Propeller drag - myrh or what?

guydickinson

guydickinson

Well-Known Member
Joined
21 Aug 2006
Messages
190
Location
Dorset, UK
Visit site
I have been trying to find out how much drag a fixed propeller really causes and whether its worth shelling out all that money on a folding/feathering etc. I haven't really found an answer so I did a VERY crude experiment today! I used an old Avon paddle (it has quite a large blade area - probably bigger than most props and of course is designed to "trap" water). I have a step on the transom and was able to slide the oar under this to a depth of about 2 ft under the water to act as a sort of fixed prop surrogate with the oar blade facing forwards. The results were v odd - perhaps someone can explain. Doing about 3 knots. Firstly under engine the speed dropped about 0.5 knots. However under sail (and I know the wind varies a lot - but I repeated it often) the speed only dropped about 0.2 knots if that. My boat is 34 ft long; 26 ft LWL and 4.5 tonnes - a Van de Stadt. Any comments (not too rude please!).
 
Re: Propeller drag - myth or what?

Perhaps the drag under sail is less than under power because the oar was actually reflecting prop wash back against the propeller itself, causing cavitation ie less grip? I can't think of why else it would be that way.
 
Re: Propeller drag - myth or what?

I thinks its more likely that the sails were producing for lack of a better word more torque then the engine, the sails on a boat will provide far more power then the engine does. Also the propeller has to 'grip' the water, the sails will just push on.

But 0.2 knots is still a large amount!
 
Interesting......

The only stats I have are based on a yacht designer called Bill Gerr :-

(1) Tank tests at a speed of 5 knots showed that a fixed prop loses 0.8 knot.

(2) As a rule of thumb, 50 sq ft of sail equals 1hp in a F4 wind.
 
When I lock my prop into reverse when sailing (2 bladed folding) I don't normally see a significant change in speed, so I suppose it is 0.2 knot or so. The main difference I notice is lack of turbulence on the helm and this alone is worth having a folder for.
 
I notice a diference in low winds if I don't lift the outboard. Usually about 1/2 a knot on 3 Knots.
 
I think Kadeena has the answer. Water coming off the propeller will be flowing backwards (relative to the sea) and the oar is interfering with the flow and reducing the propulsive effect of the propeller in addition to causing drag. Whereas water passing the oar when you are under sail is stationary relative to the sea and the method of propulsion (the sails) is not directly affected.
 
I am surprised you have not yet received an answer from a professional. Until then, you might consider a simplified one.
Your set-up was not a comparative experiment at all, because you placed the paddle in streams of different speeds, so you were bound to create different amounts of drag (roughly proportional to the square of the water-speed).
Firstly under power, the stream was accelerated by the propeller to something like 10 per cent above boat-speed, or 3.3knots: (square about 10.9)
Secondly under sail, the stream around the paddle has already been made turbulent by the fixed-blade propeller ahead of it, and possibly slowed too. It is unlikely to be more than the boat-speed of 3.0 knots; (square 9.0).
So the drag from the paddle was greater under power than under sail, by something like 20%. No wonder speed loss was greater.
It is indisputable that performance under sail is improved by reducing drag, and there have been several tank-test demonstrations of the difference in drag between fixed and folding props; typically 0.5 to 0.6 kn.
After 2 years with my Van de Stadt I swapped its 2-blade fixed for a (Gori) 2-blade folder (larger!) and gained 0.6 kn on average speed under sail over the next fifty thousand miles, as well as significantly more thrust under power.
It's a no-brainer!
 
We fitted a Bruntons Autoprop a couple of years ago and in addition to the speed gain while sailing mentioned by others, we're over 1 knot faster at the same engine revs when motoring.
 
One other consideration. A folding prop can have different pitch forward/reverse. Quick burst astern on my max-prop stops boat dead. Very helpful for manoeuvring in tight spaces. Would not be nearly as effective on fixed blade designed for full power output at max revs going forward at hull speed.
 
Hhmm, that's just raised another question! When berthing, I rely on prop walk when going astern to slow the boat AND kick the stern into the quay (I only have 1/2 a boat length max clearance in the berth).

Would I loose that prop walk kick with a folder?
 
[ QUOTE ]
Would I loose that prop walk kick with a folder?

[/ QUOTE ]

You certainly would lose it with an Autoprop. Our boat was practically uncontrollable going astern before we changed the 2-blade fixed prop.
 
About 2 years ago I suggested a very simple experiment.

Tow a yacht with a stable boat using a spring balance in the tow-line so you can observe the pull required to maintain a towing speed.
Do it with the prop free wheeling and then with it in gear.
Note any difference in the reading on the balance.

That answers the in gear/out of gear debate.

You can probably work out how to do the Folding/non-folding experiment from that.


No-one appears to have tried it though. Sigh
 
Re: Propeller drag - myth or what?

Our Kiwi prop has much less prop walk than the replaced fixed two blader.
Previously the boat would only turn one way in reverse, now I can just manage both ways with lots more power available. Having more pitch in reverse (as long as the gbox has a higher ratio in reverse) is a huge improvement. I also agree with the about 0.6kt increase in sailing speed, seems more noticeable in light airs.
 
You must log in or register to reply here.

Share this page

Top