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1996 Montauk repowered with Yamaha 70 Four Stroke
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| Finnegan |
Posted on 01/14/12 - 12:55 AM
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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.
Edited by Finnegan on 01/14/12 - 12:57 AM |
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| Tom W Clark |
Posted on 01/14/12 - 7:10 AM
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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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| Finnegan |
Posted on 01/14/12 - 1:14 PM
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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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| Tom W Clark |
Posted on 01/14/12 - 2:56 PM
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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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| madmax79 |
Posted on 05/29/12 - 5:13 PM
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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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| tedious |
Posted on 05/30/12 - 5:14 AM
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madmax79 wrote:
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?
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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| Tom W Clark |
Posted on 05/30/12 - 7:13 AM
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I got the GPS and ran 33.5 mph at 5500 rpms. That's with 20+ gal of fuel and three adults...
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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| madmax79 |
Posted on 05/30/12 - 3:43 PM
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Tom W Clark wrote:
May I presume you are running a 17" aluminum Yamaha prop?
Where did the motor get mounted on the transom?
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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| Tom W Clark |
Posted on 05/30/12 - 5:53 PM
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Well folks, you can lead a horse to water but you can't make it drink.
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| Phil T |
Posted on 05/31/12 - 5:15 AM
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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?
1992 Outrage 17 I
2019 E-TEC 90, Viper 17 2+
2018 Load Rite Elite 18280096VT |
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| tedious |
Posted on 05/31/12 - 2:10 PM
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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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| madmax79 |
Posted on 07/04/12 - 9:15 AM
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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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| Phil T |
Posted on 07/04/12 - 12:33 PM
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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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| madmax79 |
Posted on 07/04/12 - 3:23 PM
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Thanks Phil
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| Tom W Clark |
Posted on 07/06/12 - 7:20 AM
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Other than giving me a couple extra inches of bottom clearance, what does moving the motor up a couple holes do?
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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| Mr T |
Posted on 07/06/12 - 11:44 AM
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Tom W Clark wrote:
Other than giving me a couple extra inches of bottom clearance, what does moving the motor up a couple holes do?
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.
Funny!!
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| hilyert |
Posted on 07/06/12 - 1:39 PM
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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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| Tom W Clark |
Posted on 07/07/12 - 9:04 AM
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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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| hilyert |
Posted on 07/09/12 - 7:16 AM
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Tom,
the prop is a Power Tech stainless steel 3 blade 13 1/4" X 13"
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| Tom W Clark |
Posted on 07/09/12 - 7:34 AM
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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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