Are fewer blades always 'faster' than more blades?

[Philster]

I hate when I give general, well-established guidelines to people and then they refute them because some guy told them something different, which requires me to go into more detail, but also know the exceptions to the rules (so to speak).

Case in point: Friend has generic 25' center console. Wants more top speed, so decides that a 4-blade SS prop is the way to go over his 3-blade SS prop (which happens to match his engine spot on as far as his typical load and the engine RPM).

So... I think he might get a little better hole shot and scrub off 1-2 MPH and maybe enjoy better cruising characteristics, but 4-blades won't return more top end.

However, it raises the question: Can anyone cite examples of when a 4-blade prop would return more top end? My only guess would be if the 4-blader was exceptionally good at getting more bow lift over a given 3-blader and this means less wetted hull = less resistance. Not saying this is the case, but just wanted to know if there are exceptions to the rule.

 

[robert graham]

Re: Are fewer blades always 'faster' than more blades?

I gotta say, there's so many variables in prop design and application, my guess is that ALL props give some type of compromise regarding speed, bow lift, etc., etc....hopefully we have some prop guru's/engineers that can answer this. How long before that new Checkmate hits the water?...I'll be interested in your GPS top speed! Good Luck!

 

[Frank Acampora]

Re: Are fewer blades always 'faster' than more blades?

It used to be in the old days that if you wanted to race, you used a two blade prop. The reasoning was approximately--now don't quote me exactly--each blade operated in less disturbed water and was thus more efficient. On the other hand, there was also reasoning that with multiple blades, the efficiency of each blade was less but overall prop efficiency was greater.

Indeed in the 1940s. model airplane enthusiasts used single blade props on their rubber powered aircraft with great success.

However: Back to real world it seems that a lot of racers are using multiple blades. I think that with the advent of prop design and surface running all bets are off.

I think--and that is just that-- a thought. And thinking always gets me into trouble. Anyway I think on a heavy hull the greater blade area of a four blade might just return more top end IF the engine turned it at correct RPM and as you say, gave more lift for less hull wetted area.

 

[r.j.dawg]

Re: Are fewer blades always 'faster' than more blades?

One of our local custom marine shops had a Nor-Tech 4300 for sale and it had Merc 6 blade Cleavers on both drives.

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

Yeah, so I think what'll I will do from now on is tell 'em to try it out and be ready to switch back to the old prop if the new one doesn't cut it.

I never understand all this grief over a couple of miles an hour (I know; I know... from a guy with high performance boats). I could understand if there was an issue that should be addressed, but does it matter if a center console tops out at 44 or 46? Isn't the low-end and mid-range performance just 99% of the issue?

Anyway, I digress... but I'll be sure to report on the Checkmate top speed...

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

I gotta say, there's so many variables in prop design and application, my guess is that ALL props give some type of compromise regarding speed, bow lift, etc., etc....hopefully we have some prop guru's/engineers that can answer this. How long before that new Checkmate hits the water?...I'll be interested in your GPS top speed! Good Luck!

Well, she'll be mine (on paper) in a few days, but won't splash until April, when the dealer and the Merc guy get on 'er and give 'er the shakedown run (tinker with jackplate position, etc). She'll be indoors til then.

 

[tpenfield]

Re: Are fewer blades always 'faster' than more blades?

I do not know of any examples of more blades giving better top end speed ( say 4 vs 3 blades).

It may be possible in certain cases where a 3 blade is improperly matched, but that would be about it.

Let us know how your friend makes out with the 4 blade.

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

One of our local custom marine shops had a Nor-Tech 4300 for sale and it had Merc 6 blade Cleavers on both drives.

Oh good grief, isn't that a Cat? All bets are off then! That is basically a low-flying airplane that comes into contact with the water now and then at high speed, so those props are holding onto the water!

These props will chop you up!

 

[r.j.dawg]

Re: Are fewer blades always 'faster' than more blades?

Oh good grief, isn't that a Cat? All bets are off then! That is basically a low-flying airplane that comes into contact with the water now and then at high speed, so those props are holding onto the water!

These props will chop you up!

View attachment 129219

Yup, they are some sharp razors that masquerade as prop blades. The boat is actually a supervee. I looked it up and it's still for sale.
http://www.doublerperformance.com/i.../17-41-50-ft/6-2011-nor-tech-4300-supervee-li

 

[Texasmark]

Re: Are fewer blades always 'faster' than more blades?

I think Chinewalker is the guy who's Avatar is the picture of a gold and green Merc Mark 20H with a racing lower unit and a small 2 bladed racing prop. Back when I was a kid that what was used on primarily short, light 3 pt. hydroplanes. I think they were direct drive so if the engine was spinning at 6k so was the wheel. Merc used 2 blades for a long time in their production engines I think at least until they dropped seafoam white and went with phantom black back in the late 60's on their larger engines.

Published data I have read says that a 2 blade is the most efficient but it vibrates the most. Also the more surface area the blade has the better the thrust for heavier boats, higher loads.

The little 3 point hydro's were hard to get going. They had a spring loaded hand grip that you squeeze to increase rpm's. The driver would idle out to the launch area, squeeze the grip and the little prop would throw water 20' behind the boat, ventilate like crazy, rpm's through the roof. He'd release the grip and immediately get it back in an attempt to get the little blades biting into solid water. Each time he cycled it the boat would be going faster and there was less ventilation. Finally it would catch and away he would go.

I had no involvement in the sport, but a guy I paled around with and his dad did and I went to events with them...Really enjoyed it.

Mark

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

That is really interesting.

 

[R Socey]

Re: Are fewer blades always 'faster' than more blades?

The only senario I can think of is if less blades did not provide enough thrust to get you on plane, and more did.

 

[TorchedGT]

Re: Are fewer blades always 'faster' than more blades?

Interesting thread, however the aviation requirements for additional blades were brought on by very different needs and problems. In high-power applications and turbo-prop planes (the "scimitar" props for example) they must balance overall diameter with power handling - the new airbus military transport has 11,000+ shaft horsepower has 8-blade props that are still 17' in diameter.

As engines became more powerful, the blades became too long (risking prop-strikes) and also, the blade tip speeds were increased - which leads to the next problem, wave drag. In aviation applications, a prop usually reaches nearly the speed of sound, and at anything beyond induces severe drag. This is why modern props trail backwards towards the tip where it travels faster (also why fast planes have swept wings).

Neither wave drag nor diameter : power ratio limitations really apply in boating, since water is so much more dense, you absorb/transfer significantly more energy and can "load up" an engine with no problem.

In all types of marine props, cavitation seems to be the single element that dictates different designs (and there are many different types) - the exception may be large slow moving ships which optimize for efficiency, or military applications which optimize for everything, including silence. When the speed differential of the thrust versus boat speed is smaller, you see the aviation-type multi-blade variable pitch props - like on cruise / container ships or tugs. Beyond that you're looking at cavitation / performance though.

As far as tradeoffs go it's like gearing a car, the higher ratios will allow a multiplication of lower end power at the sacrifice of top end and rotational speed. When moving water from a stand-still is important, more blades are better - but as the differential between boat speed and trust/prop speed becomes closer, then the advantages run out.



.......having said that, I have idea why the performance boat I'm looking buying at has a spiffy 4 blade SS prop on it

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

Are you saying you have no idea why?

One big advantage is that the added thrust from a four-blade prop can keep your boat on a better planing angle in mid-range and lower RPM's, which translates to better fuel economy. It can help get some boats on plane faster or provide more response at low speeds.


The benefits are hardly noticeable on boats under 24'.

.

 

[Texasmark]

Re: Are fewer blades always 'faster' than more blades?

That is really interesting.

Hopefully you are serious in your statement. I mean, these little critters were small engined, light boats, with minimal water contact....nothing to compare with ocean racers with 10,000 #, 1200 HP, 2 or 3ea 6 blade cleaver props and all....different world totally.

Mark

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

No, I am sincere. Believe me, you'll know when I am being fresh. I thought those kind of conditions could tear props up though.

 

[cribber]

Re: Are fewer blades always 'faster' than more blades?

This is my personal experience going from a 3 blade 21" aluminum prop to a 4 blade 20" SS. I dropped about 4 mph off my top end from 45/44 to 40/41 mph and 100-200 rpm. I did gain a second in holeshot and dropped my on-plane speed from 21 to 19/20 mph on the gps based on wind conditions. It's all about compromise when it comes to prop selection. I have a 3.0l Penta on a Glastron gt185 at 2300 lbs dry weight running a 1.94:1 gear ratio.

Find a place that will let you experiment with your prop selection and get the one that suits how you are going to use it. I have three that I use based on load and what we're doing for the day.

 

[TorchedGT]

Re: Are fewer blades always 'faster' than more blades?

Are you saying you have no idea why?

One big advantage is that the added thrust from a four-blade prop can keep your boat on a better planing angle in mid-range and lower RPM's, which translates to better fuel economy. It can help get some boats on plane faster or provide more response at low speeds.


The benefits are hardly noticeable on boats under 24'.

.

Well I have my ideas but it's more of a top-speed type of boat, 22' Nordic.

 

[steelespike]

Re: Are fewer blades always 'faster' than more blades?

I used to hang around when Chinewalkers father was trying to get his boat set up just right for racing.
I think it was about 1958 a Mark 55A or H One of those 40+hp.I think a 10ft utility at that time. He would start out and stand and lean way over the wheel to help get the bow down( and he had some beef to work with) with an occasional throttle release to try to grab, sometimes you would wonder if it ever would make it.
Those little props we called "surface drive" because about half the prop came out of the water the blade was designed to catch some water and throw it back still creating thrust but allowing the motor to really rev. The Special "Quick Silver"lower unit I know had special gearing I'm sure Chinewalker may stop in and give us the details.
As far as a 4 blade giving more speed it can happen. See if you can find posts by HWsiii he did a lot with the al 4 blade Solas Amita.
Haven't seen much of him since the gulf oil spill.

 

[Philster]

Re: Are fewer blades always 'faster' than more blades?

Well I have my ideas but it's more of a top-speed type of boat, 22' Nordic.

I know, but what you'll find is that you can't run wide open all day, and the novelty is replaced with being able to cruise on and on at good clip with the engine RPM just right. If you have a 68 MPH boat with a 4-blade prop on 'er, you really don't miss it being a 70 MPH boat, since 99% of its life is spent at lower RPM.

You take the better cruising characteristics, etc.