Explain this one...

[croSSed]

Here's what I'm working with:
2.4L Merc 175, 2:1 gears, 27P Laser II prop, 6400 rpm at 74-75 mph, about 8% slip. Too many rpm IMHO.
I acquired a 1.87:1 lower. NOW...
Same prop, motor, everything except the gear ratio, now I'm only getting 5600 rpm at 70 mph! Same slip number.
How is that possible? In an apples to apples world it would seem to me the only difference would be rpm, which should be about 6000.
What is going on?

[FORBESAUTO]

Good ole age old argument. All the pencil pushers says should only change rpm and gear change and prop pitch will yield the same result. Depending on applications this can be true, but not always. The torque multiplication plays a huge role. With your power to weight ratio, you were better off with the extra multiplication from the lower gears. You basically ain’t got enough ump to turn the 27 now. You can drop pitches to get rpm up, but you ain’t gonna get better slip numbers with the smaller prop, and ain’t gonna go faster, more than likely slower.

[LakeFever]

There’s a lot more exciting props than a laser. The only prop that I had blow out with on my Valero was a laser. Stuff a small ear chopper on there and see how you like it. Lightning et if you want to spend the dough will transform the performance.

Also worth mentioning are you 100% sure the core is solid? When the cores go soft the hull will flex at speed, works like a brake.

[KIRCHNER]

hes #s are crazy,, 27p @ 5600? 70mph? .

[OnPad]

FORBESAUTO nailed it.

6400 rpm is perfect.
Phenomenal performance out of that setup! Really low slip for that prop.
Remind me what hull, and engine height?

I had a 16' champion with a 150 supermag. I was lucky to see 65mph with a 25p laser. Spent thousands on props, and a ltv hull, with the same 150, only see 7 or 8 mph more than your boat, at nearly 7000rpm.

[croSSed]

FOLLOW UP: I seem to have forgotten about this thread I started. I acquired an unmolested 28P big-ear Chopper. I went back to the 2:1 lower because it just performed better. Now - 2:1 lower, 28P Chopper, 6400 rpm, 78 mph, 8ish % slip. The boat is in my sig. It's a 17' Cee Bee Avenger Mach II. The hull by itself weighs about 900 lbs, give or take 50 pounds. It has a flat spot in the bottom at the keel that sort of acts like a pad. But, with all the set-up work I've done with this boat to get the best top speed one thing this boat needs for best speed is bow lift.

[LakeFever]

78! Hauling the mail now for sure. Congrats

[VkingMike]

And 6400 rpm is reasonable in my opinion.

[skialot2]

78 MPH is from a 17' Avenger with a 175 is hauling ass! Sound like you pretty much nailed the set up.

[FORBESAUTO]

Great news! Not the 1st time I’ve seen that. My boat also runs faster with lower gear ratio and higher pitch prop.

[FMP]

What's the mystery, you've effectively increased pitch going to 1.87. To make the same speed you'll need to see 6000. You put too tall a tire on , too much highway gear, the motor doesn't have the stones to push it. Drop the pitch down from 27P, get the shaft speed needed and maybe it'll do more MPH between 6000 and 6400 if it has the grunt, like 26P at 6300. Can't compare those two props, area , rake, lift, bow, over hub, thickness.... stamp numbers are give and take.

[wolfgangb]

What's the mystery, you've effectively increased pitch going to 1.87. To make the same speed you'll need to see 6000. You put too tall a tire on , too much highway gear, the motor doesn't have the stones to push it. Drop the pitch down from 27P, get the shaft speed needed and maybe it'll do more MPH between 6000 and 6400 if it has the grunt, like 26P at 6300. Can't compare those two props, area , rake, lift, bow, over hub, thickness.... stamp numbers are give and take.

Would love to see diameters and photos of the props which produce that magic. Even with a 2 to 1 ratio, I cannot see that motor swing a 14.5" dia ET. That is Merc 280 ROS territory >300hp i.e. what we here are racing in mod VP, albeit at 4200ft.
wolfgangb (South Africa)

Frank - I hope to find the time to send you a personal mail tomorrow - we are going thru some torrid times here.