Bad economy/speed limit=bad news for lake businesses

Woodsy · 04-14-2009, 02:36 PM

Quote:

Originally Posted by Dave R

I have a 25 foot boat, with a single-stepped hull, 24 degrees of deadrise at the transom, and terrible weight distribution due to the big block engine option. It's obviously a pretty bad combination for slow speed planing, but it'll easily plane out at 19 MPH with the tabs down, and 24 MPH (speeds measured with GPS, not the OEM speedometer) with the tabs retracted. I have a Bravo 3 drive, so I suppose that helps a lot, but I find it hard to believe there's many boats out there that really won't plane at 25 MPH.

Mine does get substantially better gas mileage from 28 MPH to 40 MPH compared to 25 MPH though (as measured with a Lowrance LMF-200). Literally, 40% better.

Dave...

That Bravo 3 Dual prop drive is the key.... it pushes alot of water very efficiently at lower speeds.... nothing pops a boat on plane better or quicker than the Merc Bravo 3 or Volvo Duoprop setup...

But like everything... there is a tradeoff to be made!

The dual props become less efficient at higher speeds... thats alot of blade area to spin and it requires sustantially more HP to spin the faster it goes... and water does not compress. All things being equal thats why the huge difference in gasoline consumption.

A high performance boat such as my 26' Donzi is setup (weighted, balanced, propped) to go fast... 67-69 MPH. But like everything else there is a tradeoff. The boat doesnt like 25 MPH... even with the tabs down! The propeller is cavitating, the stern squatting and the nose is high.. and the wake rivals a Carver! I am sure its compounded on the larger heavier high performance boats.

Its funny but everything settles down smoothly at 30-32 MPH. You wouldnt think 5-6 MPH would make a big difference but it does.

For the record.. I seriously doubt the MP is going to pull you over for going 30 MPH... but you never know.

Woodsy

Dave R · 04-15-2009, 10:41 AM

Quote:

Originally Posted by Woodsy

Dave...

That Bravo 3 Dual prop drive is the key.... it pushes alot of water very efficiently at lower speeds.... nothing pops a boat on plane better or quicker than the Merc Bravo 3 or Volvo Duoprop setup...

But like everything... there is a tradeoff to be made!

The dual props become less efficient at higher speeds... thats alot of blade area to spin and it requires sustantially more HP to spin the faster it goes... and water does not compress. All things being equal thats why the huge difference in gasoline consumption.

A high performance boat such as my 26' Donzi is setup (weighted, balanced, propped) to go fast... 67-69 MPH. But like everything else there is a tradeoff. The boat doesnt like 25 MPH... even with the tabs down! The propeller is cavitating, the stern squatting and the nose is high.. and the wake rivals a Carver! I am sure its compounded on the larger heavier high performance boats.

Its funny but everything settles down smoothly at 30-32 MPH. You wouldnt think 5-6 MPH would make a big difference but it does.

For the record.. I seriously doubt the MP is going to pull you over for going 30 MPH... but you never know.

Woodsy

I knew it helped, but did not realize it helped that much.

You know a B3 lower gearcase bolts right up to a B1 upper gearcase, right? Might be worth picking one up on eBay while the 45/25 law is in effect and you can't really use the top end of the B1. A 1.5:1 ratio B1 upper gearcase would be a 2:1 ratio with a B3 lower bolted on, if memory serves. The swap takes mere minutes.

Woodsy · 04-15-2009, 10:44 AM

Dave...

I have been looking for a deal on a B3 lower for awhile.... but they dont give them (or the propellers) away! LOL!

Woodsy

Dave R · 04-15-2009, 01:57 PM

Quote:

Originally Posted by Woodsy

Dave...

I have been looking for a deal on a B3 lower for awhile.... but they dont give them (or the propellers) away! LOL!

Woodsy

Yeah, they are not cheap, but if top speed is not a priority, a nice upgrade. You can get deals now and then on craigslist.

My boat model is actually faster with a small block and a B1, but I like longevity and effortless cruising RPM of the big block, and the control of the B3

04-15-2009, 10:44 AM	#3
Woodsy Senior Member Join Date: May 2004 Location: Weirs Beach Posts: 1,966 Thanks: 80 Thanked 980 Times in 440 Posts	Dave... I have been looking for a deal on a B3 lower for awhile.... but they dont give them (or the propellers) away! LOL! Woodsy __________________ The only way to eliminate ignorant behavior is through education. You can't fix stupid.