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Old 08-06-2007, 07:28   #1
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Twin props

Multi hull or prop thread? Not sure, but I'll try here. Maybe someone can help with a question that has been bothering me

On a large multihull (or indeed mobo) you have two propellers. If you just run one, the propeller 'sees' a a certain water flow rate for a given RPM (roughly RPM x pitch - slip). But when you start the second engine/propeller, the water is already passing it at speed, and so it will not be able to contribute (much) thrust until it exceeds the speed of the water flow, and hence the speed of the first engine.

This leads me to think that one prop (the one that is slightly faster at the time) would do all the work, and the other would not do much. I dont think this is the case, but why not?

At least, I would imagine that the pitch of the prop should be different if it was to be used on a twin engined cat compared to a single engine monohull. Is that the case?
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Old 08-06-2007, 08:54   #2
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To an extenet yes.On my boat I have an old synchronizer which tells you if the engines are in sync with each other.Its a light with two half circles that flash intermitintaly ...you adjust the flashing half circles until they become a cirlce... Then the engines are in sync.Which means one prop is immediately following the other .Not together ,but following, one is just a hair slower than the other.
You adjust the circles with the throttles so one prop is spinning just a shade faster or slower than the other
It delivers a very distinct sound....[hard to explain]
Old timers can tell you just by listening.
Most Captains can tell if they are familliar with the boat.
Its that sweet spot that every twin has,no matter how big or small, where the engines sound really happy and content.

wub wub wub wub wub wub wub.....is the happy sound of one prop following the other.I might add you feel it as well....less vibration

The other way I have seen it done.... is to look at the prop wash and adjust them so they look identical.
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Old 08-06-2007, 08:56   #3
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Seems True, I only get about 2 extra knots when I use both engines at the same time, but almost never do-the only time I use both engines are when entering/leaving a port for maneuverable.

I will use one engine for X amount of hours one day then use the other engine the next time to even out the running hours. This is the most economical way for me to save on fuel/oil and maintainance.

I have had to change the props because the pitch is set up from the factory for both engines running at the same time, mine came 17x16 and I changed them to 17x14, this brings the rpms to Factory spec.s when one engine is run at a time
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Old 08-06-2007, 14:36   #4
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I had assumed matching the engine speed was more due to trying to avoid 'beating' between their vibrational frequnecies that for efficienct reasons.
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Old 08-06-2007, 16:09   #5
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That too is a consideration...probably more so with diesels than gassers.
Diesels vibrate like hell....well I guess all engines do...but diesels certainly will give you bigger vibes
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Old 08-06-2007, 18:58   #6
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OK, you've got the basic idea right. Until both engines are in synch, one is pulling more load than the other. You can hear a resonance when they are in synch, or get yourself a synchronizer. Now your question about pitch is a whole lot more complicated. While pitch will usually be different between a multi and a mono, hull form is not the only determinant of pitch. Pitch works together with diameter and number of blades, and propellers are sized according to hull form, engine power, cruising speed, and intended usage. Lots of variables besides mono vs multi hull.

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Old 08-06-2007, 20:21   #7
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What a load of malarkey. Both engines contribute to net thrust. Run just one engine and you'll have assymetric thrust requiring you to helm towards the running engine to maintain your course. Bring the other engine up to half-revs and you'll still have assymetric thrust, but a lot less, and will require less helm - since both are contributing thrust at ratio. Unless you're running two props off one engine it is extremely unlikely that you will perfectly match their revs. You don't need to either. If you notice that you have to carry helm to maintain your course then adjust your engines, otherwise relax and enjoy the ride.
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Old 11-06-2007, 01:17   #8
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You are assuming a way higher propeller efficiency than propbably exists. Propeller efficiency is in the range of 50-60%. The propeller is working against drag which is a result of many factors including wetted area, hull efficency, cleanliness etc.

In a twin screw boat assuming a non-folding/feathering prop the stopped prop is probably actually adding to drag. At some point above idle you reach "zero" thrust and anything beyond that the second screw starts adding thrust.

It would be interesting to try and demo the concept to yourself with a single screw, sails and a gps especially if you have a non-folding screw. Get the sailboat going and a long tack at 4kts. If your boat normally sails in neutral with the screw windmilling, put it in gear so the screw adds drag and watch the gps speed. Start the motor and slowly accelerate the engine until you reach the original speed. This is neutral thrust. Anything beyond that and you are adding thrust.
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Old 11-06-2007, 02:43   #9
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“The Propeller Handbook”
The Complete Reference for Choosing, Installing and Understanding Boat Propellers
By Dave Gerr

On-Line at:
The Propeller Handbook: the complete reference ... - Google Book Search

***
The Propeller Handbook:
The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers (McGraw-Hill) doi:10.1036/0071381767

The Elements of Boat Strength: For Builders, Designers, and Owners
By Dave Gerr
The Elements of Boat Strength: For Builders, Designers, and Owners (McGraw-Hill) doi:10.1036/0070231591

The Nature of Boats
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The Nature of Boats (McGraw-Hill) doi:10.1036/007024233X


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Old 11-06-2007, 04:48   #10
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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

2) I notice no difference in speed between windmilling prop and locked prop. This has been done to death elswhere, but both contribute drag (a windmilling prop still has drag!). Even if it did, HOW does a prop spinning at less than 'water speed' (approaching RPM of the first prop) contribute thrust?

3) The propellor handbook does not discuss twin prop set-ups.

My question still remains, therefore.
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Old 11-06-2007, 05:19   #11
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Moby,

Based on the experience of neighbors with twin screw trawlers, I think you'll find that one propeller offset outboard relative to the center of the boat makes for an unbalanced situation. While you may not notice much in calm waters at slow speeds, trawlers do notice the imbalance easily given the higher power. My neighbor lost the fresh water pump on the starboard engine last and limped home 40 nm on the port engine. The boat still goes but it's not as efficient and the pair running. You'll take a lot more stress on the rudder with one prop too.

As far as one propeller doing all the work while the other one does not. The issue of speed is not related to power nor is an increase in speed proportional to the power. In heavier seas twin screws will have more power and more control and actually more speed. Perhaps on perfectly flat water you won't see the speed increase as much but there is an increase in power.

You are correct about the windmilling or fixed prop, the topic has been done to death here long after it had been done to death many other places and ultimately by the US Navy in their own study - anything being pulled through the water has drag. A windmilling prop may do damage to the transmission depending on the manufacturer.
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Old 11-06-2007, 05:27   #12
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I dont know if this helps?
A Study on Ship Maneuvering Characteristics of Ship with Twin Propellers & Twin Rudders...
http://www.searchpdf.com/studies/mar...pdf240/031.pdf
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Old 11-06-2007, 08:49   #13
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I had to read this thread several times before I figured out why you question remains. Let me see if I can resolve it. Propeller pitch is measured in inches. That means that for every revolution, a prop with 12 inch pitch will try to push the boat forward 12 inches. But there is slip, so perhaps the boat only moves 6". Add a second propeller, prop #2. If prop #2 is spinning at fewer RPM than prop #1 it should create drag. But there's that slip. In this example, if you spin prop #2 at anyting less than 1/2 prop 1's RPM (remember prop 1 is slipping 50%), you get drag. You get less drag as RPM's on 2 climb, so you go faster as soon as you start prop 2. As prop 2 eliminates drag, the speed on prop 1 will rise, as it has less load. You go faster. 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. Adding numbers to my example, run enigne 1 at 3/4 throttle, prop 1 spins at 2000 rpm. Start engine 2. As soon as you put engine 2 in gear, it reduces the load on engine 1 by reducing drag. Say engine 2 runs 500 rpm just off idle. Without changing the throttle on engine 1, prop 1's rpm might rise to 2100. Advance engine 2's throttle to 1/2. Say prop 2 spins at 1500 RPM and prop 1 now spins at 2200. Bring engine 2 up to 3/4 throttle, both props spin at 2500 rpm. The numbers are certainly not exact, but I hope you get the idea.

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Old 11-06-2007, 13:30   #14
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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.
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Old 11-06-2007, 15:39   #15
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When in sync one engine for example is 1500 rpm the other wil be like 1420 or so.One engine alone will skew the track significantly,stress the shaft and overload the engine in question etc. What ever the theory is mathematicaly the engines do not run @ identical rpm's when they are in sync.
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