Aqua Marina - Budget Tunnel Hull SIB

[Fenlander]

Ahh OK... yours is good value for an aluminium one though.

[chipko]

Sunny skies and freezing temps early doors today, but no swell and just a light onshore chop meant ‘get that tiny cat out for its maiden shakedown you wuss’.

First off super light rig meant haul from site, through dunes, and down to waters edge (tide out....oh no!) just doable fully loaded solo. Diy dolly/trolley/transom wheels combo worked a treat...obvious upgrade would be a proprietary super lightweight aluminium launch trolley....big bucks methinks.

On water no real surprises to be honest, just don’t expect these tunnel hull cats to work 100% straight out the box. Only running a 3.5hp Mariner 4-stroke initially and at non planing speeds pulled 5knots effortlessly at very little engine load. Amazingly, easily managed to get planing one up and topped out at a smidge over 11knots...crazy speed from 3.5hp. It also seems to track well and has decent grip in the turns. Don’t expect a dry ride though with the relatively small diameter toobs and no anti-splash/drip moulding to the rubbing strake. I didn’t even fit the seat as much prefer to sit on the (elevated) air floor, and being such a tiny boat would just be in the way.

Bad news, although not unexpected, ventilates like crazy whilst planing. A common problem evidently even on the more costly cats where fitting cupped props/hydrofoil fins is often a solution. Also on this boat the transom, at a full 15”, is just too deep for a tunnel hull craft, further compounding the problem.

Basically purchased for use as a super light, cheap muck about off the beach in ‘good conditions’ chill out sib then it should fit the bill perfectly. If we run it with the 3.5 mainly at displacement speeds then sensible to just accept the intermittent ventilation at full throttle.

Gut feel though, this little tunnel hull deserves more power, and will fly with a lightweight 6, 8 or 9.8hp motor. Of course with more power any ventilation issues will certainly need addressing, and willing to do this even if it requires trimming down the transom!

For the price I reckon this small, cheap, lightweight but basic ‘cat style’ sib will be great fun and fairly confident we can address its foibles with a little time and effort. Bottom line though, hesitant to recommend to others as at the end of the day, do not expect it to perform 100% straight out of the box.

[chipko]

Sunny and bitterly cold again this morning with a bit more chop but got out for an hour or so of more testing.

Raided the rib for the long shaft Mariner 3.5 aux motor and whacked it on the sib.

As hoped, a mahoosive improvement with no ventilation issues at all and an increase in top speed to just under 13knots. Boat transformed into a right little hooligan....easily maintaining planing speeds in tight turns and pushing through chop. Seemed ridiculously quick for 3.5hp. Got soaked playing in the white topped chop and small surf...great fun.

Realise the long shaft throws the leg too deep and reckon a good depth will be somewhere midway between that and the short shaft. So cutting down the centre section of transom by 50 to 60mm should be a good starting point to get the short shaft gripping....if not will keep going deeper in small increments.

Bit of a drastic mod for a brand new off the shelf boat, but needs must and it clearly doesn’t work as is. Now where did I put that jigsaw?

[Fenlander]

Great review and images. I would have no fear at all modding that transom to get it how you want now you've proved the fun concept... but is it worth getting the larger motor first and cutting to suit that??

So frustrated no Elling so far to see how that goes re ventilation to see if the central external sausage does the trick.

[smallribber]

Well done for getting her wet and for the review. As you say the cavitation will need to be sorted and I suspect many who might buy even at only £700 would be timid about taking a saw to the transom. Amazing performance with a 3.5 hp imagine a 9.8 2 Smoke

[smallribber]

Did you get any video ?

[chipko]

Originally Posted by Fenlander

Great review and images. I would have no fear at all modding that transom to get it how you want now you've proved the fun concept... but is it worth getting the larger motor first and cutting to suit that??

So frustrated no Elling so far to see how that goes re ventilation to see if the central external sausage does the trick.

Thanks. Yes agree sensible to wait and cut transom to suit chosen motor....still pondering 6, 8 or ‘holy grail’ 9.8, plus current supply issues aren’t helping.

Reckon it’ll need in excess of 60mm reduction for any short shaft, so might start with 50mm, temporarily seal, and just try the 3.5. Plan to leave final level and finishing to suit chosen motor.

Fingers crossed the Elling lands soon. Aware of a couple of boats both running different 9.8 four strokes with no reported ventilation issues.

[beerbelly]

was talking to a barrus dealer today and there hoping to be getting deliveries of new outboards mid to late may

[chipko]

Unfortunately no ribbing today as local tractor launch broken so, despite not having or even decided on new motor, started looking at sib transom surgery.

Took a few dims, removed inner and outer motor pads, cut and peeled back transom top pvc profiled edging and marked out said cut out.

Existing effective transom depth to u/s toobs is a whopping 485mm (19.12”). So decided to initially whip off 60mm leaving an effective depth of 425mm (16.75”). Reckon this will be as good a starting point as any.

Unlikely the tractor mended by tomorrow, so the pressure will be on to fire up the jigsaw!!

[Oldman2]

I expect to see it jigsawed and made waterproof with at least 2 coats of something today so its ready for sea trials tomorrow, you know thats what you need to do.

[chipko]

Originally Posted by Oldman2

I expect to see it jigsawed and made waterproof with at least 2 coats of something today so its ready for sea trials tomorrow, you know thats what you need to do.

On programme Oldman2.

No ribbing again today so went for it. Only have very limited tools up here but a quick trip to Screwfix for some new jigsaw blades, glass paper and a tin of yacht varnish and voila....no turning back now.

Hopefully whack transom plates on tomorrow morning and go testing!

[smallribber]

Very interested to see how this works out

[Old seahorse]

Hi Chipko, apologies for delay in coming back to you with transom height on Takacat. First chance to inflate boat( and use) was today.
I realise you have already reduced your transom height, but here are measurements on mine.
Transom to bottom of floor is 13 1/2 inches ( floor sits a couple of inches or so above water line ( I couldn’t get accurate measurement today).
Transom to a/v plate is approx 17 inches. ( Suzuki 6 he, with permatrim fitted, but uncapped prop).The earlier Takacats had the top transom tube straight, but on my version they have lowered the centre , presumably to lower transom height.
I’m still tweaking mine, adjusting the trim pin today.
I couldn’t test speed today, as there is speed limit in :Bristol docks.
Good trip though- almost the only private boat on the water, and great weather. If I can forward a photo of set up, I will. ( not tech savvy! )
Good luck with yours. You should be able to have fun with it

[chipko]

Thanks Old Seahorse for the dims.

Quick update. The good news is it didn’t sink, bad news still ventilates with the short shaft. Not ideal test conditions as quite a bit of chop.

Reduction of 60mm gives an actual transom depth of 325mm with an effective depth of 425mm to u/s tubes. Uneasy going any deeper to be honest.

Unable to further test for a week or two but at the moment best results are with a long shaft on the unaltered transom.

[Old seahorse]

It sounds like the dimensions on your transom are pretty similar to mine, after your surgery.
I looked at both propellor cupping, and fitting a permatrim. The dealer who supplied the boat was cautious about the any potential advantage in cupping.
Whilst not promoting any method or product, the permatrim has considerably improved both the ventilation issue, and performance on mine. It’s different in shape to most “fins”, with turned lips on both sides, presumably to defect any side thrust rearward.
I know there are plenty of members here more tech savvy than me.
What I have found is that these inflatable cats behave differently.
Mine is very stable, and turns in a tight, controlled manner.
They seem to perform well with a small motor, which for me, was a major consideration.
Clearly ventilation is more of a challenge, caused by the air tunnel and lack of transom below the waterline, but with some tweaking, I think you, ( and me) will get there.

[Fenlander]

Just a thought... are the Mariner 3.5 motors you have tried in long and short variants identical models apart from shaft length? If so what about the props... same OE... same pitch?

[chipko]

Yes both the same 2018 models with stock 7” pitch props.

More than likely the l/s was running so deep not a chance of ventilating, whilst the s/s is probably now at around the right depth but the skinny av plate/plastic prop is just very susceptible to ventilation. Can’t imagine these little motors are designed for operating at planing speeds!

[chipko]

Quick update.

No further on water testing with the little 3.5 Mariner’s since butchering transom due to free time and sea conditions out of sync, but have been busy in the shed.

Doubtful there’s anything to be gained going lower even though motor still ventilating, and the little 3.5’s plastic prop, AV plate and lower leg profile may just not like running at planing speeds. Have recently acquired a lightweight 8hp four stroke motor which should have enough power to easily get the little cat planing.



We also now have the flexibility for quick change transom heights as cobbled together a set of bolt on modular transom pads able to go up or down in 15mm increments.





Conscious of the fact many motors just won’t perform on cat hulls without the addition of doel fins and/or cupped props, both of which reluctant to do, so a bit more cobbling. Basically a sort of central trim tab to hopefully smooth out the water flow before and over the prop. Exact angle, height, whether it has any effect or not....not the foggiest at the moment, but simple and quick to bolt on/off for testing.



Phase two testing to commence as soon as weather gods and free time are in alignment. 🤞

[Fenlander]

Neat job... really valuable investigations for the resource pool. Any chance there might be nearby testing of a larger similar cat-like SIB.

[chipko]

Originally Posted by Fenlander

Any chance there might be nearby testing of a larger similar cat-like SIB.

Yes definitely, but confident it will be more a ‘work straight out the box’ scenario.

Reckon more interesting will be to see how the little Chinese motor pans out. 🤞