Engine Mounting Height and Large Diameter Propellers

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Author Topic:   Engine Mounting Height and Large Diameter Propellers

jimh posted 05-28-2014 11:19 AM ET (US)
Teak Oil makes an interesting observation about the Mercury ENERTIA ECO propeller in another thread:

quote:
The huge blades grab air pretty easily when mounted high.

http://continuouswave.com/ubb/Forum4/HTML/008010.html
That is a very good observation. When talking about engine mounting height, we generally have not given much consideration to the propeller diameter. Propeller manufacturers now are making propellers based on a trend to have very large diameters. This puts the blade tips of those propeller closer to the anti-ventilation plate. If the engine mounting height is raised to the point where the anti-ventilation plate is running above the water stream, this suggests that the blade tips of these very large diameter propellers will now also be right at the water surface.
In the past, many boats running at high speed would tend to use propellers of smaller diameter and with blades raked aft. Both of those features put the blade tips below the bottom of the anti-ventilation plate by perhaps as much as an inch. With these new large-diameter propellers being fitted to outboard engines where the propeller aperture will just barely accommodate them, perhaps the old rule of thumb about raising the engine mounting height to run with the anti-ventilation plate above the water can no longer be applied in all instances.
Perhaps the real dimension to watch is the relationship of the propeller blade tip to the water surface. This can be judged by observing the anti-ventilation plate's relationship to the water surface and the position of the propeller blade tip to the anti-ventilation plate.
acseatsri posted 05-29-2014 06:28 PM ET (US)
The other thing I worry about when the prop ventilates is that I would guess you'd be WAY more likely to spin a hub after it winds up and then grabs- way more stress on a plastic/rubber bushing.
Teak Oil posted 05-29-2014 11:06 PM ET (US)
I was surprised when the Enertia Eco, a prop designed for grip and low slip numbers, ventilated when the other props I tested had not.
Going from a 15.5" Rebel to a 16" Eco was enough to grab air in tight turns and when getting on plane. My 14.5" (or 14.25 I forget) only ventilates slightly on tight turns (engine is all the way up).
Before I lower the engine one hole I intend to try a Revolution 4 prop.
I prefer to run the engine as high as possible for maximum efficiency and the least amount of draft.
Tom W Clark posted 05-29-2014 11:43 PM ET (US)

quote:
If the engine mounting height is raised to the point where the anti-ventilation plate is running above the water stream, this suggests that the blade tips of these very large diameter propellers will now also be right at the water surface.
Well, yes it does, but most modern outboard propellers operate just fine with their blade tips at and above the surface of the water, even on motors with their A/V plates running below the surface of the water.
It is easy to experiment with is, by using the power trim and tilt, which the motors on most Whalers have, to trim until there is a fair amount of positive trim. This lifts the propeller blades tips out of the water and (with few exceptions) the prop will not loose all grip while ventilating, but continues to produce thrust.
I caution against making any generalizations about blade tip-to-surface dimensions. In any discussion of outboard motor propellers, the particular model of propeller needs to be considered; propellers are not generic pieces of hardware that can simply be described in terms of pitch and diameter. There is far more to it than that.
Tom W Clark posted 05-29-2014 11:46 PM ET (US)

quote:
The other thing I worry about when the prop ventilates is that I would guess you'd be WAY more likely to spin a hub after it winds up and then grabs- way more stress on a plastic/rubber bushing.
I don't believe that is true at all. Propeller hubs, both conventional press-in rubber hubs and field replaceable plastic drop-in hubs are very robust. I have spent my life "barking" outboard motor propellers (yes, even with my Revenge 25) and I have never had a hub fail.
A marginal adjustment of blade tip-to-surface distance is not going to change that at all.
jimh posted 05-29-2014 11:51 PM ET (US)
A corollary question for Tom:
If a propeller is said to have a certain diameter, say 14.5-inch, and the blades are raked aft at a significant angle, is that diameter dimension being measured as the projection of the blade tip at 90-degrees to the propeller shaft? Or is the diameter measured along the raked blade?
Tom W Clark posted 05-29-2014 11:52 PM ET (US)

quote:
I was surprised when the Enertia Eco, a prop designed for grip and low slip numbers...
No, the Mercury Enertia ECO was designed for maximum fuel efficiency, not grip or low slip numbers.
Do not confuse low slip numbers with efficiency. Do not confuse low slip numbers with higher speed. Calculated propeller slip remains the most misunderstood parameter in propeller selection. Just because a given propeller has a relatively low slip number does not mean it is more efficient.
If a given propeller offers higher fuel mileage...or top speed...or better acceleration even though it has higher relative slip, so be it, that is the way that prop is.
Low slip should not be a goal in propeller selection.
Tom W Clark posted 05-29-2014 11:55 PM ET (US)

quote:
If a propeller is said to have a certain diameter, say 14.5-inch, and the blades are raked aft at a significant angle, is that diameter dimension being measured as the projection of the blade tip at 90-degrees to the propeller shaft? Or is the diameter measured along the raked blade?
Propeller diameter is the diameter of the arc described by the blade tips as they rotate about the propeller shaft, wherever they happen to be fore and aft. Rake is irrelevant to diameter.
Teak Oil posted 05-30-2014 07:57 AM ET (US)
I thought a 16-inch-diameter propeller, even if the tips are grabbing some air, would still have plenty of propeller in the water to prevent blowout on turns and when getting on plane, similar to a four blade. I guess that's why there is no substitute for testing.
I agree that SLIP is simply based on what the manufacturer says is the pitch on any given propeller. Take a 20-pitch propeller and call it a 19-pitch, and suddenly the slip numbers are near zero.
Doug at Lockeman's told me he has seen several cooked hubs on propellers that came into his shop, but only on bass-boat-style set-up's running jack plates and higher engine set-up than anyone here will ever run.
Tom W Clark posted 05-30-2014 08:57 AM ET (US)

quote:
I agree slip is simply based on what the manufacturer "says" the pitch is on any given prop.
No, that is not quite right either. Slip depends on many things, blade geometry chief among them. Though there is one case where a manufacturer simply changed the nominal pitch of one model of propeller, that is not normally done.
Pitch is pitch, but it is actually not the simple number for a given propeller because every point on every blade has a slightly different pitch, so a whole round number is given to best describe it. But how the propeller actually performs is often different from what that nominal pitch predicts.
In the case of the Enertia ECO, I too am surprised that it would ventilate as much as it apparently does on the Outrage 22, but yes, that is why testing is so important. Propellers behave differently on different boat/motor combinations. This is why it is important not draw too many conclusions about a prop tested on a boat/motor combination different from your own.
The Enertia ECO is new so there is a much smaller body of collective knowledge around how it performs. Furthermore, it is meant as a fuel efficient cruising propeller for very high horsepower outboards on large boats. It seems to do best on 250-300 HP twins on boats in excess of 28 feet. Boats where existing propeller models were often too small to achieve the very best fuel economy.
Our body of propeller knowledge around Boston Whalers is pretty good. There are a finite number of boat, motor and propeller models and most have been explored and discussed. But a 300 HP single on an Outrage 22 is very unusual application (I can think of no other) so we may be surprised at the results of different propellers being tested on it. I find it very interesting.
jimh posted 05-30-2014 09:02 AM ET (US)
In the specific case of the Mercury ENERTIA ECO being run on an E-TEC, it sounds like there is very little clearance between the propeller blade tip and the anti-ventilation plate, practically an interference fit. In comparison, if one were running a smaller diameter propeller, the clearance would be 0.5-inch, let us say. If we set the engine mounting height by gauging the anti-ventilaiton plate, we have the two propellers operating with their blade tips in a different relationship to the water surface. The larger diameter propeller has its blade tips closer to the water surface.
Perhaps we need to measure the distance between the centerline of the propeller shaft to the bottom of the anti-ventilation plate on several different engines. I am getting the feeling that there is some variation in this distance between engines of different brands and perhaps even within a brand between engines of different gear case styles. This suggests a corollary question: is a Mercury ENERTIA ECO on a Mercury gear case going to have the blade tips in exactly the same orientation to the anti-ventilation plate as was seen with the E-TEC? Maybe not. That might affect how high the engine can be mounted before the propeller tends to ventilate.
Tom W Clark posted 05-30-2014 09:05 AM ET (US)

quote:
Doug at Lockeman's told me he has seen several cooked hubs on propellers that came into his shop, but only on bass-boat-style set-up's running jack plates and higher engine set-up than anyone here will ever run.
That has to do with the temperature of the hub, not the loading and unloading of a propeller breaking out of the surface of the water. Delrin does not like high temperatures; it becomes brittle and fractures if cooked too much.
Teak Oil posted 05-30-2014 09:49 PM ET (US)
Jim, after looking at several pictures on THT where guys were talking about having to grind the steering tab to fit the Eco prop, I am pretty sure the Evinrude gear case has the least amount of distance from the prop centerline to the a/v plate of any of the major manufacturers.
Suzuki and Yamaha have at least 1/4" more distance from the prop shaft to the a/v plate. Suzuki uses a much deeper gear ratio, so props near 16" diameter are common for them. I have to assume the Mercury gear case has plenty of room since they designed the prop.
It is clear to me that BRP never intended for a 16" diameter prop to be used on the Magnum II gear case. There just isn't enough room. I had to grind a relief right into the steering tab so the prop could even turn.
jimh posted 05-31-2014 09:03 AM ET (US)
I tested an old MIRAGE (not the plus model) propeller on my old OMC 225-HP (not the E-TEC). The diameter of that propeller was so large that it was practically an interference fit. The aluminum-zinc of the torque tab was worn away into a grove where the propeller blade tips were almost hitting, probably by erosion from the water and bubbles coming off the propeller tips.
I think the Evinrude E-TEC M2 gear case is a little bigger than the older OMC gear case, but I also think--I have not made any measurements--that it is smaller than most of the other brands, as you comment.
I speculate that there might be a new gear case design revealed by BRP in a few weeks to go with their new engine they are bringing out, and, if there is, the new gear case will have a larger propeller aperture that will allow it to turn these larger diameter new propeller designs. Again, that is just my speculation.
By the way, if the ENERTIA ECO was so large you had to fabricate a grove for it to turn, I can see why it might show some unusual characteristics like you observed.

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Author	Topic: Engine Mounting Height and Large Diameter Propellers
jimh	posted 05-28-2014 11:19 AM ET (US) Teak Oil makes an interesting observation about the Mercury ENERTIA ECO propeller in another thread: quote: The huge blades grab air pretty easily when mounted high. http://continuouswave.com/ubb/Forum4/HTML/008010.html That is a very good observation. When talking about engine mounting height, we generally have not given much consideration to the propeller diameter. Propeller manufacturers now are making propellers based on a trend to have very large diameters. This puts the blade tips of those propeller closer to the anti-ventilation plate. If the engine mounting height is raised to the point where the anti-ventilation plate is running above the water stream, this suggests that the blade tips of these very large diameter propellers will now also be right at the water surface. In the past, many boats running at high speed would tend to use propellers of smaller diameter and with blades raked aft. Both of those features put the blade tips below the bottom of the anti-ventilation plate by perhaps as much as an inch. With these new large-diameter propellers being fitted to outboard engines where the propeller aperture will just barely accommodate them, perhaps the old rule of thumb about raising the engine mounting height to run with the anti-ventilation plate above the water can no longer be applied in all instances. Perhaps the real dimension to watch is the relationship of the propeller blade tip to the water surface. This can be judged by observing the anti-ventilation plate's relationship to the water surface and the position of the propeller blade tip to the anti-ventilation plate.
acseatsri	posted 05-29-2014 06:28 PM ET (US) The other thing I worry about when the prop ventilates is that I would guess you'd be WAY more likely to spin a hub after it winds up and then grabs- way more stress on a plastic/rubber bushing.
Teak Oil	posted 05-29-2014 11:06 PM ET (US) I was surprised when the Enertia Eco, a prop designed for grip and low slip numbers, ventilated when the other props I tested had not. Going from a 15.5" Rebel to a 16" Eco was enough to grab air in tight turns and when getting on plane. My 14.5" (or 14.25 I forget) only ventilates slightly on tight turns (engine is all the way up). Before I lower the engine one hole I intend to try a Revolution 4 prop. I prefer to run the engine as high as possible for maximum efficiency and the least amount of draft.
Tom W Clark	posted 05-29-2014 11:43 PM ET (US) quote: If the engine mounting height is raised to the point where the anti-ventilation plate is running above the water stream, this suggests that the blade tips of these very large diameter propellers will now also be right at the water surface. Well, yes it does, but most modern outboard propellers operate just fine with their blade tips at and above the surface of the water, even on motors with their A/V plates running below the surface of the water. It is easy to experiment with is, by using the power trim and tilt, which the motors on most Whalers have, to trim until there is a fair amount of positive trim. This lifts the propeller blades tips out of the water and (with few exceptions) the prop will not loose all grip while ventilating, but continues to produce thrust. I caution against making any generalizations about blade tip-to-surface dimensions. In any discussion of outboard motor propellers, the particular model of propeller needs to be considered; propellers are not generic pieces of hardware that can simply be described in terms of pitch and diameter. There is far more to it than that.
Tom W Clark	posted 05-29-2014 11:46 PM ET (US) quote: The other thing I worry about when the prop ventilates is that I would guess you'd be WAY more likely to spin a hub after it winds up and then grabs- way more stress on a plastic/rubber bushing. I don't believe that is true at all. Propeller hubs, both conventional press-in rubber hubs and field replaceable plastic drop-in hubs are very robust. I have spent my life "barking" outboard motor propellers (yes, even with my Revenge 25) and I have never had a hub fail. A marginal adjustment of blade tip-to-surface distance is not going to change that at all.
jimh	posted 05-29-2014 11:51 PM ET (US) A corollary question for Tom: If a propeller is said to have a certain diameter, say 14.5-inch, and the blades are raked aft at a significant angle, is that diameter dimension being measured as the projection of the blade tip at 90-degrees to the propeller shaft? Or is the diameter measured along the raked blade?
Tom W Clark	posted 05-29-2014 11:52 PM ET (US) quote: I was surprised when the Enertia Eco, a prop designed for grip and low slip numbers... No, the Mercury Enertia ECO was designed for maximum fuel efficiency, not grip or low slip numbers. Do not confuse low slip numbers with efficiency. Do not confuse low slip numbers with higher speed. Calculated propeller slip remains the most misunderstood parameter in propeller selection. Just because a given propeller has a relatively low slip number does not mean it is more efficient. If a given propeller offers higher fuel mileage...or top speed...or better acceleration even though it has higher relative slip, so be it, that is the way that prop is. Low slip should not be a goal in propeller selection.
Tom W Clark	posted 05-29-2014 11:55 PM ET (US) quote: If a propeller is said to have a certain diameter, say 14.5-inch, and the blades are raked aft at a significant angle, is that diameter dimension being measured as the projection of the blade tip at 90-degrees to the propeller shaft? Or is the diameter measured along the raked blade? Propeller diameter is the diameter of the arc described by the blade tips as they rotate about the propeller shaft, wherever they happen to be fore and aft. Rake is irrelevant to diameter.
Teak Oil	posted 05-30-2014 07:57 AM ET (US) I thought a 16-inch-diameter propeller, even if the tips are grabbing some air, would still have plenty of propeller in the water to prevent blowout on turns and when getting on plane, similar to a four blade. I guess that's why there is no substitute for testing. I agree that SLIP is simply based on what the manufacturer says is the pitch on any given propeller. Take a 20-pitch propeller and call it a 19-pitch, and suddenly the slip numbers are near zero. Doug at Lockeman's told me he has seen several cooked hubs on propellers that came into his shop, but only on bass-boat-style set-up's running jack plates and higher engine set-up than anyone here will ever run.
Tom W Clark	posted 05-30-2014 08:57 AM ET (US) quote: I agree slip is simply based on what the manufacturer "says" the pitch is on any given prop. No, that is not quite right either. Slip depends on many things, blade geometry chief among them. Though there is one case where a manufacturer simply changed the nominal pitch of one model of propeller, that is not normally done. Pitch is pitch, but it is actually not the simple number for a given propeller because every point on every blade has a slightly different pitch, so a whole round number is given to best describe it. But how the propeller actually performs is often different from what that nominal pitch predicts. In the case of the Enertia ECO, I too am surprised that it would ventilate as much as it apparently does on the Outrage 22, but yes, that is why testing is so important. Propellers behave differently on different boat/motor combinations. This is why it is important not draw too many conclusions about a prop tested on a boat/motor combination different from your own. The Enertia ECO is new so there is a much smaller body of collective knowledge around how it performs. Furthermore, it is meant as a fuel efficient cruising propeller for very high horsepower outboards on large boats. It seems to do best on 250-300 HP twins on boats in excess of 28 feet. Boats where existing propeller models were often too small to achieve the very best fuel economy. Our body of propeller knowledge around Boston Whalers is pretty good. There are a finite number of boat, motor and propeller models and most have been explored and discussed. But a 300 HP single on an Outrage 22 is very unusual application (I can think of no other) so we may be surprised at the results of different propellers being tested on it. I find it very interesting.
jimh	posted 05-30-2014 09:02 AM ET (US) In the specific case of the Mercury ENERTIA ECO being run on an E-TEC, it sounds like there is very little clearance between the propeller blade tip and the anti-ventilation plate, practically an interference fit. In comparison, if one were running a smaller diameter propeller, the clearance would be 0.5-inch, let us say. If we set the engine mounting height by gauging the anti-ventilaiton plate, we have the two propellers operating with their blade tips in a different relationship to the water surface. The larger diameter propeller has its blade tips closer to the water surface. Perhaps we need to measure the distance between the centerline of the propeller shaft to the bottom of the anti-ventilation plate on several different engines. I am getting the feeling that there is some variation in this distance between engines of different brands and perhaps even within a brand between engines of different gear case styles. This suggests a corollary question: is a Mercury ENERTIA ECO on a Mercury gear case going to have the blade tips in exactly the same orientation to the anti-ventilation plate as was seen with the E-TEC? Maybe not. That might affect how high the engine can be mounted before the propeller tends to ventilate.
Tom W Clark	posted 05-30-2014 09:05 AM ET (US) quote: Doug at Lockeman's told me he has seen several cooked hubs on propellers that came into his shop, but only on bass-boat-style set-up's running jack plates and higher engine set-up than anyone here will ever run. That has to do with the temperature of the hub, not the loading and unloading of a propeller breaking out of the surface of the water. Delrin does not like high temperatures; it becomes brittle and fractures if cooked too much.
Teak Oil	posted 05-30-2014 09:49 PM ET (US) Jim, after looking at several pictures on THT where guys were talking about having to grind the steering tab to fit the Eco prop, I am pretty sure the Evinrude gear case has the least amount of distance from the prop centerline to the a/v plate of any of the major manufacturers. Suzuki and Yamaha have at least 1/4" more distance from the prop shaft to the a/v plate. Suzuki uses a much deeper gear ratio, so props near 16" diameter are common for them. I have to assume the Mercury gear case has plenty of room since they designed the prop. It is clear to me that BRP never intended for a 16" diameter prop to be used on the Magnum II gear case. There just isn't enough room. I had to grind a relief right into the steering tab so the prop could even turn.
jimh	posted 05-31-2014 09:03 AM ET (US) I tested an old MIRAGE (not the plus model) propeller on my old OMC 225-HP (not the E-TEC). The diameter of that propeller was so large that it was practically an interference fit. The aluminum-zinc of the torque tab was worn away into a grove where the propeller blade tips were almost hitting, probably by erosion from the water and bubbles coming off the propeller tips. I think the Evinrude E-TEC M2 gear case is a little bigger than the older OMC gear case, but I also think--I have not made any measurements--that it is smaller than most of the other brands, as you comment. I speculate that there might be a new gear case design revealed by BRP in a few weeks to go with their new engine they are bringing out, and, if there is, the new gear case will have a larger propeller aperture that will allow it to turn these larger diameter new propeller designs. Again, that is just my speculation. By the way, if the ENERTIA ECO was so large you had to fabricate a grove for it to turn, I can see why it might show some unusual characteristics like you observed.