3 Blade to 4 Blade on 45C

[Gina Marie]

Is there anyone who has converted a traditional 3 blade to a 4 blade prop? Is it worth it and does it do anything positive. Thanks Gina marie.

 

[Brian Degulis]

I don't think he was thinking of adding a blade? I think he means what's to be gained by switching to 4 blade props right? The answer is smoother better blade loading a slight loss of efficiency but well worth the gains if your trying to lose vibration.

Brian

 

[Gina Marie]

Yes I want to convert from 3 to 4 Blades not add a blade to a 3 blade wheel. Will I increase RPM, Increase or decrease load, will speed increase or decrease. I really do not mind the slight vibration since I know no difference anyway. I am looking to pick up a few more knots 1 or 2. Right now I am runnin g 17 to 18 knots at 1800/1900. But its that damn following see that I am trying to beat. Thanks Gina marie

 

[TedZ]

I would think that the best tecnology props, 4 blades or more, could add a few knots to your speed. There are many threads on the forum about such improvements. It gets expensive to experiment so a good prop supplier familiar with your boat would be helpful.

 

[TedZ]

Sorry, forgot to give you a link re: wave speed.

http://en.wikipedia.org/wiki/Ocean_surface_wave

Seems to me sometimes you can outrun them but many times you can't.

Ted

 

[REBrueckner]

Brian posts: "The answer is smoother better blade loading a slight loss of efficiency but well worth the gains if your trying to lose vibration."

And thats what Dave Geers "Propeller Handbook" also sez.

And cupping props on heavy boats like ours is also mostly a wasted effort.

 

[Brian Degulis]

If your not looking to reduce vibration then I would think a 4 blade is not the way you want to go. 4 blade is good to reduce vibration and good if your clearance between the hull and prop is to small with a 3 blade because the 4 blade can be slightly smaller for the same thrust.

Probably what you want to do is get the 3 blades you have fine tuned (Prop Scan) Or look into some new technology 3 blades but I think the Prop Scan is probably your answer. Considering only the prop 4 blade will not increase speed in fact it will reduce it slightly.

Brian

 

[Gina Marie]

Thank you folks I am staying with 3 blades

 

[TedZ]

Once again we have covered 3 vs 4 vs 5 blade props. Gerr's work is very outdated when it comes to # of blades.

There are threads here that show pictures of new Hatterases with 5 bladed props. The briefest of Google searches will show that the entire boating population form motor yachts to performance boats are all migrating to more blades for improved speed, better fuel economy and smoother running.

Buying new 3 bladed props is a waste of money and is old technology INHO.

Ted

 

[jim rosenthal]

I agree. Propeller technology has moved along quite a bit. I have thought about a set of 4 or 5 blade wheels for Blue Note, form time to time; the expense stopped me, plus my current wheels run fine. Generally speaking, most of us can go as fast as we want to and don't use the top speed of our boats very often at all. The advantages to be gained in using more efficient wheels are 1) reduced fuel usage for the same speed 2) less vibration at cruise 3) less noise at cruise.

There is also the advantage that a more efficient wheel may provide better thrust at docking, which improves maneuverability. At least in theory; it might or might not be noticeable in practice.

Given the cost of wheels for some of these boats (especially if you look at ZF or Veem propellers) you need to be doing a LOT of boating to save enough money to pay for the cost of a set of props. But they are very cool toys....like jewelry, at least when you first stick them in the water.

 

[Brian Degulis]

Once again we have covered 3 vs 4 vs 5 blade props. Gerr's work is very outdated when it comes to # of blades.

There are threads here that show pictures of new Hatterases with 5 bladed props. The briefest of Google searches will show that the entire boating population form motor yachts to performance boats are all migrating to more blades for improved speed, better fuel economy and smoother running.

Buying new 3 bladed props is a waste of money and is old technology INHO.

Ted

That's wrong Your seeing more blades and new designs as part of an entire re engineering of propulsion systems. Years ago it was not possible to use a deep reduction without a very large prop which would increase draft to much. Today there are props with very high pitchs and more blades so they can produce thrust wiyhout being overly large. Then there are props in tunnels which in the past had terrible cavitation problems new technology has helped with that also.

But unless your planing to re engineer the whole boat it's still very tough to beat a 3 blade as far as producing the most thrust per HP. I would imagine that there would be some improvement with some 3 blades built today compared to some built 20 years ago but not much probably not enough to be noticable. So unless your looking for smoother (which the poster say's he's not) your going to lose a little going from 3 to 4.

Brian

 

[Genesis]

The more blades the more drag (loss)

The higher the DAR the more loss.

However - there is a point at which loading becomes problematic for flow reasons and then you lose efficiency bigtime.

Planing boats require a compromise in these areas, which often means that the obvious (few blades = better) ends up being wrong.

 

[REBrueckner]

Brian posted:
"Today there are props with very high pitchs and more blades so they can produce thrust wiyhout being overly large. ....
But unless your planing to re engineer the whole boat it's still very tough to beat a 3 blade as far as producing the most thrust per HP."

I agree. New boats have maybe double the HP we do, often in lighter boats with different hull shape.

I'm guessing Gina's 45C has perhaps a pair of 525 HP engines...??
Put in a pair of 900 HP engines, to pick a number, and new five bladed props likely make sense...

Anyway, Gina, why not call a prop company or better two or three, tell them your engines/boat/HP/reduction/prop size and they'll immediately be able to tell you what to expect....it's not like this has never been done before....

And if they promise you, say, five knots, ask what discount you get for each knot you miss that figure...Also, ask the prop company people for the name and telephone number of a few owners of boats like yours they have repropped to see what the owners actually experienced.

Likely you'll get some interesting feedback you can share regardless what you decide. Post it here and make some of us look like goofballs!!!

 

[BUSTER]

Just a little input from my past experiences on this subject. I have made the conversion from 3 to 4 blades many times on several boats that I have owned and more often on customer-owned boats.In general the results were: A much smoother running boat,a small loss of speed at WOT,but a little gain in cruise speed,which translated into somewhat better fuel economy.

Tony

 

[Vincentc]

Once again we have covered 3 vs 4 vs 5 blade props. Gerr's work is very outdated when it comes to # of blades.

There are threads here that show pictures of new Hatterases with 5 bladed props. The briefest of Google searches will show that the entire boating population form motor yachts to performance boats are all migrating to more blades for improved speed, better fuel economy and smoother running.

Buying new 3 bladed props is a waste of money and is old technology INHO.

Ted

Who had taken Gerr's place? Sincerely, I'd like to reference any data to update the Propeller Handbook.

Perhaps getting older has set me in my ways and I am stuck in what I saw saw and did in the 70's and 80's and am just suspicious of a lot of boating innovations since then. I do question whether much of what is new in boats is better.

Based upon my appreciation of Gerr's book, perhaps the most significant reason for more blades is blade loading. You need to consider weight, HP, intended cruising speed, shaft RPM, and diameter. Unless you think oil prices are not going to go back up, efficiency may be more important than a few more knots top speed. Increased diameter is probably the best way to achieve needed bade area and it is limited by how far your shaft is from the bottom of the boat.

Gerr's book has tables on diameter, blade area, etc. Interesting to me but time consuming to sort out; however to paraphrase a philosophical addage. The more I know, the less a lot of professionals know. It pays to research even if you are going to hire someone to do it. I have been disappointed in the past by outboard prop shops decisions on what I needed.

Philosophy and Theory aside, I beleive the post started with a request for real world experience with comparable boat and sorry I can't make a contribution there.

Regards,
Vincent
43 DC 1983

 

[TedZ]

No one has taken Gerrs place, he just needs to update his excellent treatise in light of new techniques.

There are many things missing from the discussion and i hate to bring them up because all that ends up is that a "concensus" develops and you end up being a goat as opposed to a hero.

The biggest missing ingredient is stern lift. You don't need to be a rocket scientist to intuitively understand that more blades "paddle" the water more. That is where the alleged inefficiency come in. The offset is that it has the effect of producing a force vertical to the face of the propeller and can give significant lift. Some boats love to have the stern out of the water more, some do not as it "wets" the boat more and can slow it down.

Turning the props inward or outward can make a big difference. Assuming that all props should turn out is nonsense. Each boat design reacts differently and it needs to be tested.

The notion that more than three blades is for performance boats only is incorrect. See http://www.nautica.it/superyacht/533/tecnica/propellers.htm for some interesting points. (Remember 700 mm is about 28")

Remembering that an upward traveling blade has a different pitch than a downward traveling blade is very important when deciding on the amount of blades. Some hulls aerate the water more on one side of the prop and some aerate the top more and the deeper water is denser. Having more blades deeper can sometimes produce real benefits.

Nothing, absolutely nothing is certain all of the time in hydrodynamics and to embrace any "rule" about anything in moving water is foolhardy.

Have fun.

Ted

 

[Brian Degulis]

Ted This guy dosen't want to buy and try every possible combination of prop blade and design and then write a book. He wants to know if he would likely gain any speed by switching from 3 blades to 4. If you think he would tell us why based on his boat. You seem to think that new technology has made 3 blade obsolete that's just not true.

Brian

 

[TedZ]

I don't have a clue.

Three bladed props might very well be the best for his boat. I understood previous posts to suggest that they were the best. I doubt that but i don't know for sure.

Prop testing is a tedious endeavor. Comparing notes, borrowing props to try, research on your boat all can help narrow the selection.

No one can say what prop is best for any boat unless a lot of testing is undertaken and the faster the boat, the more to be gained from testing.

Ted

 

[Brian Degulis]

That's right and that's my point. In a general sense 3 blade is best for thrust per HP so without another boat to go by and without being willing to spend a lot of time and money trying to find the perfect prop. Is'nt the best aproach to work with the 3 blade props he's got? Maybe there is something that exists that might be better but there isn't any practical way to find it and keep in mind he's not looking for anything except the best top end he can get.

Brian

 

[surfdancer45]

Hulls are different, loading is different and our exspectations are different. Buster has the answer that fits most situations. A four blade prop with performace exspections equal to your current three blade prop (generally one inch less pitch) will smooth out the boats vibration, yield a slight increased in cruise speed vs a slight loss in WOT speed. Is it worth the cost to upgrade? Depends on how many boat bucks you want to spend to gain a knot or knot and a half at cruise and have slightly less hull vibration. My 2 cents. Bob