What hp to re-power a 38ft sailboat?

[nwdiver]

C&C 38, 35hp Beta Marine, moves me at 3 kn against a 6-7 kn tide…….buy a few extra horsepower not allots extra horsepower…….

[Jim Cate]

Originally Posted by nwdiver

C&C 38, 35hp Beta Marine, moves me at 3 kn against a 6-7 kn tide…….buy a few extra horsepower not allots extra horsepower…….

So, that's pretty impressive: 3 kts made good against a 6-7 kt current... that's 9 or 10 knots through the water, well above your hull speed.

Or did you mean 3 kts through the water? If so, the speed of the current has nothing to do with the achievement. Sea state and opposing wind speed do matter, and there some extra power could speed things up for you.

Jim

[sinnerman]

The Catalina 30 I sail uses a 21-23 HP, 3 cylinder diesel.

The boat is 25' LWL and 10,500 lbs. empty. Modified fin keel and spade rudder so not a lot of drag.

Your boat has almost twice the displacement of mine. The 4-107 specs per internet state it produces 40HP. 25 HP is too low. Maybe OK if sailing on a lake, but not enough going against any current or chop. The engine would be struggling most of the time.

The Betamarine site recommends a Betamarine 35 (or 38) for the Tartan 37 which is almost same length as your boat and uses the 4-107 engine. (Note the Tartan 37 has LESS displacement than yours.)

Web page: https://betamarineusa.com/portfolio/...r-replacement/

Maybe the dealer is dyslexic and mistakenly said " 25" instead of "35"...?

Ask Betamarine directly for their recommendation

cheers

[Hoodsail]

Not the size of your boat, but I displace approx 16,000, LWL about 27 ft (LOA 35.5 ft). Diesel is Westerbeke W-30, 25 hp. I run at 2000 rpm, 1/2gal/hr, gets me 5 ish knots heading into small craft advisory winds/seas. In fairly calm conditions, 2000 rpms gets me 6 ish knots. Never really used hull speed as a goal. She's a sail boat, speed under power is KNOT its design. had to do that... Not certain differences in size, weight, cost, but wouldn't argue against 30 hp..

[Adelie]

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

It looks like you are using the curve for what the engine is capable of producing at a given RPM. You should be using the prop curve which is what the prop is capable of absorbing at a given RPM.

Without seeing the engine and prop curves I can tell you that that engine is producing about 11hp at that fuel demand. Typical diesel produces about 18hp/gal/hr regardless of whether it is naturally aspirated or turbo-charged.
18hp/gal/hr x 0.6gal/hr = 10.8hp.

[MicHughV]

I learned to use 22 hp/gal/hr....can't remember where the 22 came from...not that it makes much difference...then added some losses....wasn't trying to get an exact number here, simply trying to demonstrate that if 30 hp is required to move a boat at hull speed, a 40 hp diesel will be required.

[PineyWoodsPete]

Originally Posted by Adelie

It looks like you are using the curve for what the engine is capable of producing at a given RPM. You should be using the prop curve which is what the prop is capable of absorbing at a given RPM.


Without seeing the engine and prop curves I can tell you that that engine is producing about 11hp at that fuel demand. Typical diesel produces about 18hp/gal/hr regardless of whether it is naturally aspirated or turbo-charged.

18hp/gal/hr x 0.6gal/hr = 10.8hp.

The big mystery to me here is the prop demand curve - a purely mathematical cubic relationship construct in which the fraction of lower RPM vs your max RPM is cubed. This dictates that halving RPM underway from your full-throttle max. reduces PD to 1/8x the max HP at your top RPM - 1/2 cubed - not just 1/2x as our brains would have it. But the fuel consumption curve does follow it.

[Adelie]

Originally Posted by MicHughV

I learned to use 22 hp/gal/hr....can't remember where the 22 came from...not that it makes much difference...then added some losses....wasn't trying to get an exact number here, simply trying to demonstrate that if 30 hp is required to move a boat at hull speed, a 40 hp diesel will be required.

I got 18gal/HP/hr from one of John Vigor’s books a decade or so ago.

Recently while arguing prop curve vs hp vs engine curve with somebody I checked the numbers for several engines and it varies from 17-20, average about 18.

[Adelie]

Originally Posted by PineyWoodsPete

The big mystery to me here is the prop demand curve - a purely mathematical cubic relationship construct in which the fraction of lower RPM vs your max RPM is cubed. This dictates that halving RPM underway from your full-throttle max. reduces PD to 1/8x the max HP at your top RPM - 1/2 cubed - not just 1/2x as our brains would have it. But the fuel consumption curve does follow it.

Yep, that cubic thing is pretty brutal anywhere over S/L-1.0.

[MicHughV]

Using a one line formula does not really address the issue.

I can recommend the " Propellor Handbook" authored by Dave Gerr. I know that there are other books on the subject as well, but this particular book appears to cover the subject well.

It is a very comprehensive, very detailed, very technical read and study about boats, propellors and engines. It will require one to read this book several times to gain an understanding of the science and theory behind marine propulsion.

To arrive at a solution for any given boat, requires at least a half dozen pages of calculations.

I've gone thru' this exercise several times. It is a mental challenge.

I'm retired now, but am a marine engineer by profession, and can tell you, that the above book is a must read for anyone that has an interest in this subject.

There isn't really any simple way to arrive at any meaningful solution. This thread has provided a half dozen quick and easy formula's to arrive at a probable solution, but it really requires a thorough study.

[Gripfast]

I am repowering my 36 ft beneteau 361 and beta recomended thier 30. Original engine was 3gm30f with 27 hp. Beta said the yanmar didnt realy provide 27 hp in real world conditions and his will produce as advertised. We will see. I wanted a 35 but they said it would not fit without modifications to the casework on the boat.

I ordered mine last week and they said it will be the end of May before it comes in. I would be interested in hearing if others are getting deleivery dates that far out?

[DDabs]

Originally Posted by PineyWoodsPete

Don't know how you figured that, but my 165 HP/121kW diesel burns over 8 gph/32.5 L/hr WOT on it's digital readout, or about 2 gal/hr for each 40 HP...

OK... that's a little different than going cruising speed on a 40 HP diesel motor.

[zstine]

Originally Posted by Rotten Ricky

David,

There are lots of variables in choosing the "right" engine but please do not choose one based on absolute minimum cost and fuel savings. Your life and maybe other's lives could depend not only on the engine running on cue but it actually having the power to get you out of danger. I have been there!

We all tend to think of perfect days in still waters but those are not the days to consider. Having had to bash into head winds and big seas to get away from a lee shore, and having had to tow a boat-load of people away from rocks, please do not skimp on engine size. These times do not allow you to get anywhere near hull speed - you need grunt!

A bigger engine will likely give you:

-the power to get you out of danger
-longer life and reduced maintenance costs
-a small increase in engine weight
-a more robust transmission - a frequent failure point on boats.
-probably about the same fuel economy as a smaller but harder-working engine
-a smoother engine if it has more cylinders
-a slightly slower-running engine with the right prop
-better resale value - nobody with knowledge wants an under-powered boat for all these reasons.
.........

Cheers, RR.

A bigger engine will have a shorter life due to not running it at full load. Diesel engines hate low load / half throttle running, which is what happens when you are overpowered.

Maintenance cost will likely be more than a smaller engine.

Fuel consumption will likely be higher because you will want to run the engine hard periodically to get cylinder temps up. A bigger engine will certainly suck more fuel and you are likely to cruise at a more efficient (~80% throttle) with the 30hp than a 50hp since you don't need all that much power to get to hull speed in most conditions.


I agree with your other bullets.. though I would go with a smaller, less expensive, lower maintenance engine IMO.

[kenbo]

Originally Posted by MicHughV

I learned to use 22 hp/gal/hr....can't remember where the 22 came from...not that it makes much difference...then added some losses....wasn't trying to get an exact number here, simply trying to demonstrate that if 30 hp is required to move a boat at hull speed, a 40 hp diesel will be required.

Perhaps you learned 22hp/gal-hr using a Canadian gallon. A Canadian gallon is 20% bigger than a US gallon.

[valhalla360]

Originally Posted by zstine

A bigger engine will have a shorter life due to not running it at full load. Diesel engines hate low load / half throttle running, which is what happens when you are overpowered.

Maintenance cost will likely be more than a smaller engine.

Fuel consumption will likely be higher because you will want to run the engine hard periodically to get cylinder temps up. A bigger engine will certainly suck more fuel and you are likely to cruise at a more efficient (~80% throttle) with the 30hp than a 50hp since you don't need all that much power to get to hull speed in most conditions.

As long as you don't go crazy, the engine will get up to temp and it will last a very long time. So yeah, if you put in a 500hp diesel, what you say makes a lot of sense.

If you upgrade from 40hp to 60hp, not so much.