[hpv-boats] Single person easily driven displacement hull drive systems

[Rick Willoughby]

Dennis
I expect when you get your stabilisers mounted at the right height  
you will find the narrow chord, high pitch prop will work fine.  They  
may need a redesign to work well.

Right now your boat has much more drag than it should.  In your  
summer I hope you get the opportunity to set it up correctly.  The  
stabilisers should literally skim the surface once at design speed.   
My stabilises sit about 20mm above the surface when at rest.  At  
11kph the sinkage and bow wave combine to get their stern just  
touching.  This is what you should aim for.

The prop design can be done with great precision but it is reliant on  
accurate determination of the boat drag, which equals the prop  
thrust.  Your main hull is not draggy but your stabilises will be if  
continually loaded in any way.

I have found flat bottom stabilises, 2m long and 100mm wide to be  
about the best.  They achieve close to maximum stability as soon as  
they hit the surface. So there is little tendency to flop roll even  
though they sit above the surface most of the time in calm water or  
clip waves in rougher water.

If you cannot reduce the drag then you would be better off going to a  
slightly larger diameter prop with similar pitch and slightly large  
blade area.  But you should fix the high drag problem before going to  
this measure.

My prop blades usually operate with a negative angle of attack so are  
a long way from stalling.  The 25mm chord is a little more forgiving  
in strong wind than a wider blade.

You will note that the prop my friend has copied and turned into a  
folding prop has a maximum chord of 45mm.  This will work OK on quite  
draggy hulls but is not as efficient as my 25mm chord on V14.

I am glad you find 1:4 a nice ratio and look forward to some more  
performance results when you get decent stabilises properly set.   
This is where you will get the most immediate gain.  Then try the  
25mm chord prop.

Rick

Single person easily driven displacement hull drive systems

Hi Rick

As you are aware I have suffered several design failures on my  
manufactured props caused by excessive
slipping of up to 30%.
This stalling was virtually throughout the operational range and  
predicted by Theo Schmidt in his 1999
paper on Propsim.
He advises slender type blades with high pitch /dia ratios are  
subject to stalling.
My APC 16 x16 only slipped a maximum of  2 to 3 % but when I made a  
new version with a pitch of 24" the problem started.
Your design for the constant cord prop suffered slightly worse, but  
the problem was over come by widening
the cord which I did to the extreme ( this I believe created  
increased drag due to the larger surface area).
The slipping is created by the greater resistance of my boat compared  
to your V14 boat
What this really shows is that there can be no hard and fast rules as  
the prop must be tuned to the application
ie the boat.
I like you feel that a 4 to 1 gear ratio is about right, bur like Bob  
Stuart I also feel that  if the pitch and dia
are equal the prop is more flexible
and even though the revs would be higher the efficiency might be  
better.  To sum it up regardless of the prop
design programmes, prop design it still very trial and error

Dennis a

Rick Willoughby
rickwill at bigpond.net.au
03 9796 2415
0419 104 821