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Author Topic:   Montauk: 88-HP with 21-inch Pitch
tarracott posted 09-15-2003 06:50 PM ET (US)   Profile for tarracott   Send Email to tarracott  
I have an 86 Montauk (with bottom paint) powered by a 1996 88hp Evin. The Engine has a stainless prop (12.75 x 21). At WOT (with just me and 8 gal. of fuel), I am running 4975rpm and a speed of 40.1 mph. Does this seem about?
jimh posted 09-15-2003 10:13 PM ET (US)     Profile for jimh  Send Email to jimh     
Find the gear ratio of your lower unit, then enter your numbers into the continuousWave propeller calculator.


Add up the total weight of your boat and its passengers and gear,
then calculate your expected performance using the formula shown in the series of articles on propellers. See:

Bigshot posted 09-16-2003 10:33 AM ET (US)     Profile for Bigshot  Send Email to Bigshot should be running a 17" Stainless or maybe a 19" alum. Your redline is 5500 and you should be running at least 5300 with a light load.
tarracott posted 09-16-2003 03:07 PM ET (US)     Profile for tarracott  Send Email to tarracott     
Will the prop change increase the RPM's, or do I have a bigger problem?
jimh posted 09-17-2003 08:18 AM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "Am I in the ballpark[?]" Topics like that are terrible when searching for information.--jimh]
jimh posted 09-17-2003 08:37 AM ET (US)     Profile for jimh  Send Email to jimh     
Usually the top speed of the engine is limited by the load imposed on it. In this application the load is determined principally by the pitch of the propeller.

The larger the pitch the greater the load, and thus the engine will tend to be unable to turn to its rated maximum crankcase speed when a propeller with a large pitch is used with the engine.

If the pitch of the propeller is reduced, the engine will tend to be able to rotate at faster crankcase speeds, if it can. Typically most outboards can run well over 5,000 RPM. WIth light loads they can typically run faster than the recommended maximum speed, to over 6,000 RPM or more. This can be destructive to the engine.

In your Evinrude outboard owner's manual you will find specifications for maximum recommend operating speeds for your engine and also the lower unit gear ratio.

The general recommendation is to use a propeller that permits your engine to reach the maximum rated crankcase speed when the boat is lightly loaded. (A "light load" would be just one person aboard, partial fuel tank capacity, little or no gear.)

I think most of this is explained in the series of five articles I wrote in the REFERENCE section of the website under the heading of PROPULSION ARTICLES. I provide a hyper link to that section below.

You can assess the performance of your boat/motor/propeller combination if you have the following:

--recommended maximum crankcase speed of motor
--lower unit gear ratio
--accurate observation of speed
--estimate of total boat weight
--accurate propeller pitch information

Using the above, you can predict the maximum speed of the boat using its weight and horsepower. The hull factor for a classic Boston Whaler boat like a MONTAUK is estimated to be about 200. See more information in the REFERENCE articles.

You can assess the efficiency of the propeller using the PROPELLER CALCULATOR. Enter the values for:

--Gear Ratio
--Observed Speed

and the PROPELLER CALCULATOR will compute the propeller slip.

A figure of 5-10 percent propeller slip is typical and indicative to good propeller efficiency. If you get negative numbers for SLIP, you probably have underestimated the PITCH of your propeller.

To get back to your original question and to use your ballpark metaphor, the answer is "Yes", you are in the ballpark.

If that is all you want to know, you need not worry further. If you want to find out, to continue the metaphor, if you just hit a grand slam home run, you will need to furnish more information and to make a few simple calculations.

Bigshot posted 09-17-2003 10:59 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Your engine(all 88hp) run 5500 max rpm. A 17" prop will get you close but i doubt over it. I have owned 2 17's with that engine and 17" usually put me about 5300 but your nausett is lighter so maybe closer to 5500.
Bigshot posted 09-17-2003 12:41 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Oops i see you have a 1986 Montauk. You will run about 5250-5350 WOT with a 17" prop depending if SST or alum. My 84 Newport with a 88SPL mounted on transom with bottom paint and a SST would do about 5250 with 2 people.
tarracott posted 09-17-2003 02:09 PM ET (US)     Profile for tarracott  Send Email to tarracott     
Thank you for all of the information. To ask one last dumb question, my prop says it is a 12.75 x 21. I understand that I should change to a 17 inch pitch - should I continue with a 12.75 diameter?
Bigshot posted 09-17-2003 02:47 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Can' alum it will be a 13.25x17, in SST it will usually be a 13 3/8x17. Your holeshot will be mindblowing with the new prop......well maybe not mindblowing but substantial.

I have a totally mint 17" OMC SST(stainless) prop with a new hub that I promised to another fellow forum member yesterday. If he bails, i will let you know. $125 delivered to your door and I just put a $60 hub in a few months and 10 hours ago.

Bigshot posted 09-18-2003 01:25 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Here is a nice one on e-bay:

tarracott posted 09-18-2003 03:20 PM ET (US)     Profile for tarracott  Send Email to tarracott     
Thank you for all of your help. I will let you know how the new prop runs.

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