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Old 05-11-2015, 14:42   #16
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Re: are the new sail boat overpowered?

My impression is that 4 cylinder diesels have much less vibration than 3. If I were comparing two engines, one 3, and one 4 cylinder, I would tend towards the 4 even if that meant a little extra fuel consumption.

In a car, 3 cylinder engines would be considered unusable without a balance shaft, but none of the marine diesels I know of use one. Would love to be wrong about this, would be handy for the eventual re-power.
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Old 05-11-2015, 15:00   #17
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Re: are the new sail boat overpowered?

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Originally Posted by sailnow2011 View Post
LOL
Those numbers are very wishful thinking.
We have a 63HP and burn 1.2GPH or 5 litters.
Are we OP? Probably. If I had to repower, I would do a 3 cylinder.
The boat shipped with a 55HP. We are 48' and 17-19ton.

thanks..
i calculated only the theorical engine fuel consumption...in theory with more turque and less rpm= more efficient...


your boats brands\model? gearbox ratio? prop size?

quote the last message...3 cylinder...more vibration, unbalanced...naa..



went this summer with a stefini 36ft with 76 hp....12 ton...i don't know else data but was about 3 liter per hour according to the owner...rpm counter was broken
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Old 06-11-2015, 00:43   #18
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Re: are the new sail boat overpowered?

I had 51 HP on a 33' boat.
Loved the extra power in a strong wind or a strong current and when I got stuck on a sand bank in the Bahamas: Full reverse power would solve all my problems..

The down side was the big 3 blade prop, it cost me 1/2 knot or more when sailing.
Should-have, could-a-have a Maxi or Auto Prop, but never got got around to it or the $3k it would have cost me.
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Old 06-11-2015, 02:34   #19
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Re: are the new sail boat overpowered?

If you only motor in calm conditions with no wind, a smaller engine is fine.

Your premise is flawed thinking the lower HP will give you the same performance in rough seas or a strong headwind.

The fuel savings will be negligible in any case. You have to match up the actual output HP with what the prop will take. When you do, there will be negligible difference. More importantly, if you are claiming to be a sailor who trys to minimize engine use, any fuel savings will be negligible anyway. If you are normal and use the engine a fair bit, you will really appreciate the extra power and in reality, there will be negligible fuel cost difference.

I think you see smaller HP engines on older boats mostly because big HP engines were much bigger in the past. To fit a bit slow turning naturally aspirated diesel in a 35' boat would have taken up too much space. With modern faster turning and turbo engines, it's easier to put out more HP in a smaller package.

To give you a flavor for it, my old pickup (1992) has a 7.3 liter diesel that generated a whopping 180hp. The latest version is 6.7 liters (or there abouts) and puts out more than twice the HP and drastically more torque.

1100rpm sounds low and I suspect it is for anything but idling around the dock. I suspect there is a math error in your calcualtions or you are using an overpitched prop.
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Old 06-11-2015, 09:22   #20
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Re: are the new sail boat overpowered?

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Originally Posted by CSY Man View Post
I had 51 HP on a 33' boat.
Loved the extra power in a strong wind or a strong current and when I got stuck on a sand bank in the Bahamas: Full reverse power would solve all my problems..

The down side was the big 3 blade prop, it cost me 1/2 knot or more when sailing.
Should-have, could-a-have a Maxi or Auto Prop, but never got got around to it or the $3k it would have cost me.

ok...i think this is enough...

i guess they want to sell / finish stock of 60hp engine...

i can confirm the autoprop is 3k but i guess it will be an investment over long period
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Old 06-11-2015, 09:28   #21
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Re: are the new sail boat overpowered?

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If you only motor in calm conditions with no wind, a smaller engine is fine.

Your premise is flawed thinking the lower HP will give you the same performance in rough seas or a strong headwind.

The fuel savings will be negligible in any case. You have to match up the actual output HP with what the prop will take. When you do, there will be negligible difference. More importantly, if you are claiming to be a sailor who trys to minimize engine use, any fuel savings will be negligible anyway. If you are normal and use the engine a fair bit, you will really appreciate the extra power and in reality, there will be negligible fuel cost difference.

I think you see smaller HP engines on older boats mostly because big HP engines were much bigger in the past. To fit a bit slow turning naturally aspirated diesel in a 35' boat would have taken up too much space. With modern faster turning and turbo engines, it's easier to put out more HP in a smaller package.

To give you a flavor for it, my old pickup (1992) has a 7.3 liter diesel that generated a whopping 180hp. The latest version is 6.7 liters (or there abouts) and puts out more than twice the HP and drastically more torque.

1100rpm sounds low and I suspect it is for anything but idling around the dock. I suspect there is a math error in your calcualtions or you are using an overpitched prop.

you are right, i recalculate with the formula of the other user and

beta 60 2200 rpm 6 liter hour!!
beta 75 1400 rpm 3 liter hour!

both 33 hp net to the prop shaft (according to iso 8665)

the prop will be autoprop, so it does self regulate the pitch accordin to the available power and the speed of the water on it, it absorb always about 80-85% of the available power at the prop shaft, so the engine will be always loaded to 85-90% (alternator, gearbox drag, bearing, ecc)

am i crazy or what else?

i would go for the 75 thanks....i don't want to cross all the ocean screaming at almost the WOT (2700 rpm for the 60 )
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Old 06-11-2015, 10:37   #22
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Re: are the new sail boat overpowered?

I'm no engineer however have owned a number of boats with as small as a 1cyl lister diesel on a Crealock 32 to a 80hp Lehman on a Huges 40.

I'll say when in doubt go bigger. That Huges could motor into anything so rather than being stuck in Luperon waiting for the elusive "weather window" make one.

Current 40' we have a 50hp Yanmar with a nice simple 17" 3 blade fixed and feels just about right.

Best of luck! Great problem to have (deciding which engine you want)
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Old 06-11-2015, 11:03   #23
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Re: are the new sail boat overpowered?

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I have 24hp for 11,200 lbs, which is about 470 lb/hp.

The 40-45 boat weighing 12 metric tons would have about the same ratio with a 60hp engine.

I have always heard that 500lb/hp is a good rule of thumb.

In my case the above ratio gets me hull speed at 1/2 to 2/3 throttle, leaving some in reserve for bad weather - headwind and chop.

Except that Universal undersized the heat exchanger so that it wouldn't go over 2/3 throttle without overheating. It's fixed now - installed a 3" instead of 2" version.
We have pretty much the same thing: M25 Universal (Kubota) in a 12,000 pound (nominal, most say it's more like 14 to 16,000 fully loaded), 21 hp. It had the 2 inch HX which we upped to the three inch HX, no problems in 17 years with this boat (bought it in 1998, it's a 1986 model, 2970 engine hours this week).

Mark's numbers sound right.

I have cruised with identical boats with 23 hp and 30 hp engines, made little difference in performance motoring over headwinds & chop.

We have a 100A alternator with external regulation and use Small Engine Mode to control the alternator when the house bank is depleted. Once mostly full, we go to full field on the alternator (i.e., turn off SE Mode) and have never had any issues, including the currents at the Golden Gate Bridge).

We sip less than 0.5 gallons per hour.
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Old 06-11-2015, 11:53   #24
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Re: are the new sail boat overpowered?

Modern engines are lighter than the older ones on a power / weight basis, especially turbo diesels with even better power weight ratios. You should consider your engine weight as well as its power.
Then there are accessories like a deep freeze compressor and maybe a 100 amp alternator. Even if your deep freeze / fridge is electric it still needs to get the same power from the alternator. So by the time your power gets to the prop you have lost some from your calculations. A 100 amp alternator can absorb 2 HP plus inefficiencies. Both the water pump and the gearbox are absorbing power.


I am perfectly happy to motor into a headwind if necessary. If you double the apparent wind speed there is 4X the wind resistance. A 40 knot headwind will have 4X the wind resistance of a 20 knot headwind, and that's not factoring in boat speed.


Consider having all your accessories working and you need to motor to shelter into a 40 knot headwind against a 2 or 3 knot tide and into a steep chop, you will be thankful for all the power available. Now imagine in that 40 knot headwind there are gusts of 50 knots. As far as the tide is concerned, we are considering "water speed". Your water speed can't normally be higher than your hull speed.


A 30 / 40 knot headwind against the tide is something I not infrequently experience. Then it is almost impossible to make much headway sailing as the tide sweeps us back to almost the same place on each tack.


But if when motoring the headwind is slowing you down to 4 knots water speed and the tide is 2 knots you will only be making headway at 2 knots over the ground or on your GPS. If with extra engine power you can go into that headwind at 6 knots water speed you will be making double that or 4 knots GPS speed and get to shelter twice as fast and in more safety.


You will probably have a spray dodger and perhaps a Bimini. Maybe a dinghy on deck or on davits. They are all increasing the wind drag.


I thought the sail drive motor on my (smaller) boat had ample power, and usually it has. However I sometimes wish I had gone to the next size up as the cost difference was not all that great with the installation, propellor, exhaust, etc costing the same as the smaller motor. The extra weight would not have been more than an extra battery. Then on those windless days I could have motored at hull speed using lower RPM with the greater smoothness of another cylinder; still having the option of going slower and having the same fuel consumption as the smaller motor. Your consumption is only related to the power you are actually using.



If you are throttled back and using 30 HP from a 50 HP motor your fuel consumption will be similar or better than a 30 HP motor going full throttle.
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Old 06-11-2015, 13:01   #25
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Re: are the new sail boat overpowered?

Speaking as a marine engineer I have a couple of comments/observations:1) Diesel engines are designed to work with a load. Marine Diesel Engineering 101 states that a diesel's optimum operating condition should be approximately 80% of designed maximum continuous load. Extended periods at lower loads can cause several problems. The one most common on ships is oil "pumping", unburned oil and hydrocarbons into the exhaust, causing more pollution, and a potential fire hazard when the engine is finally loaded up, after maneuvering, and the exhaust temperature heats up, (of course a wet exhaust eliminates this fire hazard in a non-turbo unit). Because of this I would agree with the Beta/Kubota rep and go for the 60hp unit.
2) MarkSF stated that a 3 cylinder engine has more vibration. Incorrect. In-line 3 cylinder 4-stroke cycle engines have perfect primary balance. There is a very slight second order axial vibration, but that is all. This is a reason triples are gaining favor in motorcycles. In-line 4 cylinder 4-stroke cycle engines are inherently unbalanced and require 2 balance shafts to smooth out the inherent vibrations. For in-line 4-stroke cycle engines: 1 cylinder, unbalanced. 2 cylinder, unbalanced. 3 cylinder, balanced. 4 cylinder, unbalanced. 5 cylinder, unbalanced. 6 cylinder, perfect balance. Modern engine design, with the used of computers, allows for almost any configuration to be smooth with the addition of balance shaft(s), various "V" angle configurations, and/or differing crank throw angles. (And 2-stroke cycle engine configurations change everything above)


Our boat, 40' lod, 34' lwl, 12' beam, 25,000-30,000 lbs loaded displacement. Westerbeke 40, (Perkins 4-108, 40hp) power with a 3 blade 16" Kiwi-Prop. Adjusted pitch on prop to maximum. 6 1/2 knots @ 2000 rpm, @ .7 gph. Engine cannot quite make redline with this pitch, but I am happy with the set-up. The engine is relatively vibration free and purrs along at ~2050 rpm at hull speed.
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Old 06-11-2015, 13:16   #26
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Re: are the new sail boat overpowered?

"2) MarkSF stated that a 3 cylinder engine has more vibration. Incorrect. In-line 3 cylinder 4-stroke cycle engines have perfect primary balance. There is a very slight second order axial vibration, but that is all. This is a reason triples are gaining favor in motorcycles. In-line 4 cylinder 4-stroke cycle engines are inherently unbalanced and require 2 balance shafts to smooth out the inherent vibrations."

Here is an interesting article on 3 vs. 4 cylinder engines :

Downsizing of Diesel Engines: 3-Cylinder / 4-Cylinder

I quote : "the vibration levels are higher for the 3-cylinder versus the 4-cylinder, without additional balance shaft(s)."

The second order rocking motion with a 3 cylinder is not very slight, it is considerable.

In this case, for a marine engine, we are comparing the two engines both without balance shafts.

For a 4 cylinder engine, the vibration is considerable at idle, lowests in the midrange, and gets more severe at higher RPM. Not a bad tradeoff for a marine diesel.
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Old 06-11-2015, 13:31   #27
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Re: are the new sail boat overpowered?

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Speaking as a marine engineer I have a couple of comments/observations:1) Diesel engines are designed to work with a load. Marine Diesel Engineering 101 states that a diesel's optimum operating condition should be approximately 80% of designed maximum continuous load. For in-line 4-stroke cycle engines: 1 cylinder, unbalanced. 2 cylinder, unbalanced. 3 cylinder, balanced. 4 cylinder, unbalanced. 5 cylinder, unbalanced. 6 cylinder, perfect balance.


Our boat, 40' lod, 34' lwl, 12' beam, 25,000-30,000 lbs loaded displacement. Westerbeke 40, (Perkins 4-108, 40hp) power with a 3 blade 16" Kiwi-Prop. Adjusted pitch on prop to maximum. 6 1/2 knots @ 2000 rpm, @ .7 gph. Engine cannot quite make redline with this pitch, but I am happy with the set-up. The engine is relatively vibration free and purrs along at ~2050 rpm at hull speed.
I absolutely agree with all that. You only need to look at an inline 6 cylinder engines 153624 firing order to see the balance of the power strokes. Unfortunately we often need to compromise on cylinder numbers.

I also have a 16" Kiwi Feathering Prop and fine tune the pitch to achieve less than maximum rpm to maintain some load. Max power is usually at a little less than max rpm anyway. The amount of coarser pitch depends on the boat and its motor. It's very easy to change pitch with maybe a half turn on each of the Kiwi blade's pitch screw.

Incidentally I feel the Kiwi has less sailing drag than my previous folder as the blades "swim" with the water flow. Reverse is even greater than a fixed 3 blade prop as the Kiwi goes into a coarse pitch in reverse. There is some maintenance needed occasionally, involving greasing and making sure the reverse rollers are free.
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Old 06-11-2015, 14:10   #28
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Re: are the new sail boat overpowered?

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Incidentally I feel the Kiwi has less sailing drag than my previous folder as the blades "swim" with the water flow.
Could you please explain this statement?

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Old 06-11-2015, 14:27   #29
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Re: are the new sail boat overpowered?

Have a few comments you might find useful. I fitted a Beta engine a few years ago and had similar dilemmas about size. The way I did it was to start at the other end and work out, with the prop engineer, what the ideal prop would be. Then check the shaft size and bearings where adequate and finally get the right engine to stick on the front of it all. Worked very well and makes more sense than trying to start with the engine. I finished up with a slow reving 4 cylinder driving a big prop and it worked very well. At 1,200 revs she motor sails at hull speed very economically in a slight sea (up to 3ft) but will also drive into heavy weather well enough to be able to hove too under engine in a gale (10ft+ sea state & 35kn wind), a very useful bonus for larger engines. I am running at slightly over 5hp per ton. (5-7hp/ton used to be rated as a 'motor sailor')
The first question the prop engineer asked was "what do you want the boat to do" because if you just want to motor in and out of harbour it is a very different set up to wanting to plug into an offshore gale for a day.
That would suggest that for 20 ton you would want 100hp+. What governs the load the engine faces is wind drag, waves and weight. For ocean cruising your 60-75hp may be a bit light and run out of steam in anything above 30kn wind speed and probably not make hull speed in much over half that.
The other thing I noticed is that you are proposing a 3 ton keel on a 16ton hull. That is a ballast ratio of under 20%. For a typical ocean cruiser I would expect to see around 40% otherwise she will be very tender or mainly reliant on form stability and have a massive neg stability index (ie capsize and she stays there!). You may want to check this as it adds several tons to you displacement.
I chose a fixed blade (15" on a 28ft water line) as I wanted maximum efficiency and reliability, I particularly don't like the lack of astern thrust for more folding props (or the cost!). The other thing I have found is that the additional drag from a fixed blade on a boat with a full keel or substantial skeg with the prop running in an aperture is negligible. Folding props are excellent on lightweight shallow hulls with spade rudders and a P bracket as they have very low drag factors anyway. On a heavy cruiser with a substantial keel you will be looking at small fractions of a kn difference.
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Old 06-11-2015, 16:29   #30
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Re: are the new sail boat overpowered?

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[B][I].
However, my Tanton 44 only had a Perkins 4-108 in it (51 hp) and it worked very well.... and fuel efficient.

The Perkins 4-108 Maximum Intermittent Rating is 46 hp@3600 rpm. Engines should not be run at max for more than 1 hour either. You were running the gut's out of your engine
Maximum continuous Rating is 37hp @ 3000rpm for the 4-108. I run our 4-107 with 1 less hp at 2450 max. What do you suppose that hp is?
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