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[Cyruscosmo]

Hello All

Reading back through "all" the posts on this site and following all the boat builds I could find I notice that the majority of the craft have a plumb stem. Is this somehow the best stem to use or is it a reflection of the trend of that era?

The Launch I am considering building will be used in the lakes around where I live first while I learn the in's and out's of a steam plants operation. She will be 30' LOA with an 8' beam and I really like the Schooner Bow/Counter Stern combination.

So since I don't know much of anything about the shape of a hull as it relates to hydrodynamics or whatever the word is used to describe how it moves through the water, I got another question.

I would like to know if the shape of the stem as far as it pertains to the steam launch variety we usually see has a significant part to play in anything other than aesthetics?

From what I have read so far it seams to be a preference. Is there anything else I should know about that combination?

Cheers,
Scott

[barts]

The angle of the bow doesn't affect much other than looks. What matters is things like length/beam ratios, prismatic coefficient and buttocks angle, which affect how easily the boat is driven at different speeds. Steamboats are usually powered at somewhere between 1 and 3 hp per ton of displacement, so having an easily driven hull is important.

Skene's Elements of Yacht design is a classic in basic naval architecture.... pick something that would look right as a steamer or a sailboat, and you won;t go too far wrong.

- Bart

[dhic001]

The plumb stem was a common shape at the period our vessels were originally used. As a result, we replicate it on traditional looking vessels. The plumb stem has an advantage over a clipper bow in that it adds waterline length, more waterline length equals more speed. The counter stern looks great, and is very traditional, but is also very useful in a seaway, where a wave from astern will lift under the counter and not slap against the stern and and possibly onto the deck as it would on a transom stern. Of course, the counter is dead weight and takes away from waterline length when in calm waters, but we don't design our boats from a purely practical standpoint.

If you like the clipper bow, by all means go for it, combined with a counter stern it looks great, but you will find that the result is a longer amd heavier vessel with very little usable space in the ends. Personally I feel that 8 feet beam is too much for a traditional 30 footer, a finer hull will be more easily driven for the hull speed.

Daniel

[barts]

Also, keep in mind that modern constraints like trailering, moorage rates, etc. affect boat designs significantly.... boat design is an exercise in trade-offs. I've seen a 42' boats that is less than 8' wide, and cruise very nicely on 10 hp (diesel) - but there's not a lot of usable space inside, and the deep keel needed for stability make a tender or dingy mandatory - no nosing into the beach for that boat.

Figure out what your constraints are:

Do you need/want to trailer the boat? How much weight can your vehicle pull?
Do you want to sleep aboard? How many people?
What kind of range do you want? Fuel(s)?
What waters will you cruise and in what sort of weather conditions? Salt or fresh?
Do you need to haul a dingy on board?

Lots of things to consider...

- Bart

[Lopez Mike]

A plumb stem gets you the best waterline length compared to the overall length. Bristol Channel cutter sailboats are very seaworthy sailboats with a plumb bow.

The only drawback that I am running into is that it's not possible to build a plywood boat with a single chine and a plumb bow without various cheats. It needs a compound curve in the lower sheet of plywood as it approaches the stem.

I second what Bart is saying about watching out for boat bloat. The whole trailering issue gets out of control with increasing length and weight. At 24' x 6' and maybe 2000 lbs. I'm at the limit of easy ramp launches and towing without a rather large towing rig.

Right now I zip around wherever I want to, launch and retrieve by myself and have no problem beaching the boat for firewood, lunch or passenger loading.

If the boat were very much larger I think that all of those issues would start to become problematical. I'd keep it light and long.

Mike

[barts]

My big boat will be just small enough so that I can move it to our place on Lopez Island with either a wide load permit or knowledge of the deputy's whereabouts . For regular over-the-road use, you're limited to a maximum of 8' 6" width and (practically) an overall-length of 34' including trailer tongue, etc. I have friends who pull Airstreams that big- but it's tricky, and tight turns are a real problem. You'd be slightly better off w/ a fifth wheel, but 30' boat length is really about all you can do.


- Bart

[fredrosse]

Another issue to consider is the "flair" in the bow section. The sidewheeler Margaret S. has a plumb stem, and almost no flair, so when a wave comes up the boat tends to cut through the wave, without the bow lifting very much. That is fine on the lake with small waves, but out in rough rater a large wave threatens to crest over the bow, so I need to have the forward deck in place to assure the boat would not be swamped in rough water.

More flair in the bow tends to force the bow upward when moving through waves, as the flaired bow provides much more lift, often less comfortable pitching of the hull, as it tends to ride over the waves rather than cutting through.

This technically independent of plumb stem or clipper stem, but in many hulls, the clipper bow tends to behave with more bouyancy when facing waves, and takes on the character of a hull with plenty of forward flair.

There are many youtube videos of full sized ships facing big ocean waves, and you can get an idea of how various hull forms act in these conditions. Almost all seagoing vessels have plenty of forward flair to face big waves.

[DetroiTug]

In Michigan as most other local states, Expressways have a legal width non-oversized of 104" IIRC and all other roads are 96". Anything over that involves Wide Load rules.

Everyone has a preference on boat size particular to their taste and for me, the bigger the better. My boat and trailer is almost 30' long and 101" wide and 12' tall and weighs around 6000 lbs. The only difficulty trailering it is on secondary roads due to it's width. Expressways are smooth sailing. We just went to Waterford New York back in September for Tugboat roundup. 1250 mile round trip - went smooth.

People seem astounded that I travel such far distances with it. The key is make it prepared for those sorts of trips. Good bearings and tires on the trailer, brakes on the trailer that work reliably. Drive like I'm pulling a load. A vehicle with enough power. Stay focused. Don't tailgate, keep it at a safe speed. Leave myself plenty of room. Don't pull in where I can't back out of, etc. Large trucks do that everyday. No reason we can't do the same with the correct equipment and same driving technique.

For loading, there is an electric winch - it's very easy to load.

Regarding rough water, here is a video from back in the spring. We were riding some pretty good swells. Double chine hull with plum stem.



-Ron

[barts]

Some interesting data from David Gerr's Propeller Handbook:

DL = displacement/length ratio = Displacement in long tons / (.01 x Waterline in feet)**3

SL = max. reachable speed/length ratio = 8.26 / (DL) ** 0.311

SHP = shaft hp at prop = LBS/((10.665/SL)**3)

[Note that these equations assume a propeller efficiency of about 60%. We can do
better w/ a big _square_ steam wheel, but only to about 70%.]

I stuffed these into a small python program

Code: Select all

#!/usr/bin/python

def DL(displacement, waterline):
        return ((displacement/2240.)/(.01 * waterline)**3)
def SL(dl):
        return (8.26/(dl**.311))
def SHP(displacement, sl):
        return displacement/((10.665/sl)**3)
for displacement in range(2000.,20000.,1000.):
        wl = 30.
        sl = SL(DL(displacement, wl))
        print "%s  %s %s" % (displacement, wl**.5 * sl, SHP(displacement, sl))

and ran it:

Disp. speed, knots SHP required
-------------------------------------------------------------
2000 15.2405840507 35.519788029
3000 13.4349863281 36.4979506595
4000 12.2851731452 37.2082611873
5000 11.461521349 37.7687268992
6000 10.8297118205 38.2329218809
7000 10.3227746965 38.629841334
8000 9.90286715423 38.9769978203
9000 9.54668336606 39.2858002204
10000 9.23893558225 39.5641059015
11000 8.96909892177 39.8175620552
12000 8.72964476848 40.0503669149
13000 8.51501772763 40.2657284431
14000 8.32101145623 40.4661543815
15000 8.14437116199 40.6536430876
16000 7.98253119558 40.8298133426
17000 7.83343644989 40.9959951805
18000 7.69541755908 41.1532950129
19000 7.56710167074 41.3026433256

So if you want to go really fast, keep it light - and have lots of power. I found it interesting that about 40 hp was all the boat could use effectively. Note that 40 hp in a 2000 lb boat is still below planning power - usually considered about 40 lbs/hp.

For fun, I ran the same equations another way - assuming a 2.5, 5, 10,15, 20 hp power plant, and seeing what speeds could be attained for a 30' WL boat of varying displacements. To trim the output, we'll use a more practical range from 5000 to 10000 lbs displacement, and clean up the output format a little:
First some code:

Code: Select all

#!/usr/bin/python

def SL2(displacement, shp):
        return 10.665 / (displacement/shp)**(1/3.)

wl = 30.
for displacement in range(5000, 11000, 1000):
        out = ""
        for shp in (2.5, 5,10,15, 20):
                sl = SL2(displacement, shp)
                out = out +  ("%5.2f  " % (wl**.5 * sl))

        print "%10s %s" % (displacement, out)

and some output:

Speed in knots
----------------------
displ. 2.5 hp 5 hp 10 hp 15 hp 20 hp
5000 4.64 5.84 7.36 8.43 9.27
6000 4.36 5.50 6.93 7.93 8.73
7000 4.14 5.22 6.58 7.53 8.29
8000 3.96 4.99 6.29 7.20 7.93
9000 3.81 4.80 6.05 6.93 7.62
10000 3.68 4.64 5.84 6.69 7.36

So we see that quadrupling engine power (and thus fuel consumption) doesn't even double our speed. Obviously, cruising at 5 hp is pretty ideal. Note that historical powering levels were 1 - 3 hp /ton displacement, so both our
15 and 20 hp examples are excessive.

Power required to move the boat at a particular speed is linear with displacement. However, for a particular power
level, since power required to drive a boat varies as the cube of the speed, adding weight to the boat slows us down only a little as the boat is much easier to drive as we slow down. For example, if we ran the engine at 5 hp, doubling the weight from 5000 to 10000 lbs costs us 21% of our speed.

We can also see that having a plant that is efficient at low power outputs is very handy if we need to extend our range.

- Bart

[Lopez Mike]

Run that last output again but start at a lower displacement, like 2000 lbs.

5000 lbs is a pretty big boat. It would more than likely rip the back off of my little Ranger pickup!

Mike