: would boats be faster if outboards had gears+transmission?

There are already CVT's in vehicles that are at 300hp, and I've got word those same CVT's were massively overdesigned, and would handle a 500 hp v8 in a boat without too much problem. (a transmission in a boat is under lighter duty than a car, since the prop can slip)

Vehicle CVT's generally don't use belts, they use metal chains with all the pins sticking out of the links. The pulleys are hydraulically powered, and controlled by a computer. (no weights/springs) Clamping force between the pulleys is insane!

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Interesting, the only ones I've seen have been belts. Learn something new every day! I can see this working in a transmission system in an inboard or in the racing stern drives with the inboard transmission. I think there's not enough room to put one in that would be terribly effective, mechanically or economically. It'd be something fun to try though. New summer project...

 

[Chris1956]

Re: would boats be faster if outboards had gears+transmission?

Actually the newest CVTs in Nissan autos, do not have belts. They use a cone-shaped pully to connect the drive pully and driven pully, which are also cone-shaped. Hydraulic pressure moves the connecting pully to swivel connecting the small part of the drive pully to the large part of the driven pully for low gear, and then swiveling to conect the large part of the drive gear to the small part of the driven gear for high gear. But I digress....

 

[mrdancer]

Re: would boats be faster if outboards had gears+transmission?

I'm a noob with outboards, but from what i've seen, an outboard basically revs in just 1-forward gear. If it had a transmission with gears like a car 1-6, would it make the outboard faster?

a dude i know got a 300HP outboard on his 20 ft bass boat and at full throttle, he says it goes up to 80mph.. I know it takes more power to move objects through water, but i expected the boat to do at least 120mph max speed with 300 HP..

Keep in mind that a motor pushing a boat is similar to a car driving uphill constantly.

Look at the high-performance sports cars and motorcycles. They generally achieve their top speed in fourth gear, although they generally have six-speed transmissions. The engine's power in the higher gears cannot overcome the wind resistance at triple-digit speeds. Now, that wind resistance at triple-digit speeds would be similar to water resistance on a boat hull at more sane speeds. So, that motor/prop combination needs to have the correct power to achieve top speed for that hull and its given water resistance (which is why the fastest boats are the pickle-fork tunnel boats that have low water resistance).

Terrestrial vehicles also need lower gears just to get moving. There is a lot of resistance (rubber on pavement), especially when moving a vehicle that may weight several tons. With a boat, the prop can "slip" in the water until the hull gets up to speed, where it can run most economically with its given pitch. In extreme cases (high-pitch props), you can have ventilation holes in the props to accelerate that slip and get up to speed faster (as mentioned by TexasMark above). If you run the prop too fast, it will cavitate or ventilate - the water can't get through the prop fast enough (again, the boat hull's resistance is holding it back) and you have prop blowout, which is form of severe "prop slip".

In summary, most boat/engine combinations don't have transmissions because there will be very little benefit from them. Certainly not enough benefit to outweigh the disadvantages of cost, weight and complexity.

 

[Captain Caveman]

Re: would boats be faster if outboards had gears+transmission?

Snowmobiles are essentially CVTs with a pulley clutch. I don't see why a boat would be losing power while shifting gears versus on a snowmobile. I don't think you have much more resistance, if any, with a boat on the water versus a sled in a foot of powder. It seems like you'd be able to put something together, but I agree with the analogy about top speeds that you probably wouldn't be able to have more than 2 or 3 gears before your hp isn't enough to compensate for the resistance of water.

 

[mbfish]

Re: would boats be faster if outboards had gears+transmission?

The only performance gain would be hole-shot to mid range acceleration. A CVT unit would allow the engine to get in it's max horsepower range right from the get-go up until WOT. So, You're hole-shot would be equal to an engine of much higher horsepower. There would be no performance gain on top end. Actually, you would probably lose a bit of top end due to slip and friction in the transmission.

It would be cool if my 90 had the hole-shot of a 140.

 

[dan t.]

Re: would boats be faster if outboards had gears+transmission?

Some drag boats with shaft drive use a 2 speed trans,a modified chev powerglide ,they dont use a torque converter just a combo pump and input drive, the valve body is modified to allow or bypass hydraulic pressure. It seems to work good but we are talking a very light boat with very high horsepower.

 

[H20Rat]

Re: would boats be faster if outboards had gears+transmission?

Some drag boats with shaft drive use a 2 speed trans,a modified chev powerglide ,they dont use a torque converter just a combo pump and input drive, the valve body is modified to allow or bypass hydraulic pressure. It seems to work good but we are talking a very light boat with very high horsepower.

In a technical sense the transmissions make the boat SLOWER, at least from a top speed perspective. What they allow is for the boat to get out of the hole quicker and get the engine up to its HP peak faster. Once you are in the top gear, the transmission is just drag that is costing you power.