lay question
 

This forum was very gracious the last time i visited. I have a practical question that generally relates to the subject of your forum.

In brief, would a boat hull move thru the water faster if were coated with a substance such as wax or silicone or would it go faster if it were bare? The hull is made of fiberglass.

I will appreciate any insight that you have to offer on the subject.

Sincerely, Brad

Re: lay question
 

The easy answer is a question: what is the friction coefficient of all of these materials?? did you get the answer?

Lay answer
 

The submarine is also a boat!!

Re: lay question
 

(1). I think, a surface covered with a smooth layer of wax "definitely" will reduce the surface drag. (2). What you need to do is to find a text book of fluid mechanics and find the chart for viscous loss of a pipe. (3). When the Reynolds number is small (very small boat, model boat, and very low speed), the viscous loss comes from the laminar boundary layer. (4). At higher Reynolds nummber(larger boat, somewhat higher speed), the viscous loss comes from the turbulent boundary layer. The loss in this range is a function of the surface roughness (a surface treated with wax will have very small surface roughness, and is normally called a smooth surface, similar to a smooth clean new copper tubing.) and the loss chart is a standard one used throughout the industries. (5). So, if you are concerned about the gas mileage of your car, the first thing you do is to keep your car clean and waxed all the time. In this way, you can help conserving the oil. By just looking at the car design, you can see that the car manufactures have speed a lot of time and effort to make the shape of a car as smooth as possible. This is the simple way to reduce the loss and conserve the energy.

Re: lay question
 

Brad,

Good to hear from you again :)

The answer is that a smoother hull surface reduces drag. The question is, how much?

The fiberglass boat hulls that I've seen are all very smooth to begin with. If you had (for example) an unpainted wood hull, then there would be any things that you could do to make it smoother and reduce the friction coefficient of the surface. If the surface is already smooth, then any improvement is likely to be marginal.

I think that applying a smoothing substance (like wax) to the surface won't hurt, so it's worth trying.

Regards, Alton

Re: lay question
 

Sharkskin - that's what you need to reduce hull drag. Alternativley get some of that simulated sharkskin tape from 3M that the Americans used on their Americas Cup boat (and then got outlawed). Anyone got any ideas on how it works?

Re: lay question
 

There are two forms of drag: the drag due to pressure difference in the front and rear of a body, which is normally the dominant one in bluff bodies like a car (that's why you want to keep a body streamlined so there is no flow seperation). The second form of drag, caused by skin friction, is normally small for a bluff body compared with the 1st one.

If the skin friction is the dominant drag, such as in the case of shark, I am not sure the smoothness of surface reduces it (in generally yes, but not always). If one looks at the shark skin closely, it contains some interesting pattern of "surface roughness" which actually reduces drag.

Re: lay question
 

(1). Yes, I think, it is possible to reduce the drag from the turbulent regime (higher drag) to the laminar regime (lower drag), by special surface texture or active surface structure. But I think, it will work only under special conditions. (2). So, applying the wax to the surface to reduce the viscous drag is not the ONLY method. (3). As a matter of fact, surface suction devices have been tested on aircraft wings to keep the flow laminar thus reduce the drag and fuel consumption of an aircraft. So, suction on the surface is another active method to reduce the viscous drag. (4). A submarine is usually shaped like a fish, so, the major drag contribution comes from the viscous drag.

Re: lay question
 

A turbulent boundary layer separates less easily than a laminar one. That's why golf balls are not smooth. If the hull is sufficiently streamlined to avoid separation in the first place, I believe keeping the boundary layer laminar by various means will reduce the drag. That is, of course, unless you provoke separation by doing so.

Re: lay question
 

The answers make interesting reading with skin friction the predominant factor as dictated by classic dimensional analysis methods.A smoother surface however achieved will give better performace than with an ordinary grained fibre glass finish.The propelling force coefficients in relation to Reynolds number represent the effects of skin friction.The variation of the force coefficients with the Froude number represents the contribution of the wave making resistance.

Great discussion
 

Thank you all very much for your input. This question came about as 'southern' bass fishermen were discussing the merits of applying various waxes and coating to the hull of their boats. Their observations seem to be in accord with what has been stated here.

Your answers have added much to my understanding of the principles involved. I knew that there would be no better place on the web to get a definitive answer!

Thank you once again.

Sincerely, Brad

Re: lay question
 

I've seen some DNS of this configuration. The skin friction is reduced by the formation of rolled up vortices on the tips of the serrations on the ribbed surface (or some such). Interesting stuff for all you boundary layer people.

Fred.

Re: lay question
 

No, the point of roughness is not at all to make a turbulent layer laminar - quite the opposite. The reason is that a turbulent boundary layer "resists" separation better than a laminar layer does at the rear of the body where the pressure gradient may become unfavorable. Where there is separation, there is significant form drag that will overwhelm by a large margin the effect of the added skin friction. This is the same reason that dimples are added to golf balls - they trip transition of the boundary layer, which keeps the layer attached longer on the ball, reducing the form drag.

Re: lay question
 

(1). If a boat is built like a golf ball, then the dimples on the surface would reduce the size of the separation bubble, and thus reduce the form drag. (2). But the golf ball is relatively small. So, when it is scaled up like a boat, the Reynolds number will be very high and the flow will be turbulent , thus, there is no need to promote the transition from laminar to turbulent flow. (3). Under this condition, adding dimples to the boat surface will not reduce the form drag, I think. Well, one can always cover the surface of a boat by the golf ball skin and check out the result. It would be very interesting.