Propeller Calculations

[stroutmail]

Seems there are two forms of prop "calculators".

Victoria Propeller has a very good one..enter known specs and it gives you HP required for hull speed and maximum speed. The calculations are based on propeller size they choose, which I assume is their "optimum".

Second method is more universally used where you input pitch and "Propeller Slip".

To get the two methods to correlate, I had to assume a "Propeller Slip" of around 20%.

Anybody have any comments?

What started my interest is a boat I am considering is running a 24" D x 14" P and my calculations tell me I should reach hull speed of 8 knots at 1650 RPM on a Grand Banks 36 Classic. Obviously, there would be more "slip" with a dirty bottom and more weight--with less speed. I am assuming that the 24 x 14 makes the boat properly "underpropped".

 

[Northern Spy]

Dave Gerr's Propeller Handbook is invaluable.

Marine diesel has calculators behind their paywall that I found very helpful when I was repowering.

 

[stroutmail]

Dave Gerr's Propeller Handbook is invaluable.

Marine diesel has calculators behind their paywall that I found very helpful when I was repowering.

Thanks----appears the "Propeller Slip" is higher than 20% for "slower" boats.
Sort of explains why anecdotal speeds required a bit higher RPM than I expected.

 

[Comodave]

I would expect that your slip is more like 30%ish.

 

[High Wire]

The Grand Banks Owners forum should have the answer since there were hundreds built. FWIW The Monk 36 is pretty close in dimensions to the GB36. Most Monks with single Perkins 135 (122 shaft hp) shipped with 25”D x 21” (+/- 1”) P 3 blade prop. Transmission is 2.57 to 1 Borg Warner.

 

[Pete Meisinger]

I know very little about propeller calculations. But it is a subject which comes up pretty often here.

My question is , Why replace what is on the boat with a different one? Unless the engine or gearbox has been changed wouldn't a guy be best served to go with what came with the boat?

pete

 

[stroutmail]

I know very little about propeller calculations. But it is a subject which comes up pretty often here.

My question is , Why replace what is on the boat with a different one? Unless the engine or gearbox has been changed wouldn't a guy be best served to go with what came with the boat?

pete

I kinda agree...in my case, the boat came with the propeller it has now. I read that some change prop to get better fuel economy (more pitch) but then they have to be careful at full throttle not to overload engine. That is not my goal...I am trying to get a feel for what speed should be achieved at certain RPM..sort of a way of evaluating engine condition. Plus, with a 36 year old boat with multiple owners, there is a bit of uncertainty as to whether prop has been changed or "adjusted". I have not seen a GB chart of speed vs rpm in any manual.

Some GB36 owners report 7 knots at 1800. Some say 7.3 knots at 1700. Same engine. So assuming bottom and weight is same. Must be difference in HP curve or prop.

 

[PacificBlue]

You don’t want pitch to be 10” less than your 24” diameter. You want to balance both and get the highest pitch number with the largest diameter you can swing, with at least 15 - 20% of your diameter as the clearance measured between the tip of your propeller and the closest location of your hull bottom.

The Michigan Wheel site should get you really close:

https://www.miwheel.com/inboard-propellers/prop-it-now/

 

[stroutmail]

You don’t want pitch to be 10” less than your 24” diameter. You want to balance both and get the highest pitch number with the largest diameter you can swing, with at least 15 - 20% of your diameter as the clearance measured between the tip of your propeller and the closest location of your hull bottom.

The Michigan Wheel site should get you really close:

https://www.miwheel.com/inboard-propellers/prop-it-now/

The Mich Wheel calc called for a 26 in dia with a 16" pitch. I think 24 in dia is largest possible for single screw. (I think twin screws can swing larger) Pretty sure GB picked the right diameter. So 24 d x 14 p seems reasonable...but insure what RPM is or should be equivalent to displacement speed.

 

[High Wire]

What is the transmission gear ratio?

 

[modernbeat]

...My question is , Why replace what is on the boat with a different one? Unless the engine or gearbox has been changed wouldn't a guy be best served to go with what came with the boat?

Not always. Many times a prop is specified for an engine to be able to run at top RPM, or as a package for an engine warranty. Or, it's specified either ignoring or paying too much attention to vibration.


If you want to work your engine harder at a lower RPM (for economy), or to get more performance out of your quiet boat, or quiet and still the vibrations in your boat at the expense of a little speed, you can do this with a prop change. Think of it as changing the type of tire your car came with.

 

[C lectric]

THere are other factors that affect determining boat speed for prop.
-how many blades?
-what is the disc area ratio.
-skewed or not. Not likely for this application.
-of course slip.
-boat weight at time of testing.
-gearbox ratio.
-specific hull type. Difficult since there are so many variations. Yet the calculators have to decide on one or two to represent all which they cannot accurately do.


Other than the diameter and pitch what is the prop now?

I definitely agree about Dave Gerr's book , the Propellor Handbook. NOt a big book but a lot of reading yet worthwhile.

Another note is that almost all of the calculators are guidlines, no more than that.
There are more items that need to be taken into account than I mentioned above which is why very few of us ever become good at this. No practice.
THere is another calculator on Boatdiesel FOrums which many use but again it is a guidline only.

TO improve the info you have what kind of tachometer are you using now? If it is not a digital tach then I will venture the rpm readings are off. Many analogue tachs read 200 or more to the optimistic side, meaning they show higher revs than the engine is actually making.

If the tachs are typical of many Lehmans they are signalled from the alternator which is not an accurate source. Good enough once you know the actual revs to use as the setting for operation but not for what you are doing now.

Before I forget which style of tach do you have?
A 120o sweep unit cannot represent rpm nearly as well as a 270o sweep unit. The divisions are too coarse.

Check your dash tachs with one from either Ebay or Amazon for about $20.00 You are looking for a laser pointer digital phototach. They won't stand rough handling but a modicum of care will make it a usefull tool.

Without accurate rpm information, a CLEAN bottom, undamaged props [actually confirmed as such] and the actual dimensions and type known you will have greater variations between calculations and actual performance.

 

[rslifkin]

The Mich Wheel calc called for a 26 in dia with a 16" pitch. I think 24 in dia is largest possible for single screw. (I think twin screws can swing larger) Pretty sure GB picked the right diameter. So 24 d x 14 p seems reasonable...but insure what RPM is or should be equivalent to displacement speed.

What does the Michigan Wheel calculator say for pitch if you tell it you want 24" max diameter? I'm guessing it'll show 16 or 17 pitch.

 

[PacificBlue]

The Mich Wheel calc called for a 26 in dia with a 16" pitch. I think 24 in dia is largest possible for single screw. (I think twin screws can swing larger) Pretty sure GB picked the right diameter. So 24 d x 14 p seems reasonable...but insure what RPM is or should be equivalent to displacement speed.

Looks like you are moving more towards a 24 x 18 then, which makes more sense. If 24” is the max diameter you can turn, focus on maximizing pitch, not reducing it. 14” pitch doesn’t seem reasonable on inspection, without any numbers, hp, rpms, gear ratio, weight and stop speed.

 

[stroutmail]

what is the transmission gear ratio?

2.03:1

 

[stroutmail]

THere are other factors that affect determining boat speed for prop.


TO improve the info you have what kind of tachometer are you using now? If it is not a digital tach then I will venture the rpm readings are off. Many analogue tachs read 200 or more to the optimistic side, meaning they show higher revs than the engine is actually making.

If the tachs are typical of many Lehmans they are signalled from the alternator which is not an accurate source. Good enough once you know the actual revs to use as the setting for operation but not for what you are doing

Check your dash tachs with one from either Ebay or Amazon for about $20.00 You are looking for a laser pointer digital phototach. They won't stand rough handling but a modicum of care will make it a usefull tool.

Without accurate rpm information, a CLEAN bottom, undamaged props [actually confirmed as such] and the actual dimensions and type known you will have greater variations between calculations and actual performance.

Thanks for pointing this out...accuracy or inaccuracy of the tach...and or the speed measurement instrument may account for all of the differences I am seeing.

I will bet the prop is correct for the boat. The sweet spot for engine fuel economy is 1400-1800 RPM and one owner reports 1700 rpm at hull speed with very good (less than 2 gph) fuel economy. This is very close to the theoretical according to calculators with a 25 to 30% slip.

I will definitely get a phototach.

 

[Nomad Willy]

With a 2-1 ratio I’d be thinking of smaller dia.

 

[PacificBlue]

Found a 1973 Boating test on a twin screw 36 Grand Banks classic here:

https://books.google.com/books?id=0...#v=onepage&q=36 grand banks boat test&f=false

Interesting that the gear ratios are different port and starboard as well as the prop pitch.

24” looks like the diameter though for a twin, not sure what a single crew is running though. And the boat test rpm was 2350.

 

[kapnd]

I know very little about propeller calculations. But it is a subject which comes up pretty often here.

My question is , Why replace what is on the boat with a different one? Unless the engine or gearbox has been changed wouldn't a guy be best served to go with what came with the boat?

pete

You’d be surprised how many boats come from the factory with poorly matched props.
They select their OEM prop to give maximum speed with a lightly loaded vessel, but as boats (and owners) age, they tend to pick up weight.
Anchors, chain, windlass, dirty bottom/running gear, additional batteries, fishing gear, ropes, cooking gear, ice, food, beer, personal items, dinghies, outboards, fuel, etc, can add enough weight to make propeller recalculation absolutely necessary.
Overpropped vessels are far more likely to overheat, the #1 cause of premature diesel motor deaths!
Measuring exhaust gas temperature against specs is a quick way to identify overpropping, as well as black smoke.

 

[C lectric]

You’d be surprised how many boats come from the factory with poorly matched props.
They select their OEM prop to give maximum speed with a lightly loaded vessel, but as boats (and owners) age, they tend to pick up weight.
Anchors, chain, windlass, dirty bottom/running gear, additional batteries, fishing gear, ropes, cooking gear, ice, food, beer, personal items, dinghies, outboards, fuel, etc, can add enough weight to make propeller recalculation absolutely necessary.
Overpropped vessels are far more likely to overheat, the #1 cause of premature diesel motor deaths!
Measuring exhaust gas temperature against specs is a quick way to identify overpropping, as well as black smoke.

You are dead on and many engines and owners eventually pay the price.

Less of a problem on slower boats but it is not to be ignored. The engine can still be overloaded. Overloading will just take longer to do it's thing.

The only way to overpitch somewhat safely is with a knowledgeable owner setting it up that way, running it that way and so the engine is not overloaded. A pyrometer is an excellent way to know how the engine is doing, if it's overloaded or noot.

Mine was overloaded after I got all my gear aboard but the Pyro saved me from ruining my engine. It told me VERY FAST I was pushing to hard and to back off. Now it's 43 yrs and a hair shy of 7,000 hrs.

 

[FF]

AS always there is ROT , a rule of thumb to see if the prop is in the ballpark.

At 1000 shaft RPM every inch of pitch (with NO slip) is about 1K.

When you do the math most displacement boats slip about 50% at cruise.

 

[stroutmail]

AS always there is ROT , a rule of thumb to see if the prop is in the ballpark.

At 1000 shaft RPM every inch of pitch (with NO slip) is about 1K.

When you do the math most displacement boats slip about 50% at cruise.

Not sure where that comes from...

1000 shaft rpm with 1 in pitch and no slip is 1000 inches per minute...x60 is 60000 inches per hour..divided by 12 gives 5000 feet per hour..divided by 6076 feet per nm is 0.822 knots..so ROT...1000 shaft rpm no slip is close to 0.82 knots.

5000/5280 ft per st mile would be 0.95 mph.

A GB36 with 2.03 trans running 1827 rpm gives 900 rpm shaft speed. For a 14 in pitch..0.82 x 0.9 x 14 is 10.32 knots no slip. 30% slip would be 7.2 knots..close to reality.

Common sense makes one think that slip would increase with load. So at speed above hull speed, slip probably increases. But, prop calculators seem to indicate the slip increase is not large.


From all the literature...ROT..diameter reduces slip, so go as big as safe for clearance..then pick pitch for rpm, speed and load... higher pitch increases speed and load, while reducing rpm. Too high and you overload engine...too low and the engine runs at a higher rpm.

 

[Nomad Willy]

Pacific Blue wrote;
“24” looks like the diameter though for a twin, not sure what a single crew is running though. And the boat test rpm was 2350.”

Many boats were overpropped at that time comming from the manufacturers. My Willard was. With a 3000rpm engine she would only make 2750. Interestingly (to me) is why? Nobody cared about a 3 to 5% fuel savings at that time however there was the embargo of 1973. Also interesting to me is the bigger props on the twins.

I’d be thinking (as I said above) of a smaller dia. I think that there is a loss of efficiency from getting the blade tips too close to the hull. The hull is a form of drag to the hydrodynamics of the prop in motion.
One should have about (off the top of my head) a 3-4 ratio as in a 20” dia. and a 15” pitch. Within a given dia too little pitch as in a 24 dia and 14” pitch means the blades aren’t pushing enough water and there’s too much surface drag loss having too much blade area and w the 24” dia too much velocity near the blade tip where a prop does it’s work. Too little dia and too much pitch (the opposite of the above) brings about big “tip losses”. Like an airplane w short wings. The tips are wide and air spills over the tip ends from bottom to top coming down on the top of the wing lowering the relative pressure causing lift to be lost. The winglets (vertical (or so)) dam the air trying to curl up on top of the wing to some extent is minimized. Same w a marine propeller. Too much pitch promotes prop tip losses providing lots of opportunity for tip losses just like on an airplane.

So balance of dia and pitch is golden. Not too much pitch or too much dia. Your 24” dia prop should have 17 or 18” of pitch. I’d be guessing a 22” dia and 16” pitch would be better for you. You can call Michigan Propeller and talk to a real expert (not me) and get better and more advice. I’d also underprop 50 to 100rpm. When you read that your engine makes 120hp at 2500rpm the 2500rpm is not a redline. It’s just where the engine makes it’s best power. And you’ll never run at 2600rpm for more than a minute or two just to ensure all is well engine and prop wise.

When I flew UL aircraft I once reduced prop dia significantly and increased pitch. In the UL community everyone thought dia was everything and ran the largest prop they could. I had tried everything else so I cut the prop down. Got dumb lucky and nailed it. Good thing as I couldn’t put the cutoff prop tips back on. The UL was then climbing at 1400 feet per minute. Spectacular performance. I was finally pushing lots of air. Same thing can happen w boat props.

 

[stroutmail]

Nomad Eric

The ratio you are referring to is generally called "pitch ratio". Pitch/Diameter.

So your 3:4 ratio that you refer to is 0.75 pitch ratio. I cannot find any source that indicates this is a magic or optimum figure.

Generally, recommended pitch ratios depend on boat speed with slower boats having lower pitch ratios in the range of 0.55 to 0.80 for tugs and trawlers and 0.65 to 1.0 for "heavy cruisers". A 24/14 prop is 0.58 pitch ratio or the low end of the generally recommended range---the perfect number being affected by a myriad of hydrodynamic factors. Your 0.75 figure would simply be on the higher end of the range with a 24/18 prop which I am pretty sure would overload/overprop the 36 year old little 120 HP Lehman at full throttle.

We will see when I sea trial the boat next week, as if the 24/14 is "underpropped" and the engine is in good shape, it would rev up to the governor at 2450-2500 (according to Ford's Performance Chart) instead of running at 90-95% of that figure (2200-2325 rpm) limited by horsepower.

Since you are an aviator (UL) you I am sure aware that "constant speed" or "ground adjustable" props allow changes in pitch. Low pitch allows engine to rev to maximum power quickly (redline) for max climb. High pitch allows engine to cruise at low rpm but with high speed and good fuel economy. But, if you run a high pitch for a long time at full throttle, you can raise EGT and CHT and overheat or shorten life of the engine. I run a ground adjustable composite prop on my cub and run the high pitch--mostly because I don't need to climb much faster than 800 FPM, am OK taking off in 500 feet of runway and I generally only fly at low 2000 feet above the ground at about 100 mph. Many other pilots with the same plane and prop choose the low pitch setting which makes the plane climb at 1200 FPM and take off in 100 feet. The guys with the low pitch go slower--maybe only 80 mph.

Since speed is not my goal, but rather maximum engine life, I want to be sure I never overload the engine. And, I hate a boat that bellows black smoke. As long as I can reach hull speed in the 1600-1800 RPM "sweet spot" I will be very happy with the 24/14.

 

[HiDHo]

When the US Navy started building gas turbine powered destroyers they solved the propeller calculations by using adjustable pitch propellers. The pitch was computer controlled based on the speed, etc of the ship. Since the gas turbines only rotate in one direction the propeller blades would be adjusted to go astern. During the first sea trials of this new system a crash back test is always preformed. I heard it worked so well it would put several feet of water over the stern of the ship. From then on the stern was always cleared of personnel before the test was preformed.

 

[PacificBlue]

Pacific Blue wrote;
“24” looks like the diameter though for a twin, not sure what a single crew is running though. And the boat test rpm was 2350.”

Many boats were overpropped at that time comming from the manufacturers. My Willard was. With a 3000rpm engine she would only make 2750. Interestingly (to me) is why? Nobody cared about a 3 to 5% fuel savings at that time however there was the embargo of 1973. Also interesting to me is the bigger props on the twins.

I’d be thinking (as I said above) of a smaller dia. I think that there is a loss of efficiency from getting the blade tips too close to the hull. The hull is a form of drag to the hydrodynamics of the prop in motion.
One should have about (off the top of my head) a 3-4 ratio as in a 20” dia. and a 15” pitch. Within a given dia too little pitch as in a 24 dia and 14” pitch means the blades aren’t pushing enough water and there’s too much surface drag loss having too much blade area and w the 24” dia too much velocity near the blade tip where a prop does it’s work. Too little dia and too much pitch (the opposite of the above) brings about big “tip losses”. Like an airplane w short wings. The tips are wide and air spills over the tip ends from bottom to top coming down on the top of the wing lowering the relative pressure causing lift to be lost. The winglets (vertical (or so)) dam the air trying to curl up on top of the wing to some extent is minimized. Same w a marine propeller. Too much pitch promotes prop tip losses providing lots of opportunity for tip losses just like on an airplane.

So balance of dia and pitch is golden. Not too much pitch or too much dia. Your 24” dia prop should have 17 or 18” of pitch. I’d be guessing a 22” dia and 16” pitch would be better for you. You can call Michigan Propeller and talk to a real expert (not me) and get better and more advice. I’d also underprop 50 to 100rpm. When you read that your engine makes 120hp at 2500rpm the 2500rpm is not a redline. It’s just where the engine makes it’s best power. And you’ll never run at 2600rpm for more than a minute or two just to ensure all is well engine and prop wise.

When I flew UL aircraft I once reduced prop dia significantly and increased pitch. In the UL community everyone thought dia was everything and ran the largest prop they could. I had tried everything else so I cut the prop down. Got dumb lucky and nailed it. Good thing as I couldn’t put the cutoff prop tips back on. The UL was then climbing at 1400 feet per minute. Spectacular performance. I was finally pushing lots of air. Same thing can happen w boat props.

You don’t want to give away diameter, with that you lose blade area , efficiency and impact the propellers ability to absorb horsepower and turn it into torque.

Forget about it.

 

[PacificBlue]

Nomad Eric

The ratio you are referring to is generally called "pitch ratio". Pitch/Diameter.

So your 3:4 ratio that you refer to is 0.75 pitch ratio. I cannot find any source that indicates this is a magic or optimum figure.

Generally, recommended pitch ratios depend on boat speed with slower boats having lower pitch ratios in the range of 0.55 to 0.80 for tugs and trawlers and 0.65 to 1.0 for "heavy cruisers". A 24/14 prop is 0.58 pitch ratio or the low end of the generally recommended range---the perfect number being affected by a myriad of hydrodynamic factors. Your 0.75 figure would simply be on the higher end of the range with a 24/18 prop which I am pretty sure would overload/overprop the 36 year old little 120 HP Lehman at full throttle.

We will see when I sea trial the boat next week, as if the 24/14 is "underpropped" and the engine is in good shape, it would rev up to the governor at 2450-2500 (according to Ford's Performance Chart) instead of running at 90-95% of that figure (2200-2325 rpm) limited by horsepower.

Since you are an aviator (UL) you I am sure aware that "constant speed" or "ground adjustable" props allow changes in pitch. Low pitch allows engine to rev to maximum power quickly (redline) for max climb. High pitch allows engine to cruise at low rpm but with high speed and good fuel economy. But, if you run a high pitch for a long time at full throttle, you can raise EGT and CHT and overheat or shorten life of the engine. I run a ground adjustable composite prop on my cub and run the high pitch--mostly because I don't need to climb much faster than 800 FPM, am OK taking off in 500 feet of runway and I generally only fly at low 2000 feet above the ground at about 100 mph. Many other pilots with the same plane and prop choose the low pitch setting which makes the plane climb at 1200 FPM and take off in 100 feet. The guys with the low pitch go slower--maybe only 80 mph.

Since speed is not my goal, but rather maximum engine life, I want to be sure I never overload the engine. And, I hate a boat that bellows black smoke. As long as I can reach hull speed in the 1600-1800 RPM "sweet spot" I will be very happy with the 24/14.

But underloading the engine you are not doing it any favors. If it is rated to 2350 rpms as in the boat test, you don’t want to use the no load governor rpm of 2500 as a design point.

You are sacrificing engine life and efficiency by focusing on slip alone, that is not how propeller geometry is designed and you are setting up for minimum returns on this approach.

Are you twin screw or single screw? What about dockside maneuvering, prop bite, all that stuff that has value in maneuvering in close quarters in high winds?

 

[Nomad Willy]

Srroutmail,
You, as I have an understanding of the basics.
But if you actually want to maximize engine life you’ll unload your engine some reducing the pressures and mechanical forces that will shorten the life of your engine.

Of course running the prop that the boat came w minus an inch or so of pitch if it’s overpropped you’ll be fine. Look up the recommendation for prop clearance prop to hull and make sure you’re don’t have a problem there.
Most trawlers probably have the lower gear ratio of 2.57-1. Ask Michigan Propeller about that too. They not only have all the graphs but the experience too. I’m sure they will steer you to loading the prop a little on the light side.

I think GB32’s have a Ford engine of 120hp at 2500rpm. Ideally you should get 2550rpm at WOT with normal loads or more. Lean toward more as people and stuff seems to accumulate on boats over time as does bottom growth. But ask Michigan Propeller if you think I’m wrong.

There is a thread by Marin Faure quite some time ago (maybe 7-8 yrs) whereas he went to a very highly respected prop shop in Seattle. Marin had/has a GB36 w two 120 Lehman’s. He was overpropped some and after having them do his props the way they recommended he had higher rpm and less fuel consumption. Find the thread if you can. The prop shop was Kruger & Sons.

Also you need not concern yourself w prop slip. Getting the WOT rpm right and Getting the balance to the dia/pitch ratio is all important. Give Michigan a call and share w us.

 

[Nomad Willy]

Pacific Blue wrote;
“But underloading the engine you are not doing it any favors.”

Yes. Underloading 50 to 100rpm will minimize the loads on the engine at all engine speeds less that achieved as above .. 50-100rpm above rated power.

Pacific Blue also wrote;
“You don’t want to give away diameter, with that you lose blade area , efficiency and impact the propellers ability to absorb horsepower and turn it into torque.”

You uncovered another ultra important function. That of blade area load. There is much published data on that too but we don’t have much need to stew about that unless you’ve got too little pitch and too much dia AND too much load. We want to load the engine and the prop for best performance from both.

 

[rgano]

Found a 1973 Boating test on a twin screw 36 Grand Banks classic here:

https://books.google.com/books?id=0...#v=onepage&q=36 grand banks boat test&f=false

Interesting that the gear ratios are different port and starboard as well as the prop pitch.

24” looks like the diameter though for a twin, not sure what a single crew is running though. And the boat test rpm was 2350.

I had a 1972 GB42 with the different gear ratios and prop pitches port to stbd. The old tech at Borg Warner informed me that there was only one ring gear for that BW tranny and that an extra pinion was installed to reverse the rotation for the port engine (I think I have the nomenclature correct). One tranny was 1:1.99 and the other was 1:2.01, IIRC. Props were both 24" diameter but one was 18" pitch an the other 17". What this allowed for was loading the two 120 Lehmans the same at the same PRM so you didn't get a beat frequency throbbing though the boat.