Twin props

[Capct]

Originally Posted by Alan Wheeler

You have some great answers and info above, but an important point is missing. This point needs to be added to the mix above and I believe will give you the info you seek.Firstly, the missing link here is a moving target. It is max Hull speed, actual Hull speed and what external influence such as wind and sea state is adding as load against forward momentum. The other important missign factor is Hp required to reach Hull speed and actual Hp you have fitted.These factors will influence the answer greatly. You may or may not see any increase in boat speed with the second engine brought online. What you will see/feel is an ease in handling adn a possible reduction in Fuel consumption. Especially if the engines are Gasoline powered.

You wont see a reduction in fuel.If running on just one engine it would burn way more fuel than normal when under double the load ....and two engines burn way more still than one under load.I run twins and have for many moons.Thats just the way it is.Two is more ~of everything~
And the increase is marginal in speed...the difference is it spreads out the work load and the boat handles.and runs much better.

[Ex-Calif]

"1) I am not assuming any prop efficiency (just it is less than 1, and both props are the same). If prop 1 has 60% efficiency, so does prop 2, so my question remains"

A prop at zero thrust is at near zero slip - to your point - it is not slipping because it is not loaded. Once it starts loading it starts slipping.

"Once you cross, as you term, the speed of the water (rpmxpitch-slip) with prop 2, it starts adding thrust, not just eliminating drag. Both engines take equal load when they run at the same rpm."

Another thing that might happen is that as you bring engine 2 up to max speed, engine one will unload as engine 2 takes up the load and on a non-governed engine will tend to gain rpm and may overspeed.

[LtBrett]

Originally Posted by Ex-Calif

"Another thing that might happen is that as you bring engine 2 up to max speed, engine one will unload as engine 2 takes up the load and on a non-governed engine will tend to gain rpm and may overspeed.

I suppose if you pin your throttles, all sorts of bad things could happen.

Brett

[mudnut]

Why wouldn't you just start the two engines at the same time and use lower revs if more speed isn't required??.Mudnut

[Moby Dick]

Maybe so ltBrett, seems to make sense - except, on a quick scan of the internet, slip factors seem to

"range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent"

So taking 25%, I should not get any added thrust from the second prop until it exceeds 75% of the speed of the first? But after that, the water will be passing prop 1 (in fact both props) faster than (RPM x Pitch-slip) (as prop 2 has added thrust). Which means ideally a corser pitch should be used. I wonder if the Autoprop works in this way on a cat?

(Assume operating below hull speed, flat water, no wind)

[Pblais]

Imagine 10 engines of equal size. You could say after one or two were running you could never go any faster and the extra engines would have no effect. On flat water it would be true. As the sea state increases it takes more power to maintain the same speed through the water and the added engines will help maintain speed. More props in the water will increase power. Adding engines would let you increase pitch on flat water but not also in an increasing sea state. The problem is the sea state is a larger variable than the pitch of the prop and an automatic prop can't really account for the whole dynamic sea state change unless you could dial in the pitch as a function of sea state. That would make the propulsion system pretty complex for a sail boat but not for a large power boat because they actually can do that. It's not an automatic function of the prop though. The whole sea state effect computation is a bit beyond high school physics. Even my autopilot can't compute it automatically. You have to set it.

You need a balance and you can clearly argue what the balance should be but you would not maximize speed to a flat water sea state in the design would you? You would clearly lose efficiency in an increasing sea state so I would expect to lose a little bit in flat water to have more when it really mattered. Over pitching the prop has some serious negative impact on transmission and engine performance / longevity. Under pitching causes cavitation to lose efficiency too. Taking everything into account it's probably true that the original boat design was correct.

Going back to the original question, one prop won't do more work when two are engaged. It's not like the props "know" about each other. They would always both perform the same amount of work based on the throttle and each exhibit the same inefficiency. The order that you start them is meaningless. You would have added power you can not realize in terms of added speed, but it would always be there though wasted from your point of view. As a lone wave strikes the bow in flat water the time to regain the prior speed would be quicker with two props. That would be how you sitting on deck would know the difference.

[LtBrett]

Originally Posted by Moby Dick

So taking 25%, I should not get any added thrust from the second prop until it exceeds 75% of the speed of the first?

Yes, that is correct. You will gain speed not from increased thrust but from decreased drag until you hit 75%. You can easliy see this with twin engines. Run one at 75% throttle. RPM's on the other will rise very quickly with little power until it approaches the speed of the water. I used easy numbers to illustrate my point.

Compare for a minute your propeller to a wheel. Instead of 12" of pitch, say the wheel has a circumference of 12". In either case, you expect to move forward one foot for every revolution. If you put a second wheel next to the first, it is easy to see that the second wheel will only add thrust when it meets or exceeds the speed of the road. Drive on ice and you get an even better analogy (slip). Spin one wheel at 20 rpm, say you slip 50% and you are only moving forward at 10 feet per minute. If your second wheel turns anything less than 10 rpm, it drags. 10 Or more, it adds thrust.

Originally Posted by Moby Dick

But after that, the water will be passing prop 1 (in fact both props) faster than (RPM x Pitch-slip) (as prop 2 has added thrust). Which means ideally a corser pitch should be used. I wonder if the Autoprop works in this way on a cat?

You just figured out why twin engines applications generally have higher pitch than a single engine with the same horsepower. Don't forget that prop 1 also speeds up when you start prop 2. Twin engines have less slip while accelerating compared with a single. At steady state rpm, slip will be roughly the same.

There's a pretty good description of propeller basics here:
Classic Whaler: Boston Whaler: Reference: Propeller Part 1

Brett