Prop Drag: The actual data...

[rustybarge]

Hi All,

I came across this graph by mistake; pray tell how else fellow Mobo owners would I have entered the world of 'The stuck-up & pretentious ones'... lol!

It makes interesting reading.....




>In the graph above, you can see that at 5 knots, a fixed three-blade prop with its shaft locked creates almost half as much drag as the entire hull. The drag can be halved by allowing the prop to spin, but the gearbox may suffer. By contrast, the drag of a feathering prop is negligible, and the drag of a folding prop is too small to plot on a graph of this scale.

>The hull resistance curve for the Océanis 323 was calculated for YM by the Wolfson Unit, at the University of Southampton, using data from the Delft University Systematic Series. The propeller drag curves are based on data from SSPA Maritime Consulting, using Volvo S-drives. This data were verified by YM’s on-the-water drag test.


Read more at:

http://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807#esLtv8qziWRJGdSx.99

 

[rustybarge]

Here's the specs:
Specifications

The 20 hp Yanmar engine, viewed from the companionway.​

http://www.popularsailing.com/488/beneteau-oceanis-323-sailboat-review-photos/

LOA : 32′ 10″
LWL : 29′ 2″
Beam : 10′ 9″
Mast length (over water) : 48′ 7″
Draft (standard) : 4′ 9″
Ballast (standard) : 2,414 lbs.
Displacement : 9,325 lbs. (approx.)
Engine : 20 hp.
Fuel Capacity : 17 gal.
Water Capacity : 42 gal.
Hull / Designer : Groupe Finot
Sail Area : 542 sq. ft

 

[rustybarge]

Just a few numbers on the back of an envelope, if you run your twin on just one engine as you're chugging along....

It takes 21hp to push a 32' 4 ton boat at 7kts with 130kg drag: 6.19hp/kg.

Therefore 20kg of drag from freewheeling prop = 3.23hp loss.

18/20hp burns about 1 gal ,so thats about 0.18 gal loss.

So running one engine of a twin at displ. Speed with one freewheeling prop produces less than 1ltr fuel/hr. In drag....

Interesting.

 

[gordmac]

Interesting, it would seem 7hp is enough to push it at 7kn so why has it got a 20hp engine?

 

[rustybarge]

Interesting, it would seem 7hp is enough to push it at 7kn so why has it got a 20hp engine?

Please correct my maths....

 

[gordmac]

I don't think your maths is faulty, power is force x speed, I just added in the prop drag which won't be there under engine I suppose but there will be other factors.

 

[MapisM]

I don't think your maths is faulty

Are you sure you don't? RB said that it takes 21hp, you said it only takes 7... :ambivalence:
Actually, I suspect that more than on a math basis, RB just took the numbers from the webpage he linked.
But tbh, I don't get your maths/logic at all...

 

[rustybarge]

I always thought that twin engined boats(on one engine) used so much more fuel at 7-8 than single engined boats because of prop drag. Nearly all reviews I've read said ' running on one engine saved no fuel'

the rule of thumb of 5hp/ton to push a displ. hull at 'hull speed' is proven by the above example; 20hp/4tons/7kts.

But we know that a free spinning prop only absorbs 3-4hp.

So maybe the cause is the extra weight?

Running both twins would of course suffer from the inefficiencies of two engines like friction and heat losses .

 

[MapisM]

Yep, extra weight and friction losses in twin vs. single engines are both valid reasons.
I suspect that the latter is more relevant than the first, because the incidence of friction losses is percentually higher when engines are used at low rpm/load (which is typical of D speed), while weight is not as critical as when going fast.
There are also other reasons, though: the drag of shafts and P brackets is not just halved in a single screw boat, it practically disappear.
And with a single engine you can use a larger diameter prop, with a more horizontal thrust, aotbe.

 

[rustybarge]

Yep, extra weight and friction losses in twin vs. single engines are both valid reasons.
I suspect that the latter is more relevant than the first, because the incidence of friction losses is percentually higher when engines are used at low rpm/load (which is typical of D speed), while weight is not as critical as when going fast.
There are also other reasons, though: the drag of shafts and P brackets is not just halved in a single screw boat, it practically disappear.
And with a single engine you can use a larger diameter prop, with a more horizontal thrust, aotbe.

I never considered that; the prop on a single is normally mounted behind the skeg/keel and doesn't need p brackets to locate it.

This would explain why some planing hulls are more economical at displ. Speeds than proper displ/semi-d hulls; they may have p brackets but they don't have keels...


A s/d hull with twins has both , causing massive drag!

 

[gordmac]

Are you sure you don't? RB said that it takes 21hp, you said it only takes 7... :ambivalence:
Actually, I suspect that more than on a math basis, RB just took the numbers from the webpage he linked.
But tbh, I don't get your maths/logic at all...

I don't know what maths was done so why criticise it for no advantage?
Power=force x speed 7kn = 3.6 m/s according to my phone 150 kg drag needs converted to Newtons so x 9.81
3.6 x 150 x 9.81 = 5.3 kw or 7.1 hp.

 

[rgarside]

I don't think your maths is faulty, power is force x speed, I just added in the prop drag which won't be there under engine I suppose but there will be other factors.

Yes there are a few other things, so while you might only need that amount of power to move that resistance at that speed you need to allow for:
- Propeller efficiency
- Gearbox efficiency
To get to the engine power needed.

Then you need to allow for air drag, and allow for the speed of head wind you would like to be able to maintain speed in, with a bit for fouling, the alternator, engine performance after a few years, etc.

The big things in that lot are probably prop efficiency and wind.

 

[AHoy2]

Interesting, it would seem 7hp is enough to push it at 7kn so why has it got a 20hp engine?

My 28ft 5.8t motorsailer requires all 27 of its Yanmar horses to get hull speed, figures from the Vicprop calculator at http://www.vicprop.com/calculator.htm

Results from calculator below:

"Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 6.56 Knots
Minimum horsepower required at propeller(s) for Hull speed: 26.7 HP (3,600 rpm)

Calculations based on desired speed and available HP
HP required at propeller(s) for desired 6.0 knots speed: 19 HP (3,250 rpm)"

Same calculator shows 11hp (2,600 rpm) for 5kts cruise speed.

Figures born out on sea trials following re-propping exercise to fit feathering prop for reasons shown by OP.
Darglow got the prop dimensions spot on first time for me thankfully.

rustybarge - figures support your quoted rule of thumb of 5hp/ton to push a displ. hull at 'hull speed'

AHoy2

 

[rustybarge]

My 28ft 5.8t motorsailer requires all 27 of its Yanmar horses to get hull speed, figures from the Vicprop calculator at http://www.vicprop.com/calculator.htm

Results from calculator below:

"Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 6.56 Knots
Minimum horsepower required at propeller(s) for Hull speed: 26.7 HP (3,600 rpm)

Calculations based on desired speed and available HP
HP required at propeller(s) for desired 6.0 knots speed: 19 HP (3,250 rpm)"

Same calculator shows 11hp for 5kts (2,600 rpm) cruise speed.

Figures born out on sea trials following re-propping exercise to fit feathering prop for reasons shown by OP.
Darglow got the prop dimensions spot on first time for me thankfully.

rustybarge - figures support your quoted rule of thumb of 5hp/ton to push a displ. hull at 'hull speed'

AHoy2

Its extraordinary how many boat builders make totally rediculous claims about the fuel economy of their boats. I've was looking at the test of the swift trawler 44 (12 tons)which claimed more than 3mpg at 8kts; in your dreams.

Here's a quote from the trawler forum:

. My friend and I have been running our boats from New Orleans to the Bahamas for years. He has a 61ft Hataras with 12-71s. This last trip he put a feathering prop on one side, giving him a maneuvering/ get home engine on that side. The boat cruises at 8kts about 20-30% more efficiently running on a single engine with the feathering prop. Honestly, if you don't run long distances, it does not matter, but for our trips where we'll burn several thousand gallons over a cruise, it does add up. One of the biggest benefits on a long cruise is not needing to change oil on the feathering side.

/QUOTE]

I think feathering props cost ££££, so just letting the prop freewheel would make perfect sense if there is only a cost of 1ltr/hr in drag.

PS. I'll try to find the blog post of a nordhaven 50' that circumnavigated and reported 1.3 mpg at an average speed of 6 kts.

Now that is totally believable!

 

[rustybarge]

People will debate the single/twin argument forever....

I Just thought I would post this photo of the simplest wing engine setup I've ever seen; piggy back on top of the prop shaft, chain drive....nice!


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