kayak catamaran hull speed?

[kayaker50]

I am thinking about building a catamaran kayak, to be paddled with a double bladed paddle. I hope to get nearly the speed of a kayak, with the stability of a multi-hull. But I am wondering if the bow waves from the hulls would interfere with each other, effectively reducing the hull speed. Any opinions? Thanks, Paul Hubert.

[Guest625101138]

The lowest drag twin hull vessel will have 40% more drag than the lowest drag single hull. For a constant input power this difference translates to a speed difference of 11%. This is the reason racing kayaks are single hulls. They put up with the instability to gain that 11% in speed.

With a catamaran there is an optimum spacing for a given speed but it is of minor significance.

If you want stability then two stabilising outriggers on a long slender hull is the least drag option. The outrigger hulls are not loaded unless the boat rolls off upright. The central hull is roughly the proportions of a lightweight rowing scull - say 7.5m long and 230mm WL beam.

If the 7.5m is too long then you are making a compromise and the catamaran style boat starts to be less disadvantaged.

Rick W

[peterAustralia]

hi

something like this

http://www.triaksports.com/

there is also something called Trilars, that is a plywood vessel, 2 outriggers, by Jim Michalak

[Guest625101138]

Originally Posted by peterAustralia

hi

something like this

http://www.triaksports.com/

there is also something called Trilars, that is a plywood vessel, 2 outriggers, by Jim Michalak

That is close to what I was describing.

However for fastest paddling I would have a sit-on that looked like these:
http://www.adventuresofgreg.com/HPB/...158-720160.JPG
http://www.boatdesign.net/gallery/sh...//ppuser/18624
But have a foot well so seating upright is comfortable.

If you want to go really fast then use pedal power.

Rick W

[Fanie]

They put up with the instability to gain...

which is one thing about boats. Any time you gain something somewhere you lose something else elsewhere. Thes compromises is what you will have to decide about, what goes and what is kept.

Rick has done his time on those things, so his advice is good to go.

[Guest625101138]

Originally Posted by Fanie

which is one thing about boats. Any time you gain something somewhere you lose something else elsewhere. Thes compromises is what you will have to decide about, what goes and what is kept.

....

Fanie
Compromise is the mentally lazy approach. I prefer to optimise. It means I am on a continuing path to find the best solution. Every now and then I get my hands dirty and actually try something but that is just the next landmark on the journey.

I do not like to compromise. The objective is to achieve the absolute best within defined constraints - optimisation.

I designed Greg's CP2 with this concept in my mind:
http://www.recumbents.com/WISIL/misc/nocom.htm
A 50+yo friend of mine just completed an 11 day unassisted crossing of Australia from Perth to Sydney (4000km) on a machine almost identical to the NoCom - who needs a car!

Rick W

[Fanie]

Hi Rick,

I don't like compromises either. I have drawed my own hull over so many times to achieve better results in one aspect only to give some up elsewhere, so it seems I ride see saw (is it pitchpole ?) with it. I wish I could ask someone like you to check it over for me

I like to be on the practical side as well, however the bigger hulls are somewhat expensive to just try.

What has 50+yo have to do with anything ?? Always thought the polish are lazy. Never thought I'd see someone lie down to paddle
My dad would have hated the thing, he always told me to sit up

[kayaker50]

Thank you all for your comments. My boat will be about 16 ft long (two pieces of plywood spliced together). I'm not too worried about drag because I can overcome it with brute force, but if the hull speed if affected I would not be able to overcome that. Thanks again, Paul Hubert.

[Guest625101138]

At a length constraint of 16ft it is line ball what has the lowest drag for a given spped. It depends on your power output. A catamaran will be better at higher power output.

Narrow hulls just keep going faster as power is increased. The required power is a function of speed cubed. There is no hard limit.

Rick W

[Squidly-Diddly]

in spite of any vague claims about "speed".

Also note that paddle length is determined by beam of the boat, not the size of the paddlers biceps.

[Ad Hoc]

"...But I am wondering if the bow waves from the hulls would interfere with each other, effectively reducing the hull speed..."

You will get interference, all multihulls have it to a less or greater degree, which can either slightly increase the resistance, or with carefully positioned hulls can actually cancel each other (theoretically - but in reality, this rarely occurs as the separation distance is beyond the 'norms'). But again, depending upon how the boats form is configured, if done correctly can actually lower the resistance with 2 hulls compared to 1. Which is also related to the displacement and the speed one is going at, and so on...

In the design of 'real' boats, by that i mean ones with engines (or large sails), carry a payload (or a crew if sailing) and/or are used for earning money, all these factors are important. And worth understanding, the main one being length displacement ratio. This is the one to really understand.

But, in your application the minor changes in % of resistance wont affect you too much and as such, unless you're going for world records, not really worth worrying about.

But if you say ".. I'm not too worried about drag because I can overcome it with brute force,.." there is a limit to how much brute force you can provide, especially in a head sea and wind condition!

Every design has an ideal location/position for its application whatever it may be. Unless you can define more than just the length, ie the mass and other aspects of the 'design', it is all just a discussion based upon thoughts. Which becomes a circular augment.

You really need to put your thoughts down on paper, draw it up...and then that forms the basis for your 'design'. Without doing this, you wont be able to tailor the design to your exacts needs. No two boats are the same...

[ancient kayaker]

A lot of sit-on-top kayaks have hulls with tunnels, virtually very narrow catamarans, presumably so they are stable enough for the paddler to go for swim and climb back on top. They are not known for performance. I borrowed one, it was already at water's side and I was too lazy to launch my own canoe, as I wanted to catch up with someone who had paddled off 10 minutes before. It was so painfully slow it was quicker to go back and get my boat from the cottage porch.

With a proper, two hull catamaran I would be concerned about the weight of the bridge and the length of the paddle I would need to clear the hulls.

[griff10]

You will run into a few large problems with this concept. One to get stability the hulls need to be fairly far apart and this compromises paddle stroke efficiency. You can't add power if you can't maintain an efficient stroke. If you narrow the overall beam to prevent the above you'll sit up too high and you won't have any stability. Kayaks work with their fairly narrow width because the paddler sits below the waterline.

Building it in plywood, unless it's S&G or compounded will result in a very heavy boat and weight works against speed. Top racing kayaks or skis are extremely light weight, often in the 20 lb. range for 20' boat or less. Darned near impossible to achieve that with plywood.

Bill H.

[peterAustralia]

dont do it

I see you are looking at a kinda tunnel hull concept.


Your best bet is one long hull, and then some smaller hulls that just touch the water for stability. With a double paddle you need a bit of space to use your paddle. So you can have the crossbeams very far aft, as per triyak, or whatever it is called (see my earlier post) or you can have the outriggers more centrally, but with the crossbeams far forward and far aft. If you go the method of 2 crossbeams and you are putting your paddle between them, think you will need almost 8ft of separation. 9ft is probably better.

Tunnel hull, not good, extra drag, probably more drag than a conventional kayak.

I built a narrow outrigger paddling canoe, had an 8ft outrigger, went OK, 8ft is about the minimum I would consider for an outrigger for a paddling craft. Mine was a deep Vee main hull, in retrospect something a bit wider and shallower would have been more stable, even taking into account the outrigger. Having an ultra narrow main hull with an outrigger did not work well for me. (my main hull was 40cm at top, about 30cm at waterline) maybe others have had more success.

Have you seen the fiberglass outrigger paddling canoes, why not just copy them.

[kayaker50]

Thanks again for the comments. I should have mentioned my intended use for the boat is fishing and hunting on local lakes (Shearon-Harris and Jordan, near Raleigh, N.C.). I own a couple of kayaks, a canoe, and a homemade dory, and was looking for something that had the dories speed with more stability.
Here is a link to a company making 10 foot cat-kayaks: http://www.wavewalk.com/ The video of their boats is what prompted my question- they look pretty slow. Their website also mentions the need for extra long paddles.
Griff10, what building material would you suggest? I need a pretty tough boat. Thanks again, Paul Hubert.