SPORT 15: Yamaha 70 Two-stroke v. F70

Optimizing the performance of Boston Whaler boats
44z
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SPORT 15: Yamaha 70 Two-stroke v. F70

Postby 44z » Wed Nov 15, 2017 9:06 pm

I have an [1985] Super Sport 15 with an [2005] Yamaha 70 two-stroke-power-cycle engine on a manual jack plate. Boat speed is 43.8-MPH SOG per GPS with just me aboard. I am considering a new Yamaha F70 for smoothness, easier starting, and quieter operation.

Can I expect similar performance with the F70?

What are the downsides of making the change?

jimh
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Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby jimh » Thu Nov 16, 2017 12:17 am

First, I am surprised you have a model year 2005 Yamaha 70-HP classic two-stroke engine. That must be close to the last year they made them for sale in the USA.

As for comparing one 70-HP engine to another, they should both be 70-HP, therefore they should both push the SUPER SPORT 15 to the same speed, assuming you have set up both engines correctly with the right propeller.

The classic Yamaha 70-HP two-stroke-power-cycle engine was not particularly powerful. I think it was only an 850-cc engine. I used to own a pair of them. They are very nice, compact, smooth running two-stroke engines, but they are not making any more than 70-HP--if that much--with that small displacement.

A new Yamaha four-stroke-power-cycle F70 will probably weigh more than the classic 70, and the extra weight will take maybe 1- or 2-MPH off the top speed.

dtmackey
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Joined: Thu Sep 28, 2017 9:29 pm

Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby dtmackey » Fri Nov 17, 2017 12:34 pm

Up until recently, the EPA maintained a large database of every engine certified with very detailed data on dyno HP ratings, RPM, and emission data. The links I save have since been broken now that the EPA retooled their website. What this allowed one to do is research outboards by model and year to see what the actual rated power was. The industry allows for a 10% variance is ratings, meaning that a 70hp motor can have a true HP rating swing from 63 to 77HP which can make a big difference. In years past when looking through all the data, generally the 4 stroke offerings were a little softer on power than their comparable 2 stroke counterparts.

I'll see if I can dig up where they moved this data to on their website and post for others if found.

D-
Last edited by dtmackey on Tue Nov 28, 2017 10:52 pm, edited 1 time in total.

El Rollo
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Joined: Wed Sep 07, 2016 12:35 pm

Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby El Rollo » Tue Nov 21, 2017 11:12 am

In my opinion, you will be very happy with the F70.

My 1988 Boston Whaler 15 had a Yamaha 70 two-stroke-power-cycle engine. I re-powered with the newer Yamaha F70 (four-stroke-power-cycle engine).

I am a two-stroke fan and was reluctant shifting over to four-stroke. I must say that I am 100-percent satisfied.

In my opinion, the weight gain is negligible. The F70 is so much more advanced over the 70 2-stroke, that there really is no comparison.

Given the choice of having either motor brand new, I would still choose the F70. (Yes--I know the two-stroke is no longer available in the US).

As for performance, I cannot say what the total horsepower is for either the engine, but I can say that it seems extremely close. If anything, I would definitely give the edge in 'overall' performance to the F70. The power just seems better spread out over the entire RPM range. There is no rough spot like there was in the two-stroke engine. The power just seems smoother, and more linear across the entire RPM range and [the F70 is] much more pleasurable to operate than the two-stroke 70.

I've used my boat for plenty of various activities, and I can say without a doubt that it has performed much better overall than [when powered by the older two-stroke Yamaha 70 engine]. For example, we loaded it up with five people--two adults and three kids--a day's worth of food, canopies, drinks, ice, and tow-behind toys) and ran up the Colorado River 13-miles. Yes--the boat was loaded beyond recommended capacity but the water was extremely glassy and we took our time. The F70 had no problem planning all that weight with a 13-1/4 x 14 four-blade stainless steel propeller. In fact, I ran the engine at about 5,400-RPM all the way to our location with throttle left. Keep in mind I also have three batteries, a 24-Volt trolling motor, and other additional weight onboard, so this was truly impressive.

You will want to take your time selecting the best propeller for your application. Tom Clark really helped me a lot in selecting the best propeller for my application.

Good Luck with your decision.

flymo
Posts: 123
Joined: Thu Oct 22, 2015 2:58 pm

Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby flymo » Wed Nov 22, 2017 9:17 am

I was an early adopter of the F70 when it first came out in the fall of 2010 as I recall. I switched from the venerable Johnson 70 triple. You can expect similar performance overall, but the characteristics are different. It took me quite a while to find the right prop for the F70, and I came to the conclusion that a stern lifting prop with less blade area worked best for me. However, others have had different experiences so it may take some experimentation to find the right one for you.

One aspect of performance that I miss from the old Johnson is that kick when you are running at about 4000 RPM, then open the throttle all the way; the F70, with the right prop, is very close but not quite as strong. One aspect of the Johnson's performance I don't miss is the relatively high minimum planing speed; go below about 3200 RPM and you'll slowly drop off plane. As all 15 owners know, it can be a challenge to keep the boat on the water in the rough stuff, and for me this was exacerbated by the power curve of that motor.

I recommend indexing the plugs on the F70. It's trivial to do, and I feel it does give you a slight performance increase.

Of course the modern F70 is much easier to live with than the old two-strokes: less noise and smoke, idles forever without fouling plugs, and dramatically better mileage.

The only downside of the F70 would be the price. At least you'll be able to reuse your controls, which will save a bit.

mkelly
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Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby mkelly » Fri Nov 24, 2017 1:56 pm

We replaced our c.1980 70-HP two-stroke-power-cycle engine with the Yamaha F60 and couldn't be happier. The F60 just purrs and burns very little fuel. As always with proper outboard performance, the propeller is all-important, as is mounting height. I'll try to check what propeller I have.

44z
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Joined: Wed Nov 15, 2017 5:59 pm

Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby 44z » Mon Nov 27, 2017 5:30 pm

Thanks to all of you for the valuable input.

jimh
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Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby jimh » Mon Dec 04, 2017 8:22 am

The new discussion topic on indexing of spark plugs has been moved to a separate thread.

jimh
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Re: SPORT 15: Yamaha 70 Two-stroke v. F70

Postby jimh » Mon Dec 04, 2017 11:46 am

The new discussion topic on cost for routine service and recommended maintenance for a Yamaha F70 engine has been split into a separate thread.