Gas hog?

[iggyw1]

I just bought a 1991,16 foot Smoker Craft aluminum boat with a 1991 70 H.P. Nissan motor on it. The motor starts and runs like a brand new motor. No missing, no hesitation at all. She just purrs beautifully! The previous owner told me that the motor probably has 50 to 60 hours on it, and it is showroom looking. Now my question: This motor seems to use a lot of gas compared to my older 70 H.P on my old 15 ft. fiberglass motor. (1984 model) I gained 11 miles per hour with this newer motor too (due to the aluminum boat I assume), but my gas usage doubled wit this newer motor. Is it a known downfall of the Nissan motor?? How can I improve on the gas mileage if at all? Thanks!

 

[iggyw1]

I should add that my old motor was NOT a Nissan, but a 70 H.P Johnson.

 

[Sea Rider]

Need to go for a wot run on calm flat no wind water cond and check max achieved wot revs as currently loaded, probably delivered prop is not opt for your combo. Test and report back your findings.

Those rebranded Tohatsu 70 swallows 8.5 gallons per hour at full hammer down at max 6 K wot revs. Are 3 cylinder OB's, probably yout older Jonny was just 2 cylinder ?

Happy Boating

 

[iggyw1]

Thanks for your input on this. My older Johnson was also a three cylinder motor. The Nissan I have now has a tilt motor on it, so maybe I should try experimenting with the tilt too. Am I correct in assuming what I should be looking for in WOT on smooth water is maximum 6,000 RPM? And if I am not getting that, it may be effecting my gas mileage? If I cannot achieve 6,000 revs, even while adjusting my tilt, then I will g a different prop to get me there. Will I want more or less pitch to achieve the max. revs? I could research this on line if need be, but I just figured I would ask while we are discussing my motor. I will report back when I find out on smooth water what revs I am getting at WOT, but it may be a few days or longer with the weather we are getting now (wind) in S.E. Mich. Thanks!

 

[Sea Rider]

All Tohatsu as well as any other brand OB needs to sit at the sweet transom/OB height along propped correctly to get all the horses out. Is that a tiller or remote with electric tilt & trim OB ? Assume you have an installed tach, right ? If not will need one, an induction tach will make the trick..

Latter will post some tech parameters to have into consideration when testing combo on flat no wind water cond. How many passengers do you like go boating with ? Wot revs for a 70 HP is 5,850 rpm.

Happy Boating

 

[iggyw1]

All Tohatsu as well as any other brand OB needs to sit at the sweet transom/OB height along propped correctly to get all the horses out. Is that a tiller or remote with electric tilt & trim OB ? Assume you have an installed tach, right ? If not will need one, an induction tach will make the trick..

Latter will post some tech parameters to have into consideration when testing combo on flat no wind water cond. How many passengers do you like go boating with ? Wot revs for a 70 HP is 5,850 rpm.

Happy Boating

In answer to your questions: I usually go out with just two of us. I use the boat strictly for fishing. At times, there is three of us, but most of the time, only two of us. It is an electric start, remote controls and steering outboard. I do have a tach in the dashboard. It was my first time out with this boat, and I did not pay too much attention to the RPM's sorry to say. I was just highly surprised (and concerned) about the amount of gas I used the last time out. We did travel a little ways on the water I can say, however, just comparing my old motor to this new one (my old Johnson compared to the newer Nissan) Both 70 horse power, in the old motor, I would use about a full 6 gallon tank to go where we went and back. In my newer motor, I used 11 gallons. Just under double the gas. But again, the motor ran so sweet. No signs of gas spillage or leaking out at all. Ran real smooth too.

 

[Sea Rider]

In order for that M70 OB to deliver full horses out need to :

Test on flat, calm no wind water cond.
Distribute deck weight evenly.
Go for a wot spin.
Trim OB for combo to ride parallel to water level when on plane.
Have someone pull head out transsom.
Check at which lower leg height is middle water flow passing bye.
Ideal is to skim right under small upper water def plate.




This is the sweet transom /M70 ideal height for that OB.
Check if need to move OB up/down to achieve that height position.
Once there will check about wot rpm, fuel consumption.

Happy Boating