Underwater running gear drag

I managed to find Masterfabs blog and have posted an excerpt re his modifications to made to the rudders:

Sea Ray 340 Sundancer Restoration.jpg
 
A plane can only fly right side up?

No, but the amount of energy needed changes as the the airfoil transitions. Steep turns require additional power as the horizontal component of lift disappears.


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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.
That is the way "some" airfoils work. There are many that are fully symmetrical and generate lift by the angle of attack.
http://airfoiltools.com/search/index?m[maxCamber]=0&m[sort]=5
 
Good stuff. Strange how this is not a commonplace thing to do. I wonder if he reported back if there were notable benefits and what model boat he has.
I managed to find Masterfabs blog and have posted an excerpt re his modifications to made to the rudders:

View attachment 48207
 
Kurt,
If you read the blog, Dale had a 340 Sundancer that he totally refurbished above & beyond any other you have ever seen which he recently sold. I'm not sure what performance benefits were gained from the running gear upgrades. He probably would have difficulty quantifying that as there were many concurrent upgrades but I guess we can only hope Dale chimes in to clarify.

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I just saw this thread - so I'll add a bit of information.

Todd is correct in that I did a host of upgrades and refinements at the same time, so I cannot quantify any improvements in speed or improvements in efficiency due to a reduction in running gear drag. Please note that I wold never have even refined the rudder shape if the rudders weren't already out of the boat for other reasons - I was re-powering, and also converting the rudder stock seals from packed seals to Tides dripless, so they were out of the boat anyway, over the winter, and I was bored one day. I would never have removed the rudders just to perform that work.

When I was "done" with the 340, she ran a two way average of 39.6 knots over the ground, in calm conditions in salt water (she was a hair slower in fresh). That was with 4 people on board, about 60% fuel, and a damn full load since we were on vacation. That speed is about 3.5 knots better than the factory got, and is due to a host of factors beyond smoothed rudders. I was running ACME 4-blade CNC props, had a fresh soda blasted bottom with smoothly applied ablative paint, and had removed some weight from the ER. Shaft alignment was within 0.001" to reduce parasitic drag as well. I had also improved exhaust flow by using mandrel bent stainless elbows in the exhaust, along with refining the muffler inlet shape. I think the rudders themselves contributed very little, if at all, to the performance increase I experienced.

As to rudder shape itself, large, foil shaped rudders are used on displacement speed vessels for several reasons. Low speed vessels require large rudders to provide for sufficient control at low speeds, as the loading per unit area on the rudder itself is low. Also, foil shaped rudders provide for minimal drag at displacement speeds, where water flow over the rudder is laminar, not turbulent. On a high speed planing vessel, large rudders impart too much drag, reducing speed and efficiency, and the foil shape can actually result in a loss of control, as the water flowing over the rudder becomes turbulent, resulting in a loss of pressure on the trailing edges of the rudder, and a subsequent loss of control. Wedge shape rudders force the transition from laminar to turbulent flow to take place aft of the rudder, and thus maintain water pressure on the entire surface of the rudder at planing speeds, maintaining control over the vessel. They are also smaller, as the pressure per unit area is higher at planing speeds. The drawback to the smaller size is lousy handling at low speeds, which we've all experienced when traveling at 2 or 3 knots in our Sea Rays, or when docking. Hence the common practice of almost (not completely) ignoring the rudders when docking, and working the shifters instead.

Bottom line - enjoy your boat. If you happen to have the rudders out anyway, and you're going to clean / blast them for paint, then a few minutes with a grinder/sander won't hurt, but it's certainly not something to lose sleep over. You'll never notice the difference at cruise speed.

Dale
 
Dale is there any other things you can recommend? I do plan to have much smoother bottom paint for this coming season. I'm also gunna take out all unessesary weight and use the bow storage for anything heavy. I will sharpen the leading edges of the rudders a bit too.
As for the propellers, having 4 blades goes against everything I've read in regards to speed. Your saying it was a speed improvement?
 
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Let's try not to generalize - it's a dangerous practice. I was running 4-blade props, and that top speed I netted may have been improved by another prop design. I am simply stating the configuration of my boat, and the results I achieved with that configuration. I selected and sized the props for the best possible cruise efficiency, not top speed. In the 5 years I owned that boat, she spent maybe 5 or 6 minutes at WOT - 30 seconds or so at the beginning and end of each season - just to verify performance and trend any changes.

Prop design is a complex science, and many factors, such as number of blades, blade area and shape, rake, diameter, pitch, and other factors affect performance. Each design choice is a compromise chosen to achieve a certain goal or group of goals, and the number of blades is only one factor in those design choices. You should discuss your goals with your chosen prop supplier, and make decisions based on results of your research.

Placing excessive weight in the bow is also a practice that can have detrimental results. While you may achieve a desired reduction in bow rise, it can come at the expense of increased pitching (porpoising). Designers typically try to locate weight closer to the center of the boat for best stability, and keep it as low in the hull as possible.

Dale
 
Thanks for your info Dale. Super clean work. I made my bilge pretty too but you got me beat there. I cant post pictures though.
 
Thanks for your info Dale. Super clean work. I made my bilge pretty too but you got me beat there. I cant post pictures though.

Yeah, but Dale cheated and did it the easy way. He took the engines out to clean the bilge. :grin:
 

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