fbh-velocity

I work on a Sunseeker Thunderhawk 43 with twin 496" 375hp engines.
I have my hands on 2 almost new bravo 3 diesel drive with 1:65 ratio.
My first question is: How will Bravo 3 drive perform on this boat vs Bravo 1 (it came with bravo 1 and twin 454 magnum 365 hp when new in 1990)

Second question: If I can use Bravo 3 with the 1:65 ratio what size props do I need?

Thanks

Finn

fbh-velocity

Nobody knows what size propeller I need for this boat with bravo 3 1.65 ratio (Dieseldrive)?

swan2

Try B Blades they will help you out the best they can with the proper prop you might need.

spectras only

Whoa, that's a big Sunseeker for twin engines.Local guy has one with triples! It's apples and oranges but here it goes.My small boat [ 6000+lbs] has twin 6.2's [ 377cu] spinning B1 26 P props on Bravo1X diesel drives with 1:65 ratio,320hp at the props to the tune of 5000-5100rpms.Wouldn't go higher than 24P counter rotating props,maybe go even lower to 22P since the engine is only 375HP.The boat might struggle getting on plane with larger props .What does a 43 weigh loaded?

fbh-velocity

Can I compare pitch on bravo 3 duoprops with pitch on single props on B1 ?
Lets say a 1:5 bravo 1 drive turning a 24P prop = 1:5 Bravo 3 24P duoprop ?

Boat weight estimated 8.7 ton

spectras only

Better call a merc rep.I think the bravo3 prop set is smaller in diameter and I don't know what the individual pitch for the 3 and four blade props are.Also rake could be different from B1 props.

Raylar

Correspond with Mark (MBAM) here on OSO or give him a call at BAM Marine. He knows the drives and props like the back of his hand and I'm sure he'll get you set up and going in the right direction. BAM probably has everything you need in stock at good pricing too.
"When you need a brain surgeon, go to a brain surgeon" !

Best Regards,
Ray @ Raylar

Mbam

Finn,

Not the best ratio for you but it should work. It might try to stall the engines when you put in gear.

You can plug the numbers into the calculator here

http://www.go-fast.com//Prop_Slip_Calculator.htm

I figure the boat will go 60 or so. At 12% slip @ 5000 rpm you will wind up with 24's

Spectras got it right.

You can also plug in the numbers to compare the B1 to the B3, but use a higher slip number, maybe 16% for a boat like that.

It all starts with knowing how fast the boat will run. There is another calculator on the site that can help with that. How fast do you think it will go?

fbh-velocity

I donīt know for sure how fast it will go but I have read about the same boat with twin 496" HO 425 HP should go 55 mph. I have twin 375 HP so my guess is about 50 mph. I canīt find the spec af drive and props it has from factory.

mcollinstn

B3 will handle much better around the dock, due to the increased BITE of the dual props.

I don't know whether this boat experiences too much slip coming out of the hole, but if it DOES, then the B3 will improve the holeshot/planing performance.

I don't know whether this boat has problems with the props losing bite in the turns, but if so, then the B3 will not lose bite in the turns.

Top speed will likely be less with the B3. Probably not horribly different, but likely slower - unless the B1 was simply not able to hook up well at moderate trim angles - in which case the B3 could bridge the gap and provide identical top speed performance.

As you can see, the benefits of the B3 are all related to "traction". Anywhere the boat could benefit from more traction, you'll see some benefit. Anywhere that the boat already has good enough traction, you will likely see no benefit or even a little less performance than now.

The biggest thing the diesel B3 will give you is stronger drives. 1990 era B1's were time bombs with heavy boats or big hp. If the boat is still somehow running 1990-gen B1 internals, then the diesel B3 will provide much more reliable service on that heavy boat.

But - hit a log - twice as many props to fix.

AND - know what you're buying - examine closely for evidence of corrosion. If the drives are repainted, then you may want to ask plenty of questions. You want to look at the threaded bearing retainer ring and examine it and the threads for corrosion. You want to examine the lube and all; of the normal stuff too.

Finally, make certain that the diesel B3 drive will bolt onto the 1990 spec bellhousings - I think there might be some problems making it fit with the old style gimbal and transom assy.

MC