1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Optimizing the performance of Boston Whaler boats
cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Fri Aug 06, 2021 9:47 am

I have a 1988 Montauk 17 that has a 2011 Mercury 90 FOURSTROKE engine. The boat is very clean. The engine only has 250 hours. The hull floats with the [engine splash well] drains just at or above the water line with no one onboard. The engine is mounted one-hole-up. The current propeller is an aluminum 13.25 x17.

PERFORMANCE OBSERVATIONS

[The Mercury 90 FOURSTROKE engine at full-throttle can only accelerate to] 5,000-RPM.

The maximum boat is about 30-MPH.

EXPECTATIONS

I believe the boat speed should be higher, based on reading other reports.

QUESTIONS

[Survey question deleted. Please do ask for information in the form of a survey. If you are seeking particular information, just ask for that information--Moderator]

Q1: Should [the engine mounting height be increased to two-holes up] or higher?

Q2: Should the propeller pitch be decreased to 15-pitch from 17-pitch?

Thanks for any insight--Chris

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Fri Aug 06, 2021 2:35 pm

The expected top speed for a typical MONTAUK 17 with 90-HP is shown in the REFERENCE article on that hull at

https://continuouswave.com/whaler/refer ... power.html

For a Mercury 90 FOURSTOKE with an engine weight of about 386-lbs, a boat speed of more than 40-MPH would be expected, based on the horsepower, weight, and hull characteristics, using the Crouch Speed Prediction Method.

For more explanation of the Crouch Speed Predicting Method, see the explanatory material that accomplies my CROUCH'S CALCULATOR that implements the method at

https://continuouswave.com/calculators/crouchCalc.php

The 43-MPH prediction for 90-HP uses a hull factor coefficient of 190 and a total boat weight of 1760-lbs. The method assumes an efficient propeller choice.

To countenance your observation of only a 30-MPH top boat speed, and assuming the engine is producing 90-HP, the boat weight would have to be much heavier, about 3,600-lbs, or the propeller would have to be very inefficient.

Or, perhaps you have given us the boat speed in NAUTICAL MILES PER HOUR. Please verify which unit of speed you intended.

With your current propeller, an aluminum propeller of 17-pitch, we can look at the propeller performance if we KNEW THE ENGINE GEAR RATIO. The GEAR RATIO depends on which model of 90 FOURSTROKE engine you have. Please give us that information.

Taking a total guess, let's assume the gear ratio is 2.38:1, which is typical for the CommandThrust models. With 17-pitch and 5,000-RPM, we would expect an efficient propeller to have a SLIP of about 10-percent, which we can use to calculate the speed of advance with a PROPELLER CALCULATOR.

There is a very useful PROPELLER CALCULATOR at


https://continuouswave.com/calculators/propCalc.php

Using the above mentioned parameters:
RPM=5000
RATIO = 2.38
PITCH =17
SLIP =10

we can solve for the anticipated speed of advance and get

MPH =30.4-MPH

This agrees with your observations.

The analysis then suggests that the problem is not with poor propeller operation.

The problem reduces to one of the following:

--the boat is very heavy, many hundreds of pounds heavier than anticipated
--the engine is not in good tune and is not producing its rated horsepower
--the hull has marine growth
--the engine is not properly trimmed for best performance at planing speeds
--the engine mounting height is too low

Without knowing the exact specifications for your engine and its MAXIMUM ENGINE SPEED range, I will guess that 6,000-RPM is more like the maximum full-throttle engine speed than 5,000-RPM.

Now the boat speed with a 17-pitch is re-calculated for 6,000-RPM, with SLIP reduced to 7: the result is 37.7-MPH.

We are still not getting the 40-MPH or higher expected speeds. Let's try 19-pitch.

Again, using the calculator with inputs
RPM=5700
RATIO = 2.38
PITCH = 19
SLIP =7

the boat speed in STATUTE MILES PER HOUR calculates to 40-MPH.

Clearly the answer is not to reduce to a 15-pitch if you ever expect to get to 40-MPH with this boat.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Fri Aug 06, 2021 2:44 pm

Some Mercury 90 FOURSTROKE engines without the CommandThrust gear case have a gear ratio of 2.07. You can use that value in the Propeller Calculator to see what outcome you'd get with a 17-pitch at 6,000-RPM. Hint 43.4-MPH.

If you really want to optimize performance, you should change to a stainless steel or painted steel propeller.

Also a modern propeller design with blade cup and blade rake can run with the blade tips closer to the water surface; that means a higher engine mounting height.

If you want to get to 40-MPH or higher, then raising the engine to two-holes-up or even three-holes-up and getting a new steel propeller may be necessary, assuming that the boat is not way overweight, the bottom surface is not fouled with growth, you get the engine trim set for optimum at high-speed planing, the engine is in perfect tune and produces its rated 90-HP.

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Fri Aug 06, 2021 3:06 pm

Wow thank you for the extensive reply Jim. Very much appreciated. I tested a couple more propeller, and I agree with what you are saying here. I am leaning towards the engine being mounted too low. The hull is very clean and have not seen any signs that it is waterlogged.

Apologies actually hadn't noticed that the new Simrad on it was reporting knots/hour. My old Garmin displayed mph.

Current load
about 15 gallons of fuel
2 batteries
slightly larger custom console
170lb person

Engine Mounted 1 hole up
The Mercury 90 EFI from 2011 I believe is geared 2.33:1
Running an SE sport 300 hydrofoil
Compression tests are all good. low hours and seems clean. Going to have my mechanic have a look though.

Last tests
Aluminum 13.25 x 17 = 4900rpm at 29 knots/hr
Quicksilver Aluminum 13.75 x 15 = 5600rpm at 28knots/hr

At good cruising speed and on good plane with motor trimmed up I do not see my anti ventilation plate or hydrofoil. It is still buried under water.

My thought is to move the engine up either one or two holes and retest with the 17 prop to see if I gain a few hundred rpm at WOT and gain a bit more speed. Does this sound like best next move?

Thank you again.
Chris

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Sat Aug 07, 2021 6:41 am

Measuring boat speed in nautical-miles-per-hour is generally done when navigating far from land. The nautical mile distance was traditionally defined as one minute of latitude change, so in traditional plotting of deduced reckoning course lines that unit was very handy. You could use the latitude scale on a chart as the boat speed reference.

Most of my boating is in the Great Lakes where the official unit of speed is (statute) miles per hour, so I always use MPH.

If you want to continue to discuss your boat speed in nautical-miles-per-hour, you can use the alternative version of the propeller calculator that computes speed in that unit.

ASIDE: a knot/hour is a measure of acceleration. The term "knot" is understood to be a value of speed of one-nautical-mile-per-hour. So a value of one-nautical-mile-per-hour-per-hour is a value of acceleration. If your boat were accelerating at 30-nautical-miles-per-hour-per hour, after one hour it would be making 30-knots, then after two hours it would be making 60-knots, and three-hours it would be making 90-knots, and so on.

To remove any confusion, just specify that the units for any value of speed you mention are in NMPH as in Nautical-Miles-Per-Hour.

Since I never use nautical-miles-per-hour as a speed unit, in order for me to figure out any speeds you mention in nautical-miles-per-hour I have to convert them to statute-miles-per-hour to have an appreciation for them. You will find that in almost all cases of references to boat speed on contiuouswave.com the units are in MPH not NMPH.

I know some people operating 17-foot boats feel that boat speed in nautical-miles-per-hour is more nautical, but since all my boating involves going from some location near to land to another location near to land, I just keep track of boat speed in statute-miles-per-hour. I suppose if I were on a 10-day voyage on the open ocean, I would change to nautical-miles-per-hour.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Sat Aug 07, 2021 6:46 am

You have not given information on the manufacturer's recommended full-throttle engine speed range.

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Sat Aug 07, 2021 10:23 am

Hi Jim. I’ll convert over my measurements to mph. Just was reporting what my Simrad displays by default.
The engine has a WOT engine rpm range of 5000-6000.
The gear ratio is 2.33

I attached a camera to the transom yesterday to observe the ventilation plate while trimmed and planing under multiple speeds. The ventilation plate is not visible and appears to be well under the water line. The engine is currently mounted one hole up. I am going to have my mechanic raise engine on Monday. I’ll report back.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Sat Aug 07, 2021 2:02 pm

I am sure the SIMRAD chart plotter can be set for statute miles per hour as the speed unit.

The gear ratio 2.33:1 sounds like you should try a 19-pitch three-bladed steel propeller after you raise the engine.

One-hole-up will be better than lowest-possible mounting height. Two-holes-up may also work.

I am not sufficiently familiar with the Mercury 90 gear case diameter. Be sure any propeller you buy will fit the gear case diameter properly.

Aim for the engine speed to be in the upper end of the 5,000 to 6,000-RPM range when testing with a light load. In four-stroke-power-cycle outboard engines the rated power is not achieved until close to the maximum engine speed.

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Sun Aug 08, 2021 12:39 am

Thank you Jim. I’ll try all of the above.

Currently the engine is mounted one hole up. And the splash plate above the anti-venitalation plate is visible. The anti-ventilation plate is not visible while on plane.

Also, the hull does very strongly list to port if any weight is out to that side of the boat under plane. My guess is with the prop positioned lower than it should be the regular list to port with a normal prop rotation is being accentuated and also confirms my thought that the engine is mounted too low. The lower the prop the more leverage it would have to pull the hull to port.

I changed the Simrad to report speed over ground in statute miles per hour going forward.

The hull is very clean and floats very well so I don’t believe it is overweight or is waterlogged. Tonight I had six people onboard including myself and the maximum engine speed and boat speed were surprisingly very similar to a light load, so I think drag is the main factor.

I’ll report back after seeing what my mechanic thinks about raising the motor mount one or two holes. I’ll also have him run a quick diagnostic on the engine to make sure it is performing within spec pification, but I don’t have any reason to believe it is not.

I would be very curious to hear if anyone else has a similar vintage Mercury 90 FOURSTROKE engine mounted on a classic 17 hull and at what hole the engine is mounted.

Thanks again for all input.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Sun Aug 08, 2021 8:54 am

cparkphoto wrote:...I had six people onboard including myself and the maximum engine speed and boat speed were surprisingly very similar to a light load...


Your adverb "surprisingly" is very appropriate. The hull weight of a c.1988 MONTAUK is only around 950-lbs. If five extra people are aboard, and they weigh an average of 180-lbs, the crew weight adds 900-lbs to the boat--which is almost as much as the hull weight. To hear that the engine speed at full-throttle and boat speed were similar to a light load is very surprising. As a general rule the smaller and lighter the boat, the more sensitive the boat's performance will be to increase in weight from additional people aboard. And this is particularly true with four-stroke-power-cycle outboard engines because of the nature of their power curve; they just do not develop particularly strong power in the lower engine speed ranges.

My boat weighs about 4,500-lbs, and I can tell immediately the difference in performance with two people aboard from having four or five people aboard.

Boat speed is always about power and weight. There is no possibility that adding significantly more weight won't cause boat speed to decline.

ASIDE: Several years ago we were visiting a friend whose fishing boat was a 17-footer with a 90-HP four-stroke-power-cycle engine. He normally operated the boat alone. We went out for a little boat ride with a total of four people aboard. The boat had a very hard time even getting on plane. The bow rise was significant, and I had to move to the bow in order to get the bow to come down so the boat could finally get on plane.

On that basis, I am really very surprised that the boat speed and engine speed are not much affected by an intrease in crew weight of about 900-lbs. Now, on the other hand, if the five people were young grandchildren that weighed 50-lbs each, then maybe that outcome is not so surprising.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Sun Aug 08, 2021 9:26 am

For some other factors to consider with regard to engine mounting height, see


Considerations in Engine Mounting Height
https://continuouswave.com/forum/viewtopic.php?f=7&t=6618

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Sun Aug 08, 2021 11:23 am

I should have clarified: the full-throttle engine speed was similar with the full passenger load albeit maybe very slightly reduced.

The boat speed was down but not as much as I was expecting. I think I recall hitting about 26-MPH with the full load when I tested briefly.

My main observation was just that the engine had the power to still comfortably handle a larger load and gave me further confidence in its operating horsepower.

On thing of note with the current engine mounting height: I am unable to make the propeller [ventilate] like I have on other boats. Also, as I increase trim I can substantially raise the engine before causing porpoising but never does the prop [ventilate]. In past boats I’ve always been able to find a trim sweet spot by raising the engine until I notice [ventilation] and then dropping it down a touch. Also the steering usually gets very light and responsive when properly trimmed. I have not been able to achieve any of these with this engine mounted where it is. Fingers crossed raising it tomorrow achieves what I am hoping for.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Sun Aug 08, 2021 1:23 pm

cparkphoto wrote:...the steering usually gets very light and responsive when properly trimmed. I have not been able to achieve any of these with this engine mounted where it is.


Your observation about the force needed at the helm to move the steering wheel being reduced when the trim position of the engine is correct is a very good observation. Usually the steering forces decrease when the engine is trimmed so the propeller shaft is parallel to the water surface.

You difficulty in getting to a "light" steering could also be related to where you have the TRIM TAB on the engine set. For some advice on positioning the engine trim tab, see the owner's manual. If you don't have the owner's manual, you can use the owner's manual I have created in HTML and put on-line in the REFERENCE section of the WHALER section of the website. See

https://continuouswave.com/whaler/reference/manual9-17/

and for the trim tab advice see

https://continuouswave.com/whaler/refer ... ml#trimTab

User avatar
Phil T
Posts: 2602
Joined: Thu Oct 22, 2015 6:08 pm
Location: Was Maine. Temporarily Kentucky

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby Phil T » Sun Aug 08, 2021 6:14 pm

When discussing propeller selection, one MUST include make and model since sizes does not equate to performance.

A Solas stainless 13x19 does not perform the same as a Yamaha painted stainless steel, or Mercury Enertia, etc., in the same size.

For the OP, you want the engine to be "two holes up" or with the top bolt in the third hole down from the top.

As for propellers, Montauk owners with that version of Mercury 90 FourStroke recommend:

13-1/2" x 20" Mercury Laser II
13-1/4" x 19" Stiletto Advantage I
13x19 Mercury Vengance

Aiming for a WOT speed of 35-NMPH or 4O-MPH at 5900-RPM running solo and no gear.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Mon Aug 09, 2021 12:22 am

Thanks Phil T. I was hoping someone else had information on this particular combination.

I am taking the boat to my mechanic who is a Mercury dealer and supposedly has experience with the same hull and engine combination. I will see if he can source one of those propellers.

Chris

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Mon Aug 09, 2021 9:10 am

As I mentioned earlier, 19-pitch is the most likely choice to get boat speed up to 40-MPH.

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Wed Aug 11, 2021 8:41 pm

Well I feel dumb. [The engine spark] coil on cylinder 2 was defective. [With a] new spark coil pack [the 2011 Mercury 90 FOURSTROKE engine could accelerate at full throttle to] 6,000-RPM. The boat speed was 38-MPH.

This test was with a propeller [of unknown material and unknown number of blades] of 13.25 x 17-pitch.

The boat had 20-gallons fuel, two batteries, and an anchor. Aboard were two adults and a dog.

The test was done at sea level, in 72 degree-water, with wind chop of one foot.

Had full-throttle been maintained longer, the boat would have probably gone faster, but I didn’t want [the engine speed to exceed] 6,000-RPM.

I don’t care about going faster than that. I [may purchase 19-pitch propeller to reduce engine speed.

I should have known the math wasn’t adding up to an engine operating at 90-HP.

Thanks for all the help.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby jimh » Thu Aug 12, 2021 8:04 am

As I commented earlier:
The problem reduces to one of the following…
--the engine is not in good tune and is not producing its rated horsepower


Does that 2011 Mercury 90 FOURSTROKE have spark coils that attach directly to the spark plugs? They are often called “pencil coils” and seem to have short service lives.

cparkphoto
Posts: 8
Joined: Fri Aug 06, 2021 9:41 am

Re: 1988 17 Montauk with 2011 Mercury 90 FOURSTROKE

Postby cparkphoto » Thu Aug 12, 2021 1:02 pm

Yes--the Pencil Coil on cylinder-2 was failing. I have heard the same about their service life from Verado owners. I am going to throw a spare in my emergency kit.

Jimh--Thanks for the help.