How many knots while motoring?

[MicHughV]

I've seen that vid...what's on the back there ???
and um.....what's the fuel consumption at that speed ??...gasoline, right ???

[MicHughV]

and...um...ask him to go around again in some waves....um..coming from the front....small one's ok......3-4'....

[smj]

Originally Posted by MicHughV

speed is really a component of water line length...LWL"...
...for your average 40' boat with a 36' water line length....about 7.8 knots, give or take....
to push that same 40' boat to 4-5 knots does not require much power....but to push it to 7.8 knots requires a lot of power....
in the same breath....motoring in a flat calm is nothing near the same as trying to motor against wind and current..
it's a complicated science, not easily explained in one paragraph.....don't be fooled into thinking you can get a lot for a little....doesn't work that way...

This would apply to monohulls, but this is the multihull forum.

[MicHughV]

ok, I get it...the Catalina 42 comment threw me off.....

[a64pilot]

Originally Posted by smj

This would apply to monohulls, but this is the multihull forum.

It actually applies to Cats as well, just not as much, but particularly the run of the mill Lagoons etc.
Although I will say they are very well motored, I’ve seen them make 7 kts into a direct 25 kt or so wind, something that I cannot approach. That has nothing to do with hull speed, it’s simply having more motor.

[chrisr]

we have a pair of turbo 55hp. 12m / 7.5mt

if flat water will motor at 6k+ into 25k headwind @ 2800rpm. no headwind say 2500rpm. 7k @ 3000 rpm

this is on one engine only

2 engines / average conditions = 9.5k @ 3000rpm (max is 3200 but don't run at that for long)

biggest issue is pounding if there is any sea. no point in smashing boat & people so slow down to whats comfortable.

cheers,

[Chotu]

Originally Posted by a64pilot

It actually applies to Cats as well, just not as much, but particularly the run of the mill Lagoons etc.
Although I will say they are very well motored, I’ve seen them make 7 kts into a direct 25 kt or so wind, something that I cannot approach. That has nothing to do with hull speed, it’s simply having more motor.

Which I still have some issue wrapping my head around.

If a can sail at 15 knots in flat water, why do they not motor at the same speed?

My last cat was a tank so none of this was relevant. It was a monohull for all intents and purposes.

A mono sails at say, 6.5 knots in good conditions, but when you turn on the motor, you’re doing 7 or 8 knots.

A cat sails at say 12-15 knots, but when you turn on the motor, you’re doing 7 knots.

Why???
This hurts my head!!

[Jim Cate]

Originally Posted by Chotu

Which I still have some issue wrapping my head around.

If a can sail at 15 knots in flat water, why do they not motor at the same speed?

My last cat was a tank so none of this was relevant. It was a monohull for all intents and purposes.

A mono sails at say, 6.5 knots in good conditions, but when you turn on the motor, you’re doing 7 or 8 knots.

A cat sails at say 12-15 knots, but when you turn on the motor, you’re doing 7 knots.

Why???
This hurts my head!!

I can't speak to the cat side of this story, but we often sail at 9+ knots but motor at 6+ in our mono. Under duress we can go faster under power, and in fact reached just over 9 knots once when we had fitted a new prop and were checking it's performance, but the fuel consumption at these power settings is ferocious. 2000 rpm is a sweet spot for us: 2.5 l/hr and a bit over 6 knots in flat water. We have also made headway into 50+ knots and chop, but pretty damn slowly. Engine is Nanni 43hp, boat in cruising trim and full tanks around 12 tonnes.

Jim

I think you need to meet some folks who have good sailing monos and become familiar with the breed!

[Tin Tin]

If you want a cat to motor as fast as it can sail, you will need the same horsepower as the sails generate. When you put a lot of horse power onto a sailing cat the sterns get sucked down and causes large drag issues, so it has be designed properly. Check out this motor sailor. For example a 38ft cat with twin 75 HPs.
https://www.spiriteddesigns.com.au/crossover

[Tupaia]

Originally Posted by Chotu

Which I still have some issue wrapping my head around.

If a can sail at 15 knots in flat water, why do they not motor at the same speed?

My last cat was a tank so none of this was relevant. It was a monohull for all intents and purposes.

A mono sails at say, 6.5 knots in good conditions, but when you turn on the motor, you’re doing 7 or 8 knots.

A cat sails at say 12-15 knots, but when you turn on the motor, you’re doing 7 knots.

Why???
This hurts my head!!

You need to convert the engine "running" hp into square area of sails or visa versa. Running hp is hard to rationalise as it depends on the shaft RPM, mechanical losses and the type of the propeller. So in the examples above a 30 or 40hp engine will not be applying 30 or 40hp to the water.

The force on the sails increases by the square of the wind speed. So in the 15 to 20 knot wind speeds the same sail will produce twice the force (hp). 100sqm of sail will produce about 35hp in 15kn of wind compared to 70hp in 20 knots. This obviously ignores the direction of wind and the redirection of the forces due to the sails.

A maxi-multi under full sail will be produce about 400hp in 15knots and 800hp in 20knots of wind.

[Chotu]

Originally Posted by Tupaia

You need to convert the engine "running" hp into square area of sails or visa versa. Running hp is hard to rationalise as it depends on the shaft RPM, mechanical losses and the type of the propeller. So in the examples above a 30 or 40hp engine will not be applying 30 or 40hp to the water.

The force on the sails increases by the square of the wind speed. So in the 15 to 20 knot wind speeds the same sail will produce twice the force (hp). 100sqm of sail will produce about 35hp in 15kn of wind compared to 70hp in 20 knots. This obviously ignores the direction of wind and the redirection of the forces due to the sails.

A maxi-multi under full sail will be produce about 400hp in 15knots and 800hp in 20knots of wind.

Yet the rigs aren’t all that different in size from mono to multi. Which is why it hurts my head.

[Tupaia]

Originally Posted by Chotu

Yet the rigs aren’t all that different in size from mono to multi. Which is why it hurts my head.

A multi with the same sail area as a mono will sail faster because of the power to weight ratio. Except in the case of your previous "tank" catamaran that performed more like a mono. Above the WL speed limitation the multi will continue to sail faster whereas the mono will be limited by the wave making drag resistance that applies when motoring also.

All motors are not equal, 2 motors are less efficient at converting the power into motion than a single motor of the same hp. So a single 56Hp (in a mono) is better than 2 x 28Hp (in a cat).
Also a (on a mono) a bigger slower turning propeller, much deeper in the water, where the water is more dense, will result in less cavitation and slip.
Similarly all cats are not equal, the propellers on a condomaran tend to be fixed 3 blade and are deeper down than the 2 bladed folding props typically found on performance cats with less draft and therefore closer to the surface.
Very light cats can motor well in calm conditions with outboards that have low ratio gearboxes and big 3 blade props, but then suffer cavitation issues in rougher conditions or simply do not have the thrust.

[MicHughV]

a cat may not be draggin' a 6' keel thru' the water.....but it is dragging two (sometimes very beamy) hulls thru' the water in addition to.....usually....substantial windage from superstructure, bridgedeck, etc....charter cats with their their 12' high steering locations a classic example. I note that your average charter cat is no lightweight either...
at the end of the day, motoring these beasts at speed thru' the water is likely to be challenging as the engine size tends to be on the smaller size per hull.
There is no one size fits all here.
Interesting thread though....good reading !!

[rslifkin]

If a cat can sail at 15 kts, there's no reason it can't motor at 15. But in many cases, they just don't have big enough engines, props, etc. to do it as that would add weight and drag to provide something many owners don't care too much about, as many sailors wouldn't want to burn that much fuel anyway.

[mikedefieslife]

Snogoose 35.
3GM30
16x12 prop
2,400 rpm

About 5 knts up to 6knts. The boat is over propped.

Yesterday in 27knts against me and very steep waves 1.5-2m and 3-4seconds apart I was motoring at 2knts. The steepness of the waves meant the prop would come close to the surface of the water, so if i pushed it too much the revs would shoot up to over 3000, and the engine would get too hot.