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Author Topic:   170 MONTAUK: Speed with 115-HP

Marsh posted 05-31-2005 09:31 PM ET (US)
I have an 2004 170 Montauk, with an 2005 Merc 115-HP four-stroke. Yesterday, per my brand new GPS, I maxed out at 42-MPH at 5,800-RPM. That was my max observed speed, on several runs, trying different trim/tilt settings, and different directions on the lake. Runs were made on a relatively flat lake, with little to no wind or current. I was alone in my boat, with approx 20 gallons of fuel (at least to start out with, LOL), a trolling motor, 3 batteries, the normal anchor/safety gear, and minimal gear otherwise. The boat handled fine. Not loose; definitely not "squirrely". I am a bit disappointed in that I was hoping for 50+ mph, but apparently have too much weight and too little hp to ever hit 50. The 170 Montauk is a great boat, with many wonderful qualities. However, it is apparently more of a plodder than a speedster.
Respectfully,
Marsh

LHG posted 05-31-2005 10:32 PM ET (US)
Marsh - Since BW says the boat will do 42 with the carbed 90, I would say you're not set up right, or have an engine problem, unless you are at high elevation. You should get more like 46-48 out of that setup I would think. Engine should probably be in third set of holes, and either an 18" Vengeance or 20" Laser II prop should do it.
What does the Mercurymarine.com prop selector program give you speed and pitch wise?
jimh posted 05-31-2005 10:42 PM ET (US)
[Separated from another discussion about a different boat and different horsepower.]
jimh posted 05-31-2005 10:45 PM ET (US)
If speed is a function of horsepower to the 0.5 power, then an increase in horsepower to 115 from 90 is
(115/90)^0.5 = 1.13 increase in speed
The boat speed with a 90 is 42, then boat speed with 115 should increase to
42 * 1.13 = 47-MPH
jimh posted 05-31-2005 11:41 PM ET (US)
One problem: if the Mercury 90 is really 98-HP, then the increase to 115-HP will only produce
((115/98)^0.5)*42= 45.5 MPH
divefan posted 06-01-2005 12:15 AM ET (US)
I don't do it as a rule but my 2004 with the 90HP 4-stroke will do that speed at same RPM. Stock prop, 12 gal. gas, 1 battery, sometimes with several people and dive gear. I can only do it if it's completely flat in the ocean or the ICW. I think LHG and jimh are correct. Something may be wrong.
Marlin posted 06-01-2005 07:28 AM ET (US)
As a point of comparison, my 160 Dauntless will do 45 with its 115 4-stroke. The 170 is just a touch lighter, and has a bit of a flatter bottom, so I'd expect you to put a couple miles an hour onto my top speed, maybe 47 or 48.
Your speed and RPM suggest to me that you're running the same 16" Vengeance prop that I am. I think the Montauk calls for a little more pitch, maybe the 18" Vengeance or a higher performance prop as LHG suggests.
-Bob
Marsh posted 06-01-2005 06:09 PM ET (US)
A few more details:
I am running the same Merc Vengance SS prop that came from the factory on the 90 hp 4 stroke that was on the boat when I bought it. I re-powered to a 115 4 stroke, and kept the same prop. The 115 now has 30 hours of running time, per the hour meter. Number on the prop says 48-16988 18P, which I understand to be an 18 pitch. Elevation of the lake is 872 above MSL at full summer pool. GPS is a brand new Lowrance 332C. There is certainly a possibility that engine, or prop, or elevation, or GPS measurement error play a part in the observed speed. However, I really think the answer is simple: my 170 is just slow. A plodder.
The boat does actually feel much faster than it did with the 90, but I realize that is subjective. Before I repowered from the 90 to the 115, the boat just did not run as fast as I wanted. I had no speedo, nor GPS, and thus no way to actually measure speed. Consequently, I upgraded to the 115, which FEELS much faster. Obviously, at only 42 mph, it's not all that fast. But compared to the 90, it FEELs much, much faster. (Maybe I'm just trying to convince myself that the repower was worth it.)
At any rate, I am satisfied with the boat's performance. I would be more satisfied at 52 mph than at 42, but that's life. There's no way I will ever re-power again in a quest for more speed. My next analytic will be to weigh the boat. I wonder if maybe it is actually heavier than I think???
Moral to the story: if speed is important to you, buy a bass boat. If it's unsinkability you want, stick with Whaler, McKee, or equivalent. You can't get in a hurry, but at least they won't sink to the bottom.
Later,
Marsh
LHG posted 06-01-2005 08:37 PM ET (US)
Everything sounds OK, including the prop. Engine should probably be in the middle set of holes if the prop will handle it. Are you sure that brand new GPS is not reading in KNOTS, which would be about the expected 47-48 MPH? I think they are factory set for knots.
Marsh posted 06-01-2005 09:13 PM ET (US)
LHG:
Nah, the GPS is set to MPH...first thing I checked. Engine is mounted in middle set of holes. Prop seems to handle it OK, as long as I trim it down a bit for take off.
One interesting thing from my runs up and down the lake: per my sonar unit, the water surface temp ranged from 57 degrees at one end of lake, in 12-feet deep tailwaters of a dam, to 72 degrees in 40-feet deep waters just 12 miles downstream. Tellico Lake, downstream from Chilhowee Dam on the Little Tennessee River, for anyone interested.
Marsh
LHG posted 06-01-2005 11:14 PM ET (US)
Current running against you? Could just be a bad tolerance powerhead. I guess that's why they have the 10% rule.
Otherwise I'm stumped. The engine should have more "go".
Even on an a larger and heavier deeper vee 18 Outrage, a 115 Merc in-line 6 will produce 42 MPH. You may have to get a Verado block 115 EFI 4-stroke instead of the Yamaha block version!
jimh posted 06-01-2005 11:40 PM ET (US)
Let's run the number through the Propeller Calculator
RPM=5,800
Ratio= 2.07:1
Pitch=18
MPH=42
Calculated SLIP = 12.1 percent
The SLIP is just a shade high, but not unusual for a single engine boat. With that propeller (and assuming about 10-percent slip), to get 48 MPH you would have to turn the engine to 6,340 RPM . That is not going to ever happen. I'd say you are running about right with the current propeller. I don't see why one would suspect low horsepower from the engine. It is turning to 5,700 RPM and that is right in the middle of where it is rated.
Let's see what happens with a 20-inch pitch and 10-percent slip:
48-MPH would happen at 5,700 RPM. However, if you can only twist the 18-inch propeller to 5,700 RPM, there is no reasonable basis to conclude you could therefore twist a 20-inch propeller to the same speed. It would be more likely you would turn it about 200-300 RPM lower. Let's see what happens at 5,400 RPM with a 20-inch pitch and the 12-percent SLIP number:
MPH= 43.5
If you really want to predict the true speed potential, you need to total up the weight on the boat.
elaelap posted 06-02-2005 12:08 AM ET (US)
With a 2004 Yamaha 115 4/s EFI/19" pitch prop turning 5600 rpm I can just get my classic Outrage 18 to hit 40 mph on the GPS. I throw these figures into the mix because I've always thought that the Montauk 170 was more closely related to the classic OR 18s than to the classic Montauks, especially as to weight and beam (the radical difference being, of course, the MT 170's lack of an internal fuel tank comparable to the OR 18's 63 gallon internal tank).
Tony
bigjohn1 posted 06-02-2005 12:11 AM ET (US)
How did Tabasco get his same setup (with same prop) to go 51MPH?
LHG posted 06-02-2005 12:18 AM ET (US)
Jim - I'm basing my assumptions on the performance data for the 170 at whaler.com. This shows the 90 HP version of the 4-stroke at 42.1 MPH, and the 2-stroke 90 (about 98 true HP) at 44.3 mph.
So it would be reasonable to assume a true 115 (even if it is made by non-performance leader Yamaha!) would move the boat more like 46-48 MPH, and be faster than the 2-stroke 90?
If this it cannot do, maybe that's why Whaler limited the HP to 90. But Marsh has said the EFI 115 is a lot faster than the carb 90, so I'm still stumped unless the Whaler published performance data is a stretch. Actually, I have found their data to be a bit conservative in most cases
jimh posted 06-02-2005 08:02 AM ET (US)
You can get to almost there, but you have to go up to a 20-inch propeller pitch:
RPM=6,000
Ratio=2.07
Pitch=20
Slip=10 (assumed)
MPH=49.4 (calculated)
Maybe with
RPM=6,000
Ratio=2.07
Pitch=21
Slip=12 (assumed)
MPH=50.7 (calculated)
Jim Bennett posted 06-02-2005 09:24 AM ET (US)
Marsh,
Remember from last summer at our Tennessee Rendezvous that you were talking about repowering your Montauk to 115. Sorry to hear results not what you had hoped but you are getting good info from all the experts. I know I am talking about apples, oranges and pears here but for more comparisons, my 1973 Outrage 21, 2003 Merc 150 2 stroke, will peak at 47mph properly trimmed and my 1968 Sakonnet, 1979 Merc 115 inline 2-stroke, will hit 45mph(untrimmed - no PT/T). Would seem that your boat weight may be a significant factor.
In mid-August last year, surface temperature at my dock(14 ft deep) on Lake Tellico was near 80F. Going 25 miles upstream to same Chilhowee Dam, surface temp was still in low 60's - the water stays chilly at Chilhowee coming out of the mountains. Who knows what temp is at many places on Tellico where water over 90 ft deep.
Keep hoping to see someone planning another TN Rendezvous!
Jim
Marsh posted 06-03-2005 07:36 AM ET (US)
Gotta get my boat on some scales...that's the one factor that has not yet been measured. Maybe this week end.

Marsh
Tarpun posted 06-03-2005 07:51 AM ET (US)
Marsh Do you have bottom paint on your Whaler ? Paint can signifiantly reduce speed. Just a thought.
imko posted 06-03-2005 09:14 AM ET (US)
I also have a Montauk 170 with a 90 Mercury 4-stroke (2003)
The prop is a Laser II 13,25 x 20".
Speed: 44,1 mph, 5700 rpm. Thats about 6% slip.
The engine is mounted in middle set of holes.
Regards,
Imko
bigjohn1 posted 06-04-2005 06:35 AM ET (US)
Amazing, I never thought the 90 or the 115efi would turn that Laser-II 20" prop to 5,700 rpms.
jimh posted 06-04-2005 09:50 AM ET (US)
Here is the data from Imko for his 170 MONTAUK with 90-HP:
RPM=5,700
Ratio=2.33 (this is from the specifications for 90-HP four-stroke)
Pitch=20
Slip= 4.8 (calculated)
MPH=44.1
That is a very low slip number. The boat/motor/propeller combination is really dialed-in. (Or, the rated pitch of that Mercury propeller is bit on the low side of actual.)
Now we have a problem. Here is the same boat, also with a four-stroke motor, and it goes faster (44.1-MPH) than Marsh's boat (42-MPH) even though it has an engine with 25-HP lower rating!
Maybe somebody switched the cowlings on these two motors! Now I am getting worried about the performance of Marsh's boat. Something is not quite right in all of these numbers.
bigjohn1 posted 06-05-2005 07:08 AM ET (US)
Agreed, there is likely something amiss in all of this. Although I have the same boat/engine combination as Marsh, I'm still running with a Black Max aluminum prop so my experience means nothing from a comparison standpoint to this. I'm wondering if the brand new GPS needs to have its speed calibrated. On my chartplotter, the manual recommends running next to a known accurate boat and compare - or do a times run over a known distance. If speed is off a bit, there is a user-performed "fine tuning" adjustment to make the speed spot-on accurate. I wonder also if its a combination of the GPS AND the two extra batteries and extra fuel onboard. If the GPS was 2-3 mph off and then corrected, and another run was made with only 6 gallons of gas (one stock tank) and ONE battery in glass calm water, I'll bet he'd get very close to that magic 50mph.
imko posted 06-05-2005 07:36 AM ET (US)
I think the Vengeance 18" is the perfect prop for a 115 EFI
4-stroke because the have a gear ratio 2,07:1.
The 90 4-stroke has a gear ratio 2,33:1.
With the Vengeance i hit: 42 mph at 6200 RPM!! To much
for the 90 (max.6000 rpm) This is with 2 pers. and 2 fuel tanks (2x 6 gallon)mounted in second hole.
The Laser II max trimmed out, 1 pers. and 1 tank (6 gallon)
Rpm 5700-5800. (with a clean bottom)i hit 44,1 mph again.
(garmin GPS)
lblanch posted 06-10-2008 02:46 PM ET (US)
I got 38 mph out of my 2006 Montauk with a Vengence 48-16988 18P prop. That makes me feel good because that means the guys with the gas guzzling 115's are paying a lot more to OPEC for that extra 4 mph.
Larry
Nauti Tauk posted 06-14-2008 08:08 PM ET (US)
Had to chime in here. 2003 90hp/4s carbed Merc. 170 Montauk. I have the engine one set of holes from as high as it'll go with a LazerII 20"p prop. Boat speed with just me and 12 or so gallons of gas,is 42.5-44+(Georgia/Florida)gps/mph at @5800rpms and 40.1@5500rpms as of yesterday on Lake Lanier with Kevin(MiniTauk) and I loaded to fish for Stripers. Something just doesn't sound right with the numbers on the 115, My .02 is raise the engine to get the rpms up and see what happens. If I REALLY trim my 90 out I can just brush 6k rpms in Florida at sea level, can't get that many in Georgia. Jim had it right, get the rpms up to max and that boat should "come alive".
Tohsgib posted 06-16-2008 11:05 AM ET (US)
What was your speed with the 90hp vs the 115? I have a Stiletto 20" if interested as well.
mikemdd posted 06-16-2008 02:02 PM ET (US)
I didn't see anyone mention it, but the RPM range for the 115HP Four Stroke is 5800-6400, assuming a later model Four Stroke. Based on the initial posting, it seems that Marsh is running at the very low rpm limit at 5800 rpm. Any lower and it will out of spec. at WOT with that prop.
Mike

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Author	Topic: 170 MONTAUK: Speed with 115-HP
Marsh	posted 05-31-2005 09:31 PM ET (US) I have an 2004 170 Montauk, with an 2005 Merc 115-HP four-stroke. Yesterday, per my brand new GPS, I maxed out at 42-MPH at 5,800-RPM. That was my max observed speed, on several runs, trying different trim/tilt settings, and different directions on the lake. Runs were made on a relatively flat lake, with little to no wind or current. I was alone in my boat, with approx 20 gallons of fuel (at least to start out with, LOL), a trolling motor, 3 batteries, the normal anchor/safety gear, and minimal gear otherwise. The boat handled fine. Not loose; definitely not "squirrely". I am a bit disappointed in that I was hoping for 50+ mph, but apparently have too much weight and too little hp to ever hit 50. The 170 Montauk is a great boat, with many wonderful qualities. However, it is apparently more of a plodder than a speedster. Respectfully, Marsh
LHG	posted 05-31-2005 10:32 PM ET (US) Marsh - Since BW says the boat will do 42 with the carbed 90, I would say you're not set up right, or have an engine problem, unless you are at high elevation. You should get more like 46-48 out of that setup I would think. Engine should probably be in third set of holes, and either an 18" Vengeance or 20" Laser II prop should do it. What does the Mercurymarine.com prop selector program give you speed and pitch wise?
jimh	posted 05-31-2005 10:42 PM ET (US) [Separated from another discussion about a different boat and different horsepower.]
jimh	posted 05-31-2005 10:45 PM ET (US) If speed is a function of horsepower to the 0.5 power, then an increase in horsepower to 115 from 90 is (115/90)^0.5 = 1.13 increase in speed The boat speed with a 90 is 42, then boat speed with 115 should increase to 42 * 1.13 = 47-MPH
jimh	posted 05-31-2005 11:41 PM ET (US) One problem: if the Mercury 90 is really 98-HP, then the increase to 115-HP will only produce ((115/98)^0.5)*42= 45.5 MPH
divefan	posted 06-01-2005 12:15 AM ET (US) I don't do it as a rule but my 2004 with the 90HP 4-stroke will do that speed at same RPM. Stock prop, 12 gal. gas, 1 battery, sometimes with several people and dive gear. I can only do it if it's completely flat in the ocean or the ICW. I think LHG and jimh are correct. Something may be wrong.
Marlin	posted 06-01-2005 07:28 AM ET (US) As a point of comparison, my 160 Dauntless will do 45 with its 115 4-stroke. The 170 is just a touch lighter, and has a bit of a flatter bottom, so I'd expect you to put a couple miles an hour onto my top speed, maybe 47 or 48. Your speed and RPM suggest to me that you're running the same 16" Vengeance prop that I am. I think the Montauk calls for a little more pitch, maybe the 18" Vengeance or a higher performance prop as LHG suggests. -Bob
Marsh	posted 06-01-2005 06:09 PM ET (US) A few more details: I am running the same Merc Vengance SS prop that came from the factory on the 90 hp 4 stroke that was on the boat when I bought it. I re-powered to a 115 4 stroke, and kept the same prop. The 115 now has 30 hours of running time, per the hour meter. Number on the prop says 48-16988 18P, which I understand to be an 18 pitch. Elevation of the lake is 872 above MSL at full summer pool. GPS is a brand new Lowrance 332C. There is certainly a possibility that engine, or prop, or elevation, or GPS measurement error play a part in the observed speed. However, I really think the answer is simple: my 170 is just slow. A plodder. The boat does actually feel much faster than it did with the 90, but I realize that is subjective. Before I repowered from the 90 to the 115, the boat just did not run as fast as I wanted. I had no speedo, nor GPS, and thus no way to actually measure speed. Consequently, I upgraded to the 115, which FEELS much faster. Obviously, at only 42 mph, it's not all that fast. But compared to the 90, it FEELs much, much faster. (Maybe I'm just trying to convince myself that the repower was worth it.) At any rate, I am satisfied with the boat's performance. I would be more satisfied at 52 mph than at 42, but that's life. There's no way I will ever re-power again in a quest for more speed. My next analytic will be to weigh the boat. I wonder if maybe it is actually heavier than I think??? Moral to the story: if speed is important to you, buy a bass boat. If it's unsinkability you want, stick with Whaler, McKee, or equivalent. You can't get in a hurry, but at least they won't sink to the bottom. Later, Marsh
LHG	posted 06-01-2005 08:37 PM ET (US) Everything sounds OK, including the prop. Engine should probably be in the middle set of holes if the prop will handle it. Are you sure that brand new GPS is not reading in KNOTS, which would be about the expected 47-48 MPH? I think they are factory set for knots.
Marsh	posted 06-01-2005 09:13 PM ET (US) LHG: Nah, the GPS is set to MPH...first thing I checked. Engine is mounted in middle set of holes. Prop seems to handle it OK, as long as I trim it down a bit for take off. One interesting thing from my runs up and down the lake: per my sonar unit, the water surface temp ranged from 57 degrees at one end of lake, in 12-feet deep tailwaters of a dam, to 72 degrees in 40-feet deep waters just 12 miles downstream. Tellico Lake, downstream from Chilhowee Dam on the Little Tennessee River, for anyone interested. Marsh
LHG	posted 06-01-2005 11:14 PM ET (US) Current running against you? Could just be a bad tolerance powerhead. I guess that's why they have the 10% rule. Otherwise I'm stumped. The engine should have more "go". Even on an a larger and heavier deeper vee 18 Outrage, a 115 Merc in-line 6 will produce 42 MPH. You may have to get a Verado block 115 EFI 4-stroke instead of the Yamaha block version!
jimh	posted 06-01-2005 11:40 PM ET (US) Let's run the number through the Propeller Calculator RPM=5,800 Ratio= 2.07:1 Pitch=18 MPH=42 Calculated SLIP = 12.1 percent The SLIP is just a shade high, but not unusual for a single engine boat. With that propeller (and assuming about 10-percent slip), to get 48 MPH you would have to turn the engine to 6,340 RPM . That is not going to ever happen. I'd say you are running about right with the current propeller. I don't see why one would suspect low horsepower from the engine. It is turning to 5,700 RPM and that is right in the middle of where it is rated. Let's see what happens with a 20-inch pitch and 10-percent slip: 48-MPH would happen at 5,700 RPM. However, if you can only twist the 18-inch propeller to 5,700 RPM, there is no reasonable basis to conclude you could therefore twist a 20-inch propeller to the same speed. It would be more likely you would turn it about 200-300 RPM lower. Let's see what happens at 5,400 RPM with a 20-inch pitch and the 12-percent SLIP number: MPH= 43.5 If you really want to predict the true speed potential, you need to total up the weight on the boat.
elaelap	posted 06-02-2005 12:08 AM ET (US) With a 2004 Yamaha 115 4/s EFI/19" pitch prop turning 5600 rpm I can just get my classic Outrage 18 to hit 40 mph on the GPS. I throw these figures into the mix because I've always thought that the Montauk 170 was more closely related to the classic OR 18s than to the classic Montauks, especially as to weight and beam (the radical difference being, of course, the MT 170's lack of an internal fuel tank comparable to the OR 18's 63 gallon internal tank). Tony
bigjohn1	posted 06-02-2005 12:11 AM ET (US) How did Tabasco get his same setup (with same prop) to go 51MPH?
LHG	posted 06-02-2005 12:18 AM ET (US) Jim - I'm basing my assumptions on the performance data for the 170 at whaler.com. This shows the 90 HP version of the 4-stroke at 42.1 MPH, and the 2-stroke 90 (about 98 true HP) at 44.3 mph. So it would be reasonable to assume a true 115 (even if it is made by non-performance leader Yamaha!) would move the boat more like 46-48 MPH, and be faster than the 2-stroke 90? If this it cannot do, maybe that's why Whaler limited the HP to 90. But Marsh has said the EFI 115 is a lot faster than the carb 90, so I'm still stumped unless the Whaler published performance data is a stretch. Actually, I have found their data to be a bit conservative in most cases
jimh	posted 06-02-2005 08:02 AM ET (US) You can get to almost there, but you have to go up to a 20-inch propeller pitch: RPM=6,000 Ratio=2.07 Pitch=20 Slip=10 (assumed) MPH=49.4 (calculated) Maybe with RPM=6,000 Ratio=2.07 Pitch=21 Slip=12 (assumed) MPH=50.7 (calculated)
Jim Bennett	posted 06-02-2005 09:24 AM ET (US) Marsh, Remember from last summer at our Tennessee Rendezvous that you were talking about repowering your Montauk to 115. Sorry to hear results not what you had hoped but you are getting good info from all the experts. I know I am talking about apples, oranges and pears here but for more comparisons, my 1973 Outrage 21, 2003 Merc 150 2 stroke, will peak at 47mph properly trimmed and my 1968 Sakonnet, 1979 Merc 115 inline 2-stroke, will hit 45mph(untrimmed - no PT/T). Would seem that your boat weight may be a significant factor. In mid-August last year, surface temperature at my dock(14 ft deep) on Lake Tellico was near 80F. Going 25 miles upstream to same Chilhowee Dam, surface temp was still in low 60's - the water stays chilly at Chilhowee coming out of the mountains. Who knows what temp is at many places on Tellico where water over 90 ft deep. Keep hoping to see someone planning another TN Rendezvous! Jim
Marsh	posted 06-03-2005 07:36 AM ET (US) Gotta get my boat on some scales...that's the one factor that has not yet been measured. Maybe this week end. Marsh
Tarpun	posted 06-03-2005 07:51 AM ET (US) Marsh Do you have bottom paint on your Whaler ? Paint can signifiantly reduce speed. Just a thought.
imko	posted 06-03-2005 09:14 AM ET (US) I also have a Montauk 170 with a 90 Mercury 4-stroke (2003) The prop is a Laser II 13,25 x 20". Speed: 44,1 mph, 5700 rpm. Thats about 6% slip. The engine is mounted in middle set of holes. Regards, Imko
bigjohn1	posted 06-04-2005 06:35 AM ET (US) Amazing, I never thought the 90 or the 115efi would turn that Laser-II 20" prop to 5,700 rpms.
jimh	posted 06-04-2005 09:50 AM ET (US) Here is the data from Imko for his 170 MONTAUK with 90-HP: RPM=5,700 Ratio=2.33 (this is from the specifications for 90-HP four-stroke) Pitch=20 Slip= 4.8 (calculated) MPH=44.1 That is a very low slip number. The boat/motor/propeller combination is really dialed-in. (Or, the rated pitch of that Mercury propeller is bit on the low side of actual.) Now we have a problem. Here is the same boat, also with a four-stroke motor, and it goes faster (44.1-MPH) than Marsh's boat (42-MPH) even though it has an engine with 25-HP lower rating! Maybe somebody switched the cowlings on these two motors! Now I am getting worried about the performance of Marsh's boat. Something is not quite right in all of these numbers.
bigjohn1	posted 06-05-2005 07:08 AM ET (US) Agreed, there is likely something amiss in all of this. Although I have the same boat/engine combination as Marsh, I'm still running with a Black Max aluminum prop so my experience means nothing from a comparison standpoint to this. I'm wondering if the brand new GPS needs to have its speed calibrated. On my chartplotter, the manual recommends running next to a known accurate boat and compare - or do a times run over a known distance. If speed is off a bit, there is a user-performed "fine tuning" adjustment to make the speed spot-on accurate. I wonder also if its a combination of the GPS AND the two extra batteries and extra fuel onboard. If the GPS was 2-3 mph off and then corrected, and another run was made with only 6 gallons of gas (one stock tank) and ONE battery in glass calm water, I'll bet he'd get very close to that magic 50mph.
imko	posted 06-05-2005 07:36 AM ET (US) I think the Vengeance 18" is the perfect prop for a 115 EFI 4-stroke because the have a gear ratio 2,07:1. The 90 4-stroke has a gear ratio 2,33:1. With the Vengeance i hit: 42 mph at 6200 RPM!! To much for the 90 (max.6000 rpm) This is with 2 pers. and 2 fuel tanks (2x 6 gallon)mounted in second hole. The Laser II max trimmed out, 1 pers. and 1 tank (6 gallon) Rpm 5700-5800. (with a clean bottom)i hit 44,1 mph again. (garmin GPS)
lblanch	posted 06-10-2008 02:46 PM ET (US) I got 38 mph out of my 2006 Montauk with a Vengence 48-16988 18P prop. That makes me feel good because that means the guys with the gas guzzling 115's are paying a lot more to OPEC for that extra 4 mph. Larry
Nauti Tauk	posted 06-14-2008 08:08 PM ET (US) Had to chime in here. 2003 90hp/4s carbed Merc. 170 Montauk. I have the engine one set of holes from as high as it'll go with a LazerII 20"p prop. Boat speed with just me and 12 or so gallons of gas,is 42.5-44+(Georgia/Florida)gps/mph at @5800rpms and 40.1@5500rpms as of yesterday on Lake Lanier with Kevin(MiniTauk) and I loaded to fish for Stripers. Something just doesn't sound right with the numbers on the 115, My .02 is raise the engine to get the rpms up and see what happens. If I REALLY trim my 90 out I can just brush 6k rpms in Florida at sea level, can't get that many in Georgia. Jim had it right, get the rpms up to max and that boat should "come alive".
Tohsgib	posted 06-16-2008 11:05 AM ET (US) What was your speed with the 90hp vs the 115? I have a Stiletto 20" if interested as well.
mikemdd	posted 06-16-2008 02:02 PM ET (US) I didn't see anyone mention it, but the RPM range for the 115HP Four Stroke is 5800-6400, assuming a later model Four Stroke. Based on the initial posting, it seems that Marsh is running at the very low rpm limit at 5800 rpm. Any lower and it will out of spec. at WOT with that prop. Mike