Surpassing hull speed?

[bowman316]

So I have a sailboat with a displacement hull. hull speed is just below 8 mph.
today I was getting towed in after a break down. And the boat had me going about 6 mph, with a 2-3 mph current going against me.
When I got going fast, the stern stated getting closer to the water. I had water comming in the cockpit, because the bailing holes were below the waterline. The holes the make the **** pit self bailing. Maybe it was doing that because the boat was trying to get up on plane?

So if you had say a 200 HP outboard on the back of my 26 ft sailboat, would it get up on plane? or would nothing get it above hull speed?

Does a current going against your boat subtract from hull speed? So with an 8 mph current, I could not go forward without getting over hull speed?

 

[saildan]

Re: Surpassing hull speed?

So I have a sailboat with a displacement hull. hull speed is just below 8 mph.
today I was getting towed in after a break down. And the boat had me going about 6 mph, with a 2-3 mph current going against me.
When I got going fast, the stern stated getting closer to the water. I had water comming in the cockpit, because the bailing holes were below the waterline. The holes the make the **** pit self bailing. Maybe it was doing that because the boat was trying to get up on plane?

So if you had say a 200 HP outboard on the back of my 26 ft sailboat, would it get up on plane? or would nothing get it above hull speed?

Does a current going against your boat subtract from hull speed? So with an 8 mph current, I could not go forward without getting over hull speed?

Hull Speed, as it's used in most demonstrations, is a simple theoretical model loosely related to real world performance. The modern displacement sailboat hull is designed to change waterline length and wetted surface as it heels in the wind. This does not, however, make it hydroplane since there's still a good portion of keel plowing the water.

You can overdrive a displacement hull with extra power, but that still will not make it ride atop the surface. Instead, it will break through its resistance (bow) wave and immediately begin piling up a new wave.

If towed beyond actual hull speed the boat would push up an increasing wave and at the same time try and climb up the back side until the force of gravity held it from rising further. At this point the towing force would drag the hull through the remaining wave height. It would not skim the surface smoothly rather it would rise and fall somewhat dramatically. It's this attempted climbing action that is probably responsible for drowning your bailer outlets.


Sailboat struggling against fast current

 

[bosn buddy]

Re: Surpassing hull speed?

You have a displacement hull. Exceding holl speed may cause her to sail under. Reserch "sail under"

 

[jay_merrill]

Re: Surpassing hull speed?

Current does not affect the speed of the hull through the water, only the speed over the bottom. Moving into the current will reduce your speed over the bottom by whatever the current speed is, while moving with the current will increase your speed by the speed of the current.



???

 

[tommays]

Re: Surpassing hull speed?

Well my J24 surfs to 12 knots pretty easy on the right kind of waves (really common on the Great South Bay here )not so much any place else

And it has done some planing towards 20 knots BUT that has allways ended REALLY badly within about 25 seconds with the mast tip hitting the water as the boat rises out of the water about 8" and thats 8" of rudder you really need

The endings were hard on the boat and crew so we really have not tryed in the last 20 years

 

[emoney]

Re: Surpassing hull speed?

Proof in the pudding is I had a buddy who towed me out from the dock into deeper water so I could do some rudder maintainence. He got bored and hot and decided to floor his throttle. I had explained the whole "hull speed" theory before we left.

Luckily the tow rope snapped in half before the eye ring got pulled out of the bow. She was only going as fast as she was built to go....regardless of the fact that there were 175 horses trying to pull her faster. She's stubborn that way.

 

[Renny_D]

Re: Surpassing hull speed?

Only way to exceed hull speed on anything other than a multi, dingy or sailing canoe is surfing. Beware of surfing on any boat with a transom hung rudder as they are apt to cavitate the rudder and have very not fun round ups. Good down wind surfing boats have very little sheer on the bow, are narrow with lots of floatation forward of the keel and fairly flat sterns. Basically the boat starts tripping over the keel as it speeds up. That is the tendancy you feel of having the stern rise as you start surfing. The more rounded the ends the worse this gets. On small boats you can stack everyone on the windward stern quarter. Bigger boats you really need one built for it and a certain level of cohones. One of the funnest downwind boats I've ever surfed was a Santacruz 70. Now that is a boat built for downwind fun.. Can anyone say sphincter tighening?

Renny