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Old 26 September 2021, 10:06   #1
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Power vs Weight- Linear scaling?

Hi! I'm new to ribnet. Looks like a great source of information! The one thing I was unable to find is this question:

Let's say you scale up a boat to have twice the weight. With twice the power will it perform the same (which would be linear scaling). Or as boats get larger do they need more or less power per kg?

The back story: my friend owns a SIB that performs ok with 10 HP and 2 adult passengers. That comes out to about 240 kg total for boat, engine, passengers, and stuff. I want to buy the larger version of his boat and load it up to about twice that weight. To perform the same, would I need twice the power? More? Less?

I realize boat performance is influenced by many factors but since I am considering buying the larger version of the same boat and taking it to the same location as I have taken his boat in the same conditions, I think the main difference will be weight.
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Old 26 September 2021, 12:41   #2
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Hi and welcome to the forum. There isn’t a calculation as such. But it would be easy to advise if we knew your friends boat type/length and the speed he gets with a 10hp. Then to know the length you are considering.
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Old 26 September 2021, 13:00   #3
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To make a boat go twice as fast you need four times the power. This is presuming other effects don't influence the performance.
In your case you're considering a different boat carrying twice the weight so you're not comparing like boats and they will be different to each other in performance. You can reasonably guesstimate the performance though since you know the performance of a similar boat. The rule is not linear, the rule follows the squared law.
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Old 26 September 2021, 14:01   #4
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Thank you both for the quick replies.

@Fenlander- my friends boat is a Takacat 340 (11'2") and the boat I am considering is a Takacat 420 (13'9").

@jwalker- good point, to go twice as fast you need more than twice the power. Will the slightly larger, heavier boat have a higher minimum planing speed? Basically I just want enough power to confidently get on a plane.

I believe I was planning around 11mph in the takacat 340, not sure.
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Old 26 September 2021, 16:45   #5
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Ahh Takacats... that gives you an advantage as they are very efficient hulls.

I have experience of a similar semi-cat design in 3.5m length with a 10hp motor. By chance SIB, motor, fuel, kit and two of us also came to 240kg. We'd be on the plane around 9kts(10.5mph) and max out at 17kts (19.5mph).

So we planed about the same as your friend and went on to a good top speed... I assume 11mph wasn't your friends maximum?

With a 420 and a similar load you may find the figures very close with the same outboard size as often a longer SIB goes well and at 50kg the 420 is still very light for a SIB.

The 420 is rated to a 20hp and I reckon it would fly with that. If you are looking for similar performance to your friend with their 340 allowing for your added load you may find 15hp is plenty.
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Old 26 September 2021, 20:53   #6
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In the opening post to this thread, I tried to simplify my question to get an answer to the linear scaling question which I am still not sure of the answer.

But in reality the 10 hp was actually under powered because it could only hit a top speed of around 13mph with mild chop on the ocean. And getting on a plane could take up to 15+ seconds of full throttle. I would like to be confident I can get on plane even if I get stuck in larger ocean chop. It’s possible the “10hp" outboard was underperforming but I don’t know and I don’t suspect that since I have nothing to compare with.
So the actual question I need to answer for myself is what power do I need to get to be confident about always getting on plane with the Takacat 420 and a weight of around 170lb*4 (people) + 60lb (stuff) + 111lb (boat) + 130lb (outboard) lb ~~ 1000lb total.

Since I am not truly doubling total weight (engine and boat only get slightly heavier), 20hp could be fine (assuming linear scaling). I am leaning towards 25hp to play it safe. Takacat lists the 420 boat as accepting up to 25hp.
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Old 26 September 2021, 21:55   #7
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That's a lot of weight.

Being under powered sucks.

I'd get a 35 over a 25 since they're basically the same.
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Old 27 September 2021, 04:55   #8
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Quote:
Originally Posted by Usain-Boat View Post
In the opening post to this thread, I tried to simplify my question to get an answer to the linear scaling question which I am still not sure of the answer....

.....20hp could be fine (assuming linear scaling)......
Please re-read the last sentence of post #3.
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Old 27 September 2021, 05:12   #9
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>>>the 10 hp was actually under powered because it could only hit a top speed of around 13mph... getting on a plane could take up to 15+ seconds of full throttle.

>>>possible the “10hp" outboard was underperforming but I don’t know and I don’t suspect that since I have nothing to compare with.

Well you do now as I've given you the example of a 3.5m cat type hull SIB with the same load weight and 10hp power (mine 2-stroke) motor making 19.5mph.

A second example is the same load and same 10hp 2-stroke motor with a V hull 3.8m SIB making 17mph... slightly down on the first example as this type has more hull drag than a cat type.

Third example same as example two with a 4-stroke 10hp motor making the same 17mph.

So all very consistent real life examples.

Which leads me to believe there is something wrong with your friends setup to only get 13mph. The setup itself... motor down on power... or given the time it takes to plane the wrong pitch prop.

Re the 420 Takacat in the USA the advised OB is 15hp and max 25hp... in the UK and some other parts of the world max rated OB is 20hp.

I'm no believer in overpowering a SIB... apart from the fact you are going above its design constraints a lighter outboard can give a better overall feel to a SIB.

Initially you said with this longer model and greater load you just wanted to get on the plane OK and match the 13mph performance of your friends with their 10hp. I reckon you would do that with a 15hp bearing in mind pitching down a size or two from the standard prop would still enable easy transition to the plane.

Above that 20hp would give you greater leeway and anyway most modern 20hp motors are the same as the 15hp so you might as well have the extra 5hp for the same weight... I'd not go to a 25 as the weight/cost jump up a fair bit.

One thing to be aware of with the Takacat design... they are prone to prop ventilation and loss of grip with certain motor types often needing a little refining of motor height and prop pitch/blade profile to avoid this problem.
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Old 27 September 2021, 06:04   #10
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I did once find such a rating on a US forum where it gave hp increases and what percentage of speed difference would be achieved. From memory it was surprisingly small difference in speed when quite large increases in hp on same sized vessels.
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Old 27 September 2021, 07:03   #11
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its not as JW says an exact science i always go for the MAX hp on my boats rather have it and not use it. my experience is it seems to me you get around two knots more for each 5hp on a sib if you are like for like weight passengers etc. then conditions can change that instantly i also like to under prop mine a bit for a more crisp response for my needs
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Old 27 September 2021, 07:11   #12
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The important thing here is the OP has asked to get on the plane OK and comfortable make 13mph.... not for those last few mph on top speed. That really isn't a big ask with the correct prop.

Of course what we don't know is how far and in what types of sea he intends operating as that would make a difference.

I do agree with Jeff and Jon that in the portable outboard sizes once you have an outboard that is easily able to get on the plane and maintain a comfortable cruise the top speed difference can be really quite small for a fair bit of extra weight and cost for each extra 5hp.
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Old 27 September 2021, 08:26   #13
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Hi there Usain-Boat

If you are numerate, this might interest you:

Power = K x Engine Speed(to the power of 4) / Boat Speed
eg – a rib has a 120hp 4-Stroke petrol engine capable of 6000rpm and 40kt. Cruising speed is 30kts at 4200rpm
Find K by putting real numbers into the formula: 120 = K x 64 / 40 = K x 1296 /40 = K x 1296 /40 = K x 32.4
K = 120 / 32.4 = 3.7
Now use K to find out how much power is being developed at cruising rpm by putting real numbers into the formula:
Power = 3.7 x 4.2(to the power of 4) / 30 = 3.7 x 311 / 30 = 38hp

If not, simply delete.

Happy ribbing!

MGx
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