A pedal powered catamaran with sail assistance

I've done it! I've splashed my new boat. All seemed well except for a lack of propulsive force. I need much wider paddles and have planned the changes I need to make.
Splash! https://mountainbiker.online/2023/04/26/splash/

 

Congratulations Andrew!

 

I read your summary with interest. A few obvious points

No Rocker. Your lack of "slope" of the keel up to the bow (flat bottom) , and the stern, is going to make turning hard, and paddling harder than it needs to be as well.
The same design flaw is also going to make the craft slower than it needs to be. A new section in the middle (longer hull) would improve things.
The angular shape at the bow and stern is also helping slow the craft down, and make paddling harder.
Bigger paddles may help a bit, but I predict you will get a lot of hard exercise while still suffering slow speed.

 

Too bad you ignored this post (above)...
What are your performance expectation of your second build?

 

Andrew, your "hull speed" is ~3.5 knots, but 2 - 3 kts in reality.
And with a self described less efficient drive system...

Don't oversize your sail area as your reserve buoyancy is small.
How much will your sheeted, standing rigging weigh, dry?
And in the rain?

 

My shorter dinghy had a completely flat transom and no rocker but could still cruise at 2.9 mph from a hull speed of 4.16 mph. What I noticed with the catamaran is that if I stopped pedaling it carried on, losing speed very slowly. I've already thought of a long middle section for touring duties and more speed. Part of the reason for the boat is exercise as well as the fascination of building. With 360 litres of displacement it felt very stable, which pleased my. Standing up presented no problem so if I fished it would be good for that. My aim is really 3 mph at a less exhausting effort than the dinghy required. I want to be better able to cruise for over 5 miles in a session.

 

Good for you Andrew, literally.
You'll get lots of exercise with your new boat.
Enjoy

 

Thanks for the report!

My guess is there are a lot of forces with interplay / tradeoff on this kind of design. There may be something like a Terminal Velocity effect because of the shape, weight, and limit to paddle area compared to a traditional water wheel. The effective gear ratio of the oscillating paddles may be too low and with very few options for increasing it, but increasing cadence might help with speed. Longer Hulls don't help if TV is in play where skin friction limits top speed compared to wave making, from what I understand. Sails might be able to apply more Force and get up to higher speeds.

But I think if anyone can solve some of these issues it's going to be you for sure, ha!

 

I've just calculated that the wetted area of my catamaran hulls is around 10% greater than that of my dinghy, which could reach hull speed and cruise at 70 % of that. What I failed to mention in my report was that spinning the pedals on water took almost no more effort than on land. I'm just not getting the power into the water. I think the only energy that did reach the water splashed my crotch!
I've decided to return to a known quantity and replace the paddles with a paddle wheel. I'll report back with data from Strava when I've made the changes.

 

Its not just wetted area, its also fluid resistance and turbulence. I bet your dinghy is a lot more efficiently shaped.
We are probably talking 30-40% lower efficiency at close to maximum speed.
I've never done it myself, but perhaps you can arrange to test the loaded dinghy and the paddleboat hulls in turn, by attaching a rope, with a weight to pull it through the water, and time the travel of each craft, you could quantify the actual difference.

 

I love the propulsion system on the plywood canoe. I have an aluminium hardtail mountain bike frame behind my shed with a cracked headtube that I was going to do something similar with. Except I was planning to use the bevel gearbox of a big angle grinder to turn the power 90 degrees to turn a conventional shaft and screw propeller or do a flapping tail propulsion system (think diving flippers without the diver!). But same idea of cutting the rear triangle off a bike and using cranks, bottom bracket gearing and rear hub. I would cut the cantilever bosses off though!

Your paddle drive system is one of the best systems for weeded waterways I have ever seen! Doubtless will lose out to an efficient screw propeller though on overall flatwater efficiency, but for thickly weeded waterways it would destroy Hobie Mirage drive, screw propeller. All of them!

A few things strike me about the propulsion system. Firstly, my understanding is that the plastic paddle is a foil and its leading edge is its far end. Have you considered building up the forward side of the paddles into a curved surface, like the upper surface of an aircraft wing? I'd also go for a SuP blade shape, so the widest part of the blade is first part to enter the water. And with a, say, 10 degree forward cant to the blade, like a SuP paddle so that it's creating lift as it's entering the water.

I'm afraid you need to bin the aluminium catamaran hulls, or recycle them into something else. Those hulls will only be very slow and inefficient. Water is very thick when moving and does not like to be made to go round corners.

When people scrap old sailing dinghy catamaran hulls for the many hundreds of pounds worth of rigging, blocks etc, they get left with the hulls to get rid of. Nobody wants them, they are a landfill only nightmare. Surely upcycling unwanted hulls would meet your project aims perfectly?

They come up periodically on Facebook Marketplace and similar, usually free to collector, or nearly free. They are efficient, curved and beautifully boat shaped!

There is a Dart 18 in a yard that was cleared of all its boats except this one 2 minutes walk from my house. The owner, presumably died or moved. It has not moved for 3 years. That's the sort of thing you want, just something a bit shorter, without moulded in skegs, and that's not locked in a pound owned by a Spanish aerospace company.

I'd say definitely stick to your walking paddles propulsion system, which is brilliant, and don't waste time building hulls when there's probably 100 of them for free or nearly free rotting in gardens and the classified ads less than an hour from your home!

 

Ps. When I say 'rotting' in gardens, I mean it metaphorically. That's the whole problem with them. They aren't rotting!

 

Thinking back about the walking paddles propulsion, it's really piqued my interest.

Thinking back to David Attenborough documentaries of my youth, don't certain birds (big, heavy waterfowl) sort of run along the water when taking off?

Got to be a parallel there. Worth getting some close up video of what these birds are doing with their feet on takeoff, it will have evolved over milenea. Maybe they close the webbed foot to pull out it out? It's the extraction I think where you have the biggest inefficiency. Maybe a marine propulsor could do what the birds do on blade/ foot extraction?

Then, of course you've got the whole powerful biomimicry marketing angle at your disposal! Investors love a biomimicry inspired marine propulsor!

 

There is not enough room between my hulls to make the paddles sufficiently large so the first system has had to go. Instead I'm building a front mounted paddle wheel since my rear mounted wheel worked so well on my dinghy. The hulls don't have the flat transom of the dinghy so slip through the water far more easily. I also need to be able to get the boat half a mile to the water so need very light weight and compactness, as mentioned in my SOR. I should be refitted in 2 weeks time for a test.

 

Fair enough, but don't forget that the average dinghy is designed to have most of the flat transom out of the water when rowing or sailing.
Your sharp edges are going to be interrupting water flow at any depth.