Why Not Run Bravo as Surface Drive?
 

I see outboard and even tunnel boat I/O people raise the X-dimension so high, they are essential running their drives as surface drives for extra top end speed. They couple this with a Speedmaster type lower to lower drag and give the prop better flow. I rarely see that done for offshore, though. Anyone have any idea why not?

Michael

The Bravo was never designed to be a surface drive. It can't handle coming out the water constantly. Also, props built for Bravos are not built to be surface piercing. Prop slip goes up, the blades crack and often break off.

"I saw it on the Internet, I'm pretty sure I can do it". :lolhit:

That's a pretty convincing argument you put out there Griff. :eek::eek:

Are the higher hp Mercury outboards are designed to run in a surface piercing state? It seems like it is done a lot.

Michael

What Griff said is correct about the Bravo style drives. My prop is surface piercing and so far I have thrown blades on three different props in three seasons.

Wow - scary. Any damage? Were they all stock props or modified/lab'd?

Do you have a shorter X-dim or do you just launch off waves as much as possible?

There is also the hamonic impact on the gears. The vibrations created when the drive is surfacing travels through the gears as well. On all acounts, it's hard for both prop and drive. When it comes to outboards, the power is often substancially less than on boats running surfaced Bravos. Which kind of prop also plays a factor. When you see outboards running completely surfaced, they run either Cleavers, Choppers ect. that are build for it. When all is said and done, outboards also has their problems with making the lowers last in larger surface installations. Just ask the guys running twins to quads on 25-35 foot cats.

I just got an email from a guy who is running his Imco SC (XR equivalent) with the prop shaft 2" below the hull line on his 21' Daytona, using a Bravo 1 prop. He has a friend who is 1/2" below hull line, with the same prop. Both report no problems so far. I keep wondering how they are getting away with this. One is running 670 hp at over 119, and the other almost 800 at over 130. They do run in calm water, and, of course, the boats are light, comparatively speaking to your typical offshore V-hull.

Michael

I gonna try this in a few months whit 560hp in front of it

http://i1007.photobucket.com/albums/...122800x600.jpg
http://i1007.photobucket.com/albums/...1/Foto0121.jpg

Prop shaft is above the bottom it works very well on the classic donzi's

with 560hp,you'll still have to be ''polite'' ,as those were designed for 400-450hp max...good luck

You kinda hit the head of the nail yourself. "So far" is the key word, and 21' cats are still easier than big heavy offshore vees and cats to move. Especially the one running a high x and 800 hp is on borrowed time. 2 inches below will still create problems down the line, but it's at the same time a height where you go from submerged to surfacing props. Now if they ran 100% surface ie. 1-3 inches above like you do with SSM, ASD, BPM ect. drives on cats, I'd bet you anything both prop and drive would be done by now.

you're right ,and I will !! And if it works on my hull I convert the second one I have in to XR by Max workx

so it will last ,I no one whit 800 hp on it whit there mods :party-smiley-004:

Keep spare parts on hand, 800hp tears XRs apart like nobodys business. The gears in the Max Works, IMCO SC, Teague drive ect. are still just stock XR gears, nothing more.

I have a short X, right about 3" below the pad. I always run labbed props, i know they are ticking time bombs, but they just run so much better. I have a 725-750hp 598ci N/A. I do get alot of air time, but am always careful on reentry.

The props always crack and then tear from the trailing edge of the blade, midway down. I have had one crack, one with a blade completely gone, and one with a blade hanging off. no damage to the drive other than a small knick where the blade hit the cavitation plate.

Philm, good information about the prop failures. I was wondering what the failure modes were.

Now you guys are going to be jealous. I just got an email from a friend with a 21' Daytona with 682 hp. He said he's had no drive failures (Imco SC), and no prop failures even though the prop shaft is only 2" below the hull. His buddy with 800 hp, and prop shaft only 1/2" below the hull has been running the same prop for 9 years! They also do full throttle hole shots.

As an engineer, I have been trying to figure out what the magic formula is. The only thing I could come up with is 1) light boat so not heavily loaded most of the time, and 2) prop rarely leaves the water (they only do lake and delta boating).

If anyone else has some theories, I'd love to hear them.

Michael

The magic formula is the amount of/lack of weight its pushing and the smooth water.

I think my freind who owned a 42 Fountain poker run with a very high x dim and stock 525efi's threw blades off 2 or 3 props in less than 50 hrs.

Thry also have been seen behind a few Cig 20's with standoff boxes and they seem to run real well in those, whats the Hull your rigging with the BH drive?

And the ability to throttle correctly

Your probably gonna want a set of these for that :D

It s a 21 ft v hull from the UK called phantom 21 normally a outboard which I converted to a inboard whit a stroked ls 2
waiting to get in soon . here a few pics ,

http://i1007.photobucket.com/albums/...1/IMG_0078.jpg

http://i1007.photobucket.com/albums/...1/IMG_0181.jpg

http://i1007.photobucket.com/albums/...1/IMG_0137.jpg

http://i1007.photobucket.com/albums/...1/IMG_0010.jpg

:eekdrop:

Wow that looks good where can I get these !!! most cost a fortune ,what are the specs on those ?

Ran one on my 25 Daytona with 570hp infront of it for 160 hrs.

The one thing that will kill them is heavy loading when planing off. If it lugs real bad, the torque kills the vert shaft. Trust me on that one.

I ended up running an airation tube to stop that issue.

If you don't power slam the hole shots and the drive is build correctly with good attention paid to back lash, it can live behind that motor.

got any more pics of the engine mounted up?

any specs on the LS2?

Who made those headers?

Why point them forward vs right out the rear ?

I honestly dont remember....another member (Cord) posted those up....i think he may have had them on his boat....PM him

it' s build by a friend of mine who is building these engines for many years for the european gt3 class car races ,it stroked to 406 whit a forged rotation assembly he converted it to dry sump whit ls7 parts and we used L92 heads ,don't know the cam specs .but we updated the springs ,ecm and wiring is mast motorsport .

The headers are lightning 's we dynoed it whit the cmi's but they have a very poor collector and bought the lightnings later for a good price we think to found another 20 hp whit them .

they point forward s because the transom is angled and they won't fit then and the engine like some exhaust length and makes more tq whit narrower pipes .

thx wally will shoot him a pm !

drives
 

Whew, glad to see it wasn't your Hustler!

Me too THE bh Will not survive THE diesel power !!!!:lolhit:

This my toy when I m alone whitout wife and kids !

Just empty your mind by going fast !

bor, does that hull have natural bow lift? You wil find the BH drive does nothing but shoot a higher rooster tale when you trim up. It takes forward speed and a natural bow lifting hull to make that drive work properly.

Bor

Have you ran that exhaust yet? From the pics it looks like it runs uphill??

Not very much but hoping that the power and big props will lift it and the hull is very light bare hull weights 800lbs otherwise I have a new bravo 1 X for it but then I probably need a -3" shorty .

Not yet ,the turns are going up yeah ,but there is a drain in the lowest point so I can drain all water out and the pipes are dry to the end due to reversion ,I will dump cool water above the gimbal

Why did Mercury discontinue the Black Hawk drives? It actually seemed like a good idea, with half the load to the prop blades, and counter-rotation, so no stern crawl.

Michael

What I heard was that the production costs where to high and the hull must be adjusted to the carastics of the drive .

Was a really good idea, just didn't handle much power. Props were a bit difficult to come by as well from what I've heard. I have a friend that has one sitting in storage, I can't tell you how many times I've thought about trying it out just to see how it handles on my boat... Just seemed a waste to destroy a perfectly good drive!

cost period. cost more to make, buy and prop. Didn't work well on many hulls they tried it on because they need natural bow lift. Somewhat tricky around docks in single applications too.

Actually, I believe those props were on a Donzi Blackhawk 22, that or maybe a Cig 20. Unfortunately, neither was my boat. I have a bunch of photos, all labeled "bniedospial".

These were my props. They were custom made by Hoss. I think the line of props were called "Hyper Drive". The idea was to use a chopper style prop so there was a bit more bow lift. Notice the solid hub.

Now for a little prop porn. Having been out of boats for a number of years, I sold everything last year so I could buy a atv for my wife. The atv has been a ton of fun, more fun than the props brought me sitting on the shelf.

Anyways, you are looking at Merc 27, 29 and 31" props. The Hoss props are on the right. The buyer also took the drive. I later found another set of 31's that I didn't take a picture of.

This is a set up sheet that Merc put out when the drive was introduced. Thought it might be helpful to somebody setting up their boat.