Underwater running gear drag

pjv911

pjv911

Member
Jan 25, 2014
116
Mount Sinai Long Island Sound NY
Boat Info
1998 Sundancer 330
Engines
2016 385ci 6.3
390hp 479 ft tq Modified Scorpions
4V magnums
1.5 Velvet V drives
Is there any benefit to cleaning up the leading edges of the shaft struts and rudders? It seems to be commonly known that the submerged running gear of an inboard becomes a heavy drag above 25-30 mph. Why would the leading edges of my rudders be flat instead of tapered to a point?
 
In a nutshell, the shape of the rudders has to do with the dynamics of the water as it passes over the surface of the rudders.

At least that's my best guess. I may not be an engineer but I did stay in a Holiday Inn Express last night.
 
So what your saying is that its already as hydrodynamic as it can be? Modifying things would have a downside?
Gofirstclass said:
In a nutshell, the shape of the rudders has to do with the dynamics of the water as it passes over the surface of the rudders.

At least that's my best guess. I may not be an engineer but I did stay in a Holiday Inn Express last night.
Click to expand...
 
pjv, my guess is that if there was a better shape to the underwater running gear, ANY manufacturer of a particular boat would have already done the testing and made the shape as "efficient" as it could be.

In this statement, the term "efficient" can mean many things. My guess would be that the manufacturers have already determined that the shape/size of any underwater gear is the most fuel efficient it can be without reducing the amount of metal present to a point where any further reduction would tend to sacrifice longevity. They might have been able to shave off some metal but at what cost? Would the rudder's life be shortened? Would it be more likely to shear off from the rudder strut if it hit some underwater object such as a log?

I guess my best comment is "IF IT AIN'T BROKE, DON'T FIX IT".
 
It is common practice for i/o guys to install a nose cone on the leading edge of the lower drive unit to improve efficiency. The manufacturers have never done such a thing but it is proven to improve speed and fuel economy.
The view count on this thread is pretty high based on the input offered. Seems to me this may be a subject that most have not considered or have experience with.
Thinking even further, how about all the water pickups and underwater exhaust outlets that are bulky and must produce serious drag at speed.
 
Gofirstclass said:
pjv, my guess is that if there was a better shape to the underwater running gear, ANY manufacturer of a particular boat would have already done the testing and made the shape as "efficient" as it could be.

In this statement, the term "efficient" can mean many things. My guess would be that the manufacturers have already determined that the shape/size of any underwater gear is the most fuel efficient it can be without reducing the amount of metal present to a point where any further reduction would tend to sacrifice longevity. They might have been able to shave off some metal but at what cost? Would the rudder's life be shortened? Would it be more likely to shear off from the rudder strut if it hit some underwater object such as a log?

I guess my best comment is "IF IT AIN'T BROKE, DON'T FIX IT".
Click to expand...

I agree with GFC but to a point.
I believe there would be gains, just not noticeable at the helm.
I believe manufacturers did a cost benefit analysis, not enough benefit for the time to manufacture.
As pjv911 eludes, there is benefit. On my performance hulls, I added nose cones (Bobs low water pick ups), cleaned up surfaces, raked and trimmed the skeg, and generally streamlined anything that look like it would drag. But then I'm talking about running 120-130 mph.
 
From memory Masterfab radiused the leading edges of the rudders and struts of his ex 340. It might be mentioned in his blog. After a prop change (ACME) IIRC his boat topped out at 40 odd knots.
 
juggernaut1 said:
From memory Masterfab radiused the leading edges of the rudders and struts of his ex 340. It might be mentioned in his blog. After a prop change (ACME) IIRC his boat topped out at 40 odd knots.
Click to expand...

Agreed! He did do some minor "cleaning" of some of the bronze leading edges of all the running gear. Search for Masterfab (Dale) he maybe able to give you some info.
 
pjv911 said:
It is common practice for i/o guys to install a nose cone on the leading edge of the lower drive unit to improve efficiency. The manufacturers have never done such a thing but it is proven to improve speed and fuel economy.
The view count on this thread is pretty high based on the input offered. Seems to me this may be a subject that most have not considered or have experience with.
Thinking even further, how about all the water pickups and underwater exhaust outlets that are bulky and must produce serious drag at speed.
Click to expand...

I have never heard of this, nor seen these cones for sale anywhere. Where can I find out more?

edit: found it online and discovered that I could expect to gain 6 mph. I'd have to be going 80 mph + to realize that performance gain though. Which, I suspect is the case with faired rudder edge.
 
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A stern drive boat is about 10 to 20% more efficient than a shaft driven boat according to all I read when I went from a stern drive to shaft drive. The saving in fuel is not worth the maintenance on a stern drive.
Pod drive boats are more efficient than stern drive. I have seen pod drives with forward facing propellers. Great idea until you hit something.
A clean hull gets me 4 more knots in speed due to reduced drag. I would expect if one was to eliminate as much drag from all under water gear they would get a bit more speed.
 
I think, as some have stated, that the gains in efficiency by streamlining things underwater are going to be extremely minimal at the speeds our boats travel at. The amount of time needed for those insignificant gains is likely the reason manufacturer's don't do it. It's just not worth the extra labor for them to say "Our boat is 1% more efficient, but it costs 10% more". I'm obviously just talking theory with those numbers, but you get the idea. Of course, it can't "hurt" to streamline things and if an owner wanted to do it on their own boat, go for it. I just wouldn't expect to see any worthwhile gains. On high-speed boats, though, it starts to make more sense - just like race cars where they are always looking for that extra 1%. Most owner's of Sea Ray style boats (even the sportier bowriders) don't do anything to their drives/running gear, but you can look at Merc racing drives for an example of what the racer's are running.

Someone with hydrodynamic or fluid dynamic education can probably elaborate on these ideas. But increases in speed don't happen linearly - often there are major increases needed in design efficiency to get minor increases in speed. But there's an old, normally accepted rule, that kind of explains this... This "rule" (for lack of a better word) states that to double your speed you need to AT LEAST triple the HP.

Northern is correct that sterndrives tend to be more efficient than inboards - primarily because of the ability to trim the drive for a better running attitude.
 
Gofirstclass said:
...I guess my best comment is "IF IT AIN'T BROKE, DON'T FIX IT".
Click to expand...

DING!!! DING!!! DING!!! DING!!! DING!!! We have a winner.

Beyond that, my $0.02 is that minimal engineering went into the shape of the rudder. It's a production boat. They are going to produce what is going to comply with the regs, do the job, and above all, turn the max profit.
 
I was told the rudders function like an airplanes wing, the leading edge is supposed to be sorta thick or blunt. When you go straight they do nothing, when you turn them they act like a wing, difference in pressure steers the boat. Making smooth the sides of the rudder would gain efficiency although on our boats I don't know if we would recognize the difference.:huh:
 
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I read on a boat design forum that inboards are not greatly affected by running gear drag until past 30 mph. My 330da cruises at 30-31mph and tops out at 40mph. So above 30 mph is an area of concern for me.
As for rudder shape being similar to an aircraft wing (foil shaped) I believe that only applies to non planing hulls. My rudders are not foil shaped.
The leading edge of my rudders are about 3/4" thick and flat.
 
Woody said:
I was told the rudders function like an airplanes wing, the leading edge is supposed to be sorta thick or blunt. When you go straight they do nothing, when you turn them they act like a wing, difference in pressure steers the boat. Making smooth the sides of the rudder would gain efficiency although on our boats I don't know if we would recognize the difference.:huh:
Click to expand...

An airplane wing is concave on one side and convex on the other creating high pressure below and low pressure above, which provides lift. If a rudder were shaped like a wing, just moving through the water would try to move the stern of the boat resulting in constantly fighting the wheel.
 
wrvond said:
An airplane wing is concave on one side and convex on the other creating high pressure below and low pressure above, which provides lift. If a rudder were shaped like a wing, just moving through the water would try to move the stern of the boat resulting in constantly fighting the wheel.
Click to expand...
I don't think the rudder would have to have that shape, actually neither would an airplanes wings.
 
Two different laws apply to an airfoil. Also the horizontal plane of the wing is important. A rudder is in the vertical plane. I don't see how Newton's law and Bernoulli's fluid principle would work. You would need a much greater rate in order for effect. Now I would be interested in the pressure properties of the props.


Sent from my iPhone using Tapatalk
 
mquiet said:
Two different laws apply to an airfoil. Also the horizontal plane of the wing is important. A rudder is in the vertical plane. I don't see how Newton's law and Bernoulli's fluid principle would work. You would need a much greater rate in order for effect. Now I would be interested in the pressure properties of the props.


Sent from my iPhone using Tapatalk
Click to expand...
A plane can only fly right side up?
 
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