Overpowering question

[Bob Noodat]

If you have a full, or near full, displacement hull boat and re-power it with an engine of higher horsepower than stock, are there not some potential negative consequences to this? Or at least might it not lead to unnecessary expense and lower fuel economy?

If Albin put a 21hp engine in the early boats, presumably this was adequate power to attain hull speed plus having a bit in reserve for headwinds, counter currents etc. If the boat is then repowered with, say, twice that, will the potential consequences not be 1. running the engine at possibly lower than recommended rpm, or 2. the risk of cavitation at higher rpm, not to mention wasting fuel?

[Dieselram94]

The way I see it is, it’s better to have too much power than not enough. Lots of factors determine rpms such as prop size and pitch, gear box reduction, how clean the bottom is, ect.
The Perkins 4.108 I have works well in my boat, top speed is about 10.5-11 knots...

[Bob Noodat]

Well Dieselram, that raises an interesting point, of course. Exactly how are these marine engines being rated? The 4.108 curve peaks at 49hp, but the graph labels this as "Max speed for approved high speed pleasure craft", at 4000rpm. "Max continuous rated speed" on the other hand is 3000rpm and about 35hp.

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Looking at the only Albin AD21 graph I could find, it looks like you get about 22hp at 2400rpm.

20200321_083941.jpg

But what rpm do people run their AD21s at, and what do you run your Perkins at? At 2200rpm the Albin would seem to be generating 20hp and the Perkins about 30hp continuous. Not such a huge difference in that case.

[hetek]

Bob - This might address some of your power questions. I found my boatdiesel.com calculations!

For an Albin A25 deLuxe (with the modified bottom)...

The original Volvo 36 hp weighed 640 lbs with gear. My Vetus 25 hp is a Mitsubishi-based engine and weighs only 270 lbs, with gear. Even though it is 11 hp less, it is a full 369 lbs lighter.

With Volvo = 10.8 knots max speed and 9.7 knots cruise.
With Vetus = 10.0 knots max speed and 9.0 knots cruise.

Less than one knot difference.

I also calculated the horsepower needed to push an A25 to hull speed:

Only 7 hp!

[DesertAlbin736]

FWIW, our top speed (early model full displacement) is 6.5 kts @ 2,400 RPM with 24 HP, 2.61:1 gear reduction, 16.5" x 12 prop. On the Albineers of BC 'Albin25 history' web page it notes Albin determined early on that the AD21 engine was 'slightly underpowered' & when they went to the DeLuxe hull they upped HP to 36 with the VP MD3B & later MD17C.

[Bob Noodat]

Jon.... very interesting figures. 7hp seems amazingly little, but then there are blue water sailboats that have as little as 12hp pushing a 35 to 40' hull up to hull speed, so it must be right. I suppose that even the early 25s were not true displacement, if my understanding is correct, since does not a true displacement hull have to be tapered at both ends, as opposed to the transom stern on the Albins? The later A25s were even less so, since they had the hard chine and scalloping near the stern to give a small planing effect. Again, correct me if I have misunderstood. DA, is that not the primary reason Albin upped the horsepower? The slightly-planing hull needed more, although I suspect that when they realised that their big market was North America, they probably felt some pressure to satisfy those power hungry customers!

Those numbers seem to prove that your engine choice is the right one Jon, since you definitely have plenty of extra oomph even with just the 25hp, but, the jury must surely be out on whether you can get that theoretical 10 knots until you actually find out if you can reach it. I would think that 8 or 9 knots would likely be wasting fuel, let alone 10 knots plus.

If my understanding is correct, you take the water line length in feet, take the square root of that, and your hull speed is a trifle (or since that is metric it would possible be a truffle) over that number in knots. So if we say 25', square root is 5, hull speed should be 5.5knots or so. It always seems you can get a little more though. Our O'Day 20 actually seems to have a hull speed of 5.5 knots, although it really should be about 5.

I still have not had an answer to my query though. Yes, more power will give more speed, but what are the effects of that extra (unnecessary?) power on engines, rpm, cavitation etc.?

[dkirsop]

I have an early model Albin with the AD21 engine. My typical average cruising speed is 6 knots. My typical engine RPM is 1600 and my reduction gearing is 2:1. This was after I tweaked the prop reducing the diameter 1/2" to 16 and having the blades cupped. Maximum engine RPM is 2100 so there is adequate reserve power. The AD21 is a constant duty rated engine and is quite happy to run at full throttle all day if this is what you want but fuel consumption and noise would outweigh the minor increase in speed.

The Deluxe model Albin is a much faster boat and does easily out run mine. A friend who recently repowered his Deluxe model with a 35HP Beta benefited from the smoother, quieter, and lighter engine. (He also has a smoother, quieter, lighter wallet.) I expect he might achieve the 10 knots at maximum RPM. Be aware that Albin was very optimistic when it came to published performance data in their sales literature so performance gains are not always what one expects.

It would be misleading to compare performance between the early full displacement hull and the deluxe hull in terms of power requirements for performance gain.

As the owner of an earlier model I would expect a newer engine to provide a smoother, quieter ride. The AD21 is a heavy unit weighing about 525 lb. with the transmission attached and has the typical vibration associated with a 2 cylinder engine. Having said that I find the unit to be very reliable, simple and sturdy. I do not foresee a re-power in my future. I do not see any advantage to re-powering beyond 25HP with this hull model. Attached below are the power and fuel consumption curves for my engine/boat combination.

albin 25 power curves.pdf

[Bob Noodat]

dkirsop, nothing seems to come in when I try to download that pdf.

Not many, if any, owners of the de luxe boats are doubling their horsepower though, but it seems that it is not uncommon to do so when repowering the early boats.

As for weight being lighter in modern engines. Nice. But those heavy old engines had heavier flywheels and rotating components meaning that the greater rotatory momentum kept them going, with less stress, at lower rpm, essentially forever. My experience of this is in tractors, but then and now marine and agricultural diesels tend to come from the same assembly lines.

[ssrig]

My A25 has a 55hp Volvo and I consider that overpowered. I believe the previous owner who did the re power thought he would have a speed boat if he stuck a big engine in there but it did not happen.

As others have said more power is more expensive to buy and keep it going, I’m just glad it wasn’t me that paid the bill for it! As for fuel economy I went with a pretty large prop to reduce my rpm’s and as a result keeps the noise down too.

I did open her up to see what it would do after the re prop, I was in a harbour so I did not prolong the high power for very long but I neared ten knots, maybe 9.5 but can’t remember exactly, my rpms due to the larger prop didn’t reach the 3000 rpm red line so the engine felt bogged down a bit. This was all before loading the boat up with all our crap for the summer and as a result I would say 10 knots is not a realistic number for this boat, wasting fuel and working the engine hard.

Sooo... if I was re powering an A25 I don’t think I would go much more than 30 hp, unless someone gave me one?

Also of note would be the size of an engine that you squeeze into there, I have a hard time getting around my engine, it fills the area up so much that getting to the impeller is difficult, a thin stick is all that gets under in under it.
Weight, we overload these boats so much, a nice light engine seems a good idea.

6.3 knots at just under 1800 rpm is what I cruise at with occasional bursts higher, no install manual with me but i think it was around 10-15 hp.

Benefits: I can goose it when docking and I get quick results in forward or reverse.
Powering up I can raise the bow up quickly like it might consider going onto a plane, fun feeling for a second until it doesn’t get onto a plane.

[DesertAlbin736]

The Deluxe model Albin is a much faster boat and does easily out run mine. A friend who recently repowered his Deluxe model with a 35HP Beta benefited from the smoother, quieter, and lighter engine. (He also has a smoother, quieter, lighter wallet.)

That wouldn't be Mike with Pagurus, would it?

It would be misleading to compare performance between the early full displacement hull and the deluxe hull in terms of power requirements for performance gain.

Exactly, apples vs oranges. Or oranges vs tangerines. Or bananas vs plantains. Or mangos vs pomegranites. (going a little stir crazy here).

roger rabbit.jpg

Our last sailboat, a Catalina 25 was roughly the same size/displacement as A25 except the addition of a 1,750 lb lead wing keel (8,300 lbs on a truck scale counting the +/- 1,600 lb steel trailer) our 'auxlliary' power was an 8 HP outboard with "power" prop and we could push close to 6 knots under power.

DSCN0634 Avalon to Dana Pt transit 7-14.jpg

DSCN0642 Guest dock at Oceanside Yacht Club 7-16.jpg

As the owner of an earlier model I would expect a newer engine to provide a smoother, quieter ride. The AD21 is a heavy unit weighing about 525 lb. with the transmission attached and has the typical vibration associated with a 2 cylinder engine. Having said that I find the unit to be very reliable, simple and sturdy. I do not foresee a re-power in my future. I do not see any advantage to re-powering beyond 25HP with this hull model.

If it ain't baroque, don't fix it.

[dkirsop]

Not sure why power curves didn't publish but here is a screen shot of it.

Performance Curves.jpg

[Bob Noodat]

Thank you dkirsop. So the red lines join hull speed of roughly 6 knots to rpm of roughly 1800?

ssrig.... what is your typical fuel consumption with that 55hp? Have you found an absolutely optimum fuel consumption and an absolutely, well let's coin a new word, pessimum fuel consumption?

It would be interesting to find the range of consumption for the two hull types with different horsepower levels. Of course there are people who like economy and others who like speed. Why a person who likes speed would buy an A25 though I don't know. As the ex-owner of a Citroën Dyane with a 602cc two cylinder boxer engine and about 55mpg, I would be firmly in the frugal court.

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[DesertAlbin736]

It would be interesting to find the range of consumption for the two hull types with different horsepower levels.

Having religiously kept a fuel log of fill ups recording every drop of fuel purchased & used in our A25 & engine hour reads at each fill up over the span of 6 years and 663 hours my average consumption is 0.45 GPH (1.7 LPH). That includes underway time at 2,100 to 2,350 RPM at speeds of 6 to 6.5 kts plus idling, docking, and setting anchor.

As the ex-owner of a Citroën Dyane with a 602cc two cylinder boxer engine and about 55mpg, I would be firmly in the frugal court.

Fun looking car, one seldom if ever seen in the US. Looks like a modernized derivation of the old Citroen 2CV. Our Ford Fusion Hybrid mid size sedan averages 42 MPG

[Bob Noodat]

Having religiously kept a fuel log of fill ups recording every drop of fuel purchased & used in our A25 & engine hour reads at each fill up over the span of 6 years and 663 hours my average consumption is 0.45 GPH (1.7 LPH). That includes underway time at 2,100 to 2,350 RPM at speeds of 6 to 6.5 kts plus idling, docking, and setting anchor.

Now, that's the start of a good data set. Probably few people keep track though.

Fun looking car, one seldom if ever seen in the US. Looks like a modernized derivation of the old Citroen 2CV.

Well spotted. The Dyane was the luxury version of the 2CV with two speed wipers and a fuel gauge, loaded. The seats unclipped and would stand stably on the ground for your picnic. Rear hatch was 2 pins to remove, and with passenger and rear seat out, you could carry huge items. Long long suspension travel, so speed bumps could be taken at any speed, you never even felt them. Wonderful minimalist car.

[nebulatech]

Bob, there is a forum member who has an A27. He has a blog post that details his thoughts and research on repowering, and evaluates considerations through a large range of HP (60 - 157):
http://motorcityboatwerks.weebly.com/al ... power.html

It's the A27, not the A25, but you may find it relevant to your research just the same. The gentleman also mentions Sound Marine Diesel. The owner of Sound Marine owns one of our A25's with a Beta in it. I've spoken with him. If you're looking at an A25 with a Beta in it, you might consider calling Sound Marine for their insight.