I hacked 1.5 inches off my prop!!

S

seafox67

Well-Known Member
Joined
14 Apr 2017
Messages
281
Location
Greenock
Visit site
I hadn't planned to do this but getting hold of a new prop fell through and I wanted to get back into the water for this nice weather.

The 2 blade prop was oversized (18 x 12). I took a gamble last year when I had a new D1-20 installed. However, after the installation I couldn't get much past 2000 RPM (although I can still get hull speed).

From calculations, I should really have a (17 x 11) 2 blade prop... So I took another gamble at the weekend and using a jigsaw and dremel, removed 2 cm from each blade (Giving a prop with around 16.5 x 12).
Today, the boat went back into the water and I went off for a sea test. Now, I was expecting two things to happen;
1: The boat would not move at all!
2: The boat would reach about 2600RPM or above

However, reducing the prop made no difference at all! Exactly as it was before when it was a 18" diameter prop.

If I put the engine into neutral, I can easily get up to the 3200 max rpm.

Can anyone think of anything else that might be causing this low RPM? Is the prop maybe a 'red herring' and it is something engine related!


Cheers
Paul
 
Just a guess but the prop pitch will wash out towards the tip to reduce induced drag, cutting this part doesn't make the prop the same as a 17 inch prop, it makes a 18" prop with more drag, this could explain your lack of improvement but full marks for giving it a go.
 
A D1 20 cannot swing a prop that big. Do the job properly and speak to Lake Engineering who will calculate the correct size for you. My D1 30 only has a 16*11 and achieves 3100. Your are more likely to need a 16*10 fixed 2 blade or a 15*11 3 blade.

Guesswork in choosing a prop is an expansive activity.

What boat is it?
 
Tranona said:
A D1 20 cannot swing a prop that big. Do the job properly and speak to Lake Engineering who will calculate the correct size for you. My D1 30 only has a 16*11 and achieves 3100. Your are more likely to need a 16*10 fixed 2 blade or a 15*11 3 blade.

Guesswork in choosing a prop is an expansive activity.

What boat is it?
Click to expand...

The 15x11 3 blade is what I was hoping to get. I didn't know what the taper was before I lifted out. I now know and it's non standard so I didn't want to wait the 6+ weeks for delivery.

I'll lift out again after the season and do it properly but I thought in the meantime this was worth a try... 'who dares wins' :)

I am a bit concerned now though that when I get the 'real' prop, it also does not make any improvement! They say 1" diameter off the prop gives you another 500 rpm.

I've got a bilge keel.... so, I could cut a bit more off with my cordless jigsaw ;)
 
seafox67 said:
I've got a bilge keel.... so, I could cut a bit more off with my cordless jigsaw ;)
Click to expand...

Admire your ingenuity;). As a matter of interest, what is vibration like?

If OK then you might want to stop the cutting for fear of damaging the cutlass bearing or something else!
 
seafox67 said:
The 15x11 3 blade is what I was hoping to get. I didn't know what the taper was before I lifted out. I now know and it's non standard so I didn't want to wait the 6+ weeks for delivery.

I'll lift out again after the season and do it properly but I thought in the meantime this was worth a try... 'who dares wins' :)

I am a bit concerned now though that when I get the 'real' prop, it also does not make any improvement! They say 1" diameter off the prop gives you another 500 rpm.

I've got a bilge keel.... so, I could cut a bit more off with my cordless jigsaw ;)
Click to expand...

No. 1" pitch or diameter is 250-300 rpm dependent on shaft speed (ie reduction ratio).

Bilge keels don't make any difference, it is boat displacement, LWL and shaft speed that are important. How did you calculate the size, or did your supplier do it for you?
 
dom said:
Admire your ingenuity;). As a matter of interest, what is vibration like?

If OK then you might want to stop the cutting for fear of damaging the cutlass bearing or something else!
Click to expand...

The vibration is no worse than before... With the bilge keel I could beach her and cut a bit more off but I guess at some point it may end up with no thrust :)

The pitch did flatten out a bit at the end of the blade which I guess could be why it didn't make any difference!

When the engine was changed, I kept the exhaust and fuel lines including filters so I was wondering if this could possibly lower horse power. The engine does start and sounds perfect though.

newprop.jpg
 
Tranona said:
A D1 20 cannot swing a prop that big. Do the job properly and speak to Lake Engineering who will calculate the correct size for you. My D1 30 only has a 16*11 and achieves 3100. Your are more likely to need a 16*10 fixed 2 blade or a 15*11 3 blade.

Guesswork in choosing a prop is an expansive activity.

What boat is it?
Click to expand...

I've used 2 calculators and one professional... although the professional company was trying to sell me a 17" diameter 3 blade! which doesn't even sound close to me;

I think the taper is 1:20 (measurements below) which doesn't appear to be 'off the shelf' stock... needs the machine shop.

My boat is a Westerly Tempest 31

Left Handed rotation
Shaft Diameter: 25mm

Overall Length:
30.58' / 9.32m

Waterline Length:
24.50' / 7.47m

Beam:
10.83' / 3.30m

Displacement:
9019 lbs./ 4091 kgs. (Although the crane guy said the boat was 4800kg when lifted out)

Maximum Hull Speed:
6.5 knots (Cruise speed: 5 knots)



Engine Details
--------------
Volvo Penta D1-20
19 Horse Power

Gear ratio:
2.63:1

Rated RPM:
3200

One Shaft with one bearing


Taper:
------
Small end: 22mm
Large end: 25mm
Taper Length: 60mm

Key Length: 54mm
Key Width: 8mm
Key depth on shaft: 2mm
Key itself is 6mm x 10mm x 55mm
 
Last edited:
When my Volvo 2001 wouldn't rev fully in gear last year, it turned out that the exhaust elbow was almost completely blocked by internal corrosion. It did reach max revs in neutral.
 
I believe there are companies who can re-pitch a prop. In other words twist the blades a bit. Since you have reduced the diameter this might now be harder to do but as a temporary fix it might be worth trying to find a company that can do it. I suspect they heat the prop to near red heat to do the job so I doubt its an amateur fix.
 
Norman_E said:
I believe there are companies who can re-pitch a prop. In other words twist the blades a bit. Since you have reduced the diameter this might now be harder to do but as a temporary fix it might be worth trying to find a company that can do it. I suspect they heat the prop to near red heat to do the job so I doubt its an amateur fix.
Click to expand...

Definitely get the prop re-pitched, you've nothing to lose. I did this several years ago and gained about 500rpm, a company in Norfolk did it but can't remember the name...
 
The 1:20 taper was the original, common with Volvo shafts which were used at the time, but uncommon elsewhere!

The problem you have is that the engine is marginally underpowered for the boat and the reduction to high, leading to a low shaft speed. A 16*13 3 blade or 17*13 2 blade would be a reasonable compromise. However that will not reach hull speed, which is 7 knots, only 6.5. You would need a 25hp to achieve that. A Beta 25 would have been a better choice, but appreciate you can't now change.

If funds allow you might be better investing in a feathering propeller such as a Featherstream which has adjustable pitch. worth speaking to Darglow Engineering to get good advice on what prop will get the best out of your engine.
 
Seafox- absolutely same prob I have. And heavy on fuel and dirty exhaust. My prop is 12" and want to knock inch off with grinder.
 
vic008 said:
Seafox- absolutely same prob I have. And heavy on fuel and dirty exhaust. My prop is 12" and want to knock inch off with grinder.
Click to expand...

If I had the choice again... I don't think I would have done it! The decision was made out of frustration and my little voice (in my head) convincing me it WOULD work ;)

I felt the jigsaw gave more precision than the angle grinder, with a dremel to clean it up. I took 1.5 inches off but the I think the biggest gamble was getting the prop balanced. I have a feeling my cutlass bearing will be destroyed by the end of the season :(
 
Last edited:
Tranona said:
The 1:20 taper was the original, common with Volvo shafts which were used at the time, but uncommon elsewhere!

The problem you have is that the engine is marginally underpowered for the boat and the reduction to high, leading to a low shaft speed. A 16*13 3 blade or 17*13 2 blade would be a reasonable compromise. However that will not reach hull speed, which is 7 knots, only 6.5. You would need a 25hp to achieve that. A Beta 25 would have been a better choice, but appreciate you can't now change.

If funds allow you might be better investing in a feathering propeller such as a Featherstream which has adjustable pitch. worth speaking to Darglow Engineering to get good advice on what prop will get the best out of your engine.
Click to expand...

Thanks for your advice... it is very sound (especially the hull speeds). If there are no boat surprises this year, I may have enough funds winter time for a feathering prop :)
 
longjohnsilver said:
Fuel starvation? I’ve had something similar happen to me in the past that was caused by a partially blocked fuel filter.
Click to expand...

That's very interesting.

The previous owner installed davits on the stern and I have wondered if it makes the boat a bit heavy on the backside!

If the exhaust is too low below the waterline, would this cause a loss in horse power?
 
Last edited:
The Volvo D120 is a perkins based engine and should rev up to 3600 out of gear are you sure the exhaust elbow isnt blocked?which is common with these engines.
 
You must log in or register to reply here.

Share this page

Top