Bryan Rose

I had the opportunity to run a labbed 28 Bravo I yesterday and have also run a box stock 28 as well. The labbed 28 had the diffuser ring cut off and the box stock one had it. Well my boat does not respond well to the lack of having a diffuser ring.
Funny thing I had no problems with blow out with the box stock one but without the ring the prop really never would hook up , it was not hard to get on plane but under acceleration from 25oo up the prop would not bite....Guess I will be having a diffuser ring on my prop. I think this ill help in my situation.






these are numbers for cruise speed .....top end i did not really test because I had the familly with me.
Box stock Bravo I 3500 = 50 mph
Labbed no ring 3500 = 35



stock 4000 = 60 mph
labbed 4000= 49 -50 mph

prop just would not hook up. I am gussing this is because the ring is not there. I have thru hull exhaust ...
What do you think.

Bryan

Beak Boater

Bryan
I think their is something else going on their. The diffuser ring would not make a 15 mph difference in cruising speed. Depending who did the Lab work to the prop, and what boat it came off of, it appears that the prop was tweaked. The pitch may have been changed, cup taken out, more changes than just the diffuser ring removed. The reason I say that is the diffuser ring primarily is to help the prop lock up and get the boat on plane by sealing off the exhaust gas from getting in the blades of the prop causing excessive slip or cavitation. Once you are up and running, it has little or no effect, and in some cases may reduce your speed a little because of drag.

Bryan Rose

I have thru hull exhaust via silent choice, but still have the provision of exhaust getting to the hub Via the input port on the drive, below the bellows I have installed an exhaust tube to get it out from the gimbal provision, and not out the hub, so there is still the open cavity with access to suck air from the front of the drive. I have a feeling the prop is pulling air down this cavity and not letting the prop hook up once under way. The lab was a standard type lab finish with cutting off of the diffuser ring and nothing else was done in the area of pitch tweaking or smaller diameter.


just my thoughts I am not a prop guru but now I remember I also tested a hydromotive Q4X with no diffuser ring and it did the same thing, and it was box stock.

Bryan

throttleup

Keep in mind that Lab Finished propellers have a fair amount of cup removed from them. This will effect your "bite" on the water and is most likely what you experienced. Removing the diffusser ring will effect your planning.

Julie