Heckler
Well-Known Member
Stop digging! Plus the load goes to the gearbox which is a spline fit in to the flywheel so no loads transmitted to the crank!
Stu
Propellor thrust - transmission to hull
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Stu
charles_reed
Well-Known Member
Most small boat installations (up to 40 hp) have "soft" drives - this means the thrust from the propellor (whether saildrive or conventional shaft) are transmitted through the engine mounts and bearers to the hull.
Some "meticulous" owners, recognising the naval architects and boat builders didn't know what they were doing, fitted a UJ (such as is made by Halyard Engineering) to give a positive drive. This does have the advantage of not demanding quite such a rigorous alignment of engine as a "soft" drive - but requires enough room to fit it and an appropriate moulded bearer. Nether of these are usually available in small sailboats.
In fact the friction grip, gearbox to shaft is a built in failsafe, slipping sufficiently to prevent gearbox or engine damage if the shaft is jammed.
Neeves
Well-Known Member
I've been one who has quoted the 100kg of thrust for every 10hp of engine. I did check it out on our cat, we have 2 x 20hp engines and at maximum cruising revs and a load cell between yacht and fixed point we get about 400kg of load, its slightly less but 400kg is a nice round number. This is with 3 bladed folding Volvo props. I have not tested on a mono, nor smaller nor bigger engines - so not statistically very sound.
I am happy, more than happy, for someone to provide a better piece of data. I am sure there are complex formula to calculate thrust, but I do like to measure things! There was an article in PBO? (YM?) recently on props - there might be more info there - but I recall they did not test a Volvo 3 bladed folding!
I do not think you can extrapolate to a 300hp engine nor an outboard.
edit, My 100kg per 10hp is a rough figure to indicate the sort of tension you can set up on an anchor rode when power setting an anchor. This load can then be equated to the data available from various sources of loads on yachts at various wind speeds. All of this data is indicative - its simply to give orders of magnitude and not suggested as definitive such that you can calculate which bolts to use in your engine mount! So you can be picky - but put the data into context. If you have better data - I would still value its availability close edit
Jonathan
I am happy, more than happy, for someone to provide a better piece of data. I am sure there are complex formula to calculate thrust, but I do like to measure things! There was an article in PBO? (YM?) recently on props - there might be more info there - but I recall they did not test a Volvo 3 bladed folding!
I do not think you can extrapolate to a 300hp engine nor an outboard.
edit, My 100kg per 10hp is a rough figure to indicate the sort of tension you can set up on an anchor rode when power setting an anchor. This load can then be equated to the data available from various sources of loads on yachts at various wind speeds. All of this data is indicative - its simply to give orders of magnitude and not suggested as definitive such that you can calculate which bolts to use in your engine mount! So you can be picky - but put the data into context. If you have better data - I would still value its availability close edit
Jonathan
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lw395
Well-Known Member
10hp is about 7500W
At say 7.5 knots or 3.8 m/second,
power = force x speed in si units, so
force = 7500/3.8 = 1956 N or 200kg if the prop was 100% efficicient.
50% efficient is more like it.
At say 7.5 knots or 3.8 m/second,
power = force x speed in si units, so
force = 7500/3.8 = 1956 N or 200kg if the prop was 100% efficicient.
50% efficient is more like it.
Neeves
Well-Known Member
Thanks Lw395
I'd fondly, well not fondly actually, imagined that the hp of our engines were at the crankshaft not prop shaft, so there must be inefficiencies of transmission and extra bits of kit, like the alternator and seawater pump. I had hoped that modern props, considering how much they cost, were a bit better than 50% efficient (but that's my naivety showing through).
But 50% efficiency equates quite nicely to 10hp offers about 100kg of thrust.
Jonathan
fisherman
Well-Known Member
Figures for assessing tug bollard pull, the first is broadly 100kg/10hp.
Tug equipped with fixed pitch propeller: (freewheeling) BHP x 0,9 x 1,10 / 100 = (t)
Tug equipped with fixed pitch propeller and kort-nozzle: BHP x 0,9 x 1,20 / 100 = (t)
Tug equipped with controllable pitch propeller: (freewheeling) BHP x 0,9 x 1,25 / 100 = (t)
Tug equipped with controllable pitch propeller and kort-nozzle : BHP x 0,9 x 1,40 / 100 = (t)
Tug equipped with fixed pitch propeller: (freewheeling) BHP x 0,9 x 1,10 / 100 = (t)
Tug equipped with fixed pitch propeller and kort-nozzle: BHP x 0,9 x 1,20 / 100 = (t)
Tug equipped with controllable pitch propeller: (freewheeling) BHP x 0,9 x 1,25 / 100 = (t)
Tug equipped with controllable pitch propeller and kort-nozzle : BHP x 0,9 x 1,40 / 100 = (t)
lw395
Well-Known Member
I just picked some vague numbers.
For a much faster boat, the thrust must be less per hp.
If the boat is going slowly, the same power engine can create more thrust.
But it's a big old force!
Car engines/transmissions often have 'torque stays' or some such to take the reaction. Never seen the equivalent on a boat....
earlybird
Well-Known Member
YM tested various propellers using a 21 hp. Yanmar. Measured "bollard pull" ranged from 270Kg to 195Kg; ie 130 to 93 Kg/10 hp.
I think that these thrust figures might be higher if the boat was underway rather than with the prop. operating in a stalled condition, but no idea of what such a factor might be.
I think that these thrust figures might be higher if the boat was underway rather than with the prop. operating in a stalled condition, but no idea of what such a factor might be.
fisherman
Well-Known Member
There are many factors to take into consideration:
http://www.dieselduck.info/machine/02 propulsion/2010 GL on Bullard Pull.pdf
http://www.dieselduck.info/machine/02 propulsion/2010 GL on Bullard Pull.pdf
Neeves
Well-Known Member
It was meant to be a simple and easily remembered blanket rule of thumb and was only suggested (though I did not mention) for 'our' sort of engines and 'our' sorts of props.
Jonathan.
fisherman
Well-Known Member
Yes, interesting the first calc comes to the same broad result though. Also that test is conducted in depth at plus 20 metres and far enough from the wall for the prop stream to develop and not be disrupted.
andygc
Well-Known Member
I have a 27-foot traditional wooden boat with the usual limited, narrow bilge space. There's plenty of room for the Aquadrive's thrust mounting which is no more than a piece of thick plywood with a hole in it fastened to the planking. Why would an AWB owner need a moulded bearer? Plywood, glass tape or mat, and suitable resin should work perfectly well.
Tranona
Well-Known Member
There is rarely enough room in today's flat bottomed boats for an Aquadrive, unlike in your type of hull. It could, however be mounted on a plywood floor bonded into the hull, but if it was done as a production item a moulded bearer as part of the inner grid would make more sense. However, the majority of such boats are now using saildrives so not relevant. Even the few shaft drives that are made manage quite well with flexibly mounted shafts reflecting both developments in engines and mounts to isolate noise and vibration from the hull structure.
andygc
Well-Known Member
Thanks, Tranona. Understood.
30boat
N/A
My Fulmar is as flat as most modern designs but still I've fitted an Aquadrive.The thrust bearing carrier is made of plywood encased in fiberglass and epoxy. Since I replaced the original Bukh with a Beta I was even able to make a conventional angled enginebox lid with steps that is quite a lot shorter than the original horror.
Tranona
Well-Known Member
That is because the engine is fitted further forward in the boat compared with other designs where there is often no space behind the gearbox to fit the extra length of an Aquadrive. Therefore to fit one would mean moving the whole engine forward. Saildrives are popular partly because there are less constraints on where they can be fitted.
STILL AFLOAT
Well-Known Member
As I was getting ready to re engine my boat, a few years ago, Either in the Vetus or Lancing Marine catalog, at the time, it clearly said, " Engines under 90 HP, do not need a Thrust Bearing "
fisherman
Well-Known Member
Whoa, hang on there: does that mean an Aquadrive under 90hp, or any engine? Does it imply that over 90hp does need a TB? My engine, 135hp, gearbox, PRM has its own thrust bearing, as does every marine G/B. An aquadrive is a bit like a car prop shaft, or carden shaft, and will not take end thrust.
STILL AFLOAT
Well-Known Member
It said, " Engines under 90 HP do not need a thrust bearing "
So yes going by the statement, I would assume that if your boat has an engine of 90 HP or more, then yes, some sort of TB is required. Obviously I have not kept the catalogs from way back when, perhaps they still say the same, but the way consumerism has risen over time, and everyone wants to boost cash & sales, it could be, that. that idea, has been scrapped. Anyway, on reading that " Engines under 90 HP do not need a thrust bearing " I did not bother, with a TB , as my engine was only 45 HP
fisherman
Well-Known Member
Having run engines up to 300hp I can tell you no TB was needed. What gearbox did you have on your 45hp? Guranteed it has a thrust bearing in the gearbox, you can't see it.