2-250HP Yamahas vs 330HP Volvo I/O diesel

Good morning,I'm building a new 34 ft prawn boat should be 7,000 lbs and at least 12,000 lbs. loaded would anyone know how my performance will be powering with 2-250HP Yamaha's vs a 330 HP Volvo I/O diesel.My comrades continue to power their commercial boats with Volvos and continually have issues I'm inclined to use outboards as long as they will give me performance the test is having sufficient thrust/torque to get up on a plane with loads.I see water taxis are believers of Volvo's but maybe they are considering fuel/hrs of use(diesel)considerations.Any experienced opinions are appreciated as this decision re power choice is very important in my competitive business.
Dan

Diesel I/O is going to be more efficient on fuel. Trouble is that at the 300hp+ rating the options for diesel I/O are very limited. Either the D6 Volvo, the V8 Yanmar on the Yanmar leg, or if your emission standards allow the 6LP Yanmar @ 315hp on a Merc leg. The Merc leg isn't the best option, so the V8 Yanmar on the still not really proven Yanmar leg or the Volvo 'Green Grenade' on the more proven Volvo leg. I don't consider the CMD diesel at 300hp+ to be a commercial option.
For outboards you quote the Yamahas, but personally I would look at the Honda 225's if you can. Set up properly they are very robust engines. Yamaha has not had the greatest design success with it's 4 strokes and have had possibly more issues than any of the other large manufacturers. They still enjoy a good reputation, but much of that comes from the fantastic 2 strokes they produced previously. There is no doubt that your fuel use will increase with the outboards, but they will provide plenty of power compared to the single I/O.

Quoting your comment"they will provide plenty of power compared to the single I/O" this is where my confusion is, the D6 Volvo turns a much bigger prop(or duo drop) and has the added torque with diesel vs gas on the outboards.Aliboy are you sure I will have more torque/power/thrust with the 2 250 Outboards this is the crux of the matter will there be less stress on this power package pushing our heavy prawn boats.Your input is appreciated as I have to have my facts right as this will be an expensive build.

The D6 370 makes 300hp @ 2400rpm and that is around the peak torque for that engine. Whilst they don't publish curves for most outboards it seems that a typical 4 stroke will make around 60% power in the 3000 - 3500rpm range. For a pair of 250's that is also 300hp, although with higher rpm's and less torque. The smaller props compensate for the lower torque. Both sets of engines give you similar hp at good cruising rpm's. If you go Honda or Suzuki you get better gear ratios than Yamaha and can swing larger props which would help you. The Yamaha however is a 4.2l engine vs the 3.6l Honda and Suzuki, so maybe slightly better on torque down low with the extra displacement.

To compare probable cruise powers, lets say you cruise the D6 at 2900rpm and the outboards at 4000rpm. Both reasonable numbers for looking after your engine loads. The D6 will make around 350hp and the 4 strokes around 75% (maybe more) of max hp, so around 375hp+. So once you are up and going there won't be much in it for performance, with a slight edge for the outboards. If you go for outboards the biggest thing will be to not over-prop them. Prop them for good WOT rpm's when fully loaded. If you did over-prop to get extra speed, that is where the difference in torque might hurt you. Do have a good think about the outboard gear ratios though as the Yamaha will swing a smaller diameter prop than the Honda or Zuke and that may be important.

Aliboy your comments are good stuff.I have paid a high price in the past for using too large propellers(2 blown blocks stupid me).Must use small propellers the top end speed not as important as being kind to motors when going to a plane under load.I've had discussions re gear ratios with Yamaha am inclined to stick with the 3.3L250 have a saving with mechanical controls yes but more importantly they have a gear ratio of 2:1 my Saltwater series had a ratio of 1.83:1 and the 4.2 has a ratio of 1.75:1 I think these figures are close if memory serves me right,with that said the Yamaha dealer felt the larger cylinders on the 4.2 would more than compensate for higher gear I don't really buy this completely these new engine designs have to prove themselves.Kind of like 2:1 with small propellers.We don't have many service outlets for Suzuki and Honda here in Nanaimo BC Yamaha is the motor of choice in this area with that said my comrade runs300 HP Mercurys on his charter boat and is quite happy(I may do some homework on these but am hesitant).On the boat weight issue I understand some Grady Whites weighing around 12,000 lbs are powered by twin 250's.My thoughts are do my aluminum build at 7,000lbs with a loaded weight of no more than 12,000lbs.If I couldn't keep within this weight I'd have to rethink my project.What are your thoughts re my weights.

Your memory is correct I think re the ratios. 7000lbs for 34ft is probably doable, but you will need to be careful how much extra gear you add. I guess 6mm or 7mm plate for the bottom with a good stringer system and 5mm (sorry but I think in metric) on the sides with reinforcing in the work areas.