Resistance vs. bottom design, small boat, slow speeds

[Russ Kaiser]

Hey everyone,

Since this is my first post maybe I should give a little background: I have lurked here for quite a while. I have wanted to design and build a small boat primarily for throwing on top of my minivan or in the back of my truck to row and fish on the many lakes near my house. I have wanted to start this project for a while and everything seems to be coming together to do it now.

As for my skills, I have worked many years with epoxy based fiberglass construction, I am a competent carpenter and wood turner, and I am blessed right now with plenty of spare cloth, resin, and all types of lumber and sheet plywood. In other words, I would not have to buy much to complete my project. It will be my first boat and I want to keep things simple.

My question revolves around slow speed resistance relative to bottom design. I created 3 very simple boats in Carene2008 all with identical bottom dimensions: length 12 ft., beam 3.5 ft., sharp prow, narrow transom, with a single plank side at 77 degrees. The first variant had a flat bottom, the second had a 7 degree dead rise, the third retained the dead rise and added a modest lift to the bottom profile both stem and stern.

Next I imported these designs into Delftship and determined equivalent drafts for each boat using two displacements 340 lbs and 500 lbs. I then ran the resistance for each boat at the two displacements. Surprisingly, there was virtually no difference in resistance for 3 knots at the smaller displacement and at the higher displacement, the flat bottom boat was slightly easier to push through the water.

Am I missing something or is the resistance of a small boat like this counter-intuitive? I thought the more complex bottomed boat would walk away from the flat bottomed boat at very slow (rowing) speeds. If this data is true, I will be somewhat torn trying to make a decision on what style bottom to use. A flat bottom boat will be much simpler for me to build. Esthetically (especially out of the water) the boat including the dead rise and curved bottom profile will look better.

Can anyone give me some insight?

[rasorinc]

why don't you give it more beam and buy or build one of these.
http://www.boatloader.com/boatloader.html

[Russ Kaiser]

Rasor,

That is a cool device for car topping, I am sure it would cost five times more than my boat budget.

I am thinking about making it a bit beamier and raising the sheer line slightly. These were quick designs specifically to test the resistance of the different bottom configurations. I have helpers to get it on the van or in the truck and it should weigh less than 130 lbs.

Here are the lines:

Flat Bottomed


Dead Rise Only


Dead Rise and Curved

[fcfc]

I would say not only the real thing is counter intuitive, but the the use of computer too is counter intuive. The fact that it is displayed on a computer screen does not mean it is true.

For the real resistance thing, the first main is the length of the waterline. the second main is the displacement. all other things (beam, draft, entry angle etc ...) have a much more limited effect.

Now, for the computed thing, the real thing is not fully understood, witness http://en.wikipedia.org/wiki/Ship_model_basin
so the computed thing does not match reality. They are 2 classes.
The mathematical view, but with simplification from reality, to make the problem human solvable. Michlet is kind of this. NB The problem MUST be human solved, before being automated on a computer. The hull and the speed must be checked to be compatible with the mathematical simplications made.
And the statistical view. Measure a bunch of hulls, and expect a hull like the one in the crowd to behave like the crowd. The hull and the speed must be checked to be within the statistical analysis range. (Holltrop is kind of this analysis).

Now, there are other issues, like resistance is estimated for a absolutely flat water, for a constant speed. Rowing in 4 inches waves is neither flat water nor constant speed.

And there are things like stability, ease of build, or ease of handling (rowing straight) , rowing ergonomics , or aesthetic aspect for the owner that may be more important than the few % of resistance difference.

[lewisboats]

Virtually all non planning boats are designed so the transom does not drag in the water and at the speeds you are talking about...most of the drag is from your transom. You are pulling a hole along behind you and creating a lot of drag. Try your boats again with the transom out of the water...I think you will notice a difference in resistance between what you have now and what I suggest. Also...reduce your bottom beam forward somewhat to narrow your entrance angle, bump out the mid beam a bit and add some beam aft to round it out a bit more. What you have resembles the old "codfish" style hulls of a few hundred years ago...not the fleetest of seagoing craft. I suspect you used 2.5-2.8 on the Power of X forward...try setting it at 2.1 and bump up the power aft to 2.5 or 2.6. Try adjusting your Position of Max beam aft by a couple of .1s too...this will automatically bump it back a percentage of the waterline.

[Russ Kaiser]

Thanks Lewisboats,

Those sound like easy things to change that may make a significant difference. I will rework the basic design and see what I get.

Russ

[rasorinc]

Nice computer pictures--real clean looking. I do all my drafting by hand
(old school) and when doing it by hand mistakes seem to leap out and scream at you. I do not do fancy stuff though and really only modify rather then create. Enjoy the process--it always takes your mind off any problems.

[lewisboats]

I did a quicky and this is what I came up with from your drawings. Once you get your basic into freeship then you can tweak things on an individual point basis. I took what I got out of Carene and worked all around the waterline tugging things around. I straightened the run aft and reduced the deadrise a smidge in the after parts, narrowed the waterline beam forwards to further tweak the entrance angle and generally nudged the points to smooth things out a bit.

[daiquiri]

Anyways, regarding your question about the apparently constant resistance vs. displacement - yes, it is possible. That's because your hull has a large wetted surface (made almost entirely by the bottom surface) and 3 kts are only 65% of the wave speed for your boat's length. Now, I am judging it by just looking at your boat's hull but I would say that you are most probably experiencing the predominance of friction drag at 3 kts, and that particular drag component will not vary much with displacement for this (wide and flat) hull shape.

[Russ Kaiser]

Lewisboats: I definitely like the lines of what you came up with. While you were turning that out I re-worked my test design quickly following your recommendations.

Base Design Parameters for Carene2008:

3.6 LWL
Max Beam 1.575
Half Max 0.48
Aft Max 0.16
Forward Power 2.1
Aft Power 2.6
Transom Angle 15
Single Plank Sides 70 Degrees
Sheer Heights - Stern 0.36 - Mid: 0.33 – Stem 0.42

Dead Rise Only added 7 degrees dead rise.

Curved Bottom Profile added 0.10 front and 0.15 back rise.

The boat has grown to around 13 feet of length and that's about the max I want to mess with without dealing with a trailer.

As far as the results are concerned, I think Daiquiri might be onto something

From Delftship - displacement 360 lbs – speed 3 knots

Flat Bottom - draft 0.222 feet and ~ 4.8 lbs. resistance
Dead Rise Only - draft 0.340 feet and ~ 5.2 lbs.
Dead Rise and Curved - draft 0.421 feet and ~ 5.8 lbs.


Flat Bottom:


Dead Rise Only:


Dead Rise and Curved Profile:

[lewisboats]

That curved profile underwater is called "Rocker" like a rocking horse. A flat bottom refers to the beam wise or athwartship flatness as opposed to deadrise or amount of difference between the keel and the bilge. A flat bottom can have rocker to get the transom out of the water. Lack of curvature aft...whether it has rocker or not is called the flatness of the run or a curved or flat run aft. Same same forward. Now that you have the basic shape to work with time to start playing with it. Here is where you go on the web and start looking at lots of lines plans to see how the pros do it. At the same time...you should start reading about why they do it a particular way so you know what you are doing when you make the transom narrow or wide, add deadrise or reduce, add or flatten rocker etc. Carene will come up with a shape that is guaranteed to be developable...but may not be the most aesthetically pleasing thing when imported into F/D-ship. First of all it will change the lines somewhat from what you had so you have to correct for that. When you look at the design in full rather than halfbreadth you will also detect things you didn't anticipate showing up. Freeship will allow you to tweak the lines but you can still check for developability with the right button menu. To see if there are potential problems reduce your resolution to medium and look for red areas other than at the seams. A red area that extends into the panel is a potential problem if you want to get the panels out of plywood. F/D-ship also does not like meshes that are not 4 pointed or sided...these will almost always come up as undevelopable...regardless whether they actually are or aren't. A concave surface will not be developable, nor will surface that is convex beyond a very small point. Think of developability as being able to be rolled out from a cylinder or a cone...if you can't roll it into one, you can't unroll it out flat properly either.

[daiquiri]

And at the end you have come up with something very close to "batana" fishing boats used in Istria (northern adriatic sea, in Croatia).

[lewisboats]

Not much anyone can come up with that hasn't been at least tried somewhere at some other time since man first mounted a log (to float on that is ) Nice looking boat(s). I notice the transom on the floating one is significantly smaller than the drawing...speculation?

[Russ Kaiser]

Thanks guys,

You have given me a lot to think about - I was born a measure twice, cut once kind of fellow; but I'm turning into a plan 100 times, measure twice, cut once fellow by this boat project. I could play with design software forever but you can't float a single sheet of paper.

This is what I normally make out of wood, brass, and fiberglass

[daiquiri]

Originally Posted by lewisboats

Nice looking boat(s). I notice the transom on the floating one is significantly smaller than the drawing...speculation?

Batana is a traditional boat for coastal fishing used in northern Adriatic, like I said. It is used in a pretty vast area spreading between Venice and Istria (what was once the medieval Venetian Republic), hence the small difference between versions. But in effect I can tell that nowadays every batana I have seen is made with a narrow transom, like the one in the right pic. It comes handy for mounting a small outboard and a lamp for night fishing.

Now, for the sake of "general culture" - they are characterised by a flat bottom which, as you can immagine, gets noisy when slamming against the waves. That's the reason why it is called "batana" - in traditional venetian language "bater" means "to slam", so it's name could can be best translated as "slamming girl".