Mercruiser 888 Unexpected Prop behavior?

[Pmt133]

Nice. This thread comes up in google searches for the subject, so at least this information is out there now (although Gemini still gets it wrong )

Also interesting that the 233 uses a rare ratio. And I thought my 1.50 ratio was a needle in a haystack... good thing I don't have that!

Sounds like more testing with the 21 pitch may be warranted. 100 under minimum ain't much... Maybe should have gone with 20... 19 too low? We shall see. But 19 was cheap (to the tune of $110 shipped for 5 blades that are the correct length and fit my existing 3-blade hub), so if it's wrong I'm not out a super large wad of moolah.

The 1.50 isn't that much of an oddball. The 1.32 is. The 1.65 from a 165 is pretty odd as it has the short input yoke. Only other odd thing, and it is no longer an issue, was on the old drives with the 1.65 you had to mark the gear mesh when resealing the upper as it was the same tooth count so they always ran the same pattern. Merc later dropped this from the service manual. The old gear sets had arrows on them if I recall from dad.

Coincidentally most engines run better the higher the gear ratio you use, it just puts you out of normal prop range most of the time. Looking around in threads here you see most people end up in the 17p to 23p range with 19 and 21 being very common. Granted we're getting into the racing end of the weeds now... but spinning a higher pitch slower is more efficient.... and would never be a noticeable difference until we're hitting double the speeds we're talking about right now.

 

[ratdude747]

Did some lake testing today in better conditions.

Bombed the 21 pitch while tubing (lost two blades but only had one spare). Didn't get to running it at WOT.

After swapping blades over lunch break, I ran the 19 at WOT. Max speed 44mph, WOT at 4200RPM. 4 people on board. May try 20 pitch blades in the future... but for now 19 works.

 

[Bondo]

Did some lake testing today in better conditions.

Bombed the 21 pitch while tubing (lost two blades but only had one spare). Didn't get to running it at WOT.

After swapping blades over lunch break, I ran the 19 at WOT. Max speed 44mph, WOT at 4200RPM. 4 people on board. May try 20 pitch blades in the future... but for now 19 works.

Ayuh,..... Probably need a 17", especially when pullin' a tube, or skiers,....

 

[ratdude747]

Ayuh,..... Probably need a 17", especially when pullin' a tube, or skiers,....

The 21 pulled the tube no problem... the biggest issue seen (that has always been the case) is that planing speed is 19-20mph which is also the max rated tube speed. We ended up blowing it a bit since I don't seem to have the magic tough to find the happy spot that keeps it on plane without overspeeding too much. My best friend seems to be better but since he drove it a lot under the prior ownership, that's to be expected. Until yesterday he'd done all the tube pulling... for the first time in 20 years he wanted to try the tube, so I got to pull him (Until I bombed the prop!).

After bombing the 21 (hit something) and swapping in the 19 blades, we didn't pull the tube any more. I can't say what if any difference that change would have made.

Given that I hit the max WOT on the 19 (per a prior post, target WOT is 3800-4200), I conclude I shouldn't go any lower than 19 at least when it comes to 14.5" 3-bladed Piranha props. Since 21 was likely too much (WOT at 3700), 20 seems to be a happy medium to try later.

The previous owner ran an 18 pitch 4-bladed comprop (along with adding the hydrofoil still installed) when they "got serious" about pulling skiers (pulling two at once, etc). I still have that prop but I broke a few chunks out of it, which is what prompted me to swap to a 4-bladed Piranha in 18 pitch. But, it had so much rake that I had to run a "high performance" trim anode to clear such. I still have this prop (blew up all my spare blades), but would need to special order a new trim anode since finding such a "high performance" anode in Mg is tricky. Hence why I elected to try different props rather than buy new spare blades and a new anode (my previous one was zinc and broken, used in fresh water by mistake that I didn't catch until I went to replace it).

 

[jimmbo]

You are running 4200 with 4 people. Do you usually have 4 people in the Boat? If fewer, you might over rev. If you have 2 or 3 in the boat and a Tube, you still be fine. A 17 might be better when tubing, if you have 4 or more in the boat.
The 17, 19, and 21, were they 3 blade or 4 blade? Myself, I avoid more than 3 blades like the Plague, as they kill top end. I wish Mercury still made 2 Blade Props for their I/Os and large Outboards

 

[ratdude747]

You are running 4200 with 4 people. Do you usually have 4 people in the Boat? If fewer, you might over rev. If you have 2 or 3 in the boat and a Tube, you still be fine. A 17 might be better when tubing, if you have 4 or more in the boat.
The 17, 19, and 21, were they 3 blade or 4 blade? Myself, I avoid more than 3 blades like the Plague, as they kill top end. I wish Mercury still made 2 Blade Props for their I/Os and large Outboards

It varies. Sometimes we have 6 people on board, sometimes as few as 2. Sometimes pulling a tube, sometimes just running around the lake/river for fun.

Here is the history of the props used on the boat, to my knowledge:

1. (Original?): Mercruiser 48-74888 15" 21P 3-blade rubber-hubbed aluminum. Prop found in barn with boat, but not usable due to hub and aluminum damage. I later found a clone on eBay for cheap, but it had a spun hub and wasn't testable.
2. Comprop M4558L 13.7" 18P 4-bladed composite props (several, multiple broken hubs were found in the barn). Hydrofoil added at same time. They ran this setup for a long time (to be able to pull multiple skiers), until it was taken off the water. I ran for a handful of outings until a chunk got taken out of a blade (I still have it)
3. Piranha 4-blade 14" "A" 18P composite modular prop. Was intended to replace the above due to Pomprop being defunct. A hair slower (really nice bottom end) and WOT RPM was good; had a lot of rake requiring a special trim anode (see above). I still have it, but not the anode to go with it.
4. Piranha 3-blade 14.5" "A" 21P composite modular prop. An attempt to replicate the original prop found (top end enhancement). Couldn't get to WOT target with two people (in somewhat rough waters). Makes me wonder what the story with the first prop was...
5. Piranha 3-blade 14.5" "A" 19P composite modular prop. Found a deal on some NOS blades (see prior post) and figured it was worth a shot to correct/quantify the WOT issue with the 21.

I also have a 4-blade 19P Solas Aluminum prop in reportedly usable condition that my manager at work (who also boats) had in his junk pile and gifted to me. If I try it, it'll be at a different lake that doesn't have as bad of a track record of bombing prop blades.

 

[Pmt133]

The planing at 18-19 mph is going to be tough to overcome. That usually never changes unless you really under prop it, at least in my experience, or have trim tabs.

 

[ratdude747]

The planing at 18-19 mph is going to be tough to overcome. That usually never changes unless you really under prop it, at least in my experience, or have trim tabs.

Exactly. Which is why I'm not faulting the 21 pitch prop.