Drag

[Guest]

A recent article, says that a 15 inch standard three bladed prop increases resistance by 22kg. This can account for 35% of a hulls resistance on "some" boats, okay perhaps a slippery racing hull with fin and spade rudder and with a total resistance of 63kg. The same article says that a three bladed folding prop only increases resistance by 3.2kgs so to fit one would reduce the total boats resistance by about 30% a fantastic improvement! I have a Warrior 35 with long keel. I have no idea what a Warrior 35´s total resistance in Kgs is (perhaps someone can tell me) but it must be loads more than 63kg. If it was say 200kg then my prop would account for 9% of the total resistance and so to swap for a folding prop at 1.6% my benefit would only be 7.4% If my calculations are correct I may not even notice the benefit so is the whole excercise worth it. I would apreciate any advice I can get on my calculations and your experience in fitting a folding Prop to a long keeled boat. The model I have in mind is Bruntons varifold.

 

[gunnarsilins]

I wonder....

...if these calculations can bee seen as linear?
I mean, lets compare two hulls, one with a resistance of 100 kgs and a second one with 200 kgs. Applying the same force on them, would one of them move with twice the speed?
No, I don´t think so, and I really hope I´m right!
Next week I´ll put on a new feathering MaxProp instead of the fixed 24" 3-blade on my old Moody 42 ketch. Definetely not a slippery hull!
If "Warriorforlife" is right I have certainly paid a LOT of money for almost nothing.
Maybe somebody is capable to enlighten us?

 

[Twister_Ken]

Hull shape

Might have a lot to do with the underwater shape of the boat in question. On a modern racing boat which sits on, rather than in the water, with only a stiletto for a keel and a dagger for a rudder, a fixed blade prop represents about the least slippy thing down there.

OTOH, an cruisy hull with a longer keel, a skeg, and probably an underwater finish that more closely resembles an old man's chin than it does a baby's bottom, is producing a lot more drag, so the prop drag is less of a problem.

Then you get to a full longkeeler, where the prop is in a keel/rudder aperture, and the water flow at the back end may be so disturbed that the effect of going to a folding/feathering prop will only be minimal.

 

[johnt]

Re: Hull shape

just a comment: Ive just changed to a 3 blade prop from a 2 blade, theoretically an increase in drag of 50% , and I cant tell the differance.

23ft waterline and it will still hammer along at up to 7.5k deep reefed in a force 5. which brings up the question ...do we put it in gear to stop the shaft from howling ; or not?

 

[Guest]

Deadwood can be handy

My feeling would be that if you have a long keeler a two bladed fixed should suit you nicely being that it is hidden behind the deadwood although Ken does have a point about turbulence in that area.

I have a longkeeler with a two blader and at low wind speeds drive my mates mad by going past them in light airs. I am convinced that in light airs you benefit GREATLY.

With regard to power under engine there is next to no benefit by using a three blader compared to 2 blades BUT in my own experience you get more bangs and less smoothness with two blades .

Back to your main point about tests. Perhaps the overriding factor is at what windspeed are the tests conducted. I remember reading an article, based on tank tests, using wind speed moving a boat (no prop) at 5 knots as a base figure. With a folder the drag was 0.01 knots and a fixed 2 bladed 0.8 knots ( hull type not mentioned).

 

[chippie]

Re: John T

I Know one sailor who engages gear to stop prop noise,but I have read somewhere that some gearboxes dont tolerate it that well.
I have also read that a spinning prop causes more drag --why? Can anyone explain the theory?
Cheers.

 

[Bergman]

I don't understand the units.

kgs are a measure of mass or weight.

Is it the case that you are saying a 3 blader is like carrying an extra 35 kg of displacement? If so I would have thought it irrelevant.

Drag is not a liner function but rises as the square of the speed. That is 35 units of drag at 3 kts is 1225 units at 6 kts.
Got that wrong its not 1225 its 140 (shouldn't try doing hard sums in my head)

Before the scientists leap on me I know there is a lot more involved than this, but the principle is right.

We have been down the locked/spinning arguement before. There is unquestionably MORE drag from a spinning prop than from a locked one. Spinning is doing work, standing still is not.



<P ID="edit"><FONT SIZE=-1>Edited by bergman on Sat Sep 29 14:15:03 2001 (server time).</FONT></P>

 

[jollyjacktar]

Oh no, here wwe go again.

Are we are going to thrash this out once again? Free running rotating props cause more drag than non rotating props. Do what I do, sail without an engine or props.

 

[Bergman]

Re: Oh no, here wwe go again.

Thats just evading the issue

 

[johnt]

Re: Oh no, here wwe go again.

I wasnt thinking drag ..I was thinking knackered gearboxes!

 

[johnt]

sorry bergman your wrong ..it IS lbs drag ..its the opposite of lbs thrust! or kilograms or whatever

 

[Bergman]

OK

Being wrong is not unusual for me

But surely it must be related to boat speed

If there was 35 kgs of drag when the boat was stationary it would set off backwards.

PS

What about Newtons?

 

[vyv_cox]

Interesting question, that has kept me thinking over the weekend.

Firstly, I doubt if a slippery racing hull has a resistance two or three times less than a long keeler. I suspect it's more like a factor of 60%. If your boat had a resistance of 200 kg I assume it would not be possible for one person to move it with a warp, whereas I have moved boats of up to about 45 ft, of every type, with little trouble.

However, why not measure it? Get someone to tow you on flat water with a spring balance in the tow line. Plot the resistance at various speeds. Then throw a 2-gallon bucket (diameter approx 15 inches) over your stern and plot again. That should answer all your questions in the best possible way. Then sell an article with the results to PBO, which will pay for the balance and diesel.

I think your supposition may well be correct. There is undoubtedly more drag in a long keeler and the prop could represent a fairly small proportion of this. The usual way that this drag can be minimised is to mark the propshaft inside the boat, allowing the prop to be positioned straight up and down, when it will be blanketed by the keel. Try this, and then turn it 90 degrees and try again. More data for your drag article!

Otherwise, I would have thought the combination of a long keeler and folding prop would produce the world's worst handling astern driver. I never heard the owner of a folding prop say he had no problems going astern, no matter what the boat type. I recently fitted a 2-blade Brunton Autoprop to my Sadler 34, also not a great reversing boat. It has made a considerable improvement to all aspects of yachting, with noticeable benefits in sailing speed, motoring speed and astern reliability.

 

[seahorse]

Re: Drag -Autoprops

Autoprop.com has some interesting (biased?) info.
Claims of 85% reduction in drag & motoring faster @ lower engine revs also.

 

[Guest]

I think you can assume they mean equivalent force.

35kg = approx 350 Newtons, if that helps.

 

[Guest]

Don't worry about it too much.

Hiding the prop behind such a long, large surface area keel, means that it will not contribute so much to drag relatively.

On your brute of a boat, the most important thing is applying enough power to get it going (big sail area needed). Prop drag is more important on light, low drag racing-type boats that are not as stiff as yours (and carry less sail much of the time).

I have the opinion that if prop drag is critical to performance, the boat must be inherently unsuitable for comfortable offshore sailing (there's a controversial opinion if I ever heard one - any takers?).

One of the main gains of many autoprops is they appear to improve going astern - possibly something that would interest you.

 

[Guest]

Re: Deadwood can be handy

On a long-keeled boat the two bladed prop has a nasty effect due to wop-wop on-off thrust created by the blind spot as it goes vertical - you always need a three-blader for peace of mind.

Three and four bladed props are often required at higher horsepowers in order to get the blade area you need to put the power in the water without cavitation, so many larger boats with two blades are effectively under-propped most likely.

Then again, most boats are under-propped, under-geared etc (including mine - who thinks 1:2.2 is a good ratio?? Are you listening Volvo?) - so whats new.