Double Outboards on a Catamaran

[catsketcher]

A couple of points

Aina - the fuel economy of a modern 4 stroke is quite good. As has been stated on this thread, modern 4 strokes are not quite as efficient as diesels but seem to be on par with petrol vs diesel cars and at least here in Australia most cars are still petrol. I get about 2 miles per litre at 6-7 knots. Also I can sail earlier and faster in light winds due to less drag so I probably use less fuel than if I put diesels with folding props on my boat.

Reliability is fabulous. The one time I had a problem with my outboard was when I tipped up the jerry can and put the dregs into my fuel tank. I was out on the reef and I stripped the carby and she purred away again. My current outboard is ten years old and has given me one issue, the one I caused. An outboard is a single unit, all designed to work together, so there are no issues with transmission and exhaust or control systems not talking to each other. Add the fact that it spends 99% of its time out of the water and you never have to worry about barnacles on the prop or checking the shaft zincs, the outboard is a great solution for a multihull. The problems of fuel storage do not occur in a cat as you can have ventilated fuel lockers as well. Monos are trickier.

Oskanush - I fear your dream is just that. Unless your boat is a powercat design the upsweep at its stern will encourage it to squat at higher speeds. I get 9 knots from one 25 hp outboard on my 4000kg cat. I think you may get close to 11 knots with two 25s but looking around the forum will show that twin outboards do not give twice the speed. They do extra push and manoeuvrability. I think you have the wrong boat for 12 knots at half throttle
unless you get twin 50s.

Phil - I would be wary of the aft beam location of the outboards. I have been happy with a single outboard for 16years. I might change it, and I have changed a lot about my boat, but it is not important for me. If your boat has the nacelle I would think about using it and bringing the outboard forward. The fine ends of the HTs means that extra weight at the ends is undesirable plus with hobby horsing, the engines may be intermittent pushers in bad conditions.

Consider the single outboard in the nacelle with its own, or coupled to, the steering system. This has helped turning in a number of single engine cats. Just ensure that the prop blast does not hit the inside of the hulls when turned or its effect it limited. It will have to be back far enough so that at full turn the prop blast goes out the tunnel, or hits the rudders. Try a 25 Yammie instead of 2 9.9s. Lots of grunt and only a little bit more expensive than one 9.9.

cheers



cheers

Phil

[oskanush]

Catsketcher - Thank you for comments. "25hp - 9kts - 4000kgs" is very satisfying example for me. Just please tell me at which ~rpm / ~hp / throttle?

In fact I have a 2013 Mercury 6hp 4stroke O/B long shaft for dingy. I can make a motor bracket for this.

I need to take my catamaran to shipyard which is about 10nm cruise, but I should pass the entrances of main harbours of Corfu Island (Greece- East Mediterranean- North Ionian Sea) , where there are many speedy cruise ships and ferries..

Do you think that my 6hp O/B can push my boat @3 maybe 4 kts? (In calm conditions)

If not, I have a chance to supply an used 2016, Parsun 30hp O/B with longshaft, 2stroke, manual start, no trim..

Because I have water penetration at my saildrives and I do not want to face to break the expensive saildrives risk..

Maybe after repair and maintanance of saildrives at shipyard, I will use 30hp O/B at a speedy dinghy..

I am Turkish, the boat is at Greece, where the repair process is quite expensive and very complicated for me, then my own country.

Again any comments welcome..

[oskanush]

@PhilipMcLaren

I apologise If I disturb your topic, and am writing at a false topic.

I do not want to be rude..

[catsketcher]

I have pushed my 38ft cat with a 3.3 and an 8. The 8 was much better but both were in calms when doing boat work. If you can borrow the 30, do that and you can get away from the ships.

As for my cat, 9 knots is a full throttle. I don't have a tacho but the revs seem nice and high.

cheers

Phil

[admiralslater]

The Pdq 36 and 32 where designed for out boards .this makes a huge difference in the way they behave .on the 36 they are about 8 feet forward of the transom . Big seas no cavitation even at full throttle . Same on the 32 . There are a number of these in the Caribbean at the moment and they perform quite well. And for those of you that think the Caribbean is flat water the channel between St Lucia and Martinique sported 6-8 footers today . I don't think that the Heavnly Twins would work with this set up because it was not designed for it . I did once see a pair of Yamaha 9.9s on a 50 foot custom cat . Two smal in my estimation but he was having a good time.

[Bill4]

Originally Posted by admiralslater

The Pdq 36 and 32 where designed for out boards .this makes a huge difference in the way they behave .on the 36 they are about 8 feet forward of the transom . Big seas no cavitation even at full throttle . Same on the 32 . There are a number of these in the Caribbean at the moment and they perform quite well. And for those of you that think the Caribbean is flat water the channel between St Lucia and Martinique sported 6-8 footers today . I don't think that the Heavnly Twins would work with this set up because it was not designed for it . I did once see a pair of Yamaha 9.9s on a 50 foot custom cat . Two smal in my estimation but he was having a good time.

I would love to be able to raise the props on our cat completely out of the water and have zero drag and no anodes to change on the saildrives and props. But 2 x Yamaha 9.9s might be just a little bit too 'interesting' in anything other than zero tide, wind and waves.

I once saw a Bahia for sale in Panama with 2 x 60 (I think) hp outboards mounted in cutaways on the transom steps but it looked so wrong and it probably didn't work much better in a rough sea.

Now if someone could develop a 30 hp extra extra long shaft high thrust outboard...

[admiralslater]

I agree ,on a boat of your size it is a non starter.

[rwidman]

Well, it might work and it might not. You won't know until you do it and then you're stuck with the results.

It seems to me that if your boat isn't big enough to suit you, you would be better off selling it (as is) and buying a bigger boat. You will be able to test it before you buy it and know that it works and meets your needs.

[VBsail]

Now if someone could develop a 30 hp extra extra long shaft high thrust outboard...

Bay manufacturing sells extension kits to turn just about any 20" L motor into a 25" Xl. (They have a kit to turn the 25" Yamaha T9.9 into a 30" shaft too).

Evinrude etec 40 has a 2.9 gear ratio, turns a large 14" diameter 9 pitch high thrust prop, add a extension and its a 25" shaft.

[Stooksy]

Originally Posted by smj

[emoji106]🏻

Unless you own a catamaran with out boards you should not comment, single Outbiard is not a good option at all, I have twin 25 Yamaha x long leg high thrust on custom mount brackets 3/4 aft. One is suicidal you need two without a doubt

[smj]

Originally Posted by Stooksy

Unless you own a catamaran with out boards you should not comment, single Outbiard is not a good option at all, I have twin 25 Yamaha x long leg high thrust on custom mount brackets 3/4 aft. One is suicidal you need two without a doubt

We’ve owned 3 catamarans with single outboards and 4 catamarans with twin outboards. I guess I probably have enough experience with both applications to comment and I also guess you’ve never owned or operated a catamaran with a single steerable out board?

[Franziska]

We have a 35 Woods Mira cat.
The hulls are similar to a Wharram, the Mira is one of Richard's oldest designs.

2x 9.9hp Yamaha get her easily to 6-7kn...

Originally Posted by oskanush

Catsketcher - Thank you for comments. "25hp - 9kts - 4000kgs" is very satisfying example for me. Just please tell me at which ~rpm / ~hp / throttle?

In fact I have a 2013 Mercury 6hp 4stroke O/B long shaft for dingy. I can make a motor bracket for this.

I need to take my catamaran to shipyard which is about 10nm cruise, but I should pass the entrances of main harbours of Corfu Island (Greece- East Mediterranean- North Ionian Sea) , where there are many speedy cruise ships and ferries..

Do you think that my 6hp O/B can push my boat @3 maybe 4 kts? (In calm conditions)

If not, I have a chance to supply an used 2016, Parsun 30hp O/B with longshaft, 2stroke, manual start, no trim..

Because I have water penetration at my saildrives and I do not want to face to break the expensive saildrives risk..

Maybe after repair and maintanance of saildrives at shipyard, I will use 30hp O/B at a speedy dinghy..

I am Turkish, the boat is at Greece, where the repair process is quite expensive and very complicated for me, then my own country.

Again any comments welcome..

[admiralslater]

Originally Posted by smj

We’ve owned 3 catamarans with single outboards and 4 catamarans with twin outboards. I guess I probably have enough experience with both applications to comment and I also guess you’ve never owned or operated a catamaran with a single steerable out board?

The first three cats that are family owned had single engines two off set and one on the centerline . The main family boat we had for almost 20 years and I took to the Bahamas and back to Toronto via the intra coastal.
No more difficult than a single engine monohull.
But it as with everything it requires skills

[Davy J]

Originally Posted by Bill4

Now if someone could develop a 30 hp extra extra long shaft high thrust outboard...

Um,.... uh,......... not an outboard,..........
but you know, diesel, Westerbeke, Sillette drive.

Already been done.

[smj]

Originally Posted by admiralslater

The first three cats that are family owned had single engines two off set and one on the centerline . The main family boat we had for almost 20 years and I took to the Bahamas and back to Toronto via the intra coastal.

No more difficult than a single engine monohull.

But it as with everything it requires skills

I would say much more maneuverable than a single engine mono if the outboard is steerable, in fact almost as steerable as twins and in some cases has benefits over the twin outboard setup.