Outboard problems Aluminium catamaran
Printed From: The Fishing Website
Category: General Forums
Forum Name: The Boat Shed
Forum Description: Discuss all things boating.
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=137350
Printed Date: 08 Jun 2026 at 3:57pm
Topic: Outboard problems Aluminium catamaran
Posted By: jules75
Subject: Outboard problems Aluminium catamaran
Date Posted: 15 Dec 2022 at 9:50pm
| Hiya. I've recently built an aluminum catamaran 4.5m and had a new 60hp Yamaha 4 stroke tiller steer on it. I'm having problems with ventilation and sluggish speeds. The boat shop that fitted the motor said they had no idea on where to fit the motor on the transom but said that I would have to test it. So that's what I've done over and over again trying to get the optimum height! It was way too low to begin with and kicked up way too much water at the back into the back of the boat and up into the cowling. So I moved it up a couple of holes and still too low I felt with more water kicking up into the cowling and still not able to have WOT without ventilation. I had no more adjustment so I extended the transom to have the motor sit higher. I've experimented with sandbags in the centre, up the front, had others in the boat, made transom wedges to get more negative trim. Ive also added more flotation to the pontoons at the back which has helped level the boat out as it was rear heavy. It seems that the motor feels very bogged down. I have though had it going very well a few times into the wind and waves, quite fast indeed and skipping over the top of waves. But with the wind behind and in anything slightly choppy it will ventilate. I know catamarans hull create much more disturbed aerated water exiting the hulls which affects the prop. I have made an aftermarket cavitation plate. I'm unsure of what to do to eliminate the problem. When planing the cavitation plate is still well below the water. If I trim up it even to where it's still just below the water it ventillates. Do I need to raise the engine further or lower it? Or is the something else I'm missing? Appreciate any help/advice on this especially from someone who has an aluminium cat and has had similar problems. Boat is similar to a Blackdog Cat. Cheers |
Replies:
Posted By: Bounty Hunter
Date Posted: 15 Dec 2022 at 10:08pm
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unfortunately two hulls do not go into one motor youll notice that Blackdog and Alloycats have spent a lot of time fine-tuning their central sponson to clean up water flow infront of the centrally mounted engine - but they still suffer from poor performance you will to go through the same trial and error process - or buy a second motor
------------- No disintegrations! |
Posted By: Schampy
Date Posted: 17 Dec 2022 at 9:08am
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Yeah...Bountys right. Cats are a bastard for cavitating. Iv got a 3m Takacat with a 5 hp 2str.... yes underpowered, but gets along well lightly loaded. A Permatrim helped a lot. A couple of weeks ago I tested it with a 6 hp 4 stroke..... and was a complete pig. Like you could not go full throttle with out cavitation, no matter how weight was distributed on boat or engine was trimmed. Strange thing is both engines have the same bottom end. Yes the 4 stroke is heavier by about 7kg. But you'll generally find all the Takas and truekit inflatables generally perform better with 2 strokes. Takacat also offer a modified rear transom bar that lowers the engine down 50mm to help mitigate cavitation issues. It would be tricky I know Jules but It would be interesting to see how it goes with a lite weight 2 stroker on the back. Sorry cant be more help... must be bloody frustrating this close to xmas .
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Posted By: jules75
Date Posted: 17 Dec 2022 at 2:12pm
| Thanks Bounty hunter and Schampy. Yes I think you're right re the centre sponson. I am planning to make a start on that modification to try and direct water away from the centre to give more clean water for the prop to operate in. I've been reading forums from overseas sites with tunnel hulls and it seems to be s good option. Will tack it on for starters and see how it goes. Thanks Schampy for your understanding and empathy re my annoying problem! The 60hp weighs 112kgs and dragged the back down quite a bit until I extended the hulls to give more buoyancy... I do have a working 30hp 2 stroke Tohatsu that I could put on to try. Will consider that...thanks. That one only weighs 53kgs. Will keep trying. I've come too far now to give up!!! Will let you know how I get on. |
Posted By: jules75
Date Posted: 02 Jan 2023 at 3:31pm
Update: So I've welded in a centre sponson, more like a wedge that directs water away from the middle where the motor is located It has helped quite a bit. It has especially helped with buoyancy at the stern meaning that I had to lower the motor down. Another issue is that it is running amazing into the wind allowing me to run full tit. into the wind. Downwind not great at all. The motor seems to bog down even when it's quite flat. Very strange. I have created a new thread on the subject. |
Posted By: smudge
Date Posted: 02 Jan 2023 at 3:47pm
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The most knowledgeable person I know when it comes to aluminium cats is Harvey Allen from Allenco boats. I'm sure he wouldn't mind a ph call.
------------- Best gurnard fisherman in my street |