Bollard pull and more speed?

[laurie65]

I have carried out tests on a 2 ton life boat she as a saab 12 hp diesel 1800 revs into a 2 to1 gear box the bollard pull with the boats own propeller was 16 inch by3 blades pulling 180 kilos at 874 revs at 12 inch of pitch on the screw. I have my own propeller designed prop that I have also tested the bollard pull was 220 kilos a the revs was 849 r,p,m, this prop is only 15 inch by 3 blades both props are about the same bar size as any one what I should do next to get this known to all boat users I need held I have photos of the test all carried out in a closed dock here in hull.I should add that both the props are open no nozzle is on the boat I need help from someone with a boat that I can use to just finish my tests as I want to take a little pitch of my watertrap prop to match the boats prop this pulls more with an open prop than if it was fitted with a nozzle ...

 

[rustybarge]

I have carried out tests on a 2 ton life boat she as a saab 12 hp diesel 1800 revs into a 2 to1 gear box the bollard pull with the boats own propeller was 16 inch by3 blades pulling 180 kilos at 874 revs at 12 inch of pitch on the screw. I have my own propeller designed prop that I have also tested the bollard pull was 220 kilos a the revs was 849 r,p,m, this prop is only 15 inch by 3 blades both props are about the same bar size as any one what I should do next to get this known to all boat users I need held I have photos of the test all carried out in a closed dock here in hull.

These are the guys who know all about this kind of stuff:http://www.boatdesign.net/forums/

 

[rbcoomer]

These are the guys who know all about this kind of stuff:http://www.boatdesign.net/forums/

That was a very tactful answer!

I have to confess to the o/p that I read 3 times and it still went over my head! At first I thought it was about the loads pulled by different props but then it started going fuzzy! I always thought I was quite good at following stuff I know little or nowt about... :ambivalence:

Edit: my initial reaction was going to be more blades, but then I thought I'd better keep quiet as I was unsure about the question!

 

[Murv]

I think the OP is suggesting that they've come up with a new, superior propeller design that and is appealing for boats to test it on.
If the OP can knock up a left handed prop to fit a Volvo 270 that has the equivalent push of a 16X23 then I'll happily give it a whirl! (no pun intended)

 

[rustybarge]

That was a very tactful answer!

I have to confess to the o/p that I read 3 times and it still went over my head! At first I thought it was about the loads pulled by different props but then it started going fuzzy! I always thought I was quite good at following stuff I know little or nowt about... :ambivalence:

I know only too well,
.......I've been caught out too many times myself......

 

[Divemaster1]

Here we go from someone who spend a lot of time thinking about stuff, and like to gain more knowledge ... so;

1) A fixed (not variable blade) boat propeller is designed so that it would slip (ie not grip perfectly) in the water at most RPM.
2) A diesel engine should reach its max RPM ... in the case of the SABB diesel here that will be 1800 RPM underway where the governor restricts the top end.
3) This means that if the lifeboat reached 1800 RPM with old prop, it may not have represented 100% load on engine, as she will have been designed to reach 1800 RPM fully laden with people.
4) Assuming that bollard pull at 874 RPM = Engine load, we can then assume that the new propeller represents a heavier load on the engine ... Ie more HP is taken out at 874 RPM than with the old propeller.
5) It is therefore logical to assume that whatever the new design is and why it does it, the propeller will be putting heavier load on the engine at any RPM
6) This therefore mean that there is a risk that the new propeller will take out 100% of the engine capacity at lower RPM, which mean that there is a risk that the engine may not achieve its designed WOT underway
7) Not achieving full RPM means that engine is overloaded at lower than designed RPM (less coolant etc., circulating to disperse of the increased heat additional injected and burned fuel generates) and thus one can expect Big Bill to arrive at some stage...

Propeller transfer engine energy (via gearbox) into the water and moves boat ... so heavier load on propeller = more HP taken out of engine ... more HP = more fuel burn = more heat generated...

Anyone available to comment or correct my understanding please ?

 

[laurie65]

divemaster I agree with all you have said but my prop has 13 inch of pitch and is turning at 848 rpm which is 40 less then the boats own prop that as got 12 inch of pitch but the bow has come up when steaming full and moving a lot faster at over 7 knots to 6.4 max with its own prop you can feel the added trust when the engine is run at full speed I think the pitch should be made 12 inch when the test can be carried out PS I started work on this idea over 5 years ago with the late Norman parsons of Kort propulsion and this propeller traps the water so there is no slippage of the prop so it thinks it is a nozzle but as nozzles slow the ship down over 16 knots with this been part of a turning prop it will work at any speed unlike a nozzle...

 

[rbcoomer]

Ah-ha - It's becoming clearer now since these last two posts!

It sounds like a very interesting project as prop-slip means lost efficiency and anything that adds a few mpg must be worth pursuing.

I can't help with a boat, but certainly wish you luck with your prop development. :encouragement: