Check out Weismann surface drives, sounds like what your talking about. No vertical shafts, easy changed gears.www.weismann.net

A current gimbal supports and transmits the load of a prop whose thrust line is far below that of a surfacing drive. For this reason, if it will handle a stern drive, then it should handle the surface drive. The prop has far more leverage than a surface drive.

As far as the "trim cylinder" load, the cylinders only have to deal with two loads: 1) the weight component of the hinged extension assy and 2) the small thrust offset by figuring that only half of the prop (the half below the centerline of the propshaft) is delivering thrust. At a theoretical 2000# of thrust, the thrust offset at the lower half of the prop, with the thrust centered at 60% of the prop diameter (figuring a 16" prop) would be 2000*60%*(8/12feet) = 800 ft-lbs.

Loadwise, I think a Bravo gimbal would have no trouble. Been a long time since I have had an Alpha apart for comparison.

Shifting and gears for the drop section are the expensive issues.

Surface footage of the gear faces? V-drives deal with the same rpm and have no issues. In fact, in order to utilize normal RightHand props, you would want to use a "three gear" setup in the drop anyhow. This keeps them around 4.5" diameter.

If shifting weren't an issue and you just wanted to start the motor in gear and have everything direct drive with no transmission, then it may not be that hard of a bird to build. Ghetto drive - I like it. (but since there is no neutral or reverse, be sure to bring along your paddle).

M

I spent a few hours trying to get terse helical gear equations to make sense, lol... All I got was a headache (I really doubt I need to use 30" wide 4-5" 8DP gears... I *did* use a 1.8 duty factor, so I was trying to test for 900hp)

I've already got the tranny figgured out, its the drop thats the problem!!

You would want 3 gears yep, but 3 4.5" gears would give you 9" of drop...

And do you really want a 6" wide skeg?

Can anyone give me a really simple quick reference for gear selection??

ie:
What makes it stronger and what makes it weaker:

Pitch Diameter (What kinda FPM gives you real trouble?)

Pitch Angle

Tooth Size vs. Number of teeth (if you have to choose between two 4" gears and one is lots of little teeth and the other is a couple bigguns, which is stronger?)

And where the HELL can you find stock helical gears with 1.5-2" faces? I can only find itty bitty gears!


Thats the problem... ok thats the one bothering me now...

--Adam

PS Anyone know why they went to straight bevels in the bravos and better? I thought helicals were stronger and smoother, is the thrust load THAT big of a problem???

Ahh a few more quick comments...

1) 6DP not 8DP

2) I am placing the drop at the back of the unit so that the majority of the drive stays out of the water... Which BTW looks kinda like what Weissman is doing, cept on a larger scale.

And another question! hehe

Anyone know what kinda loss you would expect out of a pair or CV joints running at about a 20-25% angle? hehe I imagine more than the 2% you're getting in those 2 gear meshings in a 3 gear drop...

Time for sleep...

--Adam

**lots of little teeth versus a coupla bigguns**
coupla bigguns are always stronger. What you give up is smoothness and quiet. There's also a surface footage issue. Coupla bigguns has a lower speed rating than lots of little ones.
Comes from the increased area that a given gear tooth rubs across.


**Helical versus straight-cut**
Straight cut is stronger. You give up quiet and smoothness, and have more gear lash. Helical, however, does a better job of staying lubricated at high speeds, whereas a straight-cut set tends to sling lube worse (the helicals at speed will "pull" lube thru the contact area).

M