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I own a 2002 170 MONTAUK, among the first off the production line in 2002 for the new Montauk hull style. I am shopping for (most likely a Mercury) 90-HP engine.

Q1: should I get a Mercury 90-HP Command Thrust or the standard 90-HP model?

Q2: realize the gearing is different, but with the right propeller selection, does different gear matter?

There is additional cost with the Command Thrust but availability is less.

Q3: Can I use the existing controls that came with the boat if I choose the Mercury?

Q4: is that a penny-wise and pound-foolish approach?

More criteria:

• I don't care about gauges as I will use Vessel View on my phone;
• As long as the 2002 original tachometer can still work I would be happy;
• steering and throttle work fine with the current engine so I don't see the need to replace.

Q5: is [a re-power with a new Mercury engine a] plug-and-play?

Saving installation labor time and money would be welcomed. Plus there would be less risk that collateral damage by the technician occurs in small spaces like inside the console and rigging tunnel. Drilling holes is off the table (the boat is all stock and never drilled) so other brand motors with new controls could present a problem.

--Peter

ASIDE: I have learned much from this group, but I am at re-power decision time and checked the archives for answers to a couple of questions. They have possibly been covered before so I am sorry if I could not find the answers. These are questions I would ask the dealers but many of the contributors in this group appear to have superior knowledge.

The boat is in near mint condition and has never spent a night in the water and was always covered. The boat came with the optional Mercury 60-HP FOURSTROKE BIGFOOT engine, which runs great. The only reason for a re-power is to accommodate fully-grown children and their friends for water sports like towing a tube and the thrill of going faster. The 60-HP engine is 22-years-old so there will be some point where its reliability drops off.

The Mercury dealer is a couple of miles away in the same town.

pshosho wrote:Q2: realize the gearing is different, but with the right propeller selection, does different gear matter?

The gear ratio will affect the choice of propeller pitch.

I think the Command Thrust gear case can use propellers of larger diameter, so that will affect the propeller diameter. You will need a new propeller in any case, as the 60-HP propeller is unlikely to be optimum for a 90-HP engine of either gear case model.

Look at the Boston Whaler performance reports to see if they were using the 90-HP Command Thrust or the standard gear case when they equip the 170 MONTAUK with a 90-HP.

pshosho wrote:Q3: Can I use the existing controls that came with the boat if I choose the Mercury?

If you mean "controls" to mean the mechanically linked remote shift and throttle controls, I would expect they can be reused. But check with the dealer. The only difference might be in the direction of movement of the cables for throttle or shift.

pshosho wrote:Q5: is [a re-power with a new Mercury engine a] plug-and-play?

That depends if Mercury is still using the same wiring harnesses, ignition key switch, warning horn, trim switches, and signals for analogue gauges. After 22-years Mercury may have changed something. Again, check with the dealer.

pshosho wrote:Saving installation labor time and money would be welcomed.

From that comment I infer you are going to have the dealer install and rig the new engine. That is a good plan, as typically in order for the factory warranty to be in effect the installation of a new engine must be done by a dealer or at the very least certified by a dealer that the installation was done properly.

Before committing to a particular dealer, make sure the dealer is experienced in installing new engines on old boats. Many dealer do not have much experience with this because almost all the outboard engines they sell are already installed on a new boat by the boat manufacturer. The dealer may not have installed a the current model Mercury 90 before.

Thank you for the suggestions.

MERCURY WEBSITE ENGINE PERFORMANCE DATA LACKING
The Mercury website doesn't have performance data for their engines mounted on Boston Whaler boats--which is disappointing since both are Brunswick units. The Mercury website has data mostly for Mercury engines mounted on pontoon or aluminum boats.

BOSTON WHALER WEBSITE PERFORMANCE DATA SUMMARY
The Boston Whaler website has performance data for their current fleet. To extrapolate [the data from] a 160 SuperSport (1,060-lbs dry weight) or from 2024 170 Montauk 170 (dry weight 1,700-lbs) [for comparison to my 2002 170 Montauk 170 (1,440 lbs dry weight) is hard.

For the 160 SuperSport WHALER.COM has data for a Mercury 90 with the 2.07:1 gear ratio while the Montauk 170 uses a Command Thrust with a 2.38:1 ratio.

PERFORMANCE DATA FROM INTERNET SOURCES
From other internet sources, the Command Thrust advantage appears to be for heavier or deeper-V-hull boats in the 17-foot size range, and the Command Thrust engine will be able to swing a bigger propeller or configure a propeller at the gear-ratio of the Command Thrust gear case that allows a hull to maintain plane over a wider RPM range. Some test data does appear to show that performance in some other boats. The disadvantage appears to be a small hit to top speed as the larger lower unit (same size as the one from a 150-HP) is a greater drag in the water. I am not sure my 2002 Montauk 170 is considered a heavy boat. Note that the Classic MONTAUK 17 hull was 900-lbs dry weight compared to 1,440-lbs for the originanl c.2002 170 MONTAUK, so perhaps [the classic MONTAUK 17 hull] is comparable to a 17-foot aluminum-hull boat.

I found some performance data from 2002 as part of the new model introduction publicity and the 90-HP four-stroke-power-cycle engine at that time had a 2.07:1 gear ratio. But this was before the Mercury 90-HP 2.1-liter engine was introduced (c.2014) and possibly before there was a choice of lower unit gear ratios.

I have not talked to dealers yet because I like to get the advice of this group first since you all have the perspective of a consumer while a dealer will have the perspective of a seller. That is similar how I like to fully research a car model before I walk into a car dealership to buy it.

I will certainly have an authorized dealer install it. Also, I will ask about selling the old 60-HP engine on consignment or trade-in.

I am close to making a decision for the new 90-HP engine to 2002 Montauk 170--not the classic, but the first model year of the new hull 170 MONTAUK made from 2002 to 2017. I have spent hours digging through the archives here and elsewhere. I am sorry if I missed this in the archives but I am stuck on these questions:

Q1: should a new Mercury 90-HP long-shaft (20-inch) engine be mounted "two holes up" on the 2002 Montauk 170?

Q2: or is the two-holes-up mounting ONLY for the original classic 17-foot hull MONTAUK?

I found performance data on the Boston Whaler website for my hull in 2016, which would be the same Mercury 2.1-liter 90-HP they sell today with a 2.07:1 lower unit, and also the same hull. They ran the test with a 13-1/8 x 16 VENGEANCE propeller. The acceleration was "good" and the top speed was 40.9-MPH in testing with a crew weight of 390-lbs and 25-lbs of gear. They give all the statistics and that would suit me just fine. My typical use would be cruising with a few normal sized people and maybe towing a tube, so all-round performance. I am not interested in boat speeds of over 40-MPH, as Long Island Sound conditions are too rough to go that fast, but I am certainly interested in efficiency which might be [achieved with] the same propeller. To cross check, I went on the Mercury website propeller configurator and put in all of the same data and it suggests the Vengeance 13 x 18 among others. I would have thought [the Mercury Propeller Selector] would [suggest] the same propeller [as used in the Boston Whaler test], and I found that to be frustrating.

Q3: should I rely on the official test data published by Boston Whaler?

Q4: [Inferred question] Or, should I use the Mercury Propeller Selector recommendation?

Readers: give me your answers or comments or suggestions.

ASIDE: Phil T sometimes knows the right propeller off the top of his head-- I have seen [that] in some postings.


APPENDIX FOR READERS UNFAMILIAR WITH THE TERM "TWO-HOLES-UP"

[A method for describing engine mounting height involving counting remaining unused holes was given, but this method is inconsistent because some engine mounting brackets have five holes and some have four holes. For this reason, all descriptions of engine mounting height used on continuousWave employ the method described in the article

Engine Mounting Height Nomenclature
https://continuouswave.com/forum/viewto ... ?f=7&t=739

which explains a clear and unambiguous method to describe engine mounting height that works no matter how many holes are in the engine mounting bracket.--Moderator]

Regarding the engine mounting height choice: I would presume that the engine splash well on your c.2002 170 MONTAUK was sufficiently deep that the engine mounting holes in the transom were drilled according to the standard ABYC layout, which is to say that the position of the lower engine mounting holes in the transom are spaced 8-inches from the upper mounting holes in the transom. The standard ABYC hole layout in the transom will permit you to mount the engine in any position, starting from the lowest possible position, and then raising to one-hole up, two-holes-up and so on. The limit to how high the engine can be mounted is determined by the number of holes provided on the engine mounting bracket. Some engines have five sets of holes for the upper mounting bolt, and others have only four sets of holes for the upper mounting bolt.

pshosho wrote:Q1: should a new Mercury 90-HP long-shaft (20-inch) engine be mounted "two holes up" on the 2002 Montauk 170?

Q2: or is the two-holes-up mounting ONLY for the original classic 17-foot hull MONTAUK?

The notion that two-holes-up is the best choice often depends on the particular propeller being used. Some propellers are able to tolerate running closer to the water surface than others. For example, if you choose a four-bladed propeller with a lot of blade rake, that propeller may tolerate a higher engine mounting height because the blade tips will not be as close to the water as compared to the blade tips on a three-bladed propeller with no blade rake and typically larger blades.

The notion that one-hole-up is the best choice is based on the general theory that lowest possible mounting will seldom if ever be the best choice, so why not play it safe and only go up one hole to start.

Changing engine mounting height is not particularly difficult, but it is not a five-minute job, and you need a bit of courage to do it yourself, along with some helper or an overhead support and strapping.

The installing dealer may also be reticent to mount a new engine at two-holes-up, as they dealer might think he would be liable if there were ever any mishap with the boat that could be blamed on the engine mounting height.

pshosho wrote:Q3: should I rely on the official test data published by Boston Whaler?

Q4: [Inferred question] Or, should I use the Mercury Propeller Selector recommendation?

I have used the Mercury Propeller Selector many times to assess their choice for a propeller on my boat, but I have always found that the recommended pitch is too high, based on having actually used some of the recommended propellers in a particular pitch. So on that basis--that is, on my own experience with the propeller selector--I would rely more on the actual data you found from Boston Whaler for a boat that appears to be very similar to your boat.

pshosho wrote:I am not interested in boat speeds of over 40-MPH, as Long Island Sound conditions are too rough to go that fast, but I am certainly interested in efficiency...

In my experience, the propeller which has produced the best fuel efficiency at a good cruising speed on my boat has always been a three-bladed propeller with rather large diameter, very little blade rake, and large blade area. A typical example of that sort of propeller in the Mercury Marine propeller line would be the MIRAGEplus model, using full plugs for the PVS (performance vent system) holes in the propeller hub.

I would assume--but you should verify--that if you get the Command Thrust engine the gear case hub will be suitable for use with the MIRAGEplus propeller or other large three-blade propellers.

Also, I have not made a comprehensive study of the propeller offerings from Mercury that will fit their 90-HP engine without Command Thrust (which has a smaller gear case), but my impression is that Mercury has more models and choices in propellers to fit the larger gear case that is used with the Command Thrust 90-HP engine. But, again, you might want to look more closely at that.

The general characteristics that Mercury ascribes to be provided by the Command Thrust are all very useful improvements, which leaves me wondering why anyone would NOT choose the Command Thrust model. About all I can infer is perhaps the smaller diameter gear case of the standard model might permit a very modest improvement in ultimate top speed, but would also be--if one can believe the promotional claims made--be inferior to the Command Thrust in many other aspects of overall boat performance.

pshosho wrote:ASIDE: Phil T sometimes knows the right propeller off the top of his head-- I have seen [that] in some postings.

I think Phil T has maintained a great set of notes and data about propeller performance from first-hand accounts by owners of many models in some sort of well-organized data set that he can quickly locate.

pshosho wrote:...the Command Thrust engine will be able to swing a propeller at the gear-ratio of the Command Thrust gear case that allows a hull to maintain plane over a wider RPM range.

With the typical naturally aspirated four-stroke-power-cycle outboard engine, the rated power is often only achieved at the upper RPM range of the permitted engine speed. By using a higher-numerical gear ratio, the engine can be run at a higher speed and thus produce more horsepower when trying to operate the boat close to its minimum planing speed. To maintain the hull on plane requires a certain amount of horsepower. The engine power band remains the same, that is, it does not get magically wider, but the higher-gear-ratio enables the engine to be able to turn the propeller at a propeller shaft speed that still keeps the boat on plane.

The way the propeller load--translated through the gear ratio--works is this: the engine at a particular throttle setting will accelerate until the load on the engine matches the power being produced. At that point the engine stops accelerating. Without the greater gear reduction of the higher-numerical gear ratio, the propeller load would have stopped the engine from accelerating much sooner, and at that propeller shaft speed the boat could not maintain plane.

To understand this better, see my article on propeller load:

Propeller Power Curve
Computing the load of a propeller
https://continuouswave.com/whaler/refer ... Curve.html

You can skip the initial math in which I derive the formular for the propeller power curve if you are not interested in that, and just read beginning at the subhead "Determining a Propeller Power Curve."

Thank you for the responses. I enjoy learning about this and it helps in the repower discussion and decision. But I realize that the first advice on repowering questions is usually the best: Spend your time researching the outboard dealer because they are critical in helping you decide on a large number of variables to achieve the optimal repower. Playing trial-and-error with propellers and mounting height will get pricey.


I forward to a Phil T' propeller suggestion.

I want to document my recent experience in case anyone else has the same questions that I posed when I first started this post about re-powering my 2002 170 MONTAUK with a 90-HP engine from the original 60-HP engine:

I had a new Mercury 90-HP engine intalled on my 2002 Montauk 170 by a local dealer a couple miles from my house. They are not in the business of selling used outboard engine, but they facilitated a trade-in on my boat's original c.2002 Mercury 60-HP BIGFOOT engine in an arrangement at a price I felt was fair. This was made possible because my outboard engine was very well maintained and still in demand. Again the only reason I replaced the old outboard engine was due to its small size for the boat.

The installation of the new Mercury 90-HP engine was totally plug-and-play. I was able to reuse the throttle control, steering, and all electrical. This saved substantial money on the installation. For those with 2002 and newer Boston Whalers, staying Mercury saves money.

Using my existing controls, the engine shifts are great and the steering is very easy. I am really glad I didn't spend the money on new cables and steering as they were not needed. I am totally happy with the functioning but my boat has been really well maintained and covered all its life.

The new Mercury 90-HP engine has a 14-pole connector cable and my boat is 8 pin cable. Mercury has a low-cost adapter wiring harness that plugs in to accomplish the connection.

The tachometer and voltmeter that were in stock on my boat in 2002 work perfectly with no new wiring.

The third helm gauge on my boat is for water pressure. I bought the small tap part for the engine block for $10 that will allow the hose to plug in and make that operational so the water pressure gauge will continue to work.

I am using Vessel View Mobile on my cell phone, so I don't need new gauges.

I elected to use the standard 2.07:1-ratio gear case as my 2002 170 MONTAUK boat isn't really that heavy, and to not get the COMMAND TRUST larger gear case.

The engine was mounted at two-holes-up. I can adjust if necessary. I am satisfied with the performance pulling a couple of kids on a tube and general cruising.

The initial propeller selected was Mercury Black Max 13 X 17 aluminum propeller. The advice was not to play around with expensive propellers until you see how you will use the boat and the weight of your typical crew and what area of performance you want to optimize. Once I figure out the boat's typical mission, then I can swap propellers and motor height to see what is optimal for the typical use.

I have been recording performance data; my boat's performance is really close to the Boston Whaler performance data that was published even with the aluminum propeller versus the stainless steel one in the test.

I have not had any smooth water for a maximum boat speed, wide-open throttle test.

The [time to acceleration at full-throttle from a standing start onto plane] is fine.

The engine and boat speeds at cruising are fine; my boat speed and engine speed and right on with the factory test data.

I may just keep the setup as it is now and focus on just going boating as here in New England the season ends quick.

What model of the 90-HP engine did you buy?

I think there are 90-HP engines called FOURSTROKE and others called ProXS.

When you get data about the boat speed and engine speed at full-throttle, please post that data. That performance data is perhaps the best indicator of the overall performance of the boat, engine, and propeller combination.