change from 18p cuped to 19p uncupped?

[diamondlangus]

18ft aluminum 1973 starcraft, 1998 120hp Force motor WOT rating 4750 to 5250, I currently use a Stainless 18 p prop that is CUPPED. The motor currently runs at 4850 RPM at 32mph. The cavitaion plate on the motor is right at or just a hair below the bottom of the boat. I was thinking of trying an aluminum 19p prop that is NOT CUPPED. What kind of performance change could I see?

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

work setup problems, if plate is right at or just above the bootom of boat. the motor is to low all my boats i set in between 2.5 to 4 inches above (best perfomance). remember the water comes off the back of the boat and rises so right at bottom of boat is to low. going from a cupped 18 to 19 might pick up little top end and lose some whole shot. wot rpm would'nt change much i think the 18 will be great after you get that motor up some. raising that motor will increase wot rpms , gain mph, better fuel effiecency, better hole shot. lot of people sutup cav to bottom of boat and lose a lot of performance good luck

 

[diamondlangus]

Re: change from 18p cuped to 19p uncupped?

THanks

Tried it today and the 19p prop sucked. 5050 rpm but only topped out at 27mph. Tried an aluminum 15p prop that was cupped and got 5100 to 5150 rpm and top speed was 31.5 mph. I will stick with the 15p.

Next question. To raise the motor what do I shim it with? Just a block of wood?

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

un bolt motor move it up to next set of holes. bracket does'nt have to sit on top of transom mines up 2-3 inches from top of transom. also note use a good marine caulk everytime in holes.

 

[diamondlangus]

Re: change from 18p cuped to 19p uncupped?

I raised the motor up 1.5" today, the cavitation plate is now 3/4" to 1" above the lowest point of the V on the boat. I will try both the SS 18 pitch prop and the 15 pitch alluminum. Both props are cupped.

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

a 19 ran 5050 and 15 ran 5150 even with cup it should be more like 450 not 100 the 15 is probably good at low motor height and the 19 would probably like to be closer to the surface to perform its best. almost bet the 19 will prevail with lifting motor. might even need to go up more than you did. i know it seems like it should be flush with bottom of boat. but trust me that is not the fact.

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

come back with new #'s

 

[hwsiii]

Re: change from 18p cuped to 19p uncupped?

Langus, I ran some numbers for your Real prop Slip and for Prop Power Curves versus your Motor Power Curves, and here are the results.

Real Prop Slip

Motor and Prop Curves

It will be interesting to see what happens now that you moved the motor up 1 1/2".


H

 

[diamondlangus]

Re: change from 18p cuped to 19p uncupped?

Thanks for the info. I am new to all this fine tuning the motor position and prop. What would cause the 18p ss prop to have double the slip of the 15p AL. prop (guess I don't fully understand what the slip is either)? I will be hitting the lake tomorrow so I will post my results.

 

[diamondlangus]

Re: change from 18p cuped to 19p uncupped?

Ok, here are the results. Temp was about 10* warmer, water condition the same. Only difference was I raised the motor on the transom 1.5".

18p Stainless prop 4925 RPM 36.2 mph
15p Aluminum prop 5225 RPM 33.5 mph

The hole shot on both props improved greatly, and it seamed as the the stearn was higher in the water when at cruising speed.

So it looks like since both props are in the recommended range I just use what ever one I want based on what I am doing.

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

that 19 is going to be more efficient. i'd try going up alittle more w/ the motor. youll know when its to high

 

[diamondlangus]

Re: change from 18p cuped to 19p uncupped?

I thought the higher you go with the motor the more need for a cupped prop. My 19p prop isn't cupped and has a different (more rounded) bladed design.

I was deffinantly thinking of raising it one more hole.

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

generally yes cupped props like to be rally close to surface, but if i was you id go up more untill u get blow out then go back down this will give max perfomance

 

[diamondlangus]

Re: change from 18p cuped to 19p uncupped?

I think I am going to order a manual jack plate that has 6" of set back and 4" of vertical movement. Then I won't have to mess with unbolting the motor to get my adjustments, plus I can fine tune it better.

 

[junior1113]

Re: change from 18p cuped to 19p uncupped?

I think I am going to order a manual jack plate that has 6" of set back and 4" of vertical movement. Then I won't have to mess with unbolting the motor to get my adjustments, plus I can fine tune it better.

took the words out of my mouth youll appreciate that jack plate

 

[hwsiii]

Re: change from 18p cuped to 19p uncupped?

Langus, you improved your Prop Slip Numbers very well, they improved from 20% on the 18SS to 11% and from 11% to 7% on the 15" cupped. In my opinion moving the motor any higher is not going to change the numbers dramatically, but if you want every last MPH go for it.


Real World Prop Slip

H