Crossflow vs. Looper

Hey Skools, What about drug smugglers who clamp on four yamaha 250's to the back of a boat that looks like it was built in someone's backyard?
 
Skools in 98% correct and BrainCT is a touch right as well. Technically you are looking at "diminishing returns" as the horsepower continues to increase the gains become smaller and smaller. There is a line, where when reached, it takes huge amounts of additional power to see very small gains. That is the area reffered to as hull speed. On a V-20 per say, with a 150 you can get 43ish mph, add 50 hp and you add 4 mph. Put 500 hp on one and you'll likely get mid 60's in theory. If you jumped it to 600 you'd likely see little or no increase. The hydrodynamics of the last little bit of drag just can't be overcome, efficiently at least. That's the main reason that the big boat speed records only inch up. HP is easy to find and stuff into a 40'er. For them the HP isn't the limiting factor, the hull form is.

Airslot
 
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Here's another . . . I thought max hull speed only applied to displacement hulls. I'm probably wrong but I know once you reach a certain speed . . . the power required to push the boat each additional MPH increases exponentially.
 
My dad had a 1969 johnson 55hp too on the back of a 17ft arrowglass. That motor had a green mid section a white cowling and roared like a nest of mad hornets.
 
bigshrimpin said:
ph-mo108-400.jpg


Here's another . . . I thought max hull speed only applied to displacement hulls. I'm probably wrong but I know once you reach a certain speed . . . the power required to push the boat each additional MPH increases exponentially.
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Don't remember the ratio, but remember hearin' same thing about cars...once to 100 mph, takes 10 additional HP to increase 1MPH...or somethin' like that...


...an' I'm not sure I buy into a hull having a "Max Speed"...maybe max EFFICIENT speed...but let's say a given hull speed maxes out w/ 150 hp and you strap on a jet engine ... I think it's gonna out run the 150 ;) ...
 
I know when I got my Starcraft/140 combo, I tried several different props.  I could make the holeshot better or worse, change the RPM at WOT and throughout the range, but the boat would go 43mph (gps) no matter what prop was used.  I can see why it wont go faster, with all those rivits sticking out of the bottom.

Now the guy that had the boat before me had a 150 Yamaha 2 stroke, he said he got 50 mph. But I dont know how he measured his speed.
 
Along with the V-20 I hava 1969 16' bonito walk-thru. It had a 65hp 3 banger johnson and cable steering when I bought it for 150 bucks. I replaced the deck( foam filled under it) repowered with a 115 evindude/ 21" SS prop. The boat was dangerous. At WOT it would chine walk. I still have the boat but, powered down to a 90 yam.

What's the fastest V-20s out there? The ICW limit is 25 around here and the ocean is always too rough to run WOT anyway.
 
Hammerhead said:
What's the fastest V-20s out there? The ICW limit is 25 around here and the ocean is always too rough to run WOT anyway.
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There was someone with twin 140's on the board . . . but I'll believe some of the early Alim v20 race boats were pushing 70+mph. CIA v20's with twin 100's were high 40's in 1961.



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Well looking at my motor owner's manual it says it's a loop charge. Are we talking the same here?

Also I was told by a pro that these "loopers" are equiped with low compression headers, thus running at lower compression.

Can someone translate that please? ;D

Thanks

Joe
 
Low compression heads...  Using a low compression head reduces the fuel octane requirements of the motor. It also reduces fuel economy and horsepower.

I believe the compression ratio for a 2-stroke is actually the dynamic compression ratio.  A static compression ratio is the volume of the cylinder at BDC over the volume of the cylinder at TDC.  A 2-stroke dynamic compression ratio takes the volume of the cylinder when the exhaust port closes and divides it by the volume of the cylinder with the piston at TDC.

Lowering the compression ratio, raising the exhaust port in the cylinder so it closes later, increases the top end performance of a 2-stroke motor.  Lowering the exhaust port increases the bottom end.  Since the loopers are high rpm motors, they run low dynamic compression ratios.
 
bass the compression on Loopers have 120 to 130 usually on a V4 that's the same as a crossflow V4 but the V6 motors are different the loopers are 90 to 100 usually and crossflow's are 125 to 135. the Loopers have a single carb per cylinder. they are much better on fuel than crossflow due to the fuel flow inside the motor. look at the first page of the pics they show the fuel flow. Loopers produce less heat as well. if you have a 2 stroke choice go Looper for better fuel mileage, lower temps, better all around HP range, they turn more RPM's, and easier and cheaper to fix usually.
 
Skools Out said:
bass the compression on Loopers have 120 to 130 usually on a V4
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Mine blows those numbers out of the water. I've got a 145 on one bank both cylinders, and 150 and 155 on the other side. Mine is a bare to start without dual batteries, or at least a good group 27.
 
is that on an outboard? that compression is too high for stock heads, crank, rods, and starter. i have a racing jet ski with all race mods stock was 90 PSI now 230 PSI. but once you get that high on a 2 stroke you have to weld the crank so the plates don't turn. You have to use better rods as well.
 
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