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Prop size v engine size


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 So I have an engine

i already have a 4 bladed bronze prop on our 56ft barge.

Has anyone got a 90hp or big engine ?

so how will the 90hp run with a 4 bladed prop please?

 

col

 

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Col, that question can not be answered unless we know the diameter and pitch of your four bladed prop plus the maximum engine revs. Then someone might look up the prop tables/curves and give you an answer. It will be capable of allowing the engine to turn faster than  a 50hp engine but how fast the boat will go and the maximum engine speed we have no way of assessing.

 

Personally I would not worry about it being a four bladed prop on a displacement boat. It will just have a larger blade area so might be a bit more efficient than a three bladed one.

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7 minutes ago, bigcol said:

 So I have an engine

i already have a 4 bladed bronze prop on our 56ft barge.

Has anyone got a 90hp or big engine ?

so how will the 90hp run with a 4 bladed prop please?

 

col

 

I think a bit more information is needed (my egg whisk has 4 'blades' but it is only about 2" diameter)

 

What are the dimensions ?

(ie diameter & pitch - could be something like 21 x 19, normally stamped on the prop of hub)

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4 blades are quite normal on new build barges it is difficult to get sufficient blade area on a 3 blade prop if the diameter is limited, there are 3 parameters diameter and pitch are important but so too - for obvious reasons if you think about it - is blade area.

What power and maxmimum rpm was the previous engine, was the prop ok with that, did it reach max rpm easilly, just or not at all.

Edited by Phoenix_V
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2 hours ago, bigcol said:

 So I have an engine

i already have a 4 bladed bronze prop on our 56ft barge.

Has anyone got a 90hp or big engine ?

so how will the 90hp run with a 4 bladed prop please?

 

col

 

 

There is no point in looking for problems that may not exist. 

 

The blade will have been selected to suit the likely type of engine, and you have bought exactly the right engine. So bung it in, connect it up and see how well it goes. I bet you find it is just fine. And given the state you say your finances are in, the last thing you want to be doing is fine adjustments to blade size/pitch. Wait until you have money to burn. 

 

 

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3 hours ago, bigcol said:

 So I have an engine

i already have a 4 bladed bronze prop on our 56ft barge.

Has anyone got a 90hp or big engine ?

so how will the 90hp run with a 4 bladed prop please?

 

col

 

What gearbox ratio have you got ? 1:1, 2:1, 3:1 ?

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2 minutes ago, Mike the Boilerman said:

 

Why would that matter, given the OP doesn’t know the pitch or diameter of the prop?

 

 

Yes but I thought that asking more than one question at a time may be pushing it a bit.

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6 minutes ago, BWM said:

I have no idea why, but when i ordered the new prop for our boat they strongly advised against a four bladed one for canal use.

 

That will be because they already had a three blade prop in stock I bet.

 

Four blades are rather unnecessary on a small NB though, and the downside is the much increased turbine effect.

 

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1 hour ago, BWM said:

I have no idea why, but when i ordered the new prop for our boat they strongly advised against a four bladed one for canal use.

Four bladed props can cause vibration, due to two blades at a time possibly being masked by the hull. It can be worse, with a two bladed prop on a long keel sailing boat.

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21 hours ago, Mike the Boilerman said:

 

That will be because they already had a three blade prop in stock I bet.

 

Four blades are rather unnecessary on a small NB though, and the downside is the much increased turbine effect.

 

 I think it was given in good faith, and doubt they have too many 27" × 22" in stock! 

What is the turbine effect, is it a form of cavitation? 

19 hours ago, Iain_S said:

Four bladed props can cause vibration, due to two blades at a time possibly being masked by the hull. It can be worse, with a two bladed prop on a long keel sailing boat.

I didn't realise that, i assumed that it may be more prone to fouling.

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23 hours ago, Mike the Boilerman said:

 

There is no point in looking for problems that may not exist. 

 

The blade will have been selected to suit the likely type of engine, and you have bought exactly the right engine. So bung it in, connect it up and see how well it goes. I bet you find it is just fine. And given the state you say your finances are in, the last thing you want to be doing is fine adjustments to blade size/pitch. Wait until you have money to burn. 

 

 

 

Unless the prop can be removed with the boat in the water then it's probably going to be cheaper to check and if necessary get the prop dimensions adjusted before bunging it in. It cost me £150 to get my prop re-pitched at Noris in Isleworth. 

 

It's at least worth checking with an online calculator to get some basic info before fitting everything.

 

https://www.vicprop.com/displacement_size.php

 

Edited by blackrose
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25 minutes ago, blackrose said:

 

Unless the prop can be removed with the boat in the water then it's probably going to be cheaper to check and if necessary get the prop dimensions adjusted before bunging it in. It cost me £150 to get my prop re-pitched at Noris in Isleworth. 

 

It's at least worth checking with an online calculator to get some basic info before fitting everything.

 

https://www.vicprop.com/displacement_size.php

 

 

I don't see why it would cost any more to remove the prop and get it re-pitched/whatever after fitting the engine, than before.

 

If the engine is just fitted and tried, it might well be fine. I don't understand your strong objection to fitting it and trying, given the OP has declared himself (in another thread) so skint he can barely afford £60 for a new alternator. 

 

P.S. you realise the boat is already in the water? 

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36 minutes ago, Mike the Boilerman said:

 

I don't see why it would cost any more to remove the prop and get it re-pitched/whatever after fitting the engine, than before.

 

If the engine is just fitted and tried, it might well be fine. I don't understand your strong objection to fitting it and trying, given the OP has declared himself (in another thread) so skint he can barely afford £60 for a new alternator. 

 

P.S. you realise the boat is already in the water? 

Mike I totally agree with every posy you put on this thread. And yes that’s me worrying over nothing.

engine isn’t even in yet, but regard the last post beta wants £340 for the alternator!!

thats why I asking about the 12/24 converter lol

but anyways

your right, the moment know problems, so I’ve got to stop looking for them

i will find out size of prop later on

 

thank you mike

 

col

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Col what was the old engine that was removed before you bought it? Secondly take off the 12 volt alternator and go to a lorry breaker with it they will no doubt have something the same size. Thirdly sell the old 12 volt alternator for a fat profit as its huge and desirable

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2 hours ago, peterboat said:

Col what was the old engine that was removed before you bought it? Secondly take off the 12 volt alternator and go to a lorry breaker with it they will no doubt have something the same size. Thirdly sell the old 12 volt alternator for a fat profit as its huge and desirable

Hi Peter

 

As far as I know it was a dawoo doosan L034 with a turbo 120 hp

the yard that built the boat built and still do build life boats.

 

col

F9BFDFB6-0B2D-4D30-81D3-74CEFE151C32.jpeg

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1 minute ago, bigcol said:

Hi Peter

 

As far as I know it was a dawoo doosan L034 with a turbo 120 hp

the yard that built the boat built and still do build life boats.

 

col

F9BFDFB6-0B2D-4D30-81D3-74CEFE151C32.jpeg

You might have to take a bit of pitch off but then again it might be right so Col I would just bolt it in and see what happens

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