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Old 18-01-2025, 07:05   #1
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Catamarans with outboard motors

Hi all, I'm looking for a catamaran that is below AUD300K. Most of the cats in this range will have outboard motors in Australia. I would like to hear from experienced people if outboard engines such as Yamaha 9.9's are good for offshore ocean passages. My main concerns are the cavitation and longevity under extreme stress.
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Old 18-01-2025, 08:19   #2
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Re: Catamarans with outboard motors

I have a friend that had a Mainecat 30 with Yamaha 9.9s and I did several trips to the Bahamas from Florida. The Mainecat has engine mounted amidship and cavitation was not a problem, on the other hand an acquaintance with an Endeavourcat 30 with the engine mounted near the stern said his cavitated all the time. That said, the only serious issue we were aware of with the engine Yamaha 9.9s on the Mainecat was a nearby lightning strike (hit the water,not the boat) that took out the electrical system on one of the engines. At the time one engine had been lifted out of the water and was not affected. I imagine if both engines had been down he would have lost both of them as the lightning strike was on the opposite side of the boat from the dead engine. Being a sailboat he was not disabled but it make for some sporty docking. I've personally had a couple of nearby strikes while offshore so I would think twice about being offshore in a boat with an engine that could not even take a nearby lightning strike. I suppose that includes modern diesels with engine computers.
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Old 18-01-2025, 08:30   #3
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Re: Catamarans with outboard motors

We’ve owned 8 catamarans with outboards and much prefer them over diesels, especially those with sail drives.
5 of those cats had twin outboards, 4 were the Yamaha 9.9’s and our current one has twin Tohatsu 20’s. The other 3 had steerable single outboards.
As Capt. Bill states, cavitation depends on the design and shaft length. Outboards directly on the transoms will cavitated badly, the farther forward and closer to the center of pitch, maybe no at all.
Never understood the reasoning behind needing diesels for offshore, unless the boat is a poor sailer and will motor most of the time?
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Old 19-01-2025, 13:12   #4
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Re: Catamarans with outboard motors

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Originally Posted by smj View Post
Never understood the reasoning behind needing diesels for offshore, unless the boat is a poor sailer and will motor most of the time?

What he said. Motoring offshore is futile, much better to have a cat that sails rather than one that relies on big diesel engines.
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Old 19-01-2025, 16:38   #5
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Re: Catamarans with outboard motors

You can definitely motor "offshore" actually long swells are no issue as long as it's a design that doesn't pitch much. Motored most of the way from Minerva Reef to Opua in Dec '20. 10.4m cat running a Yamaha T8. On my boats you can make headway in some chop just about until the bow starts to pound. At that point things are pretty uncomfortable. I also really sail my boat and rely on having the mainsail up when entering and exiting harbors and inlets and you probably will have more intermittent issues with outboards as they get older.

2003 motor that had about 3000 hours on it. The bolts on the connecting rod recently loosened up and the rod punched a hole in the front casting. Compression still excellent at 165 psi and no noticeable wear on cylinder walls.

Used blocks are about $150-300 on EBAY. Some are very low hour freshwater units. Complete gaskets are less than $100. The overall cost of outboards if you can work on them yourself is a tiny fraction of diesel inboards.
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Old 19-01-2025, 17:30   #6
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Re: Catamarans with outboard motors

BbayBurt I survey plenty of outboard cats, one client of mine happily circumnavigated his outboard powered cat. I bet plenty of outboard powered Wharrams have done countless miles.
There used to be the argument that by not having diesels you lose another charging source and free hot water. But now with lithium batteries, great small portable generators and good 12-volt water heaters like Duoetto that argument is mute, I think.
Check out how this lady deals with her outboard on long passages. It's about at the 4-minute mark.
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Old 19-01-2025, 17:51   #7
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Re: Catamarans with outboard motors

On an extended offshore passage you are more likely to sail than to power.


I have a sailing cat friend who has two outboards mounted in fairings along the inner side of each hull. They lift up for passage.


I have sailed with him locally. This is much better than inboards imho - easy access, no drag, frees space in the hulls, and powers very very well. I would not hesitate at all. His boat is about 36 ft long and the engines are (I think) 8hp each.



I like this solution. The boat sails well and powers well when asked. Perhaps not as effective to power in a heavy storm compared to inboards. But I like boats to sail well, not to power well. A well sailing boat gets in trouble less, and needs the engines less.


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