Re: Jack plates, whats your feeling about them.
well it seems some people do not know a surface prop works, I still don't fully understand it myself, but its very
different to how sub surface stuff works that's for sure. with the surface prop I only ever want a max of 2 blades in
the water at one time to get the best results for speed.
to say the cav plate needs to be level with the bottom of the boat is very old school and only works good with very
poorly designed old school propellers.
Now the object of the jackplate is to raise the engine a long way up, not keep it level, I could have my cav plate 2 inches above the bottom of the boat without the jackplate, that made hardly no difference at all.
Cavitation will not be a problem with the propeller I shall be using , its meant to run with the centre of the hub level
with the surface of the water !!!!
I have other props where raised up engine would cause huge cavitation and slippage, I have proved this big time.
With a omc silver streak (modern name is SRX) prop this is not the case.
Nowdays a lot of boats have jack plates on them,many have two so they can get a long way back and run very high at same time.
you have to remember that for every inch you go back, you also need to come up to just stay normal running attitude.
if you put just a setback of say 10 inches on the transom the engine would be running far too deep and never be efficient.
If you look at many of todays race boats, they use a 15 inch mid section (that is short shaft) and a jack plate on a 20 inch transom (longshaft transom).
so by using the short shaft they have already gone up 5 inches, then another 5 by using the lifting jackplate, but also gone back 6 to 10 inches.
there are hundreds if not thousands who use spacers as well as huge setbacks and lifts.
here below is a few that work very well, and they are all running a lot higher than mine.
my plates are pretty sedate compared to some that are running very good speeds with no cavitation worries.
but it really is a case of using the right prop, a normal everyday alloy prop is useless when using jackplates.
Most bass boats for example use some really exotic stainless props that cost a small fortune, they can run both high
and normal.
I tried a omc 12 3/4 x 23 pitch alloy and at just 2 inches higher than normal its rubbish and gave just 34mph at 5800rpm
with a very poor take off, the cavitation was crazy and the boat dangerous as it caused it to veer off course and hard
to get back without shutting it down quickly.
my 13x19 inch stainless sst at the same height gave 38mph with a very quick pull away and max 5800rpm.
My 13 3/4x23 stainless silver streak gave 39.1mph at just 5400rpm,it needs to run higher up to get going.
in theory the alloy should of been the quickest due to being smaller diameter, but just had no bite and too much cavitation.
check these out, and notice that one has the jack plate showing fully down and fully up, his plate sits 5 inches above
the bottom when at its lowest setting , in thise pics the rope is showing the water angle when running.
the red thing in the last pic is just a 8 inch set back used before the jackplate !!! so this guy gets about 14 inches of setback with about 10 inches of lift .
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