Merc vs Volvo Outdrives

[FAST FRED]

Have been contemplating a new build , where both speed and economy are requrements.

The designer claims that with 50 hp 16K , 100hp 20K and 300hp is required for 30K.

Those would be fine numbers IF true.
The choice gas / diesel , I'm now leaning to gas. Both for light weight and first cost.

Fuel cost shouldnt be "that" bad if only 50 hp or so is needed at 16K.

The Crusader 350ci seems a long tested product , and to grab some efficency I would hook it to an drive with dual props.

Merc or Volvo seem to be the only choices . On my past experience with small diesels , I would avoid Volvo because of parts being 1000% overpriced.

But I'm sure looking for feedback from folks with outdrive experience.

The boat will be aluminum , 39ft long , 7.5 ft wide , so as to fit into a std shipping container, and weigh 8000 lbs (with lots of work)10,000 if we get lazy.

The drive will need to be removed to get the hull into the box, does that change anything?

Comments?

FAST FRED

[Jango]

According to George Crouch, Speed / HP can be calculated as follows:

K = C ( HP/W )^ 0.5
C = 150 Average runabout, cruise, passenger vessel

K = 150 ( 300 / 10000 ) ^ 0.5
or
K = 150 x .1732

K = 25.98 w. 300 HP @ Prop or 417 flywheel Hp

Prop Hp = .72 x flywheel Hp

Disp. 8000 lb, K = 29.04 ( 300 Prop Hp)

Hope this helps - JohnG (Jango)

[FAST FRED]

C = 150 Average runabout, cruise, passenger vessel

Not sure 39 x 7 is "adverage , but thanks for the formula.


FF

[Jango]

Fred, you may be right about the , C = 150, really difficult to tell. Imperical data developed by Crouch for Basic "run of the mill" bottom configuration and planning characteristics. If anything, I would think "C" could be higher than 150. Take a look at: http://continuouswave.com/whaler/reference/prop2.html
His data is toward the end of the artical.

John

[Willallison]

Fred, in terms of efficiency, the Volvo DP is reputed to be better than the Merc (according to the European boating press at any rate).
You have a couple of other options, depending on fuel type.
Yanmar, for instance, mate their diesels to black drives
Yamaha have just re-entered the market too.

I don't know about the complexity of removing a DP leg. I helped to drop the Bravo 11 off my boat a few times (never at sea than heavens! ) and it wasn't too bad.

[marshmat]

Pretty low power requirements for such a big boat.... pretty efficient hull, I guess.
So you're trying to decide between a Bravo 3 and a Volvo (they would probably spec the DPS for a 350.) Although I see you like contra-rotating props, I wouldn't rule out the Bravo 2 yet either. All are good drives, as far as I know.
Before you get your heart set on the Crusader, take a look at the numerous 5.7 L packages offered by both Merc and Volvo. The Merc 350 Horizon, for instance, might suit you and so far is reported to be living up to its much-hyped reliability. Or Volvo's 5.7/DPS package. The nice thing about those options is that you know everything's there in the box, it works together properly, and the factory warranty covers the whole thing if you ever need it.

[FAST FRED]

In researching drives I find that the prop forward out-drive was offered for sale in the 1960's .

I though Volvo claimed to invent it , not just modify 45 year old tech.

FAST FRED