2 blade fixed vs 3 blade folding prop

[Ben998]

Hi All,
Im thinking of changing from the existing 2 blade fixed prop to a 3 blade folding prop. The boat is 37ft, fin keeled with a 3GM30F yanmar developing about 27bhp and is a shaft drive.
Will the three blade prop deliver better performance both going forward and in astern? What are pros/cons of the three blade folding prop against the two blade fixed prop?

cheers,
Ben

 

[michael_w]

I found this the other day. The definitive answer to the great prop question

http://home.mindspring.com/~pedersen1/propeller.pdf

 

[PembrokeshirePromise]

From the summary:

"The text of this report is written with the lay audience in mind"

Glad he didn't make it complicated then /forums/images/graemlins/confused.gif

 

[michael_w]

I didn't say it was going to be simple /forums/images/graemlins/cool.gif

 

[Stemar]

 &/!+/!6+9)*'1+-  ! &/(#$6/(6#$+
#$#8##$6 '1 !/!+M9'#$"@#M0"#$78+#$% ! ! @/(#M8
'1 (#$6! !=>& (7#B: !=%/!?%'1+% !66 ('&#$66+$>0 !7+.)*'1 ! 5*>& ! !6,;

WTF????

I was trying to paste the bit where he says there's significantly less drag if you let the a fixed prop spin free than if you lock it.

Well there goes another myth.

 

[FullCircle]

My fave subject.
I have 3YM30 Yanmar 29hp, 2 blade fixed
Lynn has just bought us, a 3 blade Kiwiprop and an Ambassador Stripper..

Stated advantages:

Reduced or eliminated port prop walk
Full power astern in an instant
Faster going forward
Tunable pitch for optimum drive to max revs
Lack of galvanic corrosion as its plastic
1/4 to 1/3 knot gain in speed while sailing
no rotating shaft to wear stuff out
matches Ambassador Stripper.
sell the current prop for a few quid.

Disadvantages

cost 800 quid + vat.
cant use Tiga Waves excellent rope cutter

Just do it as the Nike ads say......

 

[SvenglishTommy]

[ QUOTE ]
Disadvantages

cost 800 quid + vat.
cant use Tiga Waves excellent rope cutter

Just do it as the Nike ads say......

[/ QUOTE ]

You forgot one. Only available down to 15", so about as useful as slap round the face with a wet fish if you need a 14" prop /forums/images/graemlins/mad.gif

 

[mobeydick]

I find rotating props work much better than fixed

 

[savageseadog]

It's folding or feathering prop every time don't think about it just get on with it and buy one.

 

[davidbains]

Ben, being blinded with science is so easy!
In lay terms a 3blade prop will have greater blade area than your "sailing'" two blader, and your 3YM30 has plenty of power to drive it. Motoring performance will improve as will sailing , a double whammy hard to resist.
The real problem is to decide what diam and pitch you should go for. Also you need to form a view about whether you want a folder or featherer.
FWIW I have just taken delivery of a 15" Kiwi prop (featherer). The guy at ?Vectamarine is very helpful about sizing. They're not cheap but some rival makes are even more expensive.
If you'd rather have a folder with self pitching investigate Autoprop, supplied by Bruntons.

 

[Talbot]

Why would you sell the old one when it is a good idea to have a spare prop even if it is not a great match.

 

[FullCircle]

Hows the flu mate? mines in recession at last.

Well. I intend to carry spares to repair or replace my new Kiwiprop, as I dont see the need to return to 2 blade fixed, ever. Lack of a gearbox makes them simple.
A total of 16 blades worldwide have been replaced in 6 years, bot a bad record by any means. Kiwiprop will also supply replacement blades if asked. Other than that a couple of spare springs methinks.

 

[Talbot]

Flus not to bad, it is the chest infection thats giving me and SWMBO the most grief. The combination of lack of energy, coughing, tiredness, dizziness and the swirl of penicillin around the system is a bid much.

can you replace the blades and spring with the boat in the water? - this would be possible with a normal prop (not easy, but possible with a suba or hookah system)

 

[FullCircle]

Dont know, but I suspect that I would just take the prop off and do it on deck. I have twin rudders and the prop is easy access to stern. I can also dry out to do it......
I carry the pullers and tools to do it, and given oily calm seas in the middle of nowhere, would probably do it!


Flu: Yes the throat became the chest, and now I can sleep, but my nose has finally started running. Still cant get the headache clear though. Maybe tomorrow.

 

[thalassa]

Paid 900 EUR (616 GBP) plus VAT for a KIWI prop last week at the Amsterdam boat show. /forums/images/graemlins/smirk.gif

 

[FullCircle]

bet it was a plastic one though!

 

[Talbot]

I managed to get the headache under control by use of the standard day nurse + 2x200mg of IbuBrufen with the day nurse(as authorised by doc). Hope that helps.

 

[thalassa]

Plastic? Spot on... it actually is !

 

[Sailfree]

I went through the same loop but because HP is 75 the Kiwi was out. While more blades result in more drag I suspect the overall smaller dia may result in less rubbish and ropes on prop. It also is smoother.

I finally opted for a 4 bladed feathering Variprop having been previously impressed with the engineering on the 3 bladed one I had on a 40HP engine. I still have this prop (for a saildrive) if anyone is interested.

Boat about to be launched so will soon know the result over the previous fixed prop. One big advantage for a feathering prop is the ability to adjust the pitch to get max revs and power. Fot that reason I like to have external adjusment that nearly all have except the maxprop.

A major factor in wanting to change from a fixed to either a feathering or folding propeller is that on many modern engine manufacturers instructions are NOT to lock prop in astern as it stresses the gearbox. I just don't like the sound nor the wear of a constantly rotating prop when sailing.