Just how bad is this idea... The Peanut Galley

Let me acknowledge up front....

This is a bad idea.

I know this is a bad idea.

My question is, How bad of an idea is it?

So, a little back story. My girlfriend is a large human being, about 420 lbs. She would like to get into kayaking, but the only boat I have that fits her is my 17' 70 lb cruising yak, and even that is a bit tight. I will probably build a 'yak for her next winter and I'm thinking about designs now.

One could take the basic kayak hullform and expand it, but that would make it so beamy that it would be hard to get a good paddle stroke, though a 240 cm paddle helps with this. One could add length to increase displacement without increasing beam, but that is going to come with penalties in both maneuverability and stability.

My thought is something like this...

https://www.boatdesign.net/attachme...2/?temp_hash=2f713c3a4701b02e8b37caf60f0b0ddc

A peanut shaped boat of about 12-14 foot in length which has it's sides tight up against the cockpit ring but somewhat wider sections fore and aft of the cockpit. Beam at the cockpit would be about 26" and maximum beam at the bulges about 30". Construction would be cedar-strip, which tolerates curves well. This way the paddler would have more room for a higher angle stroke, but also have more primary stability and buoyancy. The trade-off I see is mostly one of drag, but this was never going to be a fast boat.

The name comes from the fact that it is a peanut shaped rowing vessel, hence a peanut galley.

 

I don't understand the need for the special shape. A larger person will naturally need a wider boat, which will have more buoyancy in proportion. If you are trying to reduce the draft, that's a reason, but the paddle stroke of a kayak in naturally an outward sweep. The inward sweep is called a 'C' stroke, and has its purposes, but it is more awkward.

Assuming flat water touring is your intended use, I would recommend a flat bottomed, hard chined design for maximum flotation and stability. Don't give her a lot of taper, make her sides fairly straight and the ends rounded instead of pointed. To make it a little easier on the knuckles, a slight tumble-home rise to the cockpit from the chine should be good for both more flotation and stability and to keep the knuckle scraping edges of the cockpit close in.
I'm thinking a shape similar to this

A flat bottom puts the maximum displacement at the full width of the boat.

My wife is a bigger woman and she loves her plastic Necky Rip touring kayak.

-Will (Dragonfly)

 

This is just a thought but my wife and I together must weigh well over 350lb and a tandem kayak supports us easily.

The same hull with a single cockpit would presumably support a single individual of comparable weight? Could you derive buoyancy from length without adding excessive beam? And in the process take advantage of existing designs?

 

How bad an idea it is, is relative, but it's bad.

A "better" idea would be just to customise a double kayak
into a single by making a seat in the centre.

 

You will not use the boat as drawn. All the weight will be in the back and drag will increase through the peanut. It will be really hard to paddle. The front of the peanut will drag on entry and exit, and the back of the peanut same.

Heavy loads are best deal with flat, wide bottoms. A skeg is needed for tracking.

Reconsider the shape and consider ways to make the paddle more fun/easier.

 

Maybe oars instead of paddles. Double ender facing each other and take turns rowing.

 

"240 cm paddle"

The paddle length is too long. Consider a paddle in the 220-230 cm range. I paddle an Olympic class kayak using a 222 cm paddle. If the paddle were longer, I would have to wait for it to submerge to the correct profile angle (45 degrees) to reduce the pitching moment during the stroke. A longer paddle will also take more time to exit that a shorter one. I've used a 220 cm paddle in the stern of a recreational 2-person kayak and the length was just fine. Epic Kayak makes paddles with a 10 cm adjustment so you will be able to find the correct length easily.

 

So, she's never been kayaking, right?
Before you build a boat for her,
why not try her in a rowboat?

 

I would think stability plus easy in and out would be important considerations; these are difficult to achieve with the traditional kayak shape. Maybe an efficient rowboat?

 

Just get the girl a SOT fishing kayak with a 550lbs rating. If you google for it you actually find recommendations regarding big/heavy persons and kayaking.

If you still want to build something you are going to have to make her a wide boat, over 30 inches in the cockpit, otherwise she won't be able to get in or out. Look at classic folding kayak designs for inspiration.

 

Before you can design a suitable boat for your GF, you must carefully assess her physical ability. Would she be sufficiently nimble to deal with the delicate behavior of a kayak? If you have quoted an an actual weight for her then kayaking is not the way to go. A row boat of suitable dimensions is far more practical and potentially safer.

Yes the peanut shape is a bad idea. Water does not like to flow gracefully around such a shape. To build a kayak that would be appropriate for a very large person, the boat must be more substantially built than a conventional kayak would be. The weight of such a boat might approach 80 to 90 pounds. Very well then we need a boat that is designed for at least 500 pounds of displacement. For starters lets consider a 16 foot kayak. Then how big will its' sections need to be? A little bit of math will reveal that the immersed mid section of the boat must have a considerable area. For our first draft, back of the envelope drawing, Let the DWL be 180 inches LOA will be 192 inches To give us 500 pounds of displacement in freshwater we will need .....500/ 0.03611 = 13, 846 cubic inches of displaced water..........We already know that this boat is going to have full sections..........so lets pretend that it has a prismatic coefficient of 0.55.
13846/180 (dwl) = 76.9 inches square.........but that is only the required average area for the sections. What is the size of the mid section then?.....We have to use the Cp to make our swag estimate. so>>>> 76.9/ 0.55 = 139.9 inches square.

Now we know that the mid or largest section of the boat must have in round numbers...140square inches of immersion. For that very large person I would suspect that 36 in chine width would be about minimum for any sort of comfort. So we have a 36 inch bottom. plumb sides and the draft will be 140/36 = 3.88 inches........not bad draft for a kayak. Try it some other way....say 32 inch width.....Then 140/32 = 4.37 draft................. etcetera.....

The stability of the boat will be important and the difference between a 36 inch chine and a 32 inch chine is pretty large. Go for the wider boat. the 36 inch one will have something like 25% more initial stability. Too bad. A kayak of that width would not work well with a paddle of reasonable length. Give up the kayak idea. Go for a good sized rowboat.

I suspect that CTHippo is merely messing with us about a prospective kayaker of such substantial proportions.

 

or it could be him; 420 is a very large man

I happen to have a Gilpatrick Laker canoe that is 36" or so; perhaps 32". Paddling her is hard. I am 220. Initial stability is superb.

A kayak. No.

 

I seriously doubt anyone who weighs 420 lbs could/would/should be involved in anything as active as paddling a kayak.

 

Youtube has it all, here is a 15 year old, first time on a kayak, confessing he weighs 420lbs.

 

And he wished it was more stable ... that's an easy fix: remove the frame seat so he can sit lower in it.