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How do I know the correct, or acceptable, WOT for my 2003 70 hp Yamaha 2 stroke with a 13.75 x 15 SS prop on a 16' custom Whaler? Never have had the boat in the water and want to know what to expect when this Arctic blast moves out of Georgia.

Thanks,
Zap

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The Yamaha owners manual will give you the correct WOT range for your engine.

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Joe, I bought the engine second hand and have no owners manual. i suppose I could buy one but hadn't planned on doing so just yet.

Zap

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The maximum WOT rpms is (I recall) 5500. A good target to reach with a light load, 1/4 tank of fuel and 1 person is 5200-5500.

There are two key elements to obtaining the right performance.
- mounting height of the motor (anti-cavitation above the bottom of the hull 1-1 1/2 inches
- correct prop (based on motor gear ratio)

Boat speed should be measured by GPS and factoring wind/tide.
Engine speed should be measured by a tachometer.

Remember prop pitch design varies between brands. A 13.25x17 yamaha prop will not perform the same as a Mercury/quicksilver.

What brand is the prop you have now?

Which holes in the motor bracket will be used?

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Zap, check with a dealer to make sure on the WOT RPM. The expected RPM may well be 6,000, not 5,500, and if you're down a 5,200 that will not be good - be a shame to fry the motor.

From your description, though, the prop will likely be just about right.

Tim

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The specified maximum WOT range for this motor is 4500-5500 RPM. It would be best if you prop the boat so you actually fall in the upper half of this range, i.e., 5000-5500 RPM.

If you do not have the Owner's Manual for this motor, you should buy one...and read it!

http://www.yamahapubs.com/index.do?pg...category=4

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I have another question that is a bit off topic but germane none-the-less. Carom replied to my post "be a shame to fry an engine". Is that a reference to the potential danger of lugging an engine or a reference to it not being desirable to run at less than WOT (I'm assuming for extended periods in both cases)? I prefer to not run at WOT (for extended periods) but to get the boat on plane and to run a bit faster than what is required to keep it there, at least that is what I've done in my limited outboard experience. A bit unsure as to "ideal" RPM.

Tom, Thanks for the Yamaha publication site.

Zap

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You can download or view your 2003 owners manual at the yamaha website. Try this link:

http://www.yamaha-motor.com/assets/service/manuals/2003/lit-18626-04-93_419.pdf

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Zap, my reference to frying was about lugging a 2-stroke motor for extended periods due to overpropping - there's no problem running at mid-throttle (or any other throttle setting) if you're propped right.

The issue is generation and dissipation of heat. At mid-RPMs, but full throttle, you've got all the load on the motor, and the spark advance is typically already at maximum, so you're creating a lot of heat. But you're not pulling as much air and fuel mixture through the motor as you would be at higher RPM - the air and fuel running through provide a lot of the cooling for the motor.

I think you'll find that getting the WOT RPM right up close to the manufacturer's specified redline will give you the best overall performance. The rule of thumb is that 1" pitch = 200 RPM, so see what you have, and adjust accordingly.

That's good advice about getting the specific service manual for the motor - it will let you double check the WOT RPM spec and give you a lot of other useful information.

Tim

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Thanks Tim. Excellent explanation and valuable information. I have now learned something today.

Zap

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The 2003 Yamaha 70 2-stroke has a WOT operating range between 5000 and 6000 RPM, not 4500 to 5500. That specification applies to the 60 2-stroke which is the same motor but restricted.

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According to the Yamaha manual viewed online, thanks pederj, the correct WOT is 4500-5500 RPM. I'll call the Yamaha folks and will post their reply.

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According to the Manual listed above, Whaler27 is correct....
The WOT is 5,000 to 6,000 RPM for the 70 TRB model.

The 60hp and the 90hp have a WOT of 4,500 to 5,500..

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Peter is correct, I was in error, the WOT Range of the 2003 Yamaha 70 is 5000-6000 RPM as shown on page 4-2 in the Owner's Manual.

http://www.yamaha-motor.com/assets/se...93_419.pdf

This motor should then ideally be propped to get to 5500-6000 RPM at WOT.

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Thanks to all for the info.

Zap

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When you start playing around with propellers, it is ideal to get a pitch that lets that motor spin up as close to 6000 RPM as you can. If you prop it that way, you'll notice an exhaust note change around 4400 RPM where above that the motor gets "on the pipe".

In the Yamaha K-series line of props, I'm guessing that a 15 inch pitch painted stainless steel should do it. I'm not sure what prop you have but with 13 3/4 inch diameter, I don't think its a Yamaha propeller.

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Whaler27, I have some more prop information that may be helpful to you or others. The alpha/numeric data cast into the hub of my prop is: A 992013 13.75 x 15 Vortex WI. And, it is painted staineless.

Zap

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Zap, I am guessing your prop may be a 992113 Michigan Vortex 13.75 x 15 painted aluminum prop from this listing: http://www.miwheel.com/FilesCustom/HT...043111.pdf. It does not appear to be a highly cupped prop. I think it's quite likely to be a very good prop for you - and it's paid for!

To elaborate on what others have said, it would be great to get the WOT RPM up close to 6000 - if it ends up being down around 5500, I'd drop down 2" in pitch. Keep in mind that if you do go to more highly cupped stainless prop, the cupping alone gives you 2" effective pitch, so adjust accordingly.

Good luck and happy prop testing!

Tim

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By dang. Did a scratch test on the prop and it's aluminum. The number I posted above is missing one digit; should be 992103. I also have a Power Tech Stainless Steel (unpainted) prop, the numbers on that prop are M 135RED-17P, which the manufacturer told me is suitable for use on a boat/motor such as mine (1970 16' custom w/70 Yamaha).

I was unsure of the significance of cupping, thanks for the information.

Merry Christmas

Zap

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Typically cupping provides better grip and allows the motor to be trimmed out further and/or mounted higher which can produce greater top speed.

Just an FYI but on a 15 Supersport and one person, the Yamaha 70 would turn a 13 x 19 aluminum K-series to 5900 and turn a 13 x 19 painted stainless steel K-series to 5700. Top speed was identical, mounting height was the same. The stainless steel propeller had cupping that the aluminum version did not which resulted in a lower top RPM. Basically, the cupping acted as if the stainless steel propeller had 1 more inch of pitch.

With some limited experience I've had with the Michigan Vortex on a Montauk, I think the Yamaha K series aluminum propeller is a better propeller. It's not that much better that I would scrap the Michigan outright but when it comes time to replace it, I would go with the K-series for about the same amount of money give or take 20 bucks.