# How does the angle of the bow of a boat affects its coefficient of drag? PLEASE HELP

Discussion in 'Hydrodynamics and Aerodynamics' started by Catatau, Nov 11, 2012.

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### Ad HocNaval Architect

Ok..well, that isn't really enough distance. However considering this is your school project, it is enough

So using the plateau part of your graph, you have used this for the average velocity, which is a reasonable assumption.

Last question.
When you tow the model, is the model on the bottom of the tank, or floating somewhere in the middle, i.e. with neutral buoyancy?

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### CatatauJunior Member

somewhere in the middle with neutral buyoancy

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### Leo LazauskasSenior Member

Remember, though, that when the transom is dry or partly dry, there is a loss
of hydrostatic pressure on the stern. This can, in effect, be thought of as a
component of the total resistance.

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### Ad HocNaval Architect

Indeed...but i'm trying to "keep this simple" owing to the fact this is a high school, project, not an ITTC test tank with qualified hydrodynamicists

Hmmm..problem is this means the model is moving off line as such. The model could take up an angle or trim as it is called when running, which will add more drag.

If we assume that it runs nearly level, we also need to add drag owing to is shape (front projection) to the on coming flow.

If you place your hand in water palm to the direction of flow and pull it...it feels draggy...if you rotate your hand side on, there is less resistance.

So looking at the model of beam 20cm with height of 3.9cm, this is a projected area of 78cm^2. This is also adding drag. The frontal area, will increase of course with increasing trim...so knowing how the model behaves when towed is important, hence on the bottom or floating is best.

The drag coeff from various sources would indicate a value of 1.55, for the "pointy" triangle shape. See below:

So you need to add this too. The total being the same calculation as before but the coeff being 1.55 rather than the Cf from the 0.075/(logRN-2) formula.

i.e. = 1/2 x 1000 x (78cm^2/100^2) x velcity^2 x 1.55 = drag

Does this make sense?

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### Leo LazauskasSenior Member

I know, and you're doing a great job!
But ignoring the loss of pressure could lead to negative resistance in the wash
up at the end.

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### CatatauJunior Member

Could you say what these sources are? I need to reference my work. So the drag coefficient would be 1.55 for all models? The drag that the calculation above is equal to is the frictional drag which I would have to subtract from the total drag which is the weight pulling the boat like before, right? I would then get the wave making drag and I could compare it for each speed and each model, is that right?

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### Ad HocNaval Architect

We can address that, if we need to later. Since this is the beauty of experiments...things we find we didn't expect and have to explain.

So, for now..lets get him in the right track and see what he can make of the results

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### Ad HocNaval Architect

The image posted above, i just scanned in from my "Fluid-Dynamic Drag" book by Hoerner. It is a classic, one of the, if not the best book on the subject

This is additional drag, usually termed form drag.

Since you have friction, owing to the WSA and now friction owing to the shape, or form, of the object, to the on coming flow. So you add these 2 drags together as your total friction.

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### CatatauJunior Member

Wait, so to find the friction from the WSA it is what I did before but I have to add that to the from drag which is equal to = 1/2 x 1000 x (78cm^2/100^2) x velcity^2 x 1.55? Both together will be equal to the total friction which then I have to subtract from the weight pulling the boat? If this is it wont my answers be even more negative for wave generating drag?

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### Ad HocNaval Architect

This is what you need to investigate. And if this is the case...to understand why...and is it applicable across the speed range too?

This is the whole purpose of doing experiments (tank testing). To investigate the mechanisms that cause behavior that is observed or measured. If we repeat the experiment and obtain the same results as before...quantitative testing (i.e. anyone can do the same test and get the same answer as you)....then clearly there is more to understand that first expected. Which is what learning is all about...the why's...why is it negative..why does it do this..why does it occur only over XX or YY....

Once you grasp the idea that the "why" is very important...the result is child's play...excuse the pun

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### JoakimSenior Member

If deep submerged, that would be the only (important) drag component. For that bluff bodies friction can be ignored.

You are already getting too much drag from friction only before adding the pressure drag components (form drag and "wavemaking drag"), which should be the dominant ones. Taking into account pulley friction would make it even worse.

There must be something wrong. Are you certain your velocity is correctly calculated?

12. ### tunnelsPrevious Member

Give it true to life shape !!

This may be a school project but what are we learning for heavens sake ??
If it is deeply submerged then how in the hell can you believe any of whats going on here . Are we into talking about submarines ??? .
It the bow angle and the bow rake are two differant things ? the rake determins the bow shape the angle is only a part of it from the chine down whats above the chine is determined by the deck lines . I really dont like chasing my tail and all the speculation and bull dust thats going . unless you actully make a true bow shape in its true form then how the hell can you say whats good and whats not good !!! the bow rake gives the shape of the front and the angle says if its going to be a nose diver or bow to cut through waves or and hard rider and butts into the waves . Is this what learning how to design boats is all about ?? its no wonder there are so many terrible boats getting built and there performance is terrible . seems there was more research and development went on in early time than there is in modern times all the referances are all from old books . no one seems to agree with anyone else and just goes round and round and going no where .

All the loyal follower on this site must have read this by now !! so wheres it all going ?? no where by the look of it . its taken a young guy studying and doing some exsperimenting of his own to get all you fossals excited and post silly grahps and copies of pages from books probably a 100 years old and i bet you have memorised every last word off by heart !!,is this where boat design is at ?? come on people wake up and help the young lad ?? im sorry but you have submitted nothing worth getting excited about !! not one of you !! dont you know how to post pictures and photos ?? surely some one must have something of interest worth looking at !! man you must be a bunch of boring people in real life ! wheres your enthusiasum ?? what gets you excited and fired up to do something worth while ??
.

Any one can read books and remember stuff but how many of you have actully put into practice all this mumbo bumbo and actually done some exsperimenting on you own . ?? shelves full of books and computers with loads of memory filled with meaning less information . cad drawing done by the dozen all meaningless !! .
Just now and then there is some one that does something with what they learned and puts it into practice in there back shed . I admire these people, they make things happen and its learning all the way . sitting and pushing buttons and critisizing all and everything and no one person agrees with another so whats the point !!, he said this ,but its not this, its that, and thats and this are then shot to pieces when some one else shoves his oar in and upsets the apple cart and dosent agree with anything anyone else has said and so the pointless arguements rag on .my icecreams bigger thatn your etc etc etc !!. Why dont you all read the same books that were written 100 years ago and agree with each other .

Last edited: Nov 14, 2012
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