prop design - testing results

[seadago]

Hi everyone
There are a few threads dealing with the choice of prop. Came across the attached article, with a fairly thorough comparison of different prop designs. The relevance of testing methodology is -evidently- debatable, and so are the results, but the approach seems sound. Of particular interest to me (I need to change my prop now) were test results for Kiwi prop, which many contributors to the forum appear to consider the best thing next to sliced bread.
Anyway, I found it useful as a reference.
Cheers
Rafa

[Castaway]

I missed this issue of YM; if I had seen it, I might not have bought the KiwiProp that I now have. However, it performs better on my F35 then either the Gori or Volvo folding 2 blade props I had before, and is better in every way than the 2 blade fixed prop which was original equipment. Because the pitch can be varied, it may be possible to improve the performance to match a specific boat/engine/gearbox combination, and I presume I am lucky in the setting made by the UK agent. The two folding props were much the same ahead, but very different astern, although the spec for each was identical.

Actually, I'm glad I bought the Kiwi; it's lots cheaper! However, the only way to judge them is to try them out & compare; an expensive process, and it only applies to that particular boat.

Thanks for posting the article. If I had seen it when it was written, I would have made a different choice at the time, and it gives some good hints about assessing the best prop.

[VeloFellow]

No designs included testing using the barnacle enhanced versions. Thank goodness sailboats are for sailing.
When I asked the boat to go once with the motor 1000-2800rpm results nearly the same. Worse in reverse. Overall very poor performance withe the barnacle prop.
Shaved it does better. But give me a breeze and it is better yet.

[Mike Holibar]

I think you would have to take those test results with a grain of salt. While it is usual to supply a prop manufacturer with details of the boat so they can match their prop design, they very rarely get it right first time and usually some pitch adjustment is subsequently required to get the optimum performance. An over pitched prop will result in lower speed and higher fuel consumption and, as the engine cannot achieve its rated rpm, a reduction in available power.

As a general observation, having fitted a Kiwi-prop, I seemed to lose about 1/2 knot top speed when compared to the heavy standard three blade we had. But I gained a knot in boat speed under sail and can point about 10 deg. higher. As a result we sail a lot more.

Kiwi-prop has its quirks with non yanmar/kanzaki gear boxes because of its very coarse fixed pitch astern. Our Hurth gear was 2;1 both directions where the yanmar is 3:1 in reverse, regardless of its ahead pitch. As a consequence we lost most of our astern power.

However we have now upgraded to a PRM 150 gear, 2.8:1 both ways and the Kiwi works much better. We still need to adjust the pitch up a bit more to optimise performance, but the astern performance is spot on.

Another advantage of the Kiwi is the low rotating mass, meaning less vibration and considerably less shock load in the power train when the gear is engaged.

[seadago]

Hi Mike
Thanks for your comments. In essence, I don't disagree with you. Like everything else on a pleasure boat, the choice of prop its a matter of compromise between competing and often mutually exclusive performance characteristics. Not forgetting the cost. However, if we are comparing apples, we must make sure we compare exactly that. It is in that context that I found the article useful.
In my case, the choice of design is limited by the available space. I can't fit a folding prop, so I am left with fixed blade and feathering design types. Hopefully -one never knows- I will not regularly have to steam for long periods at cruising RPMs, so speed and fuel performance within that range for my engine (a Beta 16 hp and a TCM 40 gearbox with almost identical fwd/rev gear ratio) is a lower priority. On the east coast of the UK, I use the engine to maneuver in and out of moorings and fight the tide in channels into harbour. Therefore I need a prop that will give me good bite in reverse and at high revs forward. This would suggests a traditional three-blade fixed prop. However, as you point out, a 3 blade fixed will probably represent unacceptable loss of performance under sail and pointing ability. Probably... have not done any testing myself! All the above considered, plus the price differential between a Kiwiprop and the other feathering props, makes the Kiwi an attractive choice.
Have to decide soon and can't do the testing myself, so gathering the experience of other skippers to help me choose is one of the reasons I started this thread!
Thanks again.
Rafa

[seadago]

Hi folks
Apologies for reviving an old thread, but the time has come for me to change my prop.
As per the thread, I've been thinking of fitting a Kiwiprop. However, in discussing engine specs with Vecta Marine (Kiwiprops reps in UK), I was informed a Kiwi would not be appropriate for my engine/gearbox combination. Essentially, minimum blade for a Kiwi is 14", which is apparently oversized for my Beta 16 HP (probably 14 HP at the prop) with a TMC 40 gearbox with 2:1 reduction ratio.
Confess this was a surprise, since I know of a couple of F30(28) CK owners out there that have them fitted.
So, I'd like to validate Vecta's advise (which may be 110% valid!) against practical experience from other F30(28) owners with Kiwiprops fitted. Beta's own advise re props for the 16HP is 13" X 8", but they are referring to 3-blade fixed props.
Currently I have a 14" two blade fixed.
Now, my expectations from the Kiwi are relatively modest. Attending at the test results that started this thread, I do not expect a huge improvement in forward performance, and perhaps even a small loss in reverse performance (because of the Kiwis' fixed maximum pitch setting in reverse). I DO expect however, a significant improvement in speed under sail and pointing ability. That is the key reason to put a feathering prop after all.
So I'm asking for feedback re overall performance from F30(28) CK owners with Kiwiprop fitted and similar range in engine output.
Many thanks in advance for your input
Rafa

[andygc]

I didn't post in this thread before because the boat wasn't in the water, so I didn't yet know how it handled. I have a Darglow FeatherStream prop, which I transferred from my previous boat. It is slightly smaller than Darglow recommended - the previous (smaller) boat had a 13 HP Beta and this has a Beta 20, but we changed the pitch cassette to allow for this. Darglow's advice for 5+ tons of laden Freedom 30 CK with 20HP would have been 14 x 8 3-blade, but they said that re-pitching my prop to 13 x 9 would be equivalent.

I'm as happy with the prop in this boat as I was in the previous one (ie very happy). The FeatherStream works flawlessly ahead and astern and does not have the power reduction problem of the KiwiProp. The only downside is that I don't get anywhere near as much propwalk in astern as in my previous wooden long-keeler, so turning on the spot is not as easy.

If you haven't committed to a KiwiProp you could give Darglow a call - they are very helpful - some of the best customer service I have had anywhere.

01929 556 512 (General)
01929 556 037 (Technical)

[hotstonp]

Just put a new Darglow prop on as well.... waiting for Feronia to be lowered back in the water then I shall see how it performs. The last prop had become dezincified.

[andygc]

Two boats called Feronia, and both with Darglow props ....

[hotstonp]

Well Feronia is a good name for a Freedom. That was what she was called when I got her and I wasn't going to change her name, she has had several names through her life. Her original name was Fancy Freedom when she was reviewed by PBO in June 1986. But the Goddess of Freedmen is by far a better name