Do you normally use the boat on fresh salt water?
Fresh water has quite a bit more drag than salt water.
I trimmed it to get WOT top speed then dropped back to 6k then dropped
0.5k inrements without changing trim, that probably accounts for the
increased slip at lower revs.
Which means from around 5000 down you are over trimed, which is not as significant as under trimed, till speed drops off around the 4000 mark.
The load was deliberately light, full tank of gas, and 2 POB
When propping a general use planing hull light load.. basically as it comes off the dealers floor.. as they had it when propping, one aims for the high end of the WoT range.. the hopeful theory is when loaded that will bring the WoT rpms down to around midway... In practice its far better to prop the actual boat to its normal fully loaded weight.
.. espec on a well or over powered boat.. huge differenced in speed and economy
What's the downside of having less than 4% slip?
Well not a down side .. sorta...
you like detailed info then?
Basic physics A given hp will move a given weight at a given speed.. therefore your boat car tractor aeroplane, will only travel at a given max speed
. you are matching the target WoT rpms to max speed for your boat weight..
At WoT therefore the powerhead is at max hp at that rpms.. bearings, crank, 'rods, all loaded up, down thru the gearbox and to the prop at 100% .. no torque converter or clutch in between to take any change in weight friction etc. Say hit a wake or resistance the already fully loaded engine gets suddenly overloaded...eventually something breaks.
At the other end , holeshot the engine cant produce enough power to take off, so slip is built into the poro,, and other stuff like holes in the side to allow exhaust gases around the blades, increasing slip further.. which means we can increase rpms.. and available power to get over the bow wave.
The torque/ hp curves, gear box ratio are designed (for general use boats to have reserve power at lower rpms and around 12 to 15% at cruise to around 5% woT.
(Race boats area completely different setup.. heaps slip and pitch WoT.)
In particular if min or underpowered and slip is close to ) WoT then there will be less lip at cruise, right down to trolling..the power head doesnt have the 15 to 20% reserve power in those pms to handle chop at all, and powerhead is often fully loaded and overloaded when hitting a wave, momentum lost, drops off the plane, have to work the throttle all the , chew thru heaps fuel, loading and overloading crank, etc.. and warrantee issues.
If min/ underpowewred and in a boat 4.5 thru to 5.5/6m fully loaded and propped for around 5% slip WoT @ mid range, then at lower rpms you still have a little resrve poer and 12 to 15% slip at cruise and in chop...and a balancing act as to be able to maintain reasonable sped to stay on the plane without requiring 100% hp at any rpms .
the more the WoT slip gets to 0 the more overloaded and stress on the powerhead at cruise and lower rpms.
A well powered boat.. aim for the same , thu often can afford to go to lower WoT slip if over powered.. basically because have a good 20% + reserve power at cruise and lower...and a reduculous WoT speed that would be an idiot to maintain.
eg 150hp on an old commander, fully loaded 3 PoB bimini etc and hitting around 50 mph.. starting to chine walk...and will if hit a big ripple.Like the wifes shopping basket car, it has a top speed you will never use, a side symptom so have reserve power to get up slopes fully loaded without always changing down a gear , to increase rpms to that extra power.
@ 42/43 mph open up throttle slightly too fast the rear end kicks out like a V8 on a wet road hit the throttle rear end goes loose.
Bigger heavier boats have a little more slip at WoT 6m + getting out to around 8%
less slip more speed per rpms
more pitch more speed per rpms
Propping is a balance between slip and pitch without overloading the powerhead, for best performance and economy.
Rake effects the above also, but primary to make better use of the narrower thrust cone out the back to bring a high bow down... and/or venation issues.. and improved propping performance.. due to smaller prop less hp loss, more for pushing.
Im a great fan of 4 blades espec on min and under powed boats