Question for GN7

[WESTERNAERO]

Why are struts squared off in the back? I understand for the rudder but every lower drive unit you see is tapered on the back side leading to the prop. What's different on a V-drive prop that the strut is squared off on the trailing edge.
Looking down on a strut, 2 examples of what I asking about.

[WESTERNAERO]

Maybe these 2 pics are a better example.

[wannabe]

you want the most meat where the greatest forces are. (the back of the strut.) And the more the power, the more the forces are at that back. And trust me, you want meat there! Also, making an angle can create sharp edges where a crack can begin.
With that being said, I won't let one of my boats go in the water with a squared off back of the strut. But I won't shape it like the front. Just a little to help the water pattern recover before the propeller.

[WESTERNAERO]

Thanks, I understand the fracture critical deal, yes this makes sense. The CAD drawings are crude, they were just for a visual to understand my question. But I was thinking of some taper, maybe upper and lower would have a square or full radius and from there start the taper.
What are people using for material? I was thinking 15-5PH or 17-4PH.

[WESTERNAERO]

Hey Wannabe, is this Buzz or Paul or both? My cousin bought a guitar from one of you a few months back for his son. It was black with a triangle shaped body( I know nothing about guitars , forgive me me). His kid loves it, thanks. He said you gave him a smoken deal.

[78Southwind]

Maybe these 2 pics are a better example.

Westernaero....did you make that blast plate or did you have someone do the work for you? I am trying to find out what a reasonable price would be to install and make a bigger blast plate for the Spectra. The Spectra just has a fill plate where the old strut was (acting like a small blast plate).

[WESTERNAERO]

Westernaero....did you make that blast plate or did you have someone do the work for you? I am trying to find out what a reasonable price would be to install and make a bigger blast plate for the Spectra. The Spectra just has a fill plate where the old strut was (acting like a small blast plate).

Mr. Southwind, the pics I put up were google search pics and are not mine. However, you may be in luck. I will be making my self a blast plate for my Dimarco cruiser this winter when I re-rig it. It will be a quality piece and if you're willing to split the cost of the material with me I can easily make 2. I will not install it for you, but it shouldn't be tough if you have some mechanical ability and take your time. I love to help people it's just over the years I find myself with less and less time to do labor, so I have started to encourage everyone to do it themselves. I'm also going to be machining a bunch of parts for this build, so if you need something let me know. I've been looking at all your dimensions and we are real close. So for me to make 2 pieces of something on my CNC's is just a push of the button and no sweat if I can help you out. Feel free to call me, 818-970-1872, Kevin

[78Southwind]

Mr. Southwind, the pics I put up were google search pics and are not mine. However, you may be in luck. I will be making my self a blast plate for my Dimarco cruiser this winter when I re-rig it. It will be a quality piece and if you're willing to split the cost of the material with me I can easily make 2. I will not install it for you, but it shouldn't be tough if you have some mechanical ability and take your time. I love to help people it's just over the years I find myself with less and less time to do labor, so I have started to encourage everyone to do it themselves. I'm also going to be machining a bunch of parts for this build, so if you need something let me know. I've been looking at all your dimensions and we are real close. So for me to make 2 pieces of something on my CNC's is just a push of the button and no sweat if I can help you out. Feel free to call me, 818-970-1872, Kevin

Will do...Thanks Kevin

[WESTERNAERO]

Yep no problem, let me know what you need. And Menace marine is right down the street from my shop. So maybe you could hit them up to do install if you don't feel comfortable. Teague is also around the corner, if you need parts. Talk to Eric, he's a good dude and can answer a lot of questions you might have.

[gn7]

Sorry it took so long for me to answer this. I don't venture to the v drive forum much because it tends to be dead here.
Unforgiven is part right, part wrong. Sorry Ed.

He is 100% dead on about the cracking. If a strut is going to crack or fail, it will start at the front of the strut where it is tapered to a sharp edge. I try to minimize that as much as possible, and as short a taper as possible, but its still pretty damn thin there.

But there is no forward thrust at all in the strut, and 99% of the forces it sees is upward and pretty even across the length of strut barrel. A little bit more towards the rear due to the prop being on that end, but its pretty even across the strut barrel. The wear of the bushing shows the forces, and they are almost entirely at the upper part of the bushing because the prop is trying to lift the ass end of the boat. On a hydro its entirely on the top side and virtually none anywhere else.
As to why the outdrives are tapered on the back side, and the V drives aren't, its kind of a physics thing, and a damn good thing it works out like this, or outdrives wouldn't work worth a shit.
Look at the bulk of the outdrive above the prop center line. Now compare that to the bulk of the strut above the centerline. Now look at the bulk under the prop centerline of both. Which has the most bulk in front of the prop in each case. The v drive nearly nothing above compared to the outdrive, and zero under the centerline.
The below centerline is where 90% of the work is being done on both cases. Look at a hydro. The prop is half out of the water. Same with a Ilmor, and Merc Nighthawk surface drives and in the right conditions on the right hull, a #6 and 8 Merc, and some Imcos. AND THEY HAUL ASS! At any given point in the prop rotation, the downward ear, to the mid rotation point, is doing much more work that the upward ear(s). Its the downward push that lifts the ass end, on any boat, specially a hydro. If the upward ear did the same work, they would cancel each other, and hydros wouldn't lift, and surface drives would be stuck in the water.
Another reason the bulk above the prop on a V drive means so little and has a little more effect on the outdrives is the number of blades. On a v drive the blade at the top is doing near nothing except thowing water off the perimeter of the blade into the bottom of the boat. It lends very little to the forward motion of the boat. Again, look at hydros. NO blade in the water ot the top at all. Look at the speeds of hydros and surface drives, they don't miss that balde out of the water one bit.
If the upper blade did much, the outdrive would suffer much more than the skinny ass liitle blade the strut has.
So the fact is, the tapered on both ends skeg of the outdrive is effecting the prop more than the blunt back side of the strut on a v drive, because its under the prop centerline where the prop is doing the majority of it forward thrust.
Make sense?