Ply or foam cat inspired by 3m inflatables

[Andrew Fish]

Hi everyone,

I'm a kayak fisherman getting tired of paddling into head winds and chop. I like the idea of an inflatable cat such as the takacat 3.0m (see attached) with a 6hp 4 stroke. I don't like idea of the cost, assembly time (I'd be taking it on camping trips etc and not using a boat trailer). Or the idea of a fish, hook, or spear puncturing it. I have made a ply & fiberglass kayak successfully and wondered if I could make a 3.0m ply or foam version of an inflatable. The design brief would be for it to be low cost, lightweight (roof toppable), easy-ish / quick-ish to build, strong enough for a 6hp on a choppy sea. And beach launching / punching through small surf. Unsinkable. It would need to get on a plane with just me and fishing gear. A combination of 4, 6, & 9mm marine ply and covered in 6oz fiberglass would weigh about 38kg, about the same as an inflatable the same size. The materials cost would be about 1/3 of a quality inflatable. Then I saw fiberglass covered foam which seems a better idea, I could soften the chines a bit. Attached is a mock up and the inspiration boat. I wonder if such hard chines would make it unstable to turn at speed or if it's short enough to turn easily anyway? Any idea's if this would work / handle ok? I could put a motor on my kayak but it would still be relatively slow. Thanks for any input from New Zealand

 

[rnlock]

First off, I'm not from New Zealand, and some of the following is a bit hypothetical.

I think fiberglass covered foam might be heavy, on a small boat, because the surface has to be strong enough for a certain amount of abuse, getting walked on, etc. End grain balsa core might allow a lighter laminate, because of its compressive strength, but won't be inexpensive. Plywood is probably the easiest and quickest way to go.

I have no idea about the behavior during turns at speed. LolJust how far and fast do you need to go? You might need to use two thin layers of plywood for some of those bends up front.

How did you arrive at the length? If you're car topping, something 12 or 14 feet long may be easier to put up there and take back down. Your catamaran may take a lot more work than a simpler shape. Depending on just how big "chop" is, maybe you could build something like a lightened, possibly shrunk Bolger Work Skiff.Workskiff – 15′-6″ x 4′-4″ – H.H. Payson & Company https://www.instantboats.com/product/workskiff-15-6-x-4-4/ Alternatively, if you don't need to stand up, or if you're careful about it, maybe the power version of Michalak's QT Skiff. QT Skiff Plans Printed https://duckworks.com/qt-skiff-plans-printed/ It would be a simple build and should be only 41kg according to Michalak. If you use lighter plywood and make it stitch and glue, it ought to be considerably lighter. Can you get paulownia where you live? That might help you keep things light. If you use paulownia, you might want to enlarge some of the wood sizes slightly. It wouldn't take much. You could look at his Jon Jr., too.

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A further thought: I was just imagining two long kayaks, single scull hulls, trimaran floats, etc joined to make a catamaran, with the motor and you in a platform in the middle. With the right prop, I'm thinking it might be just as fast on 3 or 4 hp as your complicated cat would be on 6. Ever watch or have a ride in a Hobie Cat on a breezy day? I doubt that a Hobie 14 flying a hull is using more than a horsepower, but it's really moving along. Maybe you could make your two hulls like the floats for the Drifter trimarans found in the free plans section of Duckworks. Those are quite simple, though you'd want to check on the buoyancy. Each piece ought to weigh less than your 38 kg, and the whole project less work. Incidentally, years ago I used to row single sculls. I am no athlete, but one time, I was being followed by someone in one of those inflatable dinghies. It was planing.
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In a less ambitious mode, it's much easier going upwind in a chop with a good rowboat than it is in a canoe or, I presume, in a kayak. You might check out some of Michalak's rowboats. I doubt any would break 3.5 knots for very long, though.

 

[Andrew Fish]

Hi rnlock, To answer your questions I usually go 5 - 20km (3- 12 miles) in my kayak and not more than 5km offshore. How fast do I want to go? Zipping around from spot to spot at 10 - 20 kph (6-12mph / 5-10knots) would be great. Point taken about the weight of fiberglass on foam. 3.0m is a bit under 10 foot, it's a popular size here and seems ideal for one person. Not so sure about the idea of a skiff in the surf, nor a dinghy for that matter, our conditions change fast and kayaks and inflatables are much safer. I'd not heard of Paulownia before, looks amazing. Also very expensive here! re using two reclaimed hulls a friend and I actually joined our kayaks with an out board between them, was quite a sight. I'm pretty sure though that a 6hp on a light planing hull will be faster than a displacement hull such as kayaks / hobies etc. Thanks very much for your suggestions.

 

[fallguy]

Hi everyone,

I'm a kayak fisherman getting tired of paddling into head winds and chop. I like the idea of an inflatable cat such as the takacat 3.0m (see attached) with a 6hp 4 stroke. I don't like idea of the cost, assembly time (I'd be taking it on camping trips etc and not using a boat trailer). Or the idea of a fish, hook, or spear puncturing it. I have made a ply & fiberglass kayak successfully and wondered if I could make a 3.0m ply or foam version of an inflatable. The design brief would be for it to be low cost, lightweight (roof toppable), easy-ish / quick-ish to build, strong enough for a 6hp on a choppy sea. And beach launching / punching through small surf. Unsinkable. It would need to get on a plane with just me and fishing gear. A combination of 4, 6, & 9mm marine ply and covered in 6oz fiberglass would weigh about 38kg, about the same as an inflatable the same size. The materials cost would be about 1/3 of a quality inflatable. Then I saw fiberglass covered foam which seems a better idea, I could soften the chines a bit. Attached is a mock up and the inspiration boat. I wonder if such hard chines would make it unstable to turn at speed or if it's short enough to turn easily anyway? Any idea's if this would work / handle ok? I could put a motor on my kayak but it would still be relatively slow. Thanks for any input from New Zealand

Getting the transom angled properly and a few other bits important.

The inside of the boat as drawn would fill with water easily. Perhaps self draining would be good, but it points to another problem.

The height of the center section needs to be 3000mm (loa) times 6% or 180mm above the waterline, not the bottom of the hulls. Otherwise, the boat bottom will be in the waves..badly.. that is midships, forward is higher still and 10% or 300mm, so your design needs to change some or it will be a cat that has problems.

 

[fallguy]

You’ll need to do a displacement calculation. Start there and establish waterline..

 

[Andrew Fish]

You’ll need to do a displacement calculation. Start there and establish waterline..

Thanks fallguy,
Hmm yes I should do some calcs, existing size shows the waterline above the floor. I'll lift the floor a bit, lengthen it to 3.2m and widen the 'pontoons' a bit and check the hydrodynamics. Does the 6% rule apply to a 3.0m / 10 foot boat? I figured if the design is not too different to the existing production inflatables I can't go too far wrong. Yes it would be self draining with a gap at the bottom of the transom. If I made it like the attached it should be quite straight forward and quite strong.