propeller dilema

[schwing]

I am currently a student in southampyon studying for an honours degree in yacht manufacturing and surveying. i have been given the task to ascertain wether simple methods (perhaps rule of thumb) can be developed to relate a set of propulsion system parameters (e.g propeller diameter:draft ratio) to the input parameters(eg. displacement, speed). i am to find out if this is possible or wether it is a case of insuffcient data, or a problem to complex to permit such a simple analysis.
Any help you may be able to give me will be greatly appreciated.

regards
Caroline Wing

[gonzo]

Well Caroline, the rule of thumb has been used succesfuly for many years. Check Nichol's "Boat data book", Gerr's "Propeller Handbook" and Herreshoff's "The Common Sense of Yacht Design" among others.

[schwing]

ok now that we've established that there is a rule of thumb, have you any guidelines on what i should be looking for because i have spent hours looking through the suggested books and can't seem to pick up what i need.

[gonzo]

What is you are looking for?

[schwing]

I am looking for an actual equation that can be used as a rule of thumb to ascertain wether, the propulsion system parameters can be used to decifer speed, loa, lwl etc. i think the case of propeller selection can only truly be done by proffesionals and computers rather than a rule of thumb calculation. in my original post i worded it exactly how it was presented to me, so if you can make any sense of it then your a better person than i am!

[dougfrolich]

Try this method for a dislp. boat - from the propeller handbook

if you know; LWL and Displacement ( 40' and 27,000lbs.) Figure the Desired speed 1.34 x sq.Rt.of LWL = 8.5 kts.
Estimate Shp 1.34= 10.665/ ^3Sq.Rt.(Displ./Shp)
Shp= 55
Assume Propeller Effi. of .55 ( Ave Value )
Find Prospective Motor, Yanmar 4JH3E 55shp@3800 RPM
Reduction Gear; 2.36:1
Find Diameter, assume 3 blade, .33 mean width ratio, normal propeller.

Diameter = 632.7xShp^.2/Rpm^.6
D=16.79"

Estimate slip to find pitch; Slip=1.4/Speed in Kts.^.57
Slip= .4133

Pitch= Kts.x1215.6/(RPMx.9)x(1-Slip)
P=12.15

In Summary
our 40'LWL 27,000lb displacement boat will need 55 Shp a 2.36:1 reduction gear and a 3 bladed 17"Dx12"P Propeller to attain 8.5kts. in calm seas and zero apparent wind speed. That is the quickest empirical method I know to get resonable estimates of Speed,Power,Prop. pitch and Dia.

Hope this helps

[gonzo]

I think that history can answer your dilemma. It is possible to prove that thousands of succesful boats have been fitted with propellers using rules of thumb. That should solve the problem. If the question is about the possibility of using rules of thumb, proving it is done makes it incontroversible.

[schwing]

but what is the rule of thumb?

[gonzo]

There are many. You say the question is whethere it is possible to use them or not. Gerr's "Propeller Handbook", "The Nature of Boats" , Herreshoff's "The Common Sense of Yacht Design", Michigan Wheels Inc's website and Volvo-Penta all have rules of thumb. Pick one and use it as an example. Dougfrolick posted an example of a rule.

hi caroline,
i can calcuate and give information about propellers...

[John Capuano]

While we are the topic of propellers I have a question maybe someone with more knowlege than I have. Why do yachts not use the same prop technology as the U.S. navy.Multibladed fan props like the ones used on subs have greatly increased thurst.There must be a reason that Iam unaware of besides cost.

[colinstone]

John - prop drag when the boat is sailing. Multi blade propulsor may also produce more thrust, but a single blade prop is most efficient - but a little rough. So yacht 2 blade is a good compromise.

Caroline - in addition to the books mentioned, there is also a useful spreadsheet sculling around. Input are all the parameters and output is prop size. Fairly accurate. I used that and Dave Gerr to get a prop made and the manufacturer was most surprised that my idea of size was exactly the same as their expensive proprietary software's idea.