4 blade vs 5 blade
 

ok im sure this has been discussed before but please give me any input you have....i have a 38 scarab kv with 1.50 drives 32 bravo props and extension boxes....im looking to increase my cruising speed....any do and don'ts about 5 blade opposed to 4 blades....what size 5 blades do i start with if i decide to go that route....

32" Maximus.

I'm not sure this will help but I'll give you the results for my 38 Fountain with 525's staggard. The boat has 1.50 gears and standard Bravo drives and was delivered with 32P Bravos. The boat ran 91.3 best every and at 4000 RPM it ran 52MPH. I went to 30 PX5 Hydromotive props and at 4000RPM's I now run 62MPH but top end dropped to 89MPH. I like the cruise speed and all though I wish top end was better I'll take the cruise increase all day long. It is important to note that the 91.3MPH top speed was when the boat was new and it now has close to 400 hours on it so performance may be down on the engines some. Also, the boat accelerates much better with the 5 blade Hydros.

Hope that help some.

Oh, one more thing you might want to know...with the 32P Bravos my top RPM was 5400+..meaning every now and then I would bump the rev limiter. Now with the 30P Hydros my top RPM is 5300-5400 but have not ever hit the rev limiter.

sounds good...its spinning the 32's on the rev limiters at 5800 so i dont know how much more it can turn them....the boat has 540/800hp blower motors.....whats up with all the talk about the 5blades eat drives....any truth to that??

seadated....what r's do you turn on top end? how many peeps and fuel do you usually have in the boat? thanks

MrSeakAttack

The boat turns very well at speed but I have never made a sharp turn at full speed. I do make ajustments for trafic, channel requirments, etc at high speed with no jumping/hopping it just make nice smooth turns.

I would say 90% of the time I have 3/4 to full tanks and four "peeps".

FYI...my boat is a 2004 and was sold as staggered engine but now is considered a 3/4 staggered bacause after 2005/6 they went full staggered. See pic.

http://www.offshoreonly.com/forums/a...38-engines.jpg

I have heard all about 5 blades eating drive too but really a lot of it comes down to the driver. If the props are hooked up tight and you slam the throttles fwd all that torque is tough on the drives. If your drives are a little loose and you slam the throttles foward you'll get a bunch of RPM's but it will take a while for the boat to gain speed like mine...not to hard on the drives. Also, coming out of the hole is the critical point...ease it up = longivity. My .02

nice set up you have.....thanks for the input...i'll keep you posted

WOW your slip numbers are off the scale with 4 blades at cruise-35%, I thought my boat had alot of slip turning 4000 rpm's at 58-60 with a 30 pitch and 1.50 gears, yours was definately a candidate for 5 blades. As far as your boat eating drives, with only 525's that shouldn't be a big issue even driving hard.
Now, as far as Mr sneak attack, with 800 hp hi tq blower motors it might be a different story, heavy boat, fairly high power, 5 blades, low slip= twisted off vert or prop shaft no matter how good he is on the sticks, broken shafts are not just a "myth" Smitty
What drives, assuming bravo ones?

Arctic being super knowledgable - and one of the very few guys on this site that really knows this drive perfectly is absolutely right.

I would say sneak attack is in for a set of bills that will make him wince with a 5 blade 800 HP deal.

My Imor 710 in a 5300 lb twin step V (it's loaded like a caddy ) is capable of laying waste to a fully built Bravo in 50 hours with a 4 blade and moderate throttling in the chop and waves on the pacific ocean. To be fair 50 fairly tough hours of boating.

I tried a 27 p5 x and was thrilled with its cruise and bite, but 5 blade would have eaten it even quicker

With a 4 blade Lake lifespans are markedly better like triple Im at 100 hours with no shavings so far with 80/20 Lakes this year.

Uncle Dave

artic and uncle dave

i spend most of the time on a river and small lake.....i will take the boat out in 2 weeks to get real numbers at cruising but it seems to me its gonna be in the 55mph range at 4000rpms....so my slippage is high i guess because of my extension boxes therefore i was leaning for 5 blade props but i dont want to be changing shafts and gears everyother time out also...i may get my current props reworked and go from there but at the same time i could put that money towards a set of 5 blades...break out another thousand :lolhit:

What has been done to your drives so far? If your slippage is too high you might need to buy 5 blades, in my opinion it sorta depends on what your slip numbers are afterwards too. Back when my boat only had 950 hp,I tried a 28 pitch 5 blade Maximus vs the 30/32 Bravos I usually ran, when I took it out to test it, damn, my slip went from its normal 18-22% to 8%-11% and after ten passes / 30 minutes I sheared off a billet Max Worx prop shaft. Now, it does depend on how you drive like Seadated mentions except in your case you do have 800 hp/and I'm sure just as much tq so even if you take it "easy" unless you don't ever hold throttle wide open you will put more strain on the drive with lower slip numbers. Now if your slip is at 30% and it ends up at 20% it might not be as much of a issue vs 10%. Have you considered drive spacers? Did you install the boxes, did you have a gain vs without them, Smitty

Isnt 18-22% alot even for a 4 blade?

Single engined...v-hull...stepped hull...tend to be fastest around the high teens for slip.

I have done alot of testing on my Nordic...it does a consistant 105...and slip is around 19% at full speed.

Arctic friends has done a lot of testing...and his slip is right there.

The Howard Bullet that did nearly 130 mph+/-, and is still the fastest single engined v-hull that I know of ....had slip numbers around 21%.

I am the fastest with a merc lab prop...and use a five blade (p5-x) for pulling my kids on an inner-tube.

The prop I most normally use is just a standard bravo....it hooks up better in the low and mid-range than the lab prop.

Chris

Yes, it is for a top end slip number. Not unusual for cruising rpms. Most props go up in slip when the rpm's drop. All things being equal. Some hull's work better with a higher slip number of fx. 14-16% slip. I recall having a long talk with Julie @ Throttle Up a few years ago about this, when dialing in a friends 38 Lightning. For your hull, it's too high.

I have tried different things that lowered my slip and gained ZERO speed or lost , broke part faster and torque loaded boat. My boat and drive is pretty happy around 20% and can hit 100mph (almost), twin engine might be a different story. The 35% that Sea dated had seems horrible, I can't imagine having 32 pitch props and cruising at 52 mph at 4000 rpm's, that boat was BEGGING for bigger props!! I would still like to hear what Mr sneakattack has for drives, heavy boat with 800 hp, if he can afford it buy beefier drives (scx or Konrad) and find the best prop be it a 5 blade or 6 blade even, Smitty

bravo's...i know ball bust is coming i understand....i had a b max with my cat and it held up good with 900 true horsepower....so future plan is maybe b maxes....

I am carrying the bow better now after adding a spacer. But I cant get good speeds with more slip now at 17% - I assume due to more hull dragging. CB-BLR, your hull is a lot more advanced and shorter (less bow to carry)

I assume a 272 with 900 HP can carry the bow due to so much horsepower at speed.

if you are on the rev limiters all the time I would try a set of 32" p5x if not try 31". the maximus is a much bigger prop and i would start with a 30" if you decide to try those but they really hook up well and may put to much load on your drives.

my boat no step 37 ft center console at 40mph with 26" bravos 20% slip 3100 rpm, with 24" maximus 10% slip at 2950 rpm.

update....i took the props off and brought them to a local prop shop and have found out they have been reworked and were only 30.5 pitch props.....so my slip isnt the issue....im getting them pitched to 33's and hopefully run this weekend and go from there..

Most Bravo props are really 1" lighter in pitch than stamped. If you go to a 5 Blade Maximus, then you really need to drop down to at least 30's.
Your rpms will still most likely decrease by about 200-300 if you go with 30 Maximus props. I would also call BBlades and/or post in the prop section.

I run 32 maximus's, I also own a set of 32 bravo's

My propshafts are 3" below the bottom of the boat,
it has Stellings boxes.


Cruise speed, at 3500 rpms the max's run 55 mph, at 3800 rpms the the bravo's run 52 mph.


The fastest I've had the max's was 94 @ 5500 on the perfect
day, ( normal average is 88-92 )the best I had the bravo's run was 89 @ 5800 ( rev limiter)

The bravo's just dont work that well on my boat.

And yes, I've broken my share of drives...but I've witnessed
the same junk XR upper gearsets break behind 500's and
4 blades.

i run the 34 MAXIMUS FOR CRUISING. 3500 RPM 62 MPH. Poker runs etc, i switch to 32 Maximus 3500 RPM 55 MPH. These are large props and both sets need the diffuser rings cut way back. With the rings i had to much transom lift. I tried 32 bravo's and had a hell of time getting on plane. Maybe because of Imco boxes and -2 shorties on SCX's. 32 maximus 102 MPH 5400