Laden dive boat...prop change?

[jwalker]

Originally Posted by Dan Worth

To close this one out....have been trialling the new 15 pitch 4 blade prop with 8 up and weight.....hole shot great, top end reduced to 32 knots (more than enough laden...really happy at 25 knots)....biggest surprise is that even when we ran with just three on board, it still didn't rev past 5300rpm....max engine revs 5700rpm according to literature....cant get my head round this...not complaining, just thought it may /would have overrevved?

Really very pleased....thanks for all the assistance....big thumbs up to steel developments....recommended

Dan

I don't understand the logic of a 4 blade prop in your situation. You were trying to reduce the load on the engine to get it back into the power band and a 15" pitch sounded about right as a starting point but then you add a blade so you're effectively spinning 1 and 1/3 propellers so increasing the load on the engine. If you needed more drive to push the heavy load, then a 13" four blade would make more sense. As an aside, your case is a prime example of where a Torqueshift prop would be ideal... if you can find one these days. The low pitch blades start at 11" and adjust up to 26" as the load allows.

Anyway, you seem to have a satisfactory setup now so good luck with your trip.

[Bigmuz7]

Originally Posted by jwalker

I don't understand the logic of a 4 blade prop in your situation. You were trying to reduce the load on the engine to get it back into the power band and a 15" pitch sounded about right as a starting point but then you add a blade so you're effectively spinning 1 and 1/3 propellers so increasing the load on the engine.

But the blades are smaller, and are cupped less.

I'm not sure what the surface area differences are for a given size perhaps a prop guru could shed some light on that ..

Because of the so called 'dark art' that are props, I think a big range of factors can have quite an impact, including the usuals such as hull weight deadrise & hull form, motor torque etc, and not to mention the depth in the water, I mean moving an engine up or down an inch can absolutely transform the perfomance of some engines, as I have experienced, thats why trial and error seems to be the best way to find that ideal match I suppose,.. so in the end therefor, its not just about physics and maths as the interaction of some of these variables doesnt always give the result expected.

Another example was when I tried a 21 pitch on my Honda 90 , It ran as sweet as a nut on a 19", so I went for glory and a few extra knots .. big mistake I could barely get that motor to spin above a fast idle, the transformation was unbelievable .. and heres a thing, the alloy prop which that engine came with was actually slightly better that a stainless equivalent, giving a superior hole shot... wierd eh ? I'm tellin ya .. theres voodo in them things

[jwalker]

Originally Posted by Bigmuz7

But the blades are smaller, and are cupped less.......

Depends on which prop was supplied I guess.

.....the alloy prop which that engine came with was actually slightly better that a stainless equivalent,

ah but, ah but..was it an exact equivalent or just supplied as being the same diameter and pitch?

... wierd eh ? I'm tellin ya .. theres voodo in them things

Nah, but a small change in shape of a prop can have a significant effect.

[Razorbill RIBs]

Originally Posted by jwalker

you seem to have a satisfactory setup now

TBH I struggle to understand what has been described on here as the 'black art' of props, and I don't have any of the other options to compare the new prop to (other than the 19 which we will also be using), but as you say we have a satisfactory setup now.....the lady at Steel developments was very helpful and very thorough in her questioning and it seems to have paid off...may try to get my head around it all one day, but just pleased to have a solution for now