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 Post subject: Launch Hulls, Power Rerquirement
PostPosted: Wed Apr 10, 2013 6:35 am 
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TYPICAL STEAM LAUNCH HULLS - Propulsion Power Required.

Hull design can (and has) occupied entire large textbooks, but for typical Steam Launch practice, some simple relationships are useful for rough quantification of propulsion requirements.

The basic rules of thumb state that a well streamlined displacement type hull will have a maximum hull speed as a function of the waterline length:

Maximum Hull Speed (Knots) = 1.34 x Square root of Waterline Length (feet)

Some slender hulls can do slightly better, and a more box-like hull will do worse, but this approximation is reasonable.

To drive the boat at “Hull Speed”, requires 1.25 to 1.75 horsepower per ton (2240 pounds) of displacement. In general this relationship requires a propeller of good efficiency, which translates to a large diameter slow turning propeller. Launch propellers should have a diameter equal to 10% of the boat’s waterline length, and have a pitch equal to, or slightly larger than the diameter.

The relationship between speed and power, up to the maximum hull speed, is a cubic function, which means low power does not hurt boat speed as much as one might think. An example here illustrates this phenomenon:

Steam Launch, 19 feet long, waterline length = 16 feet, Displacement = 1 ton with all supplies and passengers aboard. Hull speed = 5.3 knots, Horsepower required = 1.75 HP.

Speed, 5.3 Knots HP = 1.75
Speed, 5.0 Knots HP = 1.47
Speed, 4.5 Knots HP = 1.07
Speed, 4.0 Knots HP = 0.75

Conversely, driving the hull above maximum “hull speed” becomes a fifth power function, and is not a practical option:

Speed, 6.0 Knots HP = 3.25
Speed, 7.0 Knots HP = 7.03

These approximations are for pure displacement hulls, which covers the great majority of steam launch hulls. Other types of faster pleasure boats have entirely different functions.


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Last edited by fredrosse on Sat Apr 13, 2013 3:13 am, edited 2 times in total.
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 Post subject: Re: Launch Hulls, Power Rerquirement
PostPosted: Wed Apr 10, 2013 7:28 am 
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fredrosse wrote:
Launch propellers should have a diameter equal to 10% of the boat’s length, and have a pitch equal to, or slightly larger than the diameter.



Meaning a prop of 19" X 20, or presumably equivalent, i.e. smaller dia but bigger pitch?

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 Post subject: Re: Launch Hulls, Power Rerquirement
PostPosted: Wed Apr 10, 2013 3:45 pm 
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Is that prop diameter number calculated on the waterline length?

Also, for years I've been using the waterline length as measured at rest. When I'm zipping along at near hull speed, my waterline is extended at the rear by a few feet. Wonder if that should be taken into account or is that just my hull resting on it's own wave?

Mike

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 Post subject: Re: Launch Hulls, Power Rerquirement
PostPosted: Wed Apr 10, 2013 5:33 pm 
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Mike Rometer wrote: "Meaning a prop of 19" X 20, or presumably equivalent, i.e. smaller dia but bigger pitch?"

I am not sure what you mean by this, "smaller diameter but bigger pitch". I was not trying to imply that if you put on a smaller diameter propeller you would compensate with larger pitch. The guideline is to size the propeller properly, and to have a pitch which is slightly larger than the diameter, whatever the proper diameter is.

All of this involves compromises, and many hulls will not accept the ideal propeller size, and many engines will not be turning over at the right RPM to match the ideal pitch. These compromises are often needed, and the guideline is just to get in the ballpark for a reasonably sized machinery and propeller.


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 Post subject: Re: Launch Hulls, Power Rerquirement
PostPosted: Wed Apr 10, 2013 5:49 pm 
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Lopez Mike wrote: "Is that prop diameter number calculated on the waterline length?

Also, for years I've been using the waterline length as measured at rest. When I'm zipping along at near hull speed, my waterline is extended at the rear by a few feet. Wonder if that should be taken into account or is that just my hull resting on it's own wave?"

The numbers presented in the guideline are to be based on the operating (underway) real waterline length, this is the condition for which we are designing. When a longer submerged hull is supporting the weight of the boat, then this length is the relevant dimension for calculations. There can be instances where the "longer length" is merely a thin fin-like structure that submerges into the water while underway. In that case the extra length provided by the thin fin would not play in the formula, as it is not really supporting much displacement..

All of this is a bit more detail than is intended for the general guidelines, the function, involving a square root of the waterline length does not change the calculated hull speed much, using the "at rest" vs. the "underway" length. A more relevant parameter might be the streamlined shape of the hull, but we cannot go into details here with an approximate formula.


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 Post subject: Re: Launch Hulls, Power Rerquirement
PostPosted: Wed Apr 10, 2013 6:36 pm 
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fredrosse wrote:
Mike Rometer wrote: "Meaning a prop of 19" X 20, or presumably equivalent, i.e. smaller dia but bigger pitch?"

I am not sure what you mean by this, "smaller diameter but bigger pitch". I was not trying to imply that if you put on a smaller diameter propeller you would compensate with larger pitch. The guideline is to size the propeller properly, and to have a pitch which is slightly larger than the diameter, whatever the proper diameter is.

All of this involves compromises, and many hulls will not accept the ideal propeller size, and many engines will not be turning over at the right RPM to match the ideal pitch. These compromises are often needed, and the guideline is just to get in the ballpark for a reasonably sized machinery and propeller.


Sorry I was not wishing to create confusion.

I do appreciate the need for compromise in these things, and also what often drives it. My thoughts were based on 10% of your suggested water-line of 192", i.e. 19", slightly larger pitch say, 20", but where do you go when you cant get the correct calculated size in, for instance, if limited to say 16" dia.; and is there a 'required' clearance between prop and hull, and how is that calculated, or is it just a standard/minimum allowance?

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 Post subject: Re: Launch Hulls, Power Rerquirement
PostPosted: Fri May 06, 2016 1:30 am 
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When the hull cannot accept the ideal propeller size, which is very often the case, then a smaller prop must be used. To compensate for this, there are a few options:

The recommended "10% Rule", having propeller size 10% of the boat length, is generally considering a "Steam Wheel", which has relatively thin blades when compared to typical modern power boat propellers.

Using a typical power boat propeller, with broader blades than a "Steam Wheel" would have, allows smaller diameter. For example,if the 21 foot steam launch, with 18 feet waterline length, you would like to have an 18 inch diameter "Steam Wheel" propeller, typically 3 slim blades. Here you could substitute a typical 16 inch diameter 3 blade motorboat propeller, and get very nearly the same performance.

Going smaller yet, switch from a standard three blade propeller to a four blade prop, This can give about the same blade area of the larger three blade prop, and come close to the original performance objective. The boat above might get away with a 14 or 15 inch prop in four blade configuration. Some small loss of efficiency.

There are some workboat type propellers made with extra blade area, three or four blades, with some small loss of efficiency.

Fitting a smaller prop and increasing engine RPM, or increasing propeller pitch can restore design speed, with a bigger sacrifice in efficiency than the options mentioned above.


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