# Learning boat design

Discussion in 'Boat Design' started by Thule, May 4, 2023.

1. Joined: Apr 2023
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### ThuleJunior Member

Hi all,

I have been reading some books about boat design and am trying my hand at it. Eventually, I hope to build a boat that I designed. I read some great advise on here, including the excellent PDF documentation about ratios, read the John Teale book about design and made a lot of notes. However, I will greatly appreciate any critique people can offer here. I am happy to learn and not easily offended

I am hoping to build a small dinghy with sail similar to sunfish, for 1 or 2 people to sail in this.

I enclose my boat design using Freeship plus 313.

Besides any critique, suggestions or corrections, I also have a few questions.

1. leak_points_grn.jpg, you will notice the leak points in the midst of an edge! What is the reason for them?

2. cross_curves.jpg: what is the y axis? I might have misunderstood this but I thought cross_curves are for heeling angle and the righting arm on the x and y axes respectively.

3. design hydrostatics.txt
LCB is from aft perpendicular. That means from aft DWL perpendicular right.
And layer 0 COG on X is 1.515 - does anyone know if Freeship does COG from aft or bow?

4. How do I create bulkheads? When I tried to create by joining edges, it complained about edges being part of more than two faces and my boat had a negative displacement . I guess it turned to a blimp.

Thanks

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2. Joined: Oct 2007
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### bajansailorMarine Surveyor

What are the dimensions of your dinghy?
Can you post a copy of the lines plan that you have generated?

Have a look through the many small craft plans available online, to find similar boats that you can use for reference and guidance re your design.

3. Joined: Apr 2023
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### ThuleJunior Member

Hi Bajan Sailor

Here they are.
Like every newbie I am probably biting more than I can chew. Any suggestions for a smallish 1-2 people dinghy that planes easily, with free plans that I can import in to Freeship plus somehow?
I will probably buy the plan and build Alexa's rocket first before building my own plans but it is a good learning exercise too. And, who knows, if my plans come up right, I might build this first too.

Thanks

Design length : 3.300 m
Length over all : 3.107 m
Design beam : 1.100 m
Beam over all : 1.272 m
Design draft : 0.200 m

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4. Joined: Jan 2006
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The boat in the lines plan has satisfactory sections but it is not likely to plane. Way too much aft rocker. It is also "fat" forward in the plan view. Your water lines have severe reflex curves which will be a disadvantage in terms of propulsion. The topsides slope inward. That might have some minor structural advantage but it will not be as effective at keeping spray outside the boat.

The size of the boat is too small to accommodate 2 people with any degree of comfort. It is just as easy to build a 5 meter boat as it is to build a 3.3 meter boat. One of the first things to do is to make a careful estimate of the all up weight of the boat, rig, crew and whatever else it might carry. Design the boat to accommodate the total weight. The arithmetic is not difficult.

You can get a much better "feel" for the details and design concept of the boat if you do it with a drawing that you make on a large sheet of paper. Use splines and weights, a suitable scale rule, and a sharp pencil. The splines themselves will guide the practicality and extent of bending that your boat parts can comfortably tolerate. The computer, on the other hand, will let you put in curves that are extremely difficult to achieve when building the boat. The computer program is convenient but it can not do the thinking for you.

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### bajansailorMarine Surveyor

@Thule you have been given a lot of extremely useful advice by @messabout above - please do take it on board.
And do have a bash at a drawing on paper with a soft pencil - you probably don't have any splines / weights easily accessible, but you can do a lot with a standard set of French curves (obtainable from a stationer), and it should be possible to improvise a basic spline for long and shallow curves.

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6. Joined: Apr 2023
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### ThuleJunior Member

Thanks for your advice. I am going out today to the big orange store and get some strips of wood and a ruler etc to try my hand at drawing with pencil.

I am ok with it being only for 1 person. I am making it mainly for goofing around in the intercostals in Florida east coast.

The reflex curves in the front was the result of my attempt to skinny the boat at the bow, looks like I made it worse. You do mean the inbound curve of the front part of the boat sides as they are coming to the bow right.

Topsides - I can flare them out

I will estimate the total weight of the cargo+people easily. I have to weight of the ply/honeycomb panels too and once I can get the shape right and estimate the surface area, I can estimate total loaded weight.

I don't have splines but can go and get some thin strips of wood or PVC curtain liner and use that right. Any suggestions welcome.

Thanks for taking time.

7. Joined: Apr 2023
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### ThuleJunior Member

Hi Bajansailor

I certainly do and appreciate advise. I don't have any splines but am planning to go and get some PVC curtain liner or strips of wood from fabric store/home improvement store. That is my Saturday I had some French curves but probably won't be able to find them, will have to get some new ones. I don't have any weights but saw a gentleman on the youtube using a water bottle, duck tape and nail to fashion one. I will try that today.

Thanks and standby for me hand drawn calamity

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8. Joined: Jan 2006
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A small boat drawing scaled to either 2"/foot or 3"/foot will make a usable set of lines that you can scale with some degree of confidence. For a small boat at 2 inch or three inch scale, splines of straight grained softwood of about 3/16 x 1/4 will do just fine. Make the splines half again longer that the boat that you are drawing. With some experimentation you will see that some of the curves look better when the restraining weights are placed in front of and behind the actual image that you are figuring to draw. In his way you can adjust where the maximum curvature occurs on places like the sheer or chine. Use softwood like pine, spruce, or something like that. Do not use hardwood, it often fails to bend uniformly.

The little splines will let you know when you have put too much curvature in a line. They will break or you may hear a cracking sound. Trash that little stick and get another one. If you know someone who has a table saw, they can rip several splines in less than five minutes. For weight you can be pretty crude if you do not have the fancy whales or other such luxuries. Small tin cans with some sort of weight, bolts and nuts maybe, inside will work if no better weights are available. Slugs of lead melted from old tire weights make good tools for the drawing board. Best put sheet cork or rubber on the bottom of your weights.

Unlike the computer programs, doing it manually will force you to do some arithmetic before you start drawing lines. Let us say that you are doing a small rowing boat or sailboat. You have made an estimate of the all up weight that the boat will carry. The boat is....say 12 feet long (144") overall. It will have a little bit of rake in the bow so that shortens the waterline a little bit. The bottom of the boat is going to rise above the waterline a wee tad when viewed at the transom. The waterline will be shortened some more.............for example, not a suggestion, you lose 4" at the bow and about 8 inches at the aft end. OK we have a theoretical waterline of 132 inches. We want to calculate for a total displacement of 350 pounds (example only). How much volume of water is that? I am a fresh water sailor so I use 62.4 pounds of water per cubic foot. Divide 62.4 by the number of cubic inches in a cubic foot which is 1728......62.4 pounds/1728cubic inches = 62.4/1728 = 0.036111111 In your case, salt water, it is 64/1728 = o.o370 pounds per cubic inch.
Small boats for rowing or sailing often have a prismatic coefficient (Cp) in the ranges of 0.52 to 0. 58 more or less. Lets say your boat will have Cp of 0.55. Cp is something like the average of the areas of all the sections that are submerged. The front and back of the underwater parts have ever diminishing areas so the average is some proportion of the mid section or whichever section has the largest area. .............................Are you with me so far?

Take 350 pounds divide by (salt water) 0.037 = 350/o.o37 = 9459 cubic inches of displacement is required. Measure the area of the largest section. It might be a flat bottom with plumb sides. We could simply divide 9459 by the waterline to get ....9459/132= 71.65 inches need for the largest section..............WHOA!!! we forgot the Cp which is a SWAG guess at 0.55. Alright we have to divide 71.65 by 0.55 which gives us a required main section of 71.65/0.55 = 130.27 so far so good. Now how big does the main section need to be? 130 square inches that is the arithmetic goal.....Say the flat bottom is to be 40 inches wide. Divide the required area 130 by 40 which gives us 130/40 = 3.25 inches. Hooray we have determined the probable draft of the boat. Now you have at least a working idea of how much rocker to put into your boat. That will let your forefoot be just slightly ...maybe an inch below the waterline and the transom to be about 4 or more inches above the waterline.

If you are still with me after all that , then start making lines on your drawing paper. It is fun to do, I swear it is. This isonly the beginning Thule. It is a good place to start however.

Disclaimer: I apologize to my more professional peers here at the forum. I know full well that it is not as easy as all that. It does get our new student on the way however.

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### bajansailorMarine Surveyor

Absolutely!
Your thoughts above are an excellent introduction and analysis.

10. Joined: Apr 2023
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### ThuleJunior Member

Thanks a whole bunch for the detailed information there and that sounds like an approach I can bite my teeth in to.

I have been looking it older / free plans to study as well.
What do you think of the next 3 plans below?
These plans didn't come with lines plan drawings but most of the information is there in measurements and I can work on those to recreate these in lines plan format.

https://www.diy-wood-boat.com/support-files/tern.pdf
https://www.diy-wood-boat.com/support-files/zephyr.pdf
https://www.diy-wood-boat.com/support-files/falcon.pdf

I like the Tern. Don't know if these were ever built and if so, were good. That is no shade on the plans, more on my lack of knowledge

Thanks a lot

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I also like the Tern. Zephyr is a kinda fat looking little boat. Zephyr is much more likely to plane than Falcon. Falcons aft rocker is too curvy to promote planing readily. The scow will usually be sailed in light or medium air while deliberately heeled ten degrees or so. In a real breeze stand it up and it will fly. The scow will pound pretty badly when standing upright, it will pound less when heeled.

All three of the ones you have mentioned are very old designs. The building details will make all three of them a bit heavy. That's the way they did it in the old days. More up to date boats using the stitch and glue method are more likely to appeal to amateur builders. William Jackson, in his day, did a whole lot of designs for magazines like How to Build 20 Boats, Popular Mechanics and others. All of them would float but some of them can be considered obsolete.

A look at the suggested material lists will reveal that some of that material is going to be hard to get and also very expensive even if you can find it. White oak for example, is heavy. Think in terms of yellow pine or spruce for framing. For skins, plywood is the usual choice. Big box store fir plywood is not the best choice.. The problem with that sort of ply is that it has core voids that will lead to water absorption and eventual rot. It is also prone to "checking". Try to get some Okumee or Meranti ply, with a BS1088 rating, you will have the better material.

Before worrying about materials you will need to select a set of plans, either plans that you buy, or a plan that you draw, yourself. There are a gazillion posts on this form that recommend, wisely, that the aspiring builder get some plans for a proven boat from a legitimate source. You could start by looking into the Duckworks web site. I am not sure whether the plans for Phil Bolger boats are still available. They were formerly sold by Dynamite Payson. He has sailed across the bar but I believe that someone else is selling plans now. A Bolger boat that comes to mind, that is easy to build, and a good performer, is a Bolger design called "Cartopper". It is a proven design that has pleased many owner/builders. It is a twelve footer that uses a simple sprit boom rig of 65 square feet. If you want something bigger, then the Bolger Gypsy is a proven 15 foot design that is a little more work to build but it is an exceptional performer with only a small and easily managed sail. It also rows very nicely. ( it has a trapeze bottom that minimizes wetted surface area...........more about that later if we continue to communicate)

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12. Joined: Apr 2023
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### ThuleJunior Member

Your explanation for calculating the displacement, draft and sections helped immensely.
I have been playing with the numbers for fun .
I will try to draw the second one and see where I go with it.

I bought the Dynamite Payson's book. Didn't know that he sailed across the bar. I can look at the plans and you are correct. Someone [might be PAR] wrote here on the forum somewhere, that the launch day is embarrassing enough, without the boat trimming fore or aft! I might build a simple design using the bought plans first off. Especially, where I am planning to launch it, I will not live it down if it pitches or lists and sinks

So, using the Zephyr hull shape to draw up my plans and develop plates to build as a stitch and glue is not a good choice?
I am thinking of using the honeycomb panel to build these - at 18lb for a 4x7 panel, it is obviously much lighter than the 3/4 ply and something I need to include in the calculations.
[Edit: also, would like to modify the Zephyr to include flotation chambers for safety, so, that will modify most of it's behaviour I guess!!]

My two attempts at calculating the draft and displacement requirements below.

Thanks

Desired LOA 12ft = 144in
Rake and rockers lost 5+9 = 14in of waterline
Effective WLL 144-14 = 130in
Calculate for required displacement of 600lb
Displacement volume = 600lb/0.037 = 16216
[64lb /cf of sea water, so, cubic inch of water is 0.037 lb]
Cp 0f 0.55
Area of the largest section required without Cp: 16216/130 = 124
With Cp into consideration: 124 * 0.55 = 225in required main section area
Slightly flaring sides,
Beam average on waterline as a blind guess 54in [60in at the midships]
Draft needed = 225in / 54in = 4.17

Desired LOA 14ft = 168in
Rake and rockers lost 6+10 = 16in of waterline
Effective WLL 168-16 = 152in
Calculate for required displacement of 600lb
Displacement volume = 600lb/0.037 = 16216
[64lb /cf of sea water, so, cubic inch of water is 0.037 lb]
Cp 0f 0.55
Area of the largest section required without Cp: 16216/152 = 106.7
With Cp into consideration: 106.7 * 0.55 = 194in required main section area
Slightly flaring sides,
Beam average on waterline as a blind guess 44in [48in at the midships]
Draft needed = 194in / 44in = 4.4

13. Joined: Jan 2006
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Thule, don't sweat the for and aft trim of a 12 or 14 foot dinghy. Say the boat is down by the bow.......Move your own position a few inches aft and the boat will trim just fine. You can find the center of buoyancy of your boat by using a little bit more arithmetic. Use Simpsons rule and think in terms of a playground see-saw............Simpsons rule: To save space and typing strokes, consult Wikipedia or other source if you are not familiar with the process. Find the volume of the forward half of the displacement and compare to the aft part of the displacement. then do the see-saw arithmetic to find the balance point. Or simply use the sum of the displacements divided by the number of stations and you land on the location of the center of buoyancy (CB).

When Par bemoaned the possibility of discovering a poor trim angle, he was referring to big boats with ballast and furniture and machinery. That is not a worry for a conventionally designed dinghy. Calculating all that for a big boat is a lot of work but it still boils down to the see-saw principle.

I think it worthwhile to do a graph of the section areas. You can do that easily right on your drawing. Drawing that curve gives you an idea of how the water is going to be pushed aside as it flows from front to back of the hull. Actually, of course, the water does not flow, the hull moves through the water which causes what amounts to flow over the wetted surfaces of the boat. Consider the acceleration of the water particles as they are moved aside by the motion of the boat. Tinker with your design lines so to minimize particle acceleration. this is why the very skinny boats are most easily propelled.

Keep in mind that it not important to draw both sides of the boat in the plan view. (that is the birds eye view) Draw a center line and construct the sheer and chine lines and you will have the general outline. That gives you the widths of the parts of the boat at any fore and aft location, usually the sections are what we are interested in. The Profile view (side view) establishes the heights of the section elements. You can put your section area graph on the plan view if you use a suitable scale for height of the area . I hope that makes sense.

Forgive me if I am telling you stuff that you already know, like the elements of mechanical drawing.

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### ThuleJunior Member

John Teale book also has a section on the Simpson's and I found a page with description kinder to me at Basic Buoyancy Calculation https://smalltridesign.com/Trimaran-Articles/design/simpsons-rule.html

Took note of not worrying about trim on small dinghies.

When you say graph of section areas, do you mean, plot section on X and surface area at that section as Y, superimposed on the plan view to give an idea of curvature of the volume? I am probably mangling words badly but hope I am explaining myself.

Last sentence - please feel free to teach. If I already know something, it is not an insult and a refresher never is a waste.

Sincerely appreciate the time and guidance

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15. Joined: Jan 2006
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You will have laid down a base line and then erected perpendicular lines that correspond with the stations. (you need a good square or large draftsmans triangle to do this). Lay down an even number of spaces so that the stations are odd numbers. ( one more than the spaces sort of like fence posts) Simpson needs that odd number or the 4...1 multiplier does not work. The perpendiculars can extend upward/or downward depending on which view you draw on top and bottom of the paper.

When you calculate the area of the various submerged sections you will have a chart that you can extend to the plan view....or if you prefer a separate base line and perpendiculars for the several calculations that you might want to play with. say , for example that your mid section shows 98 inches square.... That is too big a number for laying out on the area curve graph. In that case use a different height scale. If you have used three inches to the foot for the main drawing, then you might want to use a much smaller scale for the height of the graph coordinates. Maybe half inch, or quarter inch to the foot or whatever fits your drawing space. The tick marks on the perpendiculars will then be connected with a spline to form a pretty....(hopefully)...nice curve. It might look something like a lazy sine curve or whatever. The spline that you have used for the general outline of the boat will be too stiff to use for the curvy displacement graph. A piece of tempered wire from the model hobby shop will work well enough.

After you have experimented with this method you will might see that you need to modify the lines of your drawing somewhat. For a small dinghy that you hope might have planing potential, explore this general idea..... Move the lowest point of the rocker a little bit forward f the mid point of the boat....maybe at 47 percent +/-.....Move the width of the chines aft a little bit....maybe 53 % +/-....fiddle with the locations of these points until you are happy with the section area graph. Try to avoid reflex curvature in the forward sections. The aft sections may come together more suddenly . That is OK for a planing hull.

If the boat is to be encouraged to plane, the after-plane needs to have the smallest rise angle that will still let you have the exit at the transom a bit above the design waterline. The afterplane is that part of the bottom, aft of the lowest point of rocker to the transom. Some angle less than about 4 degrees is the aim. You can see right away that a longer boat has the advantage in terms of what the smart guys call the aft half buttock angle. Dave Gerr's book : The Nature of Boats ,does a pretty good job of explaining this notion. In any case the aft half angle is important to minimize if you want the boat to plane readily. The why is the subject of a whole nother lecture. Believe Gerr and in the distant past some wizards like Uffa Fox. I am hesitant to get into the reasons why at this juncture. It is a mess of hydro physics that involve mass density, velocity squared, impact angle and too much other complexity to go into here. What the hell, I probably don't have a sufficient understanding of the math myself. I will leave that to my betters here on the forum.

Finally I will answer your question..... The section spacing is on the x axis and the section area is on the Y axis.

Truth to tell, you will be sailing in airs that are not as likely to power the sail enough to initiate planing. So it is prudent to consider what kind of hull you should have for light or medium air. Why is boat design so smitten with compromise??? Light air optimum design may not be completely compatible with planing design. (sigh)

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