CONTINUOUSWAVE

I'm currently re-powering a 2004 Montauk 170. I was leaning towards the Mercury 115 FOURSTROKE Command Thrust since it is the same footprint as the 90. I talked with an engineer at Boston Whaler, and he said the 90-HP would be the maximum for that boat. They do tests, and they have maximum ratings for a reason. You're just setting yourself up for liability if you are ever in an accident. I ordered the Mercury 90-HP Command Thrust. It's built [with] the larger 150-HP gear case.

http://youtu.be/mnBrqq6p7rw

When I was looking at motors for the same boat, it seems that the recommendations were that is was best not to use the command thrust with it. That was since it was intended for heavier, or pontoon boats with its gear ratio. May be worth checking into unless you got different recommendations.

See 2006 170 Montauk Re-power

Yes, the Montauk 170 is right on the edge for recommendation of the the Mercury 90 FOURSTROKE Command Thrust. [Command Thrust] allows the option of larger [diameter] propellers, especially if you're looking for better control and lower end torque. If you're going for top-end speed, then the standard Mercury [gear case] is more aerodynamic and would be a better option.

I'd go with a 115 or larger. Do you guys go out looking for someone to have a boating accident with?

[I split this subject into its own thread after it took over the discussion in another thread on a different topic]

johnlandnsea wrote:I'm currently re-powering a 2004 Montauk 170. I was leaning towards the Mercury 115 FOURSTROKE Command Thrust since it is the same footprint as the 90. I talked with an engineer at Boston Whaler, and he said the 90-HP would be the maximum for that boat. They do tests, and they have maximum ratings for a reason. You're just setting yourself up for liability if you are ever in an accident. I ordered the Mercury 90-HP Command Thrust. It's built [with] the larger 150-HP gear case.

http://youtu.be/mnBrqq6p7rw

Please let us know how the command thrust works.

Did the engineer you spoke with give any specific reasons for the 90 HP limit on the Montauk 170 or how they derived it?

Possibly the increased top speed with a 115 Mercury? Since they weigh the same, that seems like the only thing that could affect safety. Not sure if acceleration would change a huge amount or if that is a factor.

Is Command Thrust a different nomenclature for Big Foot?

Customer-facing staff at any company are carefully trained to give information only in a way that reinforces the company position - not doing so gets them fired. So expecting the "engineer" to say anything contrary to the capacity plate, or give any credible insight into the process by which maximum motor capacity is assigned, is naive.

Yes, I was posing partly as a rhetorical question. I agree it would be very unlikely a manufacturer would give their rating process.

I was curious on this boat if others here see reasons for the limit other than the obvious marketing aspect.

timf--I'll keep you posted on the performance. He didn't go into too much detail from what I remember. Sorry.

Dutchman--the Mercury Command Thrust has the larger gear case from their 150-HP FOURSTROKE model, so it has a considerable amount of torque and a much larger propeller blade than a standard 90-HP.

The Enertia propeller is a new propeller geared for this engine and center consoles.
http://youtu.be/mIB9AZdmi0Y

fno--I've never been in an accident. I said "liability." If you want to exceed your boat's maximum engine rating, go ahead. My other boat has 350-HP, if I'm looking for speed. In a Whaler, I'm not racing anybody. Explore. Cast a line. Pop a top.

The 170 Montauk topped out at 40-MPH with the Mercury 90-HP FOURSTROKE Command Thrust. It's propped with Mercury's Enertia 14.2 X 18 propeller. [The 170 MONTAUK] handled great and [the 90-HP engine] felt like more than enough horsepower to get the job done! I had two people on board, one dog, and 12-gallons of fuel.

I think you lost little if any significant top speed with the bigger gearbox. With a similar load, mine tops out at 42 MPH with the standard gearbox.

It has a Vengeance 13-1/8" x 16" prop from the factory.

Yes, I agree Timf. I believe the larger gearcase does slow it a bit. But I wanted the extra low end torque and 40mph is still plenty of speed. I use to be a competitive barefoot water-skier and I'd bet that I could foot behind it.
With that said, my main reason for low end torque is for better control. I'm in Louisville, KY (from Michigan originally) and plan on visiting the Gulf of Mexico and the Great Lakes.

For future owners considering the same motor/prop combination, can you post the motor mounting height and WOT rpm when you reached 40 mph.

BTW, I checked http://www.bostonwhaler.com for the performance test on the Montauk 170 and with a 90 hp Mercury FourStroke WOT was 5750 and 40.9 mph with a 13 1/8” x 16” Vengeance prop.
http://www.bostonwhaler.com/boat_graphics/electronic_brochure/Company94/_58_918201544114PM.pdf

John - We also skied barefoot on the river when kids, but not competitively. Then got too worried about hitting a stick and started wearing tennis shoes. Much easier, but never did learn how to stop gracefully!

Phil - I'll check RPMs next time out, but do think it was likely in the 5700-5800 range.

Phil,

At WOT I was running 5800 RPMs. Here's a pic of the engine mount height...

The motor is mounted all the way down, I would raise it 2 holes. You will gain approximately 125 rpm for each hole you go up due to less drag.

Raising the motor is doable as a DIY with basic tools if the boat is on the trailer and you have a friend to help.

Thanks Phil

Mine is at one hole up as came from the factory. I may try one more up and see how that goes.

Tim, could you give us your engine stats please (what hp, max RPMs/WOT speed, prop size)? Thanks.

It is a Montauk 170 with a 90 HP Mercury 2.1L.

It has a Vengeance 13-1/8" x 16" prop.

42 mph is the fastest I have seen (GPS). I didn't note the RPM, but will next time I have a chance.

Thanks Tim. Sounds like I need to raise my engine one or two holes.

Two holes up seems to be the common recommendation.

I'm taking it out over the weekend and will try raising when I return.

To all
I did find out that the current nomenclature for Command Thrust (larger more torque capable gear case) is what Mercury used to call Big Foot.