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Registered: 04-2010
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Sho VS Mercury


 


Though it was strange not to hear that so-familiar two-stroke growl and howl, the Yamaha gave up nothing to its two-stroke competition in the acceleration department. When we dropped the hammer from a dead in-gear idle, the four-stroke launched the boat to 30 mph in 5.5 seconds. That’s more than a second quicker than any XB-21 we’ve tested with big Merc power, and easily backs up Yamaha’s claim that this new V-6 accelerates at least as quickly as any comparable two-stroke. Its midrange punch was strong as well. From 40 mph, we accelerated to 60 mph in 4.4 seconds, again quicker than our best Merc rig tested by about a half second.

Yet, while acceleration is important, most want to know what it does at the top-end. At full pedal with two aboard and a full tank of fuel (plus a reasonable complement of rods and tackle), our test rig strung out back-to-back 86-mph runs spinning its custom Hydromotive four-blade prop at 6000 rpm. While 86 mph is faster than most would ever want (or need) to go, for Allison enthusiasts it’s sure to be considered a little slow, especially with 250 horses on tap. The last time I tested this hull, we reached 89 mph with 25 less horsepower (a Mercury OptiMax 225 Pro XS).

OUTBOARD HEIGHT
Here’s the big difference between the two setups: the SHO came equipped with Yamaha’s performance gearcase, which uses cooling water intakes mounted along the side of the housing’s “bullet,” while the Pro XS was equipped with Mercury’s Sport Master case, which features water inlets on the very bottom of said bullet. It may not seem like much of a difference, but the lower water pickups of the Sport Master allow higher engine mounting for less drag, and that allows a fast hull like the Allison to capitalize and reach higher speeds.

With our Yamaha-powered rig, we used the boat’s CMC hydraulic jacking plate to raise the engine as high as we could on the transom without risking powerhead overheating from lack of cooling water. At its highest position, the Yamaha’s propeller shaft was positioned exactly 1 inch below the boat’s pad bottom. On the Merc-powered rig, we were able to raise the engine so that the propshaft was 5?8-inch above the pad.

We were able to get our hands on a Merc 250 Pro XS-powered XB-21 at the same time we tested this Yamaha-powered rig. This Merc used a Torque Master lower unit, which is not nearly as performance-oriented as the Sport Master—in that it doesn’t have an elongated “nose cone” and the water
intakes are positioned on the side of the unit as well as at the “nose.” The results were not nearly as good as the 225 with the Sport Master, producing an 82-mph top speed at 6100 rpm turning a stock 27-inch-pitch Mercury Trophy Plus four-blade propeller. Clearly, the extra 25 horses of the 250s don’t help if the hydrodynamic efficiency of the setup isn’t optimized for the rig.

SIPPIN’, NOT DRINKIN’
Yamaha’s 250 VMax SHO also proved to be thrifty behind the Allison. At a best cruise speed of 36 mph, we recorded 5.7 miles per gallon. Though many anglers criticize Allison for including a relatively small,
36-gallon fuel tank (most 21-foot bass boats have at least 45 gallons or more), it’s enough to give this rig a 185-mile range with a 10 percent reserve. At full bore, the Yamaha’s not nearly as frugal, dropping mileage to right around 4 mpg.

Handling-wise, this Allison is not as weight sensitive as its predecessors, showing that both hull and engine manufacturers are building products that mate well with each other. The Yamaha was mounted to a CMC electric hydraulic jacking plate with a 5½-inch setback, plus a set of machined aluminum spacers that added another 2½ inches of setback for a total of 8 inches from the transom surface. This provided very good balance combined with strong acceleration from a dead stop as well as plenty of leverage for the engine and prop combination to lift the hull and enable it to run clean with a minimum of wetted surface at midrange speeds to full bore.

Yamaha 250 VMax SHOT ENGINE AS TESTED
Make Yamaha 250 VMax SHO
Horsepower 250
Number of cylinders V-6
Displacement 4.2L (254 cid)
Induction EFI four-stroke
Weight 505 lbs.
Gear ratio 1.75:1
WOT rpm range 5000-6000
Propeller Hydromotive Quad IV
15x27" stainless four-blade

TEST RESULTS

Top speed (mph) 86.0
0-30-mph acceleration (seconds) 5.5

SOUND MEASUREMENTS (dBa)
Idle at transom (700 rpm) 66
Idle at helm (700 rpm) 65
Cruise at helm (3500 rpm) 81
WOT at helm (5900 rpm) 94
    
 
 


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Sep/13/2010, 10:46 pm Link to this post Email NITROZ9   PM NITROZ9
 
Bullet20dc Profile
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Registered: 09-2004
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Re: Sho VS Mercury


Good read very interesting performance specs.

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Charlie B
Bullet20dc / 250XS
Politically correct impaired

Sep/14/2010, 5:53 am Link to this post Email Bullet20dc   PM Bullet20dc
 
NITROZ9 Profile
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Re: Sho VS Mercury


Did you notice that the fastest speed that they have gotten out of that hull hass been with a 225 pro xs. emoticon

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Sep/14/2010, 5:58 am Link to this post Email NITROZ9   PM NITROZ9
 
Gary L Profile
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Registered: 04-2010
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Re: Sho VS Mercury


Bring that Gearcase over to HydroTec here in Nixa and they can improve it's low water pickup and you can raise that motor up to the same or higher then that Sportmaster went.

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Sep/14/2010, 1:27 pm Link to this post Email Gary L   PM Gary L
 
Bullet20dc Profile
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Re: Sho VS Mercury


Im starting to hear horror stories about propping this motor for the right setup.
Maybe Jay has some ideas on this. Keeping in mind any lower unit mods will void a very expensive warranty.

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Charlie B
Bullet20dc / 250XS
Politically correct impaired

Sep/14/2010, 3:31 pm Link to this post Email Bullet20dc   PM Bullet20dc
 


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