Hydraulic Outboard Conversion????

[Ron342]

Hey Guys<br />I'm planning on doing a hydraulic drive outboard conversion by putting a hydraulic motor on a blown 200 Hp Yamaha OB in place of the powerhead and driving it with a pair of<br />Mercedes 300D diesels - 1 under each washboard and each driving a hyd. pump. These are about 120 hp engines and are reliable and now cheap. The boat is based on a 28'<br />Silverton hull set up as a fishing boat.<br />Sure don't want to reinvent the wheel though - anyone ever done or seen this done? <br />Or even converted these engines to marine??<br />Thanks - Ron

 

[Forktail]

Re: Hydraulic Outboard Conversion????

You've got a 28' boat, access to two 300 hp Mercedes diesels, and you want to drive an outboard lower unit? Doesn't make sense. <br /><br />First, the 200 hp Yamaha engine is designed to run about 5500 rpm. This means about 2960 rpm at the prop using a 1.86 ratio at the lower unit. So the outboard's drivetrain was designed to handle about 190 ftlbs of torque from the engine, and about 350 ftlbs at the prop shaft.<br /><br />I suspect each 300 hp diesel will put out about 500 ftlbs of torque. And I suspect that maximum rpm will be under 3,000 for the diesel.<br /><br />So you've got to deal with over-torque, under-rpm, and a gearing nightmare to get the prop to push the boat right. In other words, the outboard is limited in what it can handle in torque and prop size. You're basically trying to fire a cannon out of a canoe.<br /><br />Then you've got to consider how you're going to cool the diesels, cool the hydraulic pump(s), and reliably mount the unit on the outboard.<br /><br />I suppose it could be done, but it doesn't seem practical or reliable. I think you've been watching too much "Junkyard Wars".

 

[JB]

Re: Hydraulic Outboard Conversion????

Hi, Forktail. <br /><br />The Mercedes 300D engines are 3 liters and make about 120HP each. The 300TD is turbocharged and more like 160HP.<br /><br />Ron.<br /><br />Sounds like an interesting experiment, but I wouldn't call the engines cheap.<br /><br />Good luck, Ron.

 

[Ron342]

Re: Hydraulic Outboard Conversion????

Thanks JB & Forktail<br /><br />The difference in lower unit and the diesels rpm & torque are not hard to mate as you can just size the 2 diesel engine pumps to deliver together enough gals per rev. to make the hyd motor on the outboard shaft turn 5500 rpm when the diesels were at thier max power rpm.<br /><br />The thing I don't know anything about is how the 2 engines will pump in parallel to the motor.<br /><br />JB -I didn't know the 300TD were 160 hp - I have a chance to buy 2 of those engines for $1250 total from a tow operator with about 150,000 miles and thought it would be a good way to get cheap diesel power, mount the engines out of the way, have a single prop on a stern bracket and be able to up the whole drive unit out of the water all at one time. Some of the hyd pumps & motors are available as military aircraft surplus and are lite and inexpensive too.<br /><br />Looks like I need a 275 or 300 Hp lower unit though which wouldn't be a bad idea anyway since it would probably give more room to fool around with prop diameters. Anybody got one???<br /><br />Maybe I should start this with a 10 hp lower unit and my lawn tractor motor first!<br /><br />Ron

 

[dick]

Re: Hydraulic Outboard Conversion????

Saw a guy at a boat yard fitting a 7.5hp Evinrude lower in the bow of a 40' boat as a thruster, but this sounds intresting.If you get it work please post some pictures.

 

[Hooty]

Re: Hydraulic Outboard Conversion????

Sounds like an interesting project. Have you done a weight/horsepower comparison with, say a 200hp. Mercury or Evinrude?<br /><br />c/6<br /><br />Hooty

 

[russthemuss]

Re: Hydraulic Outboard Conversion????

Im sure the idea is already used in the yachting industry.From what i can remember the hydraulic set up is very noisy.

 

[russthemuss]

Re: Hydraulic Outboard Conversion????

Actualy this is how many bow thrusters are driven.I have fitted one before and to get the pump to spin fast enough you need to gear it with a gilmore belt drive.Yep i remember it sounded like a supercharger when running!Those belts are noisy!!

 

[Ron342]

Re: Hydraulic Outboard Conversion????

Thanks Russ<br /><br />Did the thruster pump run off the main engine?? If so, I guess the main was running just off idle so it would need to be geared up to spin the pump fast enough to make any headway from the thruster.<br /><br />Maybe I should check out the thruster manufactors.<br /><br />Ron

 

[russthemuss]

Re: Hydraulic Outboard Conversion????

Yea driven straight of the crank of one of the drive engines (200hp yanma turbo diesel).Engine was left in neutral so you could rev it to around 4000rpm.Electric clutch.<br />We were restricted with the room to fit it in so didn't have many pump options,like you have.<br />Yep i just did a quick search myself and i also remember the suckers are mighty expensive!!! If you can do it from surplus bits and pieces good luck.I think forktail hit the nail on the head cause its going to be tough to find the right combination to give the torque and speed you require...without it working like a on off switch.<br />If you get it to work let us know!!