Mercruiser 888 Unexpected Prop behavior?

[ratdude747]

Been playing with prop geometries this season and getting some unexpected results.

The boat is a 1976 Starcraft Montego with a Mercruiser 888. Engine is mostly stock but has been upgraded with an Edelbrock performer 289 intake, a Holley Marine 4160 4-barrel (originally off a Mercruiser 5.0) and a pertronix ignitor solid state points replacement with flamethrower ignition coil. Before my ownership, it had a non-marine 4160 (off a 1976 ford super duty v8 in a trash truck) and the original points ignition.

When I got the boat 2.5 years ago, it had an 18 pitch 4-bladed comprop. I also received what appeared to be the original prop, a 21 pitch 3-bladed aluminum Mercruiser. I replaced the comprop with a 4-bladed Piranha 18 pitch prop when the former bit the dust. Both props were good for 42-45MPH at 4200 RPM WOT. I intended to try a clone of the original prop (same model, used off eBay; the original had damage) but it failed due to a spun hub.

This season I'm trying a 3 bladed Piranha in 21 pitch to replicate the performance of the prior prop (which reportedly could hit 60MPH). However, on my one outing, I only hit 46MPH at 3700 RPM. To be fair, unlike the prior props, I did have a response to pitching up once at speed (went from 41 to 46MPH with no change in WOT RPM). My fuel economy was better. However, there was heavy chop at times with a 12mph wind (seemed to do better with a headwind, oddly). My fuel economy was slightly better... I ran 30 miles and only had the tank between 5/8 and 1/2 on the gauge. Before, I'd get about 50 miles with the tank near E (with several gallons of reserve).

So, the question is whether or not this is "good" behavior. And if not, what to do next. I do have a couple more seemingly usable aluminum props to play with:

• 4 bladed Solas in 18 pitch (a gift from my boss's junk pile, unlike the 4-bladed Piranha and like the comprop and everything else can be used with a normal trim tab without colliding)
• 3 bladed Michigan Wheel in 19 pitch (came off a used outdrive I was given and used to replace the upper half of my existing outdrive)

Since the carb isn't guaranteed to be the same tuning, that's also a variable. The old carb was unsafe and was rehomed not long after I got the boat. And the high speed was multiple decades previous (they re-propped due to wanting to pull 2 skiers at once, which the original prop wouldn't do).

WOT on an 888 is supposedly 4200-4500 RPM. 3700 is way down. But I also may not have trimmed it right? And maybe was adverse conditions?

Any thoughts? Maybe try the 19 pitch prop?

 

[Pmt133]

19 pitch 3 blade would probably be comparable to the 18 4 blade... the RPM drop sounds about right for stepping up 3 inches in pitch. While it could theoretically hit 60 with a 21p, your 3700 at 46 mph is most likely right.

Short of some magic, typically WOT speed stays the same when making reasonable prop changes in my experience. This is provided the initial prop is in the ball park which it seems the 4 blade is. And that the engine is running properly. Again, seems to be.

16 foot boat, ~200 hp. 46 sounds right.

 

[jlh3rd]

"Any thoughts?"

I prop for proper manufacturer's recommended RPM @WOT. So 3700 rpm@wot would be lugging my motor. The resulting speed is irrelevant. Prop diameter also affects rpm results.

 

[Scott Danforth]

lugging the motor is never good. get the RPM's up to near the 4500 at WOT

agreed on the speed will be similar across a range of props, this is usually defined by the hull shape and to go faster, you will need much more motor.

regarding the prop. yes, if you had enough motor to spin that 21p to 4500RPM or more, you could potentially see 60mph. but you are talking nearly double the HP you currently have.

 

[jlh3rd]

"(which reportedly could hit 60MPH)."

and can't rely on "reportedly".

 

[ratdude747]

19 pitch 3 blade would probably be comparable to the 18 4 blade... the RPM drop sounds about right for stepping up 3 inches in pitch. While it could theoretically hit 60 with a 21p, your 3700 at 46 mph is most likely right.

Short of some magic, typically WOT speed stays the same when making reasonable prop changes in my experience. This is provided the initial prop is in the ball park which it seems the 4 blade is. And that the engine is running properly. Again, seems to be.

16 foot boat, ~200 hp. 46 sounds right.

Technically, the boat (with the stock 2-barrel setup) was advertised by Starcraft as being capable of "over 50mph" in the catalog (and was the fastest offering they had in 1976). And the 888 (302) was the biggest engine they offered for the 16'; the 18' could have a 233 (351) but I'm told it was ultimately slower than the 16' with an 888.

Also keep in mind that this is a hybrid tri-v hull, which may be how Starcraft was able to get a bit more speed out of it (easier planing). It's also possible that I didn't have the drive trimmed right as on the previous prop it didn't respond to trimming up (this one did, but per the trim gauge it didn't move all that much, so maybe I need to give it more?)

That said, the boat is old and I'm not exactly light weight... so who knows. I also don't know if the 21P prop that was found in the barn was actually original (it certainly was period-correct). I do know it was (with the "original" prop) reportedly slow to get on plane, although they added a hydrofoil to the drive at the same time as the prop swap, which I still have installed. It didn't seem to be all that slow to get on plane (a bit slower, but it did get there without too much dragging).

One other factor: back then it used a pitot tube speedometer. Currently I have a GPS speedometer. Is it possible that the old speedometer (back when it was usable) was reading fast somehow?

Maybe try the 19P Michigan Wheel and hope it isn't spun? I figure that's cheaper than committing to different blades for the Piranha off the bat.

Edit: When I say "reportedly", I'm referring to what I've been told by my best friend's dad who ran it a lot and did a lot of the work on it under the prior (original) ownership. He's up there in age a bit but still seems to have a mostly solid mind (and appears to be a reliable narrator). That said, it's all memories...

 

[Pmt133]

So my pilot tube would easily read say... 35 mph when I was running 30-31 GPS... that very easily can account for some of the top speed loss. Post rebuild I gained less than half a mile an hour GPS despite being several hundred pounds lighter so theres that too.

Dad had an old 17' invader with a pad with the I6 merc i/o. That did 52 on the pilot tube with nothing but the pad in the water. It was available with the 305 and would run about 55 with that. That was also a noseconed drive to achieve that.

On my boat, the hydrofoil is worth about 3 mph on the top end. Pull it and see if it helps.

For trim, you'll hear the u joints scream when you've gone too far. Some boats respond well to it and others not so much. That invader would pick up 400 rpms just from minor trim and speed as well. I get about 200 on top trimmed to the edge of blow out which is worth about 2 mph and where the hull runs best for me.

 

[jlh3rd]

The only #'s that matter are what's current. If your concentration is on 60 mph, then the motor isn't making it for whatever reason. It's not the prop. Props are for fine tuning.
You want 4200-4500 @WOT....speed is irrelevant.....

 

[Scott Danforth]

My boat does 80...... on the trailer behind the truck.

take any sales literature stating "over 50" with a grain of salt.

 

[ratdude747]

In any case, it sounds like 21 pitch is no good now, regardless on what it was then.

I'm thinking of trying the 19 pitch Michigan wheel to see how we do. "Free" is free. Even if it feels like false advertising since I threw a vintage Piranha sticker on the stern (came with the NOS four-bladed hub I scored on eBay a couple years ago) .

I do like Piranha props, so if I get a good pitch figured out I'll get the right blades long term. But again, free is free.

Edit for more replies:

On my boat, the hydrofoil is worth about 3 mph on the top end. Pull it and see if it helps.

I'm not crying over 3MPH. Although it adding drag would make sense.

For trim, you'll hear the u joints scream when you've gone too far. Some boats respond well to it and others not so much. That invader would pick up 400 rpms just from minor trim and speed as well. I get about 200 on top trimmed to the edge of blow out which is worth about 2 mph and where the hull runs best for me.

So that's the funny thing. On both composite 4-bladed props you'd get nothing from trimming (just leave it down and let it eat). On the three blade the RPM didn't change but the speed went up. But, I was dodging rough water, so I couldn't do a lot of extensive testing.

From dock use I can confirm no howling U joints at trim limit switch cutoff, at least at idle speeds.

 

[Pmt133]

In any case, it sounds like 21 pitch is no good now, regardless on what it was then.

I'm thinking of trying the 19 pitch Michigan wheel to see how we do. "Free" is free. Even if it feels like false advertising since I threw a vintage Piranha sticker on the stern (came with the NOS four-bladed hub I scored on eBay a couple years ago) .

I do like Piranha props, so if I get a good pitch figured out I'll get the right blades long term. But again, free is free.

Edit for more replies:



I'm not crying over 3MPH. Although it adding drag would make sense.



So that's the funny thing. On both composite 4-bladed props you'd get nothing from trimming (just leave it down and let it eat). On the three blade the RPM didn't change but the speed went up. But, I was dodging rough water, so I couldn't do a lot of extensive testing.

From dock use I can confirm no howling U joints at trim limit switch cutoff, at least at idle speeds.

My experience with 4 blades is they tend to lift the stern on my hull. Usually resulting in trim making little to no difference as well.

Try the 19 and see if you get in the ballpark for RPMs. If you were closer I'd loan you my stainless 19 to test just because. That prop has run like magic on a lot of hulls.

 

[ratdude747]

My experience with 4 blades is they tend to lift the stern on my hull. Usually resulting in trim making little to no difference as well.

Try the 19 and see if you get in the ballpark for RPMs. If you were closer I'd loan you my stainless 19 to test just because. That prop has run like magic on a lot of hulls.

Nothing personal but I don't care for stainless. Too likely to blow my drive in the event of an impact (one of the lakes I semi-frequent has a bad spot that's eaten several Piranha blades!) and unnecessary at my power level.

 

[ratdude747]

Got home and looked at the Michigan Wheel prop. It's an SMC378. Discovered some dry rot starting under the grease:



Maybe not so good after all...

The Solas prop actually is a 19 pitch 4 bladed prop. Specifically a 1513-143-19. A bit smaller in diameter though (14.25) but being 4 bladed makes up for it?

Choices, choices, choices... Thoughts on which one to try next?

 

[ratdude747]

Maybe none of the above. Poking around eBay, I found a new open box Piranha 3-blade A (same as my 21P I recently bought) with 14.5x19 RH blades (5 of them) for a steal (well, well under what Piranha charges for blades). Put in an offer but I may just nab it since it's cheap. The core's code doesn't come up as valid but who cares since I don't need a core. Hmm...

Edit- Deal negotiated. Shipping tomorrow out of Green Bay so we shall see if they come in soon enough to test this weekend. But the weather is fluky enough it may not matter.