Jennifer "water-wolf"
Seaman Apprentice
- Joined
- Aug 8, 2011
- Messages
- 41
WOT RPM is way too high, want to dump 3 blade aluminum for 4 blade SS.
We recently bought a beautifully preserved, low hours 1994 20 foot Blue Water Monte-Carlo bow-rider. It has a Mercruiser LX4.3 liter 190 HP V-6 with Alpha One drive, 1.84 ratio. It still has what I believe to be the original prop which is in fine shape, but not the best one for the boat, being an aluminum 14"diameter x 19Pitch. First time out we did not do WOT test, but got up to the higher 30s MPH as indicated on the dash, at a bit over 4000 PRM., and it was sluggish to actually find plane. We fitted a SE 300 anti cavitation plate and next time we got out to the big river and had some fun....in smooth water we indicated 49 MPH at 5400 RPM, too many revs for this motor which is recommended to be 4600 to 4800. The previous owners were older folks who mostly cruised leisurely, or pulled the grand kids on a float so I doubt they ever had it at WOT and never felt the need to change the prop. In fact the whole boat was dead stock, no mods, changes or improvements except a Bimini top. I found many positive characteristics after installing the SE 300, but not the subject of this post.. but it is the prop that I need the "sage" advise on...... Oh, yes, and i carry 30 gallons at every outing, always a full tank, just in case.....and we will not be pulling toys or skiers...
I want to go to a 4 blade SS prop, and in the process drop my revs at WOT to closer to the recommended 4800 for this engine.....reading the advice on iboats, I came up with the following, ...
iBoat Note "A 2" increase in pitch (for example, from 21" to 23") typically results in a decrease of approximately 300-400 RPM"
So....if the installed 19? pitch 3 blade, WOT gives 5400 RPM , RPM is too high
NEED loose...
600 RPM loss to arrive at 4800 Maximum target RPM
800 RPM loss to arrive at 4600 Minimum target RPM
19" pitch as installed, plus 2" equals 21" pitch, results in minus 300 to minus 400 RPMs
21? pitch 3 blade will range from 5000 to 5100 RPM, still too high
19? pitch as installed, plus 3" equals 22", results in minus 450 to minus 600 RPMs
22? Pitch 3 blade will range from 4800 to 4950, almost there -
but will likely give good hole shot and 'torque' reserve for more weight loading in boat.
iBoat Note: "When switching from 3 to 4-blades, generally a 1" pitch decrease is required."
Example: a 22" pitch 3-blade prop is replaced with a 21" pitch 4-blade prop.
So if a 22" pitch 3 blade gives 4800 to 4950 RPM, then a 21" pitch 4 blade should be about the same RPM
22" 4 blade should drop the RPM 150 to 200, for a potential of 4650 to 4800, right in the target zone,
but slower hole shot. However even number pitch props are seemingly scarce, even if listed in some catalogs
21" 4 blade should hold the RPM at 4800 to 4950, slightly above target for a lightly loaded boat, but allowing for better hole shot, and more mid range push when the boat is more heavily loaded, which it will be often, as we like to bring guests aboard.
Am I correct in figuring this out? Is my conclusion that a 21"P 4 blade stainless prop be about right?
And what about prop brands and types? I like the deflection concept of the rubber hub on the Solas Rubex, and they seem to have odd number pitch available....but how about Quicksilver or Michigan Wheel? These are seemingly respected brands, ..Are there others I should consider? What about rake and other design details? I want to be tuned more for for mid range cruising speeds, and to be good with more people aboard, as compared to tuning for simple max top speed with a light load. The water may not always be smooth either....Out in the Columbia river we can find chop and swell of several feet...less than the open ocean by a long shot, but still something to deal with...
I have never yet owned an SS prop, and am interested in opinions and suggestions from those with the experience. I fear that if a 22P might be too much pitch, and is likely not available, ...going all the way to a 23P may well be far too much pitch for good mid speed and rough water performance, and it might even be too much for when we are lightly loaded on smooth water, not our typical day out.
Thanks,
Jennifer
We recently bought a beautifully preserved, low hours 1994 20 foot Blue Water Monte-Carlo bow-rider. It has a Mercruiser LX4.3 liter 190 HP V-6 with Alpha One drive, 1.84 ratio. It still has what I believe to be the original prop which is in fine shape, but not the best one for the boat, being an aluminum 14"diameter x 19Pitch. First time out we did not do WOT test, but got up to the higher 30s MPH as indicated on the dash, at a bit over 4000 PRM., and it was sluggish to actually find plane. We fitted a SE 300 anti cavitation plate and next time we got out to the big river and had some fun....in smooth water we indicated 49 MPH at 5400 RPM, too many revs for this motor which is recommended to be 4600 to 4800. The previous owners were older folks who mostly cruised leisurely, or pulled the grand kids on a float so I doubt they ever had it at WOT and never felt the need to change the prop. In fact the whole boat was dead stock, no mods, changes or improvements except a Bimini top. I found many positive characteristics after installing the SE 300, but not the subject of this post.. but it is the prop that I need the "sage" advise on...... Oh, yes, and i carry 30 gallons at every outing, always a full tank, just in case.....and we will not be pulling toys or skiers...
I want to go to a 4 blade SS prop, and in the process drop my revs at WOT to closer to the recommended 4800 for this engine.....reading the advice on iboats, I came up with the following, ...
iBoat Note "A 2" increase in pitch (for example, from 21" to 23") typically results in a decrease of approximately 300-400 RPM"
So....if the installed 19? pitch 3 blade, WOT gives 5400 RPM , RPM is too high
NEED loose...
600 RPM loss to arrive at 4800 Maximum target RPM
800 RPM loss to arrive at 4600 Minimum target RPM
19" pitch as installed, plus 2" equals 21" pitch, results in minus 300 to minus 400 RPMs
21? pitch 3 blade will range from 5000 to 5100 RPM, still too high
19? pitch as installed, plus 3" equals 22", results in minus 450 to minus 600 RPMs
22? Pitch 3 blade will range from 4800 to 4950, almost there -
but will likely give good hole shot and 'torque' reserve for more weight loading in boat.
iBoat Note: "When switching from 3 to 4-blades, generally a 1" pitch decrease is required."
Example: a 22" pitch 3-blade prop is replaced with a 21" pitch 4-blade prop.
So if a 22" pitch 3 blade gives 4800 to 4950 RPM, then a 21" pitch 4 blade should be about the same RPM
22" 4 blade should drop the RPM 150 to 200, for a potential of 4650 to 4800, right in the target zone,
but slower hole shot. However even number pitch props are seemingly scarce, even if listed in some catalogs
21" 4 blade should hold the RPM at 4800 to 4950, slightly above target for a lightly loaded boat, but allowing for better hole shot, and more mid range push when the boat is more heavily loaded, which it will be often, as we like to bring guests aboard.
Am I correct in figuring this out? Is my conclusion that a 21"P 4 blade stainless prop be about right?
And what about prop brands and types? I like the deflection concept of the rubber hub on the Solas Rubex, and they seem to have odd number pitch available....but how about Quicksilver or Michigan Wheel? These are seemingly respected brands, ..Are there others I should consider? What about rake and other design details? I want to be tuned more for for mid range cruising speeds, and to be good with more people aboard, as compared to tuning for simple max top speed with a light load. The water may not always be smooth either....Out in the Columbia river we can find chop and swell of several feet...less than the open ocean by a long shot, but still something to deal with...
I have never yet owned an SS prop, and am interested in opinions and suggestions from those with the experience. I fear that if a 22P might be too much pitch, and is likely not available, ...going all the way to a 23P may well be far too much pitch for good mid speed and rough water performance, and it might even be too much for when we are lightly loaded on smooth water, not our typical day out.
Thanks,
Jennifer