How many knots while motoring?

[a64pilot]

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!!

Not to be a smart arse, but have you sailed at 15 kts or are you going off of others reports? People have a tendency to exaggerate, and it grossly depends on conditions.
I can say without lying that I have sailed above 10 kts, but it was transient with a 25kt wind and that was surfing, something that I didn’t think my boat could do, it was also followed pretty quickly by a crash Gybe that broke my traveller.

My boat is not by any means a 10+ kt boat, unless it’s in the Gulf Stream headed North or on a trailer. Good day it’s a 7.5 kt boat. Max throttle under motor and it can hit maybe 7.5 kts, she is faster under sail.

Given enough motor and most anything can be made to go fast, for instance I think the boat in the Movie Waterworld had a very large Volvo and would go like stink under motor.

But your instance it’s the props that will hold you back, you need to get a tachometer, just a cheap inductive one will do and graph speed vs RPM, the graph will show you what’s happening and at what RPM you should stop pushing the throttle Forward and it’s not likely to be the speed you were expecting or want.
However nearly the entire ICW travels between 6 to maybe 8 kts, go faster and you will be having to constantly wait to pass people, so it’s best to just go with the flow.

If your not in the ICW, the assumption is you will be sailing anyway?

[smj]

We were out sailing about a week ago. Speeds were 7.5-10kts. hitting 12 in the gusts. Motoring with the twin 9.9’s was still just low 6’s.
A sister ship has a single Honda 50 and will top out at 15 kts under power.

[PaulvR]

My 50'. 17.5T , Perkins M90 @ 1950-2000 RPM cruises all day or all week at about 7.5 knts, running marginally cool, sipping fuel.
2400 (recommended) takes her to hull speed with increased fuel consumption, heat and noise, for very little gain.... but I love and spoil my Perkins and only rev high for the occasional decongestion for 15-30 seconds. as recommended by the doctor.

[PaulvR]

I think it is all about horse power.

An engine is always the same rated HP, a sail is not. So sails, particularly on modern powerful rigs, simply push more horses, and so more horses push.

I think?

[PaulvR]

My 50'. 17.5T , Perkins M90 @ 1950-2000 RPM cruises all day or all week at about 7.5 knts, running marginally cool, sipping fuel.
2400 (recommended) takes her to hull speed with increased fuel consumption, heat and noise, for very little gain.... but I love and spoil my Perkins and only rev high for the occasional decongestion for 15-30 seconds. as recommended by the doctor.

[a64pilot]

Originally Posted by PaulvR

I think it is all about horse power.

An engine is always the same rated HP, a sail is not. So sails, particularly on modern powerful rigs, simply push more horses, and so more horses push.

I think?

But you have to get that HP to the water, a go fast big motor outboard will just cavitate as it’s spinning a small prop at a high RPM, cause it’s designed to push a planing hull, at planning speeds.
So yes it’s about HP, if you can get that HP to the water.

[smj]

Originally Posted by PaulvR

I think it is all about horse power.

An engine is always the same rated HP, a sail is not. So sails, particularly on modern powerful rigs, simply push more horses, and so more horses push.

I think?

For the majority of monohulls and some catamarans extra horsepower will do little to get you above the theoretical hull speed. At some point it’s a waste of hp and money.

[Factor]

Originally Posted by a64pilot

But you have to get that HP to the water, a go fast big motor outboard will just cavitate as it’s spinning a small prop at a high RPM, cause it’s designed to push a planing hull, at planning speeds.

Unless its not. As in unless its designed to push a displacement hull with reducection gearing etc, eg Yamaha 25 and 60 HT engines

[Tupaia]

Not all propulsion systems are equal. A 2 blade propeller is more efficient and will give a higher speed than a 3 blade propeller in calm no wind conditions whereas the 3 blade will have more "grip" and will be better in rougher conditions or when heading into the wind where it will need to overcome the additional windage.

Going directly into the wind is not an option when under sail so you have no direct comparison and the difference noted by the OP is often a perceived one. Sailing at 15 knots will most likely be on a reach or downwind. Motoring will most likely be directly into the wind or when there is no wind at all so you cannot really compare the two.

If you have a lightweight performance vessel, mono or multi, that sails to windward well you will most likely choose a 2 blade folding propeller as the most efficient for use in no wind conditions and you will want to go as fast as possible and at the same time reduce the drag when under sail. The sailing speed "range" on such a boat will be high so there will be a big difference between the max speed under sail and the speed when motoring. This same boat especially a multihull will suffer a big loss in speed under power when heading into strong winds, someone mentioned 1.5 to 2 knots boat speed.

On a heavier lesser performance vessel you will more likely choose a fixed 3 blade propeller as its primary function will be in windy conditions to punch directly into the wind or to assist under sail, motor-sailing. The difference in speed sailing or motoring will be small. This is more evident in the case of monos where motoring will be faster unless there are strong winds.

[Chotu]

Originally Posted by a64pilot

Not to be a smart arse, but have you sailed at 15 kts or are you going off of others reports? People have a tendency to exaggerate, and it grossly depends on conditions.
I can say without lying that I have sailed above 10 kts, but it was transient with a 25kt wind and that was surfing, something that I didn’t think my boat could do, it was also followed pretty quickly by a crash Gybe that broke my traveller.

My boat is not by any means a 10+ kt boat, unless it’s in the Gulf Stream headed North or on a trailer. Good day it’s a 7.5 kt boat. Max throttle under motor and it can hit maybe 7.5 kts, she is faster under sail.

Given enough motor and most anything can be made to go fast, for instance I think the boat in the Movie Waterworld had a very large Volvo and would go like stink under motor.

But your instance it’s the props that will hold you back, you need to get a tachometer, just a cheap inductive one will do and graph speed vs RPM, the graph will show you what’s happening and at what RPM you should stop pushing the throttle Forward and it’s not likely to be the speed you were expecting or want.
However nearly the entire ICW travels between 6 to maybe 8 kts, go faster and you will be having to constantly wait to pass people, so it’s best to just go with the flow.

If your not in the ICW, the assumption is you will be sailing anyway?

YOU are getting one the first invites to go for a sail on my boat. Ha ha. Send me a PM with your email address and I’ll share some info with you about her. It’s basically a Gunboat, but lighter, hopefully, by reducing the weight of interior and components apart from the hull. It’s the least I could do for the help you’ve given on my threads. Assuming Covid is over by the time she’s completely done.

As to the second half of the post, I remember that all from the other thread. I wanted to see what speeds other sailing cats with auxiliary power were motoring at. Now I see mine is the same as everyone else’s so I don’t need to change anything. It’s a battle I don’t need to fight right now. Drops any look at my propulsion to the bottom of the list.

Although the boat smj is talking about doing 14 knots with a Honda 50 is more what I had envisioned with a pair of 30’s. My 7-8 knots will just have to do for now while I work on other things.

[thomm225]

It all depends on the tide, wind, chop, and amount of current. (plus horse power of course)

I can motor as high as 5 knots or more with my 5 hp 4 stroke outboard in the right conditions pushing my 6600 lb Disp. Bristol 27

Here though with a ripping outgoing tide it's 2.7 knots (2.2. knots as I get closer in) or anchor to avoid being taken out to sea.....it's one of the few times I ran the engine at max power

https://www.youtube.com/watch?v=q6sHpweobRM

[MicHughV]

this topic seems to negate the obvious...
where is the "horsepower" measured ?...remembering that " horse power" is derived from formula....

final gearbox ratio's are all over the map, together with prop diameters and pitch, which will also affect the actual boat performance.....the same engine, running at the same rpm, can produce wildly different "horse power" at the driveshaft, depending on the final gearbox ratio....in turn requiring wildly different prop selection...which in turn provide different motivating force...

"horse power" is the marketing tool used by engine manufacturers, but does not really tell the full story. The general public is married to the " horsepower" figure...ie...more horsepower=better !!

In the automotive world..." horse power" wars is the name of the game...interestingly enough, the new Corvette has been tested by a variety auto magazines and others, and the "claimed" horse power of the Corvette is all over the map.

I've offered this insight numerous times....go to Amazon and you can peruse dozens and dozens...and maybe a few dozen more books on engines of every type.....

[Statistical]

Originally Posted by MicHughV

this topic seems to negate the obvious...
where is the "horsepower" measured ?...remembering that " horse power" is derived from formula....

It is measured at the shaft without any auxiliary loads (BHP). Every engine produced in the past sixty years. Yes the power delivered to the shaft (SHP) will be less due to aux loads (i.e. alternator) and power delivered to the water will be even less than that.

final gearbox ratio's are all over the map, together with prop diameters and pitch, which will also affect the actual boat performance.....the same engine, running at the same rpm, can produce wildly different "horse power" at the driveshaft, depending on the final gearbox ratio....in turn requiring wildly different prop selection...which in turn provide different motivating force...

Reduction gears don't change power. If they did we would just use 1 HP engines with massive reduction gears.

Power is torque * RPMs. A 2:1 reduction gear doubles the torque and halves the RPMs. Power remains the same (minus losses).

"horse power" is the marketing tool used by engine manufacturers, but does not really tell the full story. The general public is married to the " horsepower" figure...ie...more horsepower=better !!

Well more power IS more power. Obviously you still need a properly sized propellar and there are rapidly diminishing returns beyond hull speed and higher speed requires exponentially more power (power to overcome drag = velocity ^3) to go a little bit faster and that means exponentially more fuel per mile so there are realistic limitations but HP isn't some marketing myth.

[rslifkin]

Horsepower is a useful measure, as it's a measure of how much work the engine can do in a given period of time. However, it doesn't tell the whole story. In a boat, gear ratio, prop design, size, etc. all impact how effectively that power is translated into forward motion.

Basically, if you have 50hp available, you can determine what the best performance you could expect from that is. Depending on gearing, props, etc. you may or may not reach this expected performance. Adding more power without changing other things typically won't make the boat faster. But adding more power and then optimizing the rest around it generally will add speed unless you're already overpowered and being limited by hull design.

Gearing and prop selection is all a game of tradeoffs. Adding blade area and reducing pitch may give lower slip but higher drag. So you may lose a little bit of top speed as more of your available hp is used to overcome prop blade friction and less is converted into forward thrust (and you still have the same amount of total engine power to work with). But with the extra blade area, a headwind will likely produce a smaller drop in speed, as the prop slip will increase less with the increase in resistance to forward motion.

[a64pilot]

Originally Posted by Factor

Unless its not. As in unless its designed to push a displacement hull with reducection gearing etc, eg Yamaha 25 and 60 HT engines

I specifically said big go fast motor, as in non High Thrust, but even high thrust motors have limitations, they are certainly better than non high thrust, but are still limited in prop diameter and reduction RPM.
Most bigger ones are really pontoon type boat motors etc and not really meant for sailboats and other very heavy hulls.
The OP has I believe twin 30HP Evinrudes, which are not high thrust.

But hull speed is not a limit, it’s merely a point at about which the thrust / speed curve gets real steep. Want to see a boat in displacement mode going well past hull speed? Look at the average Sportfisherman riding around bow in the air, rolling a huge wake.

Fine hulls are not immune to “hull speed” it still exists, just the curve is not as steep as it is for a broad, fat hull. A rowing shell is of course likely as close to the perfect fine hull there is.