Largest prop in diameter That fits the 25?

[notsowindyyet]

Hi

The previous owner of my albin 25 dl has installed a beta 50 engine With the original Prop.
Than he changed it to 17x16 to reduce Rpm and noise.

The engine turns rather slow With 50hp at 2800.
Gear ratio 2:1

I recon slip is quite high.
Can i fit more than 17 in diameter?

Max. Torque Starts at 1300 to 1400.
I Would love to go 6.5knots With this range.
Maybe 4 blade prop?

[RobS]

Is the boat local to you? Why not just measure your current tip clearances.

[notsowindyyet]

I bought it in sweden.
Will haul it in 2 month and want to use the time "thinking"

What is the requiered clearance?
I Heard anything allready... Fingertip... 10%. 20% diameter...

[RobS]

Oh, yes, didn't consider the boat could be in the water!

I'd go with the biggest prop you can fit and pitch down a bit if needed.

Your clearance %'s are in the recommended range. I'd stay on the north side of 10%..

[WillieC]

I'll check mine in the morning. Stand by for NEWS!

[notsowindyyet]

Standing by....

[WillieC]

Lots of clearance between blade tip and hull, almost three inches.

IMG_0223.JPG

OOPS. Not so much room at the skeg...

IMG_0224.JPG

This is a 1973 Mark II hull (pre DeLuxe nomenclature, but it is a DeLuxe model for sure.) Prop is 17" diameter by 13" pitch.

Depending on how far your prop sticks out past the cutlass, your measurement may vary, but at the gentle angle of the propshaft, not by much.

Hope this helps!

[RobS]

Good pics Willie.

Looks like 17" is it.

notsowindyyet,
What makes you think your slip is high?

Funny timing as we just had a discussion on this on the Seaboard forum: https://www.sbmar.com/community/topic/prop-slippage/

[notsowindyyet]

Thanks for the hards facts! Love that!

As I said, I have bought the boat but it resides still with the previous owner in sweden while I am waiting for my trailer to become ready.

He told me that his standard speed is 7 knots and that the engine burns 4l/h at that rate. looking at the beta 50 engine graphs, 4l is the consumption at 2100upm. given a pitch auf 16" (17x16) and a gear ratio of 2:1 this results in a slip of 50%. I find that a lot. In order to reduce noise I would love to reduce slip to make 6,5-7knots with 1300-1400 upm.
my initial thought was of a 18" diameter 4 blade prop.

but that size wont fit for what it looks like.

[dkirsop]

The word "slip" is misleading when it come to prop efficiency and design. It's an archaic term which has come from a time when propellers were thought of as screws pushing the boat through the water. In fact, it is the change in velocity that occurs when water passes through the propeller circle that generates thrust.

The propeller transfers energy to water to provide the acceleration necessary for the water to increase its velocity past the propeller. As it accelerates it pushes back against the blades of the propeller and produces thrust.

A lot of variables dictate how the energy transfer takes place and how efficiently it occurs. Water in front of the propeller has very little velocity and is essentially pushed into the propeller by the weight of the water above, hence depth of submergence is critical. As the boat travels through the water it imparts some forward velocity to the water before it enters the propeller and this lessens the change of velocity across the propeller resulting in reduced thrust. And things like keels, struts and hull shape also impact on the flow of water into the propeller and further create inefficiency in generating thrust.

There are also losses on the high pressure side of the propeller. Water that discharges at an angle to the boat will transfer less forward thrust than water that discharges in line with the propeller shaft. Think of this as water that slips sideways off the blade. This is why a small amount of cupping can significantly increase the efficiency of the propeller.

It has been a long time since I researched prop design but all props "slip" and it seems to me it is somewhere around 50%. Slippage is required in order to achieve the rotational speed necessary to transfer energy and thus generate the thrust. No slippage, no thrust.

The short answer is if you want to consider a prop change consult a professional knowledgable in propeller design and remember the changes will likely be minor when it comes to measuring performance unless the original propeller was way off the curve. Or talk to owners of similar boats with similar shaft rpm and compare notes on speed, rpm and fuel efficiency.

[Dieselram94]

So am I correct in thinking most everyone runs a 17x16?

[WillieC]

Dieselram, you need to specify what engine and gear reduction you are running. There are no blanket prop sizes, what with the multitude of replacement engines in use.

I just finished reading the highly informative missive on "slip", thinking, this guy really knows his stuff, then I checked...oh, Mr. Kirsop! Of course! His last paragraph says it all.

[notsowindyyet]

Good Morning from Europe

1) thanks for this highly interesting slippage post. I simply LOVE it when knowledgeable people elaborate on things. My damaged brain saves it for all time.

2) I hear that one of the best companies for props is "gröver". as they sit in the some city as I do, I will certainly contact them and will come back here and post their opinion.

3) so far I have only heard about 16x12 to 17x15 depending on engine of course.

[saltypaws]

Notsowindyyet,
We have a 4 blade 18 x 15 on Lola Girl. Engine 3JH4 E yanmar. The rpm maxes out at 3000 exactly as specified. Awsome performance especially in head seas.

[notsowindyyet]

Really!!??

You made my day

How Are your Rpm at 6knots and what is top speed?