G
Guest
Guest
What is a surface drive, what are its characteristics and what are their pro's and con's?
Robbie W
Robbie W
Dave_Snelson
Active member
If you would like to know what a surface drive is / does and even see one in action, then try this...
http://www.n2speedturbineboats.com
...and its an outrageously good site!
Madoc Yacht Club
www.madocyachtclub.co.uk
http://www.n2speedturbineboats.com
...and its an outrageously good site!
Madoc Yacht Club
www.madocyachtclub.co.uk
Divemaster1
Well-known member
Not easy to explain in few words, but have tried to create a short summary here.
A surface-piercing propeller is a propeller that is positioned so that when the vessel is underway the waterline passes right through the propeller's hub. This is usually accomplished by extending the propeller shaft out through the transom of the vessel, and locating the propeller some distance aft of the transom in the relatively flat water surface that flows out from the transom's bottom edge.
The important operating feature is that each propeller blade is out of the water for half of each revolution.
Propeller Efficiency: Traditional propeller design and selection is almost always an exercise in trading off diameter against several other performance-limiting parameters. Basic momentum theory tells us that for a given speed and thrust, the larger the propeller, the higher the efficiency. While there are exceptions, most notably the effects of frictional resistance on large, slow-turning propellers, it is generally borne out in practice that a larger propeller with a sufficiently deep gear ratio will be more efficient than a small one.
A number of design considerations conspire to limit the maximum feasible propeller diameter to something considerably smaller than the optimal size. These include blade tip clearance from the hull, maximum vessel draft, shaft angle, and engine location. While this may at times make life easy for the designer - the propeller diameter specified is simply the maximum that fits - it can also result in a considerable sacrifice of propulsive efficiency. And if these geometric limits on propeller diameter are exceeded, the result can be excessive vibration and damage due to low tip clearances, or a steep shaft angle with severe loss of efficiency and additional parasitic drag, or deep navigational draft that restricts operation or requires a protective keel and its associated drag. In many cases, the best design solution is to live with a mix of all of the above problems to some degree.
The surface-piercing propeller frees the designer from these limitations. There is virtually no limit to the size of propeller that will work. The designer is able to use a much deeper reduction ratio, and a larger, lightly-loaded, and more efficient propeller.
Appendage Drag: Exposed shafts, struts, and propeller hubs all contribute to parasitic drag. Inclined the exposed shafts not only produces form and frictional drag, but there is also induced drag associated with the magnus-effect lift caused by their rotation. There is a surprising amount of power loss resulting from the friction of the shaft rotating in the water flow. In fact, for conventional installations a net performance increase can often be realized by enclosing submerged shafts in non- rotating shrouds, despite the increase in diameter.
Surface propellers virtually eliminate drag from all of these sources, as the only surfaces to contact the water are the propeller blades and a skeg or rudder.
Variable Geometry: When a surface propeller is used in conjunction with an articulated drive system, the vessel operator then has the ability to adjust propeller submergence underway (trim). This has roughly the same effect as varying the diameter of a fully submerged propeller, and allows for considerable tolerance in selecting propellers - or it allows one propeller to match a range of vessel operating conditions.
When the articulated drive is used for steering, the result can be exceptionally good high-speed maneuvering characteristics. On single-shaft applications, drive steering can also be used to compensate for propeller-induced side force, without resorting to an excessively large rudder or skeg.
Shallow Draft: This is the characteristic that motivates many designers to investigate surface propeller propulsion in the first place. The vessel's navigational draft can be as low as half a propeller diameter. Compared with other options for shallow water propulsion - most notably waterjets - surface propellers enjoy a very significant efficiency avantage. This advantage is most dramatic for low-speed applications, but is still present throughout the performance spectrum.
Of course having mentioned Waterjets here, I may have opened up another kind of discussion ....
AHM
A surface-piercing propeller is a propeller that is positioned so that when the vessel is underway the waterline passes right through the propeller's hub. This is usually accomplished by extending the propeller shaft out through the transom of the vessel, and locating the propeller some distance aft of the transom in the relatively flat water surface that flows out from the transom's bottom edge.
The important operating feature is that each propeller blade is out of the water for half of each revolution.
Propeller Efficiency: Traditional propeller design and selection is almost always an exercise in trading off diameter against several other performance-limiting parameters. Basic momentum theory tells us that for a given speed and thrust, the larger the propeller, the higher the efficiency. While there are exceptions, most notably the effects of frictional resistance on large, slow-turning propellers, it is generally borne out in practice that a larger propeller with a sufficiently deep gear ratio will be more efficient than a small one.
A number of design considerations conspire to limit the maximum feasible propeller diameter to something considerably smaller than the optimal size. These include blade tip clearance from the hull, maximum vessel draft, shaft angle, and engine location. While this may at times make life easy for the designer - the propeller diameter specified is simply the maximum that fits - it can also result in a considerable sacrifice of propulsive efficiency. And if these geometric limits on propeller diameter are exceeded, the result can be excessive vibration and damage due to low tip clearances, or a steep shaft angle with severe loss of efficiency and additional parasitic drag, or deep navigational draft that restricts operation or requires a protective keel and its associated drag. In many cases, the best design solution is to live with a mix of all of the above problems to some degree.
The surface-piercing propeller frees the designer from these limitations. There is virtually no limit to the size of propeller that will work. The designer is able to use a much deeper reduction ratio, and a larger, lightly-loaded, and more efficient propeller.
Appendage Drag: Exposed shafts, struts, and propeller hubs all contribute to parasitic drag. Inclined the exposed shafts not only produces form and frictional drag, but there is also induced drag associated with the magnus-effect lift caused by their rotation. There is a surprising amount of power loss resulting from the friction of the shaft rotating in the water flow. In fact, for conventional installations a net performance increase can often be realized by enclosing submerged shafts in non- rotating shrouds, despite the increase in diameter.
Surface propellers virtually eliminate drag from all of these sources, as the only surfaces to contact the water are the propeller blades and a skeg or rudder.
Variable Geometry: When a surface propeller is used in conjunction with an articulated drive system, the vessel operator then has the ability to adjust propeller submergence underway (trim). This has roughly the same effect as varying the diameter of a fully submerged propeller, and allows for considerable tolerance in selecting propellers - or it allows one propeller to match a range of vessel operating conditions.
When the articulated drive is used for steering, the result can be exceptionally good high-speed maneuvering characteristics. On single-shaft applications, drive steering can also be used to compensate for propeller-induced side force, without resorting to an excessively large rudder or skeg.
Shallow Draft: This is the characteristic that motivates many designers to investigate surface propeller propulsion in the first place. The vessel's navigational draft can be as low as half a propeller diameter. Compared with other options for shallow water propulsion - most notably waterjets - surface propellers enjoy a very significant efficiency avantage. This advantage is most dramatic for low-speed applications, but is still present throughout the performance spectrum.
Of course having mentioned Waterjets here, I may have opened up another kind of discussion ....
AHM
ChrisP
Member
now that's what I call a man that knows his onions.
ChrisP ;o)
What do you mean the sea gull in front's walking !!!
ChrisP ;o)
What do you mean the sea gull in front's walking !!!
wakeup
Active member
Forgot to mention the wicked rooster tail a surface drive throws up.
yada yada..
yada yada..
alanharper
New member
Divemaster's pretty much nailed it, except that I doubt many designers choose surface props due to their shallow draught, because the draught's only shallow when you're up on the plane and going like the clappers.
The main motivation is to reduce appendage drag, with all the benefits that can bring in speed and fuel efficiency. Their main application is still in racing boats, although Sealine is one manufacturer currently experimenting with them (see MBY Oct), using an adaptation of a Fabio Buzzi design involving a fixed drive aft of a big hull step (which is effectively the boat's transom when up and running) and a rudder. Baia is another manufacturer that uses them, and Pershing (see MBY this month). They both use Arneson drives off the shelf, which are articulated and therefore need no rudder - rather like an outdrive. The thing about articulated drives is that you need to understand them in order to trim them properly. It's not rocket science, but there is a school of thought which says they're not ideal for leisure boats for that reason.
The other big drawback is safety: the props themselves have to be vicious sharp pointy bastards, and as they stick out the back (not in the Sealine/Buzzi set-up) they can lacerate swimmers even when they're not going round.
Sonny Levi can take much of the credit for the concept, in racing boats back in the 60s and early 70s. If you're interested in history MBY did a rather fine article about the legendary designer in the Jan 2002 edition, written by, um, me! Contact Copy Shop and they'll send you a copy.
Alan
MBY
The main motivation is to reduce appendage drag, with all the benefits that can bring in speed and fuel efficiency. Their main application is still in racing boats, although Sealine is one manufacturer currently experimenting with them (see MBY Oct), using an adaptation of a Fabio Buzzi design involving a fixed drive aft of a big hull step (which is effectively the boat's transom when up and running) and a rudder. Baia is another manufacturer that uses them, and Pershing (see MBY this month). They both use Arneson drives off the shelf, which are articulated and therefore need no rudder - rather like an outdrive. The thing about articulated drives is that you need to understand them in order to trim them properly. It's not rocket science, but there is a school of thought which says they're not ideal for leisure boats for that reason.
The other big drawback is safety: the props themselves have to be vicious sharp pointy bastards, and as they stick out the back (not in the Sealine/Buzzi set-up) they can lacerate swimmers even when they're not going round.
Sonny Levi can take much of the credit for the concept, in racing boats back in the 60s and early 70s. If you're interested in history MBY did a rather fine article about the legendary designer in the Jan 2002 edition, written by, um, me! Contact Copy Shop and they'll send you a copy.
Alan
MBY
burgundyben
Well-known member
Dont know but...
I think my computer has got one.
For sale, 1970 Triumph Spitfire-sold, 1947 Lambretta, 1922 Great grandmother, PM for details.
I think my computer has got one.
For sale, 1970 Triumph Spitfire-sold, 1947 Lambretta, 1922 Great grandmother, PM for details.
G
Guest
Guest
Given that I have had my props exposed as the stern leaves the water in big seas, coming off the tops
of very large waves and mine has standard shafts & props plus rudders and large keel, would not the surface
drives spend even more time out of the water in such conditions ?
"The Med has got me" (no not the Medway the other Med)
of very large waves and mine has standard shafts & props plus rudders and large keel, would not the surface
drives spend even more time out of the water in such conditions ?
"The Med has got me" (no not the Medway the other Med)
jfm
Well-known member
Fuuck me, authoritative new line drawn in sand
Right then, that's official. Effing and blinding is now totally in order on this BB, following AH precedent.
Divemaster, please could you explain a bit more about deeper ratios and bigger props. When you say larger prop here, you mean pitch as well as dia, right? I haven't checked, but I dont think many of the Arneson set ups are runnng bigger reduction ratios and coarser pitches, or are they? I think there are hydrodynamic drawbacks in running very coarse pitches. Can you enlighten a bit?
Later on you say surface drives allow to "use a much deeper reduction ratio, and a larger, lightly-loaded propellor". Here do you mean larger dia or larger pitch, or both? And how does this make the prop more lightly loaded (it's developing the same power, right?). Very interested if you can enlighten us, thanks.
PS how come AH can swear without the puter auto-editing it, whereas I have to mis spell "[ah em!]" (see, I typed f u c k) to get it past the machine? Do IPC staff have special swearing privileges?
<P ID="edit"><FONT SIZE=-1>Edited by jfm on 15/10/2002 20:26 (server time).</FONT></P>
Right then, that's official. Effing and blinding is now totally in order on this BB, following AH precedent.
Divemaster, please could you explain a bit more about deeper ratios and bigger props. When you say larger prop here, you mean pitch as well as dia, right? I haven't checked, but I dont think many of the Arneson set ups are runnng bigger reduction ratios and coarser pitches, or are they? I think there are hydrodynamic drawbacks in running very coarse pitches. Can you enlighten a bit?
Later on you say surface drives allow to "use a much deeper reduction ratio, and a larger, lightly-loaded propellor". Here do you mean larger dia or larger pitch, or both? And how does this make the prop more lightly loaded (it's developing the same power, right?). Very interested if you can enlighten us, thanks.
PS how come AH can swear without the puter auto-editing it, whereas I have to mis spell "[ah em!]" (see, I typed f u c k) to get it past the machine? Do IPC staff have special swearing privileges?
<P ID="edit"><FONT SIZE=-1>Edited by jfm on 15/10/2002 20:26 (server time).</FONT></P>
tcm
...
ooh good thought aabout the swimming. Stuff that. That's saved a few quid.
BrendanS
Well-known member
Can you even begin to imagine the raggies complaint letters if surface drives ever took off in a big way in the Solent! Would make them forget about wakes for a while
Divemaster1
Well-known member
I can of course not argue if your props got exposed in the following sea, but I would then have expected that you lost most, if not all all steering, as rudders must also have been exposed. However I suspect that it/they were probably cavitating, which I remember that the props in my old, semi displacement, Apollo 32 did in large following sea.
When a submerged propeller blade cavitates, the pressure on part of the blade becomes so low that a near vacuum is formed. This happens more easily than one might think - atmospheric pressure is only 14.7 psi, not a very big number considering the size of a typical propeller and the thrust it is required to produce. If the suction on the low-pressure side of the propeller blade dips below ambient pressure - then a vacuum cavity forms.
The effect can approximate that of hitting the blade with a hammer on each revolution. Cavitation, as we all know, is a major source of propeller damage, vibration, noise, and loss of performance.
Coming back to your question though, the hydrodynamics of a hull with surface drives is very diferent than a semi-displacement, or indeed a planning hull with shaft drive, and I have to confess that I would not know in detail how a surface drive would perform / act under similar conditions (lack of personal experience). However, I suspect that since they are designed to spend their time half out of water (cavitating), there would be some loss of power, but not significant and / or that one may have to bring the hull off planning mode, if the conditions are severe, and comfort and or safety are compomised.
I do not wish to be seen as pretending to be the "know all" of surface drives, but I have spent some time researching the issues around this kind of propulsion, as I have been considering buying 40-50 ft. boat where surface drives are an option. (as are shaft and sterndrives). I just wanted to do some research and found it quite interesting....
AHM
When a submerged propeller blade cavitates, the pressure on part of the blade becomes so low that a near vacuum is formed. This happens more easily than one might think - atmospheric pressure is only 14.7 psi, not a very big number considering the size of a typical propeller and the thrust it is required to produce. If the suction on the low-pressure side of the propeller blade dips below ambient pressure - then a vacuum cavity forms.
The effect can approximate that of hitting the blade with a hammer on each revolution. Cavitation, as we all know, is a major source of propeller damage, vibration, noise, and loss of performance.
Coming back to your question though, the hydrodynamics of a hull with surface drives is very diferent than a semi-displacement, or indeed a planning hull with shaft drive, and I have to confess that I would not know in detail how a surface drive would perform / act under similar conditions (lack of personal experience). However, I suspect that since they are designed to spend their time half out of water (cavitating), there would be some loss of power, but not significant and / or that one may have to bring the hull off planning mode, if the conditions are severe, and comfort and or safety are compomised.
I do not wish to be seen as pretending to be the "know all" of surface drives, but I have spent some time researching the issues around this kind of propulsion, as I have been considering buying 40-50 ft. boat where surface drives are an option. (as are shaft and sterndrives). I just wanted to do some research and found it quite interesting....
AHM
Divemaster1
Well-known member
Re: Authoritative new line drawn in sand
Cannot and will not comment on the language usage ..
Not just talking about the size of the propeller, but also general propeller design, which differs from “conventional” propeller design (sorry for failing to state this). But the general rule (friction etc., ) still applies where for a given speed and thrust, the larger the propeller, the higher the efficiency. However, you have to take into consideration the point when the frictional resistance on large, slow-turning propellers is defeating the purpose of going “larger”. At this point you look at gear ratio, pitch, and overall propeller design etc., (based upon your needs)
I have to admit that I feel that I have only scratched the surface (pardon the wordplay) on this subject in my research, and as such probably do fail to respond constructively to the questions that you may have on this subject. Afterall, the original question was "What these are". My research is still limited, as I have only looked at one application (selfishly as I am considering a boat for a specific purpose, which is not in the 50 knot range, but does involve offshore crossings. The question of rough condition is therefore an interesting one, and I will look into this in more detail and get back if and when time permits).
Also, as yet, I have not looked into a specific drive type (Arneson etc.), but more considered the general principles, pro's and cons.
To get back to the propeller question, there are a variety of propeller styles for surface piercing applications to choose from.
The propeller that most people think about when discussing Surface Drives is the cleaver propeller. The cleaver propeller ahs heavy cambered wedge sections, with a blunt, squared-off trailing edge. Up to 8-blade cleaver propellers have been used to allow smoother operation and an increase in efficiency with reduced propeller submergence. These propellers typically are used for vessel speeds in excess of 50 knots. Depending on the size of the craft, the performance gains fall off using cleavers for the slower speed applications. As a general rule, there are better styles for the slower speed applications.
Low rake propellers have a very similar appearance to conventional propellers – 4 to 6 blades. These propellers have a cambered blade section with a trailing edge cup. Sometimes, for slow speed applications, the cup is extended forward of the maximum radius on the propeller. This is the propeller of choice for most applications under 50 knots. Propeller geometry can be altered on these propellers to enhance the slow and mid speed performance.
The high rake propeller was developed primarily as a "poor man's cleaver." This is a propeller that is easily manufactured and has evolved from the standard wedge-shaped cleaver propeller. These propellers generally are for use on the light, high-speed, small vessel applications.
"Lightly loaded" prop, acually then means less pitch, but as propeller design is very different from conventional design, "less" is a relative term, but you're right in stating that there are hydrodynamic drawbacks in running coarser pitches (friction, vibration, and efficiency.)
Note the skillfull (not) avoidance of the gear ratio question, pending more research..........
AHM<P ID="edit"><FONT SIZE=-1>Edited by Dominic_Byers on 16/10/2002 14:13 (server time).</FONT></P>
Cannot and will not comment on the language usage ..
Not just talking about the size of the propeller, but also general propeller design, which differs from “conventional” propeller design (sorry for failing to state this). But the general rule (friction etc., ) still applies where for a given speed and thrust, the larger the propeller, the higher the efficiency. However, you have to take into consideration the point when the frictional resistance on large, slow-turning propellers is defeating the purpose of going “larger”. At this point you look at gear ratio, pitch, and overall propeller design etc., (based upon your needs)
I have to admit that I feel that I have only scratched the surface (pardon the wordplay) on this subject in my research, and as such probably do fail to respond constructively to the questions that you may have on this subject. Afterall, the original question was "What these are". My research is still limited, as I have only looked at one application (selfishly as I am considering a boat for a specific purpose, which is not in the 50 knot range, but does involve offshore crossings. The question of rough condition is therefore an interesting one, and I will look into this in more detail and get back if and when time permits).
Also, as yet, I have not looked into a specific drive type (Arneson etc.), but more considered the general principles, pro's and cons.
To get back to the propeller question, there are a variety of propeller styles for surface piercing applications to choose from.
The propeller that most people think about when discussing Surface Drives is the cleaver propeller. The cleaver propeller ahs heavy cambered wedge sections, with a blunt, squared-off trailing edge. Up to 8-blade cleaver propellers have been used to allow smoother operation and an increase in efficiency with reduced propeller submergence. These propellers typically are used for vessel speeds in excess of 50 knots. Depending on the size of the craft, the performance gains fall off using cleavers for the slower speed applications. As a general rule, there are better styles for the slower speed applications.
Low rake propellers have a very similar appearance to conventional propellers – 4 to 6 blades. These propellers have a cambered blade section with a trailing edge cup. Sometimes, for slow speed applications, the cup is extended forward of the maximum radius on the propeller. This is the propeller of choice for most applications under 50 knots. Propeller geometry can be altered on these propellers to enhance the slow and mid speed performance.
The high rake propeller was developed primarily as a "poor man's cleaver." This is a propeller that is easily manufactured and has evolved from the standard wedge-shaped cleaver propeller. These propellers generally are for use on the light, high-speed, small vessel applications.
"Lightly loaded" prop, acually then means less pitch, but as propeller design is very different from conventional design, "less" is a relative term, but you're right in stating that there are hydrodynamic drawbacks in running coarser pitches (friction, vibration, and efficiency.)
Note the skillfull (not) avoidance of the gear ratio question, pending more research..........
AHM<P ID="edit"><FONT SIZE=-1>Edited by Dominic_Byers on 16/10/2002 14:13 (server time).</FONT></P>
G
Guest
Guest
Re: Fuuck me, authoritative new line drawn in sand
Many thanks for your comprehensive reply and other replies. Is there a web site etc. with pictures/diagrams so that I can better visualise. For the IPC journalists, perhaps a magazine article would be appropriate.
Robbie W
Many thanks for your comprehensive reply and other replies. Is there a web site etc. with pictures/diagrams so that I can better visualise. For the IPC journalists, perhaps a magazine article would be appropriate.
Robbie W
tcm
...
No? Bastard file, bastard saw, bastard props
A bastard file is a very rough saw with big pointy teeth, likewise a bastard file. So, perfickly acceptable to have a bastard propellor imho. Obviously a shitbag c-word f-word propellor er either right out of order or very poi nty and sharp indeed.
A bastard file is a very rough saw with big pointy teeth, likewise a bastard file. So, perfickly acceptable to have a bastard propellor imho. Obviously a shitbag c-word f-word propellor er either right out of order or very poi nty and sharp indeed.
G
Guest
Guest
Re: Divemaster
It was into head seas and the props came out as we were coming off the top of the crest into the trough
probably my inexperience with the throttles at the time, or has anybody else had this.
"The Med has got me" (no not the Medway the other Med)
It was into head seas and the props came out as we were coming off the top of the crest into the trough
probably my inexperience with the throttles at the time, or has anybody else had this.
"The Med has got me" (no not the Medway the other Med)
