2008 Montauk 170 Re-power

[jim40]

Hello. I am a long time CW observer, and now a new member.

I have a 2008 Montauk 170 with the factory installed Mercury 90 FOURSTROKE. I would like to re-power with a 2016 Mercury 115 FOURSTROKE ProXS or the standard 115 FOURSTROKE. I realize that the 170 is only rated for 100-HP.

The 115 FOURSTROKE has a different gear case and ratio than the 115 FOURSTROKE ProXS, which has a Command Thrust gear case, a higher ratio, and has higher engine speed. I want to stay with Mercury to be able to keep the controls and rigging that are currently on the boat.

I am seeking advice, suggestions, and recommendations on which motor to get, along with possible propeller combinations that anyone might suggest or have knowledge of.

I am surely looking forward to any and all ideas and discussions that anyone might have to offer.

Thank you,

[Phil T]

For any repower, check the new engine weights and evaluate how the boat will sit. Add weight to the stern and observe trim at rest and general handling at speed.

Prop suggestions will be engine dependent.

[jim40]

The new engine will be 35 to 40-lbs lighter. The current 90-HP engine has a Laser II 19-pitch propeller. I am looking for propeller and mounting height advise. I am not so sure the 19-pitch propeller will be what is needed for a 115-HP.

[jim40]

Does anyone have any prop suggestions?

[jimh]

Which engine did you get?

What is the gear ratio?

[jim40]

I am getting a 2016 Mercury 115 hp. Pro XS. Gear ratio is 2.38 : 1.

[jimh]

According to the Boston Whaler test data from

http://www.bostonwhaler.com/boat_graphi ... 4114PM.pdf

the top speed of the 170 MONTAUK with a 90-HP engine is 40.9-MPH. If the engine power is increased to 115-HP from 90-HP, then we can anticipate the boat speed will improve at the square root of the power-to-weight ratio. For simplicity, I assume the 115-HP engine adds no extra weight. Then we figure the new top speed to be

40.9-MPH x (115/90)^0.5 = 46-MPH

Now we need to know the maximum RPM of the new engine. Let me go find that data. Okay, found it at

https://www.mercurymarine.com/en/us/eng ... ourstroke/

The maximum engine speed is 6,300-RPM.

Now we need a propeller calculator. The very wonderful propeller calculator from CONTINUOUSWAVE is still not fixed, so I will use Mercury's almost identical version which they implemented recently. It is at

http://www.mercuryracing.com/prop-slip-calculator/

(Note that there is a link to it in the announcement article at the top of the PERFORMANCE forum about propeller calculators.)

To use the calculator we need to enter values for the following:

--RPM
--RATIO
--BOAT SPEED MPH
--PROPELLER SLIP

and then the calculator will crunch some simple algebra and compute PITCH, The calculator is not an oracle. It just takes the data you give it and manipulates it according to very rigid and simple algebra. There is no magic in the calculator.

Here is our data for this application:

--RPM = 6300
--RATIO = 2.38
--BOAT SPEED MPH = 46
--PROPELLER SLIP = 10

SLIP as a value of 10 is just an estimate of what we will expect from the typical propeller when running at full throttle. SLIP may be lower in some cases. SLIP also depends on how the PITCH has been stated.

Now we push the CALCULATE button to get PITCH:

The calculator says PITCH = 20.4-inch

On that basis, I would begin your propeller testing with "the 19-pitch propeller" you mentioned in your post earlier. I think you said it was a LASER II 19-pitch. That should be a good propeller. It is stainless steel, and designed for good boat speed performance on light, fast boats.

Report the results from testing with the LASER II 19 propeller. Then that actual data can be used to make more informed suggestions.

There are four assumptions in this calculation. First, we calculated the new boat speed target based on the usual performance of moderate v-hull planing boats as described by naval architect George Crouch many years ago, i.e., that speed is proportional to the horsepower-to-weight ratio by the 0.5-exponent. This is just an approximation, and for a particular hull there may be some deviation from that. Next we assumed that the new engine develops its maximum horsepower at its maximum engine speed. This is typically true for many four-stroke-power-cycle outboard engines, but it may not be for the specific engine under discussion here. The engine might actually deliver its peak horsepower at a lower engine speed. Third, we just plugged in a SLIP value of 10-percent. This is a reasonable estimate. Generally, one uses the propeller calculator to calculate the SLIP factor, as implied by it being the right-most field in the calculator. Finally, I did not include any allowance for a change in boat weight. Even though the new engine may weigh a bit less than the old engine, we don't have the total boat weight of your boat. We could use the Boston Whaler boat test weight of 2,089-lbs, but a change of only 40-lbs is quite minor. For this estimate I have ignored it. These four assumptions are generally reasonable assumptions, and when the calculator cranks out 20.4-inch PITCH, that is probably a good place to start.

The LASER II 19 is close enough to the calculated PITCH to be a good choice for an initial trial. Also, propellers with a lot of blade cup tend to behave as if they had more pitch. Whether or not Mercury has included any effect of blade cupping on their calculation of the LASER II pitch rating is hard to know. Generally a propeller with a lot of blade cup will need to have one-inch of pitch added to make the calculator more accurate. That is, in effect the LASER II 19 may actually act more like a 20-pitch propeller. Give it a test run and let's see.

Also, note how the new engine has a big gear reduction. This is probably intentional. It allows the engine to turn the usual pitch propellers and also lets the engine speed accelerate well above 6,000-RPM, which it probably needs to be able to reach to make its rated power. The propellers are designed for a certain propeller shaft speed. They know nothing about engine speed. With these high-revving newer four-stroke-cycle outboard engines, the outboard uses a higher (numerical) gear reduction in order that the propeller shaft speeds will be the same as they were when the propellers were designed back in the days of two-stroke-power-cycle outboard engines that typically did not run above 5,500-RPM.

[jimh]

By the way, I am really surprised that the current 90-HP engine is running with a LASER II 19-pitch. According to the Boston Whaler test report, the stock propeller is a VENGEANCE 16 pitch. Where did you get the LASER II 19? I would expect that propeller to much be too big for the 90-HP engine to turn.

[jim40]

JimH--Thank you so much.

[jim40]

The 2008 Montauk 170 came factory rigged with the Laser II 19" pitch prop. It is the "Veradito" type 90-HP. motor.

[jim40]

Also, the 2008 90 hp 4 stroke had a 2.33:1 gear ratio, so it is close to the same as the new motor. I agree, the 19" will be a good place to start

[jimh]

Thanks for the clarification on the current engine. The confusion comes because Mercury has had so many engines called "90-HP FOURSTROKE" engines. Let's see if we can review them all:

--the 90-HP FOURSTROKE based on the 1.596-liter Yamaha-made block with carburetors and no SmartCraft with 2.33:1 ratio, sold until c.2005

--the 90-HP FOURSTROKE based on the 1.596-liter Yamaha-made engine with fuel injection but no SmartCraft with 2.33:1 ratio, sold only briefly in c.2005

--the 90-HP FOURSTROKE based on the 1.7-liter non-supercharged VERADO block without DTS, (AKA the VERADITO) with 2.33:1 ratio, appeared c.2006

--the 90-HP FOURSTROKE based on a 2.1-liter block with smaller gear case with 2.07:1 ratio, appeared c.2014

--the 90-HP FOURSTROKE COMMAND THRUST based on a 2.1-liter block with larger gear case with 2.38:1 ratio, appeared c.2014

So your 170 MONTAUK came originally with 90-HP FOURSTROKE based on the 1.7-liter non-supercharged VERADO block without DTS with the 2.33:1 ratio. Right?

[jim40]

Yes jimh, that is correct. The boat is a 2008 50th anniversary model with the normally aspirated Verado block engine.

[jimh]

In seeking more understanding of how propeller size (diameter, number of blades, and pitch) affect the engine speed, you may appreciate the presentation of the PROPELLER POWER CURVE that I give in a REFERENCE article at

Propeller Power Curve
Computing the load of a propeller

http://continuouswave.com/whaler/refere ... Curve.html

While increasing the engine horsepower on your boat to 115-HP from 90-HP will likely afford you the opportunity to have a higher top speed, it will also give you the opportunity to increase the extra power available from your engine to accelerate the propeller, as illustrated by the difference between the propeller power curve and the engine horsepower curve. I think you will find this article to be interesting.

[chuck21401]

According to the Boston Whaler test data from

http://www.bostonwhaler.com/boat_graphi ... 4114PM.pdf

the top speed of the 170 MONTAUK with a 90-HP engine is 40.9-MPH.

I was surprised to see the 40.9 top speed for the 2016 Montauk. This is slower than previous years, even with less weight on the transom. I have a 2008 170 Montauk with the 90 FOURSTROKE and with the engine trimmed out would see 43 to 44-MPH as speed over ground on GPS . My engine has a 13-1/4 x 20 LASER II propeller.

The old performance report from 2007 is still on Whaler's server, and that indicated a 42.8-MPH top speed.
http://www.bostonwhaler.com/boat_graphi ... neperf.pdf

Seems like 115 HP would generate a top speed north of 46-MPH, which is about as fast as I would ever want to go in a 170 Montauk.

[jimh]

The test data from c.2007 also shows the boat weight is higher, yet the boat runs faster. The most reasonable conclusion: the older version of the 90-FOURSTROKE engine has more power than the new version of the 90-FOURSTROKE engine.

The 2007 version of the 90-FOURSTROKE is the VERADITO engine, which has only 1.7-liter displacement. The 2016 version of the 90-FOURSTROKE is based on a larger displacement engine block with 2.1-liter volume. That is an increase in displacement of 0.4-liters, or an increase of 0.4/1.7 = 24-percent. It seems counter-intuitive that when the engine displacement was increased 24-percent the power output should go down.

Perhaps there are some environmental influences that affected the test outcome. The older test indicates the ambient temperature was 70-degrees-F. The newer test data does not include temperature information.

The boat speed decreased to 40.9-MPH from 42.8-MPH, yet the total weight decreased to 2089-lbs from 2291-lbs. If we were to attribute all of this influence to engine horsepower output, and to assume that the newer 90-FOURSTROKE was producing exactly 90-HP, then we could calculate the power output of the older 90-FOURSTROKE BASED ON 1.7-LITER VERADO. That calculation would indicate the older version was making 108-HP.

Estimated Horsepower of various Mercury 90-FOURSTROKE models, based on test data:

c.2007 90-FOURSTROKE based on 1.7-liter block = 108-HP
c.2016 90-FOURSTROKE based on 2.1-liter block = 90-HP (assumed)

Looking at the power output per liter of displacement, we find

c.2007 90-FOURSTROKE based on 1.7-liter block = 108-HP/1.7-liter = 63.6-HP-per-liter
c.2016 90-FOURSTROKE based on 2.1-liter block = 90-HP/ 2.1-liter = 42.9-HP-per-liter (assumed)

That is a decrease in power output per liter of 20.7-HP-per-liter, or a decrease of 32-percent. If the newer and larger displacement 90-FOURSTROKE BASED ON 2.1-LITER engine were to produce power at the same rate as the 90-FOURSTROKE BASED ON 1.7-LITER engine appears to have done (based on the inference calculated above from the test data) then this newer engine could produce 133.5-HP.

In the c.2016 90-FOURSTROKE engine from Mercury, the engine tuning appears to have been significantly restricted so as to limit power output. The restriction or limitation on the power output from the 2.1-liter displacement engine in the 90-FOURSTROKE model is probably intentional in order that the 90-HP version have less power output that the 115 ProXS version using the same 2.1-liter block.

[timf]

The restriction or limitation on the power output from the 2.1-liter displacement engine in the 90-FOURSTROKE model is probably intentional in order that the 90-HP version have less power output that the 115 ProXS version using the same 2.1-liter block.

Yes, this is definitely a good part of it. The 90-HP version is a de-tuned version of the 115-HP model.