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Old 05-11-2015, 08:04   #1
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Are The New Sailboats Overpowered?

Hi all

i would mean...i was looking on some boat site an i noticed a lot of sailboat having overpowered engine like a 40-45 ft boat with 60-70 hp , 10-12 ton weight, and 50-55 ft 8about 15 ton) even with 120-150 hp...

i was thinking about my project (hope not too far away)

46 ft loa (14 meters), 4,5 meters wide (about 14 ft), steel hull, about 16 ton full loaded + 3 ton keel, total lets say 20 ton, 40.000lbs)

with brunton autoprops (21 inch diameter, calculaed with a beta 60 engine and tm 345 gearbox 2,47:1)


reaching an hull speed (about 6,5-6,8) i would need about 33 shaft hp


i calculated i need about 33-34 shaft hp for cruise speed (according to iso 8665, so net power, without alternator load, gearbox bearing drag).

i saw from the graph that for 33 hp

http://www.scandiesel.it/public/pdf/...A_60_CURVE.pdf

http://www.scandiesel.it/public/pdf/...A_75_CURVE.pdf

beta 60 1600 rpm almost 3 liter hour

beta 75 1100 rpm about 2 liter hour ( a bit more than half galoon per hour, really cheap)

the 75 is 3,6 liter max 2600 rpm, the 60 is 2,4 liter max 2700 rpm, both INdirect injection and naturally aspirated diesel engine, both kubota based


so in theory the bigger and more robust 75 would be better and even if it cost about 2000 euro more, i will re gain the money it just after one year due the difference in fuel consumption.

is there any problem using the beta 75 at such low rpm? (maybe after some hours of cruise a little run of 1-2 minutes at 2500 rpm to clean all the pipe...i read thousand of post and pages about it, but the autoprop will always load the engine at about 95%)


the use of the boat will be liveaboard and charter...so medium-heavy duty.

the beta marine sales agent tell me to go for the 60 without any dubt, the 75 would be a lot overpowered without any reason (even with the 60 i would have about 25 hp as reserve power for headwind, waves, ecc)

is it the idead of overpower a sailboat just a fashion to say to the neighbor..i have 10 hp more than you or is there any reason?
thanks all
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Old 05-11-2015, 09:04   #2
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Re: are the new sail boat overpowered?

I won't comment on the "what size engine is best" question but I did notice that the power chart that you posted is for "maximum power" at a given RPM. This chart will not be applicable to a marine engine driving a propeller. For that you need to use a power chart that has a "propeller curve".

In the absence of a "propeller curve" chart, one can use a formula to get close. Here is the (rough) math for your 60 hp engine producing 32 hp.

2200 RPM divided by 2700 RPM = .81.
.81 X .81 X .81 = .53.
.53 X 60 HP = 32

The above formula works for a fixed pitch propeller that is sized to allow the engine to just make max RPM. The use of a variable pitch propeller will render the formula inaccurate but the results will be more applicable than the max power chart that you posted.

Steve

P.S. Go for the bigger engine.
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Old 05-11-2015, 10:07   #3
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Re: are the new sail boat overpowered?

point any 30-50ft boat into a 3ft chop headwind and that calculation of engine hp vs hull speed chart goes out the window.

I really have never met a single sailor who thought they needed a smaller engine than they have.......... so if you are in the position to decide between 60 & 75hp then logic, not science would say go for the 75hp engine... after all, there are thousands of shorter and lighter boats with engines that size.

picking a prop = different question.
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Old 05-11-2015, 10:21   #4
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Re: are the new sail boat overpowered?

Although I am often a proponent of lower rpm diesel use. I think 1100 is too low. You may have gearbox ratio choices though to get it up a bit...?
My heavy fiberglass 47 footer (not as heavy as your steel will be though) had an 80 hp Mercedes diesel in it. It was plenty of power. Watch those calculation schemes though. I built a 31 footer once and they said the 17 hp Volvo would be fine. It overheated when I neared max hull speed in anything but a light chop though.
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Old 05-11-2015, 11:07   #5
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Re: are the new sail boat overpowered?

thanks all for the answer

panope, did you mean that to make the propeller absorb 32hp i would run the engine at 2200 rpm? (the small one)

yeap mystic83...if 32 hp is for 6 and half knots in flat sea i guess 5 kts in bad sea would need even more tha 40hp...

in theory the autprop will adapt itself to load the engine, no matter what rpm is reached...is a fact of turque also (turque x rpm =hp, if you use this formula to compare the 2 engine, the bigger is at least 15% more efficient, ah and the bigger has 30% more turque and...is 98mm x 120mm..so long and efficient stroke, while the smaller 87 x 102..less bore/stroke ratio..)

i really can't understand why both the english division and the italian suggest me to the 60 all the way...do they need to seel the old of them before get a new engine line ?? i was 100% for the bigger but all this against it propaganda make me think...maybe just marketing..i hope

cheechako yes i could get 2,9:1 in stead of a 2,47:1, more turque, lower rpm at the shaft, bigger prop, always autoprop, more efficient nice idea your's one

how heavy is your at full load? wich brand / model is it?
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Old 05-11-2015, 11:39   #6
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Re: are the new sail boat overpowered?

cheechako yes i could get 2,9:1 in stead of a 2,47:1, more turque, lower rpm at the shaft, bigger prop, always autoprop, more efficient nice idea your's one

how heavy is your at full load? wich brand / model is it?


It was a Passport 47, 34000 lbs. net. No longer have it.
However, my Tanton 44 only had a Perkins 4-108 in it (51 hp) and it worked very well.... and fuel efficient.
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Old 05-11-2015, 12:23   #7
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Re: are the new sail boat overpowered?

Quote:
Originally Posted by Cheechako View Post
cheechako yes i could get 2,9:1 in stead of a 2,47:1, more turque, lower rpm at the shaft, bigger prop, always autoprop, more efficient nice idea your's one

how heavy is your at full load? wich brand / model is it?


It was a Passport 47, 34000 lbs. net. No longer have it.
However, my Tanton 44 only had a Perkins 4-108 in it (51 hp) and it worked very well.... and fuel efficient.
thanks.

i was looking at the tanton yacht series...your's one has only one mast alright? do you have any experience with dual mast and dual mainsail yacht?
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Old 05-11-2015, 12:28   #8
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Re: are the new sail boat overpowered?

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Originally Posted by stefano_ita View Post
panope, did you mean that to make the propeller absorb 32hp i would run the engine at 2200 rpm? (the small one)
Yes.

Unfortunately, Beta marine does not publish a "Propeller power curve" for this engine.

Here is a power chart for a similarly sized (but higher rpm) Yanmar engine that shows both the theoretical power that the engine is capable of producing (upper most curve) and the power that the engine is producing with a typical boat propeller load (lower curve).

It is the "Propeller curve" that is relevant to us boaters. The "Maximum output" curve would be beneficial if this engine was installed in a road vehicle trying to climb a steep hill (for example).

Steve

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Old 05-11-2015, 12:33   #9
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Re: are the new sail boat overpowered?

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Originally Posted by stefano_ita View Post
thanks.

i was looking at the tanton yacht series...your's one has only one mast alright? do you have any experience with dual mast and dual mainsail yacht?
No I had no experience with the Cat Ketch version of the Tanton. The Cutter I had was probably the best design I ever had though. It was basically a canoe, very long waterline. Driven easily and great interior design too. Most those boats are getting pretty long in the tooth, and were not superbly constructed either unfortunately. Although some Tanton designs are out there built in Europe or US I think... may not be true of those.
There was a 44 cat ketch up here in the PNW that had been a bit neglected maybe a year or 2 at most ago. $10k! One mast missing. Engine bad but used Yanmar 53 hp with it!
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Old 05-11-2015, 12:39   #10
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Re: are the new sail boat overpowered?

thanks panope..

i was thinking..in theory with the autoprop shouldn't i use all the power delivered at a certain rpm? meaning that the propeller power curve pratically will be the same of the delivered (and available) from the engine? because as soon as i go faster, with the same power/rpm, the propeller will get a longer pitch until a max speed for that power delivered is reached and stop raising the pitch...

or am i all wrong because the propeller power curve is also due for the 35-40% slip of the prop inside the water?

thanks
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Old 05-11-2015, 12:52   #11
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Re: are the new sail boat overpowered?

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.
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Old 05-11-2015, 12:57   #12
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Re: are the new sail boat overpowered?

Stefano: The Autoprop will certainly absorb more power that the "propeller curve" estimate. However, that new curve will likely fall well below the "max power" curve.

A vairiable pitch propeller can only be pitched so far before the blades "stall" and/or cavitate.

If an extremely large, variable pitch propeller was used, it would be possible to match the "max power" curve in a marine application. However, this would be such a large propeller that motoring efficiency during normal operations (not pretending to be a tugboat) would suffer. Not to mention the increased drag of an over-sized propeller when sailing.

Also, I believe theses engine are not designed to operate along the "max power curve" on a continuous basis.

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Old 05-11-2015, 13:18   #13
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Re: are the new sail boat overpowered?

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Stefano: The Autoprop will certainly absorb more power that the "propeller curve" estimate. However, that new curve will likely fall well below the "max power" curve.

A vairiable pitch propeller can only be pitched so far before the blades "stall" and/or cavitate.

If an extremely large, variable pitch propeller was used, it would be possible to match the "max power" curve in a marine application. However, this would be such a large propeller that motoring efficiency during normal operations (not pretending to be a tugboat) would suffer. Not to mention the increased drag of an over-sized propeller when sailing.

Also, I believe theses engine are not designed to operate along the "max power curve" on a continuous basis.

Steve

i think i got it this time


would you get the 60 with 2,47:1 gear and 21" diam prop

or the 75 with 2,9:1 (or 2,77:1) gear and maybe 22-23 prop?
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Old 05-11-2015, 14:18   #14
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Re: are the new sail boat overpowered?

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Originally Posted by stefano_ita View Post
i think i got it this time


would you get the 60 with 2,47:1 gear and 21" diam prop

or the 75 with 2,9:1 (or 2,77:1) gear and maybe 22-23 prop?
Any of those choices will work fine but I would go for the larger engine. The bigger propellers are great provided the propeller aperture is large enough to accommodate. Those gear ratio's seem plenty low especially as these Beta engines operate at fairly low RPM (compared to Yanmar).

I have what some people would say is a little too much power: 40 Hp (yanmar), 15,000 lb boat or 375 lbs per 1 horsepower. 18" fixed pitch prop. I normally cruise at about 30% power/2,600 RPM and never (yet) more than about 50% power/3,000 RPM. The engine redlines at 3,800 RPM and would drive me nuts to operate at that speed for anything longer that a minute or two. However, having the extra power on tap for close quarter maneuvering or some very high wind situation can be a real lifesaver.

On edit: My boat motors into 30 knot winds like crazy but if I did not have the wheel house, scuba gear would be needed for the spray!

Many persons believe these engines must be operated at a high percentage (75+) of their rating in order to prevent premature failure.

I, on the other hand (along with the manufacturers of these engines) believe there is no problem running these engines continuously at lower power settings provided there is at least some propeller load placed on them.

Steve
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Old 05-11-2015, 14:30   #15
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Re: are the new sail boat overpowered?

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beta 60 1600 rpm almost 3 liter hour

beta 75 1100 rpm about 2 liter hour ( a bit more than half galoon per hour, really cheap)

the 75 is 3,6 liter max 2600 rpm, the 60 is 2,4 liter max 2700 rpm, both INdirect injection and naturally aspirated diesel engine, both kubota based
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.
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