kiwiprop

[Bogan 8]

Does anyone racing have a kiwiprop instead of a folding prop?

 

I quite like the idea of reduced noise and vibration which should result from a reduced prop weight.

 

Being in a one design fleet, I'm interested in observations on the comparative drag.

[Farrari 4]

I have a kiwiprop on mine. I can't comment on drag because it was on there when I bought the boat. Ironically my mates 1020 with a three bladed Volvo folder feels a lot smoother than mine. I do like the way it bites in reverse, although the idle has had to be adjusted up a bit to stop it stalling. I have a 27HP 2003 Volvo in mine so not underpowered by any extent.

 

I understand that there is now a slightly smaller blade option that might work better for the smaller motors.

[PaulR 3]

It would be great to have some local independant testing done, comparing all the different props on the same shaft & P bracket, whilst being towed through the water, akin to sailing at 4, 5, 6 and 7 knots.

 

I have been thinking a spring weighing scale attached to a shaft, and then dragged alongside something with an outboard. The more drag from the prop, the higher the spring weight scale would read.

 

I have the spring weighing device, just need the rest of the gear.

 

Anybody more experienced Happy to help.

 

I've seen the photo of Squid in drag, but I've never had such beautiful gear to fit.

[wheels 543]

You can work it a different way Paul. The torgue on the shaft and the RPM of the shaft. That then can give you the two figures to work out Hp produced by the Prop, which is Effort X Speed.

However, in real worlkd, it is a tricky one to get accurate results from. A test tank would give far greater accuracey and as you said, a pull test would also then be able to give some reults on folding and fethering props as well. There is also a problem with varying results between boats. One prop can rect to being dragged very differently than from one boat make to another or even same boats with different Box/shafts. Each prop has a "sweet spot" in the water where it becomes the most efficient at being a "wing" when spinning. Now couple that to resistance to RPM from the shaft/gearbox and you have a point where the prop will be causing the greates drag.

[PaulR 3]

My thoughts were aimed at "finding out" in real terms, which prop creates the most drag. Yes I agree different hulls will change water flow with enclosed props, props in skegs and saildrives having additional factors.

 

I was envisaging a test boat / runabout dragging a shaft abeam of the boat and therefore in "clear" water so that the blades and boss of the prop was the only variable for drag, but taking measurements at a range of speeds for each prop, in a sailing situation seeking minimal drag, rather than when used under power.

 

Obviously size is a factor of drag and so a standard for most smaller yachts would be for a 1" to 1.5" shaft (25 to 40mm), so I envisage testing the drag for props of the same size, what ever that "common" size turns out to be.

 

Yes different props with different gearboxes will have different sweet spots and spinning starting speeds etc but I would see this as being outside of the initial drag testing = needs more money, send lots by cheque to my PO Box 25095 AKL to Campaign About Spinning Hydronamics but that is to long so just use the initials please.

 

I will make a diagram soon.

 

Meanwhile, what type of props should we compare when looking for the one with least drag:

Kiwiprop 3 bladed self feathering

Briski 2 bladed folding

2 bladed geared folding made by ?

3 bladed made by Gori?

Feathering props by ?

Fixed 2 bladed by ?

Fixed 3 bladed by ?

 

Any others. Please give details and if possible who supplies them locally.

 

Any sources of previous drag test info? to save re-inventing the wheel.

[PaulR 3]

Kiwiprop has a lot on info on their website including this graph which they told me was the testing done BEFORE they developed the kiwiprop:

[PaulR 3]

My ideas have 2 measuring options depending upon how strong the drag is.

 

Some factors will be the accuracy of the measuring scale.

My current one is el-cheapo.

 

However if the same measuring tool is used, the differences in results is what we are after, not the absolute numbers.

 

However happy to upgrade equipment if you send more money

[PaulR 3]

Does anyone racing have a kiwiprop instead of a folding prop?

 

I quite like the idea of reduced noise and vibration which should result from a reduced prop weight.

 

Being in a one design fleet, I'm interested in observations on the comparative drag.

 

 

On the S34 Panacea, we have used both whilst racing. Currently we have 2 bladed folding and seem to be sailing better but then again new sails, better crew, knowing the boat better etc are other major factors. Personally I favoured the kiwiprop, with the reverse being excellent. However I would really like to do real testing as above.

[wheels 543]

It looks like a Sailboat never wants to have a Michigan fitted. Wow! I have heard they really perform on launches.

[Bogan 8]

So according to that data all of the feathering and folding props have drag below about 3 pounds at the hull speed of a farr 1020.

 

It takes, say, 5kw to get a 1020 close to its typical upwind sailing speed (say 6.5 knots). power=force*speed. so 5000Nm/s=force*13000/3600. Force=1384N

 

So 5kw suggests a forward thrust of about 130kg or 300 pounds to reach a speed just below hull speed - and a typical upwind sailing speed in a 1020. So the drag differences among the folding and feathering props should be negligible - being less than 1% of the total drag.

[PaulR 3]

Therefore in say a 3+ hour race

say 200 minutes, 1% = 2 minutes.

But what does this mean. The other boat was 5 or 6 minutes ahead or behind you. So what

 

Better helming, sail trimming, wind shifts could all explain that.

 

So comparisions to another boat at the same time are easily dismissed as the variables are so uncontrollable, even if they were quantifible in some scientific manner. Even repeating the experiment many times (typical scientific approach) would not produce a definative trend, other than why does boat X beat boat Y so often and why are the margins so erratic

 

The focus therefore can only be on and within one yacht. Using the above, 5 minutes / 200 minutes = 2.5% but we all know a lot of races have tighter finishes than that. So lets focus on that.

 

Say a 30 second difference / 200 minutes = 0.25%

Now drag becomes relevant. If one prop has that much less drag than another design, changing the prop should produce a winning result. It is something we can do relatively easily at minimal costs, between tides, floating dock or by haulout.

 

The problem is

a). What is my current drag

. What should I change to

 

It has all been discussed many times and will still be debated keenly. The problem is really nobody knows for certain and has no definative data. They all say "I reckon . . ."

 

In answer to the force required to move a boat near hull speed is that hull design, weight, windage etc are all factors again that vary so much.

 

Tank testing would be an answer if one had the facilities, but is the salinity of the water, cleanness, temperature etc the same as Auckland harbour where we sail and want the data for

 

One way to eliminate those factors is to use the harbour. Simple and reduces costs. Again, it comes down to measuring the differences in drag, rather than absolute accuracy of equipment. Of course better measuring equipment would have finer data to see those differences sooner and show an easier to read and possibly a greater range / scale of the drag forces.

 

Practical real life testing can sometimes be better, simpler and cheaper than in a laboratory.

 

I reckon "It's worth a try."

[smithy09 50]

My ideas have 2 measuring options depending upon how strong the drag is.

 

Some factors will be the accuracy of the measuring scale.

My current one is el-cheapo.

 

However if the same measuring tool is used, the differences in results is what we are after, not the absolute numbers.

 

However happy to upgrade equipment if you send more money

 

I have a loadcell that will resolve down to 0.5kg with a digital readout and rod ends in each end that will take shackles. All self powered. Welcome to use it. Or failing that I have a digital 15kg hanging scale, but drag could exceed 15kg??? PM me if you want to use the gear! Cheers, Smithy.

[PaulR 3]

Yes & Thanks Smithy. PM'ed you.

[Bogan 8]

Actually I think the 1% difference in drag will work out less than 1% difference in time, as this is really the worst case scenario. Firstly, even the folding will have some drag. Secondly, this is below hull speed. As the boat pushes toward hull speed the hull drag goes massively up, and the relative impact of the prop should drop away to say 0.5% or less.

 

And that's drag, not boat speed. Best would be to test it while racing. Find something that creates 1 kg of drag and tow it around.