Reply
 
Thread Tools Display Modes
 
Old 12-26-2012, 05:14 PM   #21
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
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.
__________________
Advertisement

__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-26-2012, 07:44 PM   #22
Guru
 
Join Date: Aug 2011
Posts: 511
Quote:
Originally Posted by awpptdt View Post
the engine turning 2,400 rpm.
What engine do you have that you run it at 2400? Is that your regular cruise RPM? What speeds are you getting at this RPM? Is this in the boat in your avatar?
__________________

__________________
TIME well wasted
34' Mainship III
Cape Coral, FL
twiisted71 is offline   Reply With Quote
Old 12-27-2012, 03:58 AM   #23
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
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.
Marin is offline   Reply With Quote
Old 12-27-2012, 09:11 AM   #24
Guru
 
Join Date: Aug 2011
Posts: 511
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!!
__________________
TIME well wasted
34' Mainship III
Cape Coral, FL
twiisted71 is offline   Reply With Quote
Old 12-27-2012, 10:45 AM   #25
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
Quote:
Originally Posted by twiisted71 View Post
Diesels that began life as industrial motors are meant to be loaded at their max Tq range, max rpm is irrelevant.
You might want to curl up with a propeller curve.
RickB is offline   Reply With Quote
Old 12-27-2012, 10:57 AM   #26
Guru
 
SomeSailor's Avatar
 
City: Everett, WA
Vessel Name: Honey Badger
Vessel Model: 42' CHB Europa
Join Date: Oct 2011
Posts: 784
Quote:
Originally Posted by Marin View Post
The basic rule is the fewer number of blades a properly designed propeller has the more efficient that propeller will be.
That's true. The ideal propeller (in terms of efficiency alone) would be a single blade.
SomeSailor is offline   Reply With Quote
Old 12-27-2012, 11:50 AM   #27
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-27-2012, 12:47 PM   #28
Guru
 
Join Date: Aug 2011
Posts: 511
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.
__________________
TIME well wasted
34' Mainship III
Cape Coral, FL
twiisted71 is offline   Reply With Quote
Old 12-27-2012, 02:41 PM   #29
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-27-2012, 03:56 PM   #30
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Quote:
Originally Posted by SomeSailor View Post
That's true. The ideal propeller (in terms of efficiency alone) would be a single blade.
That's exactly what the prop shops said. But, they said, it would be a bitch to balance.
Marin is offline   Reply With Quote
Old 12-27-2012, 06:35 PM   #31
Guru
 
Join Date: Aug 2011
Posts: 511
Quote:
Originally Posted by manyboats View Post
I looked at several boats (lusted over) on the internet that had 4-53 engines.
I hope your lusting was satisfied when you inked the papers on your Willard. I have lusted over them from the first pic I saw of one, and yours is the sharpest looking one I can remember. It also sounds like the builder actually sat down and figured out a proper drivetrain (like most sailboat Co.s do). And as a result you likely have one of the more efficient powerboats around.
Do they make a 4x'er. Does your boat have ballast? I love its profile and rounded chines. It seems like a relatively small set of flopper stoppers and paravanes would make it rock steady. Shrimping/oystering luggers and tugboats are the only boats with rounded chines that I have personal experience with, but my next boat will have a displacement hull. After seeing how a 42 KK and numerous sailboats react to the same wakes as our boat, I'm ready for a gentler ride
__________________
TIME well wasted
34' Mainship III
Cape Coral, FL
twiisted71 is offline   Reply With Quote
Old 12-27-2012, 07:14 PM   #32
Guru
 
SomeSailor's Avatar
 
City: Everett, WA
Vessel Name: Honey Badger
Vessel Model: 42' CHB Europa
Join Date: Oct 2011
Posts: 784
Quote:
Originally Posted by Marin View Post
That's exactly what the prop shops said. But, they said, it would be a bitch to balance.
There are so many variables though, you'd have to lock down all the parameters of the props operation to determine what the best design approach would be.

Odd-numbered, high pitched, scimitar blades are extremely efficient (and quiet).



Even bladed scimitar props have been used on aircraft as well. This is one off an E-2C variant. Lots of thrust, hugely efficient.



The problem is the medium as well. Air being compressible and water not. You can REALLY get some heated discussions going on props.
SomeSailor is offline   Reply With Quote
Old 12-27-2012, 07:15 PM   #33
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,265
The hull-speed Coot's four-blade propeller seems well matched with the 2400 RPM 80-horsepower JD engine (don't recall the gear-reduction ratio). There's no audible or tactile vibration despite the thick keel, but there is a bit of visible shaft vibration between 1900 and 2100 RPM. Usually run at 1600-1800 (a knot below hull speed) or 2200 (hull speed) RPM.

__________________
Kar-KEEN-ez Koot
markpierce is offline   Reply With Quote
Old 12-27-2012, 08:06 PM   #34
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
71,
Thanks much for the compliments on Willy but I've seen other Willards w $1000 per foot paint jobs and SS window ports. Mine looks like an old fishboat by comparison but I'm actually a little more comfortable w a boat that's a little rough. And lots of upgrades is just for lack of money. We haven't sold our Alaska house yet so we're in a "heads above water" maintenance lifestyle at the moment.

If you were on the Willard web site you'd see an extensive conversation frequently very critical of Willard about how they mounted intermediate prop shaft brngs. And the W30 could be much more efficient. They have two tons of ballast and the squared and chopped off "deadwood aft that don't help efficiency at all. Could use a bigger prop (there's room), take out the ballast and streamline the end of the keel and the efficiency could double. With a 23 hp engine I would bet on 1/2 gal per hour at 6 knots. One could make it 12' wide w very very little fuel burn penalty.

But most (I suspect I'd be included) would rather have the W30 as she is except for the keels TE. And there are times I wish I had Paravanes but it's not that often. The ballast gives her a fairly gentle motion. Nothing like that fishboat I keep posting though. Oh most sailboats have a V drive and I don't consider that a proper drive at all. Actually two Willard models have a V drive. Not for me. I think a boat should be designed to carry it's weight amidships or aft but not both. And I think that's one of the wonderful things about most inboards .... they carry their weight centered in their hulls .. where it belongs in my opinion. You can design a boat to be basically balanced but it won't ride as well as a boat w her weight amidships. I digress.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-27-2012, 08:39 PM   #35
Guru
 
Join Date: Aug 2011
Posts: 511
Quote:
Originally Posted by manyboats View Post
71, They have two tons of ballast and the squared and chopped off "deadwood aft that don't help efficiency at all. .
Do you have a pic of its stern while on the hard? What do you call deadwood? What do you have as ballast?

I have been toying with the idea of streamlining my keel when she's hauled next time. I too have a rpm specific driveline vibration, and am thinking fairing the keel end might alleviate it. 1400 is a pretty happy zone for speed, momentum to deal with head-on seas, and smoothness. 1500-1700 gives a little shimmy and if taking a longer ride I like the speed/rpm/feel at 1700.

I know that lightness= better efficiency, but I like the feel of a heavy boat. Thankfully my Mainship has a fine entry which makes it handle headseas as if it had much more mass.
__________________
TIME well wasted
34' Mainship III
Cape Coral, FL
twiisted71 is offline   Reply With Quote
Old 12-27-2012, 08:40 PM   #36
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Don't know why you Willard fans keep pissing about with those little 30' boats the company turned out. () Here is a REAL Willard, one you can really go places in. We know the previous owners of this 60' boat. They bought it a number of years ago to live on after selling all their land to buy it plus using their previous 45' boat as part of the payment. The Willard is surprisingly old-- IIRC it's a 1970s boat.

The Willard then proceeded to cost them an arm and a leg to fix leaking fuel tanks and structural hull problems. In the end they sold it to a couple who take it to Alaska every summer and it lives in Bellingham marina for the winter, which is where it is currently. I've been on it once and it's a beautiful boat. But it is definitely a hole into which one pours large amounts of money. I have no idea how many of these things Willard turned out but it's a monster.

Click image for larger version

Name:	image-1193817044.jpg
Views:	108
Size:	129.4 KB
ID:	15128
Marin is offline   Reply With Quote
Old 12-27-2012, 11:37 PM   #37
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,724
Very Very few Marin. But several. All of us Willardites are aware of them but that's about it. The "real" Willards are 30', 36' and 40'. I'm guessing that about 80% of Willards are 30' boats. There are 5 different models in the 30' lineup. The odd one is a sailing version called the "Eight Ton" w an extended keel and a low fore and aft trunk cabin typical of other sailboats. Then there's the Voyager, the wide body that I wish I had. The Nomad that I do have. The Horizon that has low aspect ratio sails and sails downwind and a little more. Last and saltiest is the Searcher w it's small raised pilothouse and low aft cabin. They all have concrete and steel punchings for ballast coming in at 4000lbs.

In the first pic see the flat spot/plane just ahead of the prop as though the keel was cut off with a big saw. Much turbulence rolls of the sides of the keel and tumbles around the flat and very abrupt end of the keel. Very hydrodynamically dirty.

The second pic is of the whole hull from the side and it is very clear how the stern of a full displacement hull returns the water back to the surface. It just smoothly flows up the steeply raked stern section back to the surface. Stand on the stern underway and look at the wake. The water at the stern shows very little boiling action. It just flows up from below w/o trying to follow the boat. Square sterns w considerable submerged transoms have a wake whereas the water is dancing up and down dancing in a crazy way rolling out from the bottom that is trying to keep it down. All that action of the water is energy lost making a mess of the water and serving no useful purpose. Full displacement sterns gently guide the water back to where it came from w very little disturbance. Very little energy lost.

The third pic is showing the cross sectional shape. As you can see it's the traditional "wine glass" section. But to a greater extent than most DF hulls the W30 has a part of the bottom about half way between the sides and the keel that is close to flat. This resulting shape gives the W30 a degree of stiffness that most FD hulls lack. This element of the hull design and the ballast is probably the reason I'm not crazy to get Paravanes.
Attached Thumbnails
Click image for larger version

Name:	STH71269 copy.jpg
Views:	77
Size:	112.0 KB
ID:	15131   Click image for larger version

Name:	STH71261 copy.JPG
Views:	107
Size:	190.2 KB
ID:	15132   Click image for larger version

Name:	all to 12-15-09 396 copy.jpg
Views:	81
Size:	196.5 KB
ID:	15133  
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 12-28-2012, 01:56 AM   #38
Art
Guru
 
Art's Avatar
 
City: SF Bay Area
Country: USA
Vessel Model: Tollycraft 34' Tri Cabin
Join Date: Feb 2011
Posts: 7,986
In General:

More blades = less required prop diameter; such as is used where high hp engines need more blade surface but larger diameter three blade just won’t fit. Lower number of blades = higher efficiency... if you can maintain enough prop diameter to keep ample bite. One blade is best (but, impossible to balance), two blades is the most efficient useful design but very difficult to keep from vibration in large diameter at high rpm, plus the diameter needed for two blades usually precludes installation due to limited under hull space in coordination with required shaft-to-hull angle. High hp engines now often employ 5 or 6 blade props due to 2,000 ++ hp in a 40 to 45' boat that can’t allow enough diameter on a three blade prop. Also, the more blades on a well balanced prop = less vibration encountered at higher blade rpm.

Due to a plethora of mathematical calculations required to match the coordinates of hull design, trany reductions, hp/torque features at various rpm of specific engines, shaft angles under hull, and other items it is required that experts be employed to calc the best prop size and design in every instance. Guesses, although possibly coming close, simply will not result on the absolute best available match-ups for prop, to reduction ratio, to hull design, to horsepower, to standard usage parameters.

Our 34’ classic 1977 Tollycraft tri cabin has good condition twin 255 hp gas engines, 1.5 reduction, and straight shaft... it twists well balanced and clean 17 x 16 three blade props (I call them my “little cutters”). As a clean bottom, hard chine planing hull with 33” draft, 11 degree deadrise, and loaded weight of 21,000 lbs she performs well in exact engine synchronization with no vibration at hull speed (7 to 7.58 knots), planing cruse speed (16 to 17 knots), and WOT (21 to 22 knots). These props were mathematically designed by Tollycraft Corporation engineers in coordination with outsource engineers to match this boat’s range of requirements. I am completely satisfied with performance rendered.

I Recommend Rereading 2nd Paragraph: That’s the best way to proceed, and minimal of cost with greatly reduced aggravation compared to repeat “try and fail” prop design attempts to find the correct propeller!

Art is offline   Reply With Quote
Old 12-28-2012, 05:43 AM   #39
Member
 
DieselTrawlerGuy's Avatar
 
City: Olympia, WA
Country: USA
Vessel Name: Valhalla
Vessel Model: 38' Rawson Custom Fiberglass / Single Cat Diesel
Join Date: Dec 2012
Posts: 11
If you look at a modern-day nuclear submarine you'll see that they typically have 7 bladed props with a high EAR ratio and it's all about being smooth and quiet - and they don't have the same power issues because they have seemingly unlimited power due to the nuclear powerplant and they can simply put on a larger prop due to hull design - i.e. it's a submarine.

If you look at a 1970's to 1990's era unlimited hydroplane running the big Allison and/or Rolls Merlin engines they almost always ran a 2-bladed chopper prop as it was all about getting the power to the water and they could not care less about noise.

Other factors are not only the shape of the back of the keel (squared off or tapered/rounded) but also the distance from the prop to the keel as well as the distance from the prop to the bottom of the hull directly over the prop - all of which will add or decrease the harmonics or vibration.

I am rebuilding a 38' full displacement trawler with a 13'6" beam and 4' draft. I have a single 3160 Cat N/A 210hp on a 2:1 TwinDisc gear with a 2" shaft. The current prop is a 24"x16" LH 3-blade. As I am doing a 3'6" hull extension (underneath the swim step so most people won't even notice but I'll pick up 500 gallons of tankage as well and incorporating a stern thruster) I will be increasing the distance from my bow wave to the stern wave and should pick up a little speed and efficiency, thus I'm putting on a 24"x19" LH 4-bladed prop. I'm hoping I'll still be able to hit my max RPM of 2,800 (so I don't burn my valves) but I can have the prop re-pitched down to a 24"x17" (about 10% is all you can change pitch) if I need to in order to tune it to the new hull length, etc.

I do not agree with the statement above that you can over prop and don't need to be able to reach max RPM - and I know some very, very good diesel mechanics that agree as well as the area Cat dealers. Most will agree that you need to be able to hit the max RPM at full throttle and then that further you should run the diesel at or close to 75% speed (i.e. 2100 RPM) about 75% of the time. On my boat we tend to run between 1,800 RPM (which is what they run the same engine at when it's powering a generator) and up to 2,100 RPM. We'd frequently run the engine up to 2,200 to 2,400 RPM the last 15 minutes or so of any long runs we made just to "clean things out" a bit.

I do agree with reading up on everything you can get your hands on regarding the prop sizing and design issues and I also agree with calling and/or email Michigan Props and getting their expert input.

In my case I figure that no matter what and even if I don't like how the 4-blade works then I can always put my 3-blade back on and toss the 4-blade in the bilge as a backup/get-home prop (which is what I'll do with the 3-blade if the 4-blade does work out as I hope it will).

Just my two cents and a little food for thought.

Good luck.
__________________
M/V Valhalla - on the hard rebuilding from the hull up...
DieselTrawlerGuy is offline   Reply With Quote
Old 12-28-2012, 06:28 AM   #40
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,534
Engineers say to prop for WOT at the rated rpm. Most good propeller people will say the same thing.

Mostly its the folks that mfg or convert the engine for marine use, as any operation below WOT results in the engine being loaded very lightly , so should last the warentee period.

You might want to curl up with a propeller curve.

The "prop" curve is a theoretical, math produced, ideal guesstimate at what a non exstant prop would absorb in power.

It is very useful for anyone that does look , as one can easily see the lack of thrust,(hp absorbed) at low cruise RPM.

Hence the need for a "cruising " prop, a CPP or a 2 speed tranny for genuine efficiency.

It is doubtful that even a 30% increase in efficiency would be worth the cost or effort for a 200 hour a year vessel.

So we get "trawlers" set with 2400rpm props that cruise at 1800pm ,

where with a better prop choice (and some knowledge and an EGT) an almost silent 1300-1500 cruise would be far more enjoyable and the more heavily loaded engine would be more efficient. and longer lasting.
__________________

FF is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 04:14 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012