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Author Topic:   Normal Propeller SLIP

Nauti Tauk posted 11-17-2009 09:05 AM ET (US)
I just plugged in the numbers for my 2003 170 Montauk powered by a 90-HP Mercury FourStroke. The calculation indicates I have a SLIP factor of just over 11% at 5,800-RPM and 43.8-MPH. Is [a propeller SLIP of just over 11-percent] about right? I ask because in order to get the 47-MPH the Boston Whaler site indicates for this year boat-engine combination the effective slip would be extremely small. If this is the case, what can I do to reduce the amount of SLIP on my setup? I'm using a Laser II propeller and the engine is up two holes.
jimh posted 11-17-2009 09:19 AM ET (US)
In a single propeller installation it is normal for the calculated SLIP to be in the vicinity of ten percent when the boat is at maximum speed.
dg22 posted 11-17-2009 11:11 AM ET (US)
I think manufacture stats on on their products have about a 10% "slip factor" lol. So if the manufacture says 47 mph, I think 43 mph is about right for your boat.
Tom W Clark posted 11-17-2009 11:56 AM ET (US)
Typical prop slip for a single is usually 5 - 15 percent though figures greater or lower are not necessarily bad or uncommon.
There is too much emphasis put on propeller slip. Calculated prop slip is simply a tool to help analyze how a given propeller is performing on a given boat.
Low prop slip should NOT be a goal by itself. If the boat is faster, or more fuel efficient, or accelerates better with a high propeller slip number, then so be it.
Nauti Tauk -- Your goal should not be to lower your slip but to improve your boat speed. If that can be done by making changes that you can prop slip to help measure, that's fine, but the calculated prop slip itself is not the point of focus.
pcrussell50 posted 11-18-2009 10:09 PM ET (US)
If there was a hull change between MY 2003 and 2009, that could account for it. This is manifested most succinctly in the 13 foot sizes, where the current models are a little faster despite being twice the weight of the classics.
Peter
Santa Barbara, CA
jimh posted 11-19-2009 09:13 AM ET (US)

quote:
"If there was a hull change between MY 2003 and 2009, that could account for it."
If I understand that statement, what is mean by "it" in the above quotation is that a particular boat of a particular model year does not go as fast as the published performance for that particular model year. I do not understand how changes in hull design in subsequent model years--if indeed there were any--would affect how an existing boat of a particular model year would perform in relation to the published performance of that boat for that model year.
Boston Whaler's published performance data generally always includes a notice that the performance is only for the particular boat and motor as tested on that particular day, with the particular conditions that existed that day, and so on.
There are many things which can affect performance, however I do not believe that a change in the hull design which will be made on future models can affect the performance of existing models. Failure to match the performance obtained by Boston Whaler is more likely related to other factors.
pcrussell50 posted 11-19-2009 01:58 PM ET (US)
LOL, Jim. :) That sure was a verbose way of saying, "He was using BW performance data for his year of hull." Though I don't know where on the BW web site, he got data for a 2003 hull. I was only able to find performance data for a 2007. Did BW make a design change to the Montauk 17 between 2003 and 2007? Admittedly, (and shamefully) I don't know.
Peter
Santa Barbara, CA
Tom W Clark posted 11-19-2009 04:22 PM ET (US)
Peter,
No, there has been no design change to the hull between 2002 and today. Your 2003 and a 2007 Montauk 170 share the same hull.
Where do you see a report of the Montauk 170 doing 47 MPH with a Mercury 90 Fourstroke?
Your reported speed of 43.8 MPH is actually pretty good compared to reports at the time this model was introduced. Here we see the boat doing 43 MPH with a Mercury 90 Fourstroke:
http://continuouswave.com/whaler/reference/170Montauk/
I believe there *has* been a change to the Mercury 90 Fourstroke in that time frame, however. That may account for some difference in performance.
pcrussell50 posted 11-19-2009 05:06 PM ET (US)
Tom, just in the interest of clarity, it's not my boats we're talking about. The OP, Nauti Tauk, was the one wondering why he was only getting 43.8mph out of his 2003 Montauk 17, while the BW web site's test boat went 47. In my ignorance, I suggested that since the BW test boat was a 2007, maybe there was a hull change since his 2003 that accounted for the speed difference. Now I know there was not a hull change, so I have nothing further to add.
Peter
Santa Barbara, CA
dg22 posted 11-19-2009 05:40 PM ET (US)
I take back my comment. BW factory performance data is very accurate. I just checked out their site for myself.
jimh posted 11-19-2009 11:35 PM ET (US)
Peter--You may get a laugh out of my "verbosity," but I had to read your article about five times to figure out that the heck "it" referred to.
Nauti Tauk posted 11-20-2009 09:16 AM ET (US)
Thanks to all who replied to my post regarding the amount of propeler slip I'm getting with my rig. To address the question of where I got the 47mph figure, I printed out the performance chart from the BW site when I purchased the boat in 2006. For whatever reason,the information hadn't been updated since the introduction of the new hull design in 2003. The current speed chart on the BW site is much closer to the performance that I'm getting. Still trying to justify purchasing a 115 HO Etec! My Montauk doesn't really start to "come alive" till I reach 40mph, can only imagine how it would run/feel up around 50 or so.
Tom W Clark posted 11-20-2009 01:28 PM ET (US)
When did the Mercury 90 FourStroke stop being a Yamaha and become a Mercury?
number9 posted 11-20-2009 08:22 PM ET (US)
Tom,
Mercury 90hp L4NA started model year 2007.
Bill

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Author	Topic: Normal Propeller SLIP
Nauti Tauk	posted 11-17-2009 09:05 AM ET (US) I just plugged in the numbers for my 2003 170 Montauk powered by a 90-HP Mercury FourStroke. The calculation indicates I have a SLIP factor of just over 11% at 5,800-RPM and 43.8-MPH. Is [a propeller SLIP of just over 11-percent] about right? I ask because in order to get the 47-MPH the Boston Whaler site indicates for this year boat-engine combination the effective slip would be extremely small. If this is the case, what can I do to reduce the amount of SLIP on my setup? I'm using a Laser II propeller and the engine is up two holes.
jimh	posted 11-17-2009 09:19 AM ET (US) In a single propeller installation it is normal for the calculated SLIP to be in the vicinity of ten percent when the boat is at maximum speed.
dg22	posted 11-17-2009 11:11 AM ET (US) I think manufacture stats on on their products have about a 10% "slip factor" lol. So if the manufacture says 47 mph, I think 43 mph is about right for your boat.
Tom W Clark	posted 11-17-2009 11:56 AM ET (US) Typical prop slip for a single is usually 5 - 15 percent though figures greater or lower are not necessarily bad or uncommon. There is too much emphasis put on propeller slip. Calculated prop slip is simply a tool to help analyze how a given propeller is performing on a given boat. Low prop slip should NOT be a goal by itself. If the boat is faster, or more fuel efficient, or accelerates better with a high propeller slip number, then so be it. Nauti Tauk -- Your goal should not be to lower your slip but to improve your boat speed. If that can be done by making changes that you can prop slip to help measure, that's fine, but the calculated prop slip itself is not the point of focus.
pcrussell50	posted 11-18-2009 10:09 PM ET (US) If there was a hull change between MY 2003 and 2009, that could account for it. This is manifested most succinctly in the 13 foot sizes, where the current models are a little faster despite being twice the weight of the classics. Peter Santa Barbara, CA
jimh	posted 11-19-2009 09:13 AM ET (US) quote: "If there was a hull change between MY 2003 and 2009, that could account for it." If I understand that statement, what is mean by "it" in the above quotation is that a particular boat of a particular model year does not go as fast as the published performance for that particular model year. I do not understand how changes in hull design in subsequent model years--if indeed there were any--would affect how an existing boat of a particular model year would perform in relation to the published performance of that boat for that model year. Boston Whaler's published performance data generally always includes a notice that the performance is only for the particular boat and motor as tested on that particular day, with the particular conditions that existed that day, and so on. There are many things which can affect performance, however I do not believe that a change in the hull design which will be made on future models can affect the performance of existing models. Failure to match the performance obtained by Boston Whaler is more likely related to other factors.
pcrussell50	posted 11-19-2009 01:58 PM ET (US) LOL, Jim. :) That sure was a verbose way of saying, "He was using BW performance data for his year of hull." Though I don't know where on the BW web site, he got data for a 2003 hull. I was only able to find performance data for a 2007. Did BW make a design change to the Montauk 17 between 2003 and 2007? Admittedly, (and shamefully) I don't know. Peter Santa Barbara, CA
Tom W Clark	posted 11-19-2009 04:22 PM ET (US) Peter, No, there has been no design change to the hull between 2002 and today. Your 2003 and a 2007 Montauk 170 share the same hull. Where do you see a report of the Montauk 170 doing 47 MPH with a Mercury 90 Fourstroke? Your reported speed of 43.8 MPH is actually pretty good compared to reports at the time this model was introduced. Here we see the boat doing 43 MPH with a Mercury 90 Fourstroke: http://continuouswave.com/whaler/reference/170Montauk/ I believe there has been a change to the Mercury 90 Fourstroke in that time frame, however. That may account for some difference in performance.
pcrussell50	posted 11-19-2009 05:06 PM ET (US) Tom, just in the interest of clarity, it's not my boats we're talking about. The OP, Nauti Tauk, was the one wondering why he was only getting 43.8mph out of his 2003 Montauk 17, while the BW web site's test boat went 47. In my ignorance, I suggested that since the BW test boat was a 2007, maybe there was a hull change since his 2003 that accounted for the speed difference. Now I know there was not a hull change, so I have nothing further to add. Peter Santa Barbara, CA
dg22	posted 11-19-2009 05:40 PM ET (US) I take back my comment. BW factory performance data is very accurate. I just checked out their site for myself.
jimh	posted 11-19-2009 11:35 PM ET (US) Peter--You may get a laugh out of my "verbosity," but I had to read your article about five times to figure out that the heck "it" referred to.
Nauti Tauk	posted 11-20-2009 09:16 AM ET (US) Thanks to all who replied to my post regarding the amount of propeler slip I'm getting with my rig. To address the question of where I got the 47mph figure, I printed out the performance chart from the BW site when I purchased the boat in 2006. For whatever reason,the information hadn't been updated since the introduction of the new hull design in 2003. The current speed chart on the BW site is much closer to the performance that I'm getting. Still trying to justify purchasing a 115 HO Etec! My Montauk doesn't really start to "come alive" till I reach 40mph, can only imagine how it would run/feel up around 50 or so.
Tom W Clark	posted 11-20-2009 01:28 PM ET (US) When did the Mercury 90 FourStroke stop being a Yamaha and become a Mercury?
number9	posted 11-20-2009 08:22 PM ET (US) Tom, Mercury 90hp L4NA started model year 2007. Bill