Offset Prop

[Sayla]

I'm looking at a Herreshoff offset prop arrangement and thinking why do they do that -
Does it really sail better?
Is is really a stronger keel and post arrangement?
Was motoring so uncooth that it got put over there somewhere?
Is it easier to install and maintain?

Anyone own one?

Thanks for your thoughts

Sayla

[peter radclyffe]

the rudder usually has more effective leading edge
its harder to install as you often need a custom made bracket & log
its a stronger post & uses less timber
its still popular for at least one other reason, a better flow of water to the prop, tho its harder to steer
its better with a feathering prop

[Peerie Maa]

+1

There have been a couple of mag articles that report that sail performance and handling under sail is much better without the aperture for a prop. Peter did not mention that the dead woods are much easier to build as the bolts can go where they want, and do not need to avoid the stern tube.

[Sayla]

So - on a new build - would you do it?

There's something not quite balanced in the photo above - not 'till after some of my wife's curry anyway

Sorry - just had to say it

Sayla

[Peerie Maa]

So - on a new build - would you do it?

There's something not quite balanced in the photo above - not 'till after some of my wife's curry anyway

Sorry - just had to say it

Sayla

Yes definitely build that way. Use a Cutless bearing in the A or P bracket. If you cannot source a bracket of the right size they should not be difficult to fabricate or have cast. If the prop diameter is not too large a wooden propeller boss, aligned with the water flow, and carved to minimise dead water, may suffice.
With a hydraulic drive you can put the motor where ever suits the cabin layout best, and you can angle the shaft so that she will run straight. The rudder stock will be easier to do without the aperture.

[mizzenman]

could this be done with a saildrive?

[willmarsh3]

I'm thinking the boat is going to want to turn left until it gets a little bit of ways on. I had a side mounted trolling motor on my double ender. It wanted to turn real bad until I got a lot of ways on. Then I moved the motor back as far as possible and as close to the centerline as possible without impinging on the rudder. It worked much better and I could control the boat at much lower speeds.

Judging by the drawings though this characteristic would most likely be at a tolerable level.

Given the simplification of the deadwood and drilling the shaft log I'd definitely do it that way.

[peter radclyffe]

<a href="http://photobucket.com" target="_blank"><img src="http://i791.photobucket.com/albums/y...ME7/img288.jpg" border="0" alt="Photobucket"></a>

heres one i patterned & fitted to lulworth

[SchoonerRat]

I always thought that the primary reason to use an offset prop, is so you can remove the shaft without dropping the rudder. The only other reason I can think of is to accommodate an interior design feature. Only the first reason really makes any sense to me.

Does it affect the boat's handling? -- YOU BET IT DOES!

Under sail you'll never know it's there, but under power - at low speed - in tight quarters.

Kelpie (65' LOD Schooner) has an offset shaft. She'll turn any way you want...as long as it's left.

The time you'll really see the difference is when you are backing hard. An offset shaft will walk the stern in the direction of the offset. You need to think ahead an extra step or two so you don't put yourself into a place you can't get yourself out of. If you're smart about it you can actually use that odd behavior to your advantage.

[Jay Greer]

With a prop set in an apature, it is hydrauicaly blinded by the keel during part of each revolution. Hence it is less affective than an off set prop. It does take a bit of getting used to in reverese but, once one is used to it, the boat can be manuvered quite well. A folding or feathering prop is a very good idea.
Jay

[BarnacleGrim]

Just out of curiosity, has anyone ever made a twin screw sailboat? I guess it goes against any good judgement with that kind of cost and complexity, but think of the maneuverability in tight quarters!

[gibetheridge]

Does it have enough transom to keep it simple and mount one or two outboards? Excellent maneuverability and redundancy with two, more space inboard, no prop drag when sailing and probably some other advantages I'm not thinking of.

[Sayla]

SchoonerRat
If you're smart about it you can actually use that odd behavior to your advantage.

I wondered about that - like a mollyduka unusually good at a sport (that's Aussie for left-handed, or left facing surfer)

Mizzenman
could this be done with a saildrive?

Well, it could --- but the alloy suffers adversely (corosion) from any coppered bottom paint I would use, and I believe they're a bit of a pain with intake blockages - 2 things I'd prefer to avoid.

Peerie Maa
Yes definitely build that way. Use a Cutless bearing in the A or P bracket. If you cannot source a bracket of the right size they should not be difficult to fabricate or have cast. If the prop diameter is not too large a wooden propeller boss, aligned with the water flow, and carved to minimise dead water, may suffice.
With a hydraulic drive you can put the motor where ever suits the cabin layout best, and you can angle the shaft so that she will run straight. The rudder stock will be easier to do without the aperture

.

I'll look into all that

Thanks for discussion so far (so much to learn)

Sayla

[SchoonerRat]

Just out of curiosity, has anyone ever made a twin screw sailboat? I guess it goes against any good judgement with that kind of cost and complexity, but think of the maneuverability in tight quarters!

Lancer!

If you actually want to call it a sailboat.

Find your own pic. I'm afraid to Google it! I know I'd find it.

I can also recall some very nice sailboats with twin screws. They were also very large.

[BarnacleGrim]

This one? I suppose it makes sense with very large boats. But I'd probably consider thrusters or flap rudders as well. I'm a KISS fan, A 100 RPM thumper and a fixed pitch screw is all it takes to make Emma Maersk go.

[Jay Greer]

The Herreshoff "Meadowlark" and Errol Flynn's "Zacca" were both designed with twin screws. I believe that "Zane Grey's" 190' three masted schooner, "Fisherman" had twin screws as well. Another comes to mind, the Aberking and Rasmussen 90' ketch "Sea Diamond" which was built for William Bartholomae.
Jay

[Sayla]

They're pretty big sounding boats, well maybe not the Meadowlark - probably need twin screws - (and engineers and firemen etc)

I have wondered about the benefits of twin liftable well-mounted outboards - I know Reuel parker specifies twin outboards in wells for some pretty big boats (50')

Sayla

[Ian McColgin]

The well considered off-set prop, as LFH drew them, works out rather nicely ahead with the angle of the prop cancelling the away from center thrust. The prop aperature is a real horror in performance. When Goblin's engin was out for more than a year and I just plopped a plywood blank in the hole, I picked up almost two knots. If you have a feathering prop, as is normal with off-sets, you gain even more. The only problem is that even the best off-set installations take a bit of understanding to handle in reverse. That's a graceful understatement for the few here who have mastered that particular bit of psychosis. It can be done. It is predictable. But if you don't take the time to use it, you will suffer.

[rbgarr]

LFH designed another shallow draft ketch, Golden Ball for twin screws, and Spaulding Dunbar did likewise.

IIRC Golden Ball was 'featured' in LFH's "The Compleat Cruiser" as 'Tranquillo'.

There's one for sale here (see 47' Beautiful Dreamer): http://cppyacht.com/wood.html

Saildrive might work as offset of the boat had a very flat bottom. I doubt they'd run well if installed at a canted angle.

[BarnacleGrim]

So how is the offset prop angled laterally? Parallel with the keel, or perhaps angled towards the centre of lateral resistance?

[Sayla]

BarnacleGrim
So how is the offset prop angled laterally? Parallel with the keel, or perhaps angled towards the centre of lateral resistance?

This one has the prop shaft CL on plan as parallel

Ian McColgin
The well considered off-set prop, as LFH drew them, works out rather nicely ahead with the angle of the prop cancelling the away from center thrust. The prop aperature is a real horror in performance. When Goblin's engin was out for more than a year and I just plopped a plywood blank in the hole, I picked up almost two knots. If you have a feathering prop, as is normal with off-sets, you gain even more.

There doesn't appear to be any horizontal/lateral angle - do you mean you covered in the rudder aperture?

I've discovered another thread too - so I'll put the address here not to lose it http://forum.woodenboat.com/archive/...p/t-49304.html

Sayla

[AJZimm]

One other potential downside to an offset prop, that I can attest to from personal experience, is that it potentially can put the prop closer to the surface near the side of the hull and make it more susceptible to line tangling. Years ago, sailing a C&C 29 with an angled, offset prop, I was backing astern with the dinghy painter not snugged in enough and the prop sucked the painter down and around the shaft, breaking the line and tearing out the painter pad-eye from the float (it was an inflatable dinghy). It was an expensive lesson in dinghy towing but I am pretty sure it wouldn't have happened with a prop ahead of the rudder

[Jay Greer]

Towing the dinghy from the quarter lessons the chance of fouling the prop as keeps the dinghy and the painter on the outer edge of quarter wave away from the hull.
Jay

[Peerie Maa]

One other potential downside to an offset prop, that I can attest to from personal experience, is that it potentially can put the prop closer to the surface near the side of the hull and make it more susceptible to line tangling. Years ago, sailing a C&C 29 with an angled, offset prop, I was backing astern with the dinghy painter not snugged in enough and the prop sucked the painter down and around the shaft, breaking the line and tearing out the painter pad-eye from the float (it was an inflatable dinghy). It was an expensive lesson in dinghy towing but I am pretty sure it wouldn't have happened with a prop ahead of the rudder

The trawlers from Morecambe had their boats built with a quarter prop, the same as their conversions from sail, because it allowed them to keep the prop as far from the gear when shooting and hauling as possible. Do as Jay said and to tow the dink from the side away from the prop.

[gilberj]

I had an old British gaff cutter with an offset propeller. The offset angle counteracted the torque when going ahead, to the extent the helm was pretty neutral at half speed or better. Astern she turned to port fairly distinctly and predictably. It does allow the deadwood to be uninterrupted and allows a clear flow to the rudder and the prop. I'd say you will get more speed with a given HP, offset than on the Centre-line. the prop will be a little more vulnerable to fouling lines at least on one side.
JG

[Breakaway]

Interesting thread. Coincidentally, I was just browsing through some Atkins plans tonight that featured offset props. Here's what they have to say in the description of thier Rough Weather design.

It will be noticed that the propeller shaft is in the center of the rudder post, but the flywheel is splayed off toward the port side. Thus the shaft clears the butt of the mizzen mast, and the slip stream from the propeller (because it is kicking to starboard) will definitely keep the boat on a straight course. There is nothing experimental about this splaying of the propeller shaft. I have installed this system in five boats and the results are even better than I expected.

[purri]

Ours is offset, most efficient!

[Sailor]

I spoke with Bill Peterson about offsetting the shaftline in his 57 foot coaster design (Fritha). He wasn't really big on it. I'm pretty sure I'll do it if I build her though. I like the idea. Either that or twin screw, regenerative electric motors. LARGE slow turning screws to give me the most regenerative power at slower speeds.
Here's what he had to say way back in 06 "Offset propellers have problem is strengthening and waterflow while sailing. They work, but may not be ideal for this boat and also they should be a folding version while that protected by the deadwood could be a fixed two blade. Could also be three if you accepted the drag penalty. I recommend a Luke folding prop like Fritha has." He didn't say it couldn't be done.....

[John Little]

One other potential downside to an offset prop, that I can attest to from personal experience, is that it potentially can put the prop closer to the surface near the side of the hull and make it more susceptible to line tangling. Years ago, sailing a C&C 29 with an angled, offset prop, I was backing astern with the dinghy painter not snugged in enough and the prop sucked the painter down and around the shaft, breaking the line and tearing out the painter pad-eye from the float (it was an inflatable dinghy). It was an expensive lesson in dinghy towing but I am pretty sure it wouldn't have happened with a prop ahead of the rudder

It can happen with a centreline prop too, as I know from personal experience. I learned to always shorten up the painter to the dink before going astern, or else pull the dink alongside.

[Roger Cumming]

Francis Herreshoff gave six advantages of the off center propeller: 1. Shaft does not interfere with the keel bolts, 2. Yacht is faster and more economical under power, 3. Yacht is faster under sail, 4. Yacht steers better under power, 5. Yacht steers better under sail, 6. If the motor is removed removing the shaft is simplified. Having sailed a boat with an offset shaft I think the difficulty when in reverse, which can be severe, is a small price to pay for the other advantages. LFH's design for the Diddikai, a 36' ketch of 18,000 lbs displacement (Design #74) has an off center shaft parallel to the boat's centerline, an engine without reduction gear and a 14 x 10 feathering prop.

[kbowen]

I spent a lot of time in my teens on a boat with a folding, offset prop. This is double-trouble when trying to back up, both from it cranking in a curve, and also from the prop folding in reverse, but it otherwise was well-behaved, and the boat sailed in the faintest whisper of wind.

[Sailor]

Simplicity is what I'm after. Large slow turning fixed pitch that will freewheel and regenerate underway is what I'm looking for. Then you don't have to worry about it folding up in reverse. Angle it the right way WRT the direction of rotation and they should come pretty close to cancelling themselves out too. I once sailed with a guy who sat on the bridge for about a minute and explained the math behind it, while doing it in his head. he figured that because the screws were offset so much off the centerline of the ship, if you were to turn the azimuth thrusters so many degrees in, it would drive the ship in a straight line. We turned to the helmsman and told him to put the thruster at whatever that number was and sure enough the ship went straight if we let the other screw freewheel. He was right. The math isn't hard and we used rough numbers to figure it out. If you're building from scratch, I'd say figure out the numbers and offset it the right way. If she's a right hand screw offset the screw to Port. If a left hand screw, offset to Stbd. Hoiw much to offset, you just have to figure out where the screw is pushing on the boat (Thrust block) and then do the math to figure out the angle required to have the thrust from the screw go fwd through that point on the boat. I think we calculated 6 degrees or something. Mabye 6.5. It was interesting to see it work out in practice after figuring out the theory.
BTW, I absolutely HATE that man. Never liked him. He was an arrogant SOB and the sooner he drops off the face of the earth the better. Thankfully I'm now in the submarine world and won't have to work with him again!

[Ian McColgin]

Sailor has some interesting thoughts but on some I have a bit of disagreement: the righthand screw ought to be to starboard, lefthand to port for safety, to minimize debris in the prop, and prevent cavitation. The prop does not just screw through the water but also throws water about through cenrifical force and pulls water in a sort of spiral. The prop draws water more cleanly if the prop is moving upwards while near the hull and keel and then outward under the deadrise. If it's turning the other way, floating debris gets drawn in towards the prop and the blades are confronting generally messier, more aerated water. And certainly on auxillary sailboats the prop whose blades are turning towards the hull at the top are more dangerous to swimmers.

One might well angle the off-center prop out a little to cancel the effect of it pushing from other than the center line but this is far less necessary if the props are arranged as I mention, righthand to starboard, since the lateral prop walk will also be somewhat cancelling the off-set.

In reverse the off-set prop is regarded as psycho but that's really wrong. It's just very stubborn. But totally predictable. Use it.

A fixed blade prop actually slows a sail boat more when freewheeling than when locked. Obviously a prop that's powering a generator is even more drag. This may be a desirable trade to get some electricity into the batteries, but it will take a couple knots off your speed.

[Duncan Gibbs]

Just bumping this thread to ask you, Sailor, exactly what the formula is for calculation on the horizontal angle of offset and others if an offset scenario has any bearing on prop size/pitch?

A few of us in Hobart heard from various people that prop walk made helming under power a difficult exercise due to having to constantly compensate for the offset (like a simulated heavy weather helm) and increasing the frequency and severity of course correction in almost all seas.

[Duncan Gibbs]

Bumpety bumperama!

Another point is whether or not port side angled offset should kick to starbord or port and same question for starbord side, as well as which side is considered the best side to place an offset on?

[Ian McColgin]

If you look at a catalog of designs by someone like LFH who did lots of everything - centerline, off-set parallel to centerline, and off-set skewed - you'll see that there are varying considerations. The off-set prop has some tendency to steer the boat. If off-set to port it will push the boat starboard. If you skew the prop a couple degrees out, that will off-set the tendency to steer the boat. In terms of motoring efficiency, it's a near question as to whether some loss of efficiency due to the rudder working with the parallel prop is less or more than the inefficiency of a propwash that's not straight back. Both are more efficient in terms of clean thrust and in terms of hull turbulence than a centerline prop in a hole in the deadwood. But the latter is the most protected.

While rare, you can find power boats with off-set props. Old time lobster boats off-set the prop to port since most pot haulers are by the helm to starboard, most fishermen being righthanded. This put a nice solid skeg between the prop and any lines. Also since these guys often work right on the rocks, having the prop protected from a bump on a ledge that would only happen on the starboard side is a good thing.

For most off-set props on sail boats the down-angle of the prop shaft and the small size of the prop both mean that prop walk from the prop's rotation is very minor. Even with a centerline prop, the rudder straightens things out in forward so prop walk only matters in reverse. With an off-set prop, whatever little prop walk there is gets much overwhelmed by the effect of reversing with a prop off to one side and it will especially pull with a skewed prop. You have to really learn the boat.

[Duncan Gibbs]

Thanks Ian. Greg (Larks) has pretty much said the same difficulties are true for reversing sailing boats CL props as well. So could I rightly assume that sailboats in general are not much chop at going astern? If this is the case then all I really need to worry about is ensuring the prop pitch (on a folding prop), angle on the offset (both vertical and horizontal), transmission and engine are all carefully matched for maximum efficiency, including helping keep the boat going straight when motoring. Could I assume that a small amount of vertical angling to the shaft will also help?

[Sailor]

The formula was simple geometry. Vector addition such that the thrust of the screw was offset enough to drive the vector through the thrust blocks. Like I said, he did it in his head with rough estimate numbers. Draw it on a napkin and you should get a pretty good idea of what you need to do to make that happen. We took the distance off the CL of the screw, the distance from the Screw to the thrust block and worked out what the corresponding angle would be.Call the thrust vector the hypotenuse and the distance off the CL the short edge of the triangle and you're left with the long edge, or the CL. If you can rough in the distance off CL of the screw you've got your short edge, if you rough in the distance from the screw to the thrust block, you've got your Hypotenuse. With two sides you can figure out the rest with some High school trig. Take your 90, find the angle of the long and hypotenuse edges and you've got how much to angle in the screw. We had azimuth thrusters so were able to play with it. If you do the work out on paper first you can build the boat that way and skip the playing around with it. We stayed on course pretty easily with that setup though. It was great teaching point. Probably the only good one he ever had for us. Like I said, the man was a low life rotten SOB and I still hate him to this day. but he knew his trade. People skills not so much.

[Duncan Gibbs]

Just a quick bump for this while the question is on my mind:

What does the through hull for the shaft look like? How is it reinforced? What does the log look like and how is the bracket reinforced? I assume you'd still be able to use either bellows or a dripless stuffing box if you wanted. Could you still fit an Aqua-drive?

Pictures would be very much appreciated.

[rbgarr]

Just a quick bump for this while the question is on my mind:

What does the through hull for the shaft look like? How is it reinforced? What does the log look like and how is the bracket reinforced? I assume you'd still be able to use either bellows or a dripless stuffing box if you wanted. Could you still fit an Aqua-drive?

Pictures would be very much appreciated.

It's not much different in theory or practice from the arrangement on one of the engines in a twin screw powerboat where the shaft penetrates the planking.

[Duncan Gibbs]

I know next to nothing about powerboats Dave. Is there some kind of shaft log that butts up to the planking with a tube coming out which is caulked around its join to the planking?

[Peerie Maa]

In both cases there would be a solid block of wood faying and bedded against the inside plank, with its forward face square to the shaft cl. Water tightness would be provided by accurate fit, good bedding compound and adequate fastening. This would be bored to take the stern tube with a sliding fit. The stern tube will have also been well bedded in with putty or mastic.

[Duncan Gibbs]

Thanks Nick!

And the cutlas bearing is in the bracket, correct?

[Peerie Maa]

Yes.
If the tube goes right to the bracket, which can be stronger, there needs to be angled water inlet holes drilled into the tube to draw water in to the cutlas.

[rbgarr]

I know next to nothing about powerboats Dave. Is there some kind of shaft log that butts up to the planking with a tube coming out which is caulked around its join to the planking?

Here's a consideration of ways and means: http://www.gartsideboats.com/faq/sha...-assembly.html

[CapnJ2ds]

1. Cutless Bearing, not cutlass. ('Cos it's continuous - doesn't have a cut halfway round.)

2. Using a fixed-blade prop to generate power. Bad idea. It's been tried many times over the years, and never been really successful. A bit over 30 years ago tank tests on both sides of the Atlantic determined that while a 3-bladed prop fixed in position created a fair bit of drag, a free-wheeling one created a lot more - almost the equivalent of towing a bucket of the same diameter. Because of friction etc., the free-wheeling prop never turns quite fast enough, so presents almost a solid disc - only worse!

3. Folding/feathering props. I'd suggest a Kiwiprop. The bro-in-law installed one on his 11m sloop and reported improvement in both sailing and motoring - I've had to move the boat about the marina from time to time, and it sure goes backwards handily. A very experienced boatbuilder /designer/sailor put one on his 10m keeler, and reckoned it was worth 15% on his handicap - until the handicapper found out about it!

4. Handling: As mentioned above, it's mostly a matter of learning to make the quirks work for you. My early experience with offset props was in boats engined in the olde Englishe style - you know, a 12m keeler with and engine you could carry under your arm. They would only motor ahead very slowly in calm conditions, and handled like, well, rocks. Later on I owned a 13m cutter with an offset prop and a decent sized diesel engine. Once I got the hang of her, she'd do what you asked. Enough power was the answer.

[Duncan Gibbs]

Thanks for the replies!

I like that Gartside link Dave. I suppose one advantage of an inline prop is the thrust is more easily distributed in the structure of the vessel. I'd assume that an offset with an aqua-drive system could be the solution as it's thrust is taken up directly by a floor the shaft assembly passes through.

Nick, I do see both advantages and disadvantages of the tube going all the way to the bracket. The advantage is the strength as you've said. The disadvantage is it seems like a really good place to clamp a streamlined anode onto, as well as the possibility of starving the cutlass/cutless bearing.

Capn, I've always understood it was called "cutlass" because that was the trademarked name for the first set of stave bearings produced commercially. I've seen both spellings used by respectable nautical people everywhere.

I'd never use a prop to try and generate electricity. I've read Nicky Perriman's column on the subject in Classic Boat and it appears to have not been really successful. Anyway, the whole idea of an offset is to improve sailing performance. I've been on a boat whose prop freewheeled under sail and the sound of it was nearly as bad as having the engine running. I'm sure the vibration cannot be good for the whole shaft assembly or the gearbox. I want minimal/isolated as far as is practicable electrical installation anyway.

I've thought about a Kiwiprop, but at the same time I like the idea of a folding prop to really reduce drag to a bare minimum. It also has the advantage of reducing the likelihood of fouling the prop and will almost certainly make it easier to clear a line as well.

As far as handling goes, I really like the idea of using the prop kick to advantage to reduce prop walk and make the helm neutral when motoring as Sailor has suggested.