Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Optimizing the performance of Boston Whaler boats
fina
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Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby fina » Wed Apr 01, 2020 12:10 pm

[For a boat that is] usually rigged with 450 to 500-HP using twin 225 or twin 250-HP engines, [how will the top speed] be affected by re-power with a single 350-HP engine using four-stroke-power-cycle?

Assume the boat is a 2006 270 Outrage.

jimh
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Re: Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby jimh » Wed Apr 01, 2020 12:32 pm

Any moderate speed planing-hull boat will have its speed affected by a change in the power-to-weight ratio. Naval architect George Crouch proposed, and many examples of Boston Whaler boat hulls confirm, that the effect of the power-to-weight ratio on speed is at the 0.5-exponent. Expressed mathematically

    NewSpeed = OldSpeed × ( (NewPower/NewWeight)/(OldPower/OldWeight) )0.5

To simply, we assume the weight of the boat does not change. The relationship then simplifies to:

    NewSpeed = OldSpeed × (NewPower/OldPower)0.5

If the speed with 500-HP were, say, 40-MPH, we can estimate the speed with 350-HP by Crouch's method to be

    NewSpeed = 40-MPH × (350-HP/500-HP)0.5
    NewSpeed = 33.5-MPH

Repeat the above as desired with different new and old horsepower to find other predicted speeds when no weight change occurs based on whatever speed you wish to assume for the old configuration.

Experience applying Crouch's method to speed prediction to Boston Whaler hulls has shown very good correlation between predicted speeds and actual outcomes. So there is no basis to alter Crouch's method to calculate the effect for a 2007 270 Outrage, unless the top speed with 500-HP were way off the chart, say 65 to 70-MPH. The effect of the hull characteristics may change somewhat at extremely high planing speeds as less and less of the hull remains in contact with the water, and the submerged propeller and associated gear begin to have the dominant influence.

Without knowing the exact weight of the boat before and after the change in power, the effect of weight change is not applied in the above example. Of course, as weight decreases, performance improves, but this change also occurs at the rate of the ratio to the 0.5 exponent. If you wish to assume there was a weight reduction by a factor of 0.95 (five percent reduction), then repeat the above calculation with the 350-HP figure increased to 350/0.95 or 368-HP. The predicted speed change from the assumed 40-MPH speed would then be to 34.3-MPH.

An additional factor when changing to a single engine from twin engines will be some reduction in underwater hull drag from elimination of one engine gear case and propeller. This will also generally tend to improve performance, but predicting this will be nearly impossible. The overall efficiency for using two propellers to propel the boat might actually be higher than using one propeller is some situations, particularly with outboard engines were the propeller diameter may be limited by the aperture of the engine gear case.

Crouch's method has no factor that accounts for the number of strokes-per-power-cycle for the propulstoin engine, so there is no way to account for the new engine and the old engines being of some different design in regard to number of strokes-per-power-cycle.

Engines of different designs will have different power curves, different engine speed ranges, different gear reductions, and different propeller shaft speeds, and these many factors will require the propeller design to be altered to suit the engine in its application on the boat. The Crouch method assumes the engine, gear reduction, and propeller design and selection are of no influence, that is, that in each case those factors are equivalent for either power choice.

biggiefl
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Re: Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby biggiefl » Wed Apr 01, 2020 1:58 pm

Performance [of a 2007 OUTRAGE 27 with a single 350-HP engine] might be acceptable for you, but what about:

  • resale; most people want the maximum power or close to it when purchasing a boat
  • a smaller engine does not necessarily mean better fuel mileage
  • larger hulls will suffer in slow speed maneuvering with a single engine; docking with wind might be difficult
  • engine torque makes a big difference; a 27-foot-boat with a 300-HP outboard is no match for torque compared to a 300-HP big-block sterndrive.

Summary: many deciding factors to consider, not just top speed.
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: Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby jimh » Wed Apr 01, 2020 5:37 pm

A magazine review cites Boston Whaler information for a 2007 270 OUTRAGE with twin 200-HP OptiMax engines as follows:

According to the manufacturer's figures, the 270 turns 40.6-MPH cruising at 4000-RPM while burning 10.5-GPH, and tops out at 54.7-MPH at 5500 (wide-open throttle), burning 21.6-GPH.


Using the top speed of 54.7-MPH with 400-HP, we can predict a speed with 350-HP (with no weight change) as 51-MPH.

Cf.: https://www.saltwatersportsman.com/boats/boat-reviews/boston-whaler-270-outrage-0/

fina wrote:Assume the boat is a 2006 270 Outrage.


The Boston Whaler specifications list the MAXIMUM HP at 450-HP for a c.2007 270 OUTRAGE, and the optional engines were twin 225-HP.

What was the basis for the notion these boats would normally have 500-HP?

biggiefl
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Re: Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby biggiefl » Thu Apr 02, 2020 10:46 am

ASIDE: I want to know how twin 200's are only burning 21.6gph @ WOT unless that is per engine. My 225 burned 23.5gph @ WOT.
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: Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby jimh » Thu Apr 02, 2020 12:59 pm

ASIDE about a figure published on another website about 270 OUTRAGE fuel economy with twin 200-HP engines:

biggiefl wrote:I want to know how twin 200's are only burning 21.6gph @ WOT unless that is per engine.

The magazine says the data came from Boston Whaler. The data suggests that the 270 OUTRAGE was going 54.7-MPH and only burning 21.6-GPH, for a fuel economy of 2.5-MPG. That is really good fuel economy--maybe too good to be true.

biggiefl wrote:My 225 burned 23.5gph @ WOT.

On my boat, a single 225-HP at wide-open burns 20.4-GPH . That two 200-HP OptiMax engines would combine to burn 21.6-GPH is a bit of a stretch. More likely, as hypothesized, the 21.6-GPH might be the fuel burn on one engine.

A recent test of a 270 DAUNTLESS with twin 200-HP engines showed full-throttle the two engines consumed 35.8-GPH and got only 1.5-MPG at 53.2-MPH. The engines were very modern V6 FOURSTOKE models with excellent fuel economy, but even still the fuel economy was 1.5-MPG, far below what was reported for the 270 OUTRAGE by the magazine.

biggiefl
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Re: Effect on Performance of Reducing to 350-HP from 450 or 500-HP

Postby biggiefl » Fri Apr 03, 2020 10:51 am

With two-stroke- or four-stroke-power-cycle engines or WANKEL engines, the fuel burn at maximum throttle should be close as it is a proportion of horsepower. Efficiency at other speeds will vary--and sometimes vary greatly.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).