Is 90Hp enough?

[Rivers & creeks]

Looking at a motor sailor yacht that we will be motor a lot more than sail. She has a 90HP gardner, weighs 30 tons and is 53 foot long. The hull profile is long keel displacement.

the question is, with the right prop and that engine, will she reach hull speed and more importantly, maintain it in a bit of a sea?

Thanks for your help.

Simon

 

[jfm]

This is kinda guesswork, but 90hp seems on the small side for 30t displacement to reach 10kts. 7kts is more like a cruise speed on that engine.

As a very poor but just about relevant comparison my boat needs about 250hp to do 11 knots displacement speed cruising. Ok it is a totally different (planing) hull with flat transom, twin shafts and rudders to drag thru the water, bit less displacement than yours (26t) but those factors don't account for a 90 vs 250hp differnece and my comparison is valid so far as the main factor -speed- is concerend. So I reckon 90 is on the light side...

I guess only way you will know for sure is sea trial, if that's practical.

Happy to hear other comments/stand corrected, etc.

 

[MapisM]

10kts is well above hull speed for a 53' - as it is 11kts for a 58', on the other hand.
I bet you wouldn't need more than 150 hp at 9kts, which is more likely to be the full displacement speed for your hull.
But I agree that 90hp is pretty much on the light side, considering also that Simon talks of using the boat it in a bit of a sea.

 

[Nautical]

Agree with others but bear i mind if its an 6 LXB it has mighty grunt and 10.5 litre puts out a considerable amount of torque.

Doubt you would reach that top end though more like 7-8knts if its the five or four cyclinder thats a bit weedy for that size of boat.

 

[ccscott49]

My boat is a 57' motor sailer, 48 toins and has two 135 hp, engines, she will make her hull speed, but just and will hold it in a sea, I think 90hp is on the low side, I would have expected her to need at least 150hp. Which boat are we talking about and is there any way we could see it? Wheres it for sale, online?

 

[ValleyForge]

Might help with a pic of craft & a bit about the engine. eg a sixer or a single bulb jobby ? you know, the one you start by lighting a blowtorch first /forums/images/graemlins/grin.gif

 

[Rivers & creeks]

Thanks for your comments. I also thought that 90hp was a bit on the small side; it's the same sized engine as we saw on a 48 foot MFV. I can't find a picture of the underwater hull profile, but this is an almost sister ship.

 

[jfm]

"Well above"? What formula do you use Mapis? I thought the accepted ready-reckoner to get hull speed in knots was 1.5 or 1.4 x sq root of w/length in feet. My 58 has a w/line length of say 53 feet, sq root is 7.3, hull speed 11 ish knots. That works right in practice to - above 11kts she starts to burn much more fuel climbing up to the hump. Where am I going wrong?

 

[jfm]

That's one of those Jeremy Clarkson not-understnading-the- laws-of-physics points Trev )) This question is all about power, not torque. If his 6LXB is putting out zillions of nm torque but is only 90hp, it MUST be turning at a low rpm. Laws of physics. Hence (and assuming the right propellors/gearboxes) it aint gonna make the boat go any faster than a 90hp outbord with lots of rpm and little torque

 

[tinkicker0]

Yep the bit you need is the horsepower of the engine. Horsepower is a measure of the work done by the engine in a set time. A function of RPMs and Torque.

Torque is a measure of the turning force of the crankshaft.

1HP = 33000 ft lbs; ie 1HP will raise a 33000lb weight 1 foot in 1 minute. Or raise a 1lb weight 33000 feet in a minute.

You could have immense torque (33000 ft lbs), but if it will only turn your prop at 1 RPM, you will not go far.
You could also have your engine revving at 33000 RPM, but with 1 foot pound of torque it will only turn a very very small prop. You will not go very far.

As far as rated engine power is concerned, you will lose up to 20% due to friction in the transmission, shafts, water pumps, generators etc. So your 90 HP will only be putting 72HP into moving the boat.

Drag: A rule of thumb: To double your velocity, you need to quadruple your power. 1HP in a small boat may do 1knot. To do 2 knots in the same boat you will need 4HP. /forums/images/graemlins/shocked.gif

 

[MapisM]

I didn't mean you were talking rubbish - far from that, and sorry if that's what appeared from my previous post.
But I'd rather use 1.34, than 1.5. That alone gives 9,8 instead of 11.
And at these speeds, 1 knot makes a helluva difference (that's why I said "well above").
Did you ever check the rpm difference between 9 and 11 kts, and what that means in terms of hp/lph/mpg on the engines curves?
My guess of less than 150 hp at 9kts was based on my following experience (35T, 53', 48' w/line - so smaller but much heavier, not too distant to be meaningless):
at 1600rpm, it takes 120hp to achieve 8,8 kts (24lph - 1.4mpg - range of 1300nm or so);
at 1800rpm, she needs 180hp to cruise at 9,3 kts (38lph - 0.9mpg);
2000rpm means using 260hp to cruise at 9.7 kts (50lph - 0.7mpg).
She can happily cruise at 10kts 24/7 (engines are rated for 2800rpm), but that isn't her hull speed for sure.
And the hull is strictly designed for full displacement, so aside from being lighter, a planing boat can hardly perform better at these speeds.
But I'd be glad to hear other views, of course.

 

[jfm]

ok but the factors of 1.34 or 1.5 are each as good as the other - each particular hull shape has a different factor and the range of correct values is probably 1.3-1.5 I certianly dont think you should propose a universal factor to 2 decimal places!

I believe your data. You need an extra 60hp for a mere 0.5 knots when you just exceed the hull speed, which sounds right.

I have better data though: I have electronc fuel metering connected to GPS SOG. I can add the net fuel flow to both engines so the computer gives a reading of the boat's instantaneous litres/mile or miles per gallon. Hence I can see exactly the point when you start requiring much more bhp and fuel for not much extra speed. (you can of course "feel" it too, as I'm sure you know). On my boat that is 11knots (and, as it happens, 1050rpm). If I run at 10.5 - 10.8kts and 1040rpm I get astonishing range. I did Antibes-Portoferreiro at 1040rpm last summer and had >3/4 remaining in Elba, on a production boat with ordinary tanks. If I ran at 1150 rpm and say 12.5 knots my mpg would be MUCH lower.

This is just a function of hull design. My boat luckily has a factor of 1.45 or 1.5. Others might have 1.34 or whatever. There is no single number that is right for every hull. (And, by the way, a higher number doesn't mean a more efficient or better hull. A hull could be 1.34 and very efficient in temrs of mpg. It would simply have a lower hull speed than a same-length boat with a factor of 1.45)

Your data answers the OP's question though. Your hull dimensions are similar and you need 120hp to get 8.8 knots, which is close to his hull speed. Hence his 90hp is just on the small side, though it might be fine if he is happy to motor at say 7.5kts.

 

[jfm]

Umm yes. I dunno why you're telling me all that, I know

However, 1HP = 33000 ft lbs per minute, not (as you said) 33000 ft lbs. "33000 ft lbs" is merely an amount of work done. Power is a measure of how quickly an amount of work is done. There is a time unit (per second, per minute, whatever) in the dimensions of power.

I can drink 100 pints of beer. But not in one night. A guy who can drink 100 pints in one evening is a more powerful drink than me.

 

[tinkicker0]

>1HP = 33000 ft lbs; ie 1HP will raise a 33000lb weight 1 foot in 1 minute. Or raise a 1lb weight 33000 feet in a minute.<

Looking at the above sentence I believe I said that.

 

[MapisM]

Sensible points, I agree with all of them.
The 1.34 I used was taken by a publication of an oceanic steel trawler builder, but that's not a religion by all means. I found that it works nicely and realistically also with other pleasure boats, though of course yours might be slightly different.
What kind of fuel meter are you using, Floscan? And what kind of mpg did you get at 10.5kts?
Oh, and just for sake of accuracy, the 120hp I mentioned are taken by the engines curves at 1600rpm. It is very likely that less hps are actually necessary to keep the props spinning at that speed.
My actual consumption is in fact lower than what should be according to the curves, which shows that the prop load is less than what the engines could afford (in other words, I could use higher pitch props).
Therefore, those 90hp might possibly be good for 8 - 8.5 kts.
But some spare power would make a lot of sense anyway, no doubt about that.
Also when maneuvering, btw - not only in heavy seas.

 

[ccscott49]

If she is a sister ship I can safely say 90hp is not enough, with all that wetted surface under the water and the shape of the hull.

 

[jfm]

Initially you said "1HP = 33000 ft lbs" which no engineer would ever write. But yes you are right that you did state it correctly later as 1HP = 33000 ft lbs per minute.

 

[jfm]

Agree that. Yes the curves are not perfect at low speed. You probably need less hps that the curve says are avaialble at a low rpm, fully agreed.

Not floscan, Vovo EVC. I have the EVC D12 engines that already have fuel flow and return measurement with the data on the Canbus. I just got the Volvo electronic "plug and play" display that shows all the data on the Canbus. It also has an input for NMEA SOG from the boat's GPS. Easy peasy! It displays litres per hour, litres/gallons per mile, anything you want. For each separate engine, or the two combined.

At that 1040rpm full displacement in calm sea I think I got 1.3 or 1.4 mpg {but I'd have to check the log book on the boat to be sure) whereas at 18-20kts planing I would get 0.7 approx

 

[MapisM]

Blimey, do you keep a logbook updated with such details?
I'm having enough of paperwork on a daily basis, can't imagine bothering also when onboard...!
Anyway. 1.3-1.4 mpg at 10+kts is a good result indeed, it looks like the EVC works properly (also aside from the measurement bit).

 

[Nautical]

Agreed J /forums/images/graemlins/blush.gif, but I was kinda in my thicko way suggesting that if its the bigger lump its slower reving so HP / torque which ever you wish to use to illustrate its power are much lower down rpm range, pitch prop to suit and all that and she wont be blowing up anytime soon mind you won't be breaking any speed records either , lighter revy incl o/b would rev self to death to keep 90hp churning out hour after hour, that ole Gardner would turn 90 bhp for days on end.

Anyway what do you expect I am salesman /forums/images/graemlins/grin.gif as they say in the OEM world horsepower sells boats, torque is what moves em /forums/images/graemlins/grin.gif