Correct Cup for Prop- 2002 28 TE

[Jkraft]

I know prop discussion on here is like beating a dead horse but I read somewhere that there is a magic number for the cup on the 28 TE boats to minimize cavitation. Does anyone know what it is?

Current prop is a 19 x 17.5 4 Blade (Cup unknown)
Engine: 6lp STP2
Transmission: 2:1 ZF
Max RPM 3850-3900 Fully Loaded
Max Speed: 21 Knots

Cruise RPM: 3450
Cruise Speed: 17 Knots

I thought I read somewhere the cup should be 5 or 6.... Does that sound right?

[Ben423]

#5 seems to work best for the Yanmar 6LP.

[Jkraft]

#5 seems to work best for the Yanmar 6LP.

Thanks Ben!

[Toolmanbob1993]

Fully loaded those are good numbers

Mine during survey would do 3800 but it was lite as hell

By the time I load it down I figure it will be way overpropped

Taking my prop now to get worked on to see what they can do with it

The yard hard to get the hydraulic press to get it off probably never been off in 20 yrs

Going to see if he can make it 19x18 5 cup

[Tree]

Fully loaded those are good numbers

Mine during survey would do 3800 but it was lite as hell

By the time I load it down I figure it will be way overpropped

Taking my prop now to get worked on to see what they can do with it

The yard hard to get the hydraulic press to get it off probably never been off in 20 yrs

Going to see if he can make it 19x18 5 cup

If the prop hasn’t been off for 20 years, chances are you’ve lost your cupping. Bear in mind with the yanmar set up your prop is probably spinning at almost 1700rpm. Over time with salt water you will erode the tips of the propeller. I’ve seen it happen here with Rodman boats and simple recup has gained up to seven knots in some of these high performance boats. Cupping can work like pitch but without the load on the engine. If the propellor is having cavitation issues ( which will also lead to the tips eroding - think along the lines of very small atomic explosions at the tips as the bubbles super heat and explode) once you lose that cup the surface area changes which can lead to overloading and reduced WOT RPM. Put the cup back and a world of difference can be made. It’s an odd science and something I’ve learned from an ex nuclear submarine propellor designer a few years ago.

My starting point is 19 x 19 with a five cup which is around 71 thou if I recall. I pull my prop every couple of years to get the cupping checked and redone if required. I have run various propellor over the years which have given me varying results.

20.5 x 19.5 4 cup gave me 24 knots WOT which was a specially developed propellor, mainly so I could comfortably cruise at 18-19knots at 3300 rpm.

Last haul and a refresh of the boat running 19x19 five cup gave me just shy of 24knots WOT and a 17-18 knot cruise around 3400 rpm.

Now these figures are based on the yanmar tach which everyone will tell you is inaccurate and I agree but I have used that as a base point to compare figures.

The badge of my engine states 3800 rpm max WOT and both props make that. In reality it laser tachs at around 3700 WOT but since I run the boat around 3200 - 3300 giving me 16-17knots I’m happy to stick with that. The engine is on 2500 hours now and twenty two years old. If it was going to go bang in my opinion due to overloading, it would have done by now