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Switch to diesel?

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sharkman

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Hi, was wondering if anyone out there can help or direct to to somewhere that may be able to. Have an 8meter sea ray flybridge with twin 225hp mercruisers in it goes really well but as I,m getting older I don,t feel the need for speed or fuel bills, so thinking of replacing with single diesel and leg. I guess the boat would weigh about 2.5 ton? question is what sort of horsepower would be needed to do say 18 to 20kt cruise? many thanks in advance, cheers.
 
By "leg: do you mean and outdrive as in an inboard/outboard set up? Or are you talking about a full inboard?

I'm no expert on this topic, but I think something around 200 HP with bigger prop(s) would do the trick. I've seen several setups using Volvo 185 HP 4 cylinder diesel matched to a Volvo duo-prop Alpha outdrive. A close friend has that setup and a 24 ft Whaler which probably weighs about the same as your boat, and it is a very sweet, economical setup. However, here in the States, Volvo parts are very expensive.

My biggest concern would be weight distribution. You'll need to pay special attention to that.
 
chawk_man said:
By "leg: do you mean and outdrive as in an inboard/outboard set up? Or are you talking about a full inboard?

I'm no expert on this topic, but I think something around 200 HP with bigger prop(s) would do the trick. I've seen several setups using Volvo 185 HP 4 cylinder diesel matched to a Volvo duo-prop Alpha outdrive. A close friend has that setup and a 24 ft Whaler which probably weighs about the same as your boat, and it is a very sweet, economical setup. However, here in the States, Volvo parts are very expensive.

My biggest concern would be weight distribution. You'll need to pay special attention to that.
Click to expand...

Thanks very much for reply. Yes running thru outdrive. have thought about weight, have a fair bit of room in front of present set up, thinking of mounting motor for best weight distribution and running a jack shaft to outdrive. cheers.
 
Now you are way over my head - never seen a jack shaft run to an outdrive. Hope you or your mates are pretty good mechanical engineers. I guess it could be done, but you are in for some serious fabrication - shaft stabilization, transmission covers, cooling water routing, etc, etc.
 
Hi Bob, most of setup work would be done by friends & myself. tight budget for motor (used) about $10,000 aud
Boat builder I knew years ago had a sweet setup in a 24 foot center console, center mounted 3208 cat under console running shaft to outdrive leg no worries, thank you for the reply mate, cheers.
 
G'day Sharkman,

An interesting project, it'll have plenty of torque. What stern-drive do you have in mind? I haven't been back to OZ since 2007, so I am not too sure of the current fuel prices, but if fuel costs are one of the issues, it will take many a nautical mile to defer the 10K AUD (say 2K+ Gallons) before the project really begins to pay off.

Good on ya mate, if it can be done, an Aussie can do it a bit better.
 
Racerone - thanks for the info. I've just never seen one. How is the shaft stabilized? How do they terminate the shaft to the out drive? Is there some sort of fabricated shield that covers the part of the out drive that normally mates to the back of the engine?
 
Here are a couple of things to think about :
- To match to a stern drive you will either need to keep the hp low or the rpm's high. Even the relatively tough Bravo 2x won't reliably handle the torque of a diesel turning sub ~3200rpm and making 250hp+. Matching slow turning, high torque Cats etc to stern drives is going to give you problems unless you go to a spe******t high torque drive like a Konrad or similar. Using an alpha drive on a diesel for your purposes would be a disaster.
- Since you are looking at putting the engine forward in the hull, a V drive and shaft would be a little more work, but cuts maintenance to almost nothing when compared to a stern drive with it's 100hr oil changes and the yard lifts etc this requires. This way you can also use a larger capacity, slower turning diesel that will hugely outlast many of the higher revving, lower capacity engines usually mated to stern drives.
- You will need to modify your fuel tank(s) to take a diesel return line. This may be easy, or may be a major.
 
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