# Simplified understanding of water displacement resistance in boats.

Discussion in 'Boat Design' started by DocScience, Jan 15, 2015.

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### DocScienceWishful builder

Basic primer in understanding water displacement resistance in boats.
Comments, corrections and suggestions welcome.

This helps explain WHY and WHAT makes a rowing shell faster than other rowing "displacement boat" shapes.
If you want to build a faster boat, you need to understand this.
Most designers would understand this very well.
I have read many questions that people have ask on these boards, and there are a lot of
new people who do not have a good understanding of how this water displacement resistance works.
When you see someone ask a related question, you can point them to this thread.

This is an explanation to help illustrate the water displacement
resistance to moving efficiently, and how rocker effects this.
Other explanations show in what conditions, rocker is very useful to have.
That is not the purpose of this topic.
For a lot of smaller boats, surface area resistance is smaller than water displacement
resistance, so I am ignoring all surface area resistance in these illustrations.
For simplicity, surface area resistance should be a separate topic.

Take a look at the attached diagrams. If you click on them, you should be able to get a bigger picture.
I use a flat bottom on all illustrations, except for "D", to simplify the explanations.
I am comparing hull shapes for small boats like canoes, and small craft.
I am comparing the largest underwater frontal area cross sections of each.

For the boat to move through the water, the total boat's "underwater frontal area view"
(GREYED AREA) shows the area of water that MUST be displaced, as the boat moves through it.
It should apply to most large and small boats.
In SMALL boats, this is the largest resistance that is encountered, and is what
you need to design for, to make a faster boat. .
This is why rowing shells are faster then other shapes.
Take a look at the area labled "CC" (which is a rowing shell) on the drawing.

Diagram "A" is more like a canoe shape,
"B" a kayak , and
"C" a rowing shell, and
"D" the approximated shape of many boats like rowboats.

Boat "D" has a fine bow to cut into the waves, and a wide transom or stern, for stability, as in a typical row boat.
If the front lower edge of the bow on "D" is just at the water surface,
then the frontal area displacement would look more like "AAA".
Although "D" is a somewhat complicated case, there is no other option in that the
water must be moved around, for the boat to be able to pass through it, and that does take some energy.
I am considering all these boats to have the same mass or water displacement.
For simplicity, I am considering "A", "B","C", to be of approximately the same water depth immersion, with no
rocker, and the total front view water displacement area of them, would look like "AA", "BB","CC" respectively.
I am using "AAA", "BBB","CCC", to show the increase in total underwater
front view area in those boats with a lot of rocker on them.

When a boat has a lot of rocker, the center of the boat must sit lower in
the water to compensate for the decreased water displacement volume at
the ends, as the ends of some boats are high enough to stick out of the water.
On the diagram, you can see that the water displacement area "AAA" (boats with rocker) are
larger than the displacement area "AA" (boats with no rocker).
To simplify the diagrams, the top view of the bow of these boats, all have
the same angle on them, for parting the water.

The speed at which the water is parted, as the boat goes through the water, depends on
the speed of the boat, and on this bow angle, as shown in the top view in the diagram.
For comparison purposes, I keep this angle the same on each boat.
The faster speeds that the water is parted, uses more energy and slows the boat correspondingly.

As long as the boat stays in displacement mode, and does not do any planing, the
boat "A", must displace that volume of water "AA", as the boat "A" moves through the water.
If the boat has a lot of rocker, then it's cross section looks like "AAA".
The more water that must be displaced, as shown in the grayed areas, means
it takes more energy to displace that water, making the boat slower.
It may be difficult to easily see, but the shorter the boats are, compared
to it's width, means that they are wider, and even though they may be shallower in
the water, the total underwater frontal area WILL BE LARGER than the other shapes shown.
A good example of this would be a standard stubby rowboat.

When you look at Cat's or Tri's you must add together the frontal area of
each hull that stays below the water surface, under your set conditions.
If you have a hull that is precisely lined up behind another hull, and if they are not connected, leaving
space for water to flow between them, then they must be simply added together, as a separate hull.
The simplified explanation is that the turbulent water between them makes
them act relatively independent of each other, like a completely separate hull.

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### keroseneSenior Member

That was not simple. Also googling "water displacement resistance" did not return ANY relevant hits. Is it really useful to create yet another concept/term? How on earth does that make learning and understanding easier?

I think its worthy effort to make certain concepts more accessible for a newcomer but unfortunately I fail to see this as a very successful attempt at it.

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### DocScienceWishful builder

Please tell me what terms describe it better. Did I explain it well enough, so that you could tell what I was talking about ?? Maybe I could edit in the first line to introduce it better. Thankyou for replying.

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Doc; I have read this post two times. The points you intend to make are not clear.

You have made some statements, and I fear some amateurish assumptions, that will not withstand informed scrutiny. Example; Form drag is larger than surface drag. That is true at some velocities but definitely not all velocities. Canoes and kayaks rarely achieve the speeds that will make your statement true. You must qualify statements of that sort with suitable and well established explanations of the science behind those hydrodynamic realities.

Some of the physics that you have entertained is sound enough but there are all sorts of variables that will change the outcomes of the preliminary assumptions. Frontal area matters but section area distributions are critically important and influential. You need to address prismatic coefficients while you expound on frontal area influence.

Thank you for the effort, but please be certain of the verifiable accuracy of what you write.

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### tom28571Senior Member

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### DennisRBSenior Member

Hello. Do you have links to sources that lead you to your theory? While I am sure it is correct at some speeds it does not appear to be at all speeds. Specifically it does not appear to be correct at the lower Froude numbers a canoe or kayak will travel unless it is a racing model being raced.

Here is one source on prismatic coefficient. This is relevant to what you are talking about since 2 hulls with the same length and displacement, but with differing Cp will have a different midship area.

This can be seen on figure one of this paper which shows section area. The lowest Cp hull tested had the largest midship area or "max water displacement" as you seem to be calling it. The hull with the smallest midship had the highest Cp as you would think.

So does it follow that the high Cp hull (smallest midship "max water displacement") has the lowest drag as you suggest?

Well look at the data and find out. The answer is it depends on the Froude number (speed). The high Cp hulls usually had higher drag at lower speeds (below "hull speed"). The high Cp hulls usually need to be going fast before they have less drag than the low Cp hulls with the bigger midship area. (the grey boxes on your pic)

So given my limited amateur understanding, your theory does not seem to stack up with the evidence under many circumstances.

http://eprints.soton.ac.uk/46460/1/086.pdf

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I suspect that the OP is a well meaning person who presumes to share some observations with us. I further suspect that the OP may not realize that he/she is regaling some of the worlds most prestigious boat people who have vast knowledge, education, and experience.

I previously thanked the OP for the noble effort. I repeat thank you for the effort to enlighten us. I am taking the high road here. DocScience you are not to take these comments as a put down.

Please spend a bit of time here to learn some of the nuances and realities from the professionals. Daquiri, Ad Hoc, Tom Speer, Tansl, Laususkas, Par, and many, many, others are consummate professionals from around the globe. Their council will serve your education well if you will take the time to audit their posts.

Disclaimer: In no way do I rank myself among those many forum dignitaries. I am merely an interested observer.

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### DennisRBSenior Member

Sometimes if you THINK you have something figured out and want to check, its a valid response to put out what you think you know to have it scrutinized for accuracy. Otherwise how do you know if you are correct or not? Especially in field such as naval architecture, and you are just an enthusiast with no official tests you need to pass. I tend to do this at times. Not much BS flies past the radar on these forums. In fact, I am a member of countless forums and this forum has the most rigorous BS meter of any I have seen by far!

I don't usually get much rep here since I know so little. I can usually only come here to learn not teach. But someone repped me up for my last post in this thread, possibly the OP. So far he has not replied in fit of cut and pasting from designers websites to try and ratify his theory as many do. So it looks positive. He did ask for corrections after all.

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### JSLSenior Member

I have been designing boats for about 45 years and there are books that present more logical explanations for this topic.

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### DocScienceWishful builder

After looking up a few things commented on above, I see a couple of sentences in my OP that probably need a lot of revision.
Would it be somewhat acceptable if the title is changed to -
A simplified non academic way to understand the speed of human powered water craft. ??

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### keroseneSenior Member

Doc,
There is a major problem in the fact that you use self established ideas as facts. Its far more useful - even for a beginner - to learn concepts and terms like prismatic coefficient and wave making resistance. They are real established concepts, not THAT hard to get a grasp on and more importantly can be studied further.

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### DennisRBSenior Member

After reviewing the data in my link, and other research that you may have done based on other replies, do you think that the content may also need changing rather than just the title?

After all, it has been shown that for the same length and displacement, hulls with a larger midship area can, and usually do have LESS resistance than those with smaller midship areas in most cases below "hull speed".

Also your hulls are different lengths, and appear to have different displacements. How are you comparing them if the most important parameter in resistance (length to displacement ratio) is not being held constant?

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### Mr EfficiencySenior Member

I don't understand why water is so wet.

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### DennisRBSenior Member

Easy. Because the lack of water is dry.

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