propeller tunnels

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

 

Okay lets try this.

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?

 

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.

 

New Huh?

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.

 

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.

 

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.....

 

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.

 

10%

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.

 

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.

 

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?

 

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

 

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.

 

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