High torque engines

I have seem some steam engines that run engine in reverse. I would consider hidden electric engine for marina maneuvers. If I remember problem with steam was it taking time to develop steam. So a steam/electric would be a new concept. The bigger the prop the slow the turning the better for you idea of no tranny. At slow rpm you shouldn't get much cavitation. It looks like prop is pretty much tuck under hull.

 

ya the hull will be fitted with a electric torquedo 17 hp maneuvering thruster and two boilers one small and quicker to steam for slower speeds and one larger for cruising applications. Both can combine for flank speed. although it may end up advantageous to go with three of the smaller boilers adding each to the system as needed. Titanic ran 33 boilers for each engine and brought them on line as needed for efficiency. RIP.

 

The drag of 11669N is based on my quick sketch of the hull - see attached. The power on the hull to do 5m/s (a bit under 10kts) is 11669 * 5 = 58.3kW. Allowing for the 40 x 26 prop efficiency of 70% under these conditions, the shaft power works out at 83.5kW. I will take your word on the conversion to 112HP.

The pitch of 660mm is slightly more than 26". Don't worry about the other numbers.

The long keel aft will add a bit more drag but without getting detailed plans the results will be sufficiently accurate to do prop size comparisons. Interestingly I get a drag of 27160N at 7m/s (say 14kts) which gives very close to the installed power allowing for the prop efficiency.

Rick W

 

e gad does that mean that my first serious calculations and guessing engine size were roughly correct

40" of prop
thats huge
after reading all day on prop theory Im thinkin that the slow rpms ( 740 is nothing ) should result in higher efficiency

thing is
I got a ton of torque available in this engine
but I read little on the application of torque in a fluid medium
Im starting to think that if I gear the torque out of the system I might gain an advantage in a smaller prop that better fits this hull form

hp = Torque (rpm) / 5252

basically I can spend all that lovely torque and end up with hp by altering rpm

 

The slow rpm is only good if you have the room to swing a big prop.

My original figures were based on a 26" prop, which is about the maximum I thought you would get under that hull with the nominated draft. The best result with this is achieved with 1500rpm at 10kts. The required prop is 26 X 16. You will see the efficiency drops to 57% and the power climbs to 102kW. You start to see the advantage of big slow revving props but the penalty is more draft.

Splitting the power between two props where draft is constrained can be beneficial but then the boat needs to be set up for it.

Rick W

 

interesting
I just recalculated for a prop of 19" pitch with an engine speed of 880 rpm and a gearing ratio of .75 to 1 with a slip of 30%
that works out to an rpm of 1,173
but again this particular calculator isnt giving me prop diameter

gives me my mull speed at the optimal sustainable rpm range of the engine or about 75% of its available power

I thought that thing said a one meter prop
or 40"

an efficiency of 57% is downright grim
its your ball field but I thought that the normal efficiency range of a well designed prop was in the area of 70-75%

 

Boston

ok...so
1) as stated around 1.17ft?
2)...need to select an RPM...that the shaft will rotate at, for your most ideal speed. Either with an existing g/box, or a new one..but you need to select and RPM which you feel happy that the engine will do easily and your happy to let it for many hours..
3) Ok, at what displacement was this..roughly...is it about the same as your boat, if not roughly how much more or less?

Then a rough stab can be made.

As for getting too close to the surface, that where a quick calculation to establish the cavitation number comes into play.

As mydaulphin also noted too, you shouldn't worry too much since too many variables unknown or cant control...so just focus on the principal ones that do affect the design which you have some control over.

The key one being..who will make the prop?..if you are asking a supplier to make one, then all you need to do are these basic calc's to estbalish size and rough pitch and number of blades, then let the prop manufacturer sort out the details....if you are making it, hmmmm...different story!

 

Boston, have you priced a fully automatic hopper bin (fail safe) for your pellets? Also you will need bow and stern thrusters unless you always anchor out. With twin reversibe props operating seperatly you can delete the stern thrusters. I'm thinking what you save in fuel costs you might wind up a slave to your engine room. Also you need to really know your drive system costs. As a builder you know when building your own you keep adding and costs go up by the square factor.

 

http://www.hillpropellers.com/propcalc.html

is the calculator Im using

ya Im aware of needing to build a automated hopper feed system
no big deal they are all archenemies screws powered by small electrics
Ill use a toiled bowl float as a regulator if I have to but the thing should be simple to manufacture

Im thinking Im going to jam a 22" prop using Ricks numbers and going conservative

but dam that 57% efficiency is a pill to swallow

thing Im really curious about is all the conversions Ive done to get to this point
I started out calculating the joule's of energy used by the original engines and went from there
then I had to consider that the steam engine pounds out torque but at lower HP
bla bla bla
then there was the BTU value comparisons between pellets and diesel and the mass considerations
bla bla bla
this has turned into a bloody project to figure out if the engine size will work out
my initial guess about a month ago was that given that I could gear up the steam engine and use that torque for HP it would turn the appropriate sized prop

Mr Add
check out post # 21
its got the latest relevant answers
I think
this is going to change a lot before I get it all hammered out

oh
as I said
its going to have an electric torquedo thruster 17 hp on it
things not really designed to swivel but it will when Im done with it

 

well...57% is dream world stuff...you'll be lucky to get over 50%..i would aim at 45%. But if you wish you use software that just needs numbers, then that is your prerogative.

 

When youve built the boat, you then need a propellor to push the boat at the design speed or a little more -what ever you want. The propellor man will then tell you what Hp you will need and what ratio would suit the propellor he has made to push your boat at the speed you want.

You do not choose an engine then a gearbox then ask the propellor man to get you out of the ****.

You are doing it backwards. The horse goes in front of the cart!!

 

The 57% was for a 26" prop. If you can only jam in a 22" prop then the efficiency drops to 51% per the attached. So the pill is becoming even more bitter. Power to do 10kts now climbs to 114kW.

So roughly 50% of the hard won power from burning all the pellets just goes into churning the water and warming it up a tiny amount. As you are stoking the boiler you can think of one for the boat and one to warm the water. That starts getting you thinking about efficiency.

Rick W

 

It is possible to get prop efficiencies over 90% but you have to have it lightly loaded and this is only possible with an easily driven hull or a massively large diameter prop on a small boat.

You have a relatively small prop on a relatively large boat designed for times when fuel just oozed out of the ground and no one bothered about conserving the filthy stuff.

Rick W

 

hey Rick I found a calculator that gives me your exact numbers with the engine running at 880 rpm and 150 hp at 895 ft/lb

interesting stuff
Ill send a link in case you want to check it out
things got every bell and whistle you can imagine

http://www.lystsejleren.dk/propel/propcalc.xls

=========================================
Rick the propeller inefficiencies aply to all power sources so I dont think its a black mark on steam

Frosty I gotta design a boat that will hold the engines and type of fuel Im considering so I gotta start somewhere and then double check to see Ive got enough power to do the job
which is were the prop comes in
Im working out some details in rough form so I am sure the hole concept even flies at all ( might just go the way of the Dodo depending on the numbers )
and hey
I've owned lots of horses
even wrecked a few buggies
and yes
several of those wrecks ended up with the horse in front

Addman
I had no idea props were so inefficient
thats crazy

Rick
so there would be a benefit of leaving the prop speed slow and going with a monster prop
I read about that earlier but didnt catch part about light loading
although I like the idea of 90% efficiency Im starting to get the idea that I wont be getting quite that

 

So does your deisel fuel go into heating up air then within a split second be pushed out into atmosphere. The piston has to somehow absorb that expansion into reciprocating motion.

I think diesels are only 70% efficient. Petrol is less than that.