Pro's & con's of hp bump with re-power

[unluckycharms]

Hi all,

Been lurking here for awhile as I search for a boat to live aboard for the next few years (at least), finally making my first post.

I've already settled on a couple specific boat models that I'm interested in, and will begin looking at some in person next week, so not currently seeking advice in that department.

Where I'm hoping you guys can help is with one specific boat situation, where the PO has chosen to re-power from the old Perkins 4-236 (75hp) to a 160hp Yanmar turbo diesel. Having spoken to the broker, the PO was planning an upgrade to around 100hp, and bumped to 160hp as the installation cost increase was negligible.

I have the following concerns with this re-power:
- Increased fuel consumption (possibly 3x+ if I'm reading the specs of the new engine properly)
- Shortened engine life if run at lower RPMs to save fuel (and loss of turbo benefit)
- Added complexity/heat of turbo installation
- Zero benefit of additional power as 75hp should be more than sufficient to drive the boat at hull speed (?)

The boat in question displaces just shy of 30k pounds, with a rounded underbody, long fin keel, and skeg/rudder combo (typical 70s-80s stout bluewater construction).

Are my worries unfounded? Have I missed any other considerations, particularly any advantages of the new engine? Would it be wise to consider re-powering with an engine closer to original spec and selling the lightly used engine (2002 model, but under 100 hours)?

Appreciate all your thoughts on the matter!!

[GordMay]

Greetings and welcome aboard the CF, unluckycharms.

No, not unfounded worries. I think you've hit the nail on the head.

[SVTwilight]

I agree with Gord. Older boats were often powered with roughly 1 hp per foot of
Waterline length

Today many are powered with about 2 hp per foot.

The 4 cyl yanmars in the 77 hp range are one of the best marine engines available IMHO

My new boat has that and weighs in at about 32k lbs.

Prop choice is important as well...

[a64pilot]

I do not think there will be much increased fuel consumption at all, so long as you don't use the power, the BSFC or fuel burned to make a HP is very close to flat for these sized Diesels, all that means is roughly double the HP used, fuel consumption doubles, the inverse is true too.
Also in truth it's more of temperature thing with Diesels and low power operation and anything much more than 10 or 20% power likely gets it completely warmed up and keeps it that way.
My Duramax Diesel in my PU for example, it is a 300+ HP engine, yet likely the truck will need way less than 50 HP 99% of the time running empty, and it rarely exceeds 1500 RPM, yet they last for hundreds of thousands of miles.

All that means that in my opinion, you won't have a problem with too much motor, other than it takes up more room and is heavier than it needs to be, and I'd even go so far as to say if it's intended to be largely a motor boat for running up and down the ICW, it may be better for that than an "approiately" sized Diesel in that likely its noise, vibration and harshness will be much less.
I love my little 4-JHE as it's stone cold reliable, but hate running the thing at cruise speed cause it sounds like we have my old John Deere lawnmower in the Salon with us.

[topfrog]

I own a bunch (90+) of diesel engines on refeigeration units plus the one on my boat. The newer engines will last longer produce more hp at a lower RPM. I have found the are more efficient on fuel on an annual basis. Bottom line newer engine more efficent less overall cost. In my opinion.

[CSY Man]

Another factor is a huge prop on the 160 HP engine, it will slow you down while sailing.
Make sure you factor in the $5K for a proper folding or feathering prop.

[a64pilot]

At this power size, you may even be able to fit a variable pitch prop, a Hunstead? I think is the name?
Back to intended purpose, if it's meant to spend a large portion of its life motoring, it may be just the ticket

[UNCIVILIZED]

Know that if you plan to do a swap to a lower power engine as you mention, you won't get a lot of $ from the sale of the relatively new engine that you'll be pulling out of the boat. No where near what it'll then cost you to buy a lower power one to put into the boat. And this of course doesn't take into account the costs of both pulling the overpowered engine out of her, & the subsequent installation of a lower powered one.

So you'll lose significant amounts of money 4+ times:
- Pulling the over powered engine
- Selling the over powered engine, & prop for same
- Buying a lower powered engine, & prop for same
- Installing the lower powered engine
And this assumes that you don't need to rebuild the engine beds to match up with the engine footprints when doing the swap. Ditto engine mounts, transmissions, control levers & gauges, p-bracket, etc.

Might there be the option to buy the boat as is, with the original engine in place? And to have the price discounted to reflect the fact that she's in need of a re-power? Fiscally it might work out better. Ditto on letting you choose whatever engine you prefer. Or to even have the boat's original engine rebuilt.

[unluckycharms]

Thanks for all the replies guys, much appreciated!! Please keep them coming!

Originally Posted by a64pilot

...All that means that in my opinion, you won't have a problem with too much motor, other than it takes up more room and is heavier than it needs to be, and I'd even go so far as to say if it's intended to be largely a motor boat for running up and down the ICW, it may be better for that than an "approiately" sized Diesel in that likely its noise, vibration and harshness will be much less.
I love my little 4-JHE as it's stone cold reliable, but hate running the thing at cruise speed cause it sounds like we have my old John Deere lawnmower in the Salon with us.

I definitely hear what you're saying on lower RPMs = lower noise/vibration, but what should I make of all the sage advice to run diesels at 80%+ of max RPM at most times (including some guidance issued from Yanmar themselves)? Is this advice simply outdated?

On the space/weight issue, it shouldn't be too bad as overall displacement is actually slightly lower than the Perkins it replaced. Accounting for additional plumbing, etc, it's likely pretty negligible on a 30k displacement vessel. Visually, the engine package appears to fit just fine in the original engine compartment, save for a bit of extra exhaust pipe routing.

Originally Posted by CSY Man

Another factor is a huge prop on the 160 HP engine, it will slow you down while sailing.
Make sure you factor in the $5K for a proper folding or feathering prop.

Originally Posted by a64pilot

At this power size, you may even be able to fit a variable pitch prop, a Hunstead? I think is the name?
Back to intended purpose, if it's meant to spend a large portion of its life motoring, it may be just the ticket

I'm still waiting to find out what prop is currently fitted to the boat, but would the engine really necessitate a larger prop if gearing and RPM were adjusted accordingly (I know, I'm making a lot of assumptions about the PO here...)?

Lastly, this boat is unfortunately meant to be sailing as much as possible under my watch, and motoring as little as possible. Early life plans include the Med for a couple of years, with any future beyond that TBD.

[boatbod]

Dang. 160hp on a 30klbs displacement sailboat? Wow.... speechless!

You're never going to get that engine warm, never mind use the available power. Sounds seriously wrong to me unless there is a pto driving a genset or some other large load.

[unluckycharms]

Originally Posted by UNCIVILIZED

Know that if you plan to do a swap to a lower power engine as you mention, you won't get a lot of $ from the sale of the relatively new engine that you'll be pulling out of the boat. No where near what it'll then cost you to buy a lower power one to put into the boat. And this of course doesn't take into account the costs of both pulling the overpowered engine out of her, & the subsequent installation of a lower powered one.

So you'll lose significant amounts of money 4+ times:
- Pulling the over powered engine
- Selling the over powered engine, & prop for same
- Buying a lower powered engine, & prop for same
- Installing the lower powered engine
And this assumes that you don't need to rebuild the engine beds to match up with the engine footprints when doing the swap. Ditto engine mounts, transmissions, control levers & gauges, p-bracket, etc.

Might there be the option to buy the boat as is, with the original engine in place? And to have the price discounted to reflect the fact that she's in need of a re-power? Fiscally it might work out better. Ditto on letting you choose whatever engine you prefer. Or to even have the boat's original engine rebuilt.

I wish that were still an option; unfortunately, the swap already occurred over a decade ago, but the boat has been sitting relatively unused since then (also a concern I am addressing to ensure the engine was properly prepped, stored, and recommissioned). Still waiting to find out what happened with the propeller change as well, if any.

The plan would be to do the majority of the work myself with swapping engines, acknowledging that I would likely have to bite the bullet and hire help/cranes for the extraction/insertion processes. I have experience with similar swaps on motorcycles and cars (Japanese makes, so technical and tight packages not designed to accommodate my western hands), making the mechanical aspects seem rather straightforward, but I concede that I haven't done such work in a marine environment to date. I figure money will still find a way to flow out of my wallet during this process, but hopefully the quantity can be somewhat controlled.

Trying to bring myself down to reality with this one potential boat (there are other valid contenders, though with much higher asking prices and less 'fun of the challenge' potential)....what would be a reasonable % of the current engine's resale value to expect to recuperate, assuming proper storage and zero running issues? The model currently retails around £20k, so would it be reasonable to expect half of that amount for a 2002 model with under 100 hours?

[wayne.b]

Originally Posted by unluckycharms

I definitely hear what you're saying on lower RPMs = lower noise/vibration, but what should I make of all the sage advice to run diesels at 80%+ of max RPM at most times (including some guidance issued from Yanmar themselves)? Is this advice simply outdated?

===

It's not necessary to run the engine at 80% all of the time. 30 minutes a day should be enough to blow the carbon out of the turbo. Keeping the turbo bearings happy is what's really important.

[ebsail]

Personally, I would keep the engine as is. Full consumption should be commensurate with power demanded so it will only be pretty much the same a s asmaller engine (or even less) If the money is available, and there's room for a folding or feathering prop, I'd get one. Otherwise, I see no problem with the ultra large HP engine. There may even come a day when you're bucking a 55 K headwind and heavy chop, or similar when some extra HP may come in handy just to keep up to hull speed. Had a 43' Columbia with a 50HP Perkins and once got down to only 3K headway at full throttle. Would have liked another 50HP

[Russ]

My 55 lb steel cutter came it's a 40 hp repowered to 60 but still a dog a best 5 kts hull speed 8+. I repowered to 110 Yanmar. No regrets speed 8+ fuel consumption up from 1 gph to 2.3. But now I have power to cruise but more importantly Manuver!

[S/V Alchemy]

Originally Posted by boatbod

Dang. 160hp on a 30klbs displacement sailboat? Wow.... speechless!

You're never going to get that engine warm, never mind use the available power. Sounds seriously wrong to me unless there is a pto driving a genset or some other large load.

"HP Bump" is like what I did going from 52 to 60 HP on a steel pilothouse, also 30K pounds in weight. What you are describing is more like putting a 427 V8 in a 1970s Vega. A friend did this and it made the Vega pretty much an uncontrollable death trap that threw bolts on the highway, it was so overpowered.

While that won't happen on a boat, you are lugging in every sense engine power you can't use unless you have the huge shaft and prop to transfer that power to the water, and you willl still have the issue of max speed as a function of WL hull length.

Fuel economy will also suffer. Every diesel has a power vs. fuel economy relationship, which can be used to create a "fuel map", which tells us the most efficient and economical RPMs at which to run the engine in hypothetically flat seas. If for reasons of prop size and overpowering, you can't reach this ideal RPM, you'll never reach this RPM. Low RPMs can lead to premature wear, glazing of cylinders and other issues akin to those who run them at fast idle in neutral just to charge batteries. My understanding is that diesels are "happiest" when running at an idealized RPM and essentially left there, running warm and steadily.

Therefore you wish to match the engine size and output and prop and size of parasitic loads (like PTOs running compressors and/or big alternators) to the boat. Going from 75 to 160 is more than twice the HP. I would suggest that this is more engine and then some (unless you're running six alternators, maybe) for your boat and is a poor match unless you are a tug.