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Author Topic:   Engine Mounting Height Guidelines

jimh posted 05-18-2008 11:23 AM ET (US)
Use this thread to comment or ask questions in regard to the REFERENCE article on engine mounting height
http://continuouswave.com/whaler/reference/engineMountingHeight.html
The article presents information from Mercury and their recommendations as given in their literature.
jimh posted 05-18-2008 11:36 AM ET (US)
These recommendations are from literature published c.1988, so the recommendations for mounting height may be a bit conservative.
Tom W Clark posted 05-18-2008 11:50 AM ET (US)
Well, that was going to be my one comment; the height recommendations seem conservatively low but this is due to the fact that propeller design has come a long way in the last twenty years and there are many common propellers today that have no trouble operating as surfacing propellers, not just cleavers.
jimh posted 05-18-2008 12:06 PM ET (US)
On the other hand, there are a lot of us who are using propellers and motors made in the late 1980's or early 1990's, so these guidelines are perhaps appropriate for those boats, motors, and propellers. Mercury has often been touted as being on the leading edge of performance, so their advice should have some authority behind it.
Tom W Clark posted 05-18-2008 12:13 PM ET (US)
Certainly a lot of us are using motors from that era, but NOT many of us are using propellers from that era. I know I am not and my boats performance improved substantially when I replaced the 1988 era propellers my boat originally had on it.
I also raised my motors by one bolt holt from their original position.
In contrast, outboard lower units have not changed much in this time. In fact the lower units on my outboards are identical in outward design to what is offered today and are interchangeable with 2008 models.
To anybody still using a prop from twenty years ago, my advice is to seek out a newer design. There are but a few exceptions to this.
jimh posted 05-18-2008 01:30 PM ET (US)
The water inlet design on the gear case also plays a roll in deciding the mounting height in some instances. One trend in modern motors is to have water inlets located lower on the gear case to allow for higher mounting height.
ConB posted 05-18-2008 05:26 PM ET (US)
OK. I just raised my engine one more hole.
I have a 1987 Outrage 18 with a 1987 Johnson 150hp. It's now up two holes.
I was getting some spray from the anti ventilation plate before.
Hope fully we will try it next week end.
Con
L H G posted 05-20-2008 08:03 PM ET (US)
Under any circumstances I can't imagine raising any single engine more than 1-1/2" over the water flow past the anti-vent pate. On twin engines even less. With most Whalers, 3/4" to 1-1/2" gives maximum performance. Above that, you are in the realm of bass boats and high performance, where top speed is the only consideration, and you risk long term overheating damage without low water pickups.
If you are running higher than 1-1/2", it's is doubtful you are gaining anything, even though the prop holds. You may actually be loosing top speed and low end, rough water performance, increasing slippage without knowing it. You stop going up when top speed does not increase by doing so.
If your top speed is only 40 MPH or less, you don't need any more sophisticated propeller than a Mercury Vengeance or Mirage Plus, or equivalent. For 30 and below, all you need is a good aluminum. All of this is why high performance props are not made in pitches to provide much less than 40 mph top end on a boat. Because of higher revving 4-strokes, lower pitch performance props are now being made for them. These new lower pitch offerings can be used on a 2-stroke, but you won't gain anything.
Tom W Clark posted 05-20-2008 10:33 PM ET (US)
Larrry,
I think you have a valid point. But it should be emphasized that there is a BIG difference between "1-1/2" over the water flow past the anti-vent pate" and 1-1/2 inches above the transom (as illustrated in the Mercury literature).
Water will rise to the surface as soon as a boat's hull has passed over it and the further an outboard's lower unit is from the transom, the higher the motor needs to be.
I have seen bread-and-butter outboard A/V plates on Montauks as much as 3 inches above the keel when only set back an added 4 inches, with no adverse effects at all even in the sloppiest conditions.
jimh posted 05-20-2008 11:15 PM ET (US)
I am confused by the introduction of the notion that four-stroke motors are "high revving." While the engine speeds might be slightly higher, most of the four-stroke motors turn a gear case with more reduction, so the propeller shaft speed is back to about the same speed as always.
L H G posted 05-21-2008 04:56 PM ET (US)
Jim, it is not a "notion", it's fact.
I'm only familiar with Mercs and Merc props, so that's what I'm talking.
Merc's two stroke V-6's, Opti, carb, and EFI, generally are redlined between 5500-5750. Until the advent of the Verado, redlined at 6400, all Merc props were offered in pitches that these engines needed.
Verados can turn an extra 800-900 RPM to produce the same HP, so they generally need lower pitched props to produce the same MPH speeds. All of this is why they are introducing slightly lower pitch version of some of the perfromance lines, and going to 1" increments also.
Regarding gear ratios, my EFI's are 1.86 and the Verado's are 1.85. the big 3.0 liter Opti's are 1.76. So gear ratios don't make much of difference. And even the smaller Verados have a 2.08 ratio vs 2.0 on the Optimax 135, but the RPM difference is 6400 vs 5750.
On the mid range engines, Merc's two stroke and Opti's use the SAME gear ratios and the 4-strokes, but once again the 4-strokes turn up to 600 RPM higher, requiring a lower pitched prop assuming same HP output, to produce same running pseeds.
jimh posted 05-22-2008 01:50 AM ET (US)
Let's look at Suzuki motors, which, for example, have quite high gear ratios. Those four-strokes red line at 6,000-RPM and have a gear reduction of 2.29.
The propeller shaft speed for a Suzuki four-stroke would be
6000 / 2.29 = 2,620-RPM (slower than two-cycle)

Let's compare that to my classic two-cycle V6 3.0-liter motor
6,000 / 1.86 = 3,225-RPM
The four-stroke motor is geared down so the propeller shaft is turning slower.
Let's check Honda
6,000 / 1.86 = 3,225 (same as two-cycle)

Let's check Yamaha
6,000 / 2.00 = 3,000-RPM (slower than two-cycle)
What I see is that the four-stroke motors have the same red line as my old two-stroke, and turn the propeller shaft either the same speed or slower than my old two-stroke. The "high-revving" (i.e, higher red line) seems to be unique to the Mercury products.
L H G posted 05-22-2008 03:05 AM ET (US)
I'm sticking to what I said. The example of a single 2-stroke that redlines at 6000 is rare. You run that old Evinrude 3 liter engine like that very long and you'll blow it up in no time. They are famous for it. Robert Moore had my 200 EFI's turning 6300 for a few minutes, and I was scared to death they were going to let go.
Does anybody else here know of any other 2-stroke, DFi or regular, that is factory red lined at 6000, except the racing division Mercs. Almost all red line at 5500, as I indicated.
The Suzuki example is also unusual, since they have some crazy gearing configuration in a failed attempt to to equal the supercharged Verado acceleration. Besides I was talking Merc high performance props like the Rev 4, Tempest Plus, Enertias etc. Honda, Yamaha and Suzuki's don't generally run those, and stay with their own brand of props.
The Merc 15" Rev-4 was specifically added to the line for the Verados used in underpowered single configurations.
Tom W Clark posted 05-22-2008 09:55 AM ET (US)
The Evinrude E-TEC 40, 50, 60, 115, 250 and 300 horsepower outboards all redline at 6000 RPM.
The classic Johnson and Evinrude 56.1 cubic inch 70 HP outboard made from 1986 to 2001 also redlined at 6000 RPM.
Tom W Clark posted 05-22-2008 10:32 AM ET (US)
Other old high revving two strokes:
The old 3.0 and 3.3 liter Ficht 200, 225 and 250 redlined at 6000 RPM.
The old Yamaha Excel 225 and the 2.6 liter ProV 15, 175 and 200 redlined at 6000 RPM as well as the 2.6 liter V Max EFI 200
Tom W Clark posted 05-22-2008 10:37 AM ET (US)
Doesn't the old bread-and-butter 90 degree block carb'ed Johnson and Evinrude 225 (like Jim has on his Revenge 22 WT WD) redline at 6000 RPM?
Tom W Clark posted 05-22-2008 10:47 AM ET (US)
Since derailment of this thread's topic seems to have been sanctioned by jimh, let's look at this bizarre statement:

quote:
You run that old Evinrude 3 liter engine like that very long and you'll blow it up in no time. They are famous for it.
Famous? Really? I missed that news. So operating your Evinrude outboard within the manufactures' specifications will cause it to "blow up"?
While I do not think anybody here is suggesting operating an outboard at full throttle for extended periods is going to enhance a motor's longevity, the notion that to even dare to operate it at full throttle for fear of destroying the motor, is absurd.
The motors on my Revenge redline at 5500 RPM. I routinely operate the motors to this speed or higher. I have had them up to 6200 RPM with no ill effect.
blkmtrfan posted 05-22-2008 01:19 PM ET (US)
Tom, I am with you on that one, I have always beleved that it is easier to "blow up" a 2 stroke by underreving (ie. lugging) than allowing it to spin up. Putting too large a prop and thereby keeping the WOT RPMs low will kill a motor in no time.
And Larry, your 2.5 200EFI mercs can spin alot more than 6300 (as long as you are running good oil) before they will get hurt.
Also the main reason the DFI two strokes have a lower max rev limit is due to the DFI system more so than the motor mechanicals. With DFI, there is only a short amount of time between the closing of the exhaust port and the top of the stroke, in the mean time the fuel still needs to mix with the air before the spark. As the speed of the motor increases, there becomes a point where you can't get the fuel in fast enough.
Tohsgib posted 05-22-2008 01:42 PM ET (US)
Larry that was pretty stupid to say there are not many 2 smokes that redline at 6k. Read the old OMC brochures. 35hp OMC redlined at 6k until early 80's then 5800. 15hp redlined at 7-7300. 8 & 9.9 6k. 60hp looper 2cyl 6k. V8 6k or 6250. 120, 130, & 140 loopers 6k. 50,60,70 3cyl loopers 6k. 115 faststrike 6k. Even the 1974 brochure shows 9.9, 15, 25, and 50 at 6k or higher.
Perry posted 05-22-2008 01:51 PM ET (US)
On the other hand, Suzuki DF300 and DF250 SS redline at 6300 RPM, DF175 redlines at 6100 RPM, DF50 redlines at 6500 RPM, DF140 redlines at 6200 RPM. Honda BF 90 EFI redline at 6300 and BF 20 redline at 6600 RPM.
Don't most 2 strokes redline below 6000 RPM and most 4 strokes redline at 6000 RPM?
L H G posted 05-22-2008 02:46 PM ET (US)
Nick - Your opinion on OMC 225 3.0 liter blow up frequency?
Buckda posted 05-22-2008 06:06 PM ET (US)
LHG -
Out of curiousity - how fast was the 25 running at 6,300 RPM? Must have been blazing!
Tohsgib posted 05-23-2008 10:49 AM ET (US)
Actually other than my own 225 blowing up TWICE in 2 years, I have not heard much about it. Their early CR lower units had a bad rep for blowing up. I still see a TON of 3.0 OMC's and they stopped making them in carb form in 1998 so they seem to be pretty reliable. Mine WAS very reliable until one of the SIX carbs went lean and poof! Thirsty son of a gun though and loud...especially compared to my 175 6 degree which is still running strong(1991) with probably 3k hours on it.
jimh posted 05-23-2008 03:41 PM ET (US)
At the risk of having my remarks called "stupid," I will offer this explanation for why some four-stroke motors are specified at a higher maximum RPM ("red line"): they need the RPM to make rated horsepower. Horsepower is roughly TORQUE x RPM, so the more engine revolutions the more horsepower, even though the torque output of the engine might be less than another engine with the same rated power at a lower RPM.

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Author	Topic: Engine Mounting Height Guidelines
jimh	posted 05-18-2008 11:23 AM ET (US) Use this thread to comment or ask questions in regard to the REFERENCE article on engine mounting height http://continuouswave.com/whaler/reference/engineMountingHeight.html The article presents information from Mercury and their recommendations as given in their literature.
jimh	posted 05-18-2008 11:36 AM ET (US) These recommendations are from literature published c.1988, so the recommendations for mounting height may be a bit conservative.
Tom W Clark	posted 05-18-2008 11:50 AM ET (US) Well, that was going to be my one comment; the height recommendations seem conservatively low but this is due to the fact that propeller design has come a long way in the last twenty years and there are many common propellers today that have no trouble operating as surfacing propellers, not just cleavers.
jimh	posted 05-18-2008 12:06 PM ET (US) On the other hand, there are a lot of us who are using propellers and motors made in the late 1980's or early 1990's, so these guidelines are perhaps appropriate for those boats, motors, and propellers. Mercury has often been touted as being on the leading edge of performance, so their advice should have some authority behind it.
Tom W Clark	posted 05-18-2008 12:13 PM ET (US) Certainly a lot of us are using motors from that era, but NOT many of us are using propellers from that era. I know I am not and my boats performance improved substantially when I replaced the 1988 era propellers my boat originally had on it. I also raised my motors by one bolt holt from their original position. In contrast, outboard lower units have not changed much in this time. In fact the lower units on my outboards are identical in outward design to what is offered today and are interchangeable with 2008 models. To anybody still using a prop from twenty years ago, my advice is to seek out a newer design. There are but a few exceptions to this.
jimh	posted 05-18-2008 01:30 PM ET (US) The water inlet design on the gear case also plays a roll in deciding the mounting height in some instances. One trend in modern motors is to have water inlets located lower on the gear case to allow for higher mounting height.
ConB	posted 05-18-2008 05:26 PM ET (US) OK. I just raised my engine one more hole. I have a 1987 Outrage 18 with a 1987 Johnson 150hp. It's now up two holes. I was getting some spray from the anti ventilation plate before. Hope fully we will try it next week end. Con
L H G	posted 05-20-2008 08:03 PM ET (US) Under any circumstances I can't imagine raising any single engine more than 1-1/2" over the water flow past the anti-vent pate. On twin engines even less. With most Whalers, 3/4" to 1-1/2" gives maximum performance. Above that, you are in the realm of bass boats and high performance, where top speed is the only consideration, and you risk long term overheating damage without low water pickups. If you are running higher than 1-1/2", it's is doubtful you are gaining anything, even though the prop holds. You may actually be loosing top speed and low end, rough water performance, increasing slippage without knowing it. You stop going up when top speed does not increase by doing so. If your top speed is only 40 MPH or less, you don't need any more sophisticated propeller than a Mercury Vengeance or Mirage Plus, or equivalent. For 30 and below, all you need is a good aluminum. All of this is why high performance props are not made in pitches to provide much less than 40 mph top end on a boat. Because of higher revving 4-strokes, lower pitch performance props are now being made for them. These new lower pitch offerings can be used on a 2-stroke, but you won't gain anything.
Tom W Clark	posted 05-20-2008 10:33 PM ET (US) Larrry, I think you have a valid point. But it should be emphasized that there is a BIG difference between "1-1/2" over the water flow past the anti-vent pate" and 1-1/2 inches above the transom (as illustrated in the Mercury literature). Water will rise to the surface as soon as a boat's hull has passed over it and the further an outboard's lower unit is from the transom, the higher the motor needs to be. I have seen bread-and-butter outboard A/V plates on Montauks as much as 3 inches above the keel when only set back an added 4 inches, with no adverse effects at all even in the sloppiest conditions.
jimh	posted 05-20-2008 11:15 PM ET (US) I am confused by the introduction of the notion that four-stroke motors are "high revving." While the engine speeds might be slightly higher, most of the four-stroke motors turn a gear case with more reduction, so the propeller shaft speed is back to about the same speed as always.
L H G	posted 05-21-2008 04:56 PM ET (US) Jim, it is not a "notion", it's fact. I'm only familiar with Mercs and Merc props, so that's what I'm talking. Merc's two stroke V-6's, Opti, carb, and EFI, generally are redlined between 5500-5750. Until the advent of the Verado, redlined at 6400, all Merc props were offered in pitches that these engines needed. Verados can turn an extra 800-900 RPM to produce the same HP, so they generally need lower pitched props to produce the same MPH speeds. All of this is why they are introducing slightly lower pitch version of some of the perfromance lines, and going to 1" increments also. Regarding gear ratios, my EFI's are 1.86 and the Verado's are 1.85. the big 3.0 liter Opti's are 1.76. So gear ratios don't make much of difference. And even the smaller Verados have a 2.08 ratio vs 2.0 on the Optimax 135, but the RPM difference is 6400 vs 5750. On the mid range engines, Merc's two stroke and Opti's use the SAME gear ratios and the 4-strokes, but once again the 4-strokes turn up to 600 RPM higher, requiring a lower pitched prop assuming same HP output, to produce same running pseeds.
jimh	posted 05-22-2008 01:50 AM ET (US) Let's look at Suzuki motors, which, for example, have quite high gear ratios. Those four-strokes red line at 6,000-RPM and have a gear reduction of 2.29. The propeller shaft speed for a Suzuki four-stroke would be 6000 / 2.29 = 2,620-RPM (slower than two-cycle) Let's compare that to my classic two-cycle V6 3.0-liter motor 6,000 / 1.86 = 3,225-RPM The four-stroke motor is geared down so the propeller shaft is turning slower. Let's check Honda 6,000 / 1.86 = 3,225 (same as two-cycle) Let's check Yamaha 6,000 / 2.00 = 3,000-RPM (slower than two-cycle) What I see is that the four-stroke motors have the same red line as my old two-stroke, and turn the propeller shaft either the same speed or slower than my old two-stroke. The "high-revving" (i.e, higher red line) seems to be unique to the Mercury products.
L H G	posted 05-22-2008 03:05 AM ET (US) I'm sticking to what I said. The example of a single 2-stroke that redlines at 6000 is rare. You run that old Evinrude 3 liter engine like that very long and you'll blow it up in no time. They are famous for it. Robert Moore had my 200 EFI's turning 6300 for a few minutes, and I was scared to death they were going to let go. Does anybody else here know of any other 2-stroke, DFi or regular, that is factory red lined at 6000, except the racing division Mercs. Almost all red line at 5500, as I indicated. The Suzuki example is also unusual, since they have some crazy gearing configuration in a failed attempt to to equal the supercharged Verado acceleration. Besides I was talking Merc high performance props like the Rev 4, Tempest Plus, Enertias etc. Honda, Yamaha and Suzuki's don't generally run those, and stay with their own brand of props. The Merc 15" Rev-4 was specifically added to the line for the Verados used in underpowered single configurations.
Tom W Clark	posted 05-22-2008 09:55 AM ET (US) The Evinrude E-TEC 40, 50, 60, 115, 250 and 300 horsepower outboards all redline at 6000 RPM. The classic Johnson and Evinrude 56.1 cubic inch 70 HP outboard made from 1986 to 2001 also redlined at 6000 RPM.
Tom W Clark	posted 05-22-2008 10:32 AM ET (US) Other old high revving two strokes: The old 3.0 and 3.3 liter Ficht 200, 225 and 250 redlined at 6000 RPM. The old Yamaha Excel 225 and the 2.6 liter ProV 15, 175 and 200 redlined at 6000 RPM as well as the 2.6 liter V Max EFI 200
Tom W Clark	posted 05-22-2008 10:37 AM ET (US) Doesn't the old bread-and-butter 90 degree block carb'ed Johnson and Evinrude 225 (like Jim has on his Revenge 22 WT WD) redline at 6000 RPM?
Tom W Clark	posted 05-22-2008 10:47 AM ET (US) Since derailment of this thread's topic seems to have been sanctioned by jimh, let's look at this bizarre statement: quote: You run that old Evinrude 3 liter engine like that very long and you'll blow it up in no time. They are famous for it. Famous? Really? I missed that news. So operating your Evinrude outboard within the manufactures' specifications will cause it to "blow up"? While I do not think anybody here is suggesting operating an outboard at full throttle for extended periods is going to enhance a motor's longevity, the notion that to even dare to operate it at full throttle for fear of destroying the motor, is absurd. The motors on my Revenge redline at 5500 RPM. I routinely operate the motors to this speed or higher. I have had them up to 6200 RPM with no ill effect.
blkmtrfan	posted 05-22-2008 01:19 PM ET (US) Tom, I am with you on that one, I have always beleved that it is easier to "blow up" a 2 stroke by underreving (ie. lugging) than allowing it to spin up. Putting too large a prop and thereby keeping the WOT RPMs low will kill a motor in no time. And Larry, your 2.5 200EFI mercs can spin alot more than 6300 (as long as you are running good oil) before they will get hurt. Also the main reason the DFI two strokes have a lower max rev limit is due to the DFI system more so than the motor mechanicals. With DFI, there is only a short amount of time between the closing of the exhaust port and the top of the stroke, in the mean time the fuel still needs to mix with the air before the spark. As the speed of the motor increases, there becomes a point where you can't get the fuel in fast enough.
Tohsgib	posted 05-22-2008 01:42 PM ET (US) Larry that was pretty stupid to say there are not many 2 smokes that redline at 6k. Read the old OMC brochures. 35hp OMC redlined at 6k until early 80's then 5800. 15hp redlined at 7-7300. 8 & 9.9 6k. 60hp looper 2cyl 6k. V8 6k or 6250. 120, 130, & 140 loopers 6k. 50,60,70 3cyl loopers 6k. 115 faststrike 6k. Even the 1974 brochure shows 9.9, 15, 25, and 50 at 6k or higher.
Perry	posted 05-22-2008 01:51 PM ET (US) On the other hand, Suzuki DF300 and DF250 SS redline at 6300 RPM, DF175 redlines at 6100 RPM, DF50 redlines at 6500 RPM, DF140 redlines at 6200 RPM. Honda BF 90 EFI redline at 6300 and BF 20 redline at 6600 RPM. Don't most 2 strokes redline below 6000 RPM and most 4 strokes redline at 6000 RPM?
L H G	posted 05-22-2008 02:46 PM ET (US) Nick - Your opinion on OMC 225 3.0 liter blow up frequency?
Buckda	posted 05-22-2008 06:06 PM ET (US) LHG - Out of curiousity - how fast was the 25 running at 6,300 RPM? Must have been blazing!
Tohsgib	posted 05-23-2008 10:49 AM ET (US) Actually other than my own 225 blowing up TWICE in 2 years, I have not heard much about it. Their early CR lower units had a bad rep for blowing up. I still see a TON of 3.0 OMC's and they stopped making them in carb form in 1998 so they seem to be pretty reliable. Mine WAS very reliable until one of the SIX carbs went lean and poof! Thirsty son of a gun though and loud...especially compared to my 175 6 degree which is still running strong(1991) with probably 3k hours on it.
jimh	posted 05-23-2008 03:41 PM ET (US) At the risk of having my remarks called "stupid," I will offer this explanation for why some four-stroke motors are specified at a higher maximum RPM ("red line"): they need the RPM to make rated horsepower. Horsepower is roughly TORQUE x RPM, so the more engine revolutions the more horsepower, even though the torque output of the engine might be less than another engine with the same rated power at a lower RPM.