3 or 4 Blade Prop?

MedDreamer

MedDreamer

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This question may be too general but which prop performs better?.

What I want is to be able to get boat on plane relatively quickly and cruise steadily at 22-24 knots (around 3600-3800 revs with current 3 blader).

Engine is Mercruiser 5.7L with Alpha(Gen II) leg.

As I need to refurbish/replace my current 3 blade prop should I change it for a 4?

Martyn

<hr width=100% size=1>Do you think a Fleming 55 would look out of place on Windermere?
 
In theory 4 blades could improve things, but you would gain more advantage from superior blade design than more blades per se.The prop you have is designed to be a good all-rounder - and there's the rub! Do you want quicker to plane times with slower speeds, or reduced cruising revs, with slower time to plane?

Being as how you are going for refurb / replacement, you should talk to a company called Steel Developments who do a range of props in Aluminium, Bronze, and SS. Their advice, is generally sound. I bought a direct 3 blade replacement from them once (a Solas ali prop) for my Merc with the same pitch as the old one. The difference was astounding!! You actually need to talk to someone there to get advice rather than just surf their site. Anyway, here's both...

<A target="_blank" HREF=http://www.steeldevelopments.co.uk/Steel_Developx.html>http://www.steeldevelopments.co.uk/Steel_Developx.html</A>
020 8874 7059

<hr width=100% size=1>Madoc Yacht Club
<A target="_blank" HREF=http://www.madocyachtclub.co.uk>http://www.madocyachtclub.co.uk</A>
 
Thanks Dave

I know of Steel Developments from other posts on here and I looked up their website over the weekend. I will speak to someone there as you advise

Martyn

<hr width=100% size=1>Do you think a Fleming 55 would look out of place on Windermere?
 
Four blade prop, definately worked better for me. Or was it cos they were new??

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn
 
Just spoken to Julie at Steel Developments - She was really helpful, not trying to sell just advising.

It sounds like my existing prop is underperforming (by about 400-500 revs) so she thinks I need a smaller pitch. I am going to send it to them for refurbishment and they will then advise me if it is the optimum one. If it isn't I will keep it as a spare and buy the correct one.

Really chuffed with the great service

Martyn

<hr width=100% size=1>Do you think a Fleming 55 would look out of place on Windermere?
 
Totally nothing to do with performance but I had a 3 blade prop on my 23footer and now have a 4 blade on my 35 footer. Whenever I caught a fishing float or rope with the 3 blade prop the rope cutters would sort out the problem and the resulting rope end would drop out from between the blades. Since using a 4 blade prop the problem I have found is that on the two occaisions I have met floating rope in the Channel the rope cutters work but some rope gets caught in between the blades and becomes the devil of a job to remove. The result is that I have 3 options.
1 Put up with the resulting vibration.
2 Go the rest of the journey on one engine.
3 After much forward and reversing if it still stays put then strip off and dive down to remove it the remaining piece of rope

Result.....
Three bladed props which give the required performance are much more acceptable than a four blader.

<hr width=100% size=1>
 
Four bladers are quieter, but they tell me not as efficient as three blades at planing speeds. Submarines, to hide, have multi bladed props, quiet! But I think if you look at high performance props, they only have two or three blades. I considered four bladers for my boat, but at 5000 pounds each, they got left!!

<hr width=100% size=1>
 
But all the new princesses have 4 blade now. Recomended as faster.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn
 
I may be wrong, it has happened before, but if you look a real high performancve stuff, they only have two blades, chopper props. Maybe they are only good for very high speeds etc.

<hr width=100% size=1>
 
Re: 4 or 6 Blade Prop?

wel, dunno about 2 or three, but these are surface drives with four blades. I'm sure some have 6? I expect that being nice and big the boat will go like the clappers. The 2000hp V16 diesel on the other end might also contribute a bit.

Dscf0198_001.sized.jpg


<hr width=100% size=1>
 
Re: 4 or 6 Blade Prop?

I think surface drives, are a little different that the ones we are talking about, they are more like jet units, but I take your point, that must be one very fast lady!! Saw a similar thing in vilanova in Spain, but on a big RIB, with all kinds of tabs and trimmers etc, all made from titanium!!

<hr width=100% size=1>
 
Now have three and four bladed aluminium props for my 4.3 with Alpha Gen 2 outdrive and I am much happier with the four. Quicker planning, lower cruise RPM (so probably more efficient) and only slight loss of top end speed, if at all. I realise that this is a very simplistic view with blade design, size and material all playing a large part, but IMHO all other things being equal, buy a four blade.



<hr width=100% size=1>
 
Not sure how this translates into duo props, but in general terms you ought to know the following if you consider going from a 3 blade propeller to a new 4 blade propeller.

1) A four blade propeller has more surface area than a 3 blade propeller, hence the "grip in the water" is better.
2) More power will be required to push a four blade propeller. (Torque)
3) A 4 blade propeller is better balanced than a 3 blade propeller. (Quieter)
4) A 4 blade propeller will provide more transom "lift" as it bites into the water.
5) A 4 blade propeller has greater resistance in the water.
6) Top speed may be slower than a 3 blade propeller.
7) Cruising speed and fuel consumption is often improved using 4 blade propellers.

Generally, you will find newer, shaft driven boats having four bladed props due to a combination of reasons. Balance & being quieter, design limitations, such as maximum diameter of propeller etc.

Also you will expect a larger diameter propeller, with less pitch (less loading), to be more efficient in the water than a small one with a high pitch (something to do with the fluid flow).

Surface props are different animals...., and probably irrelevant for you...


<hr width=100% size=1>Alf
 
Re: Quite right Alf

<< expect a larger diameter propeller, with less pitch (less loading), to be more efficient >>

I am lead to believe that a single bladed prop is theoretically the most efficient. However problems with balance and the diameter leads to 2 or 3 or 4 bladed props as being better compromises. I find it a little difficult to believe that a 4 blader would be that much better balanced than a 3 blader. The trend to 4 on larger shaft boats is due to having more frequent and smaller shock waves hitting the hull.

With a sterndrive the balance and number of shock waves are irrelevant. What is more important is the pitch and blade area which can obviously be less with 4 blades than 3 and so may make getting up on the plane much easier at the cost of ultimate top speed. Also agree surface drives are a diferent kettle of fish but they do follow my understanding as they tend to have few blades acting in the water at any one time. Subs are only interested in silence, efficiency in the defence forces doesn't matter.

<hr width=100% size=1>
 
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