What hp to re-power a 38ft sailboat?

[s/v Jedi]

Originally Posted by thomm225

2 hp per pound displacement is what many experts say.....

I can get away with a 5 hp outboard on my 6600 lb displacement Bristol 27, but a boat near 20,000 lbs displacement is a different animal

So at 19,200 pounds he needs 38,400 hp?

4hp continuous rated output per ton sounds good to me.

[Bowdrie]

Forgot to add this to my last post.
In the kind of applications that most of us use, engines will not develop their rated power.
IIRC, the manufacturers compute the rating based on cool/dense/low humidity air, with cool fuel.
Stuck in a little compartment sucking-in high temp air that's of high humidity and having warm fuel can reduce output to a marked degree.

[PineyWoodsPete]

Originally Posted by MicHughV

fuel burn rate of marine diesels are sorta kinda well known, with some variables, but using the 40 hp engine as outlined previously, at cruise rpm, say 2,200 rpm, if your fuel burn rate at that rpm is about 0.6 gal (US), your engine is producing around 30 hp....which makes sense as that is about the 70% of the max. rpm rating

No, you're using the HP curve for your estimate, which is done at WOT for each reading on the curve, and represents the power need to brake the engine down to the specific RPM. In actual usage, we run at partial throttle, and the curve that represents this is the Prop Demand (PD) curve, which is vastly different. This represents the power absorbed underway by the prop at a specific RPM.The only input data needed data is your max actual WOT power picked off from your engines HP curve from your max RPM.

This is a calculated "cubic" function, in which the fraction of lower RPM's is cubed, and multiplied by your WOT power. So let's take 4000 RPM for my VW 165 HP TDi (121 kW), assuming my running WOT RPM is 4000, and run it under way at partial throttle 2000 RPM - half max RPM - and cube this difference: 1/2 x 1/2 x 1/2 = 1/8 (0.125) and multiply it times max power of 165 HP/121 kW = 20.6 HP/15.125 kW.

Or for the OP's 70% Revs, 0.7 cubed = 0.34 x 40 = 13.6 HP - only a third of full throttle HP Prop Demand. The utility of this curve is that it correlates well with fuel consumption, and it's totally independent of reduction gearing, prop size: only the WOT engine RPM and power there of the rig underway in calm conditions is needed.

Below, my engine's vastly different HP and PD curves:

[MicHughV]

Well, when you get right down to it, it's the torque curve that is important. HP numbers are always thrown around willy-nilly, but it's torque that provides the muscle, yet engine " torque" is rarely discussed in engine power wars.

[Tayana42]

Originally Posted by PineyWoodsPete

Your boat's hull speed, based on its 33' LWL, is actually 7.7kts, so you're way under it. If you could reach that w/ WOT of 3500 RPM and the full rated 40HP, your fuel burn would be near triple that @ 2500.

You are right of course. My error. My theoretical hull speed is 7.6 or 7.7 knots. And wide open at 3500 rpm I can probably do that with faster fuel burn. But I just don’t push the boat, the engine or the crew that hard.

[Tayana42]

Originally Posted by MJH

MJH here. What is the transmission ratio of your Tayana 42 and prop size?



~ ~ _/) ~ ~ MJH

Don’t have the transmission ratio at hand. The prop is a three bladed 18” maxprop.

Okay, found it. I think…
Gear reduction
Ahead 2.17 2.03:1
Astern 3.06

[valhalla360]

Originally Posted by donradcliffe

Another consideration is your fuel tankage. A 50 gallon tank will support 40 hp for 24 hours.

Sure, if you run the engine at 40hp output.

If in calm conditions, you may drop back to 15-20hp output and get more than double the efficiency...but if you do need it, you have 40hp on tap.

[MicHughV]

You run those little sailboat diesels at WOT for any period of time, you're asking for trouble.

[valhalla360]

Originally Posted by MicHughV

Well, when you get right down to it, it's the torque curve that is important. HP numbers are always thrown around willy-nilly, but it's torque that provides the muscle, yet engine " torque" is rarely discussed in engine power wars.

So all you need to do is put a deep reduction gear on a 1hp engine to generate 1000ft-lb of torque on the prop shaft and you can make hull speed in any conditions....?????

Torque determines if you can turn the prop shaft. If may turn slowly and thus speed would be slow.

It's HP that determines how fast the prop shaft turns and thus cruising speeds for a displacement boat.

Torque can have a little benefit maneuvering where you need to quickly change the rotational speed of the prop shaft but you almost never max out HP maneuvering.

[motion]

I have a yanmar 46hp in my IP38. Perfect size. I was in s running sea out of Dustin AL
The other night I needed all of the horses. Would not want any less. 25 ain't near enough.

[geoleo]

I would try to find a good rebuilt Perkins 4-108--then the switch is almost apples to apples + your tranny could be reused (or not).

[MicHughV]

in the 40 years or so that I've been afloat.....I don't think I've ever come across any sailor saying..." gee, I wish my boat had less power"....

[s/v Jedi]

Originally Posted by MicHughV

in the 40 years or so that I've been afloat.....I don't think I've ever come across any sailor saying..." gee, I wish my boat had less power"....

Well here’s the first one. I almost never go over half rpm’s and could get by with a smaller engine.

[malbert73]

4-107 is old and needs nearly 4000RPM to generate 40 Hp.
Again if you are on a tight budget feel free to rebuild it but newer Beta’s often redline at 2800 RPM which means you can extract more power at lower RPMs.

(As an aside it’s funny on these forums people often revere old engines like the 4-108/4-107 and speak of new lightweight “screamers” but I know of no new marine diesel that redlines at 4000 rpm)

I have Beta 50 on my T40. Too much engine and I deliberately overpropped to get cruising speed at lower, quiet, continuous RPMs. 7+ knots at 1800 RPMs

[jmh2002]

Originally Posted by s/v Jedi

Well here’s the first one. I almost never go over half rpm’s and could get by with a smaller engine.

Yeah, Dashew always specified quite big engines in his designs. But I guess you can motor at 10kn if you want to though? (NB: Jedi's boat is a 64ft slim monohull) And some people do want to do this...

Originally Posted by zstine

Modern advice is Wrong! Horsepower recommendations have increased significantly over time. My 1975 Tartan 41 is the same displacement as your boat (19k lbs ish). Originally it was designed by S&S to have 20hp.

zstine's example is at the opposite end of the scale, when sailboat engines really were 'auxiliary' power.

They weren't really meant for 'motoring' - they were meant for auxiliary use, getting in and out of port, on and off the anchor, and maybe some motoring in calm seas and no wind (but not for very long as fuel capacity was normally limited too).

The rest of the time you were expected to be... shock/horror... sailing!

Of course that is quite different compared to now, when people expect their (sailing)boats to be more capable, just like most other things in their life these days.

Horses for courses... (oh? pun intended or not? )