Prop Pitch??

D

dmj

Member
Joined
26 Jul 2005
Messages
30
Location
Woodbridge, Suffolk
Visit site
25 Ton 1925 Ex-Lifeboat, Russell Newbery PDM4 engine+Self- Changing Gears box (no 'actual' neutral ...makes life more difficult!). Installed 1960 & with only 500+ hrs is still 'running in'. Should do 8Kn @ 1300 rpm but only happy at max 800 rpm (@ approx 5.5-6Kn). At 850+ rpm..lots of heavy 'shake'. Recently advised that the prop (24" 3 blade) pitch may need adjusting. How do you know and how/where do we go about getting such a thing checked/sorted? Advice much welcomed.
 
I would advise you to steer clear of French Marine for this task. They are excellent engineers, (happily spent about £10k with them) but from personal experience seem to be less good at propellor size and pitch selection than perhaps others.
 
"Self-changing gears" Now that sounds fun for close quarters work.

Suggest you get in touch with a good prop company.
 
What is the power output of that engine? And how do you know that it should do 8Kn at 1300rpm?

It doesn't sound to me like a prop pitch problem because 5.5Kn at 800 rpm is equivalent in pitch to 8Kn at 1300 - so it seems that the pitch of the prop is about right for the expected speed. Or at least that would be the case if you had a normal gearbox - I'm not sure of the significance of the self-changing gears.
 
Engine output 50hp. Have the engine supplier's (Russell Newbery) info when engine and gearbox (Self-Changing Gears is the name of the Company that built them. They are hydraulic boxes ... ours just happens to be the version without an actual neutral ... neutral is hopefully located when the oil pressure is taken off either the forward or reverse clutch plates ... but it's a little imprecise) were supplied in 1960 stating that 1300rpm would deliver 8Kn on the 24" prop fitted. All this goes over my head so I tend to pick up on suggested problems ... and prop pitch was the best I had to go on. Now I'm wondering! /forums/images/graemlins/confused.gif
 
50HP to drive a 25Tonne boat sounds a little underpowered by todays standards. What is the waterline length?

It is possible that they engine is simply not powerful enough to propel the boat at 8kn. If that is the case then reducing the pitch on the prop would enable the boat to go faster and the engine to work at higher revs, but you would probably still not attain 8kns. Reducing the pitch by 10% might for example enable you to reach 7kns at 1300rpm.

Best thing to do is to ask a prop expert what they would recomend on your set up
 
You must log in or register to reply here.

Share this page

Top