I am a novice, hydraulically speaking, and was hoping to learn the feasibility of designing a hydraulically powered lift fan for a homebuilt hovercraft. Background- many hovercraft have a front lawnmower engine for the lift fan. Problem is that as the craft gets bigger, the lift engines need to also, sometimes as big as 20-25 HP at 3000-3500 rpm such as from a lawn tractor. This becomes a significant weight on the nose of the craft, and one solution has been to use a variator, which is a variable ratio pulley, and a v-e-r-y long drive belt to go from the rear propulsion engine to power the front lift fan. Since most homebuilts are about 12-20 ft long, the variator has been a decent, low cost solution, but I am planning to build a craft well over 20 ft, which would make the variator's drive belt impractically long. OK, still with me? Typical power for the craft I have in mind is a 100 HP auto engine in the rear for thrust and I am considering having this engine power a pump to provide flow to a hydraulic motor in the front for lift. I have lots of questions but figure I better quit now and get your initial take- is this workable? Thanks in advance for your help- it is truly appreciated.

It's definitely doable, although there are two things to consider. Firstly, you may only get 80% of your available power to the prop shaft. Secondly, is the added weight of these components. A 50 hp hydraulic motor could weigh upwards of 50 lbs.

Josh, Thanks for your response. I can probably live with the 20% losses you mention, since the propulsion engine will be 100+ horsepower. So, to get 25 hp to the lift fan hydraulic motor, I assume that the hydraulic pump will draw around 25hp x 120% = 30 hp? If this is right, that would leave me with 70 hp to run the propulsion fan. Does this sound right?

Weights for the hydraulic motor of around 50 lbs don't sound too bad, since the 20-25 hp gasoline lawn tractor motors sometimes used for the lift fan weigh upwards of 100-125 lbs. I was leafing through a surplus catalog and spotted Cross series 40 and 50 hydraulic gear motors which can spin at 3000 rpm continuous, which is a good speed for a lift fan; one in particuar develops 20 hp at 1800 rpm. Might this be roughly in the ball park or would the HP drop on that motor when run at 3000 rpm?

Thanks again for your response. The patience of all you knowledgable folks out there is appreciated, since I know I am blundering along here. Anything anyone can add is greatly appreciated. I am hitting the books also but there is just no substitute for the breadth of experience you guys have. Ray