4 blade props vs 3 blade props ??

[Medskipper]

Hi Everyone!

I keep seeing on the more modern cruisers with shaft drive, 4 bladed props! Now I know this is a silly question! but here goes! Are there any experts out there that can tell me if your average cruiser with a 3 blade prop could: (a) be fitted with a 4 blade prop? (b) benefit from one, i.e. more speed,less fuel etc. and (c) if its yes to the first two questions, not need a change of gear box?

Any ideas guys??

And at the risk of getting Byron going again about Birchwoods! I have a 37!

Barry Winnett

 

[Guest]

Now when I was in the states 2 weeks ago I got chance to read some of there mags, boat ones im afraid! now there was a feature on this using a Sealine S 48 with big cummins engines in, the stock props supplied by the builders were changed to various ones and makes, mainly Brit props by Teignbridge 3,4,5 bladed ones, without going into too much rocket science the best allrounder was believe it or not the stock supplied 3 blade.

4 bladers were quieter but gave less top speed by 2 knots, and slower acceleration and used more fuel.


5 bladers were very quiet, gave less vibration, were quicker at the top rpm giving slightly more speed as well but they upped the fuel consumption and cost a lot more, mid range speed was better per given rpm.

Conclusion was to keep what you got in the price, and the Brit props won I think, as they described it was to imagine the blades turning and every passing of the bottom of the hull a bang was created 3 is and odd number and flow 4 was more even and balanced, 5 was out of sync but the blade area ratio was more so less gaps for cavitation.
It is surprising what you can learn when waiting in the airport departure lounge getting bored, I did not even buy the mag either.

Getting through customs and check in was another story they picked on me every time, nearly stripped searched me both in and out of the country, and I did not even have my Binladen smile on either!, just shows you how things have tightened up.

As for the props unless they are knacked or beyond economical repair and it is a planing boat then I would not bother spending any money.

Paul js.

 

[stuartw]

As I also have a TS37, with twin 306's, I have been through the same exercise myself, contacting many prop manufacturers here, in the US & OZ. I have 21" D x 31 pitch, 3 bladed , and I do get a lot of slippage when the throttles are opened while accelerating. 3 bladed are the most effecient, ie the HP in, to thrust out. Where the 4 bladed will score is a lower blade pressure, & thus lower cavitation, and of course more rapid acceleration.
If you have cavitation/slippage a lot of the time, then a 4 bladed will help, although a lower pitch will be required, thus no need to change the g/box. Do not expect geat fuel savings though. If you are at cruising speeds and no slippage/cavitation then the 3 bladed is best.
Personally, on balance, I may well go to a 4 blade at some stage, the pitch dropping from 31" to 27", as calculated by prop makers. But, the cost. Can I really justify some £1k+, maybe not.

 

[adarcy]

Re: Prop efficiency

Barry

I can't remember exactly why/how but I remember reading that the theoretically most efficient prop is a single blader. Obviously, it's no real use because of the imbalance but, by extension a 3 blader should be more efficient than a 4 blader.

There must come a point where the power put through a certain size of blade makes it slip so sharing the power through more blades means less cavitation or avoiding going up a few sizes.

I was interested (as usual) with pauljs said/read but IMHO there is a flaw in his given reasoning. As the prop is rotating it sends a shock wave onto the hull but there is no difference in the regularity or balance if it is a 4 v 3 blader, just a different number of shocks per rev. I think it is supposed to mean 4 smaller shocks rather than 3 bigger ones at the same revs will be smoother.

Anyway, AFAIK, props are a black art and sometimes doing the "wrong" thing is right. If only we could all afford to get Rolla to make props for our own boats viz Riva.

Anthony

 

[Guest]

Re: Prop efficiency

Thank you for the following AD I must watch what I write in the future then as obviously someone reads it!, the way they described the noise was exactly that each passing of the blade under the hull created the bang a 4 blade more evenly balanced gave less noise overall, thinking about it I seem to remember that the most efficient prop was a 2 blader, reminds me of an old penta outboard we used to have.

Paul js.

 

[Medskipper]

Thanks very much guys! I am sincerely impressed! You certainly seem to know your stuff on props, I dont seem to have any problems with the 3 bladed props on my vessel, I jusy thought that perhaps they were less effective than 4 blades!

After having read what you guys have said, I bow to your knowledge and experience and will stick to my 3 blades!

Many thanks and I trust you all have a great season this summer!

Barry