propeller tunnels

[PGS]

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.

[FRANKIEFRANKIE]

What size boat and power?

[PGS]

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

[FRANKIEFRANKIE]

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

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

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?

[Nomad]

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.

[Willallison]

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?]

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.


Also, you will have to put air into the tunnel to break the prop loose. That can be done from the sides or top.

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.

[FRANKIEFRANKIE]

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

[Willallison]

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.

[Timm]

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.

[FRANKIEFRANKIE]

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.

[tom28571]

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.

[Willallison]

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....

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

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.........

[tom28571]

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.

[Timm]

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.

[Portager]

When I took aerodynamics and hydrodynamics classes (back is 19xx) we learned that counter-rotating coaxial props are more efficient because the second prop recovers the energy component that is perpendicular to the thrust vector (i.e. the swirl component). Theoretically counter-rotating coaxial props should be 15% to 25% more efficient depending on the pitch, but in the real world only 10% to 15% is actually achieved (probably due to surface friction and the addition of another prop surface).

I don't think this applies to two props that are not inline.

Cheers;
Mike Schooley

[FRANKIEFRANKIE]

First, the two props counteract the torque as with a single prop you are always steering in crab as the rudder is a steering position to run a straight line.

Propeller diameter is a product of the weight. A typical submerged three blade propeller (ie Michigan style) equals the weight divided by 21, then take the square root = propeller diameter. Two propellers divide the weight by two. There is a gain in efficency because the rudders don't have to balance the torque. Also, the larger diameter has a higher perimeter speed causing a higher torque demand in relation to the smaller two propellers. Typical speed gains usuallly amount to 10%. Now remember with are talking same total weight per horsepower. Which when I read some of your stuff it just said they put two engines outboards or inboards instead of one but never said anything about weight. My analagy was two 240HP Yanmars against one 500HP Yanmar, which calculates the same weight even a little lighter on the two.

Would you really believe that changes the engine height a few inches could cause a problem in relation to the engine on the center of the role axies. When it is a fact that the old sailers accually have been know to put an anchor in there mast to dampen the roll.

It is great that we do now have the benefit of the internet so now we can get more information out to more people.

By the way, I was on the Mercury Marine site and saw a Will. Huh?

[Timm]

I have never heard of putting an anchor up the mast to dampen roll. I and any other designer/naval architect can show you how the GM will be reduced causing a loss of stabilty by raising the CG. As far as the engine weights, it may not make a huge difference, it all depends on the rest of the boat. I agree with Portager on the two props in line. If we are talking about two, traditional inboards (not surface piercing drives), then two shafts (and their accompaning Magnus Effect), two props, two rudders, two struts will surely create more drag than a single inboard drivetrain. You are assuming the two props are half the size of a single, which is more than likely a false assumption. The amount of drag from a single rudder will be substantially less than the drag created by pulling two rudders through the water. Where exactly are you getting your test data showing these speed increases? I have never seen these tests and would love to read them.

[FRANKIEFRANKIE]

Study of History (books).

Same boat, same drives, same weight. One engine or Two, yes it does make a difference. Hp to weight is Hp to weight, same boat, same drives, same weight.

No, I didn't say 1/2 size. Work the formula.

Check Reggie's numbers. He is great at getting the most out of a boat.

Check Bam, their good.

Check Schoell Marine, they gave out a great Formula at IBEX key note speaker event in 2001.

Stephen Ditmore is a good source.

You can not make a comparison on a different hull and weight. The gain is same same and same. Hp only knows weight on that object that you are pushing and you can only do the comparisons for that object. If you go to a new then you go to a new base beginning numbers.

[Timm]

Still haven't come up with the tests showing two shafts and props are 10% more efficient. Just for yucks, I ran some hypothetical prop calcs for a 30'ish boat with twin Yanmar 250's vs. a single Yanmar 500. The twins will be slightly lighter in engine weight, but will probably balance out when you consider the two drivetrains. Using a speed of 35 knots for both boats, as both boats have the same weight and horsepower, the twin would use 18 x 19 3-blade wheels with a 1.5:1 gear ratio. E = .71 and the slip would be 11.6%. These props would absorb about 255 shp, or 15 more than our Yanmars produce. RPM's would probably be about 25-30 under the rated 3800. At 3770 rpm's, the props would use 242 shp. The single would use a 24 x 25 3-blade prop with a 1.5:1 gear. This prop would absorb 497 shp, 7 more than we have. E = .71, slip 11.9%. The interesting number is the thrust, the twins produce 1663 lbs each (3326 lbs total), while the single produces 3288 lbs, a difference of about 1.5%. At 3770 rpm's, the thrust is 1579 lbs. (3158 total), or 4% less. This calculation is based on how much power a prop will absorb at a certain speed. As thrust = resistance, both combinations should push the boat about the same speed. There is nothing in the calculation that gives you 10% extra if you have twins. These numbers are based on the KT/KQ calcs from the University of Michigan and have proven to compare very well to the Michigan Wheel prop program that the prop shops use. Like I said before, I would love to read the technical papers showing test results giving a 10% gain, but in 15+ years of designing boats, I have yet to see it.

[Willallison]

Timm,
plus you must take into account the additional appendage drag created by the 2nd driveline, (unless your formuale already do this?) so the only way I can see 10% is against the twins.....

[Timm]

I agree Will. The calcs do not take drag into account. But I would think the drag would be higher, though not double because the single would have a larger prop, rudder and shaft. As I said, I'd love to see the test reults as they could be quite useful to those of us designing boats for a living.

[FRANKIEFRANKIE]

The calulations that I post, I got from the 2000 IBEX in Ft. Lauderdale from the Schoell Marine Group, which are known for their history (since the 60's) for propulsion and hull through model towing and testing. Their copywrited calulations are available and are used by several propulsion companies and numerous boat companies. They were the key note speaker for the millinium as was put because they have been so far ahead of their time already. I, too, owed a boat company and was in awe of the questions they could answer and the things they could solve.

[captword]

this a reply to all from a boat builder and operator. a 40 knot single engine is do able but at maneuverability cost. to get 40 knots you must for go the keel, wich cost you in reverse. because in reverse a sinle engine boat will walk like a bachee. I currently own and operate a 28 foot tunnel boat wich i charter. it is powered by a single 3208 cat at 320hp. The bottom of my boat is designed as perfectly as you can get i have a keel for protection a loss of speed but the balance of it all gives me maneuverability in reverse. as far as the cg of a single verses a twin., in my experience over 40ft, it is there but hard to notice. but under 40 it is there it can be felt and observed. a single low and away is more stable. sorry guys but i have to conquer with frankie on the effeciency of twins vs sinlge. it is hard to prove but i have in personal expeiribce. I once run a twin screw that buddy harris built. it had twin 3208 cats at 350hp. his boat had a single 892 detroit at 700 hp. the boats where of similar weight and size. the only diference in the two was the fact that i held 200 more galons of fuel than he did. we fished together. we fished similar in all ways we would leave the dock at the same time run out and start fishing at the same time troll the same time and then run home. my boat would run a knot faster. we would get to the fuel pump. he would burn 90 and i would burn 70.
the balance has to be struct at getting as close as possible the same weight out of the twins as the single for it to work though. if this can be done the added drag of an extra set of running gear is more than outweighed by the added gain of thrust.
if i was building one for myself what would i do? i would build a twin. 1 the insurance of the second motor. 2 the maneuverability. and 3 takes one and two together and gives sell ability. look at the yacht brokerages on the web a similarly sized single verses a twin. the single is twice as finished out as the twin, but the asking price is 100k less than the twin.
back on the aspect of tunnel 40 knots is possible but not with a keel. and without a keel you loose manuverability in reverse. something that can be handled but with skill. it takes a a good balance of tunnel to running bottom to make up for the loss of thrust through the tunnel. the established builders say that the best that you cab get is 75% of the hull speed verses the same hull without a tunnel. it can be tweeked though. there are some builders that have started wedging the aft end of the tunnel(kort knozzle effect). they clame to gain 5 t0 15 % more speed with the wedge. on the maneuverability aspect it may be possible to gain with twin rudders set on the outboard edges of the tunnle, like twin ruddres work on a reg bottom single screw, but as of yett that is my conjequer not a tested theory yett.

[kapnD]

Thanks, capt word, I was wondeing if this thread would ever get back to the original question. I'm interested in doing this mod to my boat also, its 30x9@12deg, 250hp. I'm considering maybe not a full tunnel, more a pocket to allow a bigger prop. I'm wondering how long the pocket needs to be, or if just a flat section would be better?

[captword]

the old time builders that i learned from sAID THAT THE FURTHER FORWARD THE HEAD OF THE TUnnel runs the less power that you would loose.the tunnel gives you less draft and an opportubity for a bigger wheel. but you have to factor in the change of bottom and water flo that the tunnel creates. one old builder said that the best you could get out of a tunnel if done maximumly right was 75%. meaning that you could take a hull that had reg bottom and the hull would do 20 knts. if you took the same hull and put a tunnel in it.if you did the tunnel perfectly the best you could get was 15 knts. i havent done the math but makes sense. as far as a pocket. the pocket doesnt mean anything with out considering the flow to it. if you dont have full flow to the prop you cant get full power out of the prop. the only exception to this rule is the split hub prop where the prop is half in half out of the water. as far as pockets go I guess it depends on what you want the hull to do. a trawler hull would be minimally be effected by a pocket. a planing hull on the other hand pocket and flow are nore important. I run one boat that had what i would call a pocket. maybe not what you are talking about. but it had an indention over the prop. a pocket. it went up from forward of the wheel and then it returned back down to the hull level behind the prop. so it lost 5 inches of flow in as well as flo out.it was a 50ft planning hull with 1600 hp. it should have cruised atleast 26 or 28 for its weight and bottom but it was all the hull could do to get to 22 knts. may or may not answer your question, but I hope i gave you some points to ponder. any further questions will be answered to the best of my ability.
Capt word

[Portager]

I would be interested in knowing if there is a performance penalty of using a tunnel at displacement speeds. I was planning to use a tunnel on Portager to reduce the draft, but on a long range “Passagemaker” efficiency at displacement speed is very important. Portager will cruise at 8 knots which is 1.17 times the square root of the waterline length. I think that at this speed the performance ratio (defined as the actual speed with a tunnel / actual speed without a tunnel) would be about 98% to 99%. Does anyone know of any studies that either confirm or deny this assumption?

At maximum speed (i.e. 12 knots or 1.755 times the square root of the length, which is in the semi-displacement mode) I am less concerned about efficiency, since I will primarily using that speed when I’m in a hurry and range to the next fuel pump is short. I expect the reduced propulsive efficiency at this speed will drive up the power requirements and reduce fuel efficiency.

Thanks in advance for any information anyone can provide.

[woodboat]

Viking claims their new pocket drive mirrors the performance of there standard bottomed boat. http://www.vikingyachts.com/news_prop_pock.asp