How much is it acceptable to 'over prop'

[Laurie NB Mary M]

I have just built up a new engine for my boat, a ruston 2ywa with prm 402 box 2:1 reduction (which I'm aware is excessive, it just came at the right price) my question is; my current prop matched to the old engine is massively too small (16x10) however I have a old prop made by crowther which came off from another previous set up, that prop is 21x20. I have used the standard vic prop calculator which is suggesting something around 19x(can't remover pitch maybe something around 11) anyway, I called crowther about this issue, they said they could adjust my 21x20 to suit. And also asked if I had enough space to swing a 22, which I do, as that would be the ideal. Now the problem is obviously they don't let on the pitch they recommend. And I'm wondering if the pitch I have is usable with out shelling out on getting it adjusted. I know of that other downloadable prop calc program which I can't remember the name of, but it only works on PC, and I don't have that.

Any input much appreciated,

Laurie.

[DHutch]

Surely at least once the work is done they would tell you the pitch...

 

If the prop is excesivally pitched for the engine/gearbox/hull combination the load will overload the engine preventing the engine from reaching the desired speed.

 

A bit like trying to set of in 3rd, you might just about get away with in it, but its not going to like it, I can just about set off in 5th in the kitcar, but a heavier less powerful car would not be able, certainly not on a hill.

 

Not sure I can add much more as my experience with trying miss-fitting props is limited, although fitting double the pitch is unlikely work that well I wouldnt have said. Only other option is a smaller ratio gearbox, if a 1:1.1 or similar was available.

 

 

Daniel

[Laurie NB Mary M]

Thanks for the input. I've run largely over proped engines before, and your right they do stall when pulling away unless you apply higher revs. A smaller gearbox isn't an option. It's probably going to result in me sending this old one off to get altered, although some people I've spoken too said it should be ok, but I'm skeptical.

[ditchcrawler]

Can't you try it and see?

[DHutch]

Depends how the cost of getting it altered compared to a drydock or craning out I guess!

 

 

 

Daniel

[Laurie NB Mary M]

Try it and see costs a day in the drydock, which is again a unnecessary expense if it means I have to do it twice and get the prop altered. But that may end up what happens.

Prop altered is about £300 drydock is about £150 a day.

[by'eck]

Doesn't sound too far out for a 4" bore twin with 29bhp @ 2200rpm. I would also suggest trying it first as is. Subjective assesment will always win over theory.

[Liam]

Ariel is swinging a 22 x 20 prop, recently repitched from 22 x 18 last October whilst the shaft was replaced from mild steel to stainless duplex.

 

Ariel has a HA2 coupled to a 3:1 blackstone box.

 

Tickover speed has increased noticably more than general cruising speed which is also noticeable. There has been no difference in reverse. Ariel has always behaved impeccably going backwards, being steerable for a mile in the past.

 

Whilst discussing prop size it was mentioned that Ariel's current setup could accommodate a bigger prop, unfortunately the boat can't due to the shaft coming out off centre. There's a massive amount of room above the prop to the counter, but only 2 inches between the skeg and the bottom of the blade.

 

Getting back on topic, because of this the mention of going over square was mentioned to crowthers was mentioned, with negative comments received back.

 

One day I'll have my 2:1 box and be able to increase speed over water with slightly less revs.

[Laurie NB Mary M]

My ruston is governed to 18.5hp @1500 rpm, shaft it 1.5" I think I'm going to chance it and try it without alteration first.

You say tick

My ruston is governed to 18.5hp @1500 rpm, shaft it 1.5" I think I'm going to chance it and try it without alteration first.

You say tickover has increased Liam. Do you find there is any more strain on the engine?

This ruston is quite similar to the ha2.

Also, what was the reason for the shaft change?

[Liam]

Apologies when I say tickover speed has increased I meant for the same revs the boat moves through the water noticably quicker.

 

I wouldn't say it has put any more strain on the engine generally apart from reversing when cold, you notice an engine rev drop but it soon picks up and doesn't happen when warm.

 

I had to change the shaft, it was mild steel, battered and chewing up packing every couple of months. It's a 2" shaft that uses 1/2" packing.

[DHutch]

Try it and see costs a day in the drydock, which is again a unnecessary expense if it means I have to do it twice and get the prop altered. But that may end up what happens.
Prop altered is about £300 drydock is about £150 a day.

Middlewich is £150 for the first day, then £65 for each day after or £60 if doing a week or more.

- More reasonable for a week or so, if still the same price for a one day prop-change job!

 

 

Ariel is swinging a 22 x 20 prop, recently repitched from 22 x 18 last October .... Ariel has a HA2 coupled to a 3:1 blackstone box.

...

Whilst discussing prop size it was mentioned that Ariel's current setup could accommodate a bigger prop, unfortunately the boat can't due to the shaft coming out off centre. There's a massive amount of room above the prop to the counter, but only 2 inches between the skeg and the bottom of the blade.

....

One day I'll have my 2:1 box and be able to increase speed over water with slightly less revs.

I guess placing the prop lower was to allow better provision for 'running light' with the counter out of the water?

Why did you have the prop re-pitched? Was that part of the plan to then also go to a 2:1 box, if so why change the box?

[DHutch]

Getting back on topic, because of this the mention of going over square was mentioned to crowthers was mentioned, with negative comments received back.

 

There are good reasons for not going too far beyond 'square' as at a certain point it does become a paddlewheel not a prop! However as we have a very slow engine, direct drive, giving a very slow prop speed, we do have an over square prop as do a lot of the other steam boats.

 

Or prop is 26x32 , crusing engine/shaft speed is around 180-250 with 330 being flat out, maybe 350 briefly on a river with the wind behind us!

 

The prop on Hasty is I think something mad like 28x46.

 

 

 

Daniel

[blackrose]

Prop altered is about £300 drydock is about £150 a day.

 

My engine was over-propped. I paid £150 to have my prop re-pitched at Norris in Isleworth (reduced from 19" x 13" to 18" x 11.5"). I'e got a lot more power now.

Edited October 11, 2015 by blackrose

[peterboat]

my boat is a much nicer cruise since the prop was repitched and when we were going into keadby lock over the sandbanks it was up to 4000 rpm something it would never have done before. Mine was done at wodwards for £80 i think looked like new afterwards. A boat with the wrong prop is no pleasure to drive from my experiance

[Laurie NB Mary M]

The 300 to re pitch is from crowthers, who made the prop. And judging by the difference in quality I've had from them and others, if like them to carry out the work.

[nb Innisfree]

Crowthers repitched and reduced blade area on our Crowther prop for £40 in 2005. Original 17x14x75(ish)%BAR, reprofiled to 17x12x 55ish%

 

ETA: Took two hours while we did some sightseeing.

Edited October 12, 2015 by nb Innisfree

[madcat]

Liam if you really want a 2.1 reduction box it should be fairly straight forward to find one .

[blackrose]

The 300 to re pitch is from crowthers, who made the prop. And judging by the difference in quality I've had from them and others, if like them to carry out the work.

Which others?

 

Norris are also a very reputable firm for prop alterations, mine came back like new. I've never used others so can't comment on their work.

Crowthers repitched and reduced blade area on our Crowther prop for £40 in 2005. Original 17x14x75(ish)%BAR, reprofiled to 17x12x 55ish%

 

ETA: Took two hours while we did some sightseeing.

£40? Sounds like I paid too much and Laurie was done up like a kipper!

[Neil Smith]

I would try it first but don't over tighten it then if it needs altering you might be able to do it in shallow water without dry docking.

 

Neil

[ditchcrawler]

Try it and see costs a day in the drydock, which is again a unnecessary expense if it means I have to do it twice and get the prop altered. But that may end up what happens.

Prop altered is about £300 drydock is about £150 a day.

I am sure that Oxley Marine could change it cheaper than that

[Liam]

I guess placing the prop lower was to allow better provision for 'running light' with the counter out of the water?

 

Why did you have the prop re-pitched? Was that part of the plan to then also go to a 2:1 box, if so why change the box?

I assume it was for better running when light, keeps the blade in the water when the counter is out as you say.

 

Changing the pitch was a last minute decision (actually made in the car on the way to Crowthers!)

 

 

Liam if you really want a 2.1 reduction box it should be fairly straight forward to find one .

Yep that's the plan. I have Richard of Primrose Engineering on the case

[FadeToScarlet]

My LPWS4 (in a 72' 25 ton deep draught boat) is quite overpropped- I don't think the prop was changed from when it had a Kelvin F4. It is I think a 22x20 blade.

 

It is possible to stall the engine if you go into reverse and don't rev it when travelling fast, but other than that there's no real problems. The engine can rev to 2200 RPM but will take the boat along at about 9-10kph on the river at 1800 which is fine. I've never tried to go faster and get black smoke and see where it stops revving, but it will be higher than I need the.engine to run.

 

Not so good for towing, you have to rev it a bit.

 

If I were you I'd fit the prop and see how the boat handles- even though mine in theory shouldn't work, it does, quite satisfactorily.

[peterboat]

Which others?

 

Norris are also a very reputable firm for prop alterations, mine came back like new. I've never used others so can't comment on their work.

 

£40? Sounds like I paid too much and Laurie was done up like a kipper!

woodwards were making a 28 foot prop when i was there so i dont think they are fly by night either. i was very pleased with the result and would recommend them

[madcat]

Where are Woodwards Peter ?

[peterboat]

hull type it in google and they have loads of pretty pictures of big propellers