trimmable surface drive design/ run subsurface

[orb353]

I am looking at buying or making a drive that has the trim advantages of an i/o , but runs much closer to the surface, a little further back off the transom. I considered jackshafting to a stern drive, but I want my boat to run a little shallower than a stern drive will allow. The higher you trim an i/o, like an outboard, the more it porpoises, thus it will not run in shallow water. The jack plate solved this for outboards, but outboards are very inefficient.
With an outboard, the further it is from the transom, the higher it can run without losing its grip and blowing out. I want to accomplish the same thing with a sterndrive.

I considered a surface drive, but my boat is not a true deep, and I don't plan on powering it for high speed. the hull is a 1971 24' aquasport, with a 14 degree deadrise. I want to cruise about 30 knots, full speed, not over 40 knots. I will power it with a diesel. I am looking for economy and a decent ride. Also, if I can run it subsurface, I could use a standard prop, eliminating the need for an expensive surface piercing prop.

Are surface drives economical at speeds under 35knots?
Something like this boat in the video would be awesome.

http://www.youtube.com/watch?v=U60rx...eature=related
Notice how this drives runs subsurface just outside of the prop pocket.

this drive here looks like it would run subsurface
http://www.dbdmarine.com/DBD-marine-gallery.htm

[CDK]

You could make something like this and add trim flaps if you thing you need them:
DIY tunnel drive

[orb353]

Thanks CDK. I have read most of that before i posted. Did you complete your project? If so, how does it perform? Are you using surface piercing props?

I like the idea of a prop tunnel, or a prop pocket, I am not sure if i want to run in surface piercing mode.

I have a small flat bottom skiff with an outboard. I have a jack plate on it that pushes it out 8 inches from the transom and raises the engine 10 inches vertically. I have to start off with the jack plate down for the hole shot, then when on a plane, I can pull the engine up to the point to where it is almost surfacing, although it does have an anti cavitation/ventilation plate on the foot of the engine. It definitely increases the speed slightly, but it allow me to run in very shallow water. As i understand it, the further back the prop, the higher you can run the anti cavition plate, which should ride right at the surface, with prop just under that.

That same basic principle is what i am trying to apply to an inboard diesel drive system, although the prop will never raise or lower exactly vertically. It could almost be a stern drive- i/o but with the prop pushed further back from the transom , and running higher.

[CDK]

If instead of copying an existing and proven construction, you design and build something from scratch, mistakes are inevitable. I corrected as much as I could, but a better and more thorough approach would be to discard the prototype and build a second one.
But time is against me, so I decided that yes, the project is completed.

The boat is in use every summer, brings me wherever I want to go and has not been near a fuel station for years. I fill the original 80 gallon fuel tank with jerrycans at the start of a season and then forget about it.

At low speed the props are subsurface and the boat runs very quietly with a strong preference to go straight ahead like it was on rails. Quite a difference from the time it had stern drives and needed to be corrected constantly. So you could say the ride is superior, but there also is a price: turning with just the rudders is very sluggish. I need to make a 180 degree turn between obstacles to get to the berth and can only do that with one engine in reverse.

At increased speed the hull lifts and so do the props, so they become surface piercing which causes a lot of vibration and noise because they only have 3 blades. I only do that if the sea gets rough and I want to go back to my bay. The hull reacts more agile then, but the ride isn't comfortable and the amount of wake hints that the fuel efficiency is poor, so I pull back on the throttles as soon as I return to the shelter of the island.

[orb353]

Dbd marine, in Australia, makes a unit that is designed to run subsurface. It looks very similar to the Arneson surface drive unit. The DbD uses a standard prop. cost is 12K.

http://www.dbdmarine.com/DBD-marine-gallery.htm