Largest prop in diameter That fits the 25?

[Desertboater]

The Betamarine UK web site has the following information for a Beta 50 HP engine:

19" x 12" with a note saying "A general guidance for propeller size in inches, based upon 3 blade RH rotation and reduction ratio of 2:1. For accurate individual advice please provide us with your vessels current full specifications."

One of the larger propeller manufacturers in the US (Michigan Wheel) has a propeller sizing calculator - entering a 50 HP engine with 2:1 reduction, 6.5 knot desired speed results in a recommended propeller size for a 4 bladed prop of 19" x 11" with a 40% efficiency. Interestingly enough the 3 bladed prop recommendation was 20" x 11".

Michigan Wheel also provides the following guidance for propeller clearance:

"What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?

The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip."

I would suggest calling or emailing Betamarine UK (+44 (0)1452 723 492) as they have been very helpful in the past - I called while in Berlin visiting my daughter to ask specific questions about the extra raw water cooling connections on our Betamarine engine.

Does the Beta 50 have the Technodrive Twin Disc 60 transmission?

[WillieC]

I still think the prop ought to clear the skeg. Interesting recommendations to consider before engine swapping.

[notsowindyyet]

Its the technodrive 260

[Dieselram94]

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.

I’m running a Perkins 4.108 (50) hp with a hurth 250 (2-1) gear.

[WillieC]

OK, saltypaws has an 18" prop on what I assume is their A25. Correct me if I am wrong. I would sure like to see the clearance to the skeg with that prop. Obviously it works, and I suppose an 18" diameter would fit the WillieC, but I would want to make sure to file the bottom paint off the top of the skeg before launch...maybe a few whacks with a hammer for good measure.

The closer the prop is to the cutlass, the more clearance...simple geometry as the skeg is not parallel to the propshaft. Mine has the super duper built-in prop removal nut on the shaft as well as the collar type zinc...both ahead of the prop. This moves everything about as far south as you can go without hitting the rudder with the threads. I also have the R&D coupling at the north end of the shaft that takes up about the same thickness as the zinc. The thinking is, supposin' I ever muck up the coupler, I could remove the zinc and the shattered coupler and slide everything forward and mosey on home. I don't even look forward to THAT event. But it makes some sense.

Conversely, since the propshaft sticks out so close to the rudder, I can't fit the more preferable bullet zinc, nor can I remove the prop without dropping the rudder. As I have noted elsewhere, the kneebone is connected to the leg bone...

To further complicate matters, I recently acquired a spare shaft from another AOGer (thank you SKOL!) that I need to check out for straightness, etc. (Mine is out of spec at the prop, making for the A25 rhythm section variations we have come to enjoy.) Decisions! Assuming it is absolutely perfect, or at least fixable, needing only to be shortened... Do I shorten it up enough to eliminate the collar zinc and forget about the limp home mode? Unfortunately, this new spare shaft does not have the super duper deluxe built-in prop removal nut...DANG!

All this fuss for not much. Nothing a cool thou plus can't take care of.

To the earlier point before I got carried away, you very well might get away with an 18" prop. Measure twice, order once. And I would fully lean on the prop guys with their fancy computer programs. I got two free guesses that agreed and didn't cost me a dime. Give them everything straight out of the Per Brohall original owner's manual regarding hull measurements, displacement, etc. AND all the specs from the new engine/transmission installed. Everything. They'll come up with the correct prop.

[saltypaws]

notsowindyet RPM at 6 knots 2200 depending on wind and current. we cruise at 2400 giving 6.6 tp 7 kts

[DesertAlbin736]

notsowindyet RPM at 6 knots 2200 depending on wind and current. we cruise at 2400 giving 6.6 t0 7 kt[/quote]

That's pretty close to what we get with a 16 X 12 prop. I don't usually push it beyond 2,400 RPM & cruise at about 6.5 knots & 2,350 RPM. Our Yanmar 3GM30F is rated for 24 HP @ 3400 RPM continuous, 27 HP short term @ 3600 RPM (red line). That's engine RPM at the flywheel, not RPM at the prop, which with 2.61:1 gear ratio would be about 900 RPM.

[saltypaws]

Not so windy yet, 6 kts. low wind and sea 2100 rpm. The speed after that is totally a factor of load. During the last 4 years we drive to Florida from Canada with a half ton truck with enclosed back loaded to the roof. I suspect 700-800 pounds. On the day that we arrive we clean the interior and move in. Next day add food and liquid refreshments and about 10 gallons of drinking water.
Add to that 2 adults and 2, 20 pound dogs. With that load we cruise at 2400 rpm doing 6.6 to 6.8. That's all you should expect while engine is bigger you will still be pushing a lot of water, making a lot of noise and burning a lot of fuel.