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prop Q's... a little different than most I'm thinking..
212rowboat
Member Posts: 2,591 ✭✭✭✭✭
Howdy, folks!
I need a new prop.. I took a chunk out of a reworked one this weekend, and I don't have a spare.. I 'think' I want to make a little upgrade.. What I don't know about pitch and diameter (which is considerable) I will study by searching other threads- the question I have here may be a little 'in the weeds' for many, but I bet there are folks here who can help me... Here goes:
Does a lower pitch prop lessen load on an engine in the same manner that lower (higher numerically) gears do for a vehicle? .. I understand that a lower pitch is going to turn more RPM's at the same speed as a higher pitch, but I'm more interested in learning if it actually lessens engines load as well (as can be observed on an vacuum gauge)....
Reason I ask: it's been my long standing and often evaluated theory that RPM's aren't the specific cause of excessive fuel consumption, but instead the load on an engine.. if the engine is working without much effort to maintain stoichiometric ratio's (or close to), it is operating very economically- whereas if it is struggling to maintain speed (in terms of revs), it is often dumping more fuel than is required trying to reach the desired speed, but being 'held back', and most often because of lacking leverage.. obviously, the constants of water are much different than that of wheels, and momentum ain't going to help you much..
anyway- thoughts would be appreciated.. I'm running on a chipped up 14.5" 21p 3 blade aluminum right now.. I was thinking of dropping to a 19p four blade s/s.. alpha out-drive, 4.3L.. '05 Captiva 212.. I cruise the oceans and sounds- not much 'towing' water sports, not much racing around- just easy cruising just above plane, and often with as many as six or so passengers.
thanks in advance!!
I need a new prop.. I took a chunk out of a reworked one this weekend, and I don't have a spare.. I 'think' I want to make a little upgrade.. What I don't know about pitch and diameter (which is considerable) I will study by searching other threads- the question I have here may be a little 'in the weeds' for many, but I bet there are folks here who can help me... Here goes:
Does a lower pitch prop lessen load on an engine in the same manner that lower (higher numerically) gears do for a vehicle? .. I understand that a lower pitch is going to turn more RPM's at the same speed as a higher pitch, but I'm more interested in learning if it actually lessens engines load as well (as can be observed on an vacuum gauge)....
Reason I ask: it's been my long standing and often evaluated theory that RPM's aren't the specific cause of excessive fuel consumption, but instead the load on an engine.. if the engine is working without much effort to maintain stoichiometric ratio's (or close to), it is operating very economically- whereas if it is struggling to maintain speed (in terms of revs), it is often dumping more fuel than is required trying to reach the desired speed, but being 'held back', and most often because of lacking leverage.. obviously, the constants of water are much different than that of wheels, and momentum ain't going to help you much..
anyway- thoughts would be appreciated.. I'm running on a chipped up 14.5" 21p 3 blade aluminum right now.. I was thinking of dropping to a 19p four blade s/s.. alpha out-drive, 4.3L.. '05 Captiva 212.. I cruise the oceans and sounds- not much 'towing' water sports, not much racing around- just easy cruising just above plane, and often with as many as six or so passengers.
thanks in advance!!
Comments
Prop discussions are very interesting. I've spent quite a bit of time doing prop tuning on a 192 and a 226. There are some very good prop calculators online. You'll need some basic data to start with such as maximum rated RPM at WOT and sterndrive gear ratio.
You probably have a stern drive ratio of 1.81:1. I've attached a spreadsheet I created for various engine and drive configurations. You can create your own as well. The tabs at the bottom of the spreadsheet show the various drive ratios I played with. The graph below is from an analysis I did for a 4.3L Volvo with a 1.79:1 drive ratio. The blue line is Prop Pitch versus theoretical boat speed and the pink line is Prop Pitch versus engine RPM. You need to operate in the maximum WOT rpm range of your engine.
Now, as far a prop designs are concerned, 4 and 5 blade props are typically used for watersports and 3 blade props are more efficient when used for cruising. My gut instinct is that you would do best with a 21" cupped stainless 3-blade for cruising; however, your passenger load might necessitate a drop in the pitch.
Mark
2019 MTX20 Extreme
I do believe I have a 1.81:1 gear box, and I'm pretty sure the engines placard read max 5200RPM... I'd prefer to stay away from that, though.. :-) I'm likely going to put some rocker rollers on the heads as opposed to the stamped rockers on it- which only takes about an hour tops, and should let fly around ten ponies (at crank) due to both slightly added duration, but mostly from lessening parasitic power loss from the valve train.. they should keep things cooler a little- but (and the reason i added this) they will not allow valves to float nearly as easily as stamped rockers will... which, coupled with external harmonic balance, should allow quiet a bit higher rev speed ... not that I want that for speed, but I absolutely enjoy the wider safety margin if I break bite for whatever reason... but, I'm pretty sure there is a rev limiter in place on that thing, anyway..
those rocker rollers will let free considerable torque- likely in the 30# range- which will equate to HP, according to the ratio, of around 3 or so at the prop.. they cost around $150~$200.. yeah, I'm going somewhere with all of this.. :-)
I don't want to lug the engine with a higher pitch- and I'm thinking the 21p is sufficient for my purpose.. I'm hoping that adding a touch of torque will make up the difference in 'out of the hole' bite that dropping to a 20p would do (if I could find one)... or.. I'm hoping that touching up on torque will allow me to spin a four blade in 21p as efficiently as it spins the three blade 21p now..
I've got a confession to make: I just got this boat a couple weeks ago, and I haven't got the tach working yet... I'm thinking i'm going to hafta run a new wire.. I can't get absolute measure on RPM's to Speed ratio to know precisely where I stand, but I do know that the three blade job on there now feels good enough, other than being chipped up.
does stainless truly account for a 200rpm drop?
"Theoretically" a 23" pitched prop (without slippage) would move 23" through the water. Dropping from a properly WOT rpm'd prop say a 23 to a 21 will give a considerably better hole shot. It will place less strain on the engine BUT will exceed your boat's upper rpm limit and will burn more gas at any given rpm.
I ordered a 14" 20p 4 blade aluminum from Michigan steel... My plan is to test it out and see how I like it, and making it my spare... By my reckoning, this will allow me to bracket my target so I can fire for effect when I pull the trigger on the next one, which will most def be a stainless... Three, four, and pitch will depend on what I learn from my mistake... :-)
If I can manage this engine like I can engines in my vehicles, I'll hone in on the perfect range of economy and power, while eliminating as much parasitic drag and load on the engine that I can.. like I suggested before, it's my hard experience that rpm isn't the great enemy of economy, but instead load as registered on a vacuum gauge (for a gasser), or exhaust temperatures (for an oil burner).. guys on other forums I frequent find me hilarious in my relentless pursuit of that balance.. it's led me down all kinds of crazy trails.... And now, hopefully a channel or two.. :-)
I think you could spend a lot of money chasing the elusive 'best performance' goal. Everything is a trade-off: fuel burn rate, cruise speed, hole-shot, dealing with added weight (passengers/gear).
Remember, unlike a car, the power load on a boat engine is markedly different pushing though water. I think you would get better perfomance curves using fuel flow monitors with tach readings plotted against each other at speeds.
PC BYC, Holland, MI
I agree with you... I readily admit my insanity, though... If I know there is a better way a machine can play nicer with my reality, I've got no choice but to advance on that position.. :-)
One thing that is going to give me fits: I can't figure out a good way to sniff the exhaust... I'd love to get a wide-band o2 sensor in there somewhere, but it's not like you can just tap it at the collector... I run a carb'd engine on this thing.. (and here are words I thought I'd never hear myself say) : I wish that thing had EFI and a PCM w/OBDII port... It's been years since I worked with carbs on anything other than a motorcycle or lawn equipment motor, and I have become completely spoiled and spoiled completely with the ease of extracting data and making computed adjustments on those things.
ultimately I will track this down, but right now, as it is, I've gotta get other parts and pieces of that thing working properly (namely, the tach)..
Wow!
This discussion of props leaves me somewhat bewildered. I know there are many configurations for the infinite variables involved, but I now find myself wondering about my boat's set up and I am not sufficiently knowledgeable on the subject. I have a '07 246 Captiva BR with 350 MAG MPI, Bravo 3. The props are 24P and I don't know exactly what diameter. As near as I can measure, it appears to be 14", perhaps a little more? The boat has a dry weight of 4,900 lb. Add fuel, passengers, gear, etc. it is likely tipping the scales at 6,000-6,500. We don't care about going anywhere fast, but would like to maybe haul my 59 yr old, 200lb. body up on skis once in a while! Are these props considered to be a balance of economy and performance based on the parameters under which I would operate the boat?
I gotta break stuff down barny style, personally.. but once it's been reduced you can start building the complexities back into it...
for instance:
torque is a measurement of twisting power.. if a shaft is twisting, how much grip would you have to exert per square inch to stop it? 100 pounds per? 200?....
torque is what makes horsepower.. horsepower is just another way to look at it, but it has it's own purpose, too, which is why we toss it around.. it is a measurement of work- if the twisting shaft had a reel on the end of it, and a rope attached to a weight of 100 pounds, how quickly could it hoist it off the ground? that rate is going to be the rate of work... which we call horsepower here in the good old you ess of a..
first tricky: horsepower is based on torque... if the spinning shaft can be stopped too easily by gripping it (torque), then it will never **** 100# off the ground (rate of work)... the shaft gains it's torque by having a more constant pressure being delivered to it by an engines power stroke.. if there is sufficient torque being delivered, it can now start to be translated into work effort via gears... if a gear is introduced that turns faster than the shaft is turning, the workload (HP) increases, but at the expense of torque, as it is now much easier to stop by gripping the gear instead of the shaft..
you can have all of the horsepower in the world, but not be able to sustain motion once it encounters opposition, if you're delivering that HP at the expense of torque..
you can have all the torque in the world, but if the shaft isn't turning fast enough, you aren't performing much work or producing any horsepower..
a balance has gotta be found..
in relation to props:
think of a wood screw.. it has a high pitch on it's inclining plane.. it drills right into wood and seats quickly.. now think of trying to drive that with a screwdriver into 1/2" stainless steel.. you wouldn't be able to do it, even if you tapped a divot- you can't generate the torque (twisting power) because the pitch of that wood screw is too high to be used in steel...
think of a machine screw.. the pitch is silly low- so much so, that you can't barely see the shaft of the screw beneath the inclining plane.. you can easily generate torque enough to screw it into metal, but it will take you a long time to seat it because it will require a lot more revolutions to be seated- because the thread counts are designed for torque and not horsepower.. that is the equivalent of gearing going to work for your efforts.. you are exerting tons of torque, but you aren't performing much rate of work... if you are screwing it into wood, your not going to get a good bite into the wood because the screw is going to slip in place instead of bite in.. you're wasting effort...
your prop is a screw... your gearbox is your screwdriver.. you are the engine..
we're drilling through water.. it doesn't require much torque, but does require higher rate of work.. you have to find the balance between what you're moving (boat+gear), with what you're moving it with (engine) and how much you gotta balance out horsepower with torque with the pitch of the screw (prop), while producing little slipping in the material (water) yet getting a good enough bite but not too much- to keep the engine in it's happy range..
does that help?
and please- if I am misinforming, set me straight!!
Dream 'Inn III -- 2008 400 Express
Edited: Heheheheee.. I like how this forum censored the word t a x (ing) .. :-) that, in my opinion DOES belong in the family of four letter words... :-) ... It also censored synonym for 'tough on' that rig.. that's pretty funny!!
Funny thing: my air cleaner clearly says "left hand", and my ignition module says "left hand alpha drive"... But... Holding the prop itself in my left hand, the ball of my thumb rests easy on the blades pitch.. the other test: one blade straight up, the right hand blade is closer to me than the hull..
So.. right hand blade? Even though air cleaner and ignition module say left? ... I hope so, because I ordered a right hand blade..
Glass, that is great you still water & snow ski. I had been doing both since I could walk, but hadn't water ski for quite a while & a couple years for wno ski. I guess at 42 I'd better get back on them and not feel old. (I just need to find someone with that boat that will pull slalom because I do worse on two skiis!)
& rain, yes it is coming big time for us east coasters!
Keep the prop talk going, as I'm an EE that always enjoyed the ME stuff probably more!
Dream 'Inn III -- 2008 400 Express
it says it plain as day on the last item of engine specs: Left Hand Rotation... The ignition module also says, plain as day "Left Hand Alpha One".. but the prop on the hub is certainly a right hand prop, and when I put the thing in forward gear, forward the boat goes...
I find this strange... but, whatever, right? Maybe the dang thing IS a LH, and a previous owner swapped the gear/throttle cables around... or, maybe the thing has been rebuilt or worked on by someone who slapped parts they had extra on it, and those particular parts were for a LH.. either way- my screw turns clockwise... go figure.. my prop is certainly a right hand- and verified after I second and third guessed myself..
I'll let you fellers know how the 14.25" 20p four blade works out for me as opposed to the 14.5" 21p three blader... I talked with some locals, one of which is a 'boat guy', and he said I'd enjoy the four blade over the three- he suggested OEM's put the three blader's on most often because they are less expensive and have more range in options, but primarily because the unwarranted reputation modern four and five blade props have as 'slow', and because of past designs where they actually were 'slower' than two and three blades... he says that in times past the 3< blade designs were generally pitched lower (numerically), and built for pushing really heavy rigs at low speeds, but that isn't the case anymore.
anyhow, I'll see for myself soon enough...
I'm thinking the OEM is going to stay with the herd, and leave something as 'specific to use' to the end user... the OE supplied prop will work, but there are better choices out there for MY purpose... sorta like selecting tires for a car based on overall use of a vehicle..
back to this saga, as it has turned into: I was incorrect in my RPM range, as you fellers can see- I'm not @ 5200RPM, but between 4400 and 4800RPM... I'd guess that range is due to prop selection.. huh..
Drew,
Hope this helps (no, you are not crazy!!)...
The distributor shaft rotation is the same for RH and LH motors and it is usually almost always a distributor shaft right hand rotation as viewed from above.Use care when using a variable timing light as it must be set to zero when timing a reverse rotation engine.All Mercruiser Sterndrive engines are LH rotation.Some Mercruiser Inboard (NOT sterndrive) engines can be RH rotation.Left hand (LH) engines are standard rotation. Also known as Counter Clockwise Rotation (CCR).Usually only older twin inboard powered vessels have one engine RH reverse rotation.Right Hand Engines (RH) are reverse rotation. Also known as Clockwise Rotation (CR).Chances are you have a LH standard rotation engine especially if it's a sterndrive or a single engine installation.
Engine rotation is determined by looking from the flywheel end towards the pulley end.Do not confuse engine rotation with prop rotation. They might not (and usually are not) the same.Mark
2019 MTX20 Extreme
If your reference point is the outer cast hub, mine appears to have a bit of runout too. The blade tips are bang on.
Mark
2019 MTX20 Extreme
I'm guessing 15" is about as big as I care to go from fear of smacking something at high trim at worst, or sucking some surface down at best..
My ext prop will be a 14.5~14.75 four blade in 21p, or a three blade in 14.25 22p... Live and learn, hey?
All of these are aluminum..
I think the 23p will work fine for me, but it may be too steep if I've got a boat load.. if it does work out, I'll be happy for the rest of the season.. I'll likely move a tad backward to a 22p and larger diameter (between 14.25 and 14.75") in stainless...
I thought I had the ss nailed down, just like I wanted, but the guy posted he made a mistake in the listing and it was in fact a 25p... For $100, including hub, it may be worth grabbing up.. but I know that one has gotta be too steep..
PC BYC, Holland, MI
that dang thing is HEAVY compared to the four blade aluminum!
I'll run both the new props and see what happens.. My gut tells me you're right, and the 22p is the ticket.