Prop sizing

Status
Not open for further replies.
Ken13559

Ken13559

.
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
Curiously, prop maker asked for data, do you send original design data like Displacement, engine output etc... or do you send current 10years old couch-potato boat data ?
I was quoted a 2 blade Flexofold 15"Dx12"P by the maker but I think it looks over-pitch for my 10year old over-weight boat.
I estimate current displacement to be 9600 lbs (about 1000 lbs more than when new). My engine is Yanmar 2GM20F 16HP@3400rpm.
Anyone has a 2 blade Flexofold on similar setup, please let me know your prop Dia & Pitch. Thanks.
 
Ross

Ross

.
Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Engine RPM is meaningless you need shaft RPM which means you need your reduction gear ratio.
 
S

Steve Dion

.
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
This is why I always suggest that owners get the specs for their boat and give them to the prop shop/mfg. You cannot compare a folding/feathering prop to a fixed and you may not be able to compare two fixed props from different mfg.

Go with the mfg. specs and you will have someone to bitch at if it does not work as advertised.

This is the reply that I got from Flex-o-fold for my boat.

"With a Yanmar 3JH2E (38 HP by 3600 RPM, reduction 1:2,61) we would recommend a 3-blade 17x13-3R propeller for your Hunter Vision."
 
S

Steve Dion

.
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Ross:

I believe that they get the shaft RPM from the gear reduction that Ken may have not included in his post!
 
Don12364

Don12364

.
Dec 2, 2003
1,637
Hunter 376 Warsash, England --
The Formula

To calculate a suitable prop rather than to just make aq blind guess, the formula requires to know the following data.
If it were for an aeroplane propeller you would expect similar information so why should a boat be different?

Required details:-
  1. Type of hull. e.g. powerboat, sailboat, planing;
  2. Maximum prop diameter which could be fitted. e.g. leaving 10%-15% of diameter as clearance to hull to reduce noise and prop walk.
  3. Laden displacement of hull.
  4. Waterline length.
  5. Number of engines.
  6. Horsepower of each,
  7. Engine RPM max,
  8. Gearbox ratio.
These simple numbers are fed into the formula and it gives the optimum diameter and pitch so that the power of the engine(s) is utilised most efficiently and the boat achieves the maximum speed available from the engine's horsepower.
If you are very keen, from this it might then be necessary to tweak the pitch an inch either way.

To do this for yourself see Castle Marine's web site in Wales.
 
Ken13559

Ken13559

.
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
I believe Flexofold has all the data they need of my engine and boat since they're able to recommend a 15x12R 2blade or a 15x11R 3blade. But I suspect that's alittle over-pitch so I would like to know what other Flexofold users has to see if the recommended 15x12R is in ballpark. In anycase I'll be sending them full Hull and engine spec for just in case. My 2GM20F, ratio 2.62. Oh another question, do you send current hull displacement data or As Design data to the prop maker?
 
Prime Time

Prime Time

.
Sep 25, 2008
544
Bristol 43.3 Perth Amboy
A bit off topic but something to think about

One of the things I considered when getting a new prop for Blue Note was the ability to change the pitch. That was one of my considerations in choosing a maxprop. It turned out that I was underpropped with the recommended configuration. I was able to adjust it myself for the following season. But it is a featherer not a folder.
 
C

Cayennita

.
Nov 28, 2009
495
Catalina 30 St. Croix
I gave Maxprop my boat info. A stock Idylle 15.5 made by Beneteau and I also asked for the variable pitch propeller. The manual was very easy to understand and I started with the same pitch as the fixed prop. Almost but not enough. I dove, changed the pitch and tried again. One more adjustment and that was it. 7.5 knots at 2100 RPM even with all the extra gear and water, fuel, etc. Fully loaded I added 3200 lbs to the original 33,000. Ver satisfied with the choice.
 
Capt Fernando Martinez

Capt Fernando Martinez

.
Nov 1, 2011
9
McGregor 22 Venture Key Largo
Give them the boat/engine information and let them figure it out,, don't suggest the prop size to them. If the prop doesn't load the engine correctly then you can take it to them for correction.
 
Ken13559

Ken13559

.
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
Capt Fernando, yes that's what I plan to do. Let them have full spec and suggest they relook at their calculations and revert with their decision. So I'll send them both AS DESIGN as well as currently over-weight Displacement.
By the way, any idea if the Design Displacement is "dry" (no fuel, no water) and include engine ?
 
Ken13559

Ken13559

.
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
Can I assume that a 15Dx10P Martec fixed 2-blade prop doesn't have same thrust or engine load as a 15Dx10P Flexofold 2-blade? Both having same dia and pitch.
 
Rick D

Rick D

.
Jun 14, 2008
7,186
Hunter Legend 40.5 Shoreline Marina Long Beach CA
Actually, no...

Ken13559 said:
Can I assume that a 15Dx10P Martec fixed 2-blade prop doesn't have same thrust or engine load as a 15Dx10P Flexofold 2-blade? Both having same dia and pitch.
Click to expand
...please understand that this is from probably flawed memory, but as I recall a prop test a decade or more ago, the Martec folding prop had the same thrust as a fixed.
 
Maine Sail

Maine Sail

.
Feb 6, 1998
11,709
Canadian Sailcraft 36T Casco Bay, ME
Ken13559 said:
Can I assume that a 15Dx10P Martec fixed 2-blade prop doesn't have same thrust or engine load as a 15Dx10P Flexofold 2-blade? Both having same dia and pitch.
Click to expand
That is probably correct. The FOF is a more efficient prop and consistently tests best in forward thrust for two blade folders. Theoretically they should be the same but props vary in efficiency despite having the same diameter and pitch. For example our Campbell Sailor is a 16X9 and our old Michigan was a 16X12, both three blade props. Both props allow the engine to turn 3000 RPM but the CS is significantly more efficient.
 
tkanzler

tkanzler

.
Sep 5, 2007
689
MacGregor 26X Rochester
Ken13559 said:
So I'll send them both AS DESIGN as well as currently over-weight Displacement.
Click to expand
Out of curiosity, for a displacement hull that doesn't change wetted area or length very much when laden, how much difference does the typical increase in displacement of a loaded cruising hull vs as-designed make in real speed and engine loading? Just curious.
 
Ken13559

Ken13559

.
Oct 29, 2005
2,362
Hunter Marine 326 303 Singapore
I don't know, just wondering if it makes any difference between laden and unladen displacement boat. If the hull is laden and lowers to water by 1" due to more weight added, then its drag goes up due to more wetted surface. The added mass means needing more power to push through water. However, maybe the waterline length gets alittle longer. :confused:
 
Don12364

Don12364

.
Dec 2, 2003
1,637
Hunter 376 Warsash, England --
Sure as hell makes a big difference when under sail power i.e. when racing.
But then only a 1% speed difference means losing a 1 hour race by 120 yards and that hurts!
 
Status
Not open for further replies.

Top Bottom