another prop slip question
 

new boat to me,38 scarab avs. 3500rpm,1.5 gear,30pitch bravo 4 blade,....ready???? 47 mph. my calculation comes out to 29% slip. is this right? also the seller had even worse slip with 26pitch bravo. 2"shorties with 2"higher engine/drive rigging. the boat cavitates badly to get on plane.....Max rpm about 5100 @83 about 13% slip.would you start with a diff prop or 1"spacer?Main concern is to help it plane,not top speed...... well dont want to lose to much speed but...... thanks

Prop slip is typically in that range at midrange cruise because you still have so much hull in the water. A slip number of 13% at wot is pretty good. But since you dont care about the top end speed... You could most likely reduce this midrange number, and get on plane easier by either putting in a one inch spacer, or going to a five blade prop.... or both.

Chris

will a 5 blade reduce top end speed? what 5 blade props will work good on a Scarab?

Not if your only getting 47mph. Plus, it will plane easier with the 5 blade.




Darrell.

You would think that Scarab's AVS setup wouldnt be that far off to have someone raise the X-demension 4". Spacers might be cheaper then 5 blades?? Plus I'm wondering if that high of a drive height is going to cause a handleing issue, especially in turns.

As far as props go, I tried a set of Maximus's from Bravo's, and I picked up at least 5mph in the cruising RPM(3000 to 4200). I did loose a couple MPH at WOT though. The boat planed ok with the Bravo's, but really hooked with the 5 blades. The 5 blades are pretty pricey as well..

LE

Before putting shorties on, my Black Thunder went 50mph on gps with 26 Bravos at 3500. After putting on the -2 shorties, it went 47. Your speed numbers are awful. Not only in the midrange but top end isn't so great either. I would try spacing the drives back down some and also 26 pitch and 28 pitch P5x's.

First of all, I have a 1994 38 foot Cig, Non-Step.
Weight is 11500lbs plus fuel, cargo, etc.

I have 2 28 Pitch Maximus Props for sale right now in the Classified Section.

You might find this information helpful.

I have 2 4 Blade 26 Pitch Hydromotive props which created a 28% slip at 4100rpm.
I went to the 28 Pitch Maximus and now I have 13% Slip at 3100rpm
I am selling my 28 Pitch because I have too much prop.
I need 26 Pitch Maximus that are labbed.

Adding one blade decreases your MAX RPM by 400.
Adding 1 inch pitch decreases about 150-200rpm.

So, you might want to go with a Maximus and decrease pitch and maybe get them labbed to increase top end if you desire to do so.

I thought 83 was pretty good for a scarab w/500's??? It seems that everyone here has a different opinion about what slip should be. also ever boat will be different. Since the boat is new to me I will do some investigating in the spring. I think I will start with 1" spacers.

83 is pretty good for a 38 with 500's. I think it is right in the ball park. I would give the spacers a shot first. A 2" raised X coupled with -2" shorties seems pretty agressive. Not surprised that planning is an issue.

My apologies, I misread your first post. I didn't see you were getting 83 out of her.( good numbers) I still feel the 5 blades will help out a BUNCH in the midrange cruise department, and shouldn't mess with the top speed too much.



Darrell.


darrell.