Repowering? Bigger is better?

[gjordan]

Have you had the bottom, and the prop cleaned??? It just might stop the smoke and give you the oomph that you need, instead of the many dollars needed for a new engine. Worth trying before you spend the big bucks. ____Just a thought! _____Grant.

[cwyckham]

Originally Posted by Delancey

So, in other words, you would rather spend the weight on a larger house battery bank than spend it on a larger engine?

No, I don't think the house bank needs to be any larger. You just put back the amps slowly instead of quickly. In fact, you may need a larger bank if you're going all engine because you can only really get the bank up to about 80% with the engine before it starts eating a lot of fuel for only slow gains (ie you hit the absorption phase). That's probably a whole 'nother thread.

So no, I would think I can use my current 500 Ah bank with a 100 A Balmar for cloudy days and 250-350 W of solar and get by just fine. I certainly wouldn't upsize my engine just to do power take-offs for alternator and fridge when solar is cheaper, lighter, simpler, better for the engine, and provides a back-up system for energy generation.

[cwyckham]

Originally Posted by gjordan

Have you had the bottom, and the prop cleaned??? It just might stop the smoke and give you the oomph that you need, instead of the many dollars needed for a new engine. Worth trying before you spend the big bucks. ____Just a thought! _____Grant.

This does raise a point. Have you tried saving the existing engine? Is it smoking because it's over-propped? Does it just need a top end overhaul?

A compression tester for diesels with all the various glow plug adapters is no more than $120 and you can compression test it yourself to see if it needs an overhaul.

Depends on your budget and time. You could spend a lot of time diagnosing and end up replacing it anyways. Or you could figure out a couple problems and learn a lot about diesels while you're at it.

[GILow]

Originally Posted by gjordan

Have you had the bottom, and the prop cleaned??? It just might stop the smoke and give you the oomph that you need, instead of the many dollars needed for a new engine. Worth trying before you spend the big bucks. ____Just a thought! _____Grant.

A very good point. we got another 300 rpm after cleaning our prop, AND it looks pretty too...

[JPA Cate]

Hi, guys,

I really liked cwyckham's suggestion relative to, [I'm paraphrasing her, please forgive me] "HEY, wait a minute, let's find out what's wrong first!" It was what I thought when, on p. 1, I read about the blue smoke. Because if the problem's only worn rings, and valve guides, and maybe a few other bits and pieces, he may not "need" to re-engine at all! Must be the cheapskate in me.

Delancey, I apologize for missing this detail, but what kind of boat do you have? How would you describe the keel configuration?

For those of you unfamiliar with the Swanson 42, she's a large (high freeboard), stoutly constructed, slightly modified full keeler, with an offset prop, IIRC.
GILow, please correct me if this is incorrecct.

Ann

[GILow]

Originally Posted by Ann T. Cate

For those of you unfamiliar with the Swanson 42, she's a large (high freeboard), stoutly constructed, slightly modified full keeler, with an offset prop, IIRC.
GILow, please correct me if this is incorrecct.

Ann

Close enough, though that descriptions makes her sound a bit like Quasimodo. The prop isn't offset, but she sure is a brute of a thing from some angles. 35mm thick GRP adds to the displacement a bit too. But we love her, she makes us feel safe.

[valhalla360]

The small decrease in power will likely make no noticeable difference (if the current engine is putting out far less than it's rated power, it may even seem better).

Assuming the current engine can be repaired for a reasonable cost. I wouldn't count on much fuel efficiency improvement. The difference will be running the smaller engine harder to achieve the same speed.

[marujo.sortudo]

My boat is 32' (40' w/spars) 16,000#. WLL 29'. Engine sizing can come down to philosophy/preferences as much as anything else. Today, common wisdom is 1 hp/500#. This seems to be increasing with a growing number of sailors who tend to go to windward under power and boats with increasing windage. When engines were newer to sail boats and windage on typical boats was generally less, 1 hp/2,000# was the rule. We have lower freeboard than many modern boats and try to keep down windage maximally.

Right now, I have a 30 hp engine but I've only ever used about half of it's effective hp and tend to run it slow. I use it a bit more like an auxiliary having a preference to sail whenever possible. This is a very typical size for a boat like ours. However, if it needed replacing tomorrow, I would replace it with a 16 hp in a heartbeat. Works for my intended usage and I have friends with similar setups that very much like their engines. However, they do also use them as auxilaries.

Strangely, I don't expect the change would effect fuel consumption much as diesel fuel use strongly correlates to hp delivered and I run my current engine at about 12 hp most of the time now ( going roughly 4.8 kts which is well below hull speed but very economical.). I'd actually put a large reduction gear on the engine (3.22) so i could keep a prop of the same diameter which will help. I'd also install a house alternator cut-out switch which could provide a significant power boost, if needed. If my fuel use gains are minimal, what other gains would i be expecting?

1. Obvious cost savings on repower.

2. Improved access around engine and prop shaft make working on it more pleasant.

3. Fewer, smaller, cheaper parts. Less oil to carry aboard. Lighter engine. All this means we can carry more stuff for cruising and have more money to buy it. More locker space, etc. Extra case of rum?

4. Can motor in harbor at minimum steerage speed. I love the slow, precise approaches you can do in harbors under calm conditions. Right now that involves popping the engine in and out of gear a lot as it will push us along at roughly 2.7 kts at idle. YMMV.

5. Better engine longevity. Engines don't like low RPMs as much as high RPMs. Also, when running the engine just to charge the batteries it will be loaded closer to 50% which should all improve its lifetime.

We love either sailing or putt-putting through canals, so, for us, a small engine is a good choice. It really depends on your boat and how you'd like to use it.

[Delancey]

Thanks again for the feedback. I confess engines are not my area of expertise, far from it. I had a nice talk with Farron at Beta Marine, when he heard my boat had a 44 hp in it he immediately remarked that he felt it very big for the boat.

When I asked, he was at a loss to explain why it was paired with the boat except to say that sometimes it was hard to understand why builders do what they do except that somebody probably got a great price on a bunch of engines so that's what they used. I will have to go back and look at old brochures to see if there was any kind of marketing motive.

When asked initially about a replacement he suggested the 38 as being more than adequate which was great to hear, not only for the weight savings but for the additional space around the engine which to be gained, as marjuro mentioned, a nice perk.

I took a lead from him mentioning the 38 would be "more than adequate" to ask what would be the smallest I could get away with and he said in fact the 35 would get me where I needed to go. The two are from the same block and so weight and cost are basically the same (around $11k for stock) with the difference being in rpms at max, 38@3600 and 35@2800 so...

...back to my original question, bigger is better? Does choosing a smaller engine mean I will be more likely running closer to it's max most of the time, is that not a bad thing? Do I end up being more efficient, using less fuel? Or is bigger better because I might ordinarily be running it at less than max?

These might seem like questions I would know the answers to if I just paid more attention sometimes but I never said I wasn't here to learn so thanks for your patience.

[Delancey]

Originally Posted by marujo.sortudo

5. Better engine longevity. Engines don't like low RPMs as much as high RPMs. Also, when running the engine just to charge the batteries it will be loaded closer to 50% which should all improve its lifetime.

How does this relate to fuel efficiency? What if I put a 30 in? Other than saving an additional 100 lbs, probably decreasing the resale value which I don't really care about anyway, and a loss of oomph, is there a potentially tangible benefit to going smaller? To having a true auxiliary? Going down a size gets me back up to 3600 rpms for what that's worth.

[dannc]

Originally Posted by dannc

Per Nigel Calder's, "Marine Diesel Engines," he says, that 130-160 amp alternators are now common on boats and that it takes about 7 HP per 100 amps of alternator output for 12 volts. An engine driven compressor for a fridge is up to 2HP.

Later,
Dan

Originally Posted by Jim Cate

G'Day Delancy,

Re the alternator load issue:

1. I am pretty suspicious of the "7 hp per 100 amp" figure. We run a 150 A Balmar, usually at around 80 A and 13+ volts driving it with a single 13 mm V-belt. That belt would be VERY hard pressed to pass more than 3-4 hp, especially with the small diameter pulley involved. After all, 7HP is around 5,000 watts, and 100 amps at ~13 volts is but 1300 watts... you work it out!
...

Cheers,

Jim

Calder's number seems off to me as well but he is the "expert."

In the tractor world, the rule of thumb is that it takes 2 PTO HP to generate 1,000 watts. 1,000 watts / 110 or 120 gives 8-9 amps. Or to put it another way, Calder is saying that a boat engine needs 7 HP to generate 1,200 watts vs 2 HP for a tractor diesel.

Now, he might be talking engine HP vs PTO HP. PTO HP is the HP at the shaft turning the generator. My tractor PTO HP is 85% of the engine HP but even taking into account the differences in PTO HP and engine HP does not add up to 7 HP per 100 amp unless I am missing something or my math is wrong.

Hoping someone can explain the 7 HP per 100 amps at 12v.

Later,
Dan

[brantleychuck]

When I repowered my IF36 with the beta 37.5 I had all the same questions, bigger has got to be better. Being very new at the game and assuming that the 4-108 put out 50 hp I was very skeptical about going with a smaller motor. The engine guys were saying two things. The 4-108 is not giving 50 hp and going smaller was the way to go. Joe at sound marine was recommending the 30 hp I think but I went withe the 37.5. After sailing for 8 years around the keys, Bahamas and Belize a couple of times and once down to Roatan I have not once wished for more power. I also found out I run the engine about 1/2 the time I am moving. The sweet spot for my hull is about 5.2 knots. Pushing harder gives little speed for increased rpm. At redline I get 7.5 knots but I am making a big hole in the water . At the 5.2 speed it feels like I am just coasting along with a fuel consumption of a little less than .5/gal per hour. The engine is not working hard and very quiet. When running for 60 hours straight fuel usage, quiet and reliability are what you want.
I have turned the boat into the waves and punched through the big swells for a time but after a few minutes decided something was going to get broken and changed course. Entering reefs with waves crashing around, turning the wheel like a mad man as the waves try to turn you and pitch you onto the reef it would be nice to have the power to get onto a plane and the hell to that calm anchorage just a few minutes away would be nice but.....
Not sure how much a big alternator should affect your decision. My beta came with a 100 amp alternator. I have a 1200 amp bank so I added a 200 amp alternator in addition to the 100 amp.. I have a switch that turns the Zena off. The belts still turn but no output so not much of a load. I have found that the alternators put a load on the engine at start up but the output tapers off pretty quickly as the banks ability to suck up amps tapers. I would say alternator load has been a non issue.
So I guess boat use is what one should consider most. If your still working and will need to burn diesel to get back to slip then a little bigger may be better. If cruising then smaller. If I were to repower again I think I would go one size smaller so that I ran the engine a little harder at that 5.2 speed. Everyone says run the diesel harder, they like it. I just take "everybody's "
word on that one. Not sure we're the " double the stock power " comment comes from, ludicrous IMO.
At the present time the IF36 is for sale as I bought a very big project boat, a Schucker 436. She needs a complete refit and is getting a Beta 50. Even with my experience with the Freeport I still was tempted to go bigger, after all she is a motor sailer. I am going for efficiency so the Beta 50 it will be. Since you are considering a beta i will mention as others have that Stanley at the main beta distributer in NC is a huge resource for information. Also joe at sound marine has the best prices on engines in have found, worth giving him a call.
Good luck on your install. I enjoyed putting the 37.5 in the Freeport. Really pretty easy, did it myself. Shaft alignment has been made into a big deal but in reality is pretty easy to do, sort of like crossing over to the Bahamas. Have fun

[Stu Jackson]

Our 1986 boat is OEM 12,000# but no one ever believed that! Most of us are running anywhere from 15 to 18,000#. We have a 21 HP Universal M25. 21 HP!!! Imagine that! We do just fine here on San Francisco Bay and the ocean with it. Later models came with 23, then 30 hp engines.

I also have a 100 A alternator with a 3/8" belt, and we use a Balmar MC-612 regulator. I have been promoting the Small Engine Mode over the amp manager feature for years. Once the house bank is down (ours is 390 ah) from an overnight stay on the hook, the battery acceptance is high, but tapers off pretty quickly. I see about 50A in the morning, so turn the SEM switch to cut back to 50% AO until it reaches 25 A input, then switch it on and it goes to 30 A, usually within an hour.

I don't think your alternator concerns should be an issue if you use the external regulator features.

I think a larger house bank is preferable to a larger engine you don't need.

Good luck.

PS If you Google Small Engine Mode, you'll probably come across these:

Alternator heat, Regulator Controls, Small Engine Mode

Alternator heat, Regulator Controls, Small Engine Mode

Small Engine Mode - discussion with link to the picture of the toggle switch: Alternator heat, Regulator Controls, Small Engine Mode

[marujo.sortudo]

Hmmm...the 35 sounds like a nice option. Lower max RPM's = quieter + you could have a slower turning, bigger prop to match it if you liked. If you want to learn more about engines and props, get your hands on Gerr's Propeller Handbook. You'll get an education and easily pay for the book.

Regardless, you'll need to rethink your prop when you change your engine. Find a copy of propcalc.xls to download somewhere if you want to see factors that effect prop sizing. Always consult with the prop supplier, though.

Bigger is only better if you need it. Have to tow a disabled sailboat into weather someday? Pull another boat off a reef? Then you might be glad for the power, if you had a prop that could really deliver it. Bigger, slower turning props develop the most actual pulling power. For me, I'd rather have a more pleasant time working on the engine, save $$$, and keep the engine happy by running it hard. What do you want?

This may seem hard to believe, but diesel engines pretty much use the same amount of fuel per hp delivered regardless of engine size. As such, if you like to cruise at say, 5 kts, you'll use roughly the same amount of fuel regardless of what engine you have. Sure some engines are little more efficient than others and they might be a bit more efficient towards the top of their range, but the differences are minor. The how efficient the prop is at that speed probably makes more of a difference. A variable pitch prop would probably make the most difference in real terms, but you can pick up an old copy of Don Street's books if you want to hear about that argument. Most folks prefer to keep the shaft simpler than is required for that kind of setup.

Of course, calls to save your current engine if possible are certainly thrifty and wise. That's what I'm doing. I'll repower and downsize only when it totally fails me or major parts become unavailable. I had problems with smoke at high RPM's originally as the boat was overpropped. Adjusted the pitch of the prop 2" and now I can run her up all the way, which will, with our current big engine, take her up to hull speed no problem. More than I care to have, but I won't replace a fixable diesel engine. They last forever if well cared for.

[a64pilot]

I'd suspect the 35 and the 38 are actually the same, just you can get 38 HP when needed out of the 38.