Propellor measurment

aquaholic

aquaholic

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So I swapped out the 29hp for a 43hp using an online prop calculater it tells me I should have a 3 blade 17x11, my question is I have a 16x12 currently fitted for the 29hp, will buying a different prop with 1" difference in diameter and pitch make that much difference?
 
Yes. Your current prop is too small and will not absorb the extra power. However if I remember rightly your new engine is far too big for your boat, so you can't use the extra power anyway. So you may well find it works OK as you will only ever need to use about 2/3 of the power available from your engine - that is the same amount of power you had from your old engine.
 
Tranona said:
Yes. Your current prop is too small and will not absorb the extra power. However if I remember rightly your new engine is far too big for your boat, so you can't use the extra power anyway. So you may well find it works OK as you will only ever need to use about 2/3 of the power available from your engine - that is the same amount of power you had from your old engine.
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Thanks for your thoughts, although not sure the replacement engine is 'far too big'......its the same size as the old one :)
 
aquaholic said:
So I swapped out the 29hp for a 43hp using an online prop calculater it tells me I should have a 3 blade 17x11, my question is I have a 16x12 currently fitted for the 29hp, will buying a different prop with 1" difference in diameter and pitch make that much difference?
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I suggest you ask a propeller shop whether increasing the pitch of your current propeller, which is cheaper than buying a new prop, is a good compromise. Bigger diameter is of course always better for facing head seas but stronger pitch is better for flat water and as I said it will be a compromise.

Edit: provided you are using the same shaft!
 
Unless you have a different reduction ratio I would say that your calculations are wrong
In normal circumstances taking an inch of dia and adding an inch of pitch does not mak a lot of difference

Regardless if you turn the same prop at the same revs it will produce the same thrust it can not absorb any more
 
scottie said:
Unless you have a different reduction ratio I would say that your calculations are wrong
In normal circumstances taking an inch of dia and adding an inch of pitch does not mak a lot of difference

Regardless if you turn the same prop at the same revs it will produce the same thrust it can not absorb any more
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I believe the rule of thumb is + 2.5" pitch = approx +1" diameter.

Since the power requirements of the hull are the same, there is no need to increase the diameter, but the pitch can be usefully increased to give a lower cruising rpm and hence less noise.
As Tranona implies, the higher engine power cannot be used to go faster (assuming hull speed was being reached with the previous engine), but it can be used to maintain a higher speed in adverse conditions eg head wind, fouled hull etc (should you need or wish to)

So as you have already changed the engine, you may as well make the most of it by getting the pitch changed on your existing prop.
 
aquaholic said:
Thanks for your thoughts, although not sure the replacement engine is 'far too big'......its the same size as the old one :)
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It produces nearly 50% more power - and is turbocharged. Far too much power for the boat. The original engine was the right size and perhaps you should have replaced it with the same.
 
Assuming 40 hp @ 3000rpm &2-1 reduction the existing prop should be fine ,you do not have to use all the power but you will easily reach hull speed.The original set up could have been a bit overproped.

Jim
 
The best thing to do would be to get a yacht design book like Larsson and Elliason, and make a decent estimate of the performance with the two propellors.
Then buy a folding or feathering prop and get decent sailing performance....
 
ianj99 said:
It depends on the forum! However the well known Vicprop website states:

The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.

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

The 1:1 rule seems to be used when there is a space limitation for the recommended prop diameter and for rounding up/down..
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I'm confused. Where did the 1" = 2.5" rule come from? :confused:

Richard
 
aquaholic said:
I used the vic prop website which gave me this result

https://www.dropbox.com/s/b1k6z3xfy...cement and semi-displacement hulls 1.pdf?dl=0
Click to expand...

You have made the mistake of trying to use all of the available power and max rpm which produces an unattainable speed.

As you can see from the results, 29bhp (what you had) is enough to reach the hull speed of 6.8kts.

If you now derate the new engine to 30bhp at 2500rpm, for 6.8kts, you get a prop size of 19x13.
Compromising on using just 16bhp at 1800 for a cruising speed of 6kts the result is also 19 x 13.
This gives a good reserve of bhp and rpm for adverse conditions.

Therefore the very least you need is to have the prop repitched by 2". This is a compromise but sticking with the same diameter means no additional drag when sailing. (also shaft diameter might be a consideration if going much larger diameter)
 
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Why don't you recalculate the prop based on (say) 1500 to 1750 engine rpm and the corresponding HP at that rpm. which will be a lot quieter. You might find however that the prop will be too big to fit in the aperture.

I had the same issue fitting a 1.6 litre Ford escort engine (cos I had one) into my 21 ft MS which was overpowered but I used a 1.5:1 reduction box and an over square prop. I run the engine at 1500 rpm with prop at 1000 and get 6.5 knots.
 
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