- Feb 4, 2007
- 7,251
- Boat Info
- 1996 450DA, TNT, Caribe dink w/15hp OB.
- Engines
- 3126 427HP TD transmissions
Is that why, on my drives, they exhaust out the prop? It's more efficient to push into the air from the exhaust than pushing into pure water?
Interesting Outdrive picture
- Thread starter Loneranger
- Start date
osd9
New Member
Whew....I thought I was having a prostate problem when I was trying to pee at 100' last month when I was diving in key largo....I feel better already!!!!!
- Feb 4, 2007
- 7,251
- Boat Info
- 1996 450DA, TNT, Caribe dink w/15hp OB.
- Engines
- 3126 427HP TD transmissions
Yes my props are in the water. The exhaust from the engines exit out of the prop. So the prop would be pushing into the engine exhaust. I have no idea... never mind.
Scott1
Member
The cleaver propellers are a more effient way to move water, but they must be operated with a certain amt of air ventalation. So you move them up on the X dimension. That allows the blades to spin faster with less resistance, not boil the H20, and push more water out the bottom of the prop....where most of the thrust is created. Its the reason you see vent tubes on nearly all #6 drives and most arnesons like on the fountain 48 EC to help them get on plane. Running a proper surface application on your boat usually yields a few extra MPH with no change in power. You probably do get a benefit of more thrust in the form of a rooster tail, but there isnt any water behind the top of the prop, when running fast, so its not helping pull you fwd.
Scott1
Member
No its done for sound reasons.....keeps the boat motor quiet. My last boat, a fountain, had captains call exhaust....and it didnt run any faster coming out the prop(quiet) or the hull(loud.) The props diffuser ring is used to help flow the exaust out the hub....and a reason we tweak the diffuser ring on performance boat apps with exaust coming thru hull....its an area we can sacrifice. Sterndrives, like bravos, volvos, imcos, and teagues are a good way to move boats, just not always the best.
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I understand what you are saying now. The air being entrained in the props prevents cavitation (boiling the water) so the props can spin faster (you can put more HP in a boat). However, although I can see how the bottom half of the prop is in the water on the front side, I would think the back side of the prop would be a spray of water/air into a free air environment...
- Nov 12, 2006
- 5,334
- Boat Info
- 1999 Sea Ray 330DA
Twin 7.4 MPI (310 propshaft HP) V-drives
- Engines
- Twin Mercury Marine marinized 7.4L L-29 V8s
Cast iron block w 4-bolt mains
A friend looked at putting Arneson drives on the 38 that he was having built. He selected Marine Innovation 800s, which are normally aspirated V8s with a huge amount of torque. The Arneson drives could easily handle the engines, but the length of the drives would be a problem behind the boat when it was in her slip. The other problem with the drives is that not all hulls work well with a propeller that is so far behind the transom. His boat's hull is one of those.
This isn't my field, but I believe that the reason behind vent tubes on surface drives is engine loading. The propellers are sized to operate on the surface of the water. Deeply submerged, they overload the engines, preventing them from spinning up into the torque curve. Ventilated, the props turn more freely, the engines wind up into their torque curve, and the boat moves.
Best regards,
Frank
This isn't my field, but I believe that the reason behind vent tubes on surface drives is engine loading. The propellers are sized to operate on the surface of the water. Deeply submerged, they overload the engines, preventing them from spinning up into the torque curve. Ventilated, the props turn more freely, the engines wind up into their torque curve, and the boat moves.
Best regards,
Frank
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