39' Hunter underpowered with a 29HP Yanmar?

[Mickeyrouse]

Our boat’s original engine was 37 HP driving design weight of 20,000 lbs, actual cruise weight of 28,000 lbs. Smooth water max speed was 7.5 kts. Motoring into a steep 20 kt chop, boat speed could drop to 3 kts or less. We were underpowered. 29 HP would be fine for docking etc., but it would definitely be underpowered if long distance ICW cruising is at hand. We re-powered to a 55 HP engine. Probably over-kill, but it fit, wasn’t much more $ than a 44HP, comparable fuel consumption to the old 37, so why not?

[SailtheWind]

Originally Posted by Mickeyrouse

Our boat’s original engine was 37 HP driving design weight of 20,000 lbs, actual cruise weight of 28,000 lbs. Smooth water max speed was 7.5 kts. Motoring into a steep 20 kt chop, boat speed could drop to 3 kts or less. We were underpowered. 29 HP would be fine for docking etc., but it would definitely be underpowered if long distance ICW cruising is at hand. We re-powered to a 55 HP engine. Probably over-kill, but it fit, wasn’t much more $ than a 44HP, comparable fuel consumption to the old 37, so why not?

The original engine for our 38 Caliber (18,500lbs displacement) was a Yanmar 4JHE 44hp (cruising speed 7kts) when we repowered I went with a 4JH5 53hp, same footprint not much more money than the 44hp similar fuel burn rate but with 7.5kts at 1600 rpm and a lot more power. No regret for the extra horsepower which has come in handy in tight places with a contrary current.

[Wotname]

I'm in the "it will be OK for 99% of the time" group.

As for having the same engine as your previous boat - how often did you operate it at full power (ie max rpm / WOT). In other words, how often did use all 29 hp? I suspect it was very rare.

You might find you would use full power (ie all 29 horses) more often in the 39' but is that a bad thing.

[George_SD]

I think your previous boat had a more powerful engine than many in its size (our 33ft has 2GM20F has 16-18HP i think and it is just fine in most situations. Yes, a more powerful engine would be very nice to have on a rare occasion when that's actually called for, but that's rarely the case.

30HP engine on 39ft is the same - adequate for most jobs, but you are not going to go fast and will have go rather slowly or not at all in strong headwind and current.

As others mentioned - it is a sailboat, not powerboat. That said - repowering is doable and may even be worth doing it depending on the price of the particular 39 you are looking to buy.

[Martkimwat]

Originally Posted by Quadrille in JB

Our 12,000#, 34’ has a 25hp diesel and there have been numerous times I have wished for more horsepower. We keep a clean bottom and cruise at 6.5-7 knots but coming in the Fort Pierce inlet last week our ground speed was 1.5 knots even though the water speed was 7.1 knots.
We mainly use the engine for coming and going from our slip but there are many times an engine with adequate power is needed.
Bill in JB

If your speed through the water was 7.1kts, I would venture to say that you were already at hull speed for your waterline length, more power is not going to help you in that situation, you will just 'squat' more. Where it will come into it's own is in a short head sea that keeps knocking you back, you will accelerate back to your desired speed much quicker with more power. I remember trying to power in to the wind in a narrow Channel to get out of the harbour at Bembridge IOW in a F9, could only make 1.5Kts groundspeed with 3.5 Kts hull speed,(tide was coming in) once I was clear and cracked off to a beam wind, I was making 6Kts under reefed headsail alone. I subsequently upgraded my engine from a !GM10 to a 2GM20 and in similar conditions a few years later was able to maintain 3.9kts over the ground with 5Kts through the water.

[MooGroc]

My boat's PO repowered from 29hp to 39hp. In 10+ years
I have never needed all that HP. If I were to repower, I'd go
down to 35hp or even less. The extra room in the engine room
would be welcome far more often than the extra hp.

[wingssail]

Our 18000lb 43' boat has a 34hp engine. We have not used full thottle with this engine. If the conditions are crummy (waves and headwinds) we sail, it is easier on us and on the equipment. If there are strong currents, such as in some of the channels in British Columbia, we time our transits.

However, your Hunter may not be quite as capable a sailing vessel or you may not wish to sail in those conditions (very few cruisers do). In that case you will enjoy the power of a 40 hp engine.

[boatman61]

Originally Posted by Martkimwat

If your speed through the water was 7.1kts, I would venture to say that you were already at hull speed for your waterline length, more power is not going to help you in that situation, you will just 'squat' more. Where it will come into it's own is in a short head sea that keeps knocking you back, you will accelerate back to your desired speed much quicker with more power. I remember trying to power in to the wind in a narrow Channel to get out of the harbour at Bembridge IOW in a F9, could only make 1.5Kts groundspeed with 3.5 Kts hull speed,(tide was coming in) once I was clear and cracked off to a beam wind, I was making 6Kts under reefed headsail alone. I subsequently upgraded my engine from a !GM10 to a 2GM20 and in similar conditions a few years later was able to maintain 3.9kts over the ground with 5Kts through the water.

You beat me to it.. I remember staring at the Needles from the same angle for almost 5hrs when caught out by a Spring ebb.. I was doing 6kts through the water..

[Searles]

The hp doesn’t matter it’s the torque that drives the vsl ,a small hp with high torque ond a high ratio g box correctly proped will do the job every time ,buying high hp poorly geared is a waste of money and boat space .⛵️⚓️

[Wotname]

Originally Posted by Searles

The hp doesn’t matter it’s the torque that drives the vsl ,a small hp with high torque ond a high ratio g box correctly proped will do the job every time ,buying high hp poorly geared is a waste of money and boat space .⛵️⚓️

Horsepower equals torque multiplied by rpm, divided by a constant.

Of course it is the prop that does the work and yes, it is important to get the prop (diameter, pitch and rpm) and gearbox ratio correct but engine hp and torque are mathematically related.

[Searles]

Hp is a number people use to compare the use of a horse to a internal combustion engine,all engines only produce rpm and a measurable twisting motion we call torque ,an electric motor produces most torque at minimum rpm ,like wise a steam engine,have successfully used a low hp high rpm 4K diesel at 4.5to one swinging a 4blade prop to easily drive a 60ft 20 ton motor sailer ,don’t be fooled by sales talk .⚓️⛵️

[HeinSdL]

Diesels are happiest operating at around 80% of their rated power. Ensures temperatures are where they should be, the combustion process is complete on each cycle with minimum harmful emissions.


So this is a problem for my 56hp 4JH3E which I rarely (if ever) use above 40% of its max power, not when cruising and not while manoeuvring. Only occasionally for a couple of minutes do I crank it up to full power just to adhere to the guidelines which recommend doing this. So two things: the excess power I have is probably a bit much (stern digging in, engine trying to push the boat passed hull speed), and running the engine at such a lazy pace for most of the time might be quiet, but not ideal for the engine itself.


I would go with the 'underpowered' engine for now. Running it hard most of the time is good for the engine (or at least certainly not detrimental) and to change it our for that very rare occasion you need a bit more power is not really worthwhile I would have thought.

[sailingabe41ds]

My 2004 Hunter 36 had Yanmar with 27hp at 14,000lbs
My 2015 Jeanneau 41DS has Yanmar with 40hp at 19,500lbs

Hunter 39 at 18,000lbs looks like more hp would have been a better fit but I would not let that deter you if its a good boat at the right price.

Take it out and see if you reach hull speed with 29hp. If you are full rpm and still have a knot to go....well, it is underpowered.

I never go full power except to clear the muffler/elbow....

IMHO
Abe

[nortonscove]

Depends on the conditions where you sail. I see more days of 25-30 knots than those with 10 knots and below so that might be an issue when beating into steep seas.

[bellablue]

Thanks to all for your comments and advice - this has been very helpful.