Do much Motorsailing ?

[Nom de plume]

I have no hesitation whatever about motor sailing - to me the engine is just another propulsion option along with all the sail combination options.
With retirement our cruising has become more relaxed for sure - but even so, the knock-on effect of missing one tidal-gate can be magnified many times over on a trip from say the solent up to the Western Isles. Miss St.Cats and that can often degenerate into missing Portland, Start Pt etc. etc.
I set a 'Progress required VMG' into my passage planning - if we can sail it then great, all is perfect, but I have no hesitation in sticking the donk on if our speed drops enough to miss a vital gate.

 

[ryanroberts]

Yes the last time I had a lovely time drifting about tuned into a slow motion 8 mile game of mario kart getting into Portland at 1AM (when the wind of course finally arrived) as I was going to hit my 18 hour solo limit riding it to Studland, I think the correct thing to do would have been to have stuck it on 1200RPM 4 hours earlier adding a consistent 3kt or so..

 

[fredrussell]

Exactly.
The reason is precisely the difference between frictional and form resistance. The amount of frictional resistance obviously depends on a hull's surface area, however it only fairly gradually and linearly increases with speed and it has nothing to do with displacement. In fact a deep, narrow hull can have a smaller wetted area than a beamy lighter one.
Form resistance, on the other hand, is directly related to displacement as this represents the real bulk that needs to be pushed through the water. The crux is that form or wavemaking resistance rises exponentially with speed.
Up to a relative speed of 0.7 form and frictional resistance share the same gentle rise, after which form resistance basically goes through the roof.

For a 10m DWL heavy displacement classical hull resistance at a relative speed of 0.7 (4kts) is an bout 40kg, at hull speed 1.34 relative speed (7.7kts) it is ten times that or 400kg.

Now, as most boats are designed to reach hull speed at the end of a F4 to beginning F5, boats with a similar SA/D ratio will reach hull speed at the same time. By making SA a factor of displacement and hence form resistance (the one that rises exponentially), the heavier boat has considerably more power (SA) to overcome the predominant and lower frictional resistance at lower speeds in lighter winds. Consequently, it will be faster.

The practical speed differences between lighter and heavier boats are marginal in as much as the accepted average relative speed for displacement sailing craft is about 0.9 and most boats rarely travel at hull speed.
The ability to increase SA is limited by a boat's stability; form resistance is exponential, stability isn't.

LF, I always struggle with this. Is the summary of what you are saying that a heavier boat will be faster in light airs because it can carry more sail than a similar length lighter boat?

What about lighter boats that carry more sail by moving the ballast very low down? My 31ft, 3.2 tonne boat has a ton of lead 6 feet below the waterline. It has a sail area of 42 sq metres.

In comparison, a 32ft, 4.5 tonne Westerly Fulmar has a similar sail area to mine (43 sq metres) but has its 1.9 tonnes of ballast carried in a normal fin keel with no bulb or weight kept low down in/on keel. Am I to assume my boat can carry similar sail area to the heavier Fulmar by getting all of the ballast as low down as possible, and if this is the case where does this fit in with your formula?

 

[Laminar Flow]

LF, I always struggle with this. Is the summary of what you are saying that a heavier boat will be faster in light airs because it can carry more sail than a similar length lighter boat?

What about lighter boats that carry more sail by moving the ballast very low down? My 31ft, 3.2 tonne boat has a ton of lead 6 feet below the waterline. It has a sail area of 42 sq metres.

In comparison, a 32ft, 4.5 tonne Westerly Fulmar has a similar sail area to mine (43 sq metres) but has its 1.9 tonnes of ballast carried in a normal fin keel with no bulb or weight kept low down in/on keel. Am I to assume my boat can carry similar sail area to the heavier Fulmar by getting all of the ballast as low down as possible, and if this is the case where does this fit in with your formula?

The two boats you describe do not have the same SA/D ratio (sail area to displacement).

The first has a ratio of 20,29, the second one of 16.58 - they are not even close, at least not on paper.
But, the Fulmar is an older design with a smaller main than contemporary ones and was designed to carry a larger genny to make up for it. As SA/D is currently calculated using fore triangle and main only, the SA figures for older designs are frequently undercalculated. With an overlapping foresail in the range of a 130%, and a resulting SA/D of something alot closer to the first boat, I have little doubt that, in that case, the Fulmar would indeed be faster in light airs. The lighter boat, of course, might be able to, in high winds, reach much greater speeds with it's probable D/L of perhaps around 120 or so.

Lighter boats rarely carry more sail, even relatively so, because they have either a higher ballast ratio or carry it lower, but mostly because, as is currently the trend, they are beamier and have greater form stability. Compared to earlier designs, ballast ratios have been consistently dropping and are now often below 30%.

At low angles of heel, 20-30 degr, height of CG makes very little difference. It is here that form stability plays by far the greater role and GZ (righting arm) is increased by having as much buoyancy as far outboard as possible often coupled with a flat bottom and hard bilges.

 

[fredrussell]

Ok, I shall seek out a fulmar and see how they compare with mine in light winds. It’ll be interesting. There’s 3 or 4 Fulmars locally. I should have stated, my boat - a 1991 Parker 31 - is not a wide-arsed modern design, and not hugely dissimilar to a Fulmar in form, that’s why I’m comparing the two. The big difference being I have a lift keel with a tonne of lead at the bottom of it. I have a 150% Genoa, standard fitment on these boats.

 

[Laminar Flow]

Ok, I shall seek out a fulmar and see how they compare with mine in light winds. It’ll be interesting. There’s 3 or 4 Fulmars locally. I should have stated, my boat - a 1991 Parker 31 - is not a wide-arsed modern design, and not hugely dissimilar to a Fulmar in form, that’s why I’m comparing the two. The big difference being I have a lift keel with a tonne of lead at the bottom of it. I have a 150% Genoa, standard fitment on these boats.

Since you would be flying an overlapping headsail as well, you would also be increasing your SA/D and consequently probably now be faster. And, we would be comparing apples and oranges again.
I did state that the superiority of light air performance of heavier displacement craft rests on the premise that they have the same, or nearly so, SA/D.
To the point: My tub has a SA/D of about 18 with 71.6 sqm. In light air, as indeed I have at relative speeds of 1 (4.8kts) or less, I may very well give a much lighter boat with a similar SA/D a run for it's money. This is for the simple reason that at these low speeds I have more SA and therefore horsepower to overcome the, at this point predominant, frictional resistance.

And, as I said before, this exercise has not very much to do with sail carrying capacity, as one would expect in light going.

 

[fredrussell]

… I may very well give a much lighter boat with a similar SA/D a run for it's money. This is for the simple reason that at these low speeds I have more SA and therefore horsepower to overcome the, at this point predominant, frictional resistance…

… this exercise has not very much to do with sail carrying capacity…

Do those two statements not contradict each other?

 

[Stemar]

Milady is allergic to heel, and therefore to sails, so we motorsailed or just plain motored quite a lot in Jissel, especially if the wind was in the wrong direction (it usually is on Lake Solent). Now we've got a cat, we'll see, but I don't see us spending much time tacking to windward in a Catalac

 

[johnalison]

Milady is allergic to heel, and therefore to sails, so we motorsailed or just plain motored quite a lot in Jissel, especially if the wind was in the wrong direction (it usually is on Lake Solent). Now we've got a cat, we'll see, but I don't see us spending much time tacking to windward in a Catalac

I can see you spending a very long time tacking to windward - if you are ever permitted to try it.

 

[Laminar Flow]

Do those two statements not contradict each other?

A boat is generally designed to reach hull speed, relative speed of 1.34, somewhere around F5.
As you cab see from the graph, total resistance rises exponentially after a relative speed of, say, 1.

What this graph does not show, is that a lighter boat has a less acute rise in resistance at higher speeds, while at lower speeds the curves are much the same.


This graph is from the Delft series comparing two 10m DWL hulls, one heavy (model 1) and one light (model 25). The relative speeds here are given in Froude numbers i.e. FN 02 = a more conventional relative speed of 0.7, FM 0.3 = 1.02 and FN 0.4 = 1.34 (what is called hull speed). I
It shows that the heavier boat needs to overcome twice the resistance as the lighter to reach hull speed. Consequently, if the heavier boat were given more SA to enable it to reach FN 0.4(1.34 relative) in the same wind strength as the lighter, it would have, relatively speaking, abundant, excess power at lower speeds and therefor be faster than the lighter boat when resistance for both are near enough the same.

Total resistance (form resistance) is entirely related to displacement. This is recognized by the SA/D ratio that, obviously, correlates SA with the weight of a boat. It also means that a heavier boat will have more SA, at the same ratio as the lighter.

Hope this explains it better, LF.

 

[Stemar]

I can see you spending a very long time tacking to windward - if you are ever permitted to try it.

Only if both engines let me down. I'm a gentleman sailor