Gimme Power !!

Power-Vent Surface Drive

I've thoroughly researched all the drive options (I can think of) for O-1 and I would like to recommend the "Power-Vent" surface drive. http://www.powervent.com Power-Vent is a surface piercing drive, like the Arneson drive, however it is mounted completely under the stern in a vented tunnel. The vent is connected to the stern via a separate tunnel and is cleverly designed to provide speed compensated ventilation to the surface piercing propeller. In this way the drive requires no driver intervention as speed and conditions change and the flex couplings and hydraulic actuators are eliminated.

Here is a report that the Naval Surface Warfare Center Carderock Division filed on a test of the Power-Vent drive. http://www.powervent.com/navyreport.html. In this test, the maximum speed obtained during the trials was 40.5 kts (46.6 mph). More recent test reports show that they have increased that to 51 mph. http://www.smithboats.com/robb_report.htm You will note that the recommendation of the Carderock report was to acquire a prototype boat. As a result of this report a 40' boat driven by a Power-Vent surface drive has recently been acquired. http://www.powervent.com/APTD.html it was delivered in January 2002. http://www.smithboats.com/military_police_craft.htm. I should be able to obtain a copy of the on-going test results on this boat when they are complete.

Another informative article on Power-Vent was written in Popular Mechanics http://popularmechanics.com/outdoors/boating/2000/6/Scratching_The_Surface/. An interesting paragraph reads, "Real-world testing of Power-Vent was done by George Boynton, founder and then president of Shamrock Boats. He installed a Power-Vent system in a Shamrock 31 sport fish. Powered by twin Crusader 454-cu.-in. 330-hp engines, the boat had a measured top speed of 45 mph. Modifying the hull with Power-Vent saw the top speed jump to 52 mph--an increase of 15 percent. Fuel efficiency at 35 mph cruise speed went from 1.25 mpg to 1.5 mpg--a 20 percent gain." These results are on the low end of the range the improvements demonstrated by conventional surface drives. If these results were achieved on O-1, we could achieve 15% higher speed that an equivalently equipped "Express Cruiser", or we could achieve comparable speed with a smaller engine.

Here is a summary of my findings:
Performance: significantly better that stern drives, inboards and outboards(?) almost as good as conventional surface drive
Cost: Lower than surface drive close to inboard
Corrosion resistance: Comparable to inboard and surface drive better than stern drive and outboards.
Mass properties: Heavier than outboards comparable to inboard lighter than stern drives and surface drives. Greater flexibility in longitudinal CG that outboards and stern drives. Better vertical CG that outboards.
Engine flexibility: can accommodate either gas (petrol) or diesel engines, either raw or fresh water cooling, interface to water heater.

The main disadvantage are the Power-Vent must be integrated into the hull and the engine must be located further forward.

Cheers;
Mike Schooley

 

I remember reading about them. Great idea, but it would eat up some space. Can we just take this idea and use it, or will we have to pay a user fee.

Gary

 

Gary,
I believe that you simply buy the power vent system in the same way as you would buy any other drive system - though the hull must be designed specifically to take the unit.

As you may have noticed from a number of my other posts, I'm a bit of a fan of surface drives - but (ain't there always one of them!) they bring up a number of other issues.....
1st up, we've elected to give O-1 a max speed of about 28 knots. From memory, I think Twin Disc suggest that the Arneson surface drive becomes more efficient than other drives above 35 knots. The power vent is probably similar.
2nd you are unable to trim one of these drives up. That means riskier shore landings (though better than with conventional shafts). It also makes cleaning debri, or changing props are much more difficult job.
Lastly, I'm not so sure that I would agree with Mike about the corrosion resistance. Outboards these days have sufficient trim to enable them to be lifted completely clear of the water - zero corrosion problems. And remember that O-1 is not likely to be left in the water for lengthy periods of time anyway.

Aside from all that, I really look forward to seeing the results of the tests of this unit - and as I said, I love surface drives, so I'd love to be convinced that it was the unit for O-1......

 

When in doubt, ask someone who knows....
I've emailed Powervent and asked them whether their drive would be appropriate for a boat with a top of about 30knots and able to operate efficiently on plane down to around 15.
Hopefully I'll get a reply soon.

 

Hi all...as for outboard vs. inboard/stern drive, I think I may have a simple(albeit silly ) suggestion: the "average" express boatbuyer desires inbd/stern drive over outboards so that they can swim off the back without dodging the motors/props...plus, a swim platform makes a nice place to cookout at the slip.

 

First, welcome to the forums dweymer! I think the appearance and functionality of the transom certainly is a large part of it. Will posted one solution to this which I had not seen before where a hinged portion of the swim platform lifts with the outboard. I'm not sure how large a section this would be practical for, but it seems like an option. http://forums.boatdesign.net/showthread.php?s=&postid=2521#post2521

 

In regards to the speed, I think your on the right track contacting Power-Vent. I e-mailed them yesterday but have not received a reply yet. Arneson and Twin Disc both told me surface drives are more efficient at all speeds above planing because the drag is lower and the mechanical losses in the drive train are lower. The data I received from Twin Disc (and haven't had time to report on yet) supports this. One comparison Twin Disc provided involved changing a "fixed surface drive" on a 26' "Sea Skimmer" tunnel hull. Cruise speed, defined as 2700 rpm, increased from 27 mph to 33 mph and fuel consumption at cruise went from 10 gph to 8 gph. This represents a 20% improvement in fuel consumption rate but a 51% improvement in mileage, from 2.7 mpg to 4.1 mpg. Twin Disc also retrofit a 100' Camcraft which was converted from a supply boat to a Yacht. The engines were reduced from three 600 HP engines to two 650 HP engines. Cruise speed went from 24.3 knots to 23.7 knots with a 28% power reduction. Fuel consumption went from 100 gph to 72 gph. In the table of military applications they list a 26' Uniflite Survey Craft with a speed of 24 knots, a 70' Boghammar landing craft with a speed of 25 knots and a 50' Bristol patrol boat with a speed of 28 knots.

On corrosion, keep in mind that outboards are salt water cooled and they have dissimilar metals. Many inboards are fresh water cooled and have no dissimilar metals. Surface drives are stainless steel with zincs. While it is true that most trailerable boats are kept on the trailer, O-1 is also intended to spend a week at a time on the water. Outboards will require a fresh water flush after use which will probably not be performed each day.

You have a good point about trimming the drive for a beach landing, but I didn't see that requirement in the requirements list. The Power-Vent saves a lot of cost and complexity by eliminating the need to trim the drive.

Another disadvantage of Power-Vent that I overlooked is low speed maneuverability. You can not vector the thrust and surface piercing props do not reverse very well.

Cheers;
Mike Schooley

 

I'm not too sure that the PowerVent would suffer from the often discussed low speed manouvering problems faced by surface drives.
The problem faced by a number of systems - those such as the Arnesons on your Magnum - is that the reverse thrust from the props is directed straight into the essentially vertical transom. The section drawing of the PowerVent shows hull cutouts which should allow this thrust to pass under the boat, so I would imagine that it would be ok in this regard.
It's interesting to see the variety of applications for the Arneson's - and many of those you've noted fall within the performance spectrum of O-1, with its expected top of about 28 knots. But you will also note that most of these are commercial or military boats - designed to operate continiuously at a given service speed. O-1 must be efficient throughout its speed range in order to be succesfull - ie from about 12 knots to about 28. I'm yet to be convinced whether the surface drive is capable of this.

 

Your explanation of the Magnum reversing problem makes sense. I hope your right about the Power-Vent reversing better.

I only listed the models that were <=28 knots. There were a lot more that are >28 knots.

The military patrol boats tend to operate for very long periods at patrol speeds which are typically 12 to 18 knots depending on sea states. They only operate at maximum speed when trying to overtake someone or when responding to a rescue request. It is true their hardest requirement is generally maximum speed, but they are also very concerned with cruising range/endurance.

What Twin Disc and Arneson told me is that the surface drive is more efficient that conventional drives (specifically outboards and stern drives) at all planning speeds. Below planning speed they are claimed to be equivalent to a conventional drive. I talked to our boat driver and he confirmed this.

I think the best input would be from some Power-Vent customers. I know some guys at NSWC Carderock. I'll see if they have any feed-back on their Power-Vent boat yet.

I'd like to do some cost comparisons for the different drive options. I'm a little concerned about the cost of the Rolla surface piercing prop. What horsepower range do you think we will end up in?

Cheers;
Mike Schooley

 

Many outboard / sterndrive owners carry spare props, so that in the event of damage it can be quickly and easily replaced. Also it allows for a change in pitch to suit varying loads / conditions.
The cost of the rolla, or similar props is only 1/2 the problem: their availability could be a problem also.
From what I can gather, prop selection is vitally important to successful surface drives. And overloading the boat can render a surface drive virtually inoperable. These could all pose problems for the amateur builder.

 

As far as hp requirements are concerned, Tom recently posted the following numbers:

90hp = 24knots
115hp = 28 knots
150hp = 32 knots

ie we're definitely at the bottom end of the surface drive power spectrum!

 

Moderate Speed Surface Drive

I found an article titled "Surface-Piercing Propellers
by Paul Kamen, N.A." http://www.simplicity-marine.com/surfprop.htm which I thinks answers the question of the application of surface drives to moderate speed (i.e. 28 knots max) planing vessels.

The entire article is quite interesting and informative, but the part that is the most applicable to O-1 is, "Surface propellers have long been accepted for racing applications, where minimizing appendage drag and cavitation are the major motivations. In recent years, an increasing number of high-speed yachts and patrol boats have been propelled by surface propellers, and some of these applications have been spectacularly successful. But the use of surface propulsion for relatively heavy and slow vessels is new."

Cheers;
Mike Schooley

 

Paul Kamen

I called Paul Kamen (Paul Kamen <mailtok@well.com> is a naval architect with degrees from Webb Institute of Naval Architecture and the University of California at Berkeley. Formerly the Applications Naval Architect for Arneson Marine, he is now an independent consultant specializing in surface propulsion.) and discussed the O-1 project with him. He said that at the speeds that O-1 is designed for we would probable see a small increase in speed over a submerged drive for a given power level. He didn't know how much of an improvement the surface piercing drive would provide, but he indicated it would perform better than a submerged drive.

He said that Simplicity acquired the article that he wrote and added their name. His original article is at http://www.well.com/user/pk/SPAprofboat.html you can also down load a program that he wrote while he worked at Arneson to analyze surface drive craft at http://www.well.com/user/pk/SPA.html

This program should provide a fairly good estimate of O-1 performance with a surface piercing drive.

Cheers;
Mike Schooley

 

What's that they say about great minds?.......Poor old Mr Kamen must be wondering what's going on - I emailed him to discuss the O-1 project and another that I am working on !!
I have actually seen his article reproduced in a number of places - pitty he probably doesn't get royalties for such use!
As you say, he could see no problem with the use of a surface drive. I mentioned my concerns with the requirement to perform over a wide performance spectrum and he made no reference to this in his reply, so I think we can safely assume that it should pose no problems.
But given that the surface drive is only likely to yield a small perforomane benefit, it can't be said to be so much better than the alternatives to render them not worthy of consideration. It also means that the surface drive definitely IS worthy of consideration.

 

Surface Drives

I agree with your conclusion. The surface drive is just one of the options. I'm collecting comparison data on outboard, stern drive, surface drives and ?

I also called Power-Vent today. I talked to their sales manager (if you can call the head of a one man department a manager) and their application engineer. They both tried to convince me that O-1 wasn't a good application for a surface drive. When I pressed them on this they admitted that they had only tried one moderate speed vessel (a Sea Doo conversion) and they admitted that they had the gear ratio wrong. I concluded that for moderate speed applications it is critical to optimize the prop diameter and gear ratio for the intended speed like Paul recommends. If we do end up selecting a surface drive I think we would be wise to seek professional help. I also decided that the "Power-Vent" may not be the best surface drive option.

Cheers;
Mike Schooley