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Author Topic:   Montauk 17 Rpms-Speeds in MPH w/90 HP Rude
The Chesapeake Explorer posted 10-14-2002 10:21 PM ET (US)   Profile for The Chesapeake Explorer   Send Email to The Chesapeake Explorer  
I will be repowering my 87 Montauk probably in the spring. My 1988 Evinrude 90 has 1500+ hors and the writing is on the wall I have posted and recieved several suggestions for repowering. I just did time and distance runs at verying rpm to see what speeds I realy like to ride at. Here are the results
Boat: 1987 BW Montauk. Twin 12 gal steel tanks with about 14 gallons gas. A plywood carpeted front deck and a cooler about 60 extra pounds. Myself aboard 200lbs, standing into the wind except flat out I was sitting.The boat is botttom painted with Pettit Trinidad.
The current engine weighs 301 lbs by the manual and has 99.6 ci and 90 hp at 5000rpm. V4-Evinrude 90 1988 model. I ran between bouys R86 to 87 and 6-2 on the Potomac River at Wash. DC, These bouys are located 1 statute mile apart (NOAA chart # 12285 Potomac River). There was a very small chop and the temp. was about 60F with low humidity. The prop is a OMC aluminum 13 1/4 X 17 pitch.
Here are my results:Rounded to the nearest value.
2000 RPM =9 MPH "Tractor Speed"
2500 RPM =14MPH "Barely on Plane"
3000 RPM =20MPH "Easy Cruising Speed"
3200 RPM =26MPH "Nice Cruise Speed"
3500 RPM =27MPH "Good Cruise Speed"
4000 RPM =30 MPH "Stepping up the Pace"
5000 RPM =44 MPH " Flat Out Trimmed Up"

Flat out was 1:37 for a statute mile at 43.8 MPH. It seems the speed really came up between 3000 and 3200 RPM. The reason is the at at 300 the boat has just gotten on plane and at 3200 it is on plane real good. Planing speed is between 2500-2800 RPM or 14- 18 MPH with the bow high and the stern low. "Tractor Speed is 2000 RPM and 9 MPH, I call it "Tractor" because at this speed the bow is really high and this is the speed I slow down to in rough weather and she crawls over all kinds of bad stuff. When the seas get really bad on the Chesapeake Bay I sometimes have to throttle back to nothing if they are steep, but the tractor speed is pretty good in lots of rough weather. Looking at this chart I can see I will need a new motor that can give me at least 22 MPH for a cruise speed. I am leaning now to a new Yamaha 90 2 stroke due to weight, cost, and performance but may opt for a 4 stroke 60 yet.. When the boat was brand new with a 21 inch pitch prop she would pull 4700 RPM no bottom paint or front deck and 6 gal of gas. The boat was overproped by the dealer from day 1 with this pitch ..however it was a speedster and I did see a measured mile once at the magic 50mph number. She would have a tendency to chine walk at these speeds and you had to be wakes around. I have read some posts about over powering the Montauk with 115,s ect. You will void the manufactures certificate at the least, your insurance may not pay also if any thing happens. 90 Hp is plently on a Montauk. I am sure we all would like to hear from other Montauk owners with similar results in real life with regards to engines, props rpms and speeds. 60 divided by time = speed in MPH 60 divided by 1 minute 30 seconds as in 60/1.30 is 46.15 MPH

Bigshot posted 10-15-2002 11:27 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I think your measured mile, prop pitch or tachometer is off. A 17" alum prop spinning 5000(which is 500 under redline) should run about 38mph on that boat/motor combo. The motor(when new) would have spinned that 17 about redline and given you about 44. Being your rpm is only 5k....about 38mph.
Wild Turkey posted 10-15-2002 11:30 AM ET (US)     Profile for Wild Turkey  Send Email to Wild Turkey     

Here is the performance data for my setup. I wouldn't replace that motor, it seems pretty strong to me.

1989 Montauk, 1989 90HP V4 Evinrude
SS 19 pitch Michigan Wheel Rapture prop.
CMC setback plate (ML-65): cav. plate 2-1/2" above the bottom of transom
Light load: Me (235 lbs) and 13 gallons fuel
No bottom paint

All data by GPS:
WOT - 43.8 mph @ 5600 rpm. A little chine walk and HOLD ON!!
cruise - 31.1 mph @ 4000 rpm


PSW posted 10-15-2002 07:13 PM ET (US)     Profile for PSW  Send Email to PSW     
As is the case most of the time, Bigshot is right on the money. Your speed is accurate, but the rpm's are to low. I had a 19 with a GT100 and was a few mph faster on GPS, although not much. 90 horsepower is plenty unless you are pulling a water skier. In which case you need more power or less pitch.
The Chesapeake Explorer posted 10-15-2002 10:17 PM ET (US)     Profile for The Chesapeake Explorer  Send Email to The Chesapeake Explorer     
The original 21 pitch prop would pull about 4600 RPM. The Tach is a Tempo brand I put it on about when I got the boat its been about 12-13 years but I do remember that it could be wired several ways but I did set it for Evinrude 4 cyl,my tach may be off then by 500 rpm but on low idel it does read about 800 rpm,not 1300. A real dead slow speed is about 1000 rpm with no wake and a 1200 rpm speed was about 5 miles per hour the other day. My engine was mounted really low..I think, so low that the bottom bolts pulled a crack in the gel coat and probably the glass a bit that was done by the dealer who rigged it.I sealed the cracks long ago and made custom aluminum plates that the bolts went thur as the washers were pulling into the bottom of the transon and thats were the gel coat crack was.. This is on the inside of the transom well. They probably should have set the motor up another inch. A engine mounted too low will not pull all the rpms as a engine mounted in the right place as there is more drag on the skeg but the tach could well be off also. I have done several time distance runs and 1:37 sounds pretty much where it should be. The motor is still strong but with 1500 hours it will not last forever, I have heard that 2000 hours is pretty much max for these outboards and that is a good question for a thread. The compression is about 115/110 on three and 105/100 on the forth cyl.This checked when motor is warm ,wide open throttle and all plugs pulled,and equal turns applyed to the tested cylinder.(good subject for a thread how to check compression)
The Chesapeake Explorer posted 10-15-2002 10:41 PM ET (US)     Profile for The Chesapeake Explorer  Send Email to The Chesapeake Explorer     
Forgot to add 1.The prop is OMC 13 1/4X 17, its stamped into the hub I put it on in 1993 belive it or not! (I like deep water!) 2. the measured mile could be off but these bouys are right on the money at 1 statute mile on the chart , also the WW Wilson Bridge carring Interstate 95 is about 2 miles up river of the main set of bouys I used as my mile. The Potomac has a bend in it at this point the channel is narrow, the pilots have to line up 30000 ton 600 foot ships to pass thru the 200 ft wide bridge and the bouys are the navigation points that are very critical at this part so yes they could be off by 25 feet in a circle and that could give me a slightly off reading but I bet they are probably within 10 feet.So my marked mile is probably accurate within a mph either way. I will try to do some more runs with a 13X19 stainless and my original 21 pitch for further reference to "general" speeds with the Mountak. I say "general" as they are as close as I can test without a radar gun ..calibrated of course!
Tom W Clark posted 10-15-2002 10:59 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I don't think the measured mile, tachometer or prop pitch is off on The Chesapeake Explorer's boat, but the math is.

Covering one statute mile in 1 minute and 37 seconds represents a speed of 37.1 mph.

Bigshot posted 10-16-2002 09:56 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Whatever it is....I wowuld not run the 19 or 21 on that.....until you repower. You are already at the bottom of the rom curve. Running below can cause stress. You may want to try a can of techron or 2 just to make sure carbs are nice and clean. Compression is marginal....she is getting tired. That however does not mean it won't go another 1500 hours if treated right.
Tom W Clark posted 10-16-2002 11:29 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Agreed. That 1988 Evinrude 90 should be doing MUCH better. It should, in fact, be able to push the boat to 44 mph easily. If the best it can do is turn 5000 rpm with the 17", no way a 19" will improve things. That motor should be able to turn a 19".

As a point of comparison, last winter I was out on the Pacific off Santa Cruz with Andy Gere in his 1979 Montauk. His boat is powered by a 1979 Johnson 85 hp which is a powerhead rated motor. He has the fiberglass bow platform on his boat as well as a 15 hp kicker mounted on a bracket and dual batteries. He and I weigh perhaps 350 lbs combined and we had a bunch of gear in his boat. His boat is kept in the water and has bottom paint.

In spite of the motor's age and all the weight, we were still able to get his boat to hit 38 mph according to his GPS and it was a glassy smooth day. Had there been some wind ripples, I think it would have done even better.

Landlocked posted 10-16-2002 02:30 PM ET (US)     Profile for Landlocked  Send Email to Landlocked

Here is a link to the numbers I posted for my new merc 90/Montauk combination back in May.


The Chesapeake Explorer posted 10-16-2002 03:55 PM ET (US)     Profile for The Chesapeake Explorer  Send Email to The Chesapeake Explorer     
Tom Is right my math is off heres my results,the top speed was the one that was off much
2000 rpm 8.8 mph
2500 rpm 14.1 mph
3000 rpm 19.4 mph
3200 rpm 23.8 mph
3500 rpm 25.0 mph
4000 rpm 30.0 mph
5000 rpm 37.1 mph

when the 13 1/4 x 17 prop was put on in 93 the engine would pull 5500 with this pitch so it has lost a bit of top end, advise will be taken on the other props testing, I will give her a rest. I will have to check my old logs about that 50 mph speed when new and run that number it may be off I ll have find the minutes/seconds. I kept a detailed log for about 6 years and a less detailed log after that.

ratherwhalering posted 10-16-2002 08:42 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
You guys had me worried for a while. My '87 Johnson 90hp does 40.1 mph (GPS) at 5200 rpm (WOT & TU) with a 13.25 x 17 aluminum. (12 gallons, 215 operator and 70 labrador)
Bigshot posted 10-17-2002 10:44 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
God I love reading this stuff. Makes my 70 4 stroke just a gift from heaven. Half the gas and 95% of the speed:)
Seabrook posted 10-17-2002 12:39 PM ET (US)     Profile for Seabrook  Send Email to Seabrook     
Everytime I read one of these speed/RPM threads for Montauks I wonder even more about my boat. My 1983 Montauk came with a 17" stailess prop. Too many RPMs. I had it repitched to a 19". Still too many RPMs. The guy I bought the boat from was an old guy and said he never ran over 25-30MPH so speed was not important. He just wanted on plane in a hurry. I now have a 21" pitch, 13 1/4" stainless OMC prop. It planes quick even with a full load and still does 5500 RPMs...which is perfect since that is the top end of the RPM range for my 1983 Johnson 115. However, with a 21" pitch going 5500 RPMs, it seems my boat would be faster. I have never checked it at WOT with my GPS, but the speedo shows around 43 MPH. You guys are so good with the fast should I be running? I have a 27 gal tank that is usually full, twin batts, 2 normal size guys, 3.5 hp Nissan kicker on the back and a lot of other gear. I also run a CMC power lift and the motor has a Stingray brand fin. Help.....does the speed and RPMs I gave make sense? Just curious. Thanks for any comments you have.
Bigshot posted 10-17-2002 01:38 PM ET (US)     Profile for Bigshot  Send Email to Bigshot 1985 115 would spin a 19" alum at about 5400. Your tach may be off.
andygere posted 10-17-2002 04:11 PM ET (US)     Profile for andygere  Send Email to andygere     
To follow up on Tom Clark's post, at WOT my old Johnson 85 turns 5100-5200 rpm. The prop is a 17" pitch aluminum, and I consider the hole shot to be very good, even with a large load. I have an aluminum Whale Tail brand stabilizer on the cavitation plate as well. Last summer I took the boat to a large reservoir in the Sierra foothills, and had no problem getting on plane fairly quickly with 6 adults on board.
ratherwhalering posted 10-17-2002 08:02 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
BS: no need to rub it in :) I'm only half way there on the four-stroke fund!
Landlocked posted 10-18-2002 10:07 AM ET (US)     Profile for Landlocked  Send Email to Landlocked     
My new merc 90 2str is turning a 21" black max. Max RPM is 5250 @ 42.2 mph GPS. Not max RPM for motor but I figure close enough and I'm happy with the speed. Not sure 250 rpm would make an appreciable difference.


Bigshot posted 10-18-2002 10:44 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Nope.....taht is why you have a range. My range is 52-5800 and I only run about 5500WOT. Usually holeshot makes you prop out a boat to run near max rpms. On smaller boats if you have a decent holeshot, go for best cruise and top end in my book.
Eric posted 10-21-2002 11:08 PM ET (US)     Profile for Eric  Send Email to Eric     
This sure is a popular topic. 62 Nauset, 99 Johnson, OMC Viper stainless 13 3/8 x 17: ran 42 at about 5900+. Over revving, so I repitched it to a 19, but I wasn't able to get an accurate top speed before adding a top. I got 44 in some rather sloppy and windy conditions (not the slight chop that helps the boat get up and run, but rather ugly weather). I now have a top, and get 42 at about 5800, so I guess I'm still under propped. I have all of the speed I want, and so don't plan on changing props again, and it certainly isn't in the budget. I just have to be careful about the redline. I suppose this boat would get a little scary with a 21 pitch.
Bigshot posted 10-22-2002 09:42 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
A 2" add of pitch should have dropped you 400rpms Eric. you may want to have tach checked as well. I think your engine redlines at 6k but I may be wrong on that.
thebone12 posted 10-22-2002 12:34 PM ET (US)     Profile for thebone12  Send Email to thebone12     
I think that you guys put too much reliance into these formulas. There are just too many different variables to speed and rpm to be arguing over 2 to 3 mph and 400 rpms. Rpm guages for the most part just give you ball park not exact. Same with the formulas. My boat speed varies as much a 4 mph depending on a lot of things. just my 2 cents.
Bigshot posted 10-22-2002 02:15 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I agree there is too many variations but going from a 17 OMC prop to a 19 should drop more than 100 rpm's. The tach should be VERY accurate within 25-50 rpm's at most. Do you think the tachs in your car are just a ballpark? The problem with marine is they are universal and anything made universal usually sucks. Ever own aftermarket wheels with the universal lug patterns....try getting them to balance out to "00". I find that tachs are either on the money or way off. Most OMC, quicksilver, Yamaha, etc tachs are on the money. When you start running teleflex and Faria, etc they can get pretty nuts.
weekendwarrior posted 10-22-2002 06:35 PM ET (US)     Profile for weekendwarrior  Send Email to weekendwarrior     
I'm by far the slow poke here, with my 17' montauk and 50HP Honda I get 30mph at redline. It will creep right past redline but it hurts me to let it do that so I haven't done it long enough to see what the total top speed is. Not much more I'm sure. Anyway I can wake board all day on 6 gal of gas so I'm happy. :)
Eric posted 10-23-2002 07:13 PM ET (US)     Profile for Eric  Send Email to Eric     
By trimming out the motor more, I probably could have turned 6000, but didn't want to run that far over redline. It's 5500 with this motor.
I don't think that the repitch changed the performance much though.

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