ratdude747
Chief Petty Officer
- Joined
- Sep 30, 2023
- Messages
- 435
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:
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?
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)
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?
