Peopeller help

I am installing an inboard in the boat that sank
last year. (all cleaned up and shiny, stripped out
and ready for a second life). Instead of the usual
under hull instalation, I am just sticking the prop
out of the bottom of the stern at a 12 degree
angle. The farther it protrudes, the farther the
angle will take it under the waterline. How far
should a prop be submerged under the water line?
flush, 3",12"??? obviously I need the shaft to be
as short as possible. The engine is diesel and the
prop is 20"dia.. If I go very far then I'll need a
strut and cutless. A slight loss of effeciency is
more preferable Still, excessive cavitation or
slipage is my concern. Flush would be ideal. It
will be a slow flat bottom boat used for pushing
heavy loads, not water sking. Rpms in the 2,000 to
2500 range. The hull draws 10"" of water when fully
loaded.
Ted

 

What was the original installation?

 

140 hp Evenrude. Goes that have a bearing on prop depth?

 

Are you installing stringers for an inboard? Usually outboard powered boats have the center of floatation far aft. With an inboard it will probably float low by the bow and plow.

 

Point noted.Picture a jon boat, a big jon boat. 24' l. with a 6' w. flat bottom. It's rake (bow upsweep covers 6'. I've had it well loaded up front and it didn't plow, just sat deeper all around. I'm removing 350 lbs from the stern and sitting aprox. 400 lbs 5' farther forward. The boat is aluminium plate. 1/4" btm. 3/16" sides. double bottom.
Drawing 9" of water, the prop will stick out about 2" at the stern. extending the shaft out 5" puts the prop just under the surface. If this is enough then I won't need support and I still retain shallow water capibility.
Becaus of hazards in the swamp, this boat rarely goes much above idle. How much water does a 3 blade prop need?

 

Hi Ted

Your propeller will be operating as a surface propeller.

Operation with as little as 30-Percent of the disk wet will give adequate results, but it will require a large propeller. Your plan is to operate near 80-Percent of the disk wet, this will also yield near minimum lift.

Ventilation to the atmosphere will result in a nearly complete loss of back suction. The U. S. Navy works in the 70’s on high speed surface propellers showed that if the disk area is increased proportional to the loss of wetted area the suction loss will be compensated. Using model propellers probably of the scale of your propeller the U. S. Navy was able to demonstrate 80-Percent efficiency with surface propellers. Tunnel speeds were probably close to your operating speed.

In contemporary surface propeller designs the loss of back suction is typically compensated using large cup. This destroys low speed performance.

I would use the quoted sizing rule and be prepared to go larger if it slips too much

 

hI bANANAS
wISH i UNDERSTOOD ALL OF WHAT YOU SAID. i'M NOT THE BRIGHTEST BULB ON YJE TREE. i GOT MOST OF IT (i THINK).
mY PROP IS 20" DIA. tHE HUB IS RELATIVELY SMALL, ONLY 2 1/2" DIA. tHE BLADES ARE THIN AND LONG, 9" LONG. tHE PITCH LOOKS AGGRESSIVE BUT i DON'T HAVE A CLUE WHAT IT IS.
sO... i HAVE A SMALL HUBED, THIN LONG 3 BLADE, 20" PROP. iF i EXTEND THE SHAFT ENOUGH TO SUBMERGE IT 2" UNDER THE SURFACE, THEN i'M ok?
aNOTHER POTENTIAL PROBLEM IS REVERSING. wILL THE WATER DAM AGAINST THE TRANSOM?

 

Hi Ted

Operating 2-Inches submerged aft of the transom will most likely result in the propeller "sucking" air from the surface. This is called ventilation. I cannot calculate that it will absolutely happen, but it is likely.

If the propeller ventilates and cannot develop adequate thrust you will know it. It will shake the boat, put a massive amount of vapor in the wake and you will not accelerate.

If you need to change or adjust the propeller you will need to know rather precisely what you are running. It is likely that a local propeller shop will scan it for you for free. Then, knowing the diameter, pitch and approximate blade area you can correct the propeller using your test data.

Do you have a GPS on the boat? If not, do you have access to a hand-held GPS? Can you measure engine RPM? Do you have a transmission and gear reduction?

With boat speed and propeller RPM data, combined with knowledge about the propeller sizing it is quite likely that I can help you find a suitable solution.

But test it first. You may have a lucky guess.

Ciao

Max

 

Thanks. The boat is not in the water. It is in the middle of conversion.
As with most conversions, there are a few unknowns. An educated guess here and there sure helps. Everyones input helps to get close and prevent a major err.
Thanks again.