Speed...

[alecs123]

Hello, last weekend I've just installed a new speed log. We did 6-7 knots upwind on 9 knots winds and 5-6 running (no spin), we were sailing with old sails, the crew was myself on main sheet and tiller and a friend that it's his 2nd time on board (so genoa was poorly trimed at most of the time). Other passengers included my wife and daughter and his wife.
I was gladly surprised about this. My boat is a Merit 25 with 20 lwl. I would say that if I use my newer sails I could be doing very close to 8 knots or maybe wind speed.
What's the typical speed for a boat like this?
Thanks

[sailingdog]

My guess is that you had a current, since the hull speed for a merit 25 is about six knots, and I don't know if you'd be able to exceed hull speed in just 9 knots of wind with old sails and a poorly trimmed genny.

Seriously doubt you'll be able to do wind speed without a current. Also, seriously doubt that you'd be able to do 8 knots. Typical speeds for a boat your size is abuot 4.5-5 knots.

I can do 9-12 knots regularly, but my boat is a seriously different beast and the hull speed is much, much higher, since it is a trimaran.

[AE28]

Originally Posted by sailingdog

My guess is that you had a current, ...

I agree that 6-7kts beating into a 9kt wind doesn't seem right for a monohull.

Presuming Alejandro's new speed log is mounted in the water (not a GPS), I don't understand what current has to do with it.

[sailingdog]

The knotmeter doesn't measure boat speed, it measures speed through water. The actual boat speed is Speed Over Ground, and a knotmeter can't measure that successfully if there is any significant current—but a GPS could. If he were tied up to a mooring, and there were a three knot current in the mooring field, the boat's knot meter would show a speed of three knots...yet the boat wouldn't be moving.

Likewise, if he were sailing with both a GPS and a knotmeter, the GPS would be showing a speed either three knots higher or lower than the knotmeter, depending on whether they were sailing with or against the current.

In the case of beating into a 9 knot wind and showing a 6-7 knot boat speed.... chances are pretty good that he was sailing at about three-to-four knots and had a three knot current boosting his knotmeter speed reading. It is very likely the boat was only moving at about a knot SOG in that case.

Originally Posted by AE28

I agree that 6-7kts beating into a 9kt wind doesn't seem right for a monohull.

Presuming Alejandro's new speed log is mounted in the water (not a GPS), I don't understand what current has to do with it.

[speciald]

Did you calibrate your new knot meter? Best way is to motor in opposite directions at the same speed and compare GPS SOG to boat speed. Best to do in area with little current.

[AE28]

SD

Let's say that Alejandro is beating into a 9kt wind and his water mounted sensor says he's doing exactly 6kts. This is 6kts through the water.

Let's presume there's zero current:

• His speed through the water is 6kts.
• His speed over the ground is 6kts.

Now, presume a 6kt current on the nose:

• His speed through the water is 6kts.
• His speed over the ground is zero.

Now, presume a 6kt current from dead astern:

• His speed through the water is 6kts.
• His speed over the ground is 12kts.

The way I see it, his in-water knot meter always says he's going 6kts through the water and the current only impacts his speed over the ground.

When we talk about "hull speed", aren't we talking about the boat's speed through the water?

In your example:
In the case of beating into a 9 knot wind and showing a 6-7 knot boat speed.... chances are pretty good that he was sailing at about three-to-four knots and had a three knot current boosting his knotmeter speed reading. It is very likely the boat was only moving at about a knot SOG in that case.

This still sounds like the hull is going through the water at 6 - 7kts.

Am I having a brain f*^t?

[tommyt]

AE, I can't wait for SD's response, but the answer to your question would be yes!

If he were anchored in a river with a 3 kt. current, his reading would show that he was moving at 3 kts. Anchors can drag, but they rarely drag upstream.

With that little bit of LWL on anything heavier than a Laser, that boat speed is a lot more than a little suspect. Some calibrating needs to be done in a non current area. A gps will get you close if there is no current.

[AE28]

tommyt

I agree - the anchored (not dragging) boat would show 3kts through the water, with a ground speed of zero.

[CharlieCobra]

Considering that a Merit 25 has a Hull Speed of 6.07 knots and also is capable of planing, these numbers aren't too hard to fathom.

[tomaz_423]

Dog, you got it wrong this time.
Hull speed is speed over water.