Boat Propeller choice

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ErikBenson

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Hello again. I have replaced the missing Coventry Climax KF4 engine in my 1953 16ft Fast Motor Dinghy, with a very similar Gray Marine of the same year. The prop was missing on my boat when I got it, though I found out it was 12.5" Dia and 15.5 pitch. on a 1 1/8" shaft . This is all the space I have available under the boat.
I have now discovered that the 2:1 reduction box on the Gray Marine would prefer to have an 18" prop, according to Gray Marine.
What to do ? remove the reduction gear ?, or try for a 4 blade prop in the 12-13" range ?
Both engines 50/55hp. Cov Climax diesel -2000rpm, Gray Marine petrol -3000rpm
ANY and ALL suggestions gratefully received. . .Erik
 
If you work it out by Crouches method you could do nicely with a 17X14 and keep your 2:1 gearbox. Provided you have 15% tip clearance.

Diameter is always better than pitch for transmitting power.

However you are way overpowered, the Coventry KF4 was used on the 25 fast Navy launches, I thought the 16ft boats had the little two cylinder Coventry Diesels.
 
[ QUOTE ]
you could do nicely with a 17X14

[/ QUOTE ]
17" + 15% = 19.55"

So how will that fit into:-


[ QUOTE ]
12.5" Dia and 15.5 pitch. on a 1 1/8" shaft . This is all the space I have available under the boat

[/ QUOTE ]
 
Misread you original post about what you had was all you could accommodate as well as what you had.

12.5 limits you 19.5 pitch if three blades or 18 if you go for 4.
 
Thankyou very much for that. Could you give me your opinion on the pros and cons of retaining or removing the reduction gear please. This is all terribly new for me, but I wonder if 'original' is best, or the reduction gear and 4 blade prop might make a better option
 
It is the shaft speed that is relevant. With a 2:1 box and 3000rpm your shaft speed is 1500. If your old engine had a 1:1 box then the shaft speed was 2000.

I suggest you provide all your boat and engine data to a propeller supplier and ask for a recommendation. They will need to know how much clearance you have to determine the maximum diameter you can swing. There are programmes available on the web to do it yourself Google Propeller calculators - but as you need a new prop why not get the experts to do it for you - and then you have some comeback if it does not work.

BTW you can't actually remove the reduction gear, you still need a gearbox to give you forward and reverse. You can only replace with a different ratio.
 
Only the Navy could dream of teaming up a 50 hp engine to a 12" prop. A 15 hp engine will develop all the horsepower that a 12" prop can transmit, and you'll get a bit more space inside the boat.
Peter.
 
Sorry Peterduck, but I am trying to keep it as near the real thing as possible. The Admiralty specs stated a diesel , or petrol engine to be supplied, so that's ok. They did not specify a prop in the build programme, but just said that the builders should communicate with the Admiralty.
Unfortunately for me the boat had been stripped of all mechanics when decommissioned in 1960, added to which the Irish owner removed the prop and cut off the rudder blade in his abortive attempt to fit a huge outboard.
They built 954 of these FMD and , we believe only half a dozen remain, if that. So it is a little piece of naval history. They were known as - 'Skimming Dish'. . .and would do up to 22 knots. I don't need that speed, but, it is a semi planing hull, and there is only room underneath for a 12.5 or 13" prop. That's why the 2:1 reduction gear on my replacement engine might be a nuisance. I wonder if a 4 blade prop could solve the problem.
Don't blame the Admiralty. . it's my fault, having this 2:1 addition.
 
Sorry, didn't mean to be unhelpful. Yes, I think that a four-blader might just be the way to go, and with all that extra oomph you could have it quite high in pitch, probably 12" x 12". Can you find any of the racing hydroplane crowd? They might have some good suggestions.
Peter.
 
No Peter. . .you were not, in any way, being 'unhelpful' ! It's me who is the ignorant one in town. I have contacted a Gray Marine expert in the USA who says that I might be able to remove the gearing and go direct. . . .so, I will look into that next. I'd post you a pic of the boat, if I knew how. I am always on the look out for anyone with archive information. . .there's not a lot about
Cheers. . . .Erik
 
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