Appropriate hullsize and prop for 25 HP engine

[DetroiTug]

Frank,

Beautiful engine and boiler. Looks like an interesting project, keep us updated on progress.

-Ron

[Edward]

Dear stoomsnor ,

I'm pretty sure that the correct name for a Scotch boiler with a "dry back" is a Clyde boiler.

Regards , Edward

[fredrosse]

Your boiler can probably be pushed to high output with a very heavy fire, but for nominal cruising perhaps half that amount. With a high efficiency compound engine, and vacuum exhaust, I think your boiler is a good match for your engine.

Remember that 80% speed can be achieved with only 50% power, so 8 knots maximum speed, and at half power = 6.4 knots, still a comfortable cruising speed without working the boiler/fire at maximum.

Propeller size compromises are available. For example, if you might wish for a 40 inch three blade propeller, then a smaller diameter prop with more blades (more blade area) will do almost the same job, perhaps a four blade workboat propeller with 33 inch diameter.

Propeller efficiency is best with a diameter to pitch ratio of about 1:1 or slightly higher, and pitch somewhat more/less is OK too. For an ideal setup, 40 inch diameter x 50 inch pitch, (1.2 ratio) this has very good efficiency. If you are forced to have a smaller four blade propeller, say 33 x 50, (1.5 ratio) then propeller efficiency will be slightly lower, but still generally acceptable.

[Stoomsnor]

Hi you guys!

Thanks a lot for the compliments, the right name for my type of boiler and last but not least,
the given dimensions for the right prop for my boat-to-be !!!
I have been reading and studieing about steam, boilers and steamengines a lot
(should have done that when I was in highschool, well..... )
Still there is so much going on on these subjects, I always end up with new surprises!
It's just great to be in contact with steamaddicts all over the world!

To-morrow I will go to the wharf where I will look at the hull of a boat dated from approximatly 1930.
I do love an original older boat really but when they date to far back as 1910-1925 the quality of iron
in the Netherlands was poor. It was ok back then because it did not rust excessively but when restoring
you will have great difficulty welding in new parts.

The good news is there's a database in the Netherlands in which old shipregisters where made digital available.
All ships back then and nowadays where measured and approved, marks where chisseled in the hull and registered.
So when I can find them under layers of rust, paint and tar I should be able to read the exact history like:
On what warf was it made, where it was built for, who was the owner, what names were given and what was altered over the years.
Needless to say it would be perfect to find a hull originally equiped with a steamengine that later on has been altered to diesel.
How nice would it be to alter it back! I'll post some pictures soon!

[fredrosse]

"Gearbox:
You're very clear about the gearbox Fred. I do share your opinion! In 2014 however I visited the "Great Brittain", believed to be the first propellordriven oceangoing ship in the world. As drewling around in this magnificent work of art I ran into this gigantic multiple chain used as a gearbox..."

I made a trip to the UK just for steam machinery review, and the Great Brittian was at the top of the list. This ship is arguably the most important historical ship in the world, was the largest ship in the world at the time it was built, and was novel in having iron construction, and steam primary propulsion, with a screw propeller. Its original engine was designed as a paddlewheel engine, so very low RPM and the need to have the big chain drive gearing arrangement. While it was not the first use of screw propellers, nor the first iron ship, nor the first steamer, it was indeed the first integration of many revolutionary ship technologies on a large scale. Perhaps even more impressive: it had enough integrity, at 140 years of age, to be recovered from half way around the world, brought back to Bristol in the 1970s, and floated on its own hull up the river into its original building dock. The restoration work is excellent. However on visiting the steam engines at Crossness (in London), I went into a trance that I still have not recovered from, so be cautious if you visit the Crossness Station when it is steaming!

"propelloraxleangle" = propeller shaft angle

Attached picture, crankshaft in Great Brittan main engine.

Sorry to be "off-topic" somewhat, but interesting subject for steam people.

[steamboatjack]

As I mentioned in the other post, this boiler is similar to a type built by Cochran's of Annan. The water tube section improves the water circulation especially at start up. I think your engine is terrific! With a boat of the size mentioned you should aim for direct drive. 200 RPM is fine provided your hull can swing a big enough propeller ( note this needs to be "left hand" for this engine). Shamrock a similar size runs at less than 200RPM usually.
Regards
Jack

[Stoomsnor]

Thanks for your comments Jack!

[Stoomsnor]

Thanks a lot for your comments guys!
They really have been helpfull and interesting as well!
Last week I went to take a look for a boat dating as far back as 1930.
It was really sad to see how time had it way with it, there were holes everywhere !
At the same time it occured to me that longboat's hullframes are designed in
such a way that they can withstand forces from the sides without having a deck that
would prevent them from collapsing to the innerside.
Since a longboatmodel is what I really like I'm considering of having one build with an
old-style appearance in proper steel. I'm thinking of faking the riviting to give it
an original appearance. Is there any-one who has any experience in doing so ?

See you guys!

[gondolier88]

Frank,

An interesting proposition!

I haven't built a longship, but I have sailed on and studied the construction of Viking ships and have a few issues with what you propose;

1- At 40ft the freeboard of a longship would only be around 18 inches- this sounds ok, but in reality, they have odd stability properties and require ballast trimming for every load they took in order to maintain a safe trim- not at all practical for a pleasure boat.

2- Longships are positively bouyant- an open steel longship hull would have me wearing a lifejacket every time I stepped onboard! They are low displacement vessels and simply not suitable for carrying heavy loads.

3- They have keels parallel to the waterline, double-ended with a very round forefoot- this would be impossible to recreate with a sterntube arrangement suitable for a plant of your size.

The ocean-going longships display many different construction techniques to try to get around these problems, but at around 80ft long and using lots of wooden construction techniques that would bear very little resemblance to the vessel you propose. The smaller boats that have been discovered are really processional and leisure craft that could carry occasional light loads in very calm water.

However, have 40ft open steel steam launches been built? Yes, they have. If you want an open boat you could do a lot worse than build one of wood or steel along the lines of an early 1900's lake/river launch of which there are still lots around and have suitable lines for fitting steam plants into, propshafts and passenger accomodation. If you wanted something with a waterline pinched in to mimic the look of a longship you would be looking at something around 50-60ft long with a counter stern aft and a long keel with wine glass stern up to the counter. Your plant would be powerful enough for something with such fine lines.

If you want a hull to look rivetted then rivet it, weld it below the waterline if your boatbuilder doesn't want to/can't rivet plates below the waterline. You can always caulk joints above the waterline with a bead of weld along the inside of the platework above the waterline if you really feel you have to, but it is unnecessary. Steam launch Mosquito that I referenced to you is entirely flush rivetted 3mm steel with no welding involved- mostly open, she has a midship bulkhead, but this could be reduced in height from the gunwhales if heavier framing was used.

Greg

[Stoomsnor]

Hallo Greg!
Thanks a lot for giving me disadvantages and advantages of this type of ship.
They are realy helpfull! There is a lot to consider...
This week I am in the process of weighing the engine and boiler and measuring them both.
Together with the weight of coal mathematics can be done to find the best place to fit them in a hull.
As I want a very stabile ship I would like all attached as low as possible, perhaps with the addition of
two tanks below deck to be able to balance and supplete boilerwater eventually.