Underpropping is good!!

Uh, no. Under propping is ... no bueno, especially on IO's. Your OB will soon explain to you why its no bueno. Next, try a 12" prop and maybe you can reach 8500 rpms!
 
Put a 17" on my 20' Bowrider 5.0 L which formerly had a 19" High Five and pulled Susan out of the water repeatedly. She was happy and thought it was her conditioning. Before it was marginal, now it pulls like a ski boat. The MPI system sounds a horn 100 rpm before rev limitation but I never even checked to see how many mph I'd lost. Who needs it?
 
My 185 came with a 14x23P (or at least, that's listed in Sea Ray's prop matrix). The primary owner had a 14.5x17P on this boat when I bought it from him. I just bought a 4-blade 14x21P, based on the Merc prop tool's recommendation of a light load in my boat. Ran it a couple times - noticed significant "prop singing" at idle speeds (which I could live with if I had to) - but more to the point, I wasn't satisfied with the hole shot. So I just returned it for a 14x19P 4 blade... It was also a recommended prop in Merc's tool, but only after I loaded the boat down with occupants / weight. The tool still says I'll have 4800RPMs on the top end, which is within spec. Hope to take it out today and see how it drives!
 
Tommo said:
My 185 came with a 14x23P (or at least, that's listed in Sea Ray's prop matrix). The primary owner had a 14.5x17P on this boat when I bought it from him. I just bought a 4-blade 14x21P, based on the Merc prop tool's recommendation of a light load in my boat. Ran it a couple times - noticed significant "prop singing" at idle speeds (which I could live with if I had to) - but more to the point, I wasn't satisfied with the hole shot. So I just returned it for a 14x19P 4 blade... It was also a recommended prop in Merc's tool, but only after I loaded the boat down with occupants / weight. The tool still says I'll have 4800RPMs on the top end, which is within spec. Hope to take it out today and see how it drives!
Click to expand...
To help with the hole-shot change the prop vent plugs from closed to something more open.
 
I am unsure why you would not use the maximum diameter prop that would fit under the cavitation plate but it is only a small discrepancy. The holes are supposed to disconnect the prop from the water through enhanced cavitation. This is a crazy old theory that only makes sense if you've got a cam or a salvaged Ferrari engine that needs help getting rpm up into power range. 19" is good, depends on your use.
 
Arminius said:
I am unsure why you would not use the maximum diameter prop that would fit under the cavitation plate but it is only a small discrepancy. The holes are supposed to disconnect the prop from the water through enhanced cavitation. This is a crazy old theory that only makes sense if you've got a cam or a salvaged Ferrari engine that needs help getting rpm up into power range. 19" is good, depends on your use.
Click to expand...
Well i'll tell you that the ventilation holes are a significant tuning aid routinely used. Hardly a "crazy old theory". Every propeller i've used on my Gambler was tuned for getting out of the hole with the plugs. The 17P was plugged up, the 19P had 1/8 inch plugs, the 26P had 1/4 inch plugs, and the 25P Fury Lab had the plugs removed. If one sets up the boat correctly there is no need to grossly under pitch the propeller; the vents are one tuning feature to get things right.
 
ttmott said:
To help with the hole-shot change the prop vent plugs from closed to something more open.
Click to expand...

It's a Merc SpitFire prop, so no vent holes.

But this particular prop does have a new special "feature" - prop singing! This is the second SpitFire I've installed that has this weird harmonic resonance at idle speeds. It's kind of annoying, as it sounds like the blower is on while running at low speeds... But I hear I can shave the edges of the prop blades to get rid of it if I really want to...
 
By the way, the 19-pitch prop is a winner! Great acceleration, very good control of low & mid-speed cruising, and it runs within spec at WOT (e.g., 4600-4800RPMs). The 21P was a bit too slow to get up to speed, and definitely went too fast for my liking at cruising RPMS and WOT. The 17P I had on there originally ran RPMs above spec at WOT so had to be replaced.
 
So I had a 19" prop on a Suzuki 4cyl DF-140 4 stroke outboard hung on a 18' Campion ski/fish. The Suzi was lighter than a Yammy V-6 and had a trick reduction gear tween crank and drive gear. Better than I expected but no 2-stroke. The dealer said he was selling me a 17" prop to pull up slalom skiers but the box contained a 15". Traffic hassle to get back and trade it in so I gave it a try with wonderful results though 6500 rpm overrev protection did kick in when trimmed out. Whole thing was a bad idea as my Club ramp got worse and the Indians and the fish and probably the Cosmic Warmers vetoed dredging. I got moorage for my Searay and all is well except I had to sell the Suzi, which wasn't hard. 2 weeks later, buyer called and said it overheats after 10 minutes on plane. I suggested changing the prop.
Underpropping is a great sales technique!
 
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Let's see if we can condense this down a bit:
  • Boat didn't hole-shot as desired
  • Top end was higher than needed/wanted
  • Dropping a couple inches in pitch would help with both issues (good so far)
  • Using rev limiter to avoid engine damage was deemed a "good thing" (what?)
  • Majority of post entries input believe doing so is a bad idea (myself included)
  • At least one post noted the added benefit of reduced stress on the plywood bow cover
Am I missing anything? It's hard to believe that this thread is now onto three pages.
 
under-propping is by definition bad, hence the term "under". Otherwise, wouldn't they call it morewaybetterpropping?
 
4-stroke outboards are expensive, heavy, dogs with too many moving parts and electronics. What was wrong with 2-strokes other than the rainbow sheen on Lake Tahoe in front of Jane Fonda's place? The neatnicks are going to continue to make things up to have their way.
 
This reminds me of my nephew's severe ADHD....

Lemme see. The general think is if the engine hits WOT at design RPM, then the prop, hull and engine are matched pretty well.

If WOT RPM is too high (really bad thing causes engines to destroy themselves), then too little prop is installed. Modify by adding diameter or pitch or both or changing drive ratio.

If WOT RPM is too low (bad thing causes engine to lug and work too hard and die an early life), use less pitch, or diameter, or change drive ratio, or get a bigger engine.

Simple........

Then again, sometimes, in some posts.... Never mind
 
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An unprotected 4-stroke with insufficient load may be revved beyond the peak of the power curve if the valves don't float first. Floating valves on an interference engine result in contact with the pistons and self-destruction. Your engine probably records the number of times it has had an over-rev "fault" and your odds of a successful warranty claim diminish as the number increases.
 
All modern outboard engines have RPM limiters to keep the engine within the operating parameters.
They will not allow the human to damage them.
 
Put a 17" SS 3-blade prop on my 2003 22' Select 200 w/ 305 mpi with great results. Lost a mph or 2 but still touching 50 on GPS. I've had this boat for years but I'm getting old and only got half up on my slalom ski. This prop really pulls and may rock me over. No pull-ups for me but I put a pull down bar lifting a 100#s on pullies every day all winter. Anyway, boat came with a useless 21". I switched to 19" but it did dig a hole in the water. Then, the High-5 19" was the cat's foreskin. Now, the 17" has me just over 5000 rpm but the 5150 overspeed horn is not honking if I'm careful with trim. Very nice!
 
It gets down to what works best for you. The displacement hull rocks up and needs the locaton on the torque curve to power over the hump and rock back down into planing. The wrong prop or cam and it feels like you're sliding back down like "The Perefect Storm"rather than transitioning effortlessly.
 
The 17" SS prop from Ebay beats the expensive High Five 19" hands down. Wiley's put new bindings on Susan's 40 yr old Connely and she was up and skiing. She was so impressed that we stopped by the office and looked at props where I unsuccessfully tried to explain pitch. She did not comprehend but was very happy. Boats moored by us are now super-pigs with ballast tanks to half sink and then need the pitch to drive their semi-displacement rigs with huge wakes behind the cacophony. Bet they need some low numbers on the props too!
 
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