Author Topic: Prop size and type  (Read 1108 times)

Alphadug

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Prop size and type
« on: November 21 2019, 16:57 »
I was wondering what type (folding/feathering, fixed) prop people are using and what size for a V42 with a d2-40 engine and 130S saildrive. I think mine is too small as I can hit 3200 RPM at wide open throttle but can't hit 7 knots at cruising RPM (2600). I won't know the size of mine until I haul in the spring. It's a folding 3 blade. I had a fixed prop on my Beneteau 321 that worked fine and easily cruised at 7 knots towing a 12' RIB. Didn't really affect the sailing performance too much.
Thanks,
Doug

Symphony

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Re: Prop size and type
« Reply #1 on: November 21 2019, 18:36 »
The figures you quote suggest the prop is too small either diameter or pitch or both. You should be a be to get at least 8 knots at 3200rpm.  The diameter should be at least 17" for a 40, although the actual size will depend on the specific design of prop so it is best to take the prop manufacturer's recommendation.

In my view the best folding prop is the FlexOfold (I have a 2 blade on my B33) - you will need a 3 blade - but you might also consider a feathering prop such as the Darglow Featherstream. A Bruntons Autoprop is also worth looking at especially if you intend doing a lot of motoring or motorsailing for example long passages in light winds in the Med.

Captain Jan

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Re: Prop size and type
« Reply #2 on: December 08 2019, 08:34 »
Hi, some background for prop discussion, My B38 had a 14 x 12 which was useless. Changed to a 17 x 14 which was vg. About 5.5k at 2500 rpm, 7.3k at 3000 or thereabouts, on a D 30 thru a MS 25 S . Sailing performance was poor, fitted a Kiwiprop feathering prop from ebay, undersized at 15.5 inch blades set at 23.5' pitch giving 25% increase in sailing performance, 5.9k upwind 7.6k offwind,  and 5.3k at 2500 rpm. The Pitch can be manually set so can tune for your boat. Also , blades can be changed at  £200 a set so we will be upgrading to the 16.5 inch set probably 22' pitch recommended for us, but will manually increase to 23.5 (max) for a long inland passage we have coming up.   Talk to Vectra Marine who will advise.  A very good prop for us, but some annual maintainance ref. greasing is needed, and there are moving parts to check from time to time.  A different engine & saildrive I know, but I hope this helps.

geoff

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Re: Prop size and type
« Reply #3 on: December 08 2019, 12:56 »
I agree with Captain Jan. I have a Kiwi from Ebay , works very well indeed but rather prone to the performance tailing off if dirty. I have an 18.5 set at 23deg and get 7kt @ 2k if clean . Bav 40ocean md22l 50hp. By the way Jan I know the old owner of Macavity and sailed on her when it was at Fareham . Geoff

Captain Jan

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Re: Prop size and type
« Reply #4 on: December 19 2019, 08:37 »
Useful feedback on prop size from George, even though a bigger engine than ours on our b38, it suggest we’d run better with the Kiwiprop   17.5 blades, but the importers are keen for us to have the 16.5.  It’s good to hear you knew Macavity, who was in good health last year, when she went south to the Medway.

IslandAlchemy

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Re: Prop size and type
« Reply #5 on: December 19 2019, 09:11 »
I have a prop size calculator which seems to give good results.

If you want to post the following information, I am happy to run it through for you an give you the results.

I will need:-

Waterline length
Sailing displacement
Max engine horsepower
Max engine revs
Cruising revs
Gearbox ratio

cegri

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IslandAlchemy

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Re: Prop size and type
« Reply #7 on: December 19 2019, 11:30 »
Just found the numbers for your boat and run them through, and it says that for a 3-blade prop, you should have 17 x 13.

If you go for a 17" Featherstream, you can change the pitch by changing cassettes (which are exchangable FOC from Darglow), which at least means you're not in any danger of wasting money on the wrong size prop.

I would call Nick at Darglow and discuss it with him.  He will tell you what the options are (they also sell Flexofold too, which are the best folding props), and will do his own calcs, and also has a lot of data from boats they have fitted them to over the years.

Best of luck.

Symphony

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Re: Prop size and type
« Reply #8 on: December 19 2019, 16:06 »
I agree with Island Alchemy. Both the Featherstream and Flexofold are far superior propellers to the Kiwi, although inevitably more expensive. I have had both, Featherstream on a conventional shaft drive and FlexoFold on two different Bavarias. Although I currently have a FlexoFold, I bought this before the Featherstream was available for saildrives and if I was buying now that would be my choice. It is beautifully made (in Dorset!) of the very best materials. As suggested the pitch can be changed and also means you can have a different pitch in reverse which is useful if low speed stopping power is important for you.

Alphadug

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Re: Prop size and type
« Reply #9 on: December 20 2019, 19:39 »
Thanks everyone. According to the dealer, Volvo recommends the following: 3 blades, 18x12 if folding, 17x12 if fixed. I'm assuming diameter comes first. If this is correct, why the difference? As I mentioned I won't know what I have until I haul in the spring, but the factory prop sounds too small.

Doug

Ronald

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Re: Prop size and type
« Reply #10 on: December 20 2019, 19:53 »
Gori 16,5x13x3LHS, this is the recommendation from Gori for the configuration you mentioned as it’s the same as mine.

Can’t say how it performs as boat is still inside. 

Symphony

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Re: Prop size and type
« Reply #11 on: December 20 2019, 23:46 »
Thanks everyone. According to the dealer, Volvo recommends the following: 3 blades, 18x12 if folding, 17x12 if fixed. I'm assuming diameter comes first. If this is correct, why the difference? As I mentioned I won't know what I have until I haul in the spring, but the factory prop sounds too small.

Doug

As I said in post#2 the actual size will depend on the specific design of prop because they vary in things like blade shape and efficiency, blade area, BAR (which is the proportion of blade area to total prop area). The size recommended by the manufacturer of the prop is the one that they know works for the boat/engine combination. The Volvo folder for example is fundamentally different in blade design from the FlexoFold or the Featherstream. The prop calculators  such as Propcalc and Vicprop are based on a middle of the road fixed 3 blade (such as the standard Volvo fixed prop), so a 17*12 is a good base recommendation for your boat.

Just to give you an idea how different props can be different sizes, my 2015 B33 with a D1-30 came with a fixed 2 blade 16*13, but the FlexoFold I have is a 16*11 and works perfectly - achieving the required revs and speed specified by Volvo. I have never run it with the OE prop, but the engineer who commissioned the boat said it achieved the same performance with the folder as a standard boat.

So, if you are considering buying a non standard prop take the recommendation of the supplier. However, one thing to bear in mind that changes to the basic boat, particularly added weight may require a deviation. Many cruising boats are marginal on engine power - for example 38s with 30hp engines or 40/41/42s with 40hp. Once they are loaded with full liveaboard cruising gear (rather than kitted out for short term charters in gentle locations like the eastern Med) they struggle to maintain good speed, particularly in adverse conditions. In these circumstances, reducing pitch by say 1" will increase revs at any given speed by around 250-300 so allowing the use of more of the limited power. Alternatively you could consider buying a variable pitch prop such as a Bruntons Autoprop which automatically adjusts pitch to the load placed on the engine. These are popular with liveaboards who spend long periods motoring or motorsailing.

If you have the luxury of buying a new boat, consider the larger engine option, particularly if you intend adding significant weight for cruising.  Displacement is much more important than length of boat. As examples, the J&J designed 32/33/34 models from the late 1990s to 2010 displaced between 3800 and 4500kgs. My Farr designed 33 displaces 5500kgs, the same as my previous J&J 37. The earlier boats are fine with their standard 20hp engines, but I chose the 30hp for my new boat against the standard 20. Design displacement of 5000kgs  is the break point for going up to 25-30hp, 7000kgs for 40hp and 8500kgs for 50hp+. Liveaboard gear on a 42' can easily exceed 2500kgs, plus all the extra windage of davits, sprayhood, bimini, radar arch etc.

As with most things to do with boats, propellers are a compromise. For many people the standard prop is fine, but a folder/featherer really does give significant benefits in terms of sailing performance, particularly in light airs and most (but not all) perform as well if not better under power than the standard.

BTW if your boat is still in commission and has a reasonably clean bottom, carry out some structured speed trials, plotting speed through water against revs in 200 rpm increments from say 1600 rpm up to maximum. A key measure is being able to achieve within 150 rpm of the rated maximum. If your prop is indeed too small you will exceed this and the governor will cut in at the rated maximum. This is essentially the commissioning test on new installations.

Hope this helps.

dmfranch

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Re: Prop size and type
« Reply #12 on: December 31 2019, 11:40 »
In the proposed bavaria Table fon Blades there are two words I don´t understand:
  3 Blats "Fest"  and 3 Blats "Falt".   Could anyone explain them?

Symphony

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Re: Prop size and type
« Reply #13 on: December 31 2019, 13:34 »
I think one is "fixed" as standard and the other is "folding".

sy_Anniina

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Re: Prop size and type
« Reply #14 on: December 31 2019, 13:57 »
Fest would be fixed blade propellor

Falt would be folding.

BR,

Tommi
s/y Anniina