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I just put a new Yamaha F70 on my 1988 Montauk. I started with a 13 5/8 x 14 prop, per my dealer's suggestion. Maiden voyage today. Followed first hour break-in of varying speeds under about 2000 rpms. Then got to the second hour where you are supposed to get boat up on plane. In order to do that with just me and 20 gal fuel I had to hit nearly 4000 rpm and keep it there to stay on plane. And I was only going 16-17 mph. Not even half throttle. This seems way too slow for 4000rpms. Do I need a different prop? More pitch? I haven't run WOT yet because I'm still breaking in, and honestly I'm kinda afraid to. Seems I could redline this thing really easily.

Anyone have a new(re) F70 on a classic Montauk 17? What prop?

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Just running the numbers shows 25% to 30% slip which looks high. Is your tach set correctly and how are you measuring speed?

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Hi,
I have a Stiletto Advantage I with a 13" pitch mounted on my Montauk. The Yamaha F70 is mounted 3 holes up (all the way). It gets on plane near 3000 although it is still squatting a bit at that speed, around 11.5 mph.

At 4000rpm I am at 21mph, and max speed at WOT is almost 36mph at which point the motor rev-limits.

Going to a 15" Stiletto did not allow the motor to reach max rpm, and Stiletto does not offer a 14" model.

How high is your motor mounted? With the 2 motors I had mounted by dealers, they both wanted to mount them lower.

Dennis

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Tach seems correct. Idles at 7-800 and responds accordingly, although I am somewhat surprised at how fast it spins up. Measuring speed via gps.

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That's helpful. What's the diameter prop you are running?

I will have to check the mounting holes. I know it seemed mounted high up when looking back at it yesterday, but I'll have to double check the holes.

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El -

I think your dealer is totally uninformed. Your prop is the wrong size and probably the wrong blade design.

Note the motor needs to be mounted 3 holes up.

The Stiletto brand are great props but recently stopped production.

I strongly suggest a Yamaha stainless steel prop. Stainless Steel and aluminum K series props will work with the 13" x 19" size.
If you want a Yamaha K series Performance 3 blade, you will need the 13-1/4" x 18" size.

As spec'ed the engine's WOT speed, lightly loaded, no gear, just you driving should be 43-45 mph.

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This is a dealer who has been installing yamahas on Boston whalers for over 30 years, so I have a hard time thinking he's all wet. This prop isn't too far off from what I've researched. How are you coming up with the 13 1/4 X 18 size. That's going up in pitch a lot, more than any other suggestions I've seen with my boat and motor.

I think your WOT numbers are too fast. I got near those speeds from my 90hp two stroke before, no way I'm gonna hit that now with a 70 4 stroke. I'd be happy with 35-36mph WOT, but what I'm really looking for is to be able to cruise at 25 mph and 4200 rpm, give or take.

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Oopps, WOT speeds and prop I listed are for F90.

You don't mention the brand or model of prop but the size itself is totally wrong regardless of brand.

Correct prop info is 13 1/4" x 14" Yamaha Performance Series. A member with this prop/engine on a classic Montauk gets 38 -39 MPH at 6200 rpm's.

I see you already posted this question on a different site on the same day. I also see you got practical advice there as well.

Forum shopping is not good form and is discouraged.

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I don't know what forum shopping is. I'm just trying to get information, that's all.

please educate me, wouldn't dropping down to a 13 1/4 x 14 be even smaller and give me higher RPMs than the 13 5/8 x 14 I currently have? That would be moving backwards wouldn't it? It's a factory white aluminum Yamaha prop. What is have now sure seems to spin up fast. Ideally, I'd like to be able to cruise at 4000-4200 rpm, give or take, and be doing about 25 mph.

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Everyone needs to forget about the diameter. The engineers take care of that for us.

We buy a prop by brand and pitch for our brand of motor.
http://www.whalercentral.com/articles...cle_id=108

We also cannot expect brand X 17 pitch to be the same as brand Y 17 pitch as the rake and aggressiveness of the brands can be totally different.

So, choosing a prop just by pitch is not that easy when trying to switch brands.

If you stay within the same brand and model of prop, then going up or down in pitch makes selecting a prop much easier. The less pitch the more WOT RPM's and more pitch gives you less WOT RPM's.

What is the recommended WOT RPM's for your motor?
What WOT RPM's are you getting from the different props?
You probably need to wait until your motor is past the break-in period to go WOT as you mention above.

So far, Dennis has given you the prop he uses.


Can you tell us how your motor is mounted?
http://www.whalercentral.com/articles...cle_id=106

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Understood. Right now it's mounted one hole up. Dealer said that was standard Whaler install but the he would change it for me if I wanted.

I'll report back tomorrow after I can run it harder.

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What Phil told you is correct,I have a Yamaha F70 and the Yamaha Performance Series 13.25 x 14 is the propeller that Tom Clark recommends for an F70 on a Montauk 17 mounted all the way up, 3 holes showing.

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can some one please educate me on what effect mounting higher has? Would that give me better speed, per RPM? In essence, that's what I need I think. At least thus far, I don't feel I'm going as fast as I should given the rpms.

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If the motor is mounted too low, it is creating drag and slowing you down some.

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Do what Phil and Joe suggest! Mount the engine all the way up and the prop is 13.25x14. The part# is MAR-GYT3B-V4-14. The dealer did not want mount the motor that high but I insisted. This is a great set up for a Montauk.

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Makes perfect sense. Thanks!

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I don't want to start the next installment of Prop Wars here, but I feel that using terms like "the right prop" is simplifying the process too much. There is likely a range of pitches that could be considered for a give boat/motor combination, and the best choice for you will depend upon your anticipated usage and priorities.

I tried a 15" Stiletto (from when the motor was mounted on my 1984 15' CC), the current 13" Stiletto, as well as what was purported to be a Yamaha 14" SS found on eBay by Tom. The 15" gave me the best fuel economy, but felt really sluggish. I liked the hole shot the 13" affords, and it works great for the loads I carry typically. The Yamaha was so similar to the 13" Stiletto that we questioned if it was really the prop advertised. I'm speculating that the Yamaha Performance Series 14" pitch mentioned above will get better top end and fuel economy, but at the expense of hole shot and acceleration with heavier loads. Bear in mind that the F70 is the same displacement as the F60 (996cc) , but with a higher max RPM (5000-6000 vs 5300-6300).

It may be that the Yamaha Performance Series 14" is the right prop for you, but I'm pretty pleased with my choice as well, given my size and uses.

Dennis

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Dennis, for reference, after several years running with a Stiletto Advantage 13.25 x 15 on my 15, I just switched to a 13 x 16 Power Tech SCD3. All I can say is "wow, what a difference!" The SCD3 is more satisfactory in every aspect of performance - better acceleration, holds a plane at lower speeds, better top speed too. The difference is really dramatic - wish I'd done it years ago!

My theory is that the SCD3, with it's smaller blade area, is easier for the F70 to spin up in the midrange, allowing the motor to reach the high revs where it really makes power. Somewhat surprisingly, the 13-pitch Advantage did not have that same effect - I tried it.

I know it could be different on a Montauk, but it might be worth a try - seriously, I can't tell you what a difference it makes. The SCD3 really makes the motor come alive.

And I don't want to start a prop war either, I just played a hunch with the SCD3 and am way beyond pleased with the results.

Tim

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Hi Tim,
Seems like you found a great fit for you and your boat. I'm not sure how things would translate to a classic Montauk either, but your approach might yield a good match there too. I probably won't go to the trouble though, as I will be selling mine within a year. I have an Outrage 18 that I am repowering at a glacial pace. Probably not fair to the glaciers these days.

Dennis