Pedal Boat Design

[BG_Geno]

Hello Everyone,

I am working on a design for a pedal boat. As I am going to build it myself, and I am not a boat designer by trade, it is not as fancy as most of the designs in here. Better to be simple AND buildable.

I chose a twin hull design for the added stability over a single long thin skull or canoe design. I can handle the structure, the foam cutting and glass etc, but math is not my strongest area...I can't seem to caculate the displacement. I am including a sketchup file, and an image. I omitted the seating, propulsion, and steerage to keep it simple. I tried to import the design into FreeShip with no luck.

I can export to .dxf and will include that here as well. I am open to any and all comments. Again, can anyone tell me how to calculate the displacement for say a 300 pound total weight (includes stoker and boat)?

Length is 18 feet (with rudders, not shown) and no it wont really be that pointy =)




and here is the file in .skp and .dxf

http://geno.boxgods.com/pedal_boat_01.skp
http://geno.boxgods.com/pedal_boat_01.dxf

Thanks

[Guillermo]

I'm not sure about your question, but displacement = weight, so a 300 pounds weight is a 300 pounds displacement.
Maybe you're asking about hulls' volumes and if they are good enough to handle 300 pounds weight.
To calculate that, you should measure sections areas at several stations and then integrate volumes.
An useful calculator to that end, just posted in other thread:
http://web.nps.navy.mil/~me/tsse/Nav...e6/simpson.htm
Do it for one hull and then multiply volume times 2 and times intended water's density.

[BG_Geno]

So I make 11 evenly spaced cross sectional 'bulkheads" for the stations with one being the point of the bow (zero) and the 11th the transom or aft most station. For those cross sections do I calculate the area? What value do I use is the question I guess. What exactly do I measure?

I know a cubic foot of sea water is about 64 pounds displaced. So for 300 pounds my hulls need to displace 3.7 cubic feet. My dilema is that I do not know how to calculate the ammount of displacement at my desired water line.

its the curves in 2 directions that make it complex beyond my basic math skills.

[Guillermo]

With the calculator I provided you do not have to calculate areas. It does it for you. Just draw a line at your best guessed floatation (DWL), and then draw 7 evenly spaced waterlines from your guessed floatation to the lowest point of the hull. Measure half breath at every station and every particular waterline and enter values in the calculator (Colums are waterlines and files are stations). Then press 'Calculate Hydrostatics' It will give you a lot of information, among it the volume and displacement in salt water (inverse density: 35 ft3/ton). If that displacement is not 300 pounds you have to guess a new DWL and start again (Note it will give you displacement in tons, not pounds).

Also note: For every guessed floatation waterline you choose, FP and AP are the forward and astern extremes of such a floatation. Make LBP = length of the chosen waterline, and DWL = draught from th chosen waterline to the bottom of hull at the midship section.
Cheers.

[BG_Geno]

OK, seems complicated, but I will give it a try. I am guessing it will give me my answer as a fraction of a ton, and I just convert that to pounds?

Geno

[BG_Geno]

Underwater volume based on waterplane area integration: 2.93 cu ft
Displacement in sea water: 0.08 tons

If I understand it right, the 2.93 cu ft is how many cu ft are displaced at my projected water line. If I multiply that by 2 for the 2nd hull, and if sea water is 64 pounds per cu ft, we get the following :

2.93 x 2 x 64 = 375.04 pounds

Wow was that an ordeal. I chopped my drawing up, got my measurements, figured out where they all went on the Hydrostatic Calculator--and it said 35 tons. Then I remembered/noticed all fields were in feet, and my little boat is measured in inches (as in the numbers are all less than a foot, like 3.25" etc) so I converted them all to parts of a foot and re-entered the data.

The good news is that I am 75 pounds over the target weight for the waterline I would like (an extra 75 pounds). As my waterline is quite conservative, I can even bring my 100 pound wife along =)

A very special thanks to Guillermo for taking the time to explaine the calculator. As this is a pedal boat, do I need to be concerned with any of those other numbers it generated?

Thanks again.

[Poida]

Do we have a section here for marinizing bicycles?

[BG_Geno]

Although your humor isn't lost on me, most of the boat wil be FG and Alu. I will use the best stainless hardware I can get here--and nothing beats good old elbow grease. =)

[Guillermo]

Originally Posted by BG_Geno

As this is a pedal boat, do I need to be concerned with any of those other numbers it generated?

I don't think so, unless your wife is very interested....

[DaveB]

Hi,

I converted your dxf file into surfaces and did some very preliminary hydrostatics calculations that include the displacement and LCB over a range of drafts. I think that the LCB should be of interest as it's important in figuring out where your LCG is...

All the best,

Dave

[John Hazel]

Are you intending to incorporate some flexibility in the hull connection framework so the hulls can somewhat independely forllow waves? The rectagular geometry of the deck beams are going to do this. For a stiffer structure build a pyramid that has the seat as it's apex you could reduce the beam sizes and thereby reduce structural weight. There would be two cross beams and then four beams coming up to the seat. One from each crosbeam-hull connection.

[BG_Geno]

Originally Posted by DaveB

Hi,

I converted your dxf file into surfaces and did some very preliminary hydrostatics calculations that include the displacement and LCB over a range of drafts. I think that the LCB should be of interest as it's important in figuring out where your LCG is...

All the best,

Dave

Wow. Thank you Dave. I don't have max loaded, but I opened the Excel doc and sadly that is all greek to me. I use the "TLAR" approach to design. "That looks about right". I can build anything though, so I guess a lack of higher math skills was the trade off for smart hands in my case.

I am very serious about building this boat though, so any pointers you guys want to throw my way concerning the design are welcome. I am sourcing parts now, and doing the research on the propulsion system--pedals, gears, props etc. The people in this forum are so great that I have desided to take pictures every step of the way and keep track of parts sources etc so anyone else wanting to build one will have an easier go of it.

Thanks again

[BG_Geno]

Originally Posted by John Hazel

Are you intending to incorporate some flexibility in the hull connection framework so the hulls can somewhat independely forllow waves? The rectagular geometry of the deck beams are going to do this. For a stiffer structure build a pyramid that has the seat as it's apex you could reduce the beam sizes and thereby reduce structural weight. There would be two cross beams and then four beams coming up to the seat. One from each crosbeam-hull connection.

John, I concidered a multi-layer deck beam system with some sort of shear webbing (like the kind emplyed in an airplane wing for torsional rigidity) but ease of construction is also a factor. I confess that I am concerned the current structure of 1" square tube is not going to be rigid enough though.

One of the design goals is for the boat to easily accomodate stokers (aka the rider) of differing sizes/weights. With that in mind the entire decking can slide for and aft 12" for proper weight distribution, and the seat will also be able to slide for and aft to accomodate leg length.

My initial drawings used a structure simular to the undercarriage of your typical float plane gear, but complexity (and its associated weight and cost) creeps in. I'm posting an image of it below. Please note the port "X" brace (the diagonal load transfering member) is NOT drawn in, but would be possitioned as the starboard one is. I am wondering if that is what you had in mind? Do you think it would really make the structure that much more rigid? Square tube in that design is 1/2 inch for the under assembly, and the main seat spar is 2".




Geno

[Raggi_Thor]

Maybe it's mentioned before, but I think you should have some more (reserve) buoyanc in the ends. Imagine someone trying to stand there?

[BG_Geno]

Originally Posted by Raggi_Thor

Maybe it's mentioned before, but I think you should have some more (reserve) buoyanc in the ends. Imagine someone trying to stand there?

They would have to catch me first =)

Kidding aside, it is not designed for a passenger. The rider will be in a comfy chair. The only passengers will be any fish caught and stowed in the cooler on ice.