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View Full Version : Silly Question: Why is the prop and shaft angled down?



Laz
09-17-2007, 02:59 PM
My previous I/O and old outboard both had props that pointed straight back.

Why do the inboards have the prop angled about 15 degrees down? Isn't a bit inefficient to be pushing water down?

The answer may be obvious, but it escapes me. :?

Laz

BensonWdby
09-17-2007, 06:35 PM
In order to run straight out you would need to run the propeller off the back of the transom, instead of under it. Or run a deep well for the motor and then run it straight off the transom you would have to build under the boat. Neither of which would make for nice wake characteristics.

The angled design, especially in a direct drive unit, is the simplest with the least amount of articulation in the drive train, hence direct drive.

This slight angle is also probably responsible for the reason a mid-engine direct drive boat seems to rise out of the water instead of driving the nose way up into the air like your stern drive did.

My observations.
Dave

NH Moomba
09-17-2007, 09:52 PM
There are lots of reasons why. With an I/O or outboard, there needs to be some sort of gear train under water which results in a big fat casting creating a lot of drag and big wakes. Gears are also very inefficient power transmitters. You can lose 10-20% of power to friction in just 1 right angle set of gears (2 sets in an I/O). By tilting the powertrain at an angle, there is a much more efficient power transfer to the prop without a lot of underwater drag. Also, if you remember your high school physics and vectors, most of the thrust is pushing forward but a small portion is pushing upward which also lifts the rear of the boat for a smaller wake and a fast plane. Of course you can't change the direction of the thrust like an I/O or outboard and that is why these sort of boats don't steer well at slow speeds or reverse.

Andy

Sled491
09-19-2007, 11:24 PM
I don't think DD are all that efficient with power transfer either. I mean 325 HP and i get 44 MPH. One outboard I had was 200hp and did 107MPH. Even if you factor in the weight difference there still seems to be some loss. Granted we tested a 14 open aluminum fishing boat (say250lbs)with a Yamaha 30hp against my buddies old 2001 Nautique. The Natique blew the fising boat away out of the hole by a mile.

I realize I havn't added anything here just trying to spice up the debate

NH Moomba
09-19-2007, 11:36 PM
I would say that efficiency isn't the same as speed. I always tell people that these are high performance trucks rather then sports cars. They are made to yank you and 3 buddies out of the water and plane off in a couple of seconds. Try that with an outboard. My old Chriscraft I/O went faster with 140 HP 4 cylinder but starting on one ski took up half the distance across the lake.

Buttafewcoe
09-20-2007, 05:51 AM
I don't think I want me OB doing 70mph. I guess you could put a 24 pitch prop on it, if you could find one.
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Isn't insurance rates higher for boats that go 50+?
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deepcove
09-20-2007, 05:39 PM
My insurance rate is higher cause my boat has over 300 HP.

Sled491
09-20-2007, 05:43 PM
my rate was based on cost and size of boat. If the HP is in line with the size of boat they usually don't penalize you. If they do, look around.

BensonWdby
09-21-2007, 02:36 AM
I read somewhere, either here or on Ski Fly, that the 44-48 mph limit on inboard ski boats is a result of the hull design, not the inefficiency of the power train. Something about the angle of thrust and the hull wanting to bring the boat back down after that speed is reached. Which kind of makes sense I suppose. If you could put unlimited thrust into the drive train I suppose you would drive the back end up and the bow down since the upward thrust is behind the C.G of the boat.

Personally I love this style of boat. I can turn my boat extremely tight at almost full throttle without cavitation nor that feeling that the boat is going to flip. And I have pulled 8 skiers out of the water with no problem.

NH Moomba
09-22-2007, 11:32 AM
My insurance company quoted over 600 bucks because it was 325 hp 5 years ago when I bought the boat. I just got my SkiSafe renewal bill and it is $293. I think that reflects a bit of a discount since they know that in New England the boat is in the water for only about 4-5 months a year. The SkiSafe policy covered a lot more than what my local insurance agent offered as well.

www.skisafe.com

Buttafewcoe
09-22-2007, 10:23 PM
Besides, if it was angled up, the prop would have to be a whole lot bigger to push the air.
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Just a thought
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