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Author Topic:   1983 Outrage 18 Re-power 115-HP from 150-HP
GNolan posted 08-21-2015 12:38 PM ET (US)   Profile for GNolan   Send Email to GNolan  
Hello to All. I recently re-powered my 1983 Boston Whaler Outrage 18 with a 1992 Evinrude 115-HP four-cylinder engine; previoiusly it had a 1983 Johnson 150-HP six-cylinder engine. The mounting [bracket] has four holes. I mounted the 115-HP at the third hole [apparently counting down] from the top. The [engine mounting height] is marginal because I can not utilize the full trim range [without the propeller ventilating]. Also, I failed to change the setting on the tachometer to four-cylinders.

Performance results at a minimal load were as follows. The [acceleration onto plane from a standing start at full throtle] seemed lacking to me, just based on my years of experience. The propeller is typical 13x19 aluminum model that is in very good condition. The motor was vertical or maybe trimmed a bit in. At wide-open throttle, vertical trim or maybe trimmed a bit out, [the engine accelerated to] 4500-RPM indicated [on the tachometer] and 36-MPH [indicated on a] GPS receiver. I trimmed-out to maximum, but there was not a whole lot [of difference due to propeller venting] in turns; the [engine accelerated to] 4,600-RPM [and the boat to] 37-MPH--not a big difference. Yes, the tachometer was set wrong, but to me it seems the tachometer would still give a comparison [of engine speeds].

Can I get some thoughts from members on this data?

Should I drop the motor to the second hole [counting down from the top] so that I can use more of the trim range?

Does a 13 x 19 propeller pitch fall in line with most other 18-foot-boat and 115-HP-engine combinations out there?

Any comments and feedback is appreciated. Thanks.

Greg Nolan

jimh posted 08-21-2015 05:59 PM ET (US)     Profile for jimh  Send Email to jimh     
The tachometers for two-cycle engine generally are not set on the basis of number of cylinders in the engine. They're set on the basis of number of poles in the permanent magnet alternator.

Engine mounting height is measured by using a notation of number of increments of 0.75-inch or "hole" the engine is mounted up from the lowest. The possibilities are

--lowest mounting

--one-hole up

--two-holes up

--three-holes up

Describing the mounting using this notation is the clearest and least confusing method. I am not sure where your engine is mounted. Please describe the mounting using the recommended notation.

tedious posted 08-21-2015 08:04 PM ET (US)     Profile for tedious  Send Email to tedious     
Greg, your top speed sounds about right, based on my experience with an 18 with a 150-HP. You should sort out that tachometer setting before spending too much money on anything else.

It sounds as though you have the engine [mounted] all-the-way-up, and clearly your aluminum propeller is having trouble gripping.

My recommendation would be to do the following, in order:

--find the right tachometer setting and get a true RPM reading at WOT

--get a good stainless propeller and see if it solves your ventilation problem

--if it does not, try dropping the motor, one hole at a time, until you get the grip you like.

Regarding the hole shot, are you just used to the 150 and need to dial your expectations back a bit to match the 115?


GNolan posted 08-21-2015 10:33 PM ET (US)     Profile for GNolan  Send Email to GNolan     
Hello Jim<--Thanks for your input. My motor is mounted [two-holes] up.

[I am] not sure of the stator magnet count between the four and six cylinder engines. [I have] just assumed the setting for a four-cylinder engine and a six-cylnder engine were different.

GNolan posted 08-21-2015 10:48 PM ET (US)     Profile for GNolan  Send Email to GNolan     
Tim<--Thank you for the suggestions. I will first get the tachometer set correctly. I may drop the motor one hole to increase my trim. Or, next year, I will use a 6-inch set-back [bracket]. The tachometer will give me a good clue about the propeller pitch as well.
jimh posted 08-22-2015 02:08 PM ET (US)     Profile for jimh  Send Email to jimh     
Mounting an engine two-holes-up on a Boston Whaler OUTRAGE 18 may be too high for a run-of-the-mill aluminum propeller. Usually a higher engine mounting will need the propeller to have more cupping at the blade edges.

Since you have cut the power to 115-HP from 150-HP, the boat performance will decrease according to the change in power-to-weight ratio to the 0.5 exponent. Assuming the total boat weight is the same, this suggests the speed at full throttle will reduce by a factor of (115/150)^0.5 or 0.87.

What was the top speed reached with the 150-HP engine?

Tom W Clark posted 08-22-2015 02:29 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Leave the motor height where it is; it's the propeller that is the problem.

The 1992 Evinrude 115 25" shaft motor has a 2:1 gear ratio and a 5500 RPM redline.

If the boat is good for 40 MPH, and it should be, you'll want a 13-1/4" x 15" Turbo 1 (Stiletto Advantage) or a 13-7/8" x 17" BRP Viper

I would choose the former over the latter

tedious posted 08-22-2015 02:34 PM ET (US)     Profile for tedious  Send Email to tedious     
Greg, if you are interested in the Stiletto Tom mentions, I have one for sale. Used, but in perfect condition. You will just need a hubkit to adapt it to your motor - it currently has a Yamaha hubkit installed.

Drop me a note at the address in my profile if you are interested.


GNolan posted 08-22-2015 03:49 PM ET (US)     Profile for GNolan  Send Email to GNolan     
now that's what I'm talkin about!
Some good straight forward feed back.

Thank you a whole lot

Your analysis seems to be quite accurate. The 150 maxed out at about 41mph. Rooter tails indicated the cavitation plate was way too low. The motor was mounted at the lowest point.

Thanks for the straight forward comments. I will leave the motor where I have it and look to the prop. Thanks for the prop suggestions as well.

I am definitely interested in the SS prop that you have. Can you email me for further communication regarding the sale of your prop? GNolan.2005 at

You guys have been a big help. Maybe not so much work and cost to get 10-12% increase in my boats performance. I always kind of thought that if you get the best top end it would translate to the best efficiency say at 60% power. Or, less gph.

Tom W Clark posted 08-23-2015 10:51 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Also consider the 14" x 15" Turbo Pontoon 1 for Johnson-Evinrude (Stiletto Star) which has a lot of blade area, low blade rake and does very well on intermediate gearcase motors mounted on the Outrage 18
GNolan posted 08-23-2015 09:36 PM ET (US)     Profile for GNolan  Send Email to GNolan     
Thanks again. I will try Tim's prop and see how that works. Nothing to loose, only a gain--I hope.

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