The Steamboating Forum

Dear stevey_frac ,

Don't forget that we're dealing with three dimensional objects , so if you double the length you cube the volume . Obviously adding 6' isn't doubling the size but the same principle applies . So a 26' hull is quite a lot bigger than a 20' hull by more than might at first appear.

The North West Steam Society site shows that the power given for the engine is indicated horse power (ihp) this is usually a bit more than would be shown on a brake (bhp) .
Mosquito has a fairly big prop for her size , 20" diamete 23" pitch with 5 blades so can probably use all of the power that the engine and boiler produce . This would hardly affect top speed at all but would be a useful reserve for towing etc .
As the original hull was for a surf boat the design was probably optimized for good sea-keeping characteristics rather than "slipperiness" through the water , so this extra power may well be needed .

Even given all this I would guess that Mosquito is a bit overpowered , but not as much as a Salty would be with similar power .

Regards Edward .

Yes I would guess Mosquito's engine was a bit overkill as well. Tommy Thompson's Fire Canoe had a 4-5HP compound in the same hull and I never heard any complaints that it was underpowered.

-Mike

Ok... i have a question then, and don't laugh at me....

How is it that my buddies fishing boat has a 200 hp engine and a 6" prop (maby bigger, i didn't measure, but that order of magnitude) and cavitation isn't an issue, but a 15 hp engine can cause cavitation on 20" prop...?

What piece am I missing here?

1. No question should be laughed at!

2. Because it is probably "ventilation", not "cavitation". Ventilation is caused by the propeller's sucking air down from the water's surface. "Cavitation" is a result of: high rpm / excessive pitch / high tip speeds. The cure for cavitation is: keep rpm down / decrease pitch slightly at blade tips / keep pitch ratios as low as possible. The cure for ventilation is to get the propeller deeper under the surface. Propeller Handbook by Dave Gerr, pg. 43.

To remark on an earlier comment about appropriate HP for the 26' hull: Artemis was a 26' Navy motor whaleboat, as was Al Giles' Crest and John Campbell's Rainbow. Artemis swung a 23"D x 34"P prop using a 10HP compound at 320RPM and made 6 and a "smidge" knots. Crest swung a 25"D x 30"P using a 10HP compound at 302RPM and made 6 knots. Rainbow swung a 18"D x 24"P prop using a 5HP (Semple) simple at 450RPM and made 5 knots. Although with a slightly "flatter" underbody, Mosquito is not much lighter and 5 more HP only gets 1 more knot. The shape of the hull and the draft determines the maximum speed and the HP required. I don't think you could get much more than 5-1/2 knots out of Salty and I would use Reliable's 6HP 2.5" + 2.5" x 2" double simple with an 16"D x 20"P prop at 600 RPM - this would set the tip of the upper prop blade 6" below the designed LWL, plenty deep. Should give a really nice, stable boat with a little extra for "dusty" weather.

If i've understood other mentions of hull speed, basically, the reason why salty will go slow is mostly because she's not very long? She'll end up climbing her own bow wave to try and go faster more or less, and there's no way a steam engine is gonna be able to get a 5000lb boat to plane?

Dear stevey-frac ,

Climbing the bow wave is a not very scientific way of putting it , but it is efectively what happens .
Hull length is an important factor in determining hull speed but merely lengthening the hull won't automatically raise the speed . Given two hulls of similar proportions , shapes and surface finish and with similar power to weight ratios the longer of the two will go faster .
Once on the plane a speedboat hull is pretty efficient , but when going slowly it is essentially displacing and is very innefficient . I seem to remember reading somewhere (not on this forum) that to overcome this and get onto the plane a power to weight ratio of something like 25 hp per ton is needed . Perhaps someone out there can confirm this ?
To get this sort of performance from a steam boat is very difficult indeed . the only person I know of who has managed to make a planing steam boat is Rod Muller of Strathsteam in Australia .
His boat , Black Crow , cannot by any stretch of the imagination be called traditional . It was built specifically to achieve a higher speed than Arrow , which it MAY have done . There is a bit of information here http://www.steamboat.org.uk/register/html/reg8b.htm Rod showed me and some friends some video and talked about his attempts on the record some years ago on a visit to England .
He freely admits the boat is as ugly as sin and completely impractical for normal steam boating : almost all the components are stressed beyond manufacturers' recommended safe limits and the steam from the monotube boiler was so hot that he was using an oil developed for jet engines which unfortunately was both poisonous and extremely corrosive to organic matter , this in a total loss lubrication system ! There was no seat in the boat , partly to save weight but also because he operated it half crouched over the gun'l so he could launch himself over the side with the minimum delay if anything broke . Rod is a very competent engineer and knew what he was doing but it was definitely a case of " Don't do this at home !"

Sorry for such a long digression , I mention Black Crow to illustrate that it is POSSIBLE to build a planing steamboat , but having done so it is definitely not one in which you would want to take the family out for a pleasant day on the river/lake/canal .

Regards Edward .

I would have thought with the traditional engine and boiler plant, 25 HP per tonne is almost impossible. Things like the outboard steam engine which use very lightweight materials are specifically designed for the job.

One big thing is you need the high speed for the power, largely because of the propeller pitch problem. A 16" x 200" propeller would not only look pretty stupid, but I would imagine it would be largely useless.

Boats like Turbinia with 3000? SHP and weighed 100 tonnes were also purpose built for speed. Again, the steam turbine was so powerful because of the speed it ran at.

Dear maltelec ,

You're a bit optimistic concerning Turbinia's power and pessimistic concerning weight : http://wapedia.mobi/en/Turbinia .
With a different hull Turbinia would probably have been able to plane . When she was going flat out the stokers had to be relieved about every 15 minutes , I think she carried three shifts for extended high speed trials .

Regards Edward .

One thing that I didn't mention, but that is probably relevant..

I intend to run an alternator full time, and have an electrical system worthy of the name.

It's probably non-traditional, but then, with the monotube boiler I have planned, and the microcontroller based steam temperature and pressure regulation I've designed... Well... she won't be traditional in any sense, other then that she must burn wood.

By my best calculation, the alternator will consume about 1/2 horse even when not generating current (battery is fully charged)

Nothing wrong or sacrilege with thinking about alternators.. Not as simple as an oil lantern, but simpler than a seperate gen-set.

If you are going with one run by the main engine or a smaller engine; I would suggest using one of the small alternators that are intended for wind genertors on ocean-going sailboats. They require relatively low RPMs and HP to operate.

My uncle used a small, removable, solar panel to carge up during the day. You may want to consider this route... I think they are available at most auto parts stores.

If one used LED lightbulbs throughout the boat, the draw on the battery(s) would be very minimal.