What is the right size propeller.

[poundsterling]

i have a sloop 36' LOA 32' W/Line 10' Beam 6' Draft, the dead weight when recently lifted from the water was 12 Ton, with full fuel tanks.

She currently has a 17" diameter 2 1/2"pitch fixed prop, driven by a 55HP
Kubota Diesel, but there is plenty of room too accept a larger prop.
I have taken some external measurements and feel she could comfortably
take a 21" diameter 3 1/2"pitched pitched prop.

My question is, is there any set of tables to which one could refer too to see
if it would be worth while increasing the prop size and what would be the likely benefit from increase speed under power?

Cheers, and smooth sailing.

[Boracay]

Before you start thinking about new propellers you need to work out how good your present one is.

Q1. What engine speed can you reach with your current propeller (gearbox engaged) with the boat moving at it's maximum speed? Is this above or below the rated max. RPM for your engine.

Q2. What is the maximum speed in knots that you can reach under calm conditions?

Q3. The pitch that you mention looks wrong. Can you check?

Q4. What is your manufacturers specified maximum RPM? What is your gearbox ratio?

Q5. How clean is the propeller/boat below the waterline?

Q6. What diameter exhaust do you have? How high does it lift from the muffler to it's highest point?

[Latitude9.5]

You can also email michigan wheel with all your info and they will size your prop for you for free.

Michigan Wheel Corporation

They got back to me within 24 hours.

[David M]

There are lots of tables and formulas that can give you a close idea, but ultimately, if a healthy engine cannot reach it maximum rated RPM with a clean hull then the pitch/diameter/number of blades or any combination thereof, is too great.

There are some that subscribe to using a larger propeller saying it is more efficient. BS, diesels are most efficient at higher RPM's. If the diesel cannot reach its maximum rated horsepower then you are effectively lugging the engine and it will not be long before it is prematurely worn out. Also, not one manufacturer is going to tell you that an oversized propeller is a good thing. It is better to be a little undersized than oversized, if anything.

A Google search can bring up formulas for getting you a good idea about the approximate size that will work.

[cooper]

Hi Poundsterling have sent you a PM....

[çemberci]

Originally Posted by poundsterling

i have a sloop 36' LOA 32' W/Line 10' Beam 6' Draft, the dead weight when recently lifted from the water was 12 Ton, with full fuel tanks.

She currently has a 17" diameter 2 1/2"pitch fixed prop, driven by a 55HP
Kubota Diesel, but there is plenty of room too accept a larger prop.
I have taken some external measurements and feel she could comfortably
take a 21" diameter 3 1/2"pitched pitched prop.

My question is, is there any set of tables to which one could refer too to see
if it would be worth while increasing the prop size and what would be the likely benefit from increase speed under power?

Cheers, and smooth sailing.

Sir
Please give me the details of your vesells and Engine
Lwl = waterline
Bwl = beam waterline
Draft
* Engine Kubota
Label
engine horsepower
engine rpm
Transmission Ratio
oktay çemberci

[poundsterling]

Thanks to all who took time to reply to my question re the correct size of propeller, sorry for the time it has taken me to respond, but i have been having difficulty logging in, again thanks for the interest

[David M]

Pound,
Hows it coming along?

David

[SkiprJohn]

www.cruisersforum.com/forums/f114/proper-prop-size-5972.html

You might try thise one.

Regards,

JohnL

[poundsterling]

Originally Posted by David M

Pound,
Hows it coming along?

David

Good day David,

Well the boat is coming along slowly but surely, I am currently preparing to cut out a
section of the hull which has been affect by battery acid spill and replace same.
But having the boat out of the water and looking at the propeller in relation to the size of the hull it seemed a bit small for the job, any from the info sent by fellow members
and suggested sources of information it appears that the current prop should do the job quite adequately

Thanks for your interest.

poundsterling