Propeller choice

[nemo]

I have to choose a propeller for a 30 feet sailboat, I don't know anything about the previous propeller, just these datas: engine 10HP@2000rpm inboard diesel, shaft diameter 20mm, and the propeller should be 2 blades, fixed pitch, I don't want max-prop because of budget problems . Can any of you suggest me how to find correct diameter and pitch?

[nemo]

Errata corrige: power 10 HP@3000 rpm, reduction ratio 2.75 : 1

Nemo,

You need to tell us the designed displacement speed of the hull.

Theres no point buying a propellor with a pitch greater than the hull is designed to displace water, as that will only "lug" (overload) the engine.

So we could work backwards from hull displacement speed, if we knew it.

You only have 10 Hp at 3000 RPM...so she's not going to be any speed demon thats for sure.

How much forward speed do you need to overcome wind & and or tide /current where you sail?

Cheers!

[Stephen Ditmore]

Hi Nemo. I recommend you seriously consider an AutoProp. The fact that it's self pitching means far greater efficiency at speeds other than that for which the prop is optimised, when accellerating, and when using engine and sails simultaneously. You'll also find it improves reverse thrust while reducing the prop's tendancy to "walk the stern". Under sail with the engine off it has less drag than a fixed prop (but more than a folding or featherig prop). AutoProp can tell you the proper diameter if you supply them with an accurate displacement of your boat, rated engine HP and the RPM at which that HP is acheived, and gear redux ratio (as has been stated already).

[gonzo]

What is the displacement? It is necessary for the calculations.

[Stephen Ditmore]

I'm confused by the term "displacement speed" in Trouty's reply. The displacement is equal to the weight of the boat. In this case you should prodably use the displaement either half or fully loaded with fuel, water, provisions, and passengers.

[gonzo]

A lot has been said about "hull speed". Usually the calculation is 1 to 1.3 times the square root of the waterline. However, a very narrow hull can go much faster at displacement speeds. For example, a surface piercing catamaran never planes. It goes at three or four times the supposed "hull speed". A burdensome hull, as in a harbor tug, will have such a huge bow wave that can only do about .8 of the "hull speed". Anything over that and it will start shipping water over the bow. In other words, hull shape, prismatic coeficient and trim all affect the speed. The importance on calculating the horsepower to speed ratio is in calculating the propeller's pitch and diameter. A narrow, shallow hull can have more pitch than a deep wide one. Displacement to length ratio is a good indication of how much hull is underwater. That is the reason to include it in propeller calculations.

[lazeyjack]

nemo you offerd to help me
i will write direct
Here is a link, they are prop makers in NZ the price will be cheaper, send em all the details, beam max wl, , wl, disp, eng make model, hp, reduction cheers mate
Stu
info@bia.org.nz
http://www.imtec.co.nz/exhibitors/henleys.htm

[dougfrolich]

It might be better to concider the Cp and relate it to optimun speed to length ratio, rather than use vague terms such as "hull speed".

[CDBarry]

I generally tend to suggest that a sailboat prop be pitched so it produces full RPM at near zero speed, since handiness in manuevering, etc. is important, as is punching through waves in a sea state. This might cost some top end.

Get a prop program or spreadsheet that balances the engine curve. You will have to use B-series coefficients because segmental (Blount-Hubble) coefficients are only good for 3-5 blade props. This might result in a bit overpitch, but it should probably be OK.

[Gerald Niff]

Power 10 HP@3000 rpm, reduction ratio 2.75 : 1 for a 30' sailboat.

There is still no answer to Nemo's question. Maybe he ask the wrong question? Using the information given and assuming that the sailboat is more than 3' wide, the motor is small for that size boat. No matter what size prop you put on you will not punch into heavy seas at 5 knots! So ...... could the question be changed to ..... "Given the above information, what is the largest prop that this 10 HP setup will turn?" I know that information won't fit into the spaces in the computer program but maybe you could lie to the program?
Gerald Niffenegger

[CDBarry]

No, a prop program like PSOP, or a spreadsheet using the appropriate coefficients (from PNA for the B-series) predicts torque, thrust and cavitation given prop characteristics, RPM and velocity of advance. Set velocity of advance to a small number and select prop characteristics (diameter, pitch EAR) sure the engine has enough torque to turn it at full RPM without cavitation. That's the best you can do.

[Gerald Niff]

OK ....... Here are some facts that I have based on my 32', 7 ton yawl. The boat had a 10 HP Yanmar with a 2-1 box and a 12" two blade prop. On smooth seas the boat would lug along at 4 knots. Try to increase Rpm's, it loaded the motor and produced an enormous amount of black smoke. My conclusion ........ too much prop for the engine.
I installed a 1.9 VW in the yawl that produces around 75 HP. At 3000 RPM the boat moves along at 6.5 knots. My conclusion ........ not enough prop because I don't want to turn the engine a that high an RPM. The engine has done exactly what I wanted from it and will soon be removed and placed in a 36' Multichine steel hull. I will be putting the Yanmar back in the yawl. and probably cut the prop down to 10" for a trial.
Gerald