Rough Calculation of required HP for a planning boat??

Nautorius

Nautorius

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Help all.

If I had a 30ft planning boat (8ft beam) with a deadrise of 21 degrees what would be the required HP to make it plane? Weight approx 3500kg

So if I needed two outboard engines is there a rough calc to work out what it needs to plane?

Is there any calculation to work out speed with HP i.e 2x30hp would give 6 knots etc?

Cheers

Paul /forums/images/graemlins/smile.gif
 
Where is the dead rsie 21 degrees? All the length of the hull or is that at the back?

I cant give an answer but I do know that the power/top speed is quite a step curve, ie not much to get planning bit lots to get 40 knots!
 
At some point in the dim past I seem to remeber that it was meant to be

Petrol 100hp/ton
Diesel 70hp/ton (cos of extra torque!)

Nick
 
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Try doing a google for 'Crouch's formula'. happy hunting...

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Crouch

It suggests 60HP for the boat in question, that sounds light to me. /forums/images/graemlins/confused.gif

The 190HP sounds better.

Edit: The results made sense for my sailing boat, but were way out for my SIB. Hmmm
 
[ QUOTE ]
[ QUOTE ]
Try doing a google for 'Crouch's formula'. happy hunting...

[/ QUOTE ]

Crouch

It suggests 60HP for the boat in question, that sounds light to me. /forums/images/graemlins/confused.gif

The 190HP sounds better.

[/ QUOTE ]

60hp only showed a speed of 15 knots /forums/images/graemlins/frown.gif 190hp showed 27 knots /forums/images/graemlins/smile.gif
 
[ QUOTE ]
is there a rough calc to work out what it needs to plane?
Is there any calculation to work out speed with HP?

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No and no, I'm afraid.
I mean, there's no such thing as the LWL-based formula of displacement boats.
 
Allowing for all the kit, fuel/water and squashy things like people carried, 75hp/metric tonne of dry weight is safer than 60hp in a planing boat.
 
Thanks Guys,

That is very very useful information. I am aware it is no exact science, and I only needed ballpark figures, which you have given me. So somewhere between 60-80hp per 1000kg.

Good info. So 3500kg approx will need twin 125hp min, probably twin 140hp. To be safe twin 150Hp! Given the cost of these engines, and the fact it is mainly River use, I will advise the guy to keep down his cost, restrict his boat to a pre-planning 10 knots and buy twin 40HP petrol outboard 4 strokes!

Cheers

Paul /forums/images/graemlins/smile.gif
 
If you only want displacement speeds, then to be honest you don't need that much...

We used to have a 20' Buckingham, weighed 2000KG and that used to be pushed along beautifully at 4knots with a 6hp - one day engine broke.

Then we tried an 8hp - went better at 6kts

Then tried a 20hp - lots of wake, but only 8knts

Tried a 40hp - even more noise, wake and everything else - but still only JUST over 8knts - in other words, we'd reached displacement speed....

Dad bought the 8HP and fitted a sailprop - and we got 8 knots....

So, if you are only going for displacement speed, it's not really the power that matters it's the THRUST.... your man, if he wants to only hit displacement speeds probably could do it with a couple of 15hp sailpower outboards - or at least he won't be too far off max displacement.

To get over the hump - that's when you need the massive amounts of power - I would also have said about 75hp per 1000KG would just do it - I always used to like having 100HP per 1000KG - that always gave nice performance and meant engines could be run at an efficient 2/3 power.
 
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So, if you are only going for displacement speed, it's not really the power that matters it's the THRUST.... your man, if he wants to only hit displacement speeds probably could do it with a couple of 15hp sailpower outboards - or at least he won't be too far off max displacement.

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That is a fair point, like you I have experienced some pleasant cruising with small engines and the right prop. If you are not going for planing at all, let alone easy displacing, then smooth and steady power makes far more sense.
 
It would be interesting to know where you got those figures from! power to weight ratio means a great deal, petrols are lighter, for a planing hull power is equall be it petrol or diesel, ie 100hp is just the same, in a light hull the petrol will go faster for the same hp.
 
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