Mystery prop cavitation problems

[Fastjeff57]

I know this is a dumb question, but I'm tearing out what's left of my hair over this and would really appreciate some advice.

I running a Mark 55 type lower unit with the cav plate 1/2 " below the bottom of the boat (a deep vee design with an 8 " wide step). My totally stock (and rare) 15 pitch bronze two blade bites fine--no trouble--but my 17 pitch bronze two blade cavitates its butt off! Same set up! The blades of both props are in perfect condition, the ODs of the hubs are similar, and the props are on the propshaft the same physical way (no gaps at the housing).

Has anyone an idea why the 17 won't hold? It bites fine until the power comes on then suddenly goes berserk! If I nurse the boat onto plane (by literally sitting on the bow with only part throttle), I can eventually get the boat planed off. The prop then grabs fine--unless I stupidly allow the boat to drop off plane.

Jeff

[zul8tr]

The pitches stated are average usually taken at a specific radius. Assuming the blades have variable pitch at a given radius performance has alot to do with the progressive design of this pitch.

Do you have pitch numbers along the blades of each at different radius? Pay attention to the pitch just aft of the leading edge (water pick up edge) at various radius, if too high the blade might have problems keeping water attached (seperation) on the push side (aft) of the leading edge as the revs increase. If this happens there will be too much air on the push side and loss of bite.

Have you tried lowering the engine for the 17 pitch blade to test what differences occur? There might be excessive air being drawn in from the surface for the blade design in relation to the anti cav plate depth below the water? It just might perform better deeper in the water?

[Fastjeff57]

First of all, thank you for your response! I appreciate any and all help with this.

The prop pitches I stated are "factory" (whatever that means) for these two stock props.

I like the 'larger cavitation plate' idea--I can slap a set of hydrofoils on there and see what happens. (I hate those things, but what the heck!)

Thanks again.

Jeff

PS: One fact I forgot to mention: The 17 pitch prop WAS a thru hub exhaust prop that i adapted to the LU (which is not.) Is it possible that open "hole" at the back side of the prop might be causing this? I'm clutching for straws here, but....

[zul8tr]

Usually factory props are measured at 70 to 75% radius out on the blade from the prop shaft center. With progressive pitch they might average across the blade at that radius or....?

BTW the plate on the lower unit is for anti cavitation but is commonly called a cav plate incorrectly

By anti cav I didn't mean those extensions that are added to the engine anti cav plate but that with the 17" and the depth below the water of the present anti cav plate might be having more air being sucked in. But you can try those extensions.

The exhaust hub issue you note might be an issue. Try other things first.

[Ed Hatch]

Exhaust hub on the 17" creates a low pressure area behind the prop pulling water down from the surface possibly uncovering your blades. Suggest you drop down on the transom a bit and see if that doesn't help.

[Fastjeff57]

A couple of further observations: I've come to realize that my problem props cavitate TOTALLY and loose grip altogether. We've all experienced the usual cavitation on plane off, before the prop bits (and away we go!) This is different, for the boat looses ALL forward motion and the motor revs to the red line. (The hubs are NOT slipping--thy're pinned solid).

oth of my props that do this have an opening at the back (formerly thru exhaust props but now on a non-thru hub LU). I plugged one f them up with a large aluminum washer and it helped quite a bit: the motor still cavitated, but it didn't blow away and I was able to plane off without the usual dance (work the throttle and sit on bow!)

Gonna try closing off the other troublesome prop and see what happens.

Thanks for the inputs.

Jeff