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Gearing/prop selection

Blown72GN

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Ive been looking to switch to a 2 blade on my 20'howard from a 12×16 menkens 3 blade and would like any input regarding my current gear as well... setup is: 1150~hp th400 with 32's in a 12°... 12×16 3 blade. Gps'd 103.6 @7600rpm. Pulls hard but flattens out right around 100... like hits a wall... would a 11.75×16 2 blade help or am i running out of gear? Not trying to set the world on fire but... a lower cruising rpm would be nice as well as eclipsing the magical 110 mark...
 

obnoxious001

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Do you have a dyno sheet for the engine? Knowing where it makes peak power would help with gear selection. I ran 45's in my 20' Schiada that was set up for ski racing, approximately 1000 hp turbo engine (no dyno). 11 1/2 x 15 two blade and later 11 3/8" x 15. No GPS back then, but it would run under 6,000 where the engine was happy and could live longer.
 

obnoxious001

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By the way, while the two blade prop should give you more top speed, you can count on it to not be as smooth on take off, unless you were to step up to a Joey Grose CNC prop that costs a small fortune.
 

HighVoltage329

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I've always liked 2 blades. Not as much tail lift which helps carry the noise of the boat better at high speed.
If your boat likes the 3 blade and carries well then I stay with it and go up in gear.
I have the same HP and am running 48 gear and an 11-1/2 x 16 Grose and also have a Menkens the same size.
 

Blown72GN

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I've always liked 2 blades. Not as much tail lift which helps carry the noise of the boat better at high speed.
If your boat likes the 3 blade and carries well then I stay with it and go up in gear.
I have the same HP and am running 48 gear and an 11-1/2 x 16 Grose and also have a Menkens the same size.
I really like the way the boat takes a set and pull hard and controllable from 25-85mph. Carries the bow well. Would like to be able to cruize at a lower rpm tho... anything below 4k and the boat rides wet. Would like to be able to see 50mph at 3500~ would 39-43's be a good starting point?
 

obnoxious001

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I've always liked 2 blades. Not as much tail lift which helps carry the noise of the boat better at high speed.
If your boat likes the 3 blade and carries well then I stay with it and go up in gear.
I have the same HP and am running 48 gear and an 11-1/2 x 16 Grose and also have a Menkens the same size.
What rpm do you pull Bob? I could not find a used set of 48's when I put the 45's in. I had previously been running 43's and was looking for just a bit more top end.

Grose props are magic, aren't they? I did prop testing with Joe Mitchell after he bought Bob Stark's old Vector that I ski raced behind when it was new, prior to taking the boat apart to rebuild. When we got to testing the prop Joey sent, we looked at each other and knew the testing was complete, smoothest 2 blade I have ever felt!
 

Blown72GN

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Pulled the box out today.... top bearings/input shaft seal are toast... like 1/8'" play in all directions plus there's excessive lash in the gears.... im hoping the gears are not dicked off as well. So now that the box needs to be rebuilt... im going with 50 gears and a 11.5×16 2 blade... probably a menkens radiece.. current setup( 32's and a 11.75×15 3 blade menkens) im willing to give up some hole shot performance for higher cruising speed@lower rpm and hopefully 10~mph up top... im pretty sure i have the power to spin that combo to 6800~ rpm. Any thoughts
 

BamBam

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So you're changing from 32% to 50% as well as increasing pitch from 15 to 16. I think you're gonna tug your RPM down quite a bit. Where does the motor make peak power? If you load it too far before peak power it won't free the boat up to get the RPM you want as well. 6800 sounds like a good number. I would think you should try the gear first and see what happens.

32% gears at 7600 is a shaft speed of 10,032
50% gears at 6800 is a shaft speed of 10,200 (6700 is 10,050)
Call those numbers close to even but adding a degree of pitch will load it more and may prohibit your ability to get to the 6800 number. If the motor has any increase in HP between 6700 and 7600 you will not pull the gear or get to the 6700 number in my opinion.

That setup will reduce your cruise RPM but will probably not get you any more top end speed.
 
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Blown72GN

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Made peak hp around 6800 and carrys it all the way to 7300~ then starts to fall off. Thats @ 5.6psi av100ll. Made 964hp / 915lbft Never dynoed the current tune 13psi 111oct f&l. Should be 1100~. It likes to cruise around 4100 now and would like to be aroud 36-3700. Ib figured 400rpm/degree of pitch loss going to 16° and around the same with the gears... so 6200~ rpm WOT on the small tune... hoping 1100 would be able to pull the taller setup. This is my first time gearing a boat that makes big power. My mandella only made 475 with a little tunnel ram pump gas 468 deal. 12×15 3 blade and 22's in the box. Went 68 wot @around 5800. Wasnt really sure about throwing all that gear in it with the 3 blade on it now... have been told i should run a 2 blade on it but was afraid of loosing hole shot preformance for minimal top end gains with the 32 gears as its not the exactly lightest boat either. But dont want to shell out the money for a setup that doesnt work either... so i should put some gear into it and keep the 3 blade?
 

Blown72GN

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I believe you will drop more RPM than that changing gears 6 or so steps,
So maybe try 43's or 45's with the 16 pitch or throw a bunch of gear at the urrent 3 blade? Buddy of mine has the same boat... 83 20' howard but it has the day cruiser deck same 5° gn bottom.... and says hes getting 111mph... ill have to ask the rpm... out of 50's and a 16 pitch 2 blade. Im making a little more power than he is. Thats where i came up with the idea for the new combo. But would like any help i could get figuring this thing out. All honesty the 107mph WOT now is plenty... the boats quite a handful now would like to drop cruising rpm 600~ and still be in the 55-60 mph range.
 

obnoxious001

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So maybe try 43's or 45's with the 16 pitch or throw a bunch of gear at the urrent 3 blade? Buddy of mine has the same boat... 83 20' howard but it has the day cruiser deck same 5° gn bottom.... and says hes getting 111mph... ill have to ask the rpm... out of 50's and a 16 pitch 2 blade. Im making a little more power than he is. Thats where i came up with the idea for the new combo. But would like any help i could get figuring this thing out. All honesty the 107mph WOT now is plenty... the boats quite a handful now would like to drop cruising rpm 600~ and still be in the 55-60 mph range.
I would make one change at a time, so do the gear first, then prop. When you say the boat is a "handful", what is it doing? Too much prop lift will make it do funny things.
 

BamBam

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I agree with Barry on the "one change at a time"
As for 2 blade vs. 3 blade, When I was at Schiada our rule of thumb was you could run a 3 blade up to about 95-100. after that the extra blade hurts top end as well as the fact that a lot of three blade props have thrown ears off at high speed (not a good thing) If you wanna go fast, put a 2 blade on the thing. You want a better hole shot and good cruise without going fast, run the 3 blade
 

Blown72GN

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I would make one change at a time, so do the gear first, then prop. When you say the boat is a "handful", what is it doing? Too much prop lift will make it do funny things.
It carries the bow very high... even when burying my foot in the override pedal... have to fight torque to the left too... has a gn style rudder on it now... takes a set decently then unsettles itself and feels loose around 85-90 on. Your right i dont want to make things worst so one step at a time... so would 50's be too much of a jump... up from 32's?
 

obnoxious001

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It carries the bow very high... even when burying my foot in the override pedal... have to fight torque to the left too... has a gn style rudder on it now... takes a set decently then unsettles itself and feels loose around 85-90 on. Your right i dont want to make things worst so one step at a time... so would 50's be too much of a jump... up from 32's?
So does it end up lifting the tail at higher speeds? If you are only thinking a 600 rpm change, I believe the 50's will drop it more than that. The 20 may do better with a flatbottom style wedge rudder, the 20' Schiadas do for sure.
 

Blown72GN

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So does it end up lifting the tail at higher speeds? If you are only thinking a 600 rpm change, I believe the 50's will drop it more than that. The 20 may do better with a flatbottom style wedge rudder, the 20' Schiadas do for sure.
Ya and it carries the bow high too.... but that may just be a plate adjustment.... this is around 80~ when this pic was taken. Steering gets heavy and wants to pull left a little..
1557540204358.jpg
 

obnoxious001

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Ya and it carries the bow high too.... but that may just be a plate adjustment.... this is around 80~ when this pic was taken. Steering gets heavy and wants to pull left a little..
Steering torque is going to happen with a V drive, more torque and bigger prop probably makes it worse.

How is the strut angle on the boat?
 

Blown72GN

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Steering torque is going to happen with a V drive, more torque and bigger prop probably makes it worse.

How is the strut angle on the boat?
9°... not sure off the top of my head the dimensions... as far as transom to strut, transom to vdrive, prop from rudder, rudder to transom and rear engine plate to transom... ill have to measure tomorrow... the motor is somewhat far back as i do have a full interior
20180421_200111.jpg
 
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