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.

 

[sailhand]

Mate i would plumb your bows a fair bit to give yourself a bit more waterline, makes a huge difference in a small boat. Here's a link to my dinghy, you can see what I mean about the plumb bow. Bridgedeck clearance will give you both self draining and a better ride in rough conditions.

 

[sailhand]

My dinghy is foam core with resin infused hulls. Weight is 47kg, just over a 100lb. You couldn't get me to use ply if it was free, including the ridiculously expensive epoxy to go with it. Dinghys get knocked around too much and damage to your coatings is inevitable and allows water in. Rainwater plus ply in warm weather equals worthless rotbox. Spend your money on 80kg cubic metre pvc foam. I use 15mm for deck and 10mm for topsides. Hulls are resin infused out of a mold in solid glass. My dinghy has served as a tender to a large sailing cat for more than a decade with zero maintenance except deck paint renewal. It doesn't even have bungs in the hulls. Never had any water in them. I use stitched fabrics, 600gm2 double bias on the deck from memory. And same in 400gm2 for all other surfaces. No tie layers in chop strand you don't need it. My dinghy gets used multiple times each day and I love my fishing. That means four to five hours each day fishing. It cops an absolute flogging and never had any problems. Dont underestimate the strength of stitched fabrics on a quality foam. You've also got inherent buoyancy courtesy of the foam. It's impossible to sink. It amazes me that people are prepared to do weeks or months of hard work using inferior products in an attempt to save a few dollars. It also needs to be questioned whether or not you actually will save a few dollars. My local store now charges 112 aus dollars for a 1.2 litre kit of west epoxy. I can buy polyester resin for 12 aud a litre. Pvc foam is much cheaper now that the Chinese make it and a good quality ply has become very expensive. You can use polyester or vinylester resin with pvc foam. My dinghy is vinylester. Good luck with your design. Great to see people have a go at doing their own thing. It's a steep learning curve and it pays to seek as much advice as possible. It also pays to learn to discriminate between knowledgeable experienced advice and lesser quality opinions. Hope to see your dinghy on YouTube soon. Cheers

 

[fallguy]

If you want to plane; you don’t want the bottom to stick or hit wavetops as it’ll act like a brake…k?

 

[Andrew Fish]

My dinghy is foam core with resin infused hulls. Weight is 47kg, just over a 100lb. You couldn't get me to use ply if it was free, including the ridiculously expensive epoxy to go with it. Dinghys get knocked around too much and damage to your coatings is inevitable and allows water in. Rainwater plus ply in warm weather equals worthless rotbox. Spend your money on 80kg cubic metre pvc foam. I use 15mm for deck and 10mm for topsides. Hulls are resin infused out of a mold in solid glass. My dinghy has served as a tender to a large sailing cat for more than a decade with zero maintenance except deck paint renewal. It doesn't even have bungs in the hulls. Never had any water in them. I use stitched fabrics, 600gm2 double bias on the deck from memory. And same in 400gm2 for all other surfaces. No tie layers in chop strand you don't need it. My dinghy gets used multiple times each day and I love my fishing. That means four to five hours each day fishing. It cops an absolute flogging and never had any problems. Dont underestimate the strength of stitched fabrics on a quality foam. You've also got inherent buoyancy courtesy of the foam. It's impossible to sink. It amazes me that people are prepared to do weeks or months of hard work using inferior products in an attempt to save a few dollars. It also needs to be questioned whether or not you actually will save a few dollars. My local store now charges 112 aus dollars for a 1.2 litre kit of west epoxy. I can buy polyester resin for 12 aud a litre. Pvc foam is much cheaper now that the Chinese make it and a good quality ply has become very expensive. You can use polyester or vinylester resin with pvc foam. My dinghy is vinylester. Good luck with your design. Great to see people have a go at doing their own thing. It's a steep learning curve and it pays to seek as much advice as possible. It also pays to learn to discriminate between knowledgeable experienced advice and lesser quality opinions. Hope to see your dinghy on YouTube soon. Cheers

Hi Sailhand, that's exactly the info I need, thanks so much. Your vid & boat are awesome, love the Auzzy humour! I'm going to make the hulls a bit wider and taller, raise the bridge-deck, and plumb the bows as you suggest. 15mm 80kg PVC is $250.00 NZD per sheet here, 6mm marine glued Okoume is $73.99 per sheet. What do you pay for PVC foam over there??

 

[Andrew Fish]

If you want to plane; you don’t want the bottom to stick or hit wavetops as it’ll act like a brake…k?

Hi fallguy, yes I will raise the bridge deck to keep out of the water as much as possible

 

[sailhand]

We pay about the same as you for both ply and pvc foam. If you've got a friendly local boatbuilder that likes cash you will get it cheaper. Same goes with resin and glass. Boatbuilders usually buy 200 litre drums and will decant resin and acetone for you again only if they like cash. Just for reference my hulls are 330mm at their widest. My hulls are more displacement oriented than planing and they have slight rocker. This gives far better performance with smaller outboards as seen in my video, it also limits your speed somewhat. 15.2 knots is the maximum speed in my dinghy even with 15hp. And it does 12 with a 3hp with a modified prop. Local laws allow for no rego no licence no safety gear in boats with 4hp or less. Also in nazi Queensland we are not allowed to use tenders for fishing crabbing diving SIGHTSEEING etc. Yeah sightseeing go figure. Also you are limited to two nautical miles from your mothership. 4hp you can go to Vanuatu and even sightsee on the way if you are crazy enough, 5hp two miles that's it
I have various resistance calculators and use them as a poor substitute for tank testing. It's been over a decade since I drew my dinghy so my programs are well and truly out of date. I still use delftship pro version for design work. There's a free version but I don't think it does any hydrodynamic or resistance calculations. It's pretty cheap to buy the professional version and it also has a plate development function which I've found to be pretty accurate. Highly recommend it. It was developed by the Naval architecture department of delft university I believe. Unlike many other software programs I've used this as pretty much a one stop shop for smaller vessels. You can design your hull and make it more efficient on the computer. If your doing a straight planing hull without rocker then it's not so relevant. No rocker and it's kinda down to hp equals speed but only if you get on the plane. Extending hulls beyond the engine helps dramatically when using low horsepower and helps you plane at lower speeds. It also helps avoid getting pooped. I don't mean soiling your whitey mighties. I mean waves coming over the transom. Although I suppose one generally leads to the other. I'll let you figure out the order!!!
I imagine you will be doing foam cored hulls for your dinghy. The wider the span the thicker the core as a general rule. Also thicker keel panels can give you a semi self draining floor, ie you can leave the bungs out of your empty dinghy and rain etc will not fill your dinghy if left moored and unattended. You would need to put your bungs in it when loaded though. Measure your spans across the shortest axis and go from there. In a foam cored dinghy of this size you can get away without bulkheads, I have none in my dinghy. Actually not true, l have a small partial bulkhead above the tube for my wheels about 70mm high.
Try to be really frugal on your layup with as little resin as possible. Waste is a thing in boatbuilding. I know guys that will pour excess resin onto the laminates they just layed up to avoid waste. It definitely does not do anything for strength it just makes your dinghy heavier. I use unwaxed resin for ease and speed of build.Ii also have a pad of solid glass made from leftover resin and glass offcuts. I build this up to various thickness for different purposes, mainly solid points for winches lift points deck fittings etc. I cut various sizes from thecfinished pads as required. This way i have little to no wasted resin or glass, works better with unwaxed as you have no preparation in between. Just keep adding to the pad.
I would cut out your panels and glass the inside only. This will allow you to bend your panels into shape. I make sure I stay 25mm away from all edges when I do this. That way when i radius external edges i am not sanding any glass, just foam. I dont like a shop full of itchy dust and my lungs dont like it either. I then glue and screw my panels together with a single pack polyurethane waterproof adhesive. I like fixtech products and I think they are available in kiwi, use the one with silicone like consistency and experiment with some offcuts. Too much glue and you get foam blowing out everywhere and its messy to clean up. You want a single small bead in the centre of your join and minimum "foam out". I then remove the screws and radius the edges on the outside and glass in one hit. No taping the outsides as it adds weight. If you want to add weight add glass to the underside of your hull to about an inch above the turn of your bilge for added abrasion and impact resistance, if doing wider hulls you might want to use 600gm2 top and bottom on your keel panel. Personally i would have flat keel panels no more then 350 wide that is veed at the font for maybe 600mm. This aids dramatically with hydrodynamic lift and gives you flat floors to walk on. I have found no impact to ride comfort whatsoever but my dinghy has rocker so it doesn't get airborne, it kinda sucks to the water and rides super smooth. I have wheels that are easier to fit in 6 inches of water so my keels rarely touch anything as I stop short of the beach and put my wheels in and vice versa.
When you plumb your bows Remember to curve the bow to slightly above the waterline. This allows for floating debris that you will inevitably hit. You want to slide up and over it, if your plumb bows extend below the waterline you are likely to have a head on impact for want of a better description. Far better to go up and over than come to a dead stop. Think of it as airbags for your dinghy. Remember every piece of fixed structure you add is weight. My dinghy works as supply barge, people ferry, dive boat and fishing dinghy. It also works as beach trolley for transporting anchors chain ropes shovels etc over sand flats where I store my boat long-term. With the balloon wheels on the centre of gravity I can transport heavy loads over mud and sand on our coastal fringes.
With fixed furniture in smaller dinghys you not only add weight but restrict versatility. You cant move a single seat to trim your dinghy so you put three seats in, in case you have more people. I use plastic stepstools that stack. They weigh less than a kilo and hold 120kg and take up next to no room, they can also be positioned anywhere to trim the boat. I use tredgrip for my deck paint. Its probably no good though, its made in kiwi !!! For the skipper seat which transfers to the admiral when she is on board i use a milk crate with a padded seat. All this means i can configure my dinghy for whatever i am doing without restriction or added weight. Attachments or tie down points are spread around the dinghy and things like my anchor and rode are hung on the topsides in a mesh bag its quieter and easily removable when not needed not to mention light.
I can also launch and park it anywhere. I even had a parking inspector try and write me a ticket for chaining my boat to a no parking sign in pittwater in Sydney. I think he got sick of me laughing at him and walked off in a huff. I have 2pac polyurethane on the keel of my dinghy and it's still as good as new. I have aluminium pads under the rear of the keel panel about 150mm long. These sit in a recess so they are flush with the keel and are attached with sikaflex. When using the wheels these are the only point of the dinghy that contacts the ground. They slide on concrete gravel bitumen etc and look a little worse for wear but still totally functional.
Anyway flick me an email at apc.tender@gmail.com if you have any queries or questions, happy to help.
Cheers

 

[gonzo]

You can build two pontoons with 3 and 4 mm cheap plywood, with polyester and mat over the seams. With a couple of coats of oil base paint it will last at least 3 years. By then you will have improvements on your mind and can build a new one. There is no need to use expensive materials and build it to have the finish of fine furniture. I assure you the fish don't care.

 

[Andrew Fish]

Sailhand, I've plumbed the bows as you suggested which also softens the angle of the bow. I'll use my fishing box as a seat, just the right height and like you said I can move forward / back to trim. I used mitchlet to optimize my kayak design but since this will be a planing design, and very close to existing proven designs, the only hydrodynamic I'm considering at this stage is design draft and flotation should one hull fail at sea. The draft on the attached modified design is only about 90mm so there's now plenty of space under the bridge-deck. I'm still undecided about materials, as gonzo said above I can potentially make it at low cost as a bit of an experiment. I do get you point about all the time invested only to use cheap materials that won't last and are heavier but the cost of marine pvc foam for this would cost over $1000. A cheap inflatable the same size is $1200. I think the beauty of my design is it's very simple, all surfaces are easily developable and the curves are all simple arcs making lofting a piece of cake. It should also be quite strong as the bridge deck extends to the outside of the hulls, so the hulls are supported. I'll do more costings and see how various materials stack up cost and weight-wise. Thanks so much for all your information.

 

[Andrew Fish]

You can build two pontoons with 3 and 4 mm cheap plywood, with polyester and mat over the seams. With a couple of coats of oil base paint it will last at least 3 years. By then you will have improvements on your mind and can build a new one. There is no need to use expensive materials and build it to have the finish of fine furniture. I assure you the fish don't care.

Good point gonzo, I'm sure I'll want to refine the design after a while. I made a camper a few years ago and will be making a better one using all that build knowledge and use experience to improve the design. In fact I wish I had made the original a lot simpler for that reason.