Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 24-10-2011, 14:56   #1
Registered User
 
rotorman's Avatar

Join Date: Sep 2011
Location: South East Asia
Boat: Amel Euros 41
Posts: 64
Max RPM and Required Power for a Prop ?

Hi,

I am thinking about repowering my sailboat. I would like to use my existing prop, which is a 19x13 C7 LH 5.

How can I figure out the max rpm? (Cavitation limit)? And how can I figure out the required power on the shaft?

Cheers
Geza
__________________

__________________
rotorman is online now   Reply With Quote
Old 24-10-2011, 15:32   #2
Registered User

Join Date: Dec 2008
Location: Oriental, NC
Boat: Mainship Pilot 34
Posts: 1,429
Re: Max RPM and required power for a prop?

Buy a copy of Dave Gerr's Propeller Handbook. It is all in there.

David
__________________

__________________
djmarchand is offline   Reply With Quote
Old 24-10-2011, 15:41   #3
Registered User
 
rotorman's Avatar

Join Date: Sep 2011
Location: South East Asia
Boat: Amel Euros 41
Posts: 64
Re: Max RPM and required power for a prop?

I hope someone has read the book and can give the concrete answer :-)
__________________
rotorman is online now   Reply With Quote
Old 24-10-2011, 18:29   #4
Registered User

Join Date: Dec 2008
Location: Oriental, NC
Boat: Mainship Pilot 34
Posts: 1,429
Re: Max RPM and Required Power for a Prop ?

You want us to do your thinking for you and get opinions with no way of knowing who is right and who is wrong. Well ok, but you need to tell us the weight and length of your boat, the transmission ratio and the number of blades on your prop.

But you won't get an answer from me. Buy the book, read it and do the calculations yourself. You can believe those.

David
__________________
djmarchand is offline   Reply With Quote
Old 24-10-2011, 19:59   #5
Registered User
 
rotorman's Avatar

Join Date: Sep 2011
Location: South East Asia
Boat: Amel Euros 41
Posts: 64
Re: Max RPM and Required Power for a Prop ?

Hi David,

I am not sure why you are using this voice. I don't think I made any reason for it. If I did I apologize. However commenting that you won't help seems pointless. And I apologize for this comment, too :-).

If I could do the calculations myself I would do it. Since I have no idea thats why I am asking for help at this forum.

I got a chart from a Volvo Penta dealer which gives the cavitation limit (max rpm) for various propeller sizes (up to 18") for Volvo Penta folding propellers.

I assumed that a similar chart is available somewhere for fixed 3 blade props.

I thought that the max rpm is not affected by boat weight. Also I am looking for shart rpm so you would not need transmission ratio. And this is a fixed 3 blade prop.

Maybe what I am looking for does not exists and the only way to figure this out is to read the book.

The weight of the boat is 13T, waterline is 34 feet.
__________________
rotorman is online now   Reply With Quote
Old 24-10-2011, 20:30   #6
Registered User
 
Ziggy's Avatar

Join Date: Apr 2008
Location: U.S., Northeast
Boat: Contessa 32
Posts: 1,421
Images: 2
Re: Max RPM and Required Power for a Prop ?

Rotorman,

When I was repowering my boat I got David Gerr's book and calculated the prop size and pitch for my boat and chosen engine (Yanmar 3Y20). Just to double check, I also asked Michigan Wheel folks to calculate it for me and got the same result. To triple check, I downloaded the "propcalc" spreadsheet from the Alberg 30 Association web site and I still got the same result (within roundoff error.)

So I also recommend Gerr's book, but if it is too technical for you, try propcalc, available for download from here. Double check your numbers with the prop manufacturer.
__________________
... He knows the chart is not the sea.
-- Philip Booth, Chart 1203
Ziggy is offline   Reply With Quote
Old 24-10-2011, 20:39   #7
CF Adviser

Cruisers Forum Supporter

Join Date: Aug 2009
Boat: Custom Van De Stadt 47 Samoa
Posts: 3,743
Re: Max RPM and Required Power for a Prop ?

Quote:
Originally Posted by rotorman View Post
Hi,

I am thinking about repowering my sailboat. I would like to use my existing prop, which is a 19x13 C7 LH 5.

How can I figure out the max rpm? (Cavitation limit)? And how can I figure out the required power on the shaft?

Cheers
Geza
You can find lots of prop calculators on line, like this one

You are asking the question backwards from normal (most start with the engine and want a prop) so you will have to iterate a bit.

But looks to me like 1100 shaft rpm and 53.6 prop hp if you want to be able to attain hull speed (it suggests that 39 prop hp would give you 7kts top speed).
__________________
estarzinger is offline   Reply With Quote
Old 24-10-2011, 20:59   #8
Registered User
 
Ziggy's Avatar

Join Date: Apr 2008
Location: U.S., Northeast
Boat: Contessa 32
Posts: 1,421
Images: 2
Re: Max RPM and Required Power for a Prop ?

Quote:
Originally Posted by rotorman View Post
Hi David,

I am not sure why you are using this voice. I don't think I made any reason for it. If I did I apologize. However commenting that you won't help seems pointless. And I apologize for this comment, too :-).

If I could do the calculations myself I would do it. Since I have no idea thats why I am asking for help at this forum.

I got a chart from a Volvo Penta dealer which gives the cavitation limit (max rpm) for various propeller sizes (up to 18") for Volvo Penta folding propellers.

I assumed that a similar chart is available somewhere for fixed 3 blade props.

I thought that the max rpm is not affected by boat weight. Also I am looking for shart rpm so you would not need transmission ratio. And this is a fixed 3 blade prop.

Maybe what I am looking for does not exists and the only way to figure this out is to read the book.

The weight of the boat is 13T, waterline is 34 feet.
Assuming a 40 hp engine and 7 knots max speed, you will need 1163 shaft rpm with a 19x13 3 bladed prop.

How are you planning to achieve this shaft rpm? Marine gears come in fixed ratios and engines develop max horsepower at their max rpm. Starting the design process from a given prop seems backwards.
__________________
... He knows the chart is not the sea.
-- Philip Booth, Chart 1203
Ziggy is offline   Reply With Quote
Old 24-10-2011, 21:59   #9
Senior Cruiser
 
delmarrey's Avatar

Cruisers Forum Supporter

Join Date: Mar 2003
Location: Now in Blaine, WA
Boat: Modified Choate 40
Posts: 10,702
Images: 122
Re: Max RPM and Required Power for a Prop ?

LH prop?

You would be better off re-powering with what you want and then get the right prop.
__________________
Faithful are the Wounds of a Friend, but the Kisses of the Enemy are Deceitful! ........
A nation of sheep breeds a government of wolves!

Unprepared boaters, end up as floatsum!.......
delmarrey is offline   Reply With Quote
Old 24-10-2011, 23:14   #10
Registered User
 
rotorman's Avatar

Join Date: Sep 2011
Location: South East Asia
Boat: Amel Euros 41
Posts: 64
Re: Max RPM and Required Power for a Prop ?

Thanks for all the advice.

Yes it is a LH prop.

I am hoping to be able to reuse the existing prop but yeah I might have to replace the prop, too. I am a little bit concerned replacing it since it is an enclosed prop so space is limited.

Yanmar recommended a 55 HP engine with a 2.36 ratio gearbox. My old engine at 2000 rpm (and with a ratio of 2.91) had a shaft rpm 687 and the boat was doing 5.5.

With the Yanmar I would get the same shaft rpm at around 1500 engine rpm.
This is quite a different.

They mentioned in their quote that the prop might need to be changed. From these numbers it seems like it will need to be changed.
__________________
rotorman is online now   Reply With Quote
Old 25-10-2011, 03:55   #11
Senior Cruiser

Cruisers Forum Supporter

Join Date: Feb 2006
Location: Highlands, NJ 07732
Boat: Laurent Giles Salar 40
Posts: 260
Re: Max RPM and Required Power for a Prop ?

Be aware that Yanmar recommends against running your engine less than 85% full throttle for extended time. And that your warranty will be voided if your engine can't achieve full RPMs at WOT. Yanmar wants their engines to operate at higher RPMs. On my new Yanmar, I could achieve hull speed at 2200 rpm and would have liked to cruise at 5-6 kts at 2000 rpm, but I had to resize my prop to be able to achieve WOT RPMs Yanmar wanted. Now I will have to cruise at 2600 rpm. Since reduced noise and vibration were part of my goals for the re-power project, I'm somewhat disappointed overall in my Yanmar project.
__________________
chris07732 is offline   Reply With Quote
Old 25-10-2011, 16:42   #12
Registered User
 
Ziggy's Avatar

Join Date: Apr 2008
Location: U.S., Northeast
Boat: Contessa 32
Posts: 1,421
Images: 2
Re: Max RPM and Required Power for a Prop ?

Quote:
Originally Posted by rotorman View Post
Thanks for all the advice.

Yes it is a LH prop.

I am hoping to be able to reuse the existing prop but yeah I might have to replace the prop, too. I am a little bit concerned replacing it since it is an enclosed prop so space is limited.

Yanmar recommended a 55 HP engine with a 2.36 ratio gearbox. My old engine at 2000 rpm (and with a ratio of 2.91) had a shaft rpm 687 and the boat was doing 5.5.

With the Yanmar I would get the same shaft rpm at around 1500 engine rpm.
This is quite a different.

They mentioned in their quote that the prop might need to be changed. From these numbers it seems like it will need to be changed.
Yanmar 4JH5E (53 max HP) with 2.36 marine gear and a 19x11 3-bladed prop will move you at 7.8 knots max (assuming 26,000 lbs displacement and 34 ft waterline). You will need a RH prop though.

At 2400 engine rpm (80% max, 1000 shaft rpm) you will cruise at ~6 knots.

Seems like a pretty good fit.
__________________
... He knows the chart is not the sea.
-- Philip Booth, Chart 1203
Ziggy is offline   Reply With Quote
Old 27-10-2011, 16:05   #13
Registered User
 
rotorman's Avatar

Join Date: Sep 2011
Location: South East Asia
Boat: Amel Euros 41
Posts: 64
Re: Max RPM and Required Power for a Prop ?

Thanks Ziggi. The same this has been confirmed with a propeller expert. I also confirmed the result using PropCalc. The only thing I have to confirm now is the weight of the boat. On the original tech spec it is defined as 7T. I saw also a declaration that gross T is 13T. The boat was measured to 15T once with full cruising gear (I am not sure if I belevie that). The boat will come out of the water soon so I will know the weight.
It seems like the original setup (engine max rpm 3000, ratio 2.91) was not optimal.
__________________
rotorman is online now   Reply With Quote
Old 27-10-2011, 18:53   #14
Senior Cruiser

Cruisers Forum Supporter

Join Date: Feb 2006
Location: Highlands, NJ 07732
Boat: Laurent Giles Salar 40
Posts: 260
Re: Max RPM and Required Power for a Prop ?

Not sure what gears your old engine is using, but if it's a BW velvet drive, its a good box and you can reverse rotation depending on your requirements for the existing prop.
__________________
chris07732 is offline   Reply With Quote
Old 28-10-2011, 20:26   #15
Registered User
 
Ziggy's Avatar

Join Date: Apr 2008
Location: U.S., Northeast
Boat: Contessa 32
Posts: 1,421
Images: 2
Re: Max RPM and Required Power for a Prop ?

Quote:
Originally Posted by rotorman View Post
Thanks Ziggi. The same this has been confirmed with a propeller expert. I also confirmed the result using PropCalc. The only thing I have to confirm now is the weight of the boat. On the original tech spec it is defined as 7T. I saw also a declaration that gross T is 13T. The boat was measured to 15T once with full cruising gear (I am not sure if I belevie that). The boat will come out of the water soon so I will know the weight.
It seems like the original setup (engine max rpm 3000, ratio 2.91) was not optimal.
7 tons displacement for a 41 foot offshore cruiser seems very light. I would think 13 tons is closer to the real displacement. Also, 2 tons of cruising gear, if you include water, fuel, sails, food, other stuff, and crew, seems quite reasonable.
__________________

__________________
... He knows the chart is not the sea.
-- Philip Booth, Chart 1203
Ziggy is offline   Reply With Quote
Reply

Tags
rpm

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
The Right Way to Run a Diesel off-the-grid Engines and Propulsion Systems 80 09-12-2012 19:06



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 04:45.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.