Propeller design

Nope. It doesn't effect the design of the prop.....the clearance only affects its performance.

 

That is correct, in general but in this case there seems to be no other way than to reach a compromise between the best possible propeller for that hull and the space available for it. That is, the clearances will influence the propeller. It seems that the OP tries to figure out how to get the best possible propeller since the available space is already fixed.

 

The OP has already given different values regarding the maximum diameter. I suggest that until we have the info I requested earlier, we can only speculate. That said, unless the operator is willing to accept extremely low propulsive efficiency, there is not much to gain by the use of increased BAR or blade numbers, since the propeller loading is way outside anything in terms of "normal" propeller shapes.

Of course we could have a principal discussion on the influence of blade number on clearance. But first you must understand the difference between clearance requirement as a means to restrict the amplitude of pressure pulses on the hull shell and the influence of varying tip clearance on the fluid interaction with the rotating blade, i.e. blade efficiency.

The recommendations presented by TANSL are strictly aiming at controlling pulse amplitudes and has not much to do with the design or performance of the propeller as such, so these diagrams are not sufficient evidence to start a mudslinging campain from, should there be a wish to do so.........!

 

@baeckmo, agree with you. I think that some of those who have thought, that we have shown to know very little about this issue, we must give way to those who do know about propellers so they can give useful advice to the OP.

 

What I mean, is that it constrains the maximum diameter. If more blade area is necessary to absorb the power, the only solution is to have more blades.

 

100 HP at 330 mm diameter and 6 knots speed (even without any wake fraction) will lead to very poor efficiency (~25%) and cavitation. Maybe cavitation can be handled with a propeller designed to work in these conditions, but it will still have extremely poor efficiency and will have worse efficiency than "normal" propeller at lower powers used.

Such a small diameter at only 6 knots could be normally used up to 10-20 HP. Even at that power it would be too small to reach good efficiency, but would not suffer from cavitation. The efficiency would be 40-50% at those powers, while normally one would use at least 350 mm for 10 HP and 400 mm for 20 HP at 6 knots and reach 50+% efficiency.

What kind of vessel would only reach 6 knots at 100 HP? Must be very heavy and still have exceptionally high resistance at such a low speed.

 

A standard propeller for a 100HP outboard has a diameter of 340mm (13.5 inches) so it is only slightly below a normal range.

 

Thanks every one for suggestions

 

IF 100 HP outboard engine has one with 340 mm dia,why cant we design one with same dia for onboard engine.

 

Dear Gonzo, that propeller is designed for speeds around 30 knot and above, which makes all the difference! A propeller for the data we have at hand here, would be something like 35" (890 mm dia) or bigger in order to reach 45% efficiency, i.e. a thrust of roughly 11 kN (depending on hull shape). In comparison your 100 hp outboard with the 13.5" propeller would cavitate heavily at 6 knots, leaving you with something like 4 kN at the very best. Referring to a completely unknown operating situation, and suggesting solutions based on guesswork is, in my view, a bad practice and should be avoided here.

 

That was a response to the previous post. Joakim said that a propeller of that diameter would only be used with 10-20 HP. If you take it out of context, it makes no sense. There is not enough information to calculate the propeller, since there is no size, shape or displacement given for the vessel.

 

If you leave out "at only 6 knots" in the same sentence, it doesn't make sense. But with it it makes a lot of sense. 6 knots top speed is a pure (slow) displacement boat and you really wouldn't use that small diameter for more than 10-20 HP. Sailboats with 10 HP inboards use typically 330-380 mm (13"-15") some special cases may have 300 mm (12"). With 20 HP they typically use 380-410 mm (15-16") and top speeds are usually around 7 knots already.

Even the high thrust outboards like Yamaha T8 and T9.9 use ~300 mm propeller instead of the ~250 mm of normal ones.

 

Yes, the target speed is important to consider.