Cavitation problem
 

Hi
I got a 20 Lund Alaskan with a 50 hp Honda.

The problem is that the prop loses its bite when there is any chop in
the water (2' seas). The RPM will run up to 6500 and I (quickly) have
to back it up to about 3000 before it gets it bite again. A real
pain. The Transom height is lined up with the plane. The motor has
been maintained. The Prop is an 11.2 X 12.


The Advise I got so far is:
The Lund rep told me the engine size is too small and for that boat it
should be 75 hp min. Which I guess is what they come with now. It's
rated for up to 120 hp

Was told that a different prop size may help.

The boat will get up to 30 when empty which is more than enough for
me.
The problem is more prevalent when its got a load (800 lbs) (I lug a
lot of stuff with it).

If the problem could be fixed with a prop that does not lose its bit I
would probally opt for that since speed is not big a factor as cost of
a new engine.

Thanks for the ideas

Claude

Cavitation problem
 

On 14 Jul 2004 06:16:22 -0700, (Claude Bolduc)
wrote:

Hi
I got a 20 Lund Alaskan with a 50 hp Honda.

The problem is that the prop loses its bite when there is any chop in
the water (2' seas). The RPM will run up to 6500 and I (quickly) have
to back it up to about 3000 before it gets it bite again. A real
pain. The Transom height is lined up with the plane. The motor has
been maintained. The Prop is an 11.2 X 12.


The Advise I got so far is:
The Lund rep told me the engine size is too small and for that boat it
should be 75 hp min. Which I guess is what they come with now. It's
rated for up to 120 hp

Was told that a different prop size may help.

The boat will get up to 30 when empty which is more than enough for
me.
The problem is more prevalent when its got a load (800 lbs) (I lug a
lot of stuff with it).

If the problem could be fixed with a prop that does not lose its bit I
would probally opt for that since speed is not big a factor as cost of
a new engine.

Simple things first. What you are describing doesn't sound like
traditional cavitation. It sounds more like ventilation, where the
prop grabs an air bubble from the surface and then spins in it. From
the sounds of your performance, your engine is perfectly adequate in
H.P..

What I would check is the engine height on the transom, and more
importantly, the height of the anti-ventilation plate with respect to
the bottom of the transom. Normally the anti-ventilation plate should
be even with the bottom of the boat. High performance applications
often raise the plate up, so that the prop will actually break the
water. But this also requires a prop with a high rake, and/or cupping
of the blades. There is also the issue of the cooling water pickup. It
should never be allowed to break the surface of the water, or you will
start introducing bubbles into the cooling water, which can cause hot
spots to form.

Other than engine height, make sure you are not excessively trimmed
out. Excessive trim will eventually cause the prop to "blow out". The
remedy to this is to trim down a few notches.

A different type of prop can help this situation. But I would make
sure the engine mounting and trim are set correctly first.

Dave

Cavitation problem
 

From what you describe, I'd also suggest you try an inexpensive hydro plate.
Bob's Machine Shop makes a $35.00 "stabilization plate" that may well stop
the problem you are having.

I have had good results in the past with hydro/doel/etc. etc. plates, but
I've somewhat switched my allegiance to "smart fins" to keep my boat on the
level, but your problem sounds different than that. RichG


-- manager, Carolina Skiff Owners Group on MSN
http://groups.msn.com/CarolinaSkiffOwners