Prop Size for A25 Deluxe, Yanmar 40 HP, 2.36 trans.

[Burton]

This is a follow-up to a previous question trying to find the problem.

Short version: Looking for data as to what prop size works best for others with same hull/engine/trans combination.

DATA: OurA25 Deluxe has a Yanmar 3JH3E, 40 HP @ 3,800 RPM. Current prop is 17-16 RH. Trans ratio is 2.36. Engine will run up to max RPM no load, underway only 2850, boat at 9.5 knots. Injectors extremely dirty when serviced according to technician. Prop shaft is metric, I recall it being 30 mm. (?).

Prop calculators seem to all have "displacement hull" or "planing hull" categories, while the A25 "Deluxe" is semi-displacement. Desert Albin and others suspect it is over-propped. Distributor (Mack Boring) tech suspects over-propping and said I should get prop data from others with same boat/power combination. I contacted them to check if this situation might damage the engine. Answer: yes. He explained that the load on a boat is like your truck loaded with concrete trying to go up a hill-- the power output of the engine needs to be where it is not "lugged", and ours not able to reach full RPM is a problem that needs to be addressed "sooner than later". (Not that one runs constantly at full RPM-- it is just making sure power/load combination if correct.)

Boat is lightly loaded at present, hull has only very light slime on it, no barnacles, no water in freshwater tank. Prop has only light scale on it. So boat is not overloaded.

I would appreciate hearing from anyone with a similar boat/engine/trans, or others that have wisdom on what is best. Thanks!

EDIT:

9/2/20-- UPDATE

I took our 17/16 prop to Nautilaus Marine here to have it re-pitched and learned that to get the kind of RPM increase we needed it would take more changes than what could be done with our prop. (The rough rule is 400 RPM per inch in diameter, 100 RPM per inch in pitch.) They have a LOT of props there on consignment, and let me put a deposit on two. By running them, we would be able to figure what we needed. We tried the 16/14 and got 3450 RPM and about 10.8 knots GPS speed-- that's pretty close to specs for the engine, so I purchased it. (We actually hit 12 kts when wind and tide were helping us in the river!) This engine should get 3650 at wide open throttle... (max RPM of 3800). When we haul out, we will have Nautilus Marine repitch it to get up the correct RPM. Again, its not that we want to always go at 3650, but that RPM at WOT is what is best for the engine as to it handling the load it is pulling (or pushing).


To use a GoPro underwater for live video:

As our boat is at a float dock, we first tried to get the prop numbers using a GoPro with WiFi in an underwater housing with an iPhone, extended on a pole. This would have worked, but there was enough slime on the prop from sitting that we could not read it clearly. Many have a GoPro, and thus one does not need to buy a separate camera for underwater inspection.

This works well-- As the WiFi signal is lost in water in very short distances, one must make an antenna. I made one with an old piece of coax cable. Simply peel back the shielding so the exposed center core is 2.4 inches, or 6mm, on each end. (This makes a half-wave vertical antenna for the 2.4 Ghz WiFi of the GoPro). Ours is about 12' long, but I think you can go longer and still get a decent video signal. I sealed the coax ends exposed with contact cement and heatshrink tubing. Use electrical tape or similar to secure exposed antenna end to the button side of the underwater case for the GoPro. Then, hold the other exposed end up against your iPhone. (I don't have the Android.) This allows the Wifi signal from the GoPro to transmit to the phone from underwater. This took two of us to coordinate, of course. We took some decent screen shots, but those numbers had not been cleaned in a long time....Anyway, a shorty wetsuit/snorkeling gear here in Maine is okay in summer! That, with a bit of brushing and we had our data. This antenna length is for the GoPro. If there is some other frequency you must match, this is easy to figure for resonance. Simply convert your Ghz to Mghz, divide by 300 and this gives your full wavelength antenna length. (I reduced this to half, for convenience, for a half-wave antenna.) Addendum: length in cm.

[ssrig]

Talking to a few folk at the Albin Rendezvous with a 40hp Beta and they say they stayed with the standard 17 x 14 size prop. Said the rated power on the newer engines seem closer to the 36hp Volvo penta’s and they are happy with it.

[nebulatech]

Mine has a 17" prop. I'm on vacation so I can't work out the pitch until I return. Mine had the original MD17C engine.

EDIT: my boat is a 1979 Deluxe

[DesertAlbin736]

I imagine 17 inch diameter prop is about as big as one could go. Our 16 1/2 x 12 prop barely has an inch clearance with the skeg pipe. A 16 pitch sounds steep. Since our engine is a 24 HP Yanmar with 2.6:1 gear reduction a 12 pitch is more suitable for us, certainly no more than 14.

DSCN3631crop.jpg

[nebulatech]

20200731_185732.jpg

Interestingly, it looks like my prop is 16x19, 1.25"

This is an A25 Deluxe with an MD17C...

[hetek]

I never knew Sweden used Imperial units of measure. If I had to guess, it looks like you've had a new coupler, drive shaft and prop once upon a time.

Or at least a new prop and machined shaft.

My shaft was confirmed to be 30mm.

Wait... That didn't sound right...

[nebulatech]

My shaft was confirmed to be 30mm.

Wait... That didn't sound right...

Don't brag!

I'm not sure the shaft was changed. 30mm and 1.25" are almost the same. Who knows what the PO may have done. My calipers aren't good enough to measure the difference. Another excuse to acquire more tools!

[WillieC]

Oh, there is a difference. Not least is the taper. A cheap digital unit will be good enough for measuring your PROPELLER shaft.

[hetek]

Believe me, I've just been bit by the Imperial vs Metric debate.

I bought a 1" stainless tube for my V berth table post. It... just... didn't fit.

Evidently, there is a difference between 1"= 2.54 cm and 25 mm.

Yes, the extra 0.04 mm made a difference.

I'm returning the 1" tube and ordering a 25 mm tube and will bet it fits.

[nebulatech]

I would not try to fit a 1.25" prop on a 30mm shaft, but given some of the other stuff done to the boat, I wouldn't be surprised. Plus, isn't a 16 X 19 (or maybe 18) a bit odd, even for the deluxe? I'm thinking the PO lost the prop and bought something used that was "close enough." If they didn't bother to get the right pitch prop, I can't imagine them spending the money to replace the shaft, though they may have had it modified at a machine shop. Then again, maybe they had some lofty performance aspirations and picked that prop. A third, highly possible, option is I don't know enough about prop pitch to make assumptions.

RE: Hetek's 1" tube, could you not just sand down the last 2-3 inches of the post? Unlike a prop shaft, the table post is pretty static....

[WillieC]

An inch and a quarter shaft very likely, almost certainly has an SAE taper. What are they? 1:12? The metric 30 mm, again, very likely, almost certainly has a 1:10 taper. Mixing and matching will produce the most obscene wobble in your hands that might suggest, even to the uninitiated, that something is amiss. Also note that the taper LENGTH is critical as well. That is the hub length on the prop. You don't get a whole lot of choice there.

This is not to say that somebody got in there and morphodited the two systems together. I contemplated having a machine shop re-taper the shaft to fit standard off the shelf SAE, reel 'Merkan props. 'Cepting' in my size and rotation, there is no such thing as "off the shelf". Fortunately, I didn't have enough prop shaft length to accomplish such crimes against humanity. I ordered a real metric prop, with a real metric taper and let my prop guy do all the measurements. Which he did. In inches and thousandths! I struggled watching him, but thankfully a number is a number and accuracy, talent, here are those mad skills again at work. I trusted, and verified his work. Nailed it. (Metric is MUCH easier.)

I had to wait several weeks for the shiny new unit, so plan ahead.

OTOH, were I doing the full meal deal, I would start with a new engine, new mounts, aqua-velva drive, maybe shaft saver coupling, new shaft seal, new SAE shaft made from unobtainium that will fit an off the shelf from Stephan in Sweden new cutlass bearing, heck throw in a line-cutter and notch the rudder for a bullet bolt-on-end-of-shaft anode. Hydraulic steering and state of the art auto-pilot as well...add three feet and buy a 28TE.

The leg bone truly is connected to the knee bone.