Need more speed less slip on prop

[nparadisiotis]

Have an 1992 Chris Craft ( Caper 156 ) in excellent condition. It is 16.8 ft (5.10 mtr) lenght by 6.9 ft (2.10mtr) beam. Just installed a brand new Evinrude ETEC 150HO (150HP at 5300rpm) outboard, with a BobMachine electric jack with a setback of 8inches. Tried the following propellers:
Evinrude Raker 14.5"D x 24" 47mph (GPS) @ 5500rpm
Evinrude Raker 14.5" x 24" 53mph (GPS) @ 6300rpm
Evinrude Viper 14.75" x 21 37mph (GPS) @ 5500rpm

There seems to be high amount of slip on the prop. Note that the Raker has two vent holes on the prop, but the viper does not have one.I calculated the slip to be 30% on the raker 24pitch & 37% on the viper 21". The raker has a high rake. In all 3 cituations the throttle went only to 60% and could not reach WOT. The engine didn't seem to work hard, it was as if the prop was not bitting or as if it was venting. During the test runs the engine height was set as follows: the prop shaft was 12 inches below the bottom of transmom. At this position the prop seem to be working well. If the engine was lowered then it started to vibrade which means that the drug on the outboard increases and there was no increase in speed, also noted that that on the left and right hand side of the cavitation plate it started to create a big upwards spray. If the engine is raised then the rpm will start increasing with no increase in speed, which means that the prop is slipping/ventilating air even more. The engine can be lowered/raised by 5 inches from the ideal point. Another thing that I noted is that with the Raker 24pitch when it reached 6300rpm at 53mph the trim would start responding and lift/lower the bow, below this rpm the trim did not seem to respond. Obviously the boat was trimmed a litlle out after it comes out of the hole. While on plane the boat feels very solid (no tendancy for the nose to play up/down). Had an observer video the boat and noticed that the boat while on plane is half in the water, front half out of water. I have a suspition that if I put a higher pitch prop (Raker 28pitch) that when it reaches 53mph (estimate at 5400rpm) the slip on the prop will start to decrease (maybe due to the stern been raised) and the engine can be trimmed for more of the nose to come out thus increasing the speed. This should start to degrease the rpm as there will be less slip on the prob, and also I suspect that I will be able to use more of my throttle, hopefully go to WOT. Also I would like to ask, if a propeller with 30% slip would create any physical diformation problems on the propeller on the long run? If a boat does not reach WOT (throttle to reach 60% of WOT) but is used at the maximum allowed rpm (5800rpm) would this create any problem to the engine?

Any input/advice would be greatly appreciated.

[nparadisiotis]

sorry i made,mistake the height of the prop shaft to the bottom of the transom is 4.8 inches instead of the 12 inches mentioned in my preveous threat

[Jango]

If the Prop shaft is only 4.8" below waterline, the Prop is ventilating.

What is the total Displacement w. Driver and minimal fuel? Also, what is the Gear Ratio? Normally increasing Prop Dia and/or no. of blades decreases slip.

Make sure prop Hubs are not slipping on spline.

[nparadisiotis]

Weight of hull(no engine) with driver and minimal fuel is 1298lbs (590Kg)
Weight of engine (Evinrude 150HO) is 418lbs (190Kg)
Total weight of boat under test (2 persons & full tank ) is 2013lbs (915Kg)
Gear Ratio is 1.86:1

[Jango]

Based on the no.s provided:

Prop: 14 1/4 Dia x 21 Pitch, 55% D.A.R.
Rpm 5500
Slip 10%
Max Speed: 53.4 Mph

Keep in mind, max speed is based on Hull type, Weight and shaft HP. Unless you have more HP and/or less weight 53.4 Mph is max. with a high speed planing Hull (Race Type flat bottoms or steps will provide more speed)

Based on these calculations, I would suggest you either have prop hub slippage and/or ventilation.

[nparadisiotis]

Jango, the prop is not slipping on the spline as the Racker24" prop is brand new. You mentioned that because the prop is 4.8" below the waterline that it is venting. But in this case the engine is mounted on the transom with a jack that places the engine 8" further aft of the transom and so I think that the waterline will be higher at the position of the prop compared to the bottom of the V of the hull. The vertical height of the prop shaft to the bottom of the V hull is 4.8". As I also had the same thought initially, I lowered the engine further down and saw no change in speed or rpm reduction. Also as I lowered the engine I started getting two large streams of water comming out of the left and right hand side of the cavitation plate and felt the engine to vibrate. So this means that the water line above the prop is exactly on the cavitation plate. All suggest that there is slippage/ventilation not due to propeller height but to other factors. I suspect that the whole boat has not lifted up enough thus causing turbulance/water bubles to the water going through the prob. If the boat goes over 53mph I think the boat will start to lift more out of the water and I will be able to trim to nose up and that the prob will have less slippage as well as less drug caused by the hull.

[Jango]

Not familar with the Jack Plate impact on your boat, but am fairly confident that this is probably the cause for your venting. Lowering the drive should NOT cause a vibration. Unless you have a Surface Drive Set-up, the cavitation plate should be slightly Below the waterline when Boat is at plane.

[nparadisiotis]

Thanks Jango for taking the time to look into my problem, really apreciate your input. I am thinking of ordering either a 14*28" Raker II prop (has very high rake) or a Renagate Bass prop (14*28"). I know that this is very high pitch but as my prop is slipping (30%) I estimate to reach 63mph and still have a 30% slip at the worst case senario. I am not worried about the hole shot by judging from the 14*24" Raker I currently use, maybe because the Raker prop has vent holes it helps the prop shout out of the hole better. The vet holes are suppose to close at high speed. Do you have any idea that by running a prop with 30% slip at 63mph would create a problem to the prob (cavitation? burn the blades?)

[Jango]

Not sure what will happen to the Blades w. cavitation appearing like 30% Slip. It's probably not that high as cavitation goes.

I do know you will need 200 - 220 shaft HP (Depending on shape and flatness of the Bottom) to get 63 MPH out of a 2000# Boat.

[nparadisiotis]

Jango, what do you mean by a "2000# Boat"?

[Jango]

Your previous post, no.4,indicated total weight was 2013 lbs

[nparadisiotis]

Jango, I didn't know that with a total weight of 2013lbs I would need a 200-220HP engine. So what is the maximum speed I can get with a 150HP (Evinrude ETEC150HO, 2589cc, 150HP @5500rpm) engine provided I place on it a 28pitch prop? How did you work it out? Is it by your experience?

[Jango]

Previous post #5, indicates 53.4 MPH max. w. 150HP. This was obtained by using Formulas developed by George Crouch w. Prop Data obtained from PropCalc - Castle Marine

SPEED = C ( HP\W )^ 0.5

W = Total weight


C = Hull Type (factor) Avg Runabout, Passenger = 172
High Speed, lite weight = 218
Three Point Hydro etc = 253

Your Hull type probably has a factor of 190 - 210

Complicating the calculations is determing HP. Your engine may very well be developing more than 150HP. For instance, The in-line 6 cyc Mercury (115HP) of the 1970-80s was developing more than 150HP.

If your engine is producing no more than 150HP, a 28 P. Prop will decrease RPM and perform similar to the 24P. Raker w. less Hole Shot. If RPM drops below 5500, top speed will be less.

Precise performance predictions are all but impossible, due to the Ventilation, with your jack plate.

[nparadisiotis]

thanks for the formula. If a boat has twin engines such as twin 250HP, can I use the formula with HP being 500?

[Jango]

YES, just be sure you add the weight for the second engine

By the way, PropCalc uses 195 for your Hull Factor "C"