Rambocj7

I'm looking for options to run a different prop on my boat. I feel the RPMS are a little low.
Its a 36' boat with twin 775HP 540s on SSM3s.

I'm running 86 mph GPS at 4800 rpms. Cruise is 50 GPS at 3000 RPMs.
I feel the RPMS are a little low (limiters are set at 5500) at WOT.

Wondering if a 3 blade will make a difference? I think originally the boat ran with a 3 blade on NA 540s. 80 mph in salt water.

I dont really want to go to the new style 26 spline props ($$$), but looking to try a used set of 19 spline. I just dont want to be way off in picking a set.

Boat needs tabs to plane and will blow out occasionally.

Dont know if this video will work, but this is the actual boat when it was rigged from the factory.

https://www.facebook.com/#!/video.php?v=2240647653076

bbladesprops

Couldn't see video.
What hull is it?
19 spline props are tough to find now. They are out there but it will take some leg work to find them.
Your slip % looks good. You just need to find some 29's ish in 3 or 4 blades with big diameter that can be cut to the diameter the boat wants.

Brett

Rambocj7

Any more opinions?

I just tried some 16.5" x 29P 4 blades and they slip like crazy!! I really need large diameter, so 17 3/4.

MILD THUNDER

iTrader: (3)

There is more to "slipping" than just diameter. I've ran smaller diameter SSM props that hooked up better than some larger diameter props. I've found with the old SSM props, most of that is in the "cup" of the blades. If there isn't enough "cup", all the diameter in the world wont help.

Also, had a buddy who went from 17.5x28 five blades, to 17.5x27 four blades on his #4 drives. . His boat took FOREVER to plane with the 5 blades. 4500RPM burnouts and wait. Everyone kept telling him he would "never" get on plane with a 4 blade.

But, he listened to his friend mild thunder whos been down that road before. So, he tried those stock merc racing labbed 17.5x28 four blades, boat planed at barely 3,000 rpm in just a few seconds, and simply runs better all the way around with them.

I've had 3 blade SSM props that planed better than some 4 blade props on my boat with #4's. If the prop can't hold water, it will simply blow out. I've seen some with some pretty severe cavitation burns!

Rambocj7

Ok, what do you suggest?

These are both 4 blade props btw...

With those smaller diameter props, it would start to plane quick, then blow out. Had to be at 3500 rpms with tabs just to keep it on plane. At half throttle, I could hit 5500 rpm, 76 mph with way too much slip.
By comparision, my original 4 blade props, 17 3/4 diameter x 31P, take a while to plane but 'bite' hard when they do. I can stay on plane at 2500 rpms and have a nice cruise.

MILD THUNDER

iTrader: (3)

You may try a different set of 16.5x29 four blades and find they might not slip at all. Most of these old props have been changed over the years

I'd try to find a decent set that haven't been butchered, try them, and if need be send them to a place like bblades for fine tuning. Like bblades said, larger diameter props are a score, because they can be cut down if less diameter works better . but you can't add diameter . I've had several sets of SSM cleavers scanned on a prop scan machine. . they all pretty much were all over the place as far as pitch, cupping, etc . the sets that planed great and didn't take a mile to lay over, all had one thing in common. Cup measurements of .080 or higher on each blade. The ones that blew out like crazy, had cup measurements different from blade to blade, like one blade might have .050 , the other .070, the other .040, etc.

I'm assuming a late 80s 36 bee bottom with #3s, probably doesn't have an overly aggressive x dimension?

bbladesprops

There are many variables that affect planning.
General rules of thumb are that more blades and blade area will allow easier planning. However, bottom configurations, prop rotation, torque curves, propeller condition etc. play into the picture.
5 blades fit into this easier planning model but if the X dimension is too high it wont matter. Remember, more blades and diameter also seek the surface quicker and create more stern lift.
Like MT said above, cupping is very important. But, cupping also affects many other thing like WOT rpm and boat attitude and personality, acceleration and others. We take specific cupping seriously. I would use caution with the numbers being suggested for cup heights because every application is different. There are boats that will plane out fine with less cup than mentioned above and there are boats that will not plane out with twice the cup mentioned above.
Some of the craziest uneven propeller cupping and geometry's have been the best performing propellers some customers have had. We MRI propellers all the time and very rarely do all the numbers match. Even when they just came from the factory. People are surprised when I explain to them how imperfect CNC'd propellers can be.
There are general rules of thumb to use beginning the propping process but each application has to be dealt with on its own merit.

Brett

Rambocj7

I know there are many variables at play, that's why boats are so hard to dial in!

This boat is a 36' V-bottom, same bottom as a Cig Top Gun (splash of it), but 2 feet shorter. The #3 drives are basically on extension boxes and prop shaft is 1.5" below.

I'm putting my original props back on for now (the large ones). I need my cruise back! lol