New Yamaha 70

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Starscream
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Re: New Yamaha 70

Post by Starscream » Thu Oct 01, 2015 9:05 am

Terry wrote:Wow Some Yamaha 70 feedback!

I quite like the Yamaha 70, lots of torque and it swings a big 14 X 11 prop and I get her up to 6200 rpm and sometimes hit 6300 rpm. Curious as to what prop others are using? I don't get the speed others report, I am lucky to hit 20mph, maybe I carry to much stuff. I wonder if I could swing a 14 X 12?
My Etec 90 swings the same 14 X 11 as you are using, and at WOT it turns at 5,000 RPM unballasted and that pushes me at 24 mph. With ballast in, the motor turns at 4,800 RPM and 20 mph. I don't get it: why am I faster than you with the same prop turning slower? Are our boats that different?

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Seapup
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Re: New Yamaha 70

Post by Seapup » Thu Oct 01, 2015 9:26 am

My Etec 90 swings the same 14 X 11 as you are using, and at WOT it turns at 5,000 RPM unballasted and that pushes me at 24 mph. With ballast in, the motor turns at 4,800 RPM and 20 mph. I don't get it: why am I faster than you with the same prop turning slower? Are our boats that different?
There is a 15% gear ratio or prop speed difference between the 2 motors at the same RPMS...

Its really impossible to compare top speed of these boats once they leave the factory and anything is added since there are some many factors involved.

It does not take that much to make a large difference. When first testing my 90 it had no problem running at 28mph while totally stripped empty for winter storage. Soon as it was loaded up with normal day to day stuff it dropped to 24, once loaded for cruising it would only do 20, sometimes less. All of those speeds were within 2-300 rpm on the tach.

20 MPH & heavy loaded was an awkward ride, 25+mph and light was skipping across the surface like a normal powerboat.

Starscream
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Re: New Yamaha 70

Post by Starscream » Thu Oct 01, 2015 9:28 am

Right, gear ratios...that makes sense. Duh.

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Terry
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Re: New Yamaha 70

Post by Terry » Thu Oct 01, 2015 12:41 pm

Yup The Yamaha has a 2.33:1 gear ratio and has a full throttle rpm range of range of 5300-6300
I still think I should get more than 20mph though! :?

Starscream
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Re: New Yamaha 70

Post by Starscream » Thu Oct 01, 2015 1:53 pm

My motor has a gear ratio of 2.00:1 so I am turning the prop at 2500 RPM when I hit 24 MPH at 5000 RPM.

You have a gear ratio of 2.33:1 so you are turning the prop at 2660 RPM when you hit 20MPH at 6200 RPM.

Same prop diameter and pitch. Back to not understanding. Maybe it's an :macx: thing? Actually 20MPH doesn't sound too bad with a 70Horse. I got about 16 with my old 50 horse, unballasted, which is pretty typical. 20MPH with a 70 sounds reasonable to me.

trdprotruck
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Re: New Yamaha 70

Post by trdprotruck » Thu Oct 01, 2015 4:00 pm

Starscream wrote:My motor has a gear ratio of 2.00:1 so I am turning the prop at 2500 RPM when I hit 24 MPH at 5000 RPM.

You have a gear ratio of 2.33:1 so you are turning the prop at 2660 RPM when you hit 20MPH at 6200 RPM.

Same prop diameter and pitch. Back to not understanding. Maybe it's an :macx: thing? Actually 20MPH doesn't sound too bad with a 70Horse. I got about 16 with my old 50 horse, unballasted, which is pretty typical. 20MPH with a 70 sounds reasonable to me.
Just a side note your logic would work with a car, but with a boat there are many variables that make it a little bit difficult to make an accurate comparison. One thing that is making the comparison confusing is prop slippage. This could occur due to the outboard's height, angle, boat weight etc. Basically too many variables to do a straight comparison.

-Randy

bobbob
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Re: New Yamaha 70

Post by bobbob » Fri Oct 02, 2015 9:31 am

I'm going back and forth about re-powering with either the Yamaha 70 or the Mercury 60. The Yamaha has a little more HP but I did read somewhere that the Mercury makes maximum torque as low as 3000 RPM, which seems like a significant advantage.

EDIT: Here's a good link: http://www.walleyecentral.com/forums/sh ... p?t=252146

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fouz
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Re: New Yamaha 70

Post by fouz » Fri Oct 02, 2015 11:01 am

I know there are some that say torque is the key to these boats but I have to disagree.
Torque = get up to top speed faster. Not upping your top speed.
HP = get more top speed. Getting up to top speed a little slower.

Since these boat have basically two modes of speed. Hull speed and up on a plane. Unless you want to drag race I can't see the need for going intentionally with more torque. Torque is good to have if needed but torque is what breaks things.

I know I haven't heard of anyone breaking a transom but it is something to think about.

Or maybe I'm wrong. :?

bobbob
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Re: New Yamaha 70

Post by bobbob » Fri Oct 02, 2015 11:29 am

More torque at low end will give you better hole shot. Based on that graph it would also imply the Merc would be faster at lower RPMs (if you were interested in speed at more efficient fuel economy, or for quieter cruising). The Yamaha would give you more top speed at WOT.

Probably the differences are not enough to make a difference if you have a particular brand affinity, or if one brand or the other has a better/closer dealer to you. The biggest difference anywhere on that graph is a 15% increase in torque from the Merc at around 2250 RPM - most differences are < 10%.

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[Fraid-Knot]
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Re: New Yamaha 70

Post by [Fraid-Knot] » Thu Nov 05, 2015 2:58 pm

When I repowered, I did some extensive research and comparisons between outboards and found the Yamaha 70 has got the power to weight ratio in the bag! Honestly I was about to pick up an Evinrude E-Tec 60, but the Yammy offered more power with less weight- plus it's a four stroke and it was cheaper. I don't know how they did it. My boat came with the old Merc 50 and it was HEAVY. So adding more power and less weight was a no brainer when you have to factor in sailability.

As far as the torque, it performs to my expectation. I did not buy a Mac to race. All I ever expected was the 20 kts that was advertised. The yammy will give you that (depending on how much junk you have on board). I did a speed comparison at WOT with a fully loaded boat and was getting 15 kts. The next weekend, I took everything out and removed the mast and rigging then tried it again. I was coming up to 25 kts. It plained out nicely as well. So the weight you are carrying is key. If you are loaded up to go on a overnight sailing excursion with your buddies, don't expect to get to where you are going at 20kts unless you go with a higher HP engine. But if you do that, it will be heavier and likely cut into the sailability. So my opinion, It all comes down to what you want the boat to do for you.

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