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Old 02-01-2009, 15:24   #1
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two stroke/four stroke

I am replacing two Honda 30 hp four stroke outboards for something bigger on a 40 ft trimaran.
The hull is a fast displacement design and does NOT produce a bow wave at speed.
I have not been happy with the performance of the Honda 30hp motors even after better props were fitted etc.
My intention is to replace them with two 90 hp outboards.

I know that the newer two strokes have improved significantly over earlier models.
Should I consider two strokes this time.
The 90 hp Yamaha two strokes are much lighter than the four stroke equivalents and cheaper.
My only concern would be fuel consumption comparison when the two strokes are used at a cruising speed at 3,000 revs (1/2 throttle).

Any comments /suggestions?
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Old 02-01-2009, 15:28   #2
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Beau, First I would consider what affect the weight of 2 90 HP 4 strokes will have on the boat since cats and tris are weight sensitive. Nissan was building a 2 stroke in the size you are considering that they claim is as fuel efficient and emissions friendly as the 4 stroke.
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Old 02-01-2009, 16:47   #3
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I have a single 40hp four stroke big foot mercury with extended leg on my 36 ft tri and at 3000 rpm get 5.5 knots i have directional steering which makes manuvering into the marina easier than if it were fixed and when sailing i can isolate it on the hydraulic steering so just my rudder moves so what iam saying have you thought about having just the one outboard ?
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Old 02-01-2009, 17:12   #4
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Trinescape, thanks for your comments.
How heavy is your Tri, my aluminium trimaran is 5,200 kgs.
It is more of a motor sailer though it does have a sail for downwind cruising.
I would like to economically cruise at 7-8 knots but have the ability to achieve 15 knots to either get out of trouble or get there before dark etc.

I was a little surprised the Honda 30's did not work better, I run them at 4,500 revs for a cruising speed of 7 knots.
An earlier 29ft trimaran (1,200 kgs) I had, used to do 15 knots with one 25hp outboard

I wanted the reliability of two motors.

I have considered one large 150hp for speeds up to 15 knots and a smaller 50 hp for economical motor sailing.
I like the idea of the bigfoot high trust motors but the largest is 60 hp? and I don't think twin 60hp high thrust will give me the 15 knots speed I am looking for.
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Old 02-01-2009, 18:07   #5
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Boy, that sounds like a lot of HP. In the larger motors it's hard to beat the reliability and quiet power of the four stroke. I'm not sure about the motors in the 90hp range, but in the larger motors The Yamahas and Suzukis are superb. I'm not a Honda fan either. Avoid the Evinrudes. I managed a company that installed all of the above for 3 years, I would buy Yamamha #1 and Suzuki #2.
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Old 02-01-2009, 20:06   #6
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seems way over powered to me
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Old 03-01-2009, 03:30   #7
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hi beau my tri is approx 5t loaded the best speed ever logged while motoring was 9.5 knots flat out with a tail wind [i was late for a bridge opening] you would have to carry a fair bit of fuel to maintain 15 knots i carry roughly 110 litres and if i push the motor fuel economy goes down to buggery at 15 knots you could just about watch the fuel gauge go down! sorry i dont have any suggestions
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