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Old 07-08-2011, 14:32   #1
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Correct Prop Size for Perkins 4236

I need to replace my 3 blade bronze prop(damaged beyond repair) on my steel boat. I had someone dive and take it to a repair shop and if unrepairable obtain an accurate measurement. Unfortunately we are having some communication problems and although I received the message that it was unrepairable have not received the measurements. I tried a few prop calculation but they came up with a 24-26 inch diameter and I seem to remember reading once that perkins had a 21 inch limit on the 4236. ( I could be mistaken). Anyway when in doubt I always revert to the inevitable experts within the forum who will know.

Therefore I shall list below the known data. Before I do I must state that the existing prop was probably oversized. At just over tick over revs in flat water it will do 3.6knts. This at times can be a PITA in difficult manoeuvring situations and has the downside of less efficiency at higher revs. Therefore hopefully those of you using the 4236 will have various options I can assess.

Displacement 38000lbs. 43.3ft LOA. 12.5ft beam
Single hard chine long fin. 6ft draft keel. Water line 36ft

4236 Perkins 73 hp with BW 72c velvet ratio 2.91:1 reduction
2500-2600 Rpm; Left turning

As always I will be grateful for your input.

Best Regards

Alan
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Old 07-08-2011, 15:13   #2
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Re: help in deciding correct prop size for Perkins 4236

Michigan Wheel has a great prop size page. Fill out the form and in 2-3 days one of their techs will email you with the info.

Prop-It-Right Sizing Request // Michigan Wheel Marine
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Old 07-08-2011, 16:04   #3
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First I'd strongly urge you to get the pitch and diameter from the original prop.

When trying to sort out the prop for Boracay I went through every table and chart that I could think of, had the prop repitched then had the diameter reduced and I'm still 200 rpm short of desirable.

For the record I'm running a 2:1 ZF63, John Deere 4045D (85 hp at 2500rpm) with an estimated boat weight of 15 tonne and a LWL of about 33'. The propeller is down to 21 D x 13.293 P.

They wanted to sell me a new prop of about 22 x 12, but the cash don't flow like it used to!

As far as I know the shop didn't even look at the tables. They looked at the original prop and my rpm figures and went from there.

It can take years (and several propellers) to get the prop right on a heavy cruising yacht.
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Old 07-08-2011, 16:26   #4
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Re: Correct Prop Size for Perkins 4236

The shop can't or won't read the measurements off the prop.

Must be too hard to do..
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Old 07-08-2011, 16:30   #5
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Re: Correct Prop Size for Perkins 4236

I just got copies of our design drawings for our boat from the designer yesterday and he specified a Maxprop 3 blade feathering propeller of 22" diameter set to 15" pitch. This is swung by a Perkins 4.236 with a Hurth HBW 360 A2 gearbox at 2.22:1 reduction. This is in a 26,000 lb displacement fin keeler 46' LOA (36' DWL).

Without measuring the prop, I'm guessing that the prop was fitted as specified.

ETA: Just found this web page propellor calculator

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

When our details were entered it came back with a 3 blade prop size of 22.8" diameter and 15.5" pitch ... so the specs were pretty much spot on.
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Old 07-08-2011, 17:01   #6
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Re: Correct Prop Size for Perkins 4236

I have a Perkins 4-236, with BW Velvet Drive 1-1 and Warner vee drive 3-1 reduction in my Cal 46. I have been using a Luke feathering 3 blade 22 X 22 for 29 years. Clean bottom, calm seas at 1550 RPM's = 6.2 knots and 1.1 GPH.

Of course, I have a Bill Lapworth designed hull built by Jensen Marine.

YMMV

Tom
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Old 07-08-2011, 23:15   #7
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Don't you hate it when...

Did I mention 7.9 knots clean and 5 litres/hr at 7 knots?
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Old 16-08-2011, 18:45   #8
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Re: Correct Prop Size for Perkins 4236

Finally obtained the prop statistics. I therefore have a further question. The diameter is 23 inches with a 21 pitch. With this profile the boat when engaged at lowest revs on flat water would reach 3kts, which initially was somewhat of a PITA when manoveuring. It would never reach full WOT. Never the less was economical at cruising speed of 5-6kts. I am aware that a reduction of pitch will probably help lower the tickover speed and raise the revs at the high end.

I would be interested in any feedback as to those who had similar features which were improved/ negatively impacted by pitch changes.

Regards

Alan
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Old 16-08-2011, 19:24   #9
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Re: Correct Prop Size for Perkins 4236

First off, be sure you have at least 15% of the prop diameter between the hull and blade tip. Bow that sounds like a big prop for a 42 ft boat. If I remember right the prop on my Passport 47 with an 85 hp Nanni was 17 or 18...? The 4-236 is a great engine at lower rpm. I would get more rpms (less pitch) but not worry about getting all the way up to max rpms. (there will be some who disagree) If you know your current dia and pitch and rpm... ask a prop shop for recommendations to get you just a little more control at low speed.
My Tanton 44 was similar when I got it... fast at idle! I ended up with a Max prop and could pitch to suit.
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Old 16-08-2011, 19:58   #10
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2.91:1 reduction, 2500-2600 Rpm

I looked to see if I could find the maximum rpm for your engine. I found some data on this page.

If the maximum speed under load for your engine is 2800rpm then your previous propeller must have been pretty close to the mark. Your prop shop should be able to advise if its advisable to reduce pitch and how much.

With a 2.91:1 reduction I'd be guessing not much.
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Old 16-08-2011, 20:36   #11
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Re: Correct Prop Size for Perkins 4236

If you have to... going smaller diameter will help. You can get where you want to be. Just tell the prop shop what you had and where you want to be. Smaller will be less drag and cheaper too!
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Old 16-08-2011, 20:55   #12
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Re: Correct Prop Size for Perkins 4236

Millers Island Propeller, Baltimore........
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Old 13-09-2011, 04:04   #13
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Re: Correct Prop Size for Perkins 4236

hello ~ friend of mine started with a large prop, trimmed it incrementally himself until happy with performance, then gave it to a prop shop to finally balance. (they can change the pitch too, incrementally, if required). its easier reducing one than buying and selling speculatively.
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