propeller tunnels

Has anyone ever put a propeller tunnel on a single screw, vee-bottom sport fishing boat? The boat will go about about 40 knots, given the engine and displacement. I am trying to reduce the draft and am wondering if a centerline tunnel is a viable solution.

 

Tunnel

What size boat and power?

 

prop tunnel

Size is about 36 feet and horsepower is on the order of 500.

 

Tunnel

I guess the engine is under the console and jack shafted? If this is the case you might look at pulsedrive.net at there R drives or instead of the tunnel you could go with their single bolt on the back of your transom model which has two rudders which would help balance the torque. Also, they have shallow draft and a five year warranty. They have a data sheet and a great engineering department.

On the tunnel, I could give you a gear ratio and other calculations and the prop size but that would be only a mid range as if your drive does not trim the props are very hard to calculate. Also, you will have to put air into the tunnel to break the prop loose. That can be done from the sides or top. Give me weight, dead rise and I'll give you the info.

Why do you have a single on a fish boat? It is more reliable with a dual and you would have zero draft with a trimmable surface drive and for the same horsepower dual engines give you a 10% increase in efficency

Check with the engineering department at pulsedrive. They have some great stuff.

FrankieFrankie

 

I agree 100% with having the reliability of twins and less draft, but I've never heard you get a "10% increase in efficiency" by having duals. Why would duals increase efficiency? Does this take into account the added weight of two engines vs. one slightly larger block?

 

PGS it would help out a little if you posted some photos, or solid drawings without the tunnel. I have done it on a few boats w/ twins and single screw.

 

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I think what Frankie meant was that surface drives will give you approx 10% efficiency gains up around the 40 knot area. Unfortunately they are often very inefficient at more moderate speeds, so you really need to decide exactly how fast you plan on travelling at - few of us simply jump in the boat and roar off at 40 knots, regardless of conditions.

And you are correct of course - by going for twins rather than a single, you will immediately lose any efficiency gains you've managed to (very expensively) buy by having surface drives. The single vs twin debate will continue to rage long after we are all gone, but so long as you maintain it properly, there's no reason to be afraid of having just one engine.

This assumes you are trying to produce at least a partially ventilated drive. Few Fishboats incorporate ventilated systems because of their inefficiencies at moderate speeds. There are many single and twin engined boats which incorporate tunnels - generally the greatest advantage comes from the reduced shaft angle, lower draft and to a lesser extent the relationship between the prop and the tunnel itself. And you can certainly successfully employ a tunnel in a single engined boat.
If a ventilated system is determined to be the way to go, then I agree - better to go with a tried and tested system like the pulse drive - many a boat has been completely stuffed up by trying to build in a surface piercing system on the cheap.

 

Tunnel

No, the 10% comes out of dual propellers and yes you do have to watch the weight issue which if you are talking twin yanmars at 240hp range each compared to one yanmar at 500hp the weights would be less on the twins plus the 10%.

As far as the mid range and low speed on the Pulse Drive you have the trimmable housing which becomes buayant and you trim it down to hold you on plane longer which increases your efficency. If you normally fall off plane at 17mph, then with this system it would lower that to maybe 12 or 13 mph when you lower the drive as you slow your speed.

You are right with most surface drives that is a problem but the pulse drive has found an answer.

Five Year Warranty can't be bad.

FrankieFrankie

 

Surely you're not trying to suggest that two engines, drivelines etc is more efficient than one?? There are many reasons for having a twin installation - redundancy and manouverabilty to name just two - but I've never known anyone to say that twins are more efficient. If there is a 10% efficiency gain to be had from two pulse drives over one conventional shaft, then (provided the drive can cope with the power) one pulse drive should be 15 - 20 % more efficient.

It's true that trimmable surface drives have an improved viable speed range over fixed sytems but the problem is that in order to maximise the efficiency gains available at designed service speed - say 35 knots - the propellor must be sized accordingly. This is true of any propellor, but with a surface drive, the diameter and pitch tend to be such that at lower speeds the propellor tends to overload the engine. The solution to this is to trim the drive up, lifting the prop higher out of the water. But in so doing you not only lighten the load on the engine, you also reduce the thrust available. At higher speeds this loss of thrust is more than offset by the reduction in drag, but once you dip much below the 35 knot mark the drag becomes a less significant factor.
Most manufacturers literature suggest that there is a significant efficiency gain to be had from surface drives at higher speeds - in excess of 30 knots. Some - particularly the of the trimmable variety - also claim to be able to match conventional drives at lower speeds. I'm yet to come across a manufacturer who claims significant efficiency gains regardless of speed.

 

If this is a sportfishing boat, I doubt he wants to have a surface drive protruding 3 or more feet off the transom. Comparing apples to apples, I would think a well designed single inboard would be much more efficient than dragging two shafts, struts, props and rudders through the water. There is also the advantage of having the weight centered lower in the bilge, assuming the boat to be relatively light, this could improve stability if the boat also carries a tower.

There were some boats called Ocean Tech built in Virginia in the late 80's early 90's with surfacing props in tunnels that were fed air from the cockpit. These boats were very fast and pretty efficient if memory serves. They were also very light for their time which didn't hurt their performance.

 

Wow, I Love This.

Lets see now, on the latteral stability, lets compare this to a tight rope walker, who does not carry a manville to keep everything in the center but carries a long rod to balance which is what two engines do. Two engines are better for latteral stabiltity.

On the surface drive estruding behing a fish boat, that is true of most however, the trimmable pulse drive has a fiberglass housing that covers the props and doubles as a platform to fish from or whatever. They also have an R drive that trims and sits in a tunnel.

On the dual propeller, all dual propeller operations are 10% more efficent including the duo props of the stern drives. The dual propellers counter act the torque which equates to the 10% gain. I have also done a single split with two surface props like the drag boats are doing now.

You do not get a loss of thrust when you trim up, you get a loss of drag as from the bottom of the hub up is drag. The advantage is you can watch your rpm and speed when the speed stops and the rpm keeps going then that is the point to stop as this is the right trim for your current load at that speed.

Trimmable surface drives are more efficent at lower speeds see the data from the commercial customers. And yes one pulse drive is 15 to 20 % more efficent. One of their advantages is having the trimmable platform that acts like an afterplane and keeps the boat on plane at lower speeds.

 

Now now Frankie,

You tried to slip that tight rope walker by us but the analogy is flawed. You equated lateral stability to rotational moment of inertia. OK, they are easy to confuse but not the same at all. By that same analogy, a tall tuna tower would make a boat more "stable" and we know that is not so. The moment of inertia and roll period would be greater for a tower than without one and that might make the boat "feel" more stable.

Can't argue with the efficiency gains or losses since I don't have the experience with the different systems. I do know that with outboards, two units are less efficient than one unit of equal HP.

 

Frankie, Frankie, Frankie......

Tom as always, is humble but wise. Generally speaking, by installing twins, the engines must be located outboard of the centreline. In a conventional v-shaped hull, this means the motors must be located higher. Higher weight = higher CG = worse lateral stability.

Most fishboats are built free of any protrusions - boarding platforms included, as they make playing and landing big fish more difficult. But depending on the type of fishing you plan on doing you might be able to live with this.

But it's this business of the two separate drivelines being more efficient than one that has me stumped. Like Tom, I'm only going on my own experiences and the information from every single boat test that I've ever read - do you have any data which actually proves that two drivelines are more efficient than one. And you can't slip that duoprop thing by us as evidence - that's just one driveline with two props. But before you go looking, you might like to take a look at your last post....."all dual propellor operations are 10% more efficient" and "one Pulse Drive is 15 - 20% more efficient". Seem to have shot yourself on the foot....

This is true at high speed, but at lower speeds the appendage drag is not sufficient to be offset by the loss of thrust. If it were, then all manner of boats would oncorporate surface drives - ever seen one on a freighter or displacement cruiser?......

Lastly, don't take this the wrong way, but I noticed that a number of the customer testimonials on the Pulse Drive are addressed to someone by the name of Frankie.........

 

I forgot to add that the rod carried by tightrope walkers is also fairly heavy and limp so that it droops down. This lowers the CG of the combination and in some toys actually makes the whole system completely stable so that it remains upright unassisted.

As Will said, the two engines actually raise the CG over that with a single engine.

 

As someone who regularly performs propeller calculations, sometimes using various methods, I know of no adjustment made for having more than one prop. In fact, usually you run your calculations as if the boat had only one prop and the two engines barely enters the equation. The only place the number of engines comes into play in my calculations is when calculating wake factor, which usually has little effect on higher speed boats. It is very important on displacement vessels though. As for the CG, lower is better in most cases.