Top Speed 1989 Montauk Mercury 90-HP

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enggass
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Top Speed 1989 Montauk Mercury 90-HP

Postby enggass » Wed Oct 04, 2017 3:18 pm

When I asked a seller what speeds he achieved WOT, he replied :"per the operating instructions , the throttle is only advanced to 75% of max rpm. At that, it's about 20 mph." Does that sound right? And is he saying I can only get 75% or is it fixed to only go 75%? I would expect top speed of 40mph-ish +

So let's say, assuming top RPMs are around 5000, what should I expect for speed on a 1989 Montauk w/original 1989 Mercury at 3,750 RPMs?
The outboard has been recently tuned up.

I guess what want to know is what is the Speed-to-RPM ratio, that is, [if the boat speed is] 20-MPH at 3750-RPM, [what will the boat speed be when the engines speed is] 5000-RPM?

Thanks.

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Landlocked
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Re: Top Speed?1989 Montauk w/1989 Mercury 90 HP

Postby Landlocked » Wed Oct 04, 2017 3:36 pm

I have a 1977 Montauk with a 2002 Merc 90HP 2 stroke. With just a younger and thinner me in the boat on smooth water I once hit 41.5mph gps. With the fully grown family of 5 aboard with total weight north of 800 lbs, about 33 mph is all I can squeeze out of her. Both of these were at Wide Open Throttle which for me is just under 5000 RPM but I'm not propped exactly right. Never really a problem because 99% of my boating is at speeds in low 20's.

I've never heard of having to limit throttle to 75%. In fact, my understanding is that if you are correctly propped you should hit the max RPM rating of the engine. For my 90 I believe that is 5,200.

Speed to RPM ratio would depend on prop. Changing pitch or diameter is like shifting gears in a car.
Last edited by Landlocked on Wed Oct 04, 2017 3:44 pm, edited 1 time in total.

enggass
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Re: Top Speed?1989 Montauk w/1989 Mercury 90 HP

Postby enggass » Wed Oct 04, 2017 3:43 pm

Thanks. Was that WOT? I guess what I really want to know is how much does speed increase from 75% throttle to 100%? Is it possible that 20mph at 75% throttle could increase to 35+mph at WOT? (90hp and Montauk)

Sorry didn't somehow I missed the WOT part - thanks!

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Landlocked
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Re: Top Speed?1989 Montauk w/1989 Mercury 90 HP

Postby Landlocked » Wed Oct 04, 2017 3:48 pm

I think it might be possible. Someone else more technically savvy than I should jump in here but you may not be in the optimum power band of the engine/prop combo at 75%. Increasing RPM's that final 25% MIGHT get you closer to where you think you should be. Which with a properly propped and functioning 90 on a Montauk of normal weight should be much more than 20mph. I would think 35-40 would be what you should expect.

enggass
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Re: Top Speed?1989 Montauk w/1989 Mercury 90 HP

Postby enggass » Wed Oct 04, 2017 3:55 pm

This is off Console Speedometer - could be off. Best to test with gps.

enggass
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Montauk w/1989 90hp Mercury

Postby enggass » Wed Oct 04, 2017 5:15 pm

Can anyone recall what speed they go at 75% of Max RPMs with an average load? Speed does not increase evenly from dead stop to 100% WOT. Thanks.

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Dutchman
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Re: Top Speed?1989 Montauk w/1989 Mercury 90 HP

Postby Dutchman » Thu Oct 05, 2017 9:21 am

Yes test with GPS (use your phone, many free speedometer apps). Water activated (pitot tube) type speedometers are not the best indication of over land(water) speed.
Also you mention 75% throttle in both this and your other posting and we don't really know what RPM that is on a 30 year old OB, and as we don't know your prop diameter and pitch we never can tell what your speed "should" be, but as mentioned it should be more than the 20mph you mention.
EJO
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50th edition 2008 Montauk 150, w/60HP Mercury Bigfoot

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Re: Top Speed 1989 Montauk w/1989 Mercury 90 HP

Postby jimh » Thu Oct 05, 2017 12:38 pm


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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby jimh » Thu Oct 05, 2017 12:41 pm

Boat speed is directly proportional to engine RPM. If engine speed increases by a ratio of 1.1:1, boat speed should increase by the same ratio.

enggass
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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby enggass » Thu Oct 05, 2017 1:05 pm

jimh wrote:Boat speed is directly proportional to engine RPM. If engine speed increases by a ratio of 1.1:1, boat speed should increase by the same ratio.

So if say 20 MPH at 3900 RPMs what could one expect at 5200 RPMs?

goldstem
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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby goldstem » Thu Oct 05, 2017 3:27 pm

boat speed is not directly proportional to RPM across the entire range. think about transitioning from displacement to planing.

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby jimh » Thu Oct 05, 2017 9:39 pm

enggass wrote:So if say 20 MPH at 3900 RPMs what could one expect at 5200 RPMs?


Do the math

5200/3900 x 20

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby jimh » Thu Oct 05, 2017 9:40 pm

goldstem wrote:boat speed is not directly proportional to RPM across the entire range. think about transitioning from displacement to planing.


The discussion here is about speeds when the hull is on plane

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby Jefecinco » Fri Oct 06, 2017 9:48 am

[By the ratio method the hypothetical new speed is] 26.7-MPH.
Butch

enggass
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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby enggass » Fri Oct 06, 2017 2:29 pm

Thanks for all the responses. I’ll test her out and get back with what I find.

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby jimh » Sat Oct 07, 2017 4:08 pm

There is nothing to "test out" in the replies to your hypothetic values for engine speed and boat speed. You asked for an estimate of hypothetical boat speed at a new hypothetical value of engine speed. This is simple math: 26.7-MPH.

If you want to know what sort of TARGET BOAT SPEED you should get if you operate a real MONTAUK 17 with a real 90-HP engine in a real sea trial, the TARGET BOAT SPEED at FULL THROTTLE should be about 40-MPH, perhaps more, assuming the test conditions are in moderate or cool air temperature, in calm seas, with a light load.

If a MONTAUK 17 with 90-HP can only get to 26.7-MPH, something is seriously wrong with the engine, the propeller, the boat, or the whole set-up.

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby jimh » Sat Oct 07, 2017 4:48 pm

Also, re the seller's claim that "the operating instructions" limit the engine to being run no faster than 3/4-Throttle: that is nonsense. The only time an engine would have a limit on running at full throttle might be during a break-in period. After break-in, outboard engines can be run at full-throttle, although it would be prudent to wait for the engine to come up to operating temperature before hitting full throttle.

Running the engine up to full-throttle is a standard element of any sea trial. The sea trial should reveal if the engine can reach the manufacturer's recommended full-throttle engine speed range.

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby jimh » Sat Oct 07, 2017 4:52 pm

To understand more about the relationship between engine speed and boat speed, read about propellers. Read

http://continuouswave.com/whaler/reference/prop.html

Read the whole series of articles.

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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby pcrussell50 » Sat Oct 14, 2017 1:06 am

Another note about full throttle: I think modern outboards with jewelled bearing (and by that I mean from late 1960's onward) can be run at maximum RPM for as long as you like, provided they are in a proper state of repair. Of course that would be forsaking fuel economy.

--Peter

enggass
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Re: Top Speed 1989 Montauk Mercury 90-HP

Postby enggass » Thu Oct 26, 2017 1:59 pm

UPDATE: Turns out only 2 of 3 cylinders were in use. One of the plastic connectors to Carb was busted so Carb not in use.