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In regards to the prop size: Could over reving (5800 and climbing) of my 70 Yamaha 2 stroke on a 16 Whaler have anything to do with the size differance between my boat and the boat the engine came off of (make unknown but was much wider and 2' longer aluminum serious fishing boat)?
Zap

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Of course, boat size makes a difference. But are you sure 5800 is overrevving? The newer OMC 70 which is very similar to the Yamaha has a redline of 6000 RPM.

Tim

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Carom,
The whole ideal prop size situation is a bit mysterious to me. I get the basics but have yet to get a firm grasp on the details. How much differance in boat size would there need to be to make a significant differance in rpms? The Yamaha owners manual indicates 5500 as the max. recommended rpm. I backed off at roughly 5800 rpm to help avoid engine damage$$$$$$$$$.
Zap

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In general, each prop will provide specific speed at a rpm. So if the boat your motor came off of went 34mph at 5500rpm and your boat would normally do 38-40mph with 70mph, it going to over rev until the motor runs out of power.

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All depends on load and for me elevation too. I can wind out my ev 30 pretty easy at sea level with two aboard. At +6000' and four aboard I feel way overpropped. I could go to a more agressive prop at SL which would be faster with only two however when I load six aboard I would be dogging it again....

No perfect answer unless you always load your boat the exact same way.

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Properly rigging the motor and selecting a prop is a bit of a science with a dash of trial and error.

The good news is we (the big WE) have experience with Yamaha 70 hp 2stroke motors and the various whaler hulls.

When discussing props, you need to know what prop you have, brand and size. Two different brands in the same size do not perform the same due to blade design.

You do not want to exceed the 5500 rpm Wot limit. It can damage the motor. The fact you can tells me the prop you have has too little pitch. That is not to say it is the right brand either.

By searching on "Yamaha 70 prop" I found this thread which may be all you need. By the by, Tom Clark is a prop guru.

http://www.whalercentral.com/forum/vi...post_34145

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Thanks for the help. I talked to Gwinnett Marine for up close and personal information, hopefully this will help. I have a 17" ss prop that I'll try when they get through with the rough engine at low rpm situation. What about engine height? Could this cause an over reving problem? Presently the engine is mounted 1 1/2" above the transom. Also with a fuel load of 26 gallons and a 300 lb live load I had to maintain 3900 rpms to stay on plane. Could this also be due to engine height?
Zap

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The engine or the anti-cavitation plate is 1 1/2 inches up? It is the anti-cav plate that should be 1 1/2 " above the bottom of the boat.

What brand? Is there a part #?

As for load, is the weight all in the back? Shifting the battery or fuel tank amidships will help lower planing speed.

I presume you are trimmed all the way in (or down) when at low speed.

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Phil T,
The anti-cavitation plate is about 1 1/2" above the bottom of the boat. Would there be a crucial differance if the anti-cavitation plate were 1/2" up or down from being 1 1/2" above the bottom of the hull? In other words: How critical is the 1 1/2" measurement?

SS prop I have is a Power Tech M135RED-17P sr# is 9272242; the 2nd charachter may be a I instead of a 1. Local marine dealer told me it's been worked on a few times and won't have the same characteristics as it did when it was new but to give it a try and see what changes there are.

I have a Montauk sized Mahogany console with one battery under it and the fuel is under the RPS.

Thanks for the advise.

Regards,
Zap

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Hi Zap - I know you are finding this confusing, but it's really simpler than you are making it. Your initial expectation that the WOT RPM was a function of just the motor and prop was simply incorrect - the boat is a significant factor too. Now that you have the motor on the Montauk, you have to start over and find the right prop for the boat and motor combination - it would be very unlikely that the prop that came with the motor would work.

Think of prop pitch as the equivalent of gearing. You are exceeding the redline because your prop is pitched too low - very much the same situation as if you put your car in low gear and floored it on flat ground.

To answer some of your specific questions:

- the motor height makes a little difference, but not a lot. It sounds like you have the motor at the perfect height, so don't mess with that.
- if the SS prop you have is highly cupped and it's still a 17 pitch, it's probably too tall (too high a pitch) for you. But since it is paid for, and has been modified, it is worth a try. Just don't run it for a long time at WOT if you're only getting 5000 or less RPM.
- the reason you need 3900 RPM to stay on a plane is because your prop is pitched too low - the boat needs a certain speed to stay on a plane, and since you're "in low gear" you're having to rev way up to meet that speed.


Hope that helps!

Tim

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Carom,
Just to clarify: the prop on the boat with the 5800 rpm situation is a 15" prop. I've not had a chance to run the 17" stainles steel prop. So, theoretecally, with the over reving using a 15" prop shouldn't I be able to lower the rpms with a 17" prop.

Thanks,
Zap

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yep the 17 will lower rpm but you will notice slower time to plane.

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If both props are the same, then yes, the 17 will turn lower RPMs than the 15. However (unless I missed it) you were not specific about the 15 pitch prop, so I don't know if it is the same as your 17. The 17 is worth a try, and it will very likely lower your WOT RPMs. My concern is that it will lower them too much - your manual should tell you the acceptable WOT range, but in my opinion, for best performance you want the WOT RPM right up there near 5500.

Basically, both blade area and the "cup" of the blade also affect the RPMs you will get. A heavily-cupped SS prop often has the same effect as adding 2 inches of pitch. So if your 15 is a fairly flat prop, and your 17 is heavily cupped, you may find it's like going from a 15 to a 19.

Make sense? It's really a matter of trial and error - I am working on my 3rd prop on my new motor; hopefully I have it right this time but I won't know until spring!

Tim

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To add to the mind boggling info.

Moving up to a higher pitch prop from Power Tech will lower the WOT rpms. That is good.

Some brands are not desirable for the whaler hull so there is a chance this brand may still not be a good prop overall.

One vital item is not to change brands and size as I discussed above.

I think this may be time for me to ask for some help from the "guru".

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Carom and Phil T,

The local marine dealer told me the 15" aluminum prop didn't appear to be original to the engine (not a Yamaha prop). The 17" stainless steel prop and the 15" aluminum prop appear to be of the same basic design (neither is what one would consider a cupped prop).

Phil T,
"Moving up to a higher pitch prop from Power Tech will lower the WOT rpms. That is good." I need to understand the lingo vis-a-vie higher pitch vs. lower pitch: Is a 17" prop a higher pitched prop than a 15" prop? Or, do I have it bass ackwards?! As listed above, with the 15" aluminum prop I'm over revving and must stay 3900 rpm or greater to stay on plane with 26 gal. fuel etc. as described
previously.

If there are other 16' classic Whaler owners with Yamaha 70 engines I'd sure like to know the details of which prop has best suited your boat/engine combo.
Thanks,
Zap

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Let's just cut to the chase: What propeller would be a good fit for an old non-smirked 16'-7" Whaler with a Yamaha 70?

Let's start with the basics that we know.

- The Yamaha 70 HP two stroke has a 5000 - 5500 RPM recommended WOT range. It would be nice to get close to 5500 RPM at WOT

- The Yamaha 70 HP two stroke has a 2.33:1 gear ratio

- The old 16 foot Whaler with a 70 HP motor should be able to do about 35 MPH

- A well chosen propeller for this set up should yield (depending on the particular model of propeller) about 5-8 percent slip.

With this information we can easily use one of any number of online propeller calculators to figure out what pitch we should use.

So let's use my favorite Prop Calculator and see what we get inputting what we know.

Using 8 percent slip the prop calculator tells us we need 17" of pitch, so if Zap is using a 15" aluminum prop, that is not right. We would expect the motor to over-rev using this 15" pitch prop and that is exactly what he reports.

For every inch of pitch we typically see an inverse change in engine speed of about 150 - 200 RPM (this actually depends on the gear ratio and a few other things, but it a reasonable rough rule-of-thumb). With 2" too little pitch, we would then expect the motor to over-rev about 300-400 RPM. Zap reports that with the 15" he hits 5800 RPM, 300 RPM over. There you go.

I should comment on the dangers of over-revving an outboard; I think they are often over stated. There is nothing wrong with going a little over during propeller testing. The motor will not disintegrate or melt. You don't want to run it that way all day long but for 30 seconds or a minute, it's not going to make a lick of difference. I've run my own motors up to 600 RPM over redline during testing. Go ahead, see what your motor and prop will do. You need the data to make good decisions.

Motor height: The higher the motor is mounted on the transom, the higher the motor will rev with a given propeller.

The reason is two fold:

- The closer to the surface the prop is, the more aerated the water it turns in is and the less resistance the water gives the prop.

- The higher the motor is, the less drag from the gearcase there and the faster the boat will go. The higher the boat speed, the higher the engine speed (RPM).

If this motor is mounted on the transom say, two holes up, that is 1-1/2" of space between the top of the transom and the motor mounting bracket. For the 16'-7" Whaler that is a good place to be. No motor should ever be mounted all the way down on the transom and one hole up is pretty conservative. With a good prop, you can go three holes up and gain even more speed and efficiency.

So exactly WHAT 17" prop should Zap be using? We all have our opinions and mine is that nobody should be using cheap aluminum crap. Get a good stainless steel prop. You needn't spend a fortune.

PowerTech makes excellent propellers and they probably make more different models of propellers than anybody else on Earth, but they are spendy.

The RED3 is a decent model of theirs, but I would not be excited about one that had been reworked a few times unless it was offered for free.

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Tom,
Thanks for the education in prop size. The Power Tech was on the, defunct engine, when I got the boat; it's more or less a freebie. I'll try it out this weekend and report back to all.

Would the too small, aluminum prop be a contributing factor in the high rpms (3900) needed to stay on plane? 300 lb. live load, battery under mahogany console, 26 gal. fuel under mahogany RPS.

Thanks to all for the help.
Zap

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Yes, of course, a too small propeller will contribute to high RPM being needed to stay on plane.

Try the 17" stainless steel PowerTech RED3 and let us know exactly how it performs. We'll then go from there.

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Your aluminum prop might not also like being raised 1.5" Does it feel like its slipping?

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Dave,
I'm not atuned to the boat, or any boat for that matter, enough to tell if the prop is slipping. While running dead ahead and on plane I did feel like there was some cavitation during moderate acceleration going on. Would slipping feel a bit like cavitation?
Zap

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I have a 1961 Nauset with an 1987 70 HP two stroke Yamaha. It has a 13.5" x 17" aluminum prop. I used the boat about a month before starting renovation and found the performance to be OK.

Notes: I was not able to adequately obtain RPM ratings because the tach was only reading about 3800 and intermittent at that. The boat was quick out of the hole and maintained plane at a reasonable speed. I wasn't able to validate top speed, but with a GPS in spring I can check that. Sorry I can't be of more help but thought I would pass along where I am at so far.

According to the thread on 70 hp Yamahas, I may not be far off.

Dave

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My Whaler 15 has a 13 X 19 K aluminum prop. I want to upgrade to a SS prop. Any recommendations of manufacturer and best place to purchase?

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I've had my 15 for 25 yrs. and went through many props on the Evenrude 65 and 70 Yammys and found the Yammy SST black steel 19" was to best of all worlds. If you're planning to ski then I would have dropped to the 17.
On your boat I'd try the Yammy 17 first and make sure your tach is plugged in correctly. You need to be turning 58-5900 rpm on a one man load.