prop tunnel modification

[Captain Grog]

Looking for some suggestions from all you well versed designers out there.

Background info: I have a 40 foot Dorado Center Console, 12,500 lbs. The hull is basically a scaled down PT boat hull with a rounded trailing edge. Very interesting layout. Out house in the middle, bowling alley on one side, shuffle board court on the other. Bring thirty of your closest friends and their coolers. Small V-berth up front. Single Volvo 430 hp. 30% overdrive splitter box. Carbon fibre driveshafts to two 290 DP lower units. One engine, four props and 32 kts (without the 30 people) @ 2800 rpm with MAYBE a foot wake - the boat rides FLAT. The best part, I average 5.7 gph at 18 kts and 1000 mile range. At full throttle I can turn it around in its own length - or park it like a car in spaces a 30 footer wouldn't consider. What a dream.

Ready?

I was replacing the outdrives on the Center Console (and putting a coat of Awl Grip on her - boat ten years old - hull #1, when the better half made a decision that a real cabin with air, generator and creature comforts were more important than quick. The dogs need the A/C you know.

We found one sweet deal on a 40' Dorado Bluewater Workboat - the one with the Hardtop.
See: dorado-marine.com to get an idea what the boat looks like. I'll either post pictures here or I can be contacted at ggabert@ensercorp.com and I'll be happy to send them to you.

Dorado recessed the prop in a tunnel two feet wide at the rear and a foot vertical and tapering forward five feet to the keel on the Bluewater series. This loss of lift causes the bow of the boat to point at the moon unless the trim tabs are in the full down position - they're 9" x 18". You can't see over the bow without the tabs all the way down and yes, the helm seat is raised. Thought about going to a set of 12 x 42's but I don't think they will be enough. Underway, the prop wash looks like a jet drive blasting out the back. Max speed is 22 kts @26 gph. Cummins 480 hp straight six, 1.33:1 gearbox swinging a 21 x 22 cupped four blade at the rated 2600 rpm. Cruise at 18kts @15 gph, 2150 rpm. 15,000 lbs.

I had the opportunity to see a Pearson True North 33 or 38 close up last weekend on a lift in Boca Raton. The three foot hull extension effectively reversed the prop tunnel back to within a couple of inches of arc at the trailing edge. You'd be hard pressed to tell this was a tunnel drive from the stern at rest.

With this in mind, I thought about taking a two (or three) foot long pattern of the stern and under body of the hull in glass, reverse it and bolt it to the transom, effectively giving me a 2-3 foot hull extension - and also canting the nozzle downward some, thereby greatly reducing the nozzle opening at the stern. The trailing edge of this extension would be 2 inches lower than the plane of the hull and effectively forming a hook to eliminate the need for the trim tabs - I'll be moving them to the rear). Am I on the right track? Will this help the thrust and raise the stern so I'm not digging such a hole? The boat puts out a four foot wake and doesn't feel like its 'on top' unless its wide open -and I can't afford to run at that speed. I've already moved everything possible that's heavy to the bow - water tanks, gear, etc. I suppose I've been spoiled

Suggestions other than put a cabin on the center console that I'm required to sell? (MY preference!!!!)

[keysdisease]

How about trying a bolt on wedge in the tunnel aft of the prop? easy to fabricate, easy to take off and refabricate to adjust, unobtrusive, inexpensive. I know of sevaral manufacturers experimenting with adjustable tabs in the tunnels with good results, the thought that being in the propwash they are more effective than the same size tab mounted on the transom. A wedge should produce the same results to get you 9/10 of the way there and tabs will allow adjustment in the last 10%

Larger trim tabs are definetly in order no matter what. If you have bennett you can get just the larger tab and reuse the existing actuator (but you may have to move it to keep it centered on tab) or if you feel it necessary tee in a second actuator for the bigger tab, pump switch and wiring remain untouched, yes the same pump can handle 2 more rams. Bennett does not use dual ram as standard until above 12 x 42.

I am familiar with the Dorado's, nice boats although most of my experience is with the smaller o/b tarpon chasers. They have tried lots of things over the years, some of them pretty interesting.

Good luck,

Keysdisease

[Captain Grog]

Thanx for the idea, I can see that the reducing wedge would be a quick fix and something I hadn't thought of...the only rub is literally the rudder, which would interfere, so I see some kind of nozzle extending out the back of the hull - the reason for the extension across the whole back end was to have this and also have more surface area with a gentle incline vs a 30° speed break. I'll start looking for the bigger trim tabs - a good start in the mean time. I'll send you photo's tomorrow on the stern of the boat.