New diesel-electric hybrid installation - how to size propellers?

I contend that motorsailers should motorsail at every opportunity. I have electric drive on my motorsailer, and run my motors at a low speed when motorsailing for battery economy. My motorsailing speed is always faster than speed under sail alone or just motor at a higher power in the same conditions.
As to the ability to sail/motor up wind, that would depend on the wind, doesn't it.
Some years back, I was lead (center) tug on a 3 tug tow of a jackup oilrig. A norther blew in and we were suddenly sailing backward. I had 8 feet of 11 inch braided nylon shockline shackled between my tow wire and rigs bridles and I parted it 3 times and had to re-rig. I WAS NOT conserving power. These were 3 deep sea tugs, each rated at 4200 lbs bollard pull, or 4200 hp,(tugs daily hire rate is based on its hp; ie: bollard pull). We couldn't power up wind or even hold her against the wind untill norther passed. Wind was 40 knots with up to 55 kt gusts.Rigs air draft 140 feet; its 3 legs were 140 feet high.

 

woops, left off a couple zeros...42,000 lbs bollard pull

 

I think we have got to what Tuchman calls "folly":
i.e. the wilful pursuit of a course of action when there is unambiguous,
advice to the contrary, and when a clear, realistic alternative is available.

See you on another thread, Will.

 

2 to 3 HP per ton is NOT an opinion, it is an observation from a century+ of engines in boats.

 

You're talking about a pure power application. This is a motor sailor. Look, you're not going to dissuade me from sea trialing this, I'll post the results after launch in February. The facts can speak for themselves.

 

Ray,
Please forgive me if I take this as a calculation exercise.

Using David Gerr's charts for estimating the boat speed I get that a 56' displacement boat, displacing 50 tonnes should make almost 7 knots with 50 kW. Assuming a propeller efficiency of 60% the propeller push F is calculated as 50,000 W * 0.6 = F [N] * 3.5 m/s
F = 8.6 kN

Because it's an electric drive, the same shaft torque (or higher) can be generated at any shaft speed between zero rpm and the rpm for rated power. When the boat is just holding it's position in a very strong headwind, the propeller will still provide 8.6 kN push.

The force generated by the wind is calculated as:
F = 1/2 * air density * velocity² * Area * Cd

Air density = 1.225 kg/m³
Cd = depends on shape
Area = Cross section area, m²
Velocity = relative wind speed, m/s
F = force, N

Cd may be as low as 0.3 for an extremely streamlined shape. It's more realistic to assume a Cd closer to 1. For the sake of the calculations I assume that the cross section area exposed to the wind is 6 m * 4 m = 24 m².

If I solve the wind resistance equation for wind speed I get:

F = 1/2 * air density * velocity² * Area * Cd
8600 = 1/2 * 1.225 * velocity² * 24 * 1.0
velocity = 24 m/s (47 knots)

47 knot winds happens every now and then, but I've so far managed to avoid those conditions myself. There's always a situation where any boat is inadequate. You just need to know the limitations of your boat and yourself and act accordingly.

Back to the propeller selection. If the manufacturer says that it's OK to decrease rpm and increase torque (=current) proportionally, you must be aware that the losses in the motor are proportional to the square of current. You can increase propeller efficiency with a larger propeller but will loose motor efficiency, a pure optimisation problem. The question is also if you should maximize efficiency at maximum speed or at cruising speed when you are also using the sails. My gut feeling is that you should follow the motor manufacturers initial recommendation for rpm and select the propeller diameter accordingly.

Erik

 

I humbly remind folks, that naval architecture/yacht design isn't science. It's art. Formulas and calculations help. Following age tested principles and introducing change by small degrees is an tradition of the industry. But nobody really knows how a design will perform untill it gets wet, and I don't mean in a model tank. I doff my hat to the wonderfull artists who have created beautiful and seaworthy vessels over the centuries. And to those who create monstrous errors, like swivel chairs in the officers dining room too close to the table to swivel,have to climb over top, and that on a 1000 foot multi million dollar vessel, and worse more dangerous errors, to those, heres the bird.

 

Hi Erik,

Your figures match my own quote well (plus other advice received outside of this forum).

I'm with you - actually I personally never attempted to motor or motorsail into more than about 30 knot true winds in my last boat, a William Garden offshore ketch, which was never terribly weatherly (even with new sails) but at least sea-kindly - or at least I have attempted, but given up pretty quickly on finding actual headway over ground worse than heaving to - and having the fillings shaken out of my teeth. Anyone who has done the "Baja Bash" from Mexico to San Francisco as I have frequently will know what I mean...

Every boat has its limitation, I rather suspect the crew usually reaches their first.

 

Erik
I think this could reach about 8 knot speed based on my own VPP and also post #2 shows similar: http://www.boatdesign.net/forums/diesel-engines/handy-engine-sizing-formula-34157.html

The speed prediction formulas in the propeller handbook are not that accurate.

At 1100 rpm and 50 kW, you can't get 8.6 kN even with two propellers. Something like 6 kN is my guess.

You need to have some speed to be able to steer and then seaway drag is in the same order of magnitude as wind drag. But you also need to take into account that 24 m/s at 10 m height will be much less at 0-2 m height where most of the area is.

My earlier estimate was that about 15 m/s would be the limit for this ship. I would be interested in other views and also what is considered as "safe". Even for the acceptable 3 hp/ton there will be a wind limit for full control, but it will be much higher, maybe around 30 m/s.

 

In reply to erik818's calculations, just a few observations.
1) No real problem with the numbers if we accept the assumptions made.
2) If this were a vehicle travelling along a flat surface with a head wind then it is valid.
3) The crunch must come when we factor in what the sea surface is doing.
4) By the time 47 knots apparent wind speed is reached there is going to be quite a sea running.
5) Added resistance in waves becomes large.
6) Prop thrust now is not straight ahead due to boat pitching - small boats pitch more than large boats.
7) Wave resistance and thrust losses will likely be significant well before apparent 47 knots wind speed.
Some work still needed here?

Cheers,
Graham

 

Really!

I've made 5 knots (through the water) in a 4 ton houseboat with an 8Hp outboard.

That's 2Hp per ton, on calms sea for 100 nm...

-Tom

 

Actually, that gave me quite a chuckle, and reminded me of when I sucked a plastic bag into my engine raw-water intake in San Francisco Bay on my 25 ton ketch so I just lowered the dinghy with its 10hp outboard and tied it alongside. I ain't saying that lash-up was terribly maneuverable, and it took a while to accelerate - but I seem to have a distinct recollection of sliding along at 5+ knots in light airs... let's see, that's less than 1/2hp per ton, and vectored so badly I was dragging the rudder too.

 

Now I've added the superstructure to the Free!Ship model, I just ran the Holtrop 1988 method with 15m/s winds and 4m waves on the nose, Trawler shape and lots of other misc corrections, and at 6 knots got the figure of 5,000N for resistance. I'm sure there's a lot of room for error in my settings maybe even by a factor of 2 or 3, but I'm encouraged that in a motor-sailing application I can work with these calculations and progress to a sea trial.

It's very easy to find the analogue to pretty heavy conditions within a couple of hours sail from my dock here in San Francisco - you just head out of the Golden Gate on a NW windy day and turn right into the four-fathom shoals where there are plenty of good-size waves to play with... that way you can regularly find 12' waves and 35 knot apparent winds; properly ballasted, of course.

BTW we will be doing inclination tests as well as calculating stability.

 

Ray,

Have you considered towing the hull, loaded, and measuring the drag with a scale in the tow line?

It would give you an excellent data set at various velocities.

-Tom

 

It is NOT a motorsailor. It is an underpowered trawler with a steadying sail. It won't sail upwind.