Propeller question

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awpptdt

awpptdt

Senior Member
Joined
Nov 7, 2010
Messages
167
Location
U.S.A.
Vessel Name
Donnchaidh
Vessel Make
Marine Trader 40' sedan
I want to replace my 3 blade 26"d, 19"p, LH, 1.75"shaft, propeller with a four blade one. I have read that I should get one with one inch less pitch. T question is will a 24"d, 20"p work?
 
Guessing at diameter and pitch is fine for an outboard with a stack of used props to try.

Over about 24 inches props get way too expensive to guesstimate .

One question is why 4 blades?

Is the engine so powerful that more blade area is required?

The more blades the lower the prop efficiency , so why the change?
 
I have always understood that 3 blade was more efficient at your top RPMs but a 4 blade would be at your lower cruise ranges.
 
awpptdt,

The question here is blade area not # of blades.

If you need more blade area go to the 4 blade. Most of the time that's all one needs to know. Lots of yachts need 4 blade props because the space for the prop is limited and the prop must absorb lots of power. Trawlers have more latitude as they usually have more room for larger diameter props. For example my own Willy could use a prop w 2" more diameter.

Also it's possible to actually increase efficiency going to a 4 blade if the 3 blade has very wide blade tips. So if your 3 blade prop is starting to look like a ducted fan prop more blades w smaller tips and a higher aspect ratio could/would be more efficient.

So if your 3 blade hasn't got too much blade area for it's blade length stay w 3 blades.

As I recall there is a blade area to disk area ratio # that's used in the industry to numerically show or define the amount of blade area relative to the disk area. The disk area is the area of the circle scribed by the blade tips as they revolve. This ratio is called EAR. The EAR on my prop is .51 or .53 so just over half the disk area is filled w blades. One could not see through a prop w an "EAR" of 100.

So unless you need a 4 blade stay w the three blade.

But don't fret over getting this right as it usually makes very little difference.
 
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Another propeller question that is somewhat related. Has anyone one compared a 3 blade to a 4 blade prop on a full keel vessel? I had one, and changed to a 4 blade. The prop was tuned and computer balanced, but at higher rpms the 4 blade had a little bit of a buzz type noise. I attributed that to the fact that two blades would be parallel to the keel as they went around. The 3 blade seemed like it was pulling more clean water. Maybe just me. Just wondered if anyone else expierenced something like this.
 
Oh yes I've read about it and talked about it.

I think w wide blades (or very wide blades) the "shadow effect" would be unnoticeable depending on how wide the deadwood (now FG) is. Just another reason to go w a 3 blade prop as I see it. I think we talked about this on the Willard Boat Owners Group and as I recall no one seemed motivated to change anything. There is the efficiency aspect too but it as well seems to be fly stuff.

On your boat Don I suspect the propeller loading could be playing a part in your experience. Could be thin blades acting like tuning forks. Could be a form of propeller "singing" too. Could be the prop shaft bouncing back and forth on the rubber of the cutlass bearing in a state of "resonance". Have you asked a good prop man about it?
 
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manyboats said:
Oh yes I've read about it and talked about it.

I think w wide blades (or very wide blades) the "shadow effect" would be unnoticeable depending on how wide the deadwood (now FG) is. Just another reason to go w a 3 blade prop as I see it. I think we talked about this on the Willard Boat Owners Group and as I recall no one seemed motivated to change anything. There is the efficiency aspect too but it as well seems to be fly stuff.

On your boat Don I suspect the propeller loading could be playing a part in your experience. Could be thin blades acting like tuning forks. Could be a form of propeller "singing" too. Could be the prop shaft bouncing back and forth on the rubber of the cutlass bearing in a state of "resonance". Have you asked a good prop man about it?
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Eric, it is not on my present boat, but was on a Mainship I owned in the past. The discussion here reminded me of it. The Sabre is twin screws with no keel. It runs smoothly with no noticeable vibration. The Mainship at higher rpms did have the buzz. Seems to me that I would prefer an odd number of blades with a full keel arrangement.
 
Moonstruck said:
Another propeller question that is somewhat related. Has anyone one compared a 3 blade to a 4 blade prop on a full keel vessel? I had one, and changed to a 4 blade. The prop was tuned and computer balanced, but at higher rpms the 4 blade had a little bit of a buzz type noise. I attributed that to the fact that two blades would be parallel to the keel as they went around. The 3 blade seemed like it was pulling more clean water. Maybe just me. Just wondered if anyone else expierenced something like this.
Click to expand...

I have not made this comparison, however on the "downeast" boat forum of which I am a member (all single screw, full keels) there is complete consensus that 4 blade props are needed behind full keels and many trials to back it up. That said however, these are all high HP high speed boats which may be on the verge of cavitation due to lack of blade area with a normal 3 blade prop so naturally most have had good results w/ 4 blades.
I have often wanted to try a 3 blade on my downeast boat which is quite low powered, has a borderline small shaft diameter, and does suffer from some vibration which seems mostly to come up through the rudder. I am also of the opinion that an odd number of blades would, all other things equal, yield less vibration behind a deadwood.
My Willard 36 has the origimal 5 blade "shrimper style" prop which was in the original Garden specs. It is smooth as silk if not as efficient as a 3 blade would be.
Propellers are too expensive to fool with and the 4 to 3 size recommendation I got from Michigan/Federal some years ago made no promise that it would be either smoother or more efficent so I never did it.
 
psneeld said:
I have always understood that 3 blade was more efficient at your top RPMs but a 4 blade would be at your lower cruise ranges.
Click to expand...

'tis true. The higher the speed the fewer blades preferred. Fewer leading edges that have to "cut" through. Way back in the day of early OBs you could get a 3 blade "power" prop or a two blade "speed" prop on the same engine. Now-a-days manufacturers have had to dumb down the selection and give a prop that allows the engine to reach max revs. That has become a sticking point with surveyors and others who buy into it. This is especially true for planing hulls. On a displacement hull or any hull that is used at displacement speeds I prefer a slightly larger than "spec'd" engine and I'm going to set it up to give the speed I want at lower (but still reasonable RPMs), it can do this as it has more rpm "flexibility" due to producing more power at a lower range.

On our commercial trawlers a four blade was preferred for more "bite" when pulling trawls or pushing nets. This was true on the low speed boats, if the boat was expected to plane, to and from working grounds, they usually got a 3 blade esp. with gassers. But there were some who had 350HP 6 cyl Caterpillar powered 27' skiffs. They had the oomph to swing a 4 blade, and needed it to put all that power to use. It was an amazing sight to see a Lafitte skiff with skimmer nets sticking 16' in the air "loafing" down the pass at 35 kts! This was unheard of until the mid-late '80's when the lightweight high HP diesels began to hit the market. Before that you had a 6-71 DD, or a turbo/aftercooled 3208 Cat (bad rep as they didn't hold up putting out over 210 HP under the demands of a commercial shrimper who wanted the grunt/economy of a diesel AND the speed of a BB chevy). So while it has become regurgitated gospel that a boat HAS to reach max RPM fully loaded, remember that is so the 4 weekend/year Capt. Bligh can have a warranty. My boat is a 1983, I have no delusions about taking her out and holding her on the pins to head for the Dry Tortugas for the weekend. Common sense will play a big role in the longevity of just about anything. If you run your boat hard and it has marginal HP for the performance you want stick with the 3 blade, your engine will thank you. If you have an engine that the market has demanded the HP be doubled or more from its original design/ratings and you intend to use at its new rating, do not be alarmed when it goes KaBlooey with relatively few hours compared to what "they used to get"! The 250 HP 4BT Cummins is a prime example, as is the 375-425 3208 Cat. Just because it can be done doesn't make it a good proposition. A 40 yr old cypress Lugger with its original 4-71 DD was nothing rare. A planing skiff with a hot 3208 more than 3 yrs old without major repairs was an anomaly.

What I'm trying to say is that on low speed boats, while important its not as critical to have the "perfect" prop. I would much rather my trawler speed boat be at its Tq peak or PDC at the top of my cruising speeds. I.E. 1600 RPM at 8.5-9 kts. I don't care if it will turn 2800 with that prop/gearing configuration, its not where I operate my boat.

Let the flaming begin.....its my boat and my opinion based on my experience, not some industry concocted blanket rule. :popcorn:
 
Moonstruck said:
Eric, it is not on my present boat, but was on a Mainship I owned in the past. The discussion here reminded me of it. The Sabre is twin screws with no keel. It runs smoothly with no noticeable vibration. The Mainship at higher rpms did have the buzz. Seems to me that I would prefer an odd number of blades with a full keel arrangement.
Click to expand...

Don, another very likely reason to consider is the keel induced cavitation at high speeds causing the buzz/vibration. More blades means a higher frequency that they hit that turbulent stream behind the keel. I've noticed on newer full keeled vessels designed to operate at higher speeds that the trailing edges of the keels are shaped to give a clean stream off the keel, instead of the 90* squared off keels usually found on slower rigs.
 
twiisted71 said:
Don, another very likely reason to consider is the keel induced cavitation at high speeds causing the buzz/vibration. More blades means a higher frequency that they hit that turbulent stream behind the keel. I've noticed on newer full keeled vessels designed to operate at higher speeds that the trailing edges of the keels are shaped to give a clean stream off the keel, instead of the 90* squared off keels usually found on slower rigs.
Click to expand...

Thanks, that makes sense.
 
Here's my ridiculously "dirty" keel trailing edge. I briefly tried a Michigan Star five blade w high aspect ratio (skinny) blades and a fairly high frequency vibration resulted. So sometimes the variables will cause the "rules" to be not true.
 

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My propeller turns fairly slow as it is driven by a 2.65 transmision with the engine turning 2,400 rpm. The result is at speed there is a definate chop, chop, chop vibration. The prop has been reworked and I have been told a 4 blde propeller while not as efficient will make it smother.
 
May and may not but unless you need more blade area I don't think a 4 blade will be better enough to feel if at all.

How much clearance do you have from the top of the propeller swing to the hull. Being too close is frequently a significant source of vibration. Each blade slams water at the bottom of the boat and thousands of slammings a minute can cause lots of vibration. There is an industry standard for this distance based on blade length and hull to prop clearance.
 
All the prop shops we've talked to as well as friends in the marine propulsion industry have told us the same thing. The basic rule is the fewer number of blades a properly designed propeller has the more efficient that propeller will be. The more blades a properly designed propeller has, the smoother it will turn (less vibration). Also the fewer blades a propeller has the better the boat will back, meaning less prop walk.

Also, they all said that for a given boat a four-bladed propeller should have one inch less pitch to the blades than a three bladed propeller.

This is assuming the desired outcome is the engine's ability to reach its maximum rated rpm at wide open throttle. This is not always the desired outcome, Grand Banks boats being one example. GBs were usually deliberately over-pitched an inch or more from the factory.

Our own GB was over-pitched even though the boat's original three-bladed props had been replaced with four-bladed props. A few years ago after coming very close to buying new three-bladed props we decided instead to have four-bladed
props pitched down an inch.

There are exceptions to the basic "rules" stated above. After much trial and error, for example, our friend Carey's 420 hp lobsterboat did best with a five-bladed prop. However this was when he wanted to cruise the boat at 15 knots. Today he typically cruises at 9 knots unless the water's rough in which case he goes 12 to 15 knots. But at 9 knots he would probably do better in terms of efficiency with a different prop.
 
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LOL Marin that has to be the first time you've ever flat out backed up something I suggested. Thanks for posting that GB (a well thought of brand) knows to overprop a slow boat that is overpowered for its preferred operation parameters. It wasn't until lately that manufacturers decided to take their "trawlers" to sport fisher speeds that the max RPM rules of prop selection became needed in trawlers.

THe talk of under loading and over loading often elicits the same vociferous arguments as Max RPM capability. In over powered vessels that are never meant to operate at max range, overpropping is the way to "load" the diesel and make it "work". Diesels that began life as industrial motors are meant to be loaded at their max Tq range, max rpm is irrelevant. The newer high RPM (relative) engines seem to be designed to run like an automotive engine. These require operating in a way that some including me find "abnormal" for a diesel. They however are great for people who are fresh to boating with no pre conceived ideas generated by past experiences with low revving diesels. I can appreciate the merits of Yanmar engines but personally would prefer one for my Jeep project (currently Cummins 4BT) than in my boat. Oddly they probably find favor with owner's of some of the oldest diesels out there the Detroit Diesel 2 stroke as the sound and feel could be similar. Just don't look at the tach!!
 
twiisted71 said:
Diesels that began life as industrial motors are meant to be loaded at their max Tq range, max rpm is irrelevant.
Click to expand...

You might want to curl up with a propeller curve.
 
Marin said:
The basic rule is the fewer number of blades a properly designed propeller has the more efficient that propeller will be.
Click to expand...

That's true. The ideal propeller (in terms of efficiency alone) would be a single blade.
 
The old wife's tales sure tend to come out talking engine loading.
Willard over propped also but that dosn't mean it's right or a good thing to do.
BoatDiesel is probably the best place to go for the right information short of talking to an engineer and most engine distributors. Mechanics tend to put their own spin on engineering matters probably trying to appear knowledgable but most firmly believing their own cooked up agenda.

Usually the guys on forums like this are the last place for the real truth in matters like this but most don't care to look further. It's obvious where the real truth in this matter is but most probably don't want to hear engineering reality. Ther'e more comfortable with notions, opinions and what their friends do. It's like halving a pain in your foot and buying into what the guy across the street says to do about it. He says a guy at work did it and he's rock climbing after doing it.

This time I'm buying into what Rick says. Look at the engineering data and recognize objective and deeply involved professional data that is supported by almost all others at that level. What this guy or that guy says is just dock talk. Engineers say to prop for WOT at the rated rpm. Most good propeller people will say the same thing.

twisted71,
The proper way to load your engine is to rev it up on a properly chosen propeller and gear that achieves rated RPM at WOT. The noise a diesel engine makes is not proportional to it's load. But I'm just a guy on TF. Go get the information you want from real professional sources excluding mechanics and guys on the dock. Get data written by engineers. You can start on BoatDesign.net but it's closer to TF in the quality of information you'll get. But there are many Naval Architects there who are willing to share their knowledge w you. Other individuals that aren't a NA have worthwhile knowledge and experience all over the world but still one needs to be selective while listening. Graduate to BoatDiesel some of those there still has notions and opinions based on information coming from non-engineering sources but the percentage of objective material is WAY UP from lower sources like TF.

Naval Architects and engineers that deal w propulsion issues and design w a high level of math and physics in their education will have answers that will be knowledgeable enough and objective enough to give boaters like us something we can go to the bank on.
 
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Rick-Eric I understand what you are saying. And it is correct. Prop curve data is concrete. Engineers and scientists play with numbers and data and assume ideal circumstances. Their conclusions are a great place to begin. And that is as far as I'll go. They don't take into account the myriad of differences in the real world. Shaft angle, bottom growth, hull variations not just between mfgers but also in the same line, trim/weight distribution are just a few parameters that affect prop and power requirements. There is a ton of research done to come up with tables, charts, and curves. There are just too many possible variables to give the ideal specs for every combination possible, so they give a safe number to start with. I am a member on boatdiesel. I like their calculators, but understand they are pretty crude but they give a ballpark to begin at when doing a repower.
I also know there is a huge difference in commercial vs personal usage. I have worked in trucking, aircraft, and boating fields. Commercial equipment that has a hired operator is run much closer to 100% capacity as time is money. Those have to be tuned to run and survive at those levels. Personal equipment is usually treated much gentler and therefore can be tuned to that purpose.
owner-operators fall into a gray area.
Once again, this just my opinion and how I set up and treat "my" things. No its hardly ever the ideal way. But it is what I like.

I never said engine noise indicated loading. What. I said was the 2 stroke DDs sound like they are spinning at higher rpms and people who ran them may adjust to the new high rpm diesel's sound easier.

Eric I don't know what engine your boat has, my 34' has 200 Hp! I run it 7-8.5 kts. I don't remember the #s off the top of my head but I want to say the BD calculator said something like 60-80 Hp was all that is needed with my "general" hull type and prop and gears. My engines curve is WAY over that at the speed:RPMs I run. So there again my boat is set up to run at 14 ish. Kts. And the formula is out the window. It works but is far from ideal. I'm looking for a 353 or 453 to repower one day.
 
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Very well 71,
Does your user name relate to 6-71 as in DD?
Our "toy boats" as Marin says are just toys and they are 100% ours and we can treat them any way we want. I think rated rpm at WOT is the best way but I know there is a small fuel burn penalty - probably 5% or so (don't really know) to run at less than probably about 50% load and you can load the engine a bit better by over propping but I personally think the price to pay is too much and not very clear either. Propped right you can't hurt your engine. Over propped you can and nobody really knows (I don't think) at what rpm on any given engine setup the situation becomes abusive to the engine. Obviously an over propped engine at WOT or near WOT is very bad dynamics and should never be done but where is the redline .. safe below/dangerous above? It's anybody's guess. If you over loaded a FL 120 250 rpm and ran it at 1700 rpm is that safe and not overloading the engine? A properly mounted EGT could possibly tell. Who would be able to establish the redline temperature? Too many if's and too much unknown. All for the purpose of saving a very small amount of fuel and a little extra noise.

BUT if you over propped the FL 120 150 rpm and NEVER went over 1500 rpm in gear you'd be safe I think on overloading but at what engine speed will propper engine loading occur. It could be done safely and effectively but achieving it may be mostly an accident. Pleasure boat engines get abused to death and this could be a big contributing factor. I see it as too little to gain and too much to loose.

My boat is the 30' 8 ton Willard w 40 hp and propped to it's rated rpm of 3000. I cruise at 2300 most all the time but run at 2500 about 5% of the time. And I don't need an EGT to tell me my engine is loaded correctly .. that is "IDEALLY". Actually I could run 2700 rpm continuously and as long as I warmed up slowly and didn't cool down abruptly i wouldn't be abusing my engine at all. I'm not guessing ... it's in the engine operating specifications.

4-53, 3-53 ... Terrific. I really like the DDs. I looked at several boats (lusted over) on the internet that had 4-53 engines.

And yes .. I had a fisherman alongside me mooring in Thorne Bay that had a V6-53 and he ran it at least half the time he was aboard and I was miles off guessing what rpm he was running. The DDs have an "I'm the boss" sound especially when they start up.
 
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