Advice on Preventing Prop Ventilation

It would be useful to know what the clearance is between the underbelly and the prop tips, to know whether there is scope to install anti-ventilation plates there.

 

You mentioned Power and speed, but what is the propeller dia, pitch and shaft speed?

These numbers are relevant for the thrust loading of the propeller, and the sensitivity for ventilation is a function of loading. Depending on the prop loading, other measures may be necessary.

As for the hull shape, I feel there might be an air path down along the "corner" between main hull shape and the "keel wedge". The air coming that way may be "peeled off" from the flow entering the prop disc by a horizontal anti-ventilation plate (like what you have on an OB leg) above the prop; as suggested by Gonzo (if I got him right).

You would also consider something like bilge keels along the aft part of the hulls in order to cut off the aeration path, but first give us the prop info.

 

Is the painted waterline accurate ? Does not seem to be much boat under the water, maybe it needs some ballast if all else fails !

 

There is great knowledge in this forum.
baeckmo, you are absolutely correct that the air travels down the corner between the hull and the keel wedge. The prop info is: 17.5 dia 15" pitch 2:1 gearbox. The boat seems to over rev at max 16.5kn.
Mr efficiency, the boat was designed for cedar/polyester layup @8t. I built it with an epoxy/balsa duraflex/duracore s/glass layup. It came out @ 6.5t. I put 1000 ptrs of water per side and 500 ltr of fuel per side to counteract but I feel it is still too light.

 

Normally you would say extra weight is poison for a boat like this, it certainly would not be the first choice to rectify your problem.

 

Considering the proportions of the hull, primarily the wl length versus tunnel width, I'd say that there is a greater risk of added ventilation from the wave system within the tunnel, than from the outside of the hulls. The diverging waves are difficult to control, and the sum of the amplitudes will probably be critical in the upper speed range of your boat. So what you observe from the outside may not be relevant for what goes on in the tunnel.

Now, again what is the shaft speed (or the engine speed, now that we know the ratio)?

 

max rev 3300 rpm.

 

The very soft chines in the vicinity of the prop aren't helping IMO. Boat would probably be better off w a flat bottom above the propeller. Then if the boat didn't come out of the water in the aft quarter it should't suck air.
As a long shot would be to change prop rotation.

 

This is just my opinion, but may not hold true. You're running a four blade prop. In a quartering sea , you have lift causing the cavitation ,plus vortexing due to the RPM of the prop. If your running 3200rpms that gives the prop 1600rpms . At that point due to the pitch 15 the rpm's have to be lowered to 600rpm's to clear the vortex. It would seem to me a three blade with a few more degree pitch would be a better choice , and you will have some cavitation but the prop would be less in the differential rpm's (not as far apart. ) I believe the three blade would not hold the air on the back side of the prop as long, not have the high pressure from the fourth blade pushing the air forward on the prop blade , holding the cavitation at high rpm's . If that makes any sense.

 

As mentioned, trailing edge of keel looks pretty crude.
- Looks like the prop is painted... what on earth for?????
- re-do the prop calcs and check your prop blade loading. If on the high threshold, it does not take much to trigger cavitation.

 

The prop is now prop speed. Will fair the trailing edge and maybe the rudder. I was advised that a smaller dia prop with more pitch might help? with a cav plate.

 

Other than the ventilation drama, are you happy with the boat ?

 

Don't go that route, your props are comparatively highly loaded as is! I'll get back on this subject later, but have to focus on Another job right now, sorry!

 

Yeah it really suits me well. There is lots of room, really stable, economical ( 1ltr per 1nm @ 8.5kn). The ventilation only occurs at max speeds for which the boat was never designed..If I built again I would design narrow deeper hulls, little rocker with outboards. I believe outboards are cheaper, easier to install/replace and simpler than diesel inboards.

 

No good?