How big is a 21" propeller ?

J

Jassira

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Egham, Surrey
www.jassira.com
Sounds like a silly question I know but.....

I've just taken deliver of a new folding prop from a well know manufacturer. The spec as advised by them was for a 21 x 14 four blade prop, this is what they've delivered.

IMG_0178lr.JPG

I emailed them querying the size and this is their reply

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I think the reason it measures short of 21” is that we haven’t accounted for the shrinkage in the casting from the original pattern and blade design – school boy error on our part.

To get a more accurate diameter measurement it is better to measure the radius and multiple by 2. I think it will be closer to 20”.

Please be advised that we are 100% sure that your propeller will match your installation.
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I'm struggling to believe this is anything other bullshit, your thought would be appreciated

Update: As close as I can measure it, it's 19 3/4" I'm going to try and measure the pitch but not entirely sure how to at the moment
 
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Yes it's just under 20" diameter.

I don't understand their reasoning - they admit they made a school boy error but don't offer any sort of resolution.

I think you need to talk to them; I presume as you've bought a four blade prob, you are short of available space and were struggling to get as large a diameter as possible. If so, it's not what you ordered. Whist it might 'work', that's quite a significant drop in size.
 
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The suppliers having admitted making an error, the OP has a decision to make, in which he is surely on fairly firm ground. He might choose to try the prop, and if it suits the boat/engine, all well and good. However, he might perhaps write now to the supplier (who seem also to be the manufacturer, which makes matters simpler), stating that he notes their admission of error, and requiring to be indemnified in full should the prop not suit. This may take the form of an alternative prop or reimbursement in full.

If they agree (and they should be pressed to do so), he's just bought a prop from the nautical equivalent of Marks & Sparks. Quids in.

Whatever he does, he should assert his legal rights as soon as possible (if he hasn't already done so), otherwise keeping the prop can become de facto acceptance of it.
 
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Did the manufacturer fit it?
I'd be inclined to remove one of the blades and see what is stamped on it inside the hub.
 
macd said:
The suppliers having admitted making an error, the OP has a decision to make, in which he is surely on fairly firm ground. He might choose to try the prop, and if it suits the boat/engine, all well and good. However, he might perhaps write now to the supplier (who seem also to be the manufacturer, which makes matters simpler), stating that he notes their admission of error, and requiring to be indemnified in full should the prop not suit. This may take the form of an alternative prop or reimbursement in full.

If they agree (and they should be pressed to do so), he's just bought a prop from the nautical equivalent of Marks & Sparks. Quids in.

Whatever he does, he should assert his legal rights as soon as possible (if he hasn't already done so), otherwise keeping the prop can become de facto acceptance of it.
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I have emailed them, told them I'm not happy and will be seeking 3rd party advice. If push comes to shove is there somewhere I can go for an authoritative opinion ?
 
Jassira said:
I have emailed them, told them I'm not happy and will be seeking 3rd party advice. If push comes to shove is there somewhere I can go for an authoritative opinion ?
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Plenty of reliable stuff on-line: Citzens Advice Bureau, Trading Standards as mentioned, and so on. Consumer law in the UK changed markedly not long ago, so you'll find loads of precis of the effects of the new law. Both CAB and TS may have offices near you.

Assuming you bought on the basis of an ad, the item is not 'as described', in a manifestly material way, which should be all the grounds you need. Additionally, your order (which you'll presumably have a record of unless placed by phone) no doubt included the expression "21 inch"? If you paid for it by credit card, your card provider owes you to exactly the same extent as the vendor (which isn't a route to being compensated twice ;))

If you bought the prop on-line you may have even more rights under what was 'distance selling' legislation, now Consumer Contracts regulations.
 
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Poecheng said:
Is that a parallax error? The prop and the tape looked to be both pushed up against the solid wall on the left but the tape is a bit higher than the prop.
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To be sure of the diameter you could go to a mechanical workshop that has a large vernier caliper, measure with htis and take a picture of the caliper applied to the prop.
You are lucky it is a 4 blade, a 3 blade would have been much more difficult.

Best wishes, you deserve to be refunded.

Sandro
 
You need to be clear about what you specified. Did you specifically ask for a 21" diameter prop, or did you do what is more common and specify a prop to suit your boat engine combination?

The difference is crucial. If the former you are taking responsibility for the prop being suitable and if you can show what was supplied does not meet YOUR spec then return it for a replacement or refund.

If on the other hand you did the latter then it is the supplier's responsibility to provide a prop that works for your boat - which is what they are now claiming it does. In a sense therefore its exact size is irrelevant. If it does not work you can return it and demand one that does. If it achieves what they claim then you have got what you ordered. I would ask them for the calculations they used, in particular the speed and revs they expect at WOT.

There is no absolute correct size for a specific boat. My boat for example was delivered from the factory with a 16*13 2 blade fixed but the folding prop (FlexoFold) is a 16*11 2 blade. Both achieve essentially the same performance - just different designs. Any decent prop manufacturer should know or be able to calculate an appropriate size for most boats/engines.
 
Tranona said:
You need to be clear about what you specified. Did you specifically ask for a 21" diameter prop, or did you do what is more common and specify a prop to suit your boat engine combination?

The difference is crucial. If the former you are taking responsibility for the prop being suitable and if you can show what was supplied does not meet YOUR spec then return it for a replacement or refund.

If on the other hand you did the latter then it is the supplier's responsibility to provide a prop that works for your boat - which is what they are now claiming it does. In a sense therefore its exact size is irrelevant. If it does not work you can return it and demand one that does. If it achieves what they claim then you have got what you ordered. I would ask them for the calculations they used, in particular the speed and revs they expect at WOT.

There is no absolute correct size for a specific boat. My boat for example was delivered from the factory with a 16*13 2 blade fixed but the folding prop (FlexoFold) is a 16*11 2 blade. Both achieve essentially the same performance - just different designs. Any decent prop manufacturer should know or be able to calculate an appropriate size for most boats/engines.
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The order I placed was specifically for a 21" x 14" 4 blade prop and the invoice I paid listed it as the same, they also sent me a drawing of the prop for me to check it would fit, I've measured as best I can and it seems to be smaller everywhere
VF-108 21 inch, 1.75 inch 1in12 Actual Size.jpg
 
Then they have not supplied what you asked for. There is a risk it will not do the job so return it unless they agree to replace prop and associated costs if there is pa problem.
It's like getting a focus rather than a Mondeo it may do the job but it is not what was ordered
 
Jassira said:
The order I placed was specifically for a 21" x 14" 4 blade prop and the invoice I paid listed it as the same, they also sent me a drawing of the prop for me to check it would fit, I've measured as best I can and it seems to be smaller everywhere
View attachment 64741
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Ok then they are in breach of contract for not supplying to your specification so you have the right to demand a replacement or a refund.

However, the risk is all yours if they supply a prop of the size you specify and it does not work. Personally I would not take that risk with something as expensive as a 4 blade folding prop unless it is an exact replacement, same size and design as a previous prop that did work. I have done quite a lot of prop testing and remember one session with a number of competing designs of the same size, same boat and I was surprised by the difference in performance.

So I would just specify a prop to suit (boat/engine/reduction and intended use with any maximum constraints such as diameter, folded length etc) and then the onus is always on the supplier - assuming your data is accurate.
 
Maybe suggest to them that you will try it, see if it works well but reserve the right to send it back at the end of the season if you think a bigger prop would be better?
I assume that a 4 blade prop was in order due to plenty of area being needed, so making it smaller could be the wrong compromise.

Also I would expect a smaller prop to cost less, in general!

In other fields, a 'nominal' size is not always the actual size. If a superior design makes a 19cc widget perform like everyone else's 21cc, the superior widget co might label their 19cc model '21', although being careful not to clam 21 actual cc's.
 
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