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

[S/V Alchemy]

Originally Posted by 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!

Hard to say without displacement, but maneuvering is a function of many elements. As I said above, I went from 52 HP to 60 HP because the cost of rebuild equalled the cost of repower and the newer engine can run a bigger alternator and is far more economical and cleaner than the old block. But a far bigger change was going from an 18 x 13 three-blade fixed prop to a 19 x 15 four-bladed Variprop feathering prop. The prop blade area is significantly greater and, although I have to tweak the pitch a bit to hit the fuel map sweet spot, I maneuver with far greater "authority" thanks to the slightly greater torque and the greater mass of water set in motion.

Linear speed is more expensive as one approaches hull speed in a displacement boat; you are pushing increasingly large volumes of water out of the way of the hull. By contrast, low-speed maneuvering is not going to eat much fuel, because it's brief and you are essentially just overcoming (or aiding) inertia of a nearly stopped boat. You might have bought a 75 HP and a bigger or four-bladed prop and accomplished the same goals at a more economical fuel burn rate.

If your boat was 55 feet in steel, and therefore probably over 25 tons displacement, 40 HP sounds severely underpowered, so I understand your rationale for going higher. I'm just not sure it could not have been accomplished by a combo of middle-sized diesel and different prop.

[donradcliffe]

How big is the boat?? Are we really talking about 30,000 lbs (40 ft) or 30T (55 ft)?

Did the PO re-engineer the drive train to take the higher hp?? You should be looking at a prop shaft diameter of 1.75-2".

Did the PO re-engineer the exhaust system to meet the engine requirements?? You should be looking at an exhaust diameter of 4".

If the installation was done well, I wouldn't lose any sleep over the extra hp. It will require opening up the throttle once every 24 hours or so, but it won't take 3x the fuel under normal operation. The worst that can happen is that you might have to replace the turbo, but that is a small fraction of the cost of a repower.

[donradcliffe]

How big is the boat?? Are we really talking about 30,000 lbs (40 ft) or 30T (55 ft)?

Did the PO re-engineer the drive train to take the higher hp?? You should be looking at a prop shaft diameter of 1.75-2".

Did the PO re-engineer the exhaust system to meet the engine requirements?? You should be looking at an exhaust diameter of 4".

If the installation was done well, I wouldn't lose any sleep over the extra hp. It will require opening up the throttle once every 24 hours or so, but it won't take 3x the fuel under normal operation. The worst that can happen is that you might have to replace the turbo, but that is a small fraction of the cost of a repower.

[unluckycharms]

Originally Posted by donradcliffe

How big is the boat?? Are we really talking about 30,000 lbs (40 ft) or 30T (55 ft)?

Did the PO re-engineer the drive train to take the higher hp?? You should be looking at a prop shaft diameter of 1.75-2".

Did the PO re-engineer the exhaust system to meet the engine requirements?? You should be looking at an exhaust diameter of 4".

If the installation was done well, I wouldn't lose any sleep over the extra hp. It will require opening up the throttle once every 24 hours or so, but it won't take 3x the fuel under normal operation. The worst that can happen is that you might have to replace the turbo, but that is a small fraction of the cost of a repower.

Sorry for leaving out this important detail, boat is 43ft LOA, 30k lbs (not 30 tons).

I'm still awaiting details on the supporting upgrades, if any, that were done to accompany the re-power. If none, clearly that will raise additional questions about the overall quality of the work performed.

Thanks for the input everyone, really appreciate it!!

[FionaJC]

My 34ft 6.5 Imperial Ton (15K lbs) motor sailer ketch was meant to have a 38-40hp by design. I have used a Peugeot XD2 70hp since I bought it. The problem was I could never get faster than 6.5kts with more power as the stern dug in. The engine failed so I have replaced it with a 50hp Perkins 4107. The result is that its quieter, more economic and still goes 6.5kts the engine is 15 years older. I have had no end of problems with modern turbo diesel vehicles so the simpler the better. The recovery service at sea is not as good as on land.

[Elie]

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.

In my humble opinion, I think your are buying yourself lots of trouble. The speed limit of a boat is related to the water length and a bit on the displacement. Doubling the horse power will require a new and much larger propeller, witch in turn will create more drag under sail.This engine is grossly exagerated for your boat, and you won't be able to use it at its peak performance curve. Such heavier engine means beefed engine bed, change of the drive shaft, cutless bearing, among other things. There is no benefit what so ever, unless you want to use your boat as a tugboat. Sometimes one wonder why we are looking for trouble... At great cost...

[seabreez]

Originally Posted by 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!!

Over power is not a bad thing. Yes it is more fuel burning but not that noticeable. It will run a little less rpm for the work and will remain at the same temperature. It will not be screaming at high rpm to get you there. We installed an 110 hp in a small trawler where it was 60 hp on the old engine. The owner wanted it and he got it. The fuel burn is .8 gallon per hour and the rpm's are lower. He installed a second alternator to charge all his batteries and he just loves his new conversion. You will get accustomed quickly to the new HP and less screaming engine. Good sailing...

[groundtackle]

On my Cal 2-46 which is. Also 30,000lbs I have the original 85 HP Perkins 4-236 and it will drive the boat at 7.5 knots all day long at 1600 rpm. It has plenty of power for the boat and I honestly can't imagine needing anything larger. It'll do that and push a 200 amp alternator without even the slightest hesitation. If I was to repower I'd go with an m90 which is as far as I understand the modern version of the same engine.
Nick

[leftbrainstuff]

Diesels like to work hard. Having an oversized diesel that always runs at low load is not good.

We're also talking about displacement hulls on monohulls. Double the power is not going to give you double the benefits.

Fuel consumption will increase at full power but will probably be negligible at lower power settings.

You want to ensure the engine at least runs at operating temperature, is suitably propped and the trans is properly rated.

On my own 44000# Liberty 458 with an 85 hp Perkins 4-236 and a Maxprop I can't see any benefit in having more power. If I had to repower I'd just overhaul the original Perkins. Cheap, simple and reliable. No electronics.

A couple of months back we saw a 35' sailboat up on the plane moving fast with a big wake. Maybe it was this boat...

[FionaJC]

Originally Posted by leftbrainstuff

On my own 44000# Liberty 458 with an 85 hp Perkins 4-236 and a Maxprop I can't see any benefit in having more power. If I had to repower I'd just overhaul the original Perkins. Cheap, simple and reliable. No electronics.

As an electronic engineer I can confirm that the simpler the setup i.e. the less electronics the more reliable.

[groundtackle]

I agree with the rebuild and thats what I would do as well, I was just speculating if I did for some reason replace I'd go with very similar HP
N

[Stumble]

Everything everyone is saying is absolutly true. This engine is rediculiously oversized, which will eventually lead to glazing, more fuel burn, extra weight, etc.

But, I really just wouldn't worry much about it. The fuel economy will suffer, but not enough to notice at the same output. The weight, meh it's a big heavy full keel boat anyway what's a couple hundred pounds low and in the middle. Even the cylinder glazing I don't see as a huge problem, run it hard for an hour out of 24 and it will be fine enough, after all 25 years for an engine is a pretty good deal and it's already at 15.

If I was looking at two sisterships one with a reasonable spec engine and the other with this I would lean towards the smaller engine, but it wouldn't be the determining factor.


What really does concern me is the turbo. I hate those things on marine engines. But I would contact Yanmar and see what it would take to just pull it off. Which would mitigate two issues, 1) the turbo, 2) excess hp, at the same time.

[Cadence]

Originally Posted by 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!!

If making hull speed stay with the original or close to it.

[DeepFrz]

Just 2 words of advice. Keep looking.

[Cadence]

Originally Posted by FionaJC

My 34ft 6.5 Imperial Ton (15K lbs) motor sailer ketch was meant to have a 38-40hp by design. I have used a Peugeot XD2 70hp since I bought it. The problem was I could never get faster than 6.5kts with more power as the stern dug in. The engine failed so I have replaced it with a 50hp Perkins 4107. The result is that its quieter, more economic and still goes 6.5kts the engine is 15 years older. I have had no end of problems with modern turbo diesel vehicles so the simpler the better. The recovery service at sea is not as good as on land.

Just out of curiosity, is an Imperial ton what I recall as a long ton. 2200 lbs. A guess is a1000 kilo. I guess I could look it up.

Yep hull speed and an old reliable engine. Turbo charged engines are probably good for go fast planning hulls and deep pockets.