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Author Topic:   1987 MONTAUK, 1997 Johnson 70

scottfarm posted 08-16-2007 09:00 PM ET (US)
Is 30-MPH top end what I should expect out of [a 1987 MONTAUK powered by a 1997 Johnson 70-HP] with a stainless steel 17-inch propeller? My 1996 montauk would run 44-MPH with a 75-HP Mercury. I suspect the Mercury is under rated and the Johnson is over rated on power. Is [the difference in the speed of these two boat and motor combinations caused by] my propeller? Seems like I had a 21-inch pitch on the 75-HP Mercury.
2manyboats posted 08-16-2007 10:16 PM ET (US)
I have a 1977 Montauk with a 1992 70 and I run 36-MPH. Is the 1997 a two-stroke or four-stroke?
Sal DiMercurio posted 08-16-2007 11:10 PM ET (US)
I'm sorry but I'm having a tough time swallowing that a 75 hp merc can twist a 21 pitch prop on a Montauk and get 44 mph. I doubt that 75 merc could twist 4200 rpm on that boat with a 21 pitch prop. I would say 38 - 40 mph with a "15 or 17" pitch prop for the 75 hp merc, and maybe 35 - 38 mph for the 70 Johnson, with either a 15 or 17 pitch prop. turniong 6000 rpms but not a 21p for either engine.
That 70 Johnson is rated to 6000 rpms & you better make sure she can reach it, [ at least 5500 ] , or your gonna score the middle cylinder.
That 70 Johnson is probably the best engine OMC ever made, but it has to be able to reach the top end of the rpm range, or she will kill herself.
Thats one engine [ 70 Johnson ] you don't dare over prop.
Sal
outragesteve posted 08-17-2007 03:01 AM ET (US)
The classic rig had got to be Montauk with a Johnson (or Evinrude) 70 looper. Excellent combo for most owners. Those engines were great on fuel and very dependable. (And I am a Merc guy!) I had a pair on a 17-foot low profile ski boat, a Fiberform splash of a Kona, and it still has to be one of my favorite boats owned.
jimh posted 08-17-2007 08:18 AM ET (US)
The predicted speeds for a great many motors used on a classic Boston Whaler MONTAUK are given in the REFERENCE section's several articles on the 16 and 17-foot hull. See:

http://continuouswave.com/whaler/reference/16-17/repower.html
Your boat's maximum top speed of 30-MPH is way off the mark.
I'd look around for a simpler explanation, say perhaps a problem with your motor or set up, before I'd join the brand zealots in trying to make a universal rule about horsepower ratings.
scottfarm posted 08-20-2007 05:52 AM ET (US)
Ok, what would be the best stainless prop for this motor boat combination and where is the cheapest place to buy. I ran it yesterday and barely eeked out 33 mph at 5800 rpm and I really have to almost trim out of the water to get it to 5800. It has a stainless 17 pitch but have not taken it off to check the diameter. The prop looks pretty rough, but no vibration.
Tohsgib posted 08-20-2007 11:26 AM ET (US)
Right prop but something is wrong. GPS in Stat MPH and not knots? Carbs might be a little cruddy, try some techron or Berryman's B-12 in a tank. 5800 full tilt boogie is good.
Tohsgib posted 08-20-2007 11:27 AM ET (US)
PS...no way did you get 44 out of a 75hp, they won't do it with a 90hp merc.
scottfarm posted 08-23-2007 07:56 PM ET (US)
Clark Roberts witnessed the 44mph with 75 merc. I trimmed it up where it was porpoising perty bad but it did hit 44. Cross my heart. Motor was up maximum height with doel fin.

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Author	Topic: 1987 MONTAUK, 1997 Johnson 70
scottfarm	posted 08-16-2007 09:00 PM ET (US) Is 30-MPH top end what I should expect out of [a 1987 MONTAUK powered by a 1997 Johnson 70-HP] with a stainless steel 17-inch propeller? My 1996 montauk would run 44-MPH with a 75-HP Mercury. I suspect the Mercury is under rated and the Johnson is over rated on power. Is [the difference in the speed of these two boat and motor combinations caused by] my propeller? Seems like I had a 21-inch pitch on the 75-HP Mercury.
2manyboats	posted 08-16-2007 10:16 PM ET (US) I have a 1977 Montauk with a 1992 70 and I run 36-MPH. Is the 1997 a two-stroke or four-stroke?
Sal DiMercurio	posted 08-16-2007 11:10 PM ET (US) I'm sorry but I'm having a tough time swallowing that a 75 hp merc can twist a 21 pitch prop on a Montauk and get 44 mph. I doubt that 75 merc could twist 4200 rpm on that boat with a 21 pitch prop. I would say 38 - 40 mph with a "15 or 17" pitch prop for the 75 hp merc, and maybe 35 - 38 mph for the 70 Johnson, with either a 15 or 17 pitch prop. turniong 6000 rpms but not a 21p for either engine. That 70 Johnson is rated to 6000 rpms & you better make sure she can reach it, [ at least 5500 ] , or your gonna score the middle cylinder. That 70 Johnson is probably the best engine OMC ever made, but it has to be able to reach the top end of the rpm range, or she will kill herself. Thats one engine [ 70 Johnson ] you don't dare over prop. Sal
outragesteve	posted 08-17-2007 03:01 AM ET (US) The classic rig had got to be Montauk with a Johnson (or Evinrude) 70 looper. Excellent combo for most owners. Those engines were great on fuel and very dependable. (And I am a Merc guy!) I had a pair on a 17-foot low profile ski boat, a Fiberform splash of a Kona, and it still has to be one of my favorite boats owned.
jimh	posted 08-17-2007 08:18 AM ET (US) The predicted speeds for a great many motors used on a classic Boston Whaler MONTAUK are given in the REFERENCE section's several articles on the 16 and 17-foot hull. See: http://continuouswave.com/whaler/reference/16-17/repower.html Your boat's maximum top speed of 30-MPH is way off the mark. I'd look around for a simpler explanation, say perhaps a problem with your motor or set up, before I'd join the brand zealots in trying to make a universal rule about horsepower ratings.
scottfarm	posted 08-20-2007 05:52 AM ET (US) Ok, what would be the best stainless prop for this motor boat combination and where is the cheapest place to buy. I ran it yesterday and barely eeked out 33 mph at 5800 rpm and I really have to almost trim out of the water to get it to 5800. It has a stainless 17 pitch but have not taken it off to check the diameter. The prop looks pretty rough, but no vibration.
Tohsgib	posted 08-20-2007 11:26 AM ET (US) Right prop but something is wrong. GPS in Stat MPH and not knots? Carbs might be a little cruddy, try some techron or Berryman's B-12 in a tank. 5800 full tilt boogie is good.
Tohsgib	posted 08-20-2007 11:27 AM ET (US) PS...no way did you get 44 out of a 75hp, they won't do it with a 90hp merc.
scottfarm	posted 08-23-2007 07:56 PM ET (US) Clark Roberts witnessed the 44mph with 75 merc. I trimmed it up where it was porpoising perty bad but it did hit 44. Cross my heart. Motor was up maximum height with doel fin.