have you changed the prop supplied on a new boat for performance reasons??

[paul salliss]

I have read reports of various performance gains from changing the prop, but would assume that the OEM has done massive research to ensure that the original is the best suited from the off, and I am wrong??

 

[Johntheoil]

Hi Paul, the only thing I can think of is that sometimes the builders are tempted to overprop by the idea of being able to quote a high top speed. I have a Sessa Key Largo 34...lovely boat, but only once have I ever seen builders quoted top speed ( and engine quoted max revs) ......with my 'lightweight' crew.........and we ran out of fuel a minute later!
Get a few big mates aboard, a full tank of fuel, 'essential' supplies, some barnacles on the props and it feels a different boat.
Going to try moving a size 'down' on the props this season, to see if we can get more 'usable' performance

 

[oldgit]

Perhaps not the same sort of boat but...
RCC. has around 8 or nine Fairline Turbo 36s on the moorings of all flavours,.
Owners of two of the 255HP versions were always at drawn handbags due to one of them being noticably quicker by a fair margin.This grudge match went on for ages until it was decided between the two of them that it was all down to props.
Yea right......
One had 3 blade props fitted the other 4 bladed.
The owner of the slower boat finally gave in at bought himself a set of brand new propellors to match the number of blades on the faster boat.
Of course the result was basically 1K worth of props changing nothing.
Any guesses which boat was faster .
The one with 3 blades or the one with 4.

 

[paul salliss]

cheers guys, all read with great interest, thank you

 

[Bandit]

The important thing is to allow the prop to reach full rpm, then to absorb the power.

My old boat was over propped in a bid to increase top speed which did not work, I should have had the pitch reduced to increase speed.

The real test is how well it sits at cruising speed and how it gets over the hump.

Talk it through with Clements marine assuming you are shaft drive.

If it's lower fuel consumption you want and a good cruising speed, keep the weight down, have a smooth hull and keep the hull weed and barnacle free, ie don't haul around your own private Eco system.

Most manufacturers of identical shaft drive boats have a selection of two or three sets of props of different pitch and use one of these not all the same.

 

[grumpy_o_g]

Perhaps not the same sort of boat but...
RCC. has around 8 or nine Fairline Turbo 36s on the moorings of all flavours,.
Owners of two of the 255HP versions were always were at drawn handbags due to one of them being noticably quicker by a fair margin.This grudge match went on for ages until it was decided between the two of them that it was all down to props.
Yea right......
One had 3 blade props fitted the other 4 bladed.
The owner of the slower boat finally gave in at bought himself a set of brand new propellors to match the number of blades on the faster boat.
Of course the result was basically 1K worth of props changing nothing.
Any guesses which boat was faster .
The one with 3 blades nor the one with 4.

I've been told that the less blades the more efficient the prop, with a hypothetical one-bladed prop being the most efficient - more blades is smoother though. On aircraft it's common to add blades to "absorb" more horsepower but aircraft are usually limited by diameter (either ground clearance or tip speed) - boats seem to be very different.

 

[Spi D]

The important thing is to allow the prop to reach full rpm, then to absorb the power.

My old boat was over propped in a bid to increase top speed which did not work, I should have had the pitch reduced to increase speed.

The real test is how well it sits at cruising speed and how it gets over the hump.

Talk it through with Clements marine assuming you are shaft drive.

If it's lower fuel consumption you want and a good cruising speed, keep the weight down, have a smooth hull and keep the hull weed and barnacle free, ie don't haul around your own private Eco system.

Most manufacturers of identical shaft drive boats have a selection of two or three sets of props of different pitch and use one of these not all the same.

+1

Talking planing hulls, a certain amount of hp will be able to push a given boat/weight up to a max. speed. The factors are power/weight ratio when the engine is allowed to reach the rpm range allowing max(ish) hp to be developed and hull properties. So it actually is a matter of calculating back from that, based on achieved rpm, pitch, and gearbox ratio. When talking recommended rpm range, you aim for the upper limit to get better hole shot and less load on the engine.

Changing a prop won't give you more hp but might allow your engine to perform as designed to.

Weight is a crucial factor and reduction is by far the easiest and cheapest way to get a higher top speed (and mpg). As the manufacturers have no chance of guessing each owner's 'ready-to-go' weight with fuel, water, gear and crew their selection of prop commonly is a bit off.

Displacement hulls are different as their top end is dictated mainly by waterline length.

 

[rbcoomer]

Perhaps not the same sort of boat but...
RCC. has around 8 or nine Fairline Turbo 36s on the moorings of all flavours,.
Owners of two of the 255HP versions were always at drawn handbags due to one of them being noticably quicker by a fair margin.This grudge match went on for ages until it was decided between the two of them that it was all down to props.
Yea right......
One had 3 blade props fitted the other 4 bladed.
The owner of the slower boat finally gave in at bought himself a set of brand new propellors to match the number of blades on the faster boat.
Of course the result was basically 1K worth of props changing nothing.
Any guesses which boat was faster .
The one with 3 blades or the one with 4.

Going on my own experiences playing with props on a much smaller vessel, I'd say the 4 blade. Yes, theoretically three blades is faster but if the boat is relatively heavy then you can just get too much slip. The extra blade adds bite and below 40kts I don't think the extra drag is enough to offset this. (IMHO)

 

[omega2]

been there done it got the T shirt, as stated got worse not better, however with our present boat the props were in a poor state as were the shafts, so we had new shafts and the old props breathed on by Paul Williams at www.clementsmarine.co.uk, his work transformed the boat talk to him you will not regret it.

 

[oldgit]

3 blades on the faster boat !
However when the slower boat had similar props fitted it made BA difference apart from big hole in a bank balance.

 

[rbcoomer]

3 blades on the faster boat !
However when the slower boat had similar props fitted it made BA difference apart from big hole in a bank balance.

That's how it should work in theory due to less drag. It's an expensive game when you start buying new props! Thankfully being smaller, more common outboard props I find there's quite a few around second hand for mine. I doubt that's true of props for the Fairline? I went from a 19p alloy prop with which I could get 36kts (but over-revving by 500rpm) to a 20p 4 blade borrowed from a friend that gave 35kts and revs on the limit. I then tried a 21p & 23p stainless (Laser IIs) - both topped out at 34kts but 20% slip on the 23p! I now have a 22p 4-blade waiting to try, but not had chance to run yet. It's the 'ideal' prop according to the various prop calculators I've tried, but I'll believe it when I see it! The most expensive was the 22p (Trophy Plus) but even that was only £150 - all would be around £500 new so I shudder to think what the 4 blade sets for the Fairline cost...

I've always assumed manufactures would go for the best 'all round' balance as they don't know what load the customer will run with. You'd need something with enough bite to get on the plane when fully loaded, but not so much that it would never reach advertised speeds or risk over revving when running light.

 

[epervier]

Back in the day (late 70's) of a much younger man looking for the need for speed, I owned a sidewinder an American boat, a clone of dateline I believe, with 175 2stroke mercury outboard bolted to the transom, the engine arrived in the box with a 21" pitched prop, general all-rounder prop, which gave the boat a general all-round performance as you'd expect, after a few weeks of this prop bought a 19" for waterskiing, it was very quick out of the blocks but top end loss of about 10kts, then bought a 23" slower out of the blocks but topped out at about 48kts then after speaking to the prop sales people bought a 25" 3 blade SS surface cutting clever,no collar for the exhaust, no rubber in the centre hub,blade roots straight from the centre, jacked the motor up by 4" on the transom and scared the shit out of myself 62kts running on a knife edge very very unstable Calshot beach to Cowes public town quay 4 and a half minutes on a flat as a flat thing water no wind, the rooster tail was worth it though, didn't bother using it much after that, took forever to get out of the hole, and far too skittish to live with.