Another Help with prop

[muzbomb]

Hey guys so looking for a little prop help. This is one area of my boat I suck at is figuring out what prop is best for what. I just sold my high five 19 pitch prop and looking to buy new one. Here is some info on boat

2001 sea ray 190BR 5.0L EFI(240hp) Alpha 1 leg 1:62 ratio.

With the high five 19pitch to me the boat takes a bit to get on plane so at WOT 4800rpm will do 48mph but there is still throttle left if I put it all the way down just over revs to 5300rpm with no change in speed or power motor just revs up. I am not sure what prop to get and pitch?? I have been looking at the turning point brand props. I mainly use boat for just cruising(so speed is not to important to me) towing kids on wake boards, skis and tubing. I would like a really good hole shot prop and not to worried about a speed prop. What you guys think? Thanks for help all

 

[DaveKamp]

If you're able to exceed 4800 swinging the 19p, then one of three things is happening:

Either you don't have enough prop pitch and surface to transmit power, or either the hub or engine coupling is slipping... or you're ventilating the prop.

With a 5.0 V8 and 1.62:1 ratio, you'd have 10% slip and 48mph... which is about right.

Going to 21p without changing design, would put you at 53mph at 4800 with 10% slip... and 12mph at 1200rpm IF your hull will plane, and yield 10% slip. More likely, you'll need to be at 2100rpm with 15% slip, and see 20mph, which is at the bottom end of where you'll have wakeboards and light skiers.

The 5-blade is generally not a good choice for your 19ft bowrider, especially not at low pitch... it's just too light a boat for that thrust class. Better to go with 3 blades, higher pitch, so engine comes up the powerband farther when coming out of the hole. (higher slip ratio on holeshot increases thrust because there's more HORSEPOWER available in the higher-slip state (work=force*distance, power = work * time, the engine is spinning FASTER at the same, or greater torque).

Important to realize that once on plane, hydrodynamic drag falls, which results in much lower wetted surface drag AND reduced prop slip.

Frankly, I'd be inclined to try a 21 with LOTS of cup, or even go to a 23p with ventilation holes in the hub... if you have any friends that have one you can borrow.

If you go LIGHT, with a 23, at 4800, you'd be at 58mph with 10%, but if your hull drag drops enough, you'd make 60mph at 7% slip... you'd probably need the bow covered to keep wind drag from taking away what you gained. With this combo, 1200rpm ath 20% slip would be 13mph, so barely on plane... but it'd climb out of the hole about same as your 5-blade would.

Realize, having more blades isn't magic- it adds more surface area for power transmission, but it doesn't equate to 5/3rds more thrust... the improvement over a 3-blade occurs mostly in the range between displacement and low-planing speed. Above that, the drag of two extra blades eats most of the gains. Running three blades at planing speeds will prove more efficient in most aspects.

 

[muzbomb]

Wow Dave that is a lot and I thank you for this very very much. I am going to read this a few times as it is a lot of info to take it great info that is!!! So basically you are saying a 3 blade props 23” pitch. I have been doing lots of reading on the turning point props and have been thinking of trying one of them for sure. I will say I am done with the 5 blade props that is for sure. I am thinking of getting 2 props a 4 blade and a 3 blade. Now the 4 blade I was Looking at a 21 pitch or 19 pitch (not sure) turning point aluminum squeezed cast dual geometry prop and then a 3 blade 23 pitch aluminum squeezed cast turning point prop.

 

[DaveKamp]

if you were sitting in my driveway, i'd loan you a 21p 3 blade stainless, and a 23p same.

if you're in circumstance where you'd v expect hard bottom contact, aluminum would be sensible, but the thinner blade profile of stainless will yield better performance.

do double check your drive info to verify the gear ratio... most v6/v8 MCMs would be 1.5:1, rather than 1.65... at least, down here. if you had a high altitude option, i wouldn't be surprised... but at my elevation, only inline sixes saw the 1.65ish (usually 24:24t combo) up top.

 

[muzbomb]

I was reading the Turing Point brand props aluminum are a squeezed cast which makes them very strong I talked to one boat owner he has both a aluminum squeezed cast and a stainless steel in same pitch and same 4 blade. He said the squeeze cast prop was very very close to the stainless Performane. The stainless was 4mph faster top end and the hole shot was a shade better on squeeze cast we are talking hundredths of a second only. I checked the boat it is a 1.62 ratio

 

[Arminius]

Can't beat the 19' High Five you had.

 

[muzbomb]

My buddy had a 21pitch high five I could try but honestly i don’t know what to do or try there is just so many dam prop choices

 

[DaveKamp]

if he's exceeding 4800 with the 5, he's not properly coupled, and losing efficiency, economy, and performance.

 

[muzbomb]

if he's exceeding 4800 with the 5, he's not properly coupled, and losing efficiency, economy, and performance.

So you think couple me my hub?

 

[DaveKamp]

coupled, meaning the drive system is not able to transmit its output to the water.

Prop blade thickness, surface area, quantity, diameter, pitch, and tip velocity, distance from surface, exhaust relief presence, hull weight and forward velocity all combine to define load coupling efficiency.

 

[Arminius]

I think I have the same rig, the mpi with the 555 processor, the 19" high 5. 260 hp. 305/5 liter. Tried a 19" 3 blade SS and my hole shot was much worse. Put my Android cell in the cupholder with the big GPS app and got it up to 50 mph on flat water. At 5150 rpm, I heard the tone and just trimmed down a bit but kept it firewalled. Think overrev protection kicks in at 5200. Torque times rpm equals hp and this boat is set up right. Lots of other less expensive things to play with besides your prop. 21" would be useless. This is lots of fun.

 

[GypsmJim]

With the high five 19pitch to me the boat takes a bit to get on plane so at WOT 4800rpm will do 48mph but there is still throttle left if I put it all the way down just over revs to 5300rpm with no change in speed or power motor just revs up.

If there is still throttle left then 4800 is NOT WOT. Thus, your data is cornfused.

If you are exceeding the max rpm spec and ALSO having a planing problem, your prop is either spun or you have one that is totally inappropriate for your rig.

Insert you data into the Mercury prop selector and take their recommendation. A 3-blade Enertia would be my first choice, but you must be willing to spend the $. Buy you won't be disappointed...

 

[DaveKamp]

I think I have the same rig, the mpi with the 555 processor, the 19" high 5. 260 hp. 305/5 liter. Tried a 19" 3 blade SS and my hole shot was much worse. Put my Android cell in the cupholder with the big GPS app and got it up to 50 mph on flat water. At 5150 rpm, I heard the tone and just trimmed down a bit but kept it firewalled. Think overrev protection kicks in at 5200. Torque times rpm equals hp and this boat is set up right. Lots of other less expensive things to play with besides your prop. 21" would be useless. This is lots of fun.

If your engine RPM is exceeding 4800 for any length of time, it is NOT 'set up right'... you're either running too low a pitch, or you've got the drive raised into ventilation or a slipping hub.

The R/MR/Alpha/Alpha One/Alpha One SS drives will NOT sustain over 4800 for any length of time, and the reason is very simple:

The drive is cooled by water. The drive gears are lubricated by oil circulation caused by the TAPER of the vertical driveshaft. Most of the time, the upper gearcase exterior has no contact with water. On plane, the bottom portion of the drive leg (from beneath the anticavitation plate) is submerged, thus, able to transfer heat through the lower drive housing's cast aluminum housing and skeg.

Heat from lower gears passes mostly through the bearings into the housing, into the surrounding water.
Heat from the upper gears is absorbed MOSTLY by drive lube, which was drawn up from the bottom of the drive by virtue of taper of the vertical drive shaft, to to the upper drive bearing and lower pinion gear, where it lubricates the upper gearset and absorbs it's heat. From there, the oil is flung off onto the interior of the upper case, then flows down the case to the bottom, where it sheds most of it's heat. As it goes down, it passes the portion where water pump output flows into a 'band' void travelling to the gimbal to supply the engine with raw water... and this area of the casting absorbs a significant amount of heat from the drive lube.

When a R/MR through Alpha One SS series drive is spun in excess of it's rated 4800rpm, drive lube is flung off the shaft long before it can get to the upper gears.

An immediate visual clue of cooling and lubrication problems, is a 'chalky' appearance to the paint on the top half of the drive, especially from the drive labels upward. The operational visual clue doesn't appear until the gears assume a plastic state, at which time they become very smooth, and the rev limiter does it's real job, which is to prevent the connecting rods from checking the depth of water through the block and bilge. The failure tends to occur extremely quickly, as excess gear heat causes the upper gear's lip seal to melt away, allowing drive gear lube to pump into the void of the bellows (between the gimbal face and gimbal bearing), leaving no indication that your drive lube is about-half-absent.

The common MerCruiser drive can withstand considerable torque, but exceeding it's RPM limitations will result in a very limited warning, and an extremely abrupt failure. Get that prop pitch up, and if your on-plane time isn't satisfactory, employ exhaust bypass holes slightly past, but ahead of the leading edge of each blade to introduce artificial ventilation at speeds below plaining. Exhaust bypass will do so only when there's considerable exhaust pressure, which means, you're well-buried into the throttle.

And another note- Horsepower = (torque x rpm)/ 5250, but this is entirely irrelevant for a continuous-duty hydrodynamic thrust application. Spinning the engine faster at planing speed does NOT equate to 'more horsepower'.