Sailboat Repower Question

[rostyvyg]

I am planning to repower my sailboat replacing 75hp/3800rpm Yanmar 4JH3-TE with 59hp/2700 rpm Nanni N.65.The boat is an Island Packet 420 year 2000, LWL 37.33'. Dry weight is 30,000 lb but loaded as is I would say she is closer to 38,000lb. The engine is a 75HP Yanmar 4JH3-TE, the transmission is Kanzaki KM4A, 2.68:1, and the current shaft diameter is 1.25". I believe the transmission has a down angle of 7 degrees. The propeller is a four-blade MaxProp Easy, 19-inch. The pitch is currently set to 17 degrees (I tweaked it a few times to get 3700 rpm at WOT and 17 seems the best pitch for that now). I could have Nanni N.65 with either a 2.1:1 or 2.45:1 transmission. Could you, please, help me confirm that 59hp would be enough and what pitch will I have to set the MaxProp to?

[Lee Jerry]

Originally Posted by rostyvyg

I am planning to repower my sailboat replacing 75hp/3800rpm Yanmar 4JH3-TE with 59hp/2700 rpm Nanni N.65.The boat is an Island Packet 420 year 2000, LWL 37.33'. Dry weight is 30,000 lb but loaded as is I would say she is closer to 38,000lb. The engine is a 75HP Yanmar 4JH3-TE, the transmission is Kanzaki KM4A, 2.68:1, and the current shaft diameter is 1.25". I believe the transmission has a down angle of 7 degrees. The propeller is a four-blade MaxProp Easy, 19-inch. The pitch is currently set to 17 degrees (I tweaked it a few times to get 3700 rpm at WOT and 17 seems the best pitch for that now). I could have Nanni N.65 with either a 2.1:1 or 2.45:1 transmission. Could you, please, help me confirm that 59hp would be enough and what pitch will I have to set the MaxProp to?

A 21% reduction in power seems like a lot. I can't tell you if that 59hp will be enough. How did she perform with the 75hp? Why are you going down?

You will definitely want the 2.45 transmission. That will give you similar torque at the prop (at 2700 rpm at engine) as the Yanmar had at 3700 rpm. That would imply to me that the same pitch should be close. The nice thing about adjustable props like the MaxProp is that you can tweak it. I'd highly recommend that you set it to reach the rated 2700 rpm engine speed at WOT.

[rostyvyg]

I currently cruise at 6 knots at 2400 rpm, almost never run my engine over 2800 rpm, and pretty much never over 3000 rpm so I think my Yanmar might be too much for my boat. I am deciding between Nanni N4.80 and Nanni N4.65 and like the idea of not having an intercooler (which makes for larger engine dimensions and an extra point of failure).

[grantmc]

The questions being asked are very specific, and would perhaps be better answered by a competent marine engineer on whom you can rely and trust. Or possibly post in a forum devoted to Island Packets. I'd ask less specific questions, maybe asking for people to share their experiences in repowering. Maybe even go and visit a boat or two where they’ve done so to gain first hand knowledge. That often gives rise to questions not thought of. Your engineer might also be able to suggest some owners he/she has done such work for as references.

It's just that I think your expectations of the responses here on CF may be inflated. Yes there will be opinions, and some here are wonderfully competent and knowledgeable, but they can never give the same advices as someone there on the ground. Besides the technical issues, matters of budget, location, access to parts/engineering shop may also be big factors.

Whilst I don't want to to make this about me, I'll just share that I am going through something very similar currently. My boat is in Fiji and the engineering facilities, particularly parts availability are very basic. Importing parts is incredibly difficult too. So I've chosen to replace the old engine with the exact same make/model (rebuilt). If the boat were here in Wellington where I have access to much more sophisticated parts suppliers and workshop facilities, plus a network of people I trust and know, then my decision would have been different. But I had to adjust for the reality. And yes, sailing the boat here to NZ was considered.

The Yanmar 4JH3-TE was a common engine fitted to 420s (presumably some sort of bulk deal done with Yanmar). Changing to a another engine make brings a host of other factors (all over come-able). But mounts, morse controls, exhaust etc. need thought.

When I replaced a Yanmar (different boat to above) as a silly example, I discovered very late that I needed a new prop because the old Yanmar was left hand and new engine right hand. (Yes we did consider just changing forward for reverse.) And props are really expensive. But all though the repower process problems presented, and it struck me they all resulted in more and unanticipated cost. Although to be fair the firm that quoted had made a good allowance for non specifics and so the overall job came in under budget.

Dropping down nearly 10% in power has pros and cons. In theory less fuel, and I read your reply about not going above 2,400 rpm. But that other 600rpm is insurance for when the current or wind is against you. Or maybe you need to help a fellow sailboat with a tow. Who knows, Weather or events can turn against us. I always want to get my grand kids home safe and sound, and knowing I've extra coal to throw into the fire gives me a warm feeling.

Good luck with your project.

[MicHughV]

I've posted this on many similar threads

Get yourself a copy of Dave Gerr's "Propellor Handbook". It's available online and also on this forum.

In it, you will find a step by step procedure to figure out an answer to all your questions.

[donradcliffe]

The Nanni n65 is a common rail engine, and I would avoid it if I could.

[rostyvyg]

Originally Posted by MicHughV

Get yourself a copy of Dave Gerr's "Propellor Handbook

I have this book and read it and it appears that my boat needs 60hp on the lower end and 75hp on the higher end. Thus I am thinking of Nanni N4.65 (59 hp)

[MicHughV]

Horsepower alone is insufficient information. One must also consider many other variants, aperture size, blade clearance from hull, etc, etc.

The Island Packet has quite some heft for it's LOA. A 60 hp engine might do the trick, but at what rpm is the engine running to push that boat at hull speed. These and other factors are the questions I'd be asking.

[grantmc]

Originally Posted by donradcliffe

The Nanni n65 is a common rail engine, and I would avoid it if I could.

So is the Yanmar 4JH3-TE that it's replacing.
The Nanni is actually a marinised Kubota.
And both engines are turbocharged.

My boat has a 72hp motor (common), but I've seen boats the same in for sale listings with all manner of different engine types and sizes, many down as low as 27hp. Presumably they all get along fine.

OP
What's wrong with the existing engine?
Why can't it be repaired?

[PippaB]

To me, the OP's question are a math(s) issue. The relevant data are available:

Yanmar 4JH3-TE

Nanni N4.65

OP reports 3000 rpm max use = 40hp (prop curve), 70hp (crankshaft) and 175Nm torque, @2400 cruising RPM = 20hp/56hp and 170Nm torque.

The Nanni reports 20hp@prop at 2000 RPM, with 47hp at the crankshaft, and 164Nm torque. At WOT the Nanni reports 59hp (prop and crank since it is WOT) and 148Nm.

In prop hp, running the Nanni at WOT is about the same as the Yanmar at 3500rpm, which the OP reports never using.

OP reports the current transmission is 2.68:1, although I saw a spec of 2.63 for the same transmission in literature.

For WOT, 2.68*2700/3700 = 1.95:1, and if we assume cruising at 2000rpm (based on same prop hp/torque) for the Nanni, 2.68*2000/2400 = 2.23:1. I would probably opt for the 2.1:1 from Nanni, with the thought that I might have to up the prop pitch just slightly for cruising.

To match up with the reported Yanmar usage, the Nanni will likely be used at ~2000rpm in cruise, and ~2500rpm for max power, leaving a little additional headroom to WOT. IOW, the OP does not appear to be using anything close to the 75hp available in the Yanmar, and the 59hp in the Nanni should suffice.

All this worth what was paid for it, YMMV.

[rostyvyg]

Thank you for the detailed explanation. My (different) way of thinking was that if I was hitting (with the correct prop pitch) the boat hull speed of 8.3 knots at less than 3000 rpm with Yanmar, then extra available power from Yanmar would do nothing for me with strong opposing current since the speed of the hull through the water can't get over 8.3 knots anyway. If I need to go against high wind, I would rather use some sail to help push the boat.

[Na Mara]

Yanmar engines like being run hard. The recommended rpm for the OP’s existing engine is 2700. If you aren’t going to run the engine at that then changing to a lower raving engine is a good idea.

It does take some getting used to motoring around at what sounds like WOT on other boats. I myself can’t quite bring myself to do it and aim for 2300-2500. At 2700 the 4kh2 in my boat is loud, thirsty and my boat is digging a hole in the ocean. Overpowered? Probably but then there are those days with steep headseas or a tight maina when I’m very thankful for the extra reserve oomph.

[rostyvyg]

Originally Posted by donradcliffe

The Nanni n65 is a common rail engine, and I would avoid it if I could.

Well, it is not! Nanni and Beta are the only old-school mechanically fed and controlled diesels still available in the US. And the fact that they are Kubota-based is an icing on the cake.

[Wandering1]

Originally Posted by grantmc

The questions being asked are very specific, and would perhaps be better answered by a competent marine engineer on whom you can rely and trust. Or possibly post in a forum devoted to Island Packets. I'd ask less specific questions, maybe asking for people to share their experiences in repowering. Maybe even go and visit a boat or two where they’ve done so to gain first hand knowledge. That often gives rise to questions not thought of. Your engineer might also be able to suggest some owners he/she has done such work for as references.

It's just that I think your expectations of the responses here on CF may be inflated. Yes there will be opinions, and some here are wonderfully competent and knowledgeable, but they can never give the same advices as someone there on the ground. Besides the technical issues, matters of budget, location, access to parts/engineering shop may also be big factors.

Whilst I don't want to to make this about me, I'll just share that I am going through something very similar currently. My boat is in Fiji and the engineering facilities, particularly parts availability are very basic. Importing parts is incredibly difficult too. So I've chosen to replace the old engine with the exact same make/model (rebuilt). If the boat were here in Wellington where I have access to much more sophisticated parts suppliers and workshop facilities, plus a network of people I trust and know, then my decision would have been different. But I had to adjust for the reality. And yes, sailing the boat here to NZ was considered.

The Yanmar 4JH3-TE was a common engine fitted to 420s (presumably some sort of bulk deal done with Yanmar). Changing to a another engine make brings a host of other factors (all over come-able). But mounts, morse controls, exhaust etc. need thought.

When I replaced a Yanmar (different boat to above) as a silly example, I discovered very late that I needed a new prop because the old Yanmar was left hand and new engine right hand. (Yes we did consider just changing forward for reverse.) And props are really expensive. But all though the repower process problems presented, and it struck me they all resulted in more and unanticipated cost. Although to be fair the firm that quoted had made a good allowance for non specifics and so the overall job came in under budget.

Dropping down nearly 10% in power has pros and cons. In theory less fuel, and I read your reply about not going above 2,400 rpm. But that other 600rpm is insurance for when the current or wind is against you. Or maybe you need to help a fellow sailboat with a tow. Who knows, Weather or events can turn against us. I always want to get my grand kids home safe and sound, and knowing I've extra coal to throw into the fire gives me a warm feeling.

Good luck with your project.

My boat was originally powered by a 4 cylinder 72hp Perkins. The previous owner replaced the engine with an 86hp Perkins m92b. The engine is 4.4L. Max torque is at 1200rpm. No turbo. It's a stump puller. 42,000lb 44 ketch. We cruise at 6kts at 1200rpm. We can drop to 1000rpm for 5 kts. Fuel consumption at 1200rpm is 3.3l/hr. We swing a 22" prop. The gearbox is hydraulic so you can reverse the pipes and turn the prop thr other way.
We can powerminto 7' seas at 1900rpm at over 7 kts. Fuel consumption at those revs would be high but it's there if we need it. Maximum theoretical revs is 2400rpm but we are over propped so only make 2000rpm at 8kt hull speed

[FPNC]

59 hp is marginal for the displacement of the vessel, but it will work with the right propeller. You have a Max Prop. Call PYI in Lynnwood, WA (800-523-7558) and ask them which gear reduction will work best with that prop and what the recommended pitch setting would be.