Aqua Marina - Budget Tunnel Hull SIB

[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. 🤞

[Old seahorse]

Very impressed at your efforts ( and determination!)
The trim tab may do the trick.
I have been trying out options to partially enclose my transom
(It’s made up of two parallel tubes with transom plate bolted in centre), just to avoid losing things out the back.
Still working on options.
Good luck with your trials

[Fenlander]

>>> confident it will be more a ‘work straight out the box’ scenario.

I completely forgot to draw attention to it but when I was on my private test track yesterday the Elling popped on the plane with the OB gripping the water like a good un and not a hint of ventilation. Most impressive... and thank goodness.

The 8hp will be the experiment as you say. You and I plus others have been vocal in our opinions in the past... based entirely on assumption and logic... so I'm looking forward to a fair real world test from someone with loads of portable OB experience.

[chipko]

Finally managed a few hours on the water this week and now getting little to no ventilation in chop. Different heights suiting different motors so the quick change riser pads useful and now getting a solid 14/15 knots from the 27kg Chinese Orca 8hp (feels more like 6hp tbh) 4-stroke running stock 7.5 x 8” prop.

To be honest, although great fun, reckon these little cat’s are probably more suited to small motors at displacement speeds. Once planing you’re in for an exciting twitchy ride in anything more than glass smooth water. With small dia. tubes and no drip moulding to the rubbing strake it’s a fairly wet ride in any wind and chop. Better performance to be had going into wind/chop when the air cushion works well to keep bows up. Going downwind slower to plane and rather susceptible to a fair bit of bow steering and semi submarining (losing air cushion and sucking bow down) in choppy conditions. Razor sharp reflexes required but great fun nonetheless.

All in all a good value no frills little boat and ideally suited as a super lightweight beach based fun runabout or tender. Wouldn’t recommend as your only/main boat for more serious use, and in no way as competent as say the much larger tubed cats like the Takacat/TrueKit/Elling.

Had great fun experimenting and getting this tiny super light cat to work, but realistically after a bit more seakeeping to cope with mixed conditions use.

[Old seahorse]

Hi Chipko, good to see you are having fun with your boat. It’s certainly a different experience! My takacat has generally been a really dry ride, but I launched off the beach into a bit of surf recently. I took a couple of waves head on, and took on plenty of water. Thankfully, it went straight out through the open transom, but it certainly cooled me down on a very warm day!
As you say, they have their limitations, but in the right conditions, they are great

[PapaWhiskey]

Hi, I just joined the forum after reading your experiences with the Aqua Marina 2.98 and hoping you can advise on the best engine choice.

I've just ordered one as a smaller light weight replacement for my existing 3.2m airdeck and Mariner 6 which at 48kg + 27kg is just getting too heavy for its main use as a tender to my offshore cruiser.

Having read your thread, I'm thinking the 2.98 with a Mariner 3.5 at 28kg + 17kg would be a well matched rig that seemed to work well for you at speeds up to 13knots using the long shaft version with no modifications to the transom height.

I'd very much appreciate your opinion and any further comment/advice on the above configuration.

Thanks, Peter.

[beerbelly]

that should work fine i bought one also and found the 3.5 worked fine but mines a short shaft and suffered bad propslip at the higher end of its speed range

[PapaWhiskey]

Thanks beer belly. Just need to be fairly sure there aren’t any issues going for a long shaft that would make it a bad choice.

[PapaWhiskey]

I’m hoping to order one this afternoon and save 10% on a promotion that ends at 5pm.

[PapaWhiskey]

Hopefully, chipko will drop in with his experience...

[PapaWhiskey]

I’m not looking for outright speed but will want to run it as fast as a 3.5 can go without obvious issues or needing transom modifications etc. I’ll be using it as a boat to shore taxi and exploring creeks, rivers and estuaries. I’ll be happy if it does 10-15 knots without any handling issues or skittishness

[PapaWhiskey]

If the long shaft proves too long, I could fit a raising pad like chipko if that would work better

[chipko]

Sorry just seen this. I’ll try and reply later but bottom line, don’t bother unless you just want a beach toy. All my findings are well documented earlier in this build thread.

[chipko]

Originally Posted by chipko

Finally managed a few hours on the water this week and now getting little to no ventilation in chop. Different heights suiting different motors so the quick change riser pads useful and now getting a solid 14/15 knots from the 27kg Chinese Orca 8hp (feels more like 6hp tbh) 4-stroke running stock 7.5 x 8” prop.

To be honest, although great fun, reckon these little cat’s are probably more suited to small motors at displacement speeds. Once planing you’re in for an exciting twitchy ride in anything more than glass smooth water. With small dia. tubes and no drip moulding to the rubbing strake it’s a fairly wet ride in any wind and chop. Better performance to be had going into wind/chop when the air cushion works well to keep bows up. Going downwind slower to plane and rather susceptible to a fair bit of bow steering and semi submarining (losing air cushion and sucking bow down) in choppy conditions. Razor sharp reflexes required but great fun nonetheless.

All in all a good value no frills little boat and ideally suited as a super lightweight beach based fun runabout or tender. Wouldn’t recommend as your only/main boat for more serious use, and in no way as competent as say the much larger tubed cats like the Takacat/TrueKit/Elling.

Had great fun experimenting and getting this tiny super light cat to work, but realistically after a bit more seakeeping to cope with mixed conditions use.

Can’t really add much more to my previous post above.

Had a blast last year messing about and modding the tiny Aqua Marina, although in the end not for us and sold on.

In summary, couldn’t eliminate ventilation with our Mariner 3.5 short shaft despite cutting down transom by 60mm. Long shaft 3.5 worked ok on uncut transom with no ventilation although not an elegant or efficient solution…already had a long shaft aux on the rib and certainly wouldn’t buy one for this boat.
Best suited motor turned out to be an Orca 8 (aka Yamaha 6hp clone) which suffered little to no ventilation with transom lowered around 20 to 30mm, seeing a scary 15 knots solo in ideal conditions.

It’s a wet, twitchy, ride at planing speeds in anything other than glass smooth water requiring razor sharp reflexes…immense fun though.

[PapaWhiskey]

So did you find it wet and twitchy with the 3.5 long shaft as I don’t want a motor heavier than 20kg?

[ShinyShoe]

Originally Posted by chipko

Nice contribution to the thread Limecc, much appreciated....not!

Your fast becoming ‘troll’ of the month here. Very close to reporting this shite... hope your proud of your efforts.

Mrs Chipko has noticed this thread hasn't she...?