Resizing prop

ErikBenson

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Help !. . .I have bought the perfect prop for my ex RN FMD. . .except it is 13" and not 12.5". can you give me any ideas on how to reduce it. I have all the necessary tools I reckon.
I have a photo
 

Tranona

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Would not bother as it is unlikely to make much difference unless there are severe space restrictions. Try it and see if it works.
 

Andydent2000

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Speak to Clements Marine

They will be able to tell you the perfect prop size and pitch for your boat based on engine size, gearbox reduction, displacement etc.

I just had my props refurb'd through them. The original size and pitch was too big. Even half an inch can make a difference as the actress said to the bishop.

They were very reasonable I thought. I paid £150 per prop to have them repitched and refurb'd.

Even if you decide not to change anything it is still worth the conversation.
 

Andydent2000

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I really don't think that would be a good idea. It seemed to me when I went through the process at Clements that the process of altering and balancing a prop is a real engineering fete. Everything was measured by computer to ensure the blades were not only in balance but also in line. If they are not you will simply shake your boat to bits.

Not something for the DIY enthusiast I'm afraid.
 

Tranona

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Assume this is a displacement boat - the small difference in diameter is highly unlikely to make any difference in performance. Props are not normally made in 1/2" increment and the "size" is adjusted by the pitch. If you think it is too large then flatten off the pitch a bit - much easier for a prop specialist to do.

If you use a size calculator like Propcalc they will give you diameter and pitch to 2 decimal places, but in reality you chose a regular size closest to the thoeretical best. As I said earlier the only constraint is if you are tight on tip clearance (15% diameter) when you may want to go down in diameter and up in pitch.
 

DAKA

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I agree with your first post, 1/2 inch bigger prop isnt going to make much difference as long as enough clearance just try it.

As others have said the pitch is what alters the gears, 12.5 to 13 would be like changing up from 5th to 5.5 in a car and not even making it to 6th gear.

The 13 inch size is going to cause a fraction of extra resistance as the prop turns, bit like driving a car with an aerial fitted , you wouldnt need to change down because of it.



A prop manufacturer can change the pitch easily.

taking 1/2m inch off the size however is a long and potentially expensive operation if done properly as all the blades are tapered from the boss to the tip and you dont want wide chunky blades as they will create a lot more resistance than extra 1/2 inch bigger but thinner blade edges.
 

ErikBenson

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Thankyou for your very useful tips. I just did not want to damage the underside of the boat from the prop being too close. I think I saw once that someone attached a strip of copper sheet to the hull to protect from the pressure from the prop water flow. Is that just a bodge ?
 

Tranona

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Check your clearance. 15% diameter is the rule of thumb, but not definitive. Unlikely ever to do any damage to the hull. Symptoms of insufficient clearance are noise and turbulence. If you really are so tight then 13" is too big so go down to 12" with a coarser pitch. The sizing programmes usually ask for max prop diameter based on the 15% rule to avoid recommending a prop that is too big for the aperture.
 

ErikBenson

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Thanks again. . .I will do a trial fitting to get the actual gap. . .on paper it looks ALMOST possible.. . I suppose it is only a quarter of an inch difference anyway.
 

ErikBenson

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Thanks for that. . .who is/are Clements please?
What do you know? When I came to trial fit the prop - -even tho' it is stamped , and sold as, a 1 1/8" one. . it turns out to be 1'16'th too small !
The company, in the US are baffled, but will pay for me to have the taper taken out by 1/16th.
What a palaver !
 

Tranona

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Clements is a UK prop manufacturer with a strong following among power boaters as well as supplying some of the major builders.

I am surprised you went to the US for a prop - there are plenty of mail order suppliers in the UK who machine exactly to order. (I know you are in France, but much easier dealing with a UK supplier).

Just to check you are looking for around 2" clearance to the tip of the prop ideally, but probably 1 1/2" would be OK.
 

Tranona

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In my case there was none of that - but the engine came 600 rpm short of max revs.

Another vote for Clements, who sorted it

That may well be the case on your boat, but my understanding is that the OPs boat is a low speed displacement boat and low clearance would not have that dramatic effect.
 

ErikBenson

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Hi guys. . no. . .my boat is a 1953 wooden high speed Captain's shuttle. . sometimes known as a "skimming dish" I'll try to include a pic for you.
I went to the US because the prop was half the price of anything I could find even near the Admiralty specs for the FMD.
Admiralty tests had them running at 19 knots. . . .with Coventry 2 stroke diesel with rootes blower. . .50hp at 2000rpm.There are only 4 or so left out of 954 built.
 
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