# Prop size limited question

Discussion in 'Props' started by milo12, Jun 27, 2015.

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### milo12Junior Member

I have used the prop calculator at http://www.vicprop.com/displacement_size.php?action=calculate

Basically tells me I need a 20 inch diameter prop with a 20 inch pitch.

My dilema is I am planning on using a duoprop sterndrive and am limited to a 16 inch prop.

So is this possible or is the 16" too small? What pitch is needed if the prop is 4 inches smaller than recommended size? Is there a way to estimate efficiency?

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### milo12Junior Member

I found that by changing the gear ratio I can get the recommended calculated prop size to be 16 inches. My original calcs were based on a 2.18 ratio but 1.65 results in higher prop speed and smaller diameter.

I'm sure the higher rpm hurts efficiency but I have no idea how to quantify the penalty.

I am confused about prop efficiency.

Does a reduced prop efficiency mean you are wasting fuel at cruise or do you just have a lower top speed?

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### Mr EfficiencySenior Member

It is a slow displacement boat you are powering ?

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### jehardimanSenior Member

Efficiency is power out/power in so it means that you are wasting energy if the efficiency lower. The whole idea of prop selection is to hit the powering curve where the maximum prop efficiency occurs at the desired vessel speed and engine RPM.

FWIW, with hydro propellers, for maximum efficiency you want to have the largest slowest turning wheel you can. Not having a large enough prop available for the sterndrive implies that it was selected poorly or that there is some other fault in the propulsor selection or design. Maybe more info, like the hull type, displacement, speed, HP, etc. will let the forum guide you better.

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### JSLSenior Member

FYI, as the VicProp calculator notes, it does not allow for prop blade loading/area which is a critical factor.
From my experience a 16" duoprop is about equal to a 20" prop with a DAR of about .65

Last edited: Jun 28, 2015
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### baeckmoHydrodynamics

You can not apply a prop calculation method for single props to tandem propellers; neither counter- nor corotating, since the two props are not equally loaded and you don't know the power balance.

If you are talking about a Volvo Penta duoprop unit, the manufacturer has recommendation tables for various engine - speed - boat weight combinations.

The same goes for Mercruiser; they have a propeller selection booklet available to their workshops.

Both manufacturers have a variety of gear ratios for their z-units, you have to select the right one that matches engine rpm, boat speed and propeller pitch (since the diameter is more or less fixed).

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### milo12Junior Member

Do you have a source for the Volvo tables for prop selection? I have searched and the online prop guide pdf is useless. There is mention of software but it looks like only dealers can download the software.

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### baeckmoHydrodynamics

There was a booklet once.......will have a dive into my sediments later today.

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### baeckmoHydrodynamics

Ok it has the Volvo Penta publication nr 7731045-6.

If you tell me what engine you are using, I can scan the pages that come close to your application.

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### milo12Junior Member

I found the old version of the propeller guide and it had good info regarding size and speed vs prop. The problem is it doesn't have any new engines or drives.

I am interested in the D4-260 DP.

Thanks

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### baeckmoHydrodynamics

The "paper software" is now substituted by the sc Volvo MPS, read "Marine Propulsion Software". This is available to the VP dealers and to boatbuilders. Volvo are very restrictive about their propellers; guess they don't want anybody else to play in their backyard. You should get your supplier to check what you need for your specific boat.

Now, FYI, the dp pitch numbers refer to an advance ratio; boat speed/(shaft speed*diameter), with boat speed in m/s, shaft speed in rps and diameter in m.

If memory serves me right, no 4 corresponds to advance ratio of ~1.0, and each step corresponds to a change of 0.1 unit.

This would mean that with the D4-260-dph (Engine rpm=3500 and z=1.85:1) the G4 propellers should work in the 25 knot domain a.s.o.

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