Larger boat feasible?

[barts]

From Gerr:

sl_ratio = 10.665 / ((disp/hp)**.3333)
speed = sl_ratio * ((length) ** .5)

So yes:

Speed in knots for various hp, displacement

HP| lbs: 5000 6000 7000 8000 9000 10000
-------------------------------------------------------------
1.000000 3.743220 3.522527 3.346116 3.200459 3.077252 2.971065
2.000000 4.716053 4.438003 4.215744 4.032232 3.877005 3.743220
3.000000 5.399180 5.080177 4.825987 4.616076 4.438003 4.285003
4.000000 5.941718 5.591404 5.311382 5.080177 4.884608 4.716053
5.000000 6.400474 6.023113 5.721470 5.472414 5.261745 5.080177
6.000000 6.802384 6.400474 6.081091 5.815757 5.591404 5.399180
7.000000 7.161014 6.738289 6.400474 6.123391 5.887280 5.683832
8.000000 7.485922 7.044565 6.691767 6.400474 6.154078 5.941718
9.000000 7.788239 7.329058 6.962012 6.658955 6.400474 6.179817
10.000000 8.063905 7.588471 7.208434 6.894650 6.629230 6.400474
11.000000 8.327514 7.835580 7.442517 7.118077 6.843707 6.607281
12.000000 8.573700 8.063905 7.661517 7.329058 7.044565 6.802384
13.000000 8.805510 8.285154 7.869414 7.526274 7.236103 6.986069
14.000000 9.022103 8.492820 8.063905 7.714811 7.419262 7.161014
15.000000 9.233821 8.686514 8.255418 7.893942 7.588471 7.329058
16.000000 9.436488 8.875391 8.434118 8.063905 7.755771 7.485922
17.000000 9.625248 9.064617 8.608325 8.231934 7.913787 7.639741
18.000000 9.818239 9.233821 8.775149 8.389564 8.063905 7.788239
19.000000 9.989433 9.406438 8.931307 8.539627 8.214495 7.928802
20.000000 10.159649 9.560654 9.081848 8.686514 8.352113 8.063905

To go a particular speed takes power in direction proportion to displacement -
but for a particular power, displacement only has a small effect on speed.

- Bart

[Lopez Mike]

Took me a little squinting at that table to get things sorted out (is this a test for astigmatism?)

Finally picked a number and started scanning about.

Starting at 5 hp and 5000 lbs = 6.400474 kts.
Then 6 hp and 6000 lbs = 6.400474 kts. and so on.

So fickle me. I will scale up your boat, Otter, to 24 ft. x 6 ft. and build it using stitch and glue. If the weight goes up as the cube of the length, it will weigh twice as much. Around 1000 lbs. Add my iron mongery and we are getting near 2000 lbs. I'm sure there is a flaw in all this theorizing.

Tomorrow a different plan.

[barts]

Took me a little squinting at that table to get things sorted out (is this a test for astigmatism?)

Finally picked a number and started scanning about.

Starting at 5 hp and 5000 lbs = 6.400474 kts.
Then 6 hp and 6000 lbs = 6.400474 kts. and so on.

So fickle me. I will scale up your boat, Otter, to 24 ft. x 6 ft. and build it using stitch and glue. If the weight goes up as the cube of the length, it will weigh twice as much. Around 1000 lbs. Add my iron mongery and we are getting near 2000 lbs. I'm sure there is a flaw in all this theorizing.

Tomorrow a different plan.

What does your boat weigh now?

- Bart

[Lopez Mike]

Dunno. My machinery weights are from the manufacturers brochures and parts I have weighed myself. Most probably not over 2K but 'we be guessing'.

[fredrosse]

Margaret S. at 19ft-3in weighs 2150pounds, about 550 pounds overweight, been adding too much stuff over a few years.

If you don't have a scale for your boat's weight, any number of establishments with a truck scale can give an accurate weight, drive on one day with the boat/trailer, then another day with trailer only.

I was told a minimum of 1 horsepower per ton of displacement, and two horsepower per ton can get maximum displacement hull speed (1.34* SQRT( LWL) ), with margin for wind and waves. Of course this assumes an efficient propeller, = big and slow RPM.

[Lopez Mike]

I'll ask around the island about some scales. If all else fails I can cobble together a thing to measure each wheel and the tongue with a plank and a bathroom scales. Archimedes to the rescue.

The builder rates my little engine at 5 hp/500rpm. I've never been able to reach 500. I've settled on a wheel 18 x 24) that gives me a nice cruise around 4.5 kts at perhaps just under 300. With the valve gear as far in the corner as it will go and wide open, I can hit 6.5 to 7 but there is a giant stern wave and things are thrashing around.

There is an extreme low tide mid morning today and Barbara and I are going tide pooling. We tow and inflatable and take along a waterprrof camera, a viewing tube with glass in the end and (my latest toy) a hydrophone. We are near where a pod of Orcas feed and hang out so I have hopes of hearing them. A quiet boat is nice so that I don't have to drift to listen (In theory!). I'll let you know what a primitive steam plant sounds like from under water.

Bart is looking in to finding some lines for Otter and a scaled up version is sounding better and better to me. 24 x 6 and perhaps 1000 lbs for the hull. Plywood with glass inside and out. Stitch and glue with a laminated up counter. Simple sandwich panels for the superstructure to keep the C.G. under control.

Barbara is already tuning up about too many projects!!

So we've totally hijacked your thread. Apologies.

[Scotty]

Hallo Jstemwedel,

your plan seems practical, provided you find a suitable engine/boiler.

At 30 ft length and 8.5 ft wide I would guess at appr. 7500 lbs displacement.
With a good compound engine you could get 12 HP at appr. 40 lbs of wood/hr fuel
(depending on the efficiency of the boiler of course).
This would give appr. 7 knots, so your 100 miles will take 12.5 hrs of steaming
at a total fuel consumption of 550-600 lbs.
This amount of wood will be no large problem in a boat this size.

In calculating the displacement, always add the weight of crew/passengers, fuel, beer and so on.
Otherwise you could be out by 1000 lbs easily.

Scotty

[fredrosse]

My software is closer to agreement with Barts numbers, with 6.96 knots on 7500 pounds displacement with 6 shaft horsepower.

However making 6 horsepower would require about 70 pounds of ordinary wood per hour.

With a high efficiency steam plant (compound condensing or uniflow), high efficiency boiler (70%), and high quality dry hardwood fuel (6500 BTU/pound), you could get the fuel consumption for the 6 horsepower down to around 35 pounds per hour. The FAQ section addresses all of these numbers.

[Bob Cleek]

Slow down, guys! Pete Culler's launches were specifically designed for steam. The ones I pictured were all I had handy from google pics. The ones pictured (two boats) have little slow turning diesels in them. The design contemplated the weight of a steam plant and with the diesel, they are a bit above their lines. I can't remember the displacement off hand, but they are neither super lightweights, being built of conventional carvel with a combination of steamed and sawn frames, nor overweight. (You may have found some displacement data that included the steam plant and full fuel and water tanks. As I recall, the 24' launch has the pilot house as pictured with a canvas canopy and the steam plant aft. It looks much different with the stack and canopy. The 34'ers have a small cuddy, but no pilot house, and canvas canopies, as are traditional with steam launches. The addition of a pilot house and cabin would be easily accomplished, but that assumes one with the proper eye for proportion and sufficient construction abilities. Those rounded pilot houses are a bear to build. There isn't a straight stick in them, except for the staving and window sashes. Their sides are inclined slightly, not straight up and down, the fronts are curved and the roofs have camber. (The lack of this is what makes so many amateur designed deck houses look boxy.)

[S. Weaver]

Nice Culler hulls, Bob. Thanks for the pics.