Boat Design

Izzy46 · 14 March 2025, 08:25

About 6 months ago I purchased a 430 inflatable soft floor catamaran from Aliexpress along with a 18HP Hangkia two stroke. The intention was to explore the backwaters and side rivers of Saigon with my family. Despite having some amazing adventures I’ve always been a little disappointed with how the boat handles, in particular, I’ve been disappointed with the constant ventilation and cavitation at various speeds.

The boat is based on the truekit design and has a very large and thin floor.

The biggest problem with this boat is that, even at maximum floor inflation (15 psi) it has a huge bulge like a pregnant whale which moves up and down bow to stern, “giving birth” out the back in the form of ventilation and loss of speed. I’ve tried different foils and pressure settings but I can’t get rid of the problem.

The boat also has a solid transom which doesn’t allow water to wash out the back of the boat. As the day goes on we end up with water sloshing around the boat.

In search of a better design
I’m intending to find a better boat design and I’m hoping for some advice to steer me in the right direction. Firstly, it’s probably best to outline my requirements.

Must deflate and fold away into a laundry sized area (I live in an apartment in the city)
Must have enough room for 4 adults (430 is fine, I expect a 380 would also do)
Stable and agile in rivers (ocean stability is also a plus)
Be able to expel water from heavy rains and splashes
Maximum speed and efficiency (catamarans seem to have the edge here…?)

I’m considering 2 different designs.

380 Takacat style catamaran

Have thicker narrower floor which should not flex as much causing less ventilation / cavitation
Floor looks like it’s out of the water causing less drag
Shorter length meaning less flex (also easier to transport and store)
Open back for easy expulsion of water

Soft floor or hard floor rib around 380 in length

More overall rigidity
Better turning and handling…?
Cheaper
Do these boats have more drag? (less efficient)
How is the water expelled out the back?
Does the hard floor fold up well?

From my research it seems I have to sacrifice boat rigidity for ease of assembly, transport and storage. I am happy to sacrifice a little but at this point my current boat is not really functional. The Hangkai outboard is great but I’ve never managed to get above ¾ throttle without cavitation/ventilation. It planes easily but the top speed is under 30 km/h.

So it seems I've got two options; a standard SIB or a Takacat style Cat with a firmer floor. Any advice or suggestions would be much appreciated. 🙂

Fenlander · 14 March 2025, 11:21

Hi Izzy and welcome to the forum. That's a very well framed first question which highlights the dilemma many of us face getting just the right boat and biasing any compromise in the right direction for a particular use.

Regarding use how many family members is usual, always 4 or sometimes less? Do you carry any heavy kit apart from fuel and normal items for a day out?

We have a member on here called Chipko who's owned/used probably the greatest selection of conventional and cat type SIBs and tested them all extensively. I'm sure he will post before too long.

From a personal perspective over 15yrs of sibbing has caused us to settle to a 380 air floor with conventional transom and closed bow. This is for leisure days out mostly on the sea or an estuary. We are often two adults plus dog then sometimes with an additional light adult. On the odd occasion we've had four adults the 380 feels quite full and restricts the way you can use the boat at sea. Having said that an air floor 380 is the sweet spot for handling, setting up and storing in smaller spaces. I would be wary of a conventional style model over 4m and with alloy floor as they are really suited to a trailer, having said that they'd not have any ventilation issues and would be super roomy. That's what I mean about compromise.

chipko · 14 March 2025, 14:27

Hi Izzy and welcome.

Sorry to hear about floor deformation issues on your sibcat, and to be honest up until last year never would have thought this would be a problem on a cat/tunnel hull sib. A common issue though on conventional mono hull air floor boats over 3.6m, creating what in effect is a hooked hull. Over inflating the hp floor to 1.0bar (max 0.8 manufacturer) cured this on our Honwave T38.

Not familiar with the make of your boat but at 4.3m long and heavily loaded, then not too surprised that the thin depth hp floor is struggling. We’ve run/tested/piloted more than a few cats from 3 to 3.8m and only one, a budget Kolibri 380 SeaCat, ever experienced any hull deformation. This only occurred rarely on glass smooth water at speeds over 18 knots and quite mild, but the air cushion was lost and the rear of the hull sucked down with the air floor felt mildly bulging near the transom. Coming off the plane and accelerating again would break the cycle. The main difference between this and other cats we’ve had was the lower height of the deck above underside of sponsons…100mm as opposed to the more common 120 to 150mm. This was probably the main culprit.

If we’d kept this boat then I would have tried adding a full width piece of 9mm plywood under the hp floor near to the transom.
This may be worth trying on yours.
Probably doesn’t need to be full length or in one section but will need to be full width and start tight up to the transom. Wear will be an issue so round off and feather all edges and cover said edges with say duck tape. Also sand/grit will further wear tubes/floor so clean out regularly.

I doubt a 3.8 Takacat or 4.0 TrueKit Discovery would exhibit these problems but heavily loaded with four adults and 18hp is a big ask to maintain planing speeds and air cushion.

Although not tried one, my favoured cat would be the 4m TrueKit Discovery, ticks all the right boxes for me, just a bit pricey to get one over here in the UK by the time you’ve added import duties, taxes and shipping.

Izzy46 · 18 March 2025, 08:30

Thanks for all your suggestions.

I guess there is sweet spot when it comes to Sicats...? It seems the longer you go the less you can actually weigh the boat down.

Usually we are 4 people (90, 65, 45 and 30kg), a tank of fuel and some food.

What kind of outboard are you running Fenlander?

I'm curious to see the underside of the hull and measure the distance between the underside of the deck and the sponsons. This distance is measured right up agains the transom?

I think I've got some 9mm ply in the workshop to cut and bevel into the right shape. Maybe something like this to seal it?

Chipko you mentioned: "If we’d kept this boat then I would have tried adding a full width piece of 9mm plywood under the hp floor near to the transom."

Would you put the wood between the floor and the sponsons or underneath the floor and above the material holding the sponsons together?

Fenlander · 18 March 2025, 11:28

>>>What kind of outboard are you running Fenlander?

As I'm an older guy weight is crucial so I run a Yamaha 15 2-stroke which is fine for our 43kg Bombard, the two of us, medium size dog and day out kit. When we were more often four folks we had a 20hp but that was a bit too heavy to move about off the boat so once the kids no longer came out with us I sold it. We also have a Tohatsu 9.8 2-stroke which is a bit marginal for power normally but so light well worth keeping for the days we use the river or if I'm on my own it's fine at sea.

chipko · 18 March 2025, 15:16

Quote:

Originally Posted by Izzy46

Chipko you mentioned: "If we’d kept this boat then I would have tried adding a full width piece of 9mm plywood under the hp floor near to the transom."

Would you put the wood between the floor and the sponsons or underneath the floor and above the material holding the sponsons together?

Yes, below the high pressure air deck and above the pvc fabric hull holding the sponsons together.

Start at the back tight up to transom, one may be enough to cure problem, if not add more. Make boards full width and as long as practical for handling/storage.

Izzy46 · 04 April 2025, 02:46

So I took the boat out last weekend with a new hydrofoil and it handles the best it ever has. While it still has the giant hump it performed without severe ventilation. On the next trip I will add the plywood, hopefully improving the performance further.

I also measure the distance from the bottom of the deck to the floor while the boat was up on the inflated sponsons. It seems to be only 30mm. Would this be the main reason for the boat's poor performance....?

https://photos.app.goo.gl/7ZzWz5stmdzjDHW98

Fenlander · 04 April 2025, 08:48

It certainly seems at odds with a cat design to have the floor set almost as low as the tubes, does it continue like this forward or is there a much greater clearance towards the bow?

In some ways it looks as if this maker has produced a cat type in looks alone.

Izzy46 · 13 April 2025, 02:20

Yes, it looks like the floor is low most of the way to the front of the boat. It does bend slightly upwards about a meter before the front of the boat.

I guess I rolled the dice with the Chinese made boat and it didn't really work out. The sad thing is that the manufacture won't take any responsibility which means they can't/won't be able to improve their design. More junk in this world I guess.....