Learning boat design

messabout said:
A small boat drawing scaled to either 2"/foot or 3"/foot will ......... 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.
Click to expand...

Hi messabout

Finally managed to get back to my project :-)

I was going over the thread again, now that I have a wee bit more idea of how to approach the design. The above part was hard for me to understand. The jump between knowing how much draft is needed / wanted to how to spread that between forefoot and aft rocker. Is it like a rule of thumb, for example, 3 in draft means bow an inch in water and transom up by 4in leading to an average of 3in below water line?

Thanks for your help
 
A planing hull will have the transom submerged at rest and little or no rocker in the aft 2/3 of the hull. The bow will be above the water at rest.
 
gonzo said:
A planing hull will have the transom submerged at rest and little or no rocker in the aft 2/3 of the hull. The bow will be above the water at rest.
Click to expand...

Thanks Gonzo

So, a planing hull is already in a position even at rest, where forward movement against water will push the front end above and rest on the flat aft - is my understanding correct?

I am guessing that the bow being above water line and transom being submerged are not only the function of waterline design but also the trim, and in a dinghy it will be the weight of the sailor right.

I understand that to design a hull for planing will require higher wetted area and will reduce efficiency at low speeds

Then, taking from what messabout said "plan the boat to function reasonably in light to medium airs too", what will be a reasonable compromise to improve light/medium air performance while keeping a possibility of planing?

Thanks
 
It is up to you what kind of performance is expected. Build the boat and test the design. Most likely you will find that it lacks in many aspects. Particularly considering that it is your first design and build. I suggest you don't worry about finish or high quality materials, but save those for a future boat. Designing and prototyping are fun if you don't spend too much time trying to finish the boat like fine furniture. Keep in mind that is will be far from perfect. You can test it and then, cut, add section, etc. to see if there are improvements.
 
gonzo said:
It is up to you what kind of performance is expected. Build the boat and test the design. Most likely you will find that it lacks in many aspects. Particularly considering that it is your first design and build. I suggest you don't worry about finish or high quality materials, but save those for a future boat. Designing and prototyping are fun if you don't spend too much time trying to finish the boat like fine furniture. Keep in mind that is will be far from perfect. You can test it and then, cut, add section, etc. to see if there are improvements.
Click to expand...

Indeed. That is what I am planning with some cedar chine logs and light plywood or some form of quick and dirty construction to test it.
 
Gonzo is entirely correct in that a planing hull will probably have the bottom to transom immersed when at rest. That is the norm for power boat planing hulls. Sailboats or under powered power boats not the same deal.

You can readily observe that a submerged transom /bottom juncture will cause a lot of turbulence when moving at low speed. Very well, why does that matter? The turbulence is the result of the expenditure of energy. Something has to make those swirls and bubbles. The water has to be stirred into motion by some force or other. Do what you can to mitigate the turbulence. That eans that if the boat is going to be moving slowly then we'd better so what we can to reduce one of the sources of turbulent activity. Get the transom lower edge up above the waterline. That ploy does not eliminate all the turbulence but it does reduce it by a large margin.

Draw a horizontal line that will be below the waterline in the side view of your drawing. That is a base line. Raise the forefoot some amount above the base line, on some designs a little above the waterline. You can let it be a little below the waterline but if it is a long way below, the boat will be a bit "twitchy" when quartering waves or wakes. At the other end, raise the bottom curve a little bit above the waterline so that the transom does not drag.

Here is where some major compromise comes into play. Suppose that you have laid out the bottom curves to correspond to an expected all up displacement of exteen pounds for single handing the sailboat. But now you want to bring along another person for a pleasant sail. But you have designed the boat for single handing. Your only remedy is to move the skipper and new crew a little bit forward so that you can get the transom up a little bit. But now it is down by the bow by some amount or other. Part of the fun is to guess how much you can do to diminish the effect of extra weight in the boat. By now you can see that designing a good working dinghy is not as simple as drawing something that is cool looking., or telling your computer program what to do.

Dammit all we did so want the boat to be able to plane readily. Think of what we need to have happen. We have to get the boat moving at a sufficient speed and we have to present part of the bottom to the oncoming water at a favorable angle to the bottom. Let us call that angle of attack or AOA. That means that the bow must come up in a a distance so that the afterplane, the area of the bottom that is behind the point of maximum depth, is now allowing the water to smash into the bottom at some critical angle that will generate some dynamic lift. ( I hope that you are still trying to interpret the poor explanation that I am trying to do)

Now imagine the boat in side or profile view. It is at the beginning of a planing attitude. The bow has raised above the normal position. Now the afterplane is pretty much parallel to the flow of the water...............You cannot plane in this attitude because there is almost no dynamic pressure exerted in an upward direction. You are in transition mode here. You will need to raise the bow some more so that the flow of water is pushing upward on a sufficient area of the afterplane that can support some of the total weight of the boat and all its contents. That is why you need to move the point of maximum depth of the bottom some amount ahead of the mid point of the boat. Why? so that we can reduce the upward angle of the afterplane as much as we can while still getting the transom up when in displacement mode.

When you draw the side view and then try to work in the most favorable curves and locations you will begin to see what a dilemma all this design stuff is. If you are into puzzles, then this is a real fun project.
 
Hi @Thule! I certainly bit off a great deal. I'm chewing it, but it's taking a very long time. Longer than I expected, although to be fair, the bystanders generally thiugh I would fail utterly. So here's to you and I proving them wrong.
 
DogCavalry said:
Hi @Thule! I certainly bit off a great deal. I'm chewing it, but it's taking a very long time. Longer than I expected, although to be fair, the bystanders generally thiugh I would fail utterly. So here's to you and I proving them wrong.
Click to expand...

I am in the same boat - pun intended.
My family and friends think of my boat drawing and book reading is a disaster waiting to happen :-)
Hope they are wrong...
 
Thule said:
...So, a planing hull is already in a position even at rest, where forward movement against water will push the front end above and rest on the flat aft - is my understanding correct?...
Click to expand...

This HERE... may help to understand a bit more too.
 
@Dolfiman
@Ad Hoc
There is a lot to read and chew on this forum. I am slowly going through.
Thanks both for the links.
 
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