position of boiler and engine in new launch

[lighthousekeeper]

Hi I'm Currently building a 21 foot fantail launch I have the hull built just finishing the inside. I'm just curious if anyone can help me or give me some idea on were my boiler and engine should be postioned in the boat. I have a vertical tube boiler apox weight 500lbs single cyl engine apox weight 200lbs. I ca't figure out how to put pics on this site haha here is a link to the exact boat I built 5 years ago.

http://fantailsteamlaunch.webs.com/

[fredrosse]

General rule of thumb: Locate machinery such that the boat floats level with the design waterline. Then when one or two persons run the boat from behind the engine, pitch is slightly high forward, this is OK and typical. Added passengers in the forward area can bring the boat back to level. Pitch with bow down should be avoided for all typical load conditions.

The hull lines, if available, will show the Center of Bouyancy for various draft conditions. Freeware like Freeship or Deftship (I am sure there are many others) does all these calculations without effort on your part, but you need the hull lines modeled. I know of no other way to figure placement of machinery, and unless someone has an identical hull with similar machinery, copying another boat's arrangement might not be correct.

[Lopez Mike]

Hard to beat laying down some temporary engine and boiler stringers, launching the boat with the engine and boiler lashed down and moving them around to get the boat level. Maybe add a bunch of big waterjugs to simulate the boiler contents. The same goes for other masses.

[DetroiTug]

I third the previous two. Temporarily mount the equipment and put it in the water. It is the only sure way to get it right. I was not able to float mine with equipment and spent the next year finishing it up not sure how it would set with everything located. Would have been nice to have known. I got lucky and it all worked out ok.

-Ron

[fredrosse]

"Temporarily mount the equipment and put it in the water. It is the only sure way to get it right."

Sorry Ron, but I have to disagree, this is not the only sure way to get it right. Although for many of us actual physical testing in the water is the most practical way to go, Naval Architecture has not used this method for at least a couple of hundred years, they calculate the numbers. Of course there can be errors in the design with either method.

If the boat is being built, then lighthousekeeper has the lines for the hull, and can work them up in the freeware "Deftship" or "Freeship". That takes me about 4 hours for a typical launch hull, or far less time for something like my plywood Sharpie hull. If the lines were generated with hull software, then the originator may well have all of this information already. At that point one must input the hull weights, and start adding the machinery weights. In my view much less work than hauling everything to the water and shifting things around on the actual hull.

[mtnman]

"Temporarily mount the equipment and put it in the water. It is the only sure way to get it right."

Sorry Ron, but I have to disagree, this is not the only sure way to get it right. Although for many of us actual physical testing in the water is the most practical way to go, Naval Architecture has not used this method for at least a couple of hundred years, they calculate the numbers. Of course there can be errors in the design with either method.

If the boat is being built, then lighthousekeeper has the lines for the hull, and can work them up in the freeware "Deftship" or "Freeship". That takes me about 4 hours for a typical launch hull, or far less time for something like my plywood Sharpie hull. If the lines were generated with hull software, then the originator may well have all of this information already. At that point one must input the hull weights, and start adding the machinery weights. In my view much less work than hauling everything to the water and shifting things around on the actual hull.

Yes this is the correct way but, not everyone is as good with numbers and computer programs as others. Puttin' it in the water leaves no room for error.

[Lopez Mike]

The nature of my power plant (small engine and VFT boiler) lends it to being assembled as a unit on a pair of common stringers and lowered in place with provision for levering it fore and aft a bit to get things right.

That said, I find that I do an amazing amount of passenger hauling at public events. There is little hope of getting the trim anywhere near right from trip to trip. One venture from the dock, the prop doesn't get a decent bite until I'm moving fairly well and the next load is guys all crowded around the engine. I can barely stoke the firebox.

Don't sweat it if you are off a bit. There are other masses to install and endure. As well, I can't detect that it goes any faster with everything in trim. Just looks funny in the pictures.

[cyberbadger]

If you are going to do the test method, using mass simulators(bricks/cylinder blocks) for the boiler and engine has somewhat of an appeal.

My new boiler is going around 1000 lbs, I do not want to have to salvage it if their is a capsize from a weight distribution test. For a fancy engine, being submerged in water from a capsize is not a fun idea.

I'm building a launch around a pontoon boat - My plan for myself is sort of a hybrid but conservative. I'm not going to crunch the numbers like crazy - but Probably some simplified rough calculations like, were is the center of gravity, where are the significant masses going to be.

Then my buddy has a small shallow pond 60ft or so in diameter. I'm going to float the hull, and test with mass simulators. Then I can move them around, and if it sinks, not to much of a problem - private property - small shallow pond - he has a big a tractor.

Then when I'm happy with that - bolt the boiler and engine, test it again.

-CB

[Lopez Mike]

I think that some simple 'back of the envelope' calculations would show if your design was that far out of whack.

Try to keep in mind that the sort of tests we have ben talking about are related to static weight and balance, not dynamics. There are way too many examples of boats that sat their looking fine until the first wake came along and then over they went. I feel that a steam launch should be stable, meaning that it will return from a given angle of heel to upright, to at least 45 degrees. I don't know what it would take to get Folly over that far but it won't happen on my watch!! It's called seamanship.

To get back to what I thought we were discussing, fore and aft trim, either simulated weights, like your bricks, or the real machinery would work.

I have had long conversations with Sam Devlin, a designer and builder of many successful small boat designs. He speaks favorably of model testing, both for performance and stability. Make 1 to 12 scale model and scale the weights to 144 to 1. A 24' foot boat down to 24" and a 144 lb, weight to 1 lb. In the case of plywood hulls it is easy to build a model cheap and quick with door skins and hot glue. Your pontoon hull would be an easy job. ABS pipe anyone?

You could even get all carried away and power the model with an electric motor and measure the power to speed ration though I suspect strongly that the little propellors will scale so poorly that not much useful data will result.

[cyberbadger]

Mike,

For my Nyitra I I'm only going to take it to small fresh water lakes, that are calm and I've checked the weather. The most ambitious for me would be Lake Chautauqua in Western New York State.

I'm going into this knowing the Nyitra I will be limited to severe restrictions on where it can steam, when, and weather condition.

The hull was very cheap, and I may reincarnate the boiler and/or engine into the Nyitra II with a different hull. For the Nyitra 1 I'm just trying to learn some fundamentals first hand. My weight balance test is more about whether I need to add a third pontoon.

-CB