Getting up to hull speed

Posted by Mehfisto666@reddit | sailing | View on Reddit | 38 comments

Hi all!

I have fairly recently switched from a 28ft to an old 5.5ton 34 feeter. It's a medium weight ocean racer/cruiser which I'm really liking.

It has a theoretical hull speed of 7kn, but I find it actually pretty rare to reach it. With the old md11c 28hp engine i cruise finely at 6-6.2kn at 2000rpm (max 2500rpm), and when sailing I mostly also comfortably reach around 6kn, but i think i can count the times i got over 6.5kn.

For example a couple days ago we had some nice wind like 15-16kn of tws (close to 20kn of aws) on a close reach, and i think we could only get between 6 and 6.5kn of speed.

I did get over 7 a few times in pretty overpowered conditions.

So now I'm just wondering how easy it should be to reach hull speed. Is it something that i should achieve pretty easily or if it's normal to only get there with pretty strong winds.

The hull is clean but the sails are pretty worn out. The genoa is ok but the main while solid is really blown out i can tell that it really doesn't keep the shape that should have. So I'd be inclined to think of that as the main reason but having no experience on other boats I'm just wondering

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svsonora@reddit

no, CCA rules is why they have overhangs, to extend the waterline. IOR overhangs aren't as pronounced as CCA boats.

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TriXandApple@reddit

Sure

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dwkfym@reddit

P40 and B40. One IOR, other CCA.

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the-montser@reddit

Long overhangs are definitely pre-IOR

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Plastic_Table_8232@reddit

Those overhangs allow more canvas area. Recently boats have much higher aspect rigs so they are creating more sail area aloft in the absence of the overhang.

I do agree that stern design prevents the hull from dragging by breaking tension with the surface.

You can watch boats from the early 2000’s with the sugar scoop strait case sterns submarine in the rear. When sailing them I can actually hear the sucking noise it creates. A wineglass rear section tapered from a mid ship with descent tumblehome allows water to detach from the hull beautifully.

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the-montser@reddit

No, those overhangs are designed specifically to increase waterline when heeled. Boats of that era used long bowsprits when the goal was increase sail area.

There is plenty of literature about this.

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Plastic_Table_8232@reddit

It does both mate. The point was already made about increased lwl when healed. Some classes get penalties for a sprit under certain rating systems. For instance meter boats do not use sprits but have huge overhangs.

The basic point is the design was driven by class rules to increase speed cheating lwl and increasing sail area.

Many of the sprit boats were from a time when boats were head sail driven, smaller mains and substantial sail overlap.

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the-montser@reddit

This thread is about the origins of the long overlaps, which were first developed under rules that did not harshly penalize bowsprits, and predate the meter boats, and the J class I showed in the picture.

You’re right that longer overhangs are an alternative to bowsprits to allow increased sail area on a given waterline length, but the specific reason the overhangs came into fashion was to increase the waterline length as a way to take advantage of a rating rule, not necessarily increase sail area.

We can see evidence of this in boats like Reliance, the first four Shamrocks, and other early racing yachts.

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dwkfym@reddit

You call that an overhang? This is an overhang. (Trying my best oz accent and I can't attach photos from mobile but look up Bristol 40. Which is the type of boat a person 40 and the like replaces under ior rules)

Also let's try not to be sarcastic on the Internet.

nroose@reddit

What's the waterline length?

Rednmrfer@reddit

Think of hull speed like a theoretical max speed on a displacement boat.

Anything above hull speed you're pushing a bigger wave and pulling a bigger one behind you, making your boat dig itself into a little trough. The more poweet you put in above your hull speed, the more water you push and the farher you sink down into the trough.

That's why older boats have overhangs. When they heel, the portion of the boat in contact with the water is longer than the water line l, increasing the size of the waveform they sit in, which increases their hull speed.

Modern planning hulls avoid this by reaching speed and rising out of the water on a plane. They stop pushing that wave and rise above it, allowing them to reach higher speeds because they aren't constrained (as much) by pushing the water ahead of them.

Long winded way of saying routinely hitting max speed is really only doing to happen in ideal conditions. Any wind or waives while under motor and you're not pushing at hull speed.

Your boat will favour a point of sail (look up your polar diagram) and you'll see what conditions you sail fastest under.

I regret posting first. I would gladly get into it with all of you at the bar. (In a friendly way)

Raneynickelfire@reddit

That's not why they have oberhangs. IOR racing rules is why.

Oh that predates ior my friend. Waterline as a spec for a class is defined, then you put overhangs to increase your waterline length on heel up to the boats max loa.

nmperson@reddit

But also simple economics, no? Purchase price, maintenance, and slip fees all scale with LOA, if I can give you a better top displacement speed or LWL at a lower LOA, I will.

Overhangs increase the length of a boat for a given LOA not decrease it.

Ah, I see what you mean. But really almost all boats are designed to have a longer LWL when heeled so the OP’s intro to that paragraph is really just a non sequitur?

StumbleNOLA@reddit

This is just not true. It is a rule breaker under IOR but that’s about the only reason to do it.

hellowiththepudding@reddit

Overhangs are the opposite of that.

You’re both right.

Older boats have overhangs because they increase waterline length, increasing speed potential. The reason they did this instead of build boats with longer waterlines is because racing rules penalized waterline harshly.

Really it’s more related to the rules that predate IOR like CCA, Universal Rule, etc.

Overhands got shorter when IOR came to town.

Just-Smart-Enough@reddit

Well, that and they're pretty.

blame_lagg@reddit

A little bit of a silly question but how are you measuring it?

Do you have a pitot tube / paddle wheel to measure STW directly or using GPS for SOG?

J109@reddit

Is your stern pinched?

Mehfisto666@reddit (OP)

What do you mean pinched?

a lot of old ior boats had narrow sterns that were pinched not wide ones.

frak357@reddit

Good comments pointing at new sails and prop tuning however, one thing many people overlook is weight. Look at how overloaded you might be with everything added to the boat. 🤔

gomets1969@reddit

This right here. We carry 90 gallons of water and 40 gallons diesel, not to mention everything else on board, so I learned to stop fixating on reaching theoretical hull speed.

Bad sails make it difficult and I’d guess you’re probably not sailing flat enough.

But really I would also just stop worrying about the hull speed. It’s just a theoretical number to describe the speed at which the wavelength of the bow wave is equal to the length of the boat. Some boats can reach this speed easily, others can’t. Some can exceed it easily, others can’t. It’s useful for a quick rough estimation of a boats speed potential but it’s not very useful beyond that.

garage149@reddit

This.

jh937hfiu3hrhv9@reddit

Sailing at hull speed is not easy sailing. It requires maximum pressure on the sails. You can most easily achieve it on s reach.

A lot of boats won't do hull speed going upwind. Try again on a broad reach. My boat won't do hull speed going upwind until it gets piping up 20knots or so.

5cheinwerfer@reddit

Maybe take a look at your antifouling?

pdq_sailor@reddit

You may not have an optimal prop in terms of pitch for your engine... Keep in mind that you likely have a two to one reduction gear in your transmission so shaft RPM is half what engine RPM is.... Try running it at 2500 and see how your speed is but it sounds about right for longevity and durability being below hull speed..

In terms of sailing - there are a lot of factors to optimize performance... mast tuning is a big one, sails are another.. a bagged main sail with deep camber is never going to be as fast as a flat sail... We have a fast set of sails and regularly for the same sized boat achieve hull speed or better... but our boat is optimized. Good sails, new lighter mast, new design rudder with less wetted surface and proper balance... Your boat sounds like a good one... We went up against a J34C on the weekend... 12,000 pounds, fractional rig good sails guy knew what he was doing... That boat carried more sail and has a longer water line... We were faster and not by a little bit.. We are lighter by a bit but again less sail area and less LWL... sail trim was perfect, mast is well tuned, we are a foot narrower - that helps.. We heel less - that helps a lot... I have also been doing this racing and cruising for fifty years and that helps too... I learned as a crew on a very successful ocean racer...

daysailor70@reddit

I have a 1967 40' 20,000lb full keel centerboard yawl and I routinely hit and exceed hull speed. My theoretical hull speed is 7.02, but with long overhangs, heeling increases waterline length substantially. I have hit 8.4 knots on a beam reach in a stiff wind. But, I have new sails and I think that's your issue. Right now, your sail foil shape is so bad, it tends to push the boat over, not forward. It's comparable to having too much sail area for the wind. You can actually go faster reducing sail area then relaxing the sheets and traveler to depower the sails. Also, if you LOA and LWL are close, you won't get the waterline increase from heeling. My LWL is 28', LOA, 40'.