80 Year Old Trawler - Nice Lines

[fredrosse]

Got these lines for a 1930 Motor-Sail Trawler for the North Sea, 91 feet long, 170 tons fully loaded.
Scaling it to 30% size, it would make a very nice steam launch hull, a bit less beam than most,
and not quite a plumb stem, but just about right for efficient running and a seaworthy design.
Probably would need bilge keels.



At 30% size, these are the scaled parameters:
Length: 27ft - 4in, Waterline Length, 26ft - 7in
Beam: 5ft - 11in Draft: 2ft - 3 in at 4.6 Long Tons Displacement

Original Speed - Length Ratio gave 10 knots for the little ship, translated to the scaled launch:
5.5 Knots with 5.02 shaft horsepower, swinging a 24 x 26 prop at 350 RPM.

Pushing speed up to the maximum, = 1.3 x SquareRoot of Waterline Length = 6.7 Knots
for the scaled launch: 9.2 shaft horsepower, swinging a 24 x 26 prop at 425 RPM.

[Rainer]

Oh, I am always interested in such calculations. Especially the HP!

5.5 Knots with 5.02 shaft horsepower, swinging a 24 x 26 prop at 350 RPM.

Which slip?
When I am calculating with 5.5 knots = 6.3 mph = 10.2 km/h
with 350 rpm I get
a pitch without slip of 19.1"
makes about 27% slip to get 26.2" pitch.

6.7 Knots for the scaled launch: 9.2 shaft horsepower, swinging a 24 x 26 prop at 425 RPM.

6.7 knots with 425 rpm needs a 19.2 pitch according to my calculation.
So with 27% slip again this is 26.2" pitch...

I have read in different books to calculate with a pitch of 40% up to 50% for boats like ours?

I would like to know some real prop/speed figures from all of your boats to get a better overview!

I would also love to learn more about your estimated shaft hp calculation to put it on my webpage.
In the moment you can calculate the pitch here:
http://www.steamboating.de/rami/rami-de-4010.php
- in German only - but numbers are international...

[fredrosse]

The information I posted just assumed 25% slip, as a first estimate of typical performance. Most pleasure cruising boats have undersized propellers, which waste power, but cost less, and do not have clearance problems of larger diameters. These would have 40 - 50% slip range, but would not be a wise selection for our low powered steamboats.

I have some old software that develops power and other parameters for displacement hulls, but it needs some familiarity with the process to be useful. For propeller software, the US "Boat Design Forum" has many members that have recommended this site for good basic propeller technology. It is freeware, and I have not yet used it, but evidently it treats the technology well: http://www.mh-aerotools.de/airfoils/javaprop.htm

[Albert]

Hi Rainer,

I would like to offer some figures regarding the relationship RPM-Speed-Slip for my boat. These figures resulted from a GPS measurement carried out two years ago. They clearly show that the slip of the wheel depends on how close the actual speed comes to the hull speed of the boat.

The figures for this 20 x 30" wheel were:

a) 86 RPM - 2.0 knots. Corresponding propeller slip: 6%

b) 135 RPM - 3.0 knots. Propeller slip: 11%

c) 258 RPM - 5,3 knots. Propeller slip: 20%

The hull speed is something around 5.9 knots. The total weight was about 1,100 kg (2,425 pounds).

Albert

[fredrosse]

Albert, would you be so kind as to put up your boat information on the "Member's Websites and Boats" Section, within the thread "Member's Steamboats - A Simple List for All".

It is very good to have the technical information you have supplied here, illustrating that slip increases at higher powers, but now with some quantification.

[Lopez Mike]

I'm traveling and away from my HP-48 calculator just now but if anyone is interested some data from my boat testing it is posted under On the Water Testing of Folly. I never thought of reducing the data for slip.

The rpm is from a digital tach and the speed is from a GPS in calm water and fairly light winds.

Mike

[Albert]

Mike,

I have taken the liberty to reduce your data for slip, with interesting results...

100 RPM - 1.6 kn - Slip 3%

200 RPM - 3.0 kn - Slip 10%

300 RPM - 4.5 kn - Slip 10%

400 RPM - 5.2 kn - Slip 26%

520 RPM - 6.1 kn - Slip 40%

These figures seem to confirm your perception of a "sweet spot" around 300 RPM. Beyond 400 RPM something in the system starts to oppose violently against an easy progress.

Albert

[Lopez Mike]

Probably just the old square law sneaking up to grab me. The boat is 24' over all and I'll guess at maybe 21' water line. Square root of 21 is about 4.6. Times 1.2 is 5.5 kts. Things get really difficult above that speed. It does drag a heck of a stern wave at those speeds.

I guess those pointy headed naval architects know what they are talking about.

When the boat goes back in the water it will have a 24" pitch wheel on it for an experiment. My idea is to see if it makes the boat nicer to be in at moderate speeds. Overdrive, so to speak. I'll take some numbers then.

Mike

[Rainer]

Hello All!

I am back from a two day business trip - so thanks for all the data and calculations. After all of this I think it is good to calculate with at least 40% slip for the upper non economic speed level...


off - topic....
Some pictures from my trip:
This is Munich in the morning. Unfortunately this canals around Nymphenburg Palace are not navigable any longer:


http://www.schloss-nymphenburg.de/engli ... /index.htm

At my way home I did a little detour over the Harz (little mountain 1142 m over sea level - train goes up to 1125 m without cog rail! - blue line in map) to visit the famous Harzquerbahn - Harz-Cross-railroad. It has a 140 km long narrow gage system with an all year full steam operation on most regular trains


This picture was taken by me tonight. The locomotives are always under (on?at?) steam. Last steam train here in Wernigerode for today was 18:45. Next steam train tomorrow morning is 8:55 - so no time to cool down...

They have still 17 of this engines running. They where build in 1954-56 right after WWII in the GDR. After "the wall" came down they transfered this from a hard all day business to a main tourist attraction in the Harz region - about 1 hour by car from my home...

Here you can listen to some whistles if you like
http://www.hsb-wr.de/hsb_barrierefrei/klingeltoene/

Here are some data about this engines
http://www.hsb-wr.de/hsb_barrierefrei/h ... 72date.htm

hopefully less than 50% slip on the mountain ramps

[Lopez Mike]

What is the gauge (the distance between the rails)? What wonderful pictures!

Mike