Aqua Marina Aircat 335 Ventillation Issues

[knwatkins]

Hello all, new to sibbing and thought I'd drop a post on here as we've bought a used Aqua Marina Aircat 335 and we're having a few teething troubles.

We bought the cat mainly to cruise around Poole Harbour and take our two little girls (4 & 5 years) out exploring around the harbour, landing on some beaches for picnics etc.

Here she is just before her maiden voyage in our possession a couple of weeks ago:


With two adults and two little ones, I would expect our all total weight to be about 200kg-ish.

She was mated with a little 5hp four stroke Tohatsu. We had a great first day but unfortunately I found that I couldn't increase the throttle much as the engine would suck in air and ventilate.

In order to try and resolve this, I went out and bought a Stingray Airo hydrofoil from Avon Marine in Christchurch.


However, before fitting the hydrofoil, I accidentally (on purpose) put an offer in on a 9.8 four stroke Tohatsu on eBay which was accepted. After getting over the bollocking from the wife for the unauthorised purchase I fitted it to the new-to-us 9.8 ready for our next outing, which was Saturday just gone.

Unfortunately after throttling up, the issue was there with the 9.8 even with the hydrofoil fitted Very frustrating as I feel like the engine is revving but we aren't getting anywhere at any pace.

So from some further research, I think I have two options:
Option 1. Cut the transom down so the prop is lower in the water, which I don't really want to do unless I really have to as I don't want to ruin the integrity of the transom and also don't want to cause excess splash with the leg being lower.
Option 2. Fit a cupped prop.

Regarding option 2, I found the following post on another forum where a guy had the same issue as me but with a Takacat, mated with the same engine. He changed the prop from a 8.9x8.3 to a 8.5x7 and had it cupped at a local prop shop. He reported that this 100% resolved the ventillaton issues.
https://www.cruisersforum.com/forums...ew-167930.html

I'd appreciate any thoughts from you all. Would changing the prop to one with shorter pitch and having additional cupping added, be the road you would go down?

Thanks in advance!

Kev

[Fenlander]

Hi and welcome to the forum. Have a look at this lengthy thread on this very forum which has loads of information regarding your issue.

https://www.rib.net/forum/f50/aqua-m...sib-85721.html

[knwatkins]

Thanks Fenlander. Yes, I've read chipko's thread a few times. It's an interesting journey he went on. It appears that whilst he made the adjustable transom, he wasn't able to solve the ventillation problem entirely unfortunately which suggests that cutting down the transom may not be the optimum solution.

[smallribber]

With the same aircat and my 9.8 Tohatsu 2 stroke with just me onboard it seemed to be ok. Not sure if the 9.8 4 stroke and the 9.8 2 stroke are different lengths in the same length category. I am wondering if you had too much weight up front ?. It would not take much to lift the back und a little which might be all it needs to ventilate

https://www.youtube.com/watch?v=ma21XdQr-nk

[knwatkins]

I watched your videos before I bought the cat Dennis - thank you for the great YouTube content. I am now one of your subscribers.

I don't think it's a weight up front issue. My wife and I are generally positioned either side of the seat, perched on each of the sponsons, with the two little ones in between. We have all our gear behind us. We have tried shifting all our weight forward but unfortunately it doesn't make a difference

[smallribber]

Originally Posted by knwatkins

I watched your videos before I bought the cat Dennis - thank you for the great YouTube content. I am now one of your subscribers.

I don't think it's a weight up front issue. My wife and I are generally positioned either side of the seat, perched on each of the sponsons, with the two little ones in between. We have all our gear behind us. We have tried shifting all our weight forward but unfortunately it doesn't make a difference

Very interesting. I think you might have to cut the transom. It should be possible to do it in such a way that the removed piece could be put back on. I surpose the issue is how much do you cut off. I think I would start with about 25mm. But that is just a guess.

Thanks for the Subscription

[Fenlander]

>>> he wasn't able to solve the ventilation problem entirely unfortunately which suggests that cutting down the transom may not be the optimum solution.

Reading through the thread linked in post 2 it seems Chipko found the best result from a 30mm reduction in transom height using his 8hp.

From the many posts I've read on these small cats here and elsewhere it seems each persons problem/resolution is quite personal to their load, trim (weight), outboard make and prop type. I would have no hesitation in shaving 30mm from the transom first but if you are reluctant then why not try the cupped prop? It would be good to get a mini tach first to see what revs you are achieving at maximum speed as it may give a guide to a decision on changing the pitch at the same time if appropriate.

Once you have that info I'd have a discussion with someone like Steel Developments who could supply a new (or perhaps recon) prop ready cupped.

https://www.steeldevelopments.net

As an aside I've only suffered ventilation to a problematical degree on one outfit (air floor SIB but not cat) and all it took was a swap of prop to a different brand to cure it... the problem can be that fickle.

[beerbelly]

As an aside I've only suffered ventilation to a problematical degree on one outfit (air floor SIB but not cat) and all it took was a swap of prop to a different brand to cure it... the problem can be that fickle.[/QUOTE]

wasn't the elling was it?? .im considering one for use with this 6hp but with the issues with the aqua marina tunnel hull sib i bought im hesitating

[Fenlander]

No it was a Zodiac air floor.

The Elling issue for us was it just didn't suit seniors as an outing/touring SIB. Neither of us are lightweights then add in fuel and the usual kit it only needed small waves to stop it acting as a CAT and that flat floor would slam down causing Mrs F's knees great discomfort. An associated issue was they're quite a shallow boat to sit in (particularly compared to the great depth at bow of the Aerotec) and when it did slam down Mrs F felt quite exposed to being fired out over the bow.

As I've said recently as a fun lightweight solo boat to enable a smaller OB to be used I'd consider one again.

[beerbelly]

Originally Posted by Fenlander

No it was a Zodiac air floor.

The Elling issue for us was it just didn't suit seniors as an outing/touring SIB. Neither of us are lightweights then add in fuel and the usual kit it only needed small waves to stop it acting as a CAT and that flat floor would slam down causing Mrs F's knees great discomfort. An associated issue was they're quite a shallow boat to sit in (particularly compared to the great depth at bow of the Aerotec) and when it did slam down Mrs F felt quite exposed to being fired out over the bow.

As I've said recently as a fun lightweight solo boat to enable a smaller OB to be used I'd consider one again.

ok thanks i understand now unfortunately myself at 17.5 stone and my mrs being north of a size 10 with bad arthritis in her knee and both hands i think the boatworld 330 air v floor might suit us better .tbh its only for leisurely trips up the local river and a mile off shore fishing trips when i dont want the chew of launching the rib. thanks for your input

[Fenlander]

>>bad arthritis in her knee...

Yes that's the same with Mrs F and she's confident willing crew so it was no good having something that in any way put her off.

[knwatkins]

Thanks for the feedback folks. I've set the Aircat up in the garden so I could study the engine height more closely against the transom and the bottom of the sponsons. I think you guys are right, transom work is necessary...




You mention a techo Fenlander, I did order one on the weekend coincidentally as I want to know what my RPMs are. Here's the one I ordered:

https://www.aliexpress.com/item/1005002986529200.html
Can't go wrong for a tenner including UK VAT and delivery.

[smallribber]

Before you cut away it occurs to me.... Which hole have you got the trim pin in. Have you tried trimming in a bit more ?

[knwatkins]

I've got it in the bottom hole so the leg is closest to the transom as it can be. I did attempt to experiment with it when I was out on the water but unfortunately the pin fouls on the wheel brackets so I'd have to remove the motor to trim it to a different position.

Am I right in thinking that when trimmed down/in, this is best position for reducing ventilation?

[Fenlander]

Re the tacho take the waterproof claim with a pinch of salt. They are splash proof at best so I usually have one cable tied to the tiller for gaining the readings while testing then either remove or relocate under the hood for recording engine hours.

Re the fins did the outboard come with them or were they the ones you'd bought and if so have you tried without? Some have found they improve things, others the opposite.

[knwatkins]

Thanks for the tip on the tacho

Originally I had a 5hp Tohatsu fitted on our first outing and that ventilated. I then bought the hydrofoil with the intention of fitting it to the 5hp in an attempt to combat the ventilation, but instead picked up the 9.8hp and put the hydrofoil on that one instead.

I think the hydrofoil does keep the bow down when accelerating so it should help keep the craft trimmed horizontally.

[Fenlander]

I do wonder if it's providing too much lift in your case?

[knwatkins]

You could be right Fenlander. Perhaps I should remove it and take the 9.8 out to see what happens.

[Fenlander]

I would. It’s a test without extra spending or modification.

[Nasher]

I would agree that the combination of the fins and the engine being trimmed right in might be lifting the stern and forcing the Bow down too much.

I think you need to test the 9.8 trimmed out a couple of holes and without the fins as a more normal starting point.

.