Are there established hull speeds for particular boats?
Also, when I refinished my Rapier 18 hull, I put the final coat on with a roller and it left what I would call a slight “orange peel” finish on the boat. It’s not rough to the touch, but you can definately feel a fine texture to it. (I will probably add more coats and sand after a well deserved break from the boat shop!)
What effect on speed might this “orange peel” finish have?
Any guesses?
One MPH or more?
Hmmm
Hull speed. I used to have a 25’ sailboat that had a definite hull speed. Definitely a displacement boat, wouldn’t go above it. The problem with kayaks is that even though they’re technically displacement boats, since they’re so lightweight and float in the, what, top 4-6" of the water, they really don’t seem to have that well-defined maximum hull speed. Instead of using the formula, I would recommend that you look at the water resistance tables that are out there that give data on a bunch of boats (water resistance vs. speed)
Orange peel. Lot of discussion here. There seems to be some data that indicates that some orange peel can actually lower the boats resistance to the water. Something about it creates a thin layer of turbulence next to the hull that makes it more slippery. I would think that it would depend a lot on the thickness of that layer, which could be difficult to control. Not sure that there’s a clear consensus on this though!
Always an interesting discussion though.
4 billion golf balls can’t be wrong
Maybe your boat will drive farther and straighter, with fewer slices.
If you want to get a smoother painted finish, try a “roll and tip” method on subsequent coats. Basically you roll on a small area, and then immediately run a dry brush through it.
If I recall my little bit of aerodynamic
correctly, you get less resistance from turbulent flow than laminar flow but if you are getting to that level,it ain’t the orange peel - it’s your paddling ability.
yes and no
In theory there is max. hull speed. Unless you are an Olympic level paddler you will never hit it. Most of us just are not strong enough to squeeze everything possible out of our boats.
Orange peel, again unless you are an Olympic level paddler you are never going to see the difference. Smooth or rough. And some studies are saying that a textured finish is better but I don’t know. I can’t paddle fast enough for it to matter.
the “sharkskin” approach
– Last Updated: Sep-01-09 10:15 AM EST –
Some racing sailboats have tried using a textured finish to their bottomsides in the hopes that creating a looser boundary layer near the hull would result in less drag, but it's unproved that it actually does anything.
In the case of a human-powered kayak, it's unlikely that a little orange peel in the outer surface will result in measurable drag, so I wouldn't worry too much about it. Look on the bright side; you can always fix it with some 400/600/1200 grit wet-dry sandpaper and a few hours' time.
A few comments:
First, hull speed is real, even for kayaks, but the degree to which hull speed is an “absolute limit” depends on the overall “sleekness” of the boat. Boats that are very narrow relative to their width don’t “hit the speed limit” as abruptly, which means that a strong paddler can push such boats faster than hull speed. I can’t reach hull speed in a solo canoe, but I can in both of my rowboats. In the rowboats, GPS measurements show that hull speed is, for practical purposes, an absolute speed limit (meaning I CAN go a little faster than hull speed, but the amount of effort it takes is FAR too extreme to maintain for more than a few seconds).
That faintly orange-peel finish shouldn’t have much effect on your boat’s speed. There will certainly be “some” effect, and as some have noted, it might even be a benefit, but whatever effect it has, it seems unlikely that you will be able to notice it. At speeds near hull speed, skin friction is a very tiny componant of what’s holding you back. At normal cruising speeds, I would bet that the small difference in skin friction due to the orange peel probably won’t be as great as slight differences in resistance between two very similar boats.
Read that
I just can’t remember where. Seems I even read that subs were getting a new textured coating somewhere?
My last SOF has a course textured skin on it and I didn’t try to fill the weave either. It’s only 15’ so it stinks in sprints but it is much faster (as in easier to paddle) at cruising speeds than any of my other boats. I don’t know that skin helped but it doesn’t appear to have hurt it either.
What a drag
There are two kinds of drag…
Form drag: as the boat moves through the water, it pushes water out of the way, which then has to move back when your boat is gone. Unfortunately, it doesn’t move all the way back to it’s original position (this is the wake), and the energy required to move it from original to final results in form drag.
Viscous drag: The water directly in contact with your boat moves with your boat, the next layer of water gets pulled along a bit, the next layer pulled along a little less, etc. This shearing motion leaves little vortices which actually heats the water. It requires energy. This is called skin friction or viscous drag.
Golf balls: The dimples on a golf ball are intended to induce turbulence. A turbulent boundary layer carries with it slightly more momentum, helping the displaced water to return closer to its original position (smaller wake), thereby reducing FORM drag, which more than makes up for the increase in viscous drag.
Sailboats: In 1987, Stars&Stripes used 3M Riblets to reduce drag. They worked (10-12% reduction in VISCOUS drag). Well enough that now sailing and rowing both ban Riblets (and the like) to make it more a contest of skill and ability than an arms race. Riblets are suspected to work by inhibiting stream-wise vorticies and reducing the amount of momentum convected towards the moving surface, but no one knows for sure (although there is at least one tall, good looking fellow who is working on it).
Kayaks: The flow over a kayak hull is well into the turbulent regime by the time it reaches your forward hatch cover. There is no question that turbulent flow generates more skin friction than laminar flow, but on a kayak, there is little to be done. So given that the boundary layer will be turbulent, the best you can do is reduce the turbulent drag. Every bump in the orange peel extends a good portion into the boundary layer, thereby interacting with fluid moving at a greater speed. In a nutshell, smooth is fast.
Sharks: A close-up view of a sharks skin shows that the ridges on the skin cells / scales are actually aligned in the streamwise direction and behave somewhat like Riblets. There has been some research into more randomly dispersed ridges rather than continuous streamwise grooves, but they are extremely expensive to fabricate, even at the laboratory scale. It is not well documented whether or not shark-skin or Riblets are more effective.
As guideboatguy indicates, as you approach hull-speed, the uphill nature of overcoming the bow-wake is far more influential.
I use a mobrien-esque series of wet sanding on my boat. 1200 leaves it baby’s butt smooth.
For what it’s worth…
D
Drag Figures
– Last Updated: Sep-01-09 5:12 PM EST –
Sea Kayaker tested the Rapier 20 in the June 2007 issue. I don't know how different it is for your Rapier 18. The review has calculated drag figures (the online pdf does not).
While it doesn't give you the 'hull speed' of the boat it gives you a good idea of where the drag escalates.
BTW, SK noted that the Rapier 20 was the fastest boat they had ever tested ;-)
hull speed
All displacement boats have a maximum speed at which they can’t realistically exceed. More so with paddling craft as there is a real limit to how much power a paddler can generate. Because racing kayaks are so narrow compared to their length the multiplier that’s normally used for the speed/length ratio is incorrect and thus it “seems” that the rule doesn’t apply. Still does just at a higher speed and you run out of power before you reach it.
Bill H.
Question me this
"DRESSMEISER SAID: …Every bump in the orange peel extends a good portion into the boundary layer, thereby interacting with fluid moving at a greater speed. In a nutshell, smooth is fast."
Any figures on what kind of difference this makes? Is this one of those Olympic quality issues where every nanosecond gained is important?
Or is this something that the average paddles would see a benefit from smoothing his hull? I am guessing it’s a very small increase and unless your race it’s not a big deal?
Regardless it is interesting reading. Any sources for the data?
Thanks Cap’n Brittanica
I have to say, THAT is why I love this site. We will read about orange peel questions that will lead to answers and jeers and Banana peel/fire stories and coffee grounds at your camp site. THEN turn around and read all about types of friction and boating regulations.
This is a magical site.
Liveoutside
(who has been missing on here for a while. Yes I know. Been busy and drinking the Kool Aid that cyclists offer. Now the number of bikes I own match the number of Boats I own. AND my future wife and our family will be closing on a house this friday! if I ever have time I will post pictures of the toy-box/Man-cave I will have. Paddles and Pedals…wow I should make a shop with a name like that…anyways, hello to everyone, I have missed reading and posting here)
in the olden days…
20-30 years ago when I was racing small sailboats, the accepted wisdom was that you should wet-sand all wetted surfaces with 400-600 grit, and do the final sanding aligned with the flow direction. A waxed finish was considered bad – the goal was a surface that water would form a sheet on instead of beading up. I don’t know if it was science or voodoo, but the fast guys all seemed to take it seriously.
For touring, I wouldn’t worry about it. A good marine polish will make the hull easier to clean and give some UV protection.
My brother in law
is a sailor and engineer and (and google) agree that ‘hull speed’ for displacement boats is 1.34 times the water line length.
BUT imo and (limited) experience that’s a very rough, an often inaccurate, calculation.
According to my GPS Iv’e had my 17’ canoe over hull speed and I’m no olympic caliber athlete. Put two of us in there and I bet we can exceed it by a large margin. Racers to do it all the time. What was the average speed of the winner in the 340 mile race? 9 mph? That would equate to a 45’ boat!
There are too many variables - a canoe and a kayak can be the same length but I’d bet the kayak would go faster for the same effort - it’s a lot narrower. The shape and finish of the hull will be a factor as well as water conditions, and current and wind will factor in the real world ability to make way.
Numbers and sources
If you look up something called a Moody chart, you can get some ball park “friction factors” for the difference in drag between the rough gel coat and the sanded. The Reynolds number is well above 10e6, so you can look to the right half of the Moody chart. Empirically speaking, it is probably around a 1.5-2% reduction in friction factor when you go from rough to sanded. This is because the flow is so turbulent for most of the hull, the bumps in the gel coat are smaller than majority of the turbulent structures (swirls) in the Boundary Layer.
Good reading would be in Frank M. White’s text titled simply Fluid Mechanics. I think Chapter 7 covers boundary layers. Javier Jimenez (sp?) from the University Politecnica Madrid also wrote a great summary of turbulent boundary layers over rough surfaces. Ronald Panton’s “Incompressible Flow” text also does a good job covering wall shear, which leads to viscous or skin friction.
It’s a lot of Greek letters and math. I don’t know of a good qualitative description of the above topics.
D
Here’s some clarification
– Last Updated: Sep-03-09 10:22 AM EST –
First of all, the point about hull speed being exceeded by very skinny boats and racing boats has been mentioned already. However, the fact that such boats are able to exceed hull speed is NOT because the hull-speed formula is not accurate, but because a skinny boat can force its way into its own bow wave much more easily than a wider boat can, and it doesn't "squat" so much when doing so because the waves at the bow and stern are so small (I'm sure there're more details involved too). Thus "hull speed" becomes a true speed limit for typical boats, but a "soft speed limit" for skinny hulls, especially those made for especially reaching high speeds (hull speed is related to the wavelength, and therefore the travel speed, of the boat's wake, and this is something that is a constant since a boat won't create a wake with a wavelength longer than its own waterline length).
Bringing wind and current into a discussion of hull speed is not relevant. Going 10 mph downstream on a river that is flowing 5 mph doesn't mean you have exceeded hull speed, but your way of mentioning these items incorrectly suggests that it would be.
I'll assume that your failure to say that you use the square root of the waterline length in the hull-speed calculation was an oversight, but you also must use the right UNITS. You are using the hull-speed formula which gives the speed in knots (nautical miles per hour) when speaking of miles per hour. That creates a fairly minor error at paddling speeds, but I question how well you understand what's going on because of that. To find hull speed in miles per hour, multiply the SQRT of the waterline length by 1.54.
Just where is this 340-mile race? You say the average speed is 9 mph, but you don't say what kind of boats are used or whether current, tides, or surfing waves might be involved. Folks here have pointed out in the past that racers on surf skis and similar boats can maintain such a speed, but as has been mentioned, such boats can easily exceed hull speed for reasons not related to the accuracy of the hull-speed calculation.
I'd like to SEE you push a 17-foot canoe faster than hull speed by yourself, on STILL water, because I don't know a single person who can even come close to doing that (assuming a 16.5-foot waterline length, hull speed on that boat would be 6.3 mph). You say that with two paddlers you could exceed hull speed by a large margin, and with a typical canoe, I know of no two paddlers who can do that either, that is, beating hull speed by "a large margin" anyway. My paddling technique is pretty decent, and I can't get a solo canoe which isn't hard-tracking enough to be good for sit-and-switch up to hull speed. I've had my 15-foot guide-boat up to 13 mph when surfing lake waves but gravity is doing most of the work in that case. On flat water the fastest sustained speed I can hold in that boat is 6.0 mph (which is exactly equal to hull speed, since the waterline length and total length are equal on that boat). It's worth noting that in an impromptu race using that boat, I can blow away any solo-canoe paddler I've met so far, though I have no doubt that a good racing paddler in a the right kind of canoe could go faster. The in-water portion of a guide-boat is a sleeker design than that of "normal" non-racing canoes, but the "wall" that is encountered once you hit hull speed is so "hard" that applying huge amounts of extra effort into propulsion do almost nothing to increase the speed (I can only get another few tenths of a MPH, and only for very brief spurts).
Voodoo
– Last Updated: Sep-02-09 9:26 PM EST –
Water sheets on cars/boats because of molecular attraction of water to the planar surface of the car/boat. When you wax your car/boat, the water beads because the hydrogen bonds in the water molecules have a higher attraction to each other than they do to your car/boat.
So, do you want your car/boat to stick to the water or slip through it?
Wax it.
It probably doesn't make a significant difference, though.
Be careful of simple explanations
– Last Updated: Sep-02-09 9:30 PM EST –
When things are as simple or as "obvious" as this line of reasoning, there's a good chance of overlooking what's really happening. If the people who study this stuff aren't saying "wax your boat" after all these years, I'm betting it's because what happens where the hull meets the water is more complex than this.
But as you added, it probably makes no measurable difference for most of us!
Hmmm
Dug out “Boat Tuning for Speed”, Imhoff & Pranger,1975:
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"For minimal frictional resistance one needs a hull surface which causes no turbulence whatsoever but to which a thin water film can remain attached by means of a microscopic roughness. This thin layer ensures that friction only acts through adjacent water layersand is thus at a minimum.
“…high gloss will repel water and this causes friction between paint and water which is higher than the friction resulting between adjacent water layers”
“It is well known that the dolphin is a fast mover and its skin has a surface layer of slime which holds a thin layer of water. here we see the same effect where the friction is only between adjacent water layers and hence is low”
“The ideal surface for small boats has about .005mm granular roughness and this can be obtained by rubbing down wet with 400 grade abrasive paper”
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Work has been done to mimic dolphin skin mucus. Ablative polymer coatings were developed that did work but have been banned by most racing organizations.
This claims to be a non-ablative hydrophilic coating:
http://www.hyspeedkote.com/sailing.htm