Blow outs and bad porpoise, how do I fix??
 

So I am finally getting some seat time in the scarab and realize that its time to focus on props!

I ran the boat quite a bit this weekend and even took the time to capture some data to hopefully help with the diagnosis. So here is what I know so far.....

Boat came with a 26 three blades. No idea of the make as they have no part numbers or names on them just the pitch and direction stamped on the hub. Maybe you can tell from the picture?

The boat is a 84 30' scarab with twin 502's and trs drives. The boat weighs about 7500lbs. The below numbers were captured with two people on board and about 90 gallons of fuel. The water conditions were 1 foot chop fresh water

2500 rpm.....30mph
3000 rpm.....41mph
3500 rpm.....47mph
4000 rpm.....55mph
4500 rpm.....61mph
4800 rpm.....66mph

These numbers were all with the trim set at -1 because anything 0 or above will create a porpoise that gets progressively worse until the boat is literally jumping out of the water which is one of the major problems. The second problem is that I have to baby it out of the hole or the starboard prop will blow out.

WOT is 5300

I am hoping to find a prop that can get me in the 75-78mph @ 5300 while at the same time allowing me to actually trim the boat out of the water and not have to baby it so much out of the hole!!

Thoughts??

http://i69.photobucket.com/albums/i4...b/d40593c0.jpg

A good 4 blade will work wonders , call bblades propellers

I would also give a 4 blade prop a try.. Will the boat turn to 5300 with the current props? You will need to go down in pitch when you go to a 4 blade..


John jr

I really thought scarabs like round ear props for bow lift, i dont think choppers are the correct prop. Maybe some 4 blades or mirage plus blades .

I dont see any diffusion rings on those props ? Those would help you get out of the hole better. I also agree on the 4 blade...maybe a 26 Bravo....maybe a 24. I dont think you'll see 75-78 mph (your at 66 mph now @ 4800, and I highly doubt a prop change will get you 12 mph)....but good luck. :drink:

Agreed that 12 mph is a lot but remember that I am seeing 66 with -1 trim. The boat is plowing quite a bit. In addition I did see 73mph at 5300 and 0 trim but it was with a borrowed set of 26 3 blades (round not chopper) but was still having the blow out problem!

Sounds like I should plan to rent a 4 blade 24 from bblades. I remember reading somewhere that you lose or gain a certain rpm depending on if you go up or down with the pitch. Do you know how many rpm you typically gain or lose for each change in pitch?

Depends on the boat...and the setup (motor hp, drives, and if the props have been worked on), but usually between 200-300 rpm increase as you go down each number in pitch (in general). Obviously, rpm will DECREASE as you go up in pitch. :drink:

If your saying it is plowing, doesn't respond to trim, then a bravo 4 blade is what you are looking for.. They are a bow lifting prop.. Def talk to bblades or throttle up.. I have saw 7 mph from swapping to a diff prob on my stinger...

John jr

Our boats are similar setups. I had porpoising problems when I ran cleaver style blades, Hydromotive 4 blades. It would try to carry the bow and then it would fall. This turned into bad bow steering and it felt like a death trap. I also had serious issues with props blowing out and could not get on plane with my 3 blade mirages when I first bought my boat. All of the cupping was washed out of my props. My best prop results were adding cupping to my 3 blade mirages (or a Mirage+) or my 2 different style Spinelli's. Round ear blades. I would put on a 25 Mirage+ or a try a 26 Bravo1.