How big is a 21" propeller ?

[Jassira]

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.

I spoke to a number of manufacturers and they all specified 21/22" props of various flavours.... I eventually settled on this prop after they (the manufacture) specified a best match for my boat as either a 21x14 or a 22x13 with a slight preference to the 21x14 as it would likely perform better astern. At no point did I claim to have any knowledge of prop sizing and ordered exactly what they recommended.

 

[Tranona]

I spoke to a number of manufacturers and they all specified 21/22" props of various flavours.... I eventually settled on this prop after they (the manufacture) specified a best match for my boat as either a 21x14 or a 22x13 with a slight preference to the 21x14 as it would likely perform better astern. At no point did I claim to have any knowledge of prop sizing and ordered exactly what they recommended.

OK then the onus is back on them - not supplying what they recommended - but they now claim it is still suitable. This could of course be just an excuse for their mistake. The loss of an inch in diameter is quite significant, but you may not be able to quantify the difference in performance without having a benchmark to measure against.

Suggest you insist on 4 new blades to make up the size that they recommended. I don't buy their shrinkage explanation as it is over 1/2" length of each blade and if the shrinkage is even a loss of blade area. If they are competent they would know what the shrinkage is and allow for it in the pattern.

 

[JumbleDuck]

Suggest you insist on 4 new blades to make up the size that they recommended. I don't buy their shrinkage explanation as it is over 1/2" length of each blade and if the shrinkage is even a loss of blade area.

All the copper alloys have roughly the same linear coefficient of thermal expansion, about 20 microstrain per ^{oC. Melting point could be as high as 1900oC, which would give a total shrinkage of 0.04, or 0.8" across 21". Add a bit of contraction at solidification and 1/2" per blade isn't implausible ... but it seems most unlikely to me that they would specify propellers by pattern size and not by finished product size.}

 

[Tranona]

Yes - not the pattern that drives the boat, but the finished prop blade. No discussion of the pitch, but an extra 1" pitch to 15" might give the same performance. Not so easy to measure at home but maybe worth discussing with the manufacturer as it is now clear the prop size was specified by them, not the buyer.