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After owning our 1996 Montauk 17 for 6 years I had almost given up on it. There were problems every year with the original Evinrude 90 2-stroke and it was becoming more work than fun to own.

My wife loves the Whaler (looks, layout, handling) and she wanted to fix it up but I was sick of it. I looked at new boats and was close to a deal on one when I found a good price on a new Yamaha 70 4-stroke. My wife was glad and I obliged (spending 1/3rd of what a new boat would cost).

I've had it in the water 3 times since the swap and am still breaking it in but I now have a renewed love for the Whaler. It runs so quiet and smooth and barely uses any fuel compared to the old 90.

I haven't run it WOT yet but will be sure to add an update when I do. Pics of the old and new setup are in my personal page.

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Hi Max,
I'm very interested to know how you like the F70 on your Montauk. I am planning on moving one from my 15' sport onto a newly acquired '88 Montauk. I really love the motor, and hopefully it will suit the boat well.

Dennis

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I'm interested in this as I think I'm going to have to do something about my '92 Johnson 90 in the off-season--whether to have someone go through it and rebuild as necessary or to source a new engine. Frankly, I'm not too enthused at the idea of spending what a new motor costs.

John K.

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When you say "New" is this the newer version of the Yamaha 70 Four Stroke that is lighter than the previous version?

Is this a 2011 model? or?

The new version weighs in at 257 pounds. I am not sure what the prior version weighed.
http://www.yamaha-motor.com/outboard/.../home.aspx

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Joe -- There is only one Yamaha F70. It was introduced last year.

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Yes, brand new. The dealer had ordered two; one for a customer and one to put in the store stock.

So I bought the one that wasn't accounted for and he gave me a good trade-in on my evinrude that had waaaayyyy low compression in one cylinder.

I'm trying to run it at least every other week for a couple hours so I should have the break-in done the first weekend in November. I'll give a full report then.

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Im also interested in performance. I am finally gonna chunk my 96 merc 90 this offseason. I had it completely rebuilt and it runs great, but I want a 4 stroke!

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Thanks.

I was thinking about the Honda models that have been getting lighter than the previous versions...

This Yamaha four stroke weighs almost identical to my old 1985 Evinrude 70 two stroke.

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Well, here's a mini-update; after the break-in period, we've opened it up a little and it doesn't jump on plane quite as quick as the 90 but it cruises just fine and feels like there not a loss of speed. The main test was pleasing the commander in chief (my wife) and she hasn't complained one bit.

When I finally remembed to get my gps out of the Jeep so we could test the top end, the gps was dead. So I asked Santa for a new gps for christmas. If that comes thru, I'll post results.

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I am now in the same boat so to speak. My 17' 88' Montauk had an old 75 Johnson. I was going back an forth with the dealer, I wanted to repower with a 90hp 4-stk Yamaha, he talked me into putting the 70hp 4-sty Yamaha on it.
All I can say is WOW, how quite, only have 2 hours on it breaking it in. We had quite a chop in the bay but it quickly got up on plane. In two hours it did not burn any gas so to speak.
We're going to love it.

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I forgot to mention how quiet it is. The first time I fired it up I thought it wasn't even running :) And, it sips so little gas it's like the Prius of the river.

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You guys are making me jealous! I need to sell my old merc and get one of those.

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Christmas is over!
What did you get for the top speed?

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Hi Madmax! How much did you pay for it? I'm thinking getting one for my 86 montauk

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Hey Madmax or hilyert - any chance you've been able to get some speed results? Are you able to pull skiiers out of wather easily?
My 88 model Yamaha 90 2-stroke still runs good but I always worry about it - plus the thought of a quite, smooth, clean & efficient 4 stroke sounds good - assuming it doesn't sacrafice too much power. Hence the questions above =)

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I've only taken it out for the second time. Six total hours on it.
I don't have a speedo on it, but it ran up to 6200 rpms quickly.
Too cold to pull anyone skiing yet. It felt like 35 to 40 mph which is fast enough for me. I had two people total weight approx 325lbs.

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I have a friend with a 2002 Montauk and a new F70 on it. He reports 38 MPH @ 6200 RPM using the 13-1/4" x 14" Yamaha Performance Series three blade propeller

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Tom, those numbers did not sound right to me and when I put them in to the prop calculator I got a negative number for slip. I wonder if he is reporting his prop size incorrectly or if he is not running the published 2.33:1 gear ratio?

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There is nothing unusual about negative slip calculations with the Yamaha Performance Series Three Blade propeller. This propeller is the same propeller as the Stiletto Advantage and the Turbo 1. All three are made by Precision Propeller Industries, Inc. which is now owned by Yamaha.

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Tom, my experience with my Montauk using a Stilleto Advantage 19 pitch on a 75 HP two-stroke Mercury with a 2.33 gear ratio runs 38 MPH @ 5200 RPM it calculates out to +5.37 slip. While the Yamaha has a higher max RPM it has the same gear ratio as my Mercury, same prop style and hull style so I do not understand why running a 14 pitch prop and calculated -7.87 slip would be correct? If the Yamaha had a 16 pitch prop then that would calculate to +5.75 slip which I would definitely understand and what I would expect.

For clarification the numbers I used were:
5200 RPM / 2.33:1 Gear Ratio / 19 pitch / +5.37 slip / 38 MPH
6200 RPM / 2.33:1 Gear Ratio / 14 pitch / -7.87 slip / 38 MPH
6200 RPM / 2.33:1 Gear Ratio / 16 pitch / +5.75 slip / 38 MPH

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According to the propeller engineers at Mercury, a negative slip is a physical impossibility. The boat could have no forward motion even with a zero slip calculation. There must be positive slip for a boat to be able to move forward.

If a calculation is coming up negative, one or more of the input numbers is incorrect.

According to Mercury, if a very low slip calculation is coming up on a single engine boat, the propeller diameter is too large, and energy is being wasted. My experience is that twin engine boats tend to run with lower slip than the same prop on a single, because of the larger amount of blade surface.

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Not all propellers are the same. There is a lot more to propeller design than just pitch, diameter and number of blades. Number of blades and diameter are simple things to understand and measure but pitch is another matter. Every propeller is assigned a pitch, usually expressed in inches, but not modern outboard propeller has "a" pitch; it has varying pitch all over the surface if its blades. The pitch is an approximation of how it would travel forward in one full rotation if it were able to rotate through a solid. Water is a fluid and air is a gas. BIG difference.

Every model of propeller is different and will yield a different typical range of calculated slip. The Yamaha Performance Series Three Blade is the same design as the Stiletto's Advantage model and Turbo's Turbo 1 model and all three models typically have very low or negative calculated slip figures. I've seen it over and over and over. I am not particularly concerned with why, I just know it to be the case so take it into account when crunching propeller performance numbers.

Larry's comments about actual negative slip are true, or close to it, but he continues to confuse calculated slip with actual slip which are not the same thing. Calculated slip is the ONLY kind of slip that is measurable.

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Tom, just for the record, I am only the messenger here. What I quoted was directly from Mercury's propeller reference material, available on line at their website.

So I guess you are saying MERCURY is confused about prop slip calculations, and what those calculations mean. You are certainly entitled to your opinion, but that is a pretty bold statement.

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Larry -- I never said Mercury is confused. Mercury is not confused. Mercury is talking about actual slip. They understand the difference. Apparently you do not.

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Sorry all, I've been off for a while. I travel a lot for work so I spend most of my home time on the boat or Jeep when I can. I got the GPS and ran 33.5 mph at 5500 rpms. That's with 20+ gal of fuel and three adults on pretty smooth water. I've been out every other weekend since March and have used less total fuel than I used to in a day with the Evinrude 90.

I'm running a 17 pitch prop and may switch to a 19 since I cruise about 60 miles round trip on a weekend and like keeping it below WOT. Any input from the experts?

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Max, I'm no expert, but if the numbers you quote are the WOT numbers, I'd be looking to go down in pitch, not up. You don't specify the type of prop (or maybe I missed it in an earlier post) but you could probably go down to a 13-pitch and get better performance. The WOT range of the F70 is 5300-6300, so technically you're OK with your current prop, but the guy who sold me my F70 was adamant that Yamaha 4-strokes like to have the WOT RPM right up near the redline, for best longevity. A 19-pitch will take you outside of the manufacturer's recommended range, so you certainly don't want to go that direction.

Tim

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That sounds about right. With just yourself on board the boat will probably go to about 37 MPH and your WOT engine speed will climb to over 6000 RPM without that extra 350 pounds of passengers (assuming normal sized, 175 pound-ish people).

May I presume you are running a 17" aluminum Yamaha prop?

Where did the motor get mounted on the transom?

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Yes on the Yamaha 17 aluminum propand the motor is mounted on the top holes which I guess is the lowest setting.

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Well folks, you can lead a horse to water but you can't make it drink.

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Mad -

You want to raise the motor two holes up.

Top set of holes:
O <---- you are here?
O
O<------should be here
O

I would have the dealer move it for no charge.

Tom (a prop guru) has repeatedly stated and many agree that aluminum props are not ideal (cheap poo poo). You should have the dealer swap the prop for a stainess (painted) Yamaha 17" prop.

After spending big bucks why not get it right?

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Max, before spending on a new prop, move the motor up 2 holes. That's easy enough to do yourself with a chainhoist, rope over a tree branch, or just your trailer, and it won't cost you anything other than perhaps some marine caulk for around the bolt holes.

Next try the performance again - your top speed, handling, and mileage should all improve, and you'll pick up some WOT RPM as well. Report back on your results and we'll go from there!

Good luck!

Tim

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1. Tracking on getting a stainless prop. I just didn't want to spend the extra money until I was sure on the size I need. It sounds like I should stick with the 17.

2. Other than giving me a couple extra inches of bottom clearance, what does moving the motor up a couple holes do?

(sorry if I'm ignorant on boat dynamics, If I was setting up a suspension on a Jeep, I'd be all over it)

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Short Answer= Less engine in the water is less drag and better performance. For each mounting hole you raise the motor, the engine gains 150-200 rpm's.

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Thanks Phil

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Moving the motor up will offer higher top speed, improved fuel mileage, faster acceleration, less bow rise on acceleration, lighter steering effort, reduced tendency to porpoise, less list to port from prop torque and, as noted, the ability to get into shallower water without a prop strike.

But other than that, not much.

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Funny!!

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I have the same set up 1988 17'Montauk with new 70hp 4 Yamaha. I mounted all the way up. Yes it is quicker, less fuel consummed but you tend to loose any help from you trim and tilt most of the time. on slick water it can run up to 6600 rpms but I think that is too much so I tone it down and try and keep it at 5500
or below. What RPM should I be turning? I also get some secondary spray comming off the transom / prop planed out unless I trim the motor nearly half the way down. I don't know what to make of that.
The heavier the load (people) the more it needs to be trimed down.

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Motor mounting height has to be set based on what propeller is being used. Not all propellers will tolerate high mounting height and still maintain grip. For example, no aluminum prop will tolerate being mounted so high on a Montauk.

hilyert -- What propeller is on your new F70?

The redline for the Yamaha F70 is 6300 RPM.

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Tom,
the prop is a Power Tech stainless steel 3 blade 13 1/4" X 13"

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Which one? PowerTech makes no less than 15 different models of "stainless steel 3 blade" propellers that will fit a Yamaha F70.

Whatever model you have, it sounds as if it is not a good fit on the Montauk/F70 combination.

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The F70 redlines at 6300, and starts dropping ignition / FI cycles at 6450 RPM. So if you're hitting 6600, you are definitely leaving performance on the table - meaning you'll increase your top speed and improve your mileage by going to a different prop.

Tim

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Tom,
The actual model I have is a PowrTech RED 3R13PYM90.
back on 1/13/12 you replied to me that the RED3 is a good prop which
PowerTech describes as ..... best all purpose prop, similar to Stiletto (Advantage)

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I guess I have to amend my opinion. The PowerTech RED3 does not seem to be like the Stiletto Advantage which has no trouble on a Yamaha F70 (or Evinrude E-TEC 90 for that matter) mounted all the way up on a Montauk.