Propeller sizes

[plesbit]

Apologies to all who've seen my various threads on other forums about this but it has been suggested that I would do well to post here on this subject.

Basically I am looking for an optimum size of props for my boat. Keyparts suggested 15x17, which is slightly "smaller" than we had expected but no matter. We have since found out then when she was originally built she was fitted out with 16x16's.

I'm deliberately leaving out the type of boat and engines because all that has already been discussed at length. All I am looking for here is some idea as to whether there is likely to be any difference between these two props sizes - one with an extra inch of pitch, the other with an extra inch of diameter. Are they really just two slightly different ways of achieving the same result or are they completely different animals?

Many thanks,
Simon

 

[DAKA]

Not the same at all.
The pitch is how far it will move in one rotation.

For ease of thinking about this forget the water is liquid and think of the prop. screwing on a thread.

16 inch will move 16 inch in one rotation.

The size may help prevent some prop slip.

If it had 16 inch diameter before I would have thought you should stick to 16 inch diameter.

If you over rev ed then still keep 16 inch diameter and increase to 17 inch pitch which will reduce revs. ie/ 16 x 17

Keypart know what they are doing, is there another issue they are trying to resolve at the same time ??????????

 

[plesbit]

Well the original issue was what the correct prop sizes would be. That was before we had spoken to Keyparts and before we had the info through from Fairline so we had nothing to go on. Keyparts said 15x17 or perhaps 16x15, then the original spec came through with 16x16. I've now spoken to Lancing Marine and they say 16x17 (because the 16x16 is now obsolete) but advise against actually ever revving the engines up because apparently they were a very bad engine (VP AQD32A's) and fell apart quickly when run hard which is why parts and info is hard to come by now - because there are very few left.

I think it is clear we are not going to get the expected performance. /forums/images/graemlins/frown.gif

 

[Gumpy]

Best people I have found for props are Crowther marine.
They have re propped two boats for me now both of which have been a great imrovement. Not cheap, my last prop (23x16) was 575+vat but worth every penny.

http://www.crowthermarine.co.uk/


Julian

 

[tico]

From my experience, I would go with Mike Bellamys recommendation.

(not connected but happy customer)

 

[MapisM]

[ QUOTE ]
Keyparts said 15x17 or perhaps 16x15

[/ QUOTE ]
Daka said that Keyparts know what they are doing, but if these alternatives are similar in their view, I'd recommend you to ask elsewhere!

 

[omega2]

www.clementsmarine.co.uk beat the rest.

 

[plesbit]

[ QUOTE ]
[ QUOTE ]
Keyparts said 15x17 or perhaps 16x15

[/ QUOTE ]
Daka said that Keyparts know what they are doing, but if these alternatives are similar in their view, I'd recommend you to ask elsewhere!

[/ QUOTE ]
When I spoke to Keyparts I did not have the info from Fairline and so no idea of what she ran originally. Keyparts said 15x17 Volvo HS short hub props would be the best solution. He said the AQD32A's probably wouldn't turn any long hub prop with enough power to climb over the hump. In his opinion to get a boat of that size up on the plane would require the engines to be able to spin right up to the governor and 15x17 were the biggest he thought they would turn at that speed. He said if we were determined to try long hub props then we could try 16x15.

Lancing Marine, as indicated, felt that running AQD32A's at high revs was a recipe for a short engine life as they were a poor design and used to burn out valves very quickly. His suggestion was to forget trying to plane at all and stick at around 10-11kts but fit bigger props so those speeds could be achieved using lower revs and therefore avoid the higher rev ranges which cause the burnt out valves. However that performance does not meet what we are looking for. We were hoping to be able to maintain a cruising speed of around 17kts (not 10!!) at Volvo's recommended rpm of 3600.

Whilst I don't doubt Lancing's expertise, a large amount of time during the call was spent rubbishing my engines and telling me I should expect nothing but trouble and buy new ones / or a kit to marinise some old Ford lumps. It forces me to bear in mind that these people aren't in business to give free advice.

But ultimately it leaves us with something of a dilemma. We know these engines are in good nick because they've been completely stripped down in the last few months and the engine hours are low. We know the heads are true and all the gasket sets new. We know the fuel lines, lift pumps and filters are new. We also know we've replaced every pump and valve in the domestic systems, plus fixed a lot of problems with the electrical systems so she's in a pretty fit shape. The outdrives are currently undergoing a major overhaul, including new UJ's etc. After all that, putting her back into the water and straight on the market makes no sense at all as we would get little benefit from the vast amounts of money (not all of it ours) that has been put into her over the last few months.

Our choices seem to be:

a) accept that she's only really good for inland cruising, which defeats the whole reason we bought this particular boat when we could have paid half for a Birchwood 25 with much lower fuel and maintenance costs as well

b) sell it and write off all the money we've ploughed into this boat and run the risk of going through similar problems with the next one

c) plough a whole load more money into this boat to re-engine but at least know we have a good boat and drives

d) thrash the AQD32A's and run the risk that they as fragile as Lancing say they are (which is contrary to advice from other sources)

Option A is unattractive because we bled ourselves dry to get a seagoing boat and river cruising makes no sense because there are other boats far cheaper to buy and run if that's all we wanted - which we don't. Option B is unattractive because we've blown our budget for maintenance so if we sold and bought another at a similar price we'd have no cash left to throw at any problems we might encounter. Option C is a non-starter because there is no money left in the budget. Option D is a possibility but there's no telling how long the engines would last. If it was a couple of years we might have the funds to replace them, if it was a couple of months we'd end up with a useless boat, costing us a fortune in mooring fees, insurance etc but unable to leave the yard.

All in all, I am not presently the happiest bunny. /forums/images/graemlins/frown.gif

 

[omega2]

Talk to Paul Williams at www.clementsmarine.co.uk he does not bite, but boy he knows his stuff. No connection just a happy bunny who took his advise.

 

[DAKA]

go for the 16x17.

If they dont give you what you want get them re pitched, if you cant find anyone to re pitch aluminum then at least have them reduced to 15 inch diameter(quick grind off and balance) then you will have tried two options.

You will not get full revs but hopefully close.

After all this effort,time and cash you need to try it.

I have found old engines do give max power as long as they are looked after.

good luck /forums/images/graemlins/smile.gif

 

[Chris_d]

Is your boat a Princess 32 or Fairline Phantom 32? in which case the VP 32's (106hp?) are unlikely to give more than 10/12knts under any cricumstances. I would also support anything Lancing marine have told you, they are very experienced. Just think of the fuel you will be saving /forums/images/graemlins/grin.gif

 

[[2068]]

It's hard to know where to start, as you don't say what the boat is. How heavy she is would help, too.

As an example, trying to get a 4500kg boat planing comfortably with 2 elderly 106hp engines just isn't going to happen, whatever propellors you fit.

dv.

 

[tony345]

Hmm - with just over 200hp in a 32ft boat, I'd forget the idea of sustained cruising at 17knots

10 - 12 knots is much more realistic in anything but a flat calm sea. This should be achievable at about 2,500 rpm which will

1 - reduce wear on engines
2 - maximise fuel economy
3 - reduce wear on the skipper - its hard work cruising any distance at max speed

Given this - In would have thought that the choice of 15x17 or 16x17 would make little difference. The smaller prop will make life easie for the engine and probably make it slightly more responsive

 

[plesbit]

I didn't say what the boat was because all I really wanted from this thread was to find out whether there was any real difference between 16x16 or 15x17 - in other words, does the extra inch of pitch roughly equate to the loss of an inch of diameter or not. I didn't say what the boat or engines were initially because I knew that everyone would simply discuss that instead - and it's all already been discussed at length elsewhere.

However, since the thread has veered in that direction anyway (largely through my own subsequent post) the answer is she's a Fairline Mirage 29 and she does plane, or at least did when the tabs were working, but only barely, and that's on the current 15x15's. But it's long been suggested to us that's she's underpropped, the main reason for suspecting so is that from a standing start if you throttle up she'll bounce off the governor instantly - and most of the crew will get catapaulted over the stern as she takes off. At the time we actually had no idea what props we were running but advice from others was that if she could spin up to max instantly from a standing start then we should really be running bigger / coaser props. Unfortunately we neither knew what she had or what she should have. Now we know what she has and we're trying to work out what she should have. The original 16x16's no longer exist so an alternative must be found and that is what I am trying to do.

Despite Lancing's comments a number of other marine engineers, respected around our area, disagree completely and think the AQD32A's were very good engines and will happily run at high revs without issue. In any case, our sea usage will be so little that she won't get a regular thrashing anyway. However if there's any possibility that she can cruise on the plane instead of in the water I'd much rather do that for obvious reasons.

 

[[2068]]

You can't separate the propellor selection from the boat: the two are inextricably linked!

You still haven't said how heavy she is. I've looked around, and some adverts claim around 3000Kg displacement, so with 212Hp, you should (just) be able to get her planing if this is the case.

As to the engines, I don't think it makes sense to re-engine if you still have two working engines. Diesel Mirages are still in demand. Just because the AQD32 series might not be Volvo's finest creations doesn't mean your individual engines won't be kind to you. It is true that parts are getting harder to source for these (not sure: Peugeot derived?) blocks.

If it were me, I would try and get agreement to trial a couple of different props e.g. 16x17, 15x17 and see what happened if I only wanted occasional sea use, or be looking for a boat with more power if I was going to do more time on the wide blue yonder.

All imho,

dv.

 

[macnorton]

Got this from a propeller supplier web site?

(use the below as a "rule of thumb" for prop sizes

One inch diameter = 2.5 inches of pitch.

two Inches of pitch will alter revs by 500 rpm)

Hope this helps

 

[plesbit]

Thanks for all the comments. I don't believe there is anything further to add now. We've got a pretty good idea of the kind of thing we want to go for so we'll leave it at that. We won't really know for sure until we actually get a chance to try out something different.