190 MONTAUK with 115-HP; Compared to OUTRAGE 18

Optimizing the performance of Boston Whaler boats
Whalerdog
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190 MONTAUK with 115-HP; Compared to OUTRAGE 18

Postby Whalerdog » Sat Sep 05, 2020 12:53 pm

[Moderator's note: In a thread that was dormant for two-and-a half years, the thread was revived to add new comments. The new comments have been separated into their own thread. The reply that revived the old thread was rather cryptic. I have tried to figure out exactly what the author was trying to say by adding some additional text based on the many prior comments in the lengthy and dormant thread to which they were added two-and-a-half years late.]

A [190 MONTAUK with a 115-HP engine is a] 37-MPH boat with two people [aboard, and a] 39[-MPH boat] with [only] me [aboard] and [with a] half tank of fuel.

[In model year] 2007 [Boston Whaler set the power maximum for the 190 MONTAUK to] 115-HP. [In] 2008 [Boston Whaler] increased [the maximum horsepower rating of the 190 MONTAUK] to 135-HP.

[For a 190 MONTAUK that is powered with a 115-HP engine the performance is as follows:]
  • two people [aboard the performance is] OK.
  • [With] four people [aboard the performance is] not so good, and
  • with eight [people aboard, which is the rated] capacity [the performance] would be horrible.

biggiefl
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Re: 190 MONTAUK with 115-HP is Under-power

Postby biggiefl » Tue Sep 08, 2020 12:47 pm

Personally I can not fathom that a 190 [MONTAUK] with a 115-HP is faster than a classic 18 Outrage with a 115-HP.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

jimh
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Re: 190 MONTAUK and OUTRAGE 18 Hull Comparison

Postby jimh » Thu Sep 17, 2020 6:47 am

The hull of a 190 MONTAUK has a smaller length to beam ratio than does an OUTRAGE 18, that is, the 190 MONTAUK hull is wider.

COMPARISON OF LENGTH TO BEAM RATIO
OUTRAGE 18 = 2.58 (Length = 18.5; Beam = 7.166)
190 MONTAUK = 2.42 (Length = 19.33; Beam = 8)

Hull forms with more beam are generally easier to get onto plane and easier to maintain on plane as the hull form provides more lift, and particularly so when the hull is not a distinct V-hull shape and the hull deadrise is not particularly great.

jimh
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Re: 190 MONTAUK with 115-HP; Compared to OUTRAGE 18

Postby jimh » Thu Sep 17, 2020 11:34 am

Regarding how the performance--which is probably most characterized by the amount of time it takes for the hull to transitions to hydroplane mode from displaement mode and the ultimate maximum boat speed possible obtained--varies when the number of persons aboard is increased to eight persons from one person:

For the sake of an example, let us assume that the 190 MONTAUK boat with engine, fuel, required safety gear, and general accessories might weight about 2,795-lbs. This is a reasonable assumption. It is based on the reported weights from a Boston Whaler boat performance test of the 190 MONTAUK with a 115-HP engine.

Let us assume that each person added weights 180-lbs. This is a reasonable assumption for an adult male, and, in fact, might be a bit on the low side of real weight.

With one person added, the boat weight increases to 2,975-lbs. We can refer to this as the baseline boat weight.

Now we can compute the power-to-weight ratio: 115-HP/2975-lbs = 0.038-HP/lbs or about 25.9-lbs-per-HP.

Boston Whaler reports the boat with two people at total weight 3,125-lbs can hit 41-MPH. The power to weight ratio for that is 115/3125=0.0368. From this we can calculate a hull factor constant: c = 214.

With this information we can predict how performance will change as more people are added. Let's go for the worst case, there are eight adults on the boat. We have to add seven more people at 180-lbs each, or 1,260-lbs, and add that to the baseline boat weight, 2,975-bs. The boat with eight people should weigh about 4,235-lbs. Now the HP-to-weight ratio becomes smaller:
    115-HP / 4235-lbs = 0.027
Using the speed prediction method as above, the top boat speed should now be:
    MPH = 214 × 0.027^0.5
    MPH = 35

Now we must assess what a change in top boat speed means when the boat speed decreases to 35-MPH from 41-MPH. Can we describe this 6-MPH decrease in top speed as being characterized as "horrible performance"?

We can look at this another way. What increase in horsepower would be needed to maintain the boat at 41-MPH with the load of eight passengers? This can also be calculated easily. We just need to maintain the same power-to-weight ratio as before. The 41-MPH boat speed with two aboard occurred with the power-to-weight ratio of 0.0368-HP/lbs. To maintain that ratio at 4,235-lbs of boat and passengers, we need 4235 × 0.0368 = 156-HP.

The 190 MONTAUK is only rated for 150-HP maximum, so we cannot compensate for seven additional passengers and achieve the 41-MPH we got with the 115-HP engine and only two people aboard.

None of this is particularly surprising to anyone who has operated a small boat. Because small boats do not weigh very much, the addition of seven adult passengers increase the boat weight by 1,260-lbs and is certain to have a negative effect on boat performance. To look at the likely boat speed reduction--which has been characterized as being "horrible"--and to find the top speed might reduce by 6-MPH seems quite tolerable as opposed to "horrible."

The notion of having eight people aboard a 19-foot boat should also be considered. If the goal in buying a boat is to be able to operate the boat on a continual basis with eight people aboard, there are probably some other considerations that might need to be taken beside the reduction in top speed by 6-MPH to 35-MPH from 41-MPH with only two aboard.

It is important to note that in the speed predictions above, there is no allowance for a propeller pitch. We just assume that the propeller pitch is suitable for allowing the engine to accelerate to its rated engine speed for maximum horsepower. In real world outcomes, it will be very likely that a propeller that permitted the 115-HP engine to accelerate to 6,000-RPM with one or two people aboard may no longer allow the same engine to accelerate to that same speed when eight people are aboard. A change in the propeller may be necessary to get optimum performance with eight people aboard. If the propeller pitch were not reduced, the performance of the boat, having been tuned to optimum for a much lighter load, would likely be less than optimum.

jimh
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Re: 190 MONTAUK with 115-HP; Compared to OUTRAGE 18

Postby jimh » Thu Sep 17, 2020 11:56 am

Whalerdog wrote:A [190 MONTAUK with a 115-HP engine is a] 37-MPH boat with two people...

A Boston Whaler performance report says such a boat makes 41-MPH.

(edit - added performance report)
perfreport.jpg
perfreport.jpg (138.46 KiB) Viewed 5464 times

jimh
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Re: 190 MONTAUK with 115-HP; Compared to OUTRAGE 18

Postby jimh » Fri Sep 18, 2020 9:39 am

Also to be noted in the performance report from Boston Whaler on the 190 MONTAUK with 115-HP engine is the inclusion of a T-Top. There is considerable air drag from a T-Top. The effect of air drag increases with an exponential rate with boat speed, which means the presence of a T-Top has the most effect on boat speed at the highest boat speeds.

A T-Top also adds weight to the boat. All added weight reduces speed at all speeds.

On that basis one can quite reasonably assume that were the test boat a 190 MONTAUK without a T-Top, the top boat speed would have been higher than 41-MPH. Let us assume the top speed would increase to 43-MPH.

Now we have to judge the performance. Is a 19-foot boat powered by a 115-HP engine that can go 43-MPH just "OK" performance?

Exactly how fast should a 19-foot boat go in order for its performance to improve to "Good" from just "OK"?

jimh
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Re: 190 MONTAUK with 115-HP; Compared to OUTRAGE 18

Postby jimh » Fri Sep 18, 2020 1:27 pm

Regarding the ability of a boat to rapidly accelerate its boat speed, we have no good method to predict this. Some insight can be gained by comparing the propeller power curve to the engine power curve. Unfortunately, we do not have any of those curves.

For a good discussion of how a propeller curve can be estimated and how and engine power curve can be compared, see my article in REFERENCE on propeller power curves at:

Propeller Power Curve
Computing the load of a propeller

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

In 2008 article I provide some graphs that simultaneously plot a propeller power curve and an engine power output curve, both variables as a function of engine speed. This method gives insight into how easily the populsion engine can accelerate a particular propeller and thus accelerate a boat.