Boat speed with different winds speeds and directions

[dmksails]

Hi All -

I've a Tashiba-31 rigged as a cutter. The typical boat speed for this boat is said to be 6 knots. I haven't had a chance to test this as the indicator doesn't work, I'm new to the boat and it is currently 800 miles away and dry docked for the next 3 months. In the mean time I'm practicing navigation and trip planning but I need to know how to estimate speed in order to get elapsed time for my planning.

How would I determine the minimum wind required to get the boat up to 6 knots on a reach. Would it be different when close hauled? Is there a formula for this?

[Don C L]

Interesting question but really there is quite a bit more than just wind speed involved to answer it. Ball park, with a big genoa, on a reach, 10-15 knots. To weather, working jib, at least 15 knots. Now that is just a stab in the dark.. need more info on loading, sails, trim, sea conditions and more

[sy_gilana]

What you are looking for is called a "Polar Diagram" for your boat.

[Stumble]

6kn on that boat is probably close to a high average in perfect conditions. Realistic planning speeds are more like 4kn. The easy way to find out is a cheap GPS or chartplotter but that will only give you SOG. It's also nice to have STW (sped thru the water) since it help you find currents and surfable waves.

As Gilana mentioned you are asking for a polar diagram for your boat and it is very possible you can buy one from US Sailing. Or the designer for your boat I find they are still in operation. But that will only give you a place to start. Polaris assume light loads, good condition sails, and a clean bottom. With modern software it is now pretty easy to create custom pillars but it takes a lot of time on the boat to do so.

[MarkJ]

I have a 39 foot boat with a hull speed of 9.96 knots. When I sail it solo I plan a passage speed of 5.5knots.

I think u should plan 4.5 knots. It will be near enough.

[StuM]

I've got a reasonably performing 43ft cat (well, reasonable for a condomaran!) and I only plan on 6 - 6.5 knots for passages.

Having done quite a few longish passages in a range of monohulls from 30-45 foot, I fully agree with MarkJ. Those are realistic overall speeds in practice.

[Sandero]

theoretical hull speed is 1.34 x square root of the LWL in knots

if you have a 36' LWL the hull theoretical hull speed is 1.34 x 6 = 8 knots

Shiva's ~30 LWL does 150 - max 175 miles a day offshore in good conditions

[StuM]

Originally Posted by Sandero

theoretical hull speed is 1.34 x square root of the LWL in knots

if you have a 36' LWL the hull theoretical hull speed is 1.34 x 6 = 8 knots

Shiva's ~30 LWL does 150 - max 175 miles a day offshore in good conditions

How often are conditions that good?
6.25 - 7.3 knots average for 24 hours? That's more than I'd plan on.

The OP has a 25.4 ft LWL making his theoretical hull speed about 6.75 knots.
And with a full keel, cutter rig and displacement not much less than your Contessa 36, it's not going to be a speed demon.

Edit: Sorry Sandero, I misread your profile as Contessa 36, not Contest 36

[Don C L]

So, to help out the OP, he was asking how much wind is required to get the boat up to 6 knots or its hull speed. Anybody know a calculation for that?

[MLOI]

Originally Posted by MarkJ

I have a 39 foot boat with a hull speed of 9.96 knots.

How do you manage that then? An extra magic fold out 10ft of waterline? Can I have some too?

But I agree with your point - I've got 33 ft of waterline and plan on 5 knots. Only time we seem to do better than that is when we have a tide restriction and don't want to arrive early.
And for shorter hops, don;t forget to add in 1/2 hour of futzing around getting sails / reefing lines / dock lines / fishing gear / drinks sorted before you actually get up to speed... Or an hour if you have someone new on board who wants to help.

Mike

[StuM]

Just a WAG, but I'd say somewhere around 15 knots to hit 6 knots on a reach.
I doubt that it would hit 6 knots close hauled or running with white sails in any reasonable conditions.

[Paul Elliott]

Originally Posted by Don C L

So, to help out the OP, he was asking how much wind is required to get the boat up to 6 knots or its hull speed. Anybody know a calculation for that?

We are a heavy cruiser, and a little "sticky" in very light air. I figure that if we are in good form we can very roughly sail about 1/2 the wind speed up to the point where we are approaching hull speed. Perhaps slower hard on the wind, or dead downwind. In light air and confused seas it's going to be slower. If we are flying the spinnaker it's going to be faster.

[MarkJ]

Originally Posted by MLOI

How do you manage that then? An extra magic fold out 10ft of waterline? Can I have some too?



Mike

7.96 knots!

Sorry, typo in my original post that I can't now edit!

My hull speed is 7.96.

As a long range sailor I slow the boat when I am near 8 knots as the forces exerted increase exponentially.

The OP has a Hull Speed of 6.8 but as Stu says, unlikely to achieve that easily unless dropped by a crane.

As for 175 nms per day I think I have achieved that, in the last 40,000 nms a hand full of times only... and most of them were on the Galapagos-Marquesas run with 25 knots at 120 degrees for 20 days and current up the bum.

A heavy 31 foot boat with 25 ft lwl will average much, much less than my boat even in excellent (not just 'good') conditions.
In PLANNING one is a fool to plan on excellent conditions and plan on having no problems, no lulls, no tricky sail changes that stop u for 1/2 an hour in 24 hours, current never against you.
Its only a fool who PLANS a perfect trip. Because perfection does not exist in long passages (nor short).

So if the OP plans at 4.5, 4.25, he should arrive as planned, maybe a few hours earlier.


On this last trip of 1,300nms I arrived 4 DAYS earlier than I planned. Wind up the bum instead of having it dead on the nose. So I had extra naughty snacks, plenty of meat still in the freezer, and felt good to be always in front of the plot. (Yes psycholgy is important on long passages).


So plan slow, sail slow, break nothing, enjoy the days and nights, and most importantly: Get There!


Mark

[MarkJ]

Originally Posted by MarkJ

I have a 39 foot boat with a hull speed of 9.96 knots. When I sail it solo I plan a passage speed of 5.5knots.

I think u should plan 4.5 knots. It will be near enough.

Filthy liar!

My hull speed is 7.96.
Thats was a typo saying 9.96

Sorry

[MarkJ]

Originally Posted by dmksails

How would I determine the minimum wind required to get the boat up to 6 knots on a reach. Would it be different when close hauled? Is there a formula for this?

1/3 of reaching wind speed. And never close hauled.

So for you to get hull speed you want 20 knots at 120 degrees off the bow. You might get there at 18knots but not cinsustantky to average 6 knots.

Remember every sailor lies: "How fast did I go? I saw 9 knots! I saw 9 knots often on the dial. Average? Oh, I averaged 6 nms that hour.........