Propellor Pitch

[ctelfer38]

Opinions welcome from those who might know. Current engine is Penta 2003 with Propellor 15in diameter and 11in Pitch. Displacement weight is 17 tonnes. Volvo literature suggests that 2003 can cope with a 15in pitch. Extra drive is an obvious attraction, offset by disadvantage of extra prop walk especially when going astern. The question is - will the 2003 labour too much with anything above 11in pitch at this displacement weight. Thus far the engineering professionals have offered a selection of conflicting opinions. Anyone care to comment? /forums/images/graemlins/cool.gif

 

[roly_voya]

You don't say what the power output is so don't know how this relates to the boat but a 15x11 does not sound oversize for a 17ton boat. Around 3hp+ per ton should give displacement speed on flat water so if you have more than 50hp you should be achieving at least 1.3x sqroot LWL. If not pitch might need increasing.

A good test would be to take the boat out on flat water and check if you can reach between 1.2 & 1.3x sqroot LWL. If this is OK do the same test again in F3 with a bit of chop. If boat does not reach its previous speed then you are at the limit of pitch already. Do the test again in F4/5 and a noticable head sea. If she still achieves the same speed you have power to spare and can think about a bigger prop.

The fly in the ointment is that if you increase the pitch you might also need to increase the diameter. 15" is not a huge prop for a 50+hp engine so although in theory increasing pitch should increase speed if ther is not enough blade area the prop cant transfer the power to the water, More pitch = more speed, more area = more power.

 

[brianrunyard]

I believe the 2003 is 28hp, therefore I would have thought you were underpowered for 17 tonnes.

 

[ctelfer38]

The 2003 is only 28hp. Whilst that is not much for a 17 tonne displacement, the engine area is narrow and limited in its height. It is positioned under the cabin sole in the narrow aft end of a classic long keel. Therefore no prospect of considering larger dimension engine. As an auxiliary the 2003 performs quite well; but even a little more drive from the prop would be useful. The prop diameter is limited to 15ins by the keel/rudder aperture. So with these limiting factors the only immediately available option is to have more bite from the prop by increasing the pitch - as long as I then do not have an engine which is labouring in the ' higher gear'. I might just have to toss a coin to make a decision. /forums/images/graemlins/cool.gif

 

[roly_voya]

Ok so this is a classic Aux. You can still check the prop with a load test. If you are running the engine at its power limit on flat water it will overload into wind/sea so you want a bit in reserve. Whatever revs the engin will pull on flat water it should still be able to achieve with a bit of wind and sea otherwise it will overload. So try in on a poor day, if its struggling to rev and maby black smocking a bit its overloaded if its running fine and pulling full revs with good accelleration you may have some spare and can try a courser prop.

The other idea that occurs to me is a veriable pitch folding prop - ideal for an aux as it minimises drag, you can 'tune' the pitch. Downside is that the cost a fortune and you loose power in reverse

The other option if you wanted to re-engine (you metion space is limited) have you looked at deisel electric drive. Allows you to resite the engine to where there is more space

 

[Mudplugger]

Bearing in mind the positive comments on file, have you looked @ the Kiwi prop spec? would probably suit and not break the bank! unfortunately I've got too much HP.