Stooriefit
Well-Known Member
I want to replace the prop on my Victoria 34 with one which gives more thrust for a lower rpm. Anyone tell me what I need to look for ?
Fixed props
- Thread starter Stooriefit
- Start date
VARNE
Well-Known Member
I'm no engineer , but props are matched to boat & engine,particularly HP. & RPM.
Upping prop size will reduce RPM which will up the torque.You may not get full power & may just bugger the engine completely.
Talk to engine manufacturer or engineer.
Upping prop size will reduce RPM which will up the torque.You may not get full power & may just bugger the engine completely.
Talk to engine manufacturer or engineer.
Stemar
Well-Known Member
Varne is right that the prop should be matched to boat and engine, but they don't always get it right. If your current prop is matched to the engine, there are two ways to go:
1. Get an Autoprop - This is self pitching, so it gives a more efficient drive over a range of revs, while a fixed prop is only at its best at a fixed RPM and boat speed.
2. Get a more powerful engine
In both cases, you probably need to be sitting down when you find out the price...
If your current prop is undersized, your engine will reach full revs without effort, and you'll be short of drive. In that case a bigger and/or coarser pitched prop will help.
If you're badly overpropped, you won't get full revs in gear, and the engine will probably produce black smoke at full throttle. In that case you'll need to go down a bit.
There is software out there to help you find out what you need - try Castle Marine's website - but it's not an exact science.
All this assumes a clean prop and boat bottom. It won't take a lot of barnacles on a prop to make the prop vastly less efficient, or on the bottom to make the engine struggle.
1. Get an Autoprop - This is self pitching, so it gives a more efficient drive over a range of revs, while a fixed prop is only at its best at a fixed RPM and boat speed.
2. Get a more powerful engine
In both cases, you probably need to be sitting down when you find out the price...
If your current prop is undersized, your engine will reach full revs without effort, and you'll be short of drive. In that case a bigger and/or coarser pitched prop will help.
If you're badly overpropped, you won't get full revs in gear, and the engine will probably produce black smoke at full throttle. In that case you'll need to go down a bit.
There is software out there to help you find out what you need - try Castle Marine's website - but it's not an exact science.
All this assumes a clean prop and boat bottom. It won't take a lot of barnacles on a prop to make the prop vastly less efficient, or on the bottom to make the engine struggle.
roger
Well-Known Member
Repitch the prop
As said prop sizing in not an exact process and the prop originally supplied may not be right.
You can take your prop along to a specialist and get the pitch altered for a modest price £35 - £75 in my case. You have to decide what pitch you want now.
The second time I had mine altered the engineer told me smugly the different pitches of the individual blades as altered by the first engineer.
My guess (subject to this forum) is that amount of thrust is proportional to pitch.
As said prop sizing in not an exact process and the prop originally supplied may not be right.
You can take your prop along to a specialist and get the pitch altered for a modest price £35 - £75 in my case. You have to decide what pitch you want now.
The second time I had mine altered the engineer told me smugly the different pitches of the individual blades as altered by the first engineer.
My guess (subject to this forum) is that amount of thrust is proportional to pitch.
charles_reed
Well-Known Member
Always best to proceed from known to unknown..in this case we start in the dark.
A few pointers - the more blades, the more effective the propellor, larger slow-rev, fine-pitch propellors tend to be better at low speeds (as on displacement vessels).
So, unless you want to spend a mint of money, and assuming the original builders didn't make a mess of the calculations, increase the number of blades, and the diameter of the prop, but reduce the pitch.
It's a fairly arcane art and various people use different sets of calcs - speak with Les Hill of Lake Engg. 01202 621631, but don't waste his time or patience if you don't have the details of your current prop - he supplies most of the marine engineers on the S Coast.
A self-feathering prop, like the Brunton Autoprop, I've got is probably what you're looking for, as they're more effective over a wider range of conditions and rpm, but investment is considerable.
Other names to look at are Kiwiprop, Maxprop and Gori.
A few pointers - the more blades, the more effective the propellor, larger slow-rev, fine-pitch propellors tend to be better at low speeds (as on displacement vessels).
So, unless you want to spend a mint of money, and assuming the original builders didn't make a mess of the calculations, increase the number of blades, and the diameter of the prop, but reduce the pitch.
It's a fairly arcane art and various people use different sets of calcs - speak with Les Hill of Lake Engg. 01202 621631, but don't waste his time or patience if you don't have the details of your current prop - he supplies most of the marine engineers on the S Coast.
A self-feathering prop, like the Brunton Autoprop, I've got is probably what you're looking for, as they're more effective over a wider range of conditions and rpm, but investment is considerable.
Other names to look at are Kiwiprop, Maxprop and Gori.
FAITIRA
Well-Known Member
Probably, more pitch and smaller dia, but you need proff advice. It is a 3bladed prop?, (must be on a Vic 34), otherwise it would be very inefficient anyway.
kengill
Well-Known Member
To check if your current prop is the correct one for the boat you need to do a flat out run in flat water. If the speed of the boat is the same as the maximum hull displacement speed and you have a clean exhaust with no smoke or dirty water exhaust then the prop is most probably as good as you're going to get it. If the engine easily exceeds the max power rpm then the prop is too small in either diameter or pitch. If you have ablack exhaust and the engine will not acheive the specified max rpm then you are overpropped and need to either reduce the pitch or the diameter.
If you have black smokey stains from your exhaust then its probably too big a pitch or diameter or both.
But remember prop selection is an art - not a science. With the best will in the world most of the "expert prop selection Programmes" will only get it approximately right and you will still have to fiddle with the pitch and / or clip the diameter to get it exactly right for your boat.
Remember that a prop which works on a sister boat may not work as well on yours 'cos your boat may be loaded down with cruising goodies and the comparator could be a stripped out racer.
If you have black smokey stains from your exhaust then its probably too big a pitch or diameter or both.
But remember prop selection is an art - not a science. With the best will in the world most of the "expert prop selection Programmes" will only get it approximately right and you will still have to fiddle with the pitch and / or clip the diameter to get it exactly right for your boat.
Remember that a prop which works on a sister boat may not work as well on yours 'cos your boat may be loaded down with cruising goodies and the comparator could be a stripped out racer.
