propeller

[johnp]

So, if ihave a hull with a speed of 4.472136*1.34=5.99knots(6.89mph) my pitch should be 18"? And the Diameter to run at 400rpm should be?

[fredrosse]

Something more like 20 inch diameter, 24 inch pitch, pushing a one ton boat with about 3 horsepower at 400 RPM. The RPM, boat speed, and speed roughly determines the pitch. The diameter is aproximately a function of the transmitted horsepower, and I just guessed that the boat, at about 20 feet waterline length would displace about one ton.

A lighter more streamlined boat would displace less, and could use a smaller propeller diameter. For that case you could probably use somewhat higher RPM, with smaller pitch and smaller diameter.

[barts]

The larger the prop diameter, the lower the slip... ideally, for a displacement hull you'd like about 1:1 ratio of diameter to pitch, since that ratio is most efficient. This is hard to fit under a lot of hulls as the pitch is more or less determined by RPM; most steamboats end up running a lot more pitch than diameter.

For a detailed way of doing this that takes lots of factors into account, I can recommend http://www.amazon.com/Propeller-Handboo ... 0071573232.

Try and get the pitch right, the diameter doesn't matter nearly as much. If your boat is very heavy, the diameter needs to be large as well for best efficiency.

- Bart

[johnp]

it's an old glass lifeboat hull 7 1/2' beam about 21' L.O.A she draws about 2 1/2', i'm building a 2.25"x3" simple.

[artemis]

it's an old glass lifeboat hull 7 1/2' beam about 21' L.O.A she draws about 2 1/2', i'm building a 2.25"x3" simple.

Offhand I know of one lifeboat hull in steam that meets those dimensions. I suggest you look at Charlie Coghill's Simon Willard in the "boats&c" section of the NWSS website: http://www.northweststeamsociety.org/Pa ... SSBsim.htm. You will note that it has the same basic characteristics as yours (the "draft" of the Simon Willard is the distance from gunnel to keel amidships which is close to 2+ feet depth of water to float her) and - despite the fact its steel - the power requirements will be similar. But look at the engine and propeller noted in the description. You may want to rethink your power plant. And a lifeboat hull has a conversion constant of about 1, not 1.34

[johnp]

I'm on a small land inland lake, she will just be a cottage toy. i thought that if it is that bad i would build another and jint them have a twin simple.

[johnp]

after recalculating, hopefully correctly, i think i need a 17"x17" prop. I used 1 as the sqrt. waterline multiplier since it's a lifeboat hull. And hopefully my 3 hp will push me along.