8.1 prop selection for 23-24ft boats

[SS24V]

Just picked up a 2006 Supra 24V with Indmar 8.1. I realize this is a Malibu site but your 23-24ft boats with the 8.1 got almost the same stock prop as mine and must suffer similar fates.The stock ACME 497 13x17.5 is not impressive out of the hole at all and only turns 4800 rpm at 50 mph with an 800lb load. Thought the 8.1 could run much better so called Supra, Indmar, Acme, Wakemakers, and Bakes to help make a new prop selection. Very long story very short....... not much knowledge out there on propping the 8.1. I got suggestions of 13x16, 14x15.5, and 14x14.25 from the flow charts based on usage or they simply said I can't surf and foot behind the same prop. Guys......that's why I bought the 8.1. The 450hp, 525 lb ft of torque, and revs to 5400 rpm can do a lot. Thats 125hp, 175lb ft, and 600 more rpms than the 5.7 Assault motor. After much online reading on " the Malibu Crew" about others' 8.1 prop experiments and significant calculation regarding pitch, diameter, cup, RPM, and finally slip percentage estimation, I made a choice. The ACME 1273, 14.5x14.25, .150 cup, arrives tomorrow and I will record RPM and MPH info on both props to share with the forum. The lower pitch pulls harder, the larger diameter moves more water, and the increased cup controls WOT RPM. My calculations show the 1273 will run 47.5mph at 5200 rpm with a barefooting load and carry 3000lb ballast at 11mph easily for the surfers. Fingers crossed!

[SS24V]

Update:  The 1273 greatly improved the hole shot, holds a great surf wake, dropped the prop slip by 7% for better fuel economy, and ran 46.5mph at 5000 rpm with an 800lb load. Performed pretty close to what I was looking for so will probably run this for the remainder of the year and then decide if I want to try the 1847 which is the same prop with 1/2: less diameter. to get the revs up another 200-300. Talked to Larry at Indmar today and he said peak power is made closer to 5400 rpm so that is the goal.  FYI, he also stated that peak torque on this motor is 500 lb. ft at 4000 rpm. Anyone have similar prop data on their 8.1?

[Eagleboy99]

8 hours ago, SS24V said:

ran 46.5mph at 5000 rpm

If that is GPS verified, that is impressive for  a boat that size.

 

[SS24V]

On 8/20/2019 at 11:33 PM, Eagleboy99 said:

If that is GPS verified, that is impressive for  a boat that size.

 

Yes, that  46.5 mph was from GPS and replicated on several runs. The stock 13x17.5 ran 50 mph on GPS with three of us on board. Talked with an ACME engineer yesterday that suggested the 1615 or 2773 to turn closer to 5300-5400 rpm empty. He was super helpful and very knowledgeable. Dropping an inch of pitch may cost 1 mph but I should gain that back by getting to the motor's peak power. Also sent off the stock prop to have the pitch reduced to 16" and cup added so I have a useful spare.