Semi displacement and quartering seas

This is how the RNLI sorted the problem of getting a lifeboat to track straight in a following sea; it took 8 years of research!




It looks a bit like the fins on a surfboard! Not unlike the entry/stern sections on the Hatt 48 LRC.

interesting jet power

Rustybarge said:

This is how the RNLI sorted the problem of getting a lifeboat to track straight in a following sea; it took 8 years of research!

It looks a bit like the fins on a surfboard! Not unlike the entry/stern sections on the Hatt 48 LRC.

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Being jet powered it has it own set of slow speed tracking issues. I imagine large power plants and plenty of speed.

Sure but the good thing that those Fins do w basic tracking turns into a bad thing at times while broaching. The stern moves sideways quickly and the drag of the slab side of the hull between the WL and the chines or the even lower fins tend to or do cause capsizing. It's usually called chine tripping and a double chine is seen as a good solution to that problem. Double chine hulls are often or usually called Sampan hulls. Small racing outboards of the 50's w "anti tripping chines" are an excellent example.

Speaking of Sampan hulls I'd propose that the classic GB's would benefit from a double chine. One could make the hull even wider and reduce the power needed for propulsion.

manyboats said:

Sure but the good thing that those Fins do w basic tracking turns into a bad thing at times while broaching. The stern moves sideways quickly and the drag of the slab side of the hull between the WL and the chines or the even lower fins tend to or do cause capsizing. It's usually called chine tripping and a double chine is seen as a good solution to that problem. Double chine hulls are often or usually called Sampan hulls. Small racing outboards of the 50's w "anti tripping chines" are an excellent example.

Speaking of Sampan hulls I'd propose that the classic GB's would benefit from a double chine. One could make the hull even wider and reduce the power needed for propulsion.

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Im No NA, but having a guess: lol....

The wave energy from behind tries to push the hull sideways. The boat naturally rolls as its breaching towards the side which is closest to the actual wave face.

A big keel/skeg will get pushed sideways by all that wave energy causing a broach.

Maybe The little fins act like fins on a surf board, only helping to keep the boat straight when its hurtling down the back of a wave. I can't see how they could stop broaching?

But..
the lifeboat has virtually no draft, so maybe that's what stops it being pushed sideways; there's not enough hull underneath the waterline to get pushed about.

manyboats said:

Many sit-on-top kayaks employ the same hull displayed in the Corvette.

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My wife's Hobie Revolution looks like the that!

Just by looking at the lines of the Hatteras it appears to have a few differences than most power boats.

the thin bow entry with a thin middle
decent sized rudders for a twin
cut away after section of the keel
sloping hull sides aft vs the normal slab sides of a semi displacement boat
the combination of rudders/props being far aft and outboard
the apparent aft cg/ce of the boat due to house and mechanical location
and they probably have a powerful steering system.

all the factors more than likely add up to a great handling boat.. they have always been a favorite of mine.. now if I could find a well kept one with Luggers vs the D.D. 4-53's and get the Admiral on board to go cruising again...
HOLLYWOOD

and yes we do use the term of "Admiral" in our house.. at her request.

Legend said:

There seems to be only one handling characteristic of my Halvorsen that I dislike. It is when I find myself in quartering seas. They lift up a corner of the transom and with the semi displacement deep forefoot she will catch an upcoming wave at an angle creating bow steer.

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As people have already mentioned, when the waves are from directly astern the trick is to match your speed to that of the waves and settle into the trough for a smooth ride.

Sometimes though, like when conditions are getting worse and it's time to hide in a protected bay, you have to move downwind diagonally across the direction of the waves, which is what your question is about, right?

We live at the head of a narrow 60 mile mountainous channel which funnels the winds, so have a bit of experience with this.

We stay in the trough and turn to take the waves on a slight angle, but not so far that there is a danger of going out of control and taking waves over the side. Speeding up slightly is needed to stay in the trough.

If it turns out the comfortable angle to the waves isn't enough to get us to the protected bay, instead of increasing the angle and risking broaching we'll wait for a flat section in the waves, then make a fast turn into the waves and take them diagonally on the bow until at a better angle for another downwind shot at it.

Just some thoughts from a relative rookie...

In wind driven large quartering seas we have noticed this in our Bayliner as well. In large rollers, it is not so bad, but when the wind stacks the waves is when there is an issue.

What I found, (except for avoidance) is to hand steer in these conditions. As Marin indicated the autopilot only knows about what is happening, and tries to correct for it. As humans we have the ability to anticipate, and we can swing the rudder to pre-steer making for a much more pleasant ride.

My autopilot has a option of a joy stick or knob for steering. I was thinking about buying one of them and experimenting with it.

manyboats said:

Sure but the good thing that those Fins do w basic tracking turns into a bad thing at times while broaching. The stern moves sideways quickly and the drag of the slab side of the hull between the WL and the chines or the even lower fins tend to or do cause capsizing. It's usually called chine tripping and a double chine is seen as a good solution to that problem. Double chine hulls are often or usually called Sampan hulls. Small racing outboards of the 50's w "anti tripping chines" are an excellent example.

Speaking of Sampan hulls I'd propose that the classic GB's would benefit from a double chine. One could make the hull even wider and reduce the power needed for propulsion.

Click to expand...

A photo of an Ed Monk designed Ocean Alexander hull. The boat cruises on the "inner" hull width/chine at slow speeds. When it begins to squat with application of higher power, the outer width provides additional lift. Moonen has employed the same design concept on at least one of their current "efficient" (their word) hull designs. OA also pointed out in their literature that the double chine functionally acts to "soften" the hard chine in operation. This particular hull is exceptionally efficient both at hull speed +, and in the trans "hump" to low semiplaning speed range. It also has a very nice roll characteristic that is somewhere between a hard chine and a rolled chine. While it experiences the typical quartering seas steer issue....the relatively large rudders and a pretty smart and quick W-H (Will Hamm) autopilot go a long way toward mitigating the phenomena. So, here's a hull with some genuine engineering applied to the efficiency aspect of design. When you read an OA ad that says engineering and performance are their overriding design objectives, they aren't blowing smoke.

Agree that GB and most other SD hulls could benefit from the double chine...but they're plenty wide...actually too wide to begin with. Going inward from the current width would make more sense from an efficiency perspective.

Semi,
Yes the rudders do look bigger than on the Hatt.
As to double chine the OA pictured is not what I had in mind but very similar. A much wider flat between the bottom and the topsides as found on the Payson "Diablo" skiff. Would have the dynamics of a much narrower boat. The more I think about it the more I can't understand why it's not common. There are reasons of course and it's probably not just because no one has thought out of the box. But that's always a remote posibility. Kinda like a prospector finding the mother lode but I suspect most good NA's probably get well known by designing convential hulls.

I agree w Kevin and Marin that anticipation is the name of the game on following seas and in other circumstances like plunging into a strong side current.

HOLLYWOOD,
The Hatt is a very popular and respected boat. And got there by being a very good design.

Rusty Barge,
Thanks so much for the very interesting link.
What is puzzling to me is how the exit lines at the aft end of the bulge would not be enough drag to prevent the boat from getting anywhere near 20 knots. The abrupt termination of the bulge would be too much drag .. IMO. But appearently it's not. However if shown the lines beforehand I would have said the speed would be limited to 13 or 14 knots.

manyboats said:

Semi,
Yes the rudders do look bigger than on the Hatt.
As to double chine the OA pictured is not what I had in mind but very similar. A much wider flat between the bottom and the topsides as found on the Payson "Diablo" skiff. Would have the dynamics of a much narrower boat. The more I think about it the more I can't understand why it's not common. There are reasons of course and it's probably not just because no one has thought out of the box. But that's always a remote posibility. Kinda like a prospector finding the mother lode but I suspect most good NA's probably get well known by designing convential hulls.

I agree w Kevin and Marin that anticipation is the name of the game on following seas and in other circumstances like plunging into a strong side current.

HOLLYWOOD,
The Hatt is a very popular and respected boat. And got there by being a very good design.

Rusty Barge,
Thanks so much for the very interesting link.
What is puzzling to me is how the exit lines at the aft end of the bulge would not be enough drag to prevent the boat from getting anywhere near 20 knots. The abrupt termination of the bulge would be too much drag .. IMO. But appearently it's not. However if shown the lines beforehand I would have said the speed would be limited to 13 or 14 knots.

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What's even more surprising is the original power installed...

>'The engines favoured by the original builders were twin 106 hp Volvo Penta D32s, linked to sterndrives, and 105 hp Mercedes-Benz OM352s, with V-drives, top speed being about 16 knots in each case. This configuration generally required more use of the trim tabs to travel at the optimum angle'

That's some efficient hull!

Our Halvorsen 57 will tend to wander when in a following sea , but they need to be fairly big sea's. On my last boat a 50 footer when originaly launched it apparently tendered to broach , round up. So the original owner had the keel extended to almost a full length keel and this fixed the problem to a point where it was one of the most straight tracking vessels I have ever had. It had twin 210 hp cummins and was a pleasure to be on in following sea's, the bigger the better. I suppose when docking it needed a bit of extra grunt to kick the stern around , but still fairly responsive.
I have been considering doing the same with Liberty, and I recently had the original Owner who commissioned Liberty on board for a couple of weeks whilst Circumnavigating Tasmania and he agreed with me that extending the keel in length may make her shoot straighter in a following sea.
We have active stabiliser fins, but they still have the original control system which reacts to roll. We have been advised that if we were to update the control system this would dramatically improve the performance in following sea's also.
So these are two items I have on the agenda at present.

Codger2 said:

As has been posted...Trim for "bow up" in a following sea. Also, trimming in a quartering sea really helps with the ride.

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You're right Walt. More by necessity and good luck than intention, because most of my tankage is in the aft part of the hull, (PO did not remove the old fuel tanks, so they are still there as just buoyancy now), while not giving an obvious bow up trim, it does put more weight than usual aft, and my Clipper tracks beautifully in a following or quartering sea. I have seen over 11 kn, actually surfing, on big following waves, with a nice light feel on the wheel, and not a hint of broaching. Although I must admit Marin's comments re anticipating rudder corrections are spot on.

Best way to control yawing in following seas I have found is to tow a drogue.
We used to take a 39' Mariner full displacement down to Tasmania annually and whenever we had following seas (and they were big!) (we were young and stupid) we deployed a torpedo shaped hard plastic drogue called a Sea Squid, made in New Zealand.

As soon as it is deployed on a long line, with bridle, and chain just ahead of it to keep t down it is happy days! Literally straightens the boat, autopilot just steers happily, where without it forget it, no hope, you would have to steer all night long. Much harder to hand steer downwind at night without any reference points for anticipation. The drogue is incredible.

Limits speed to no more than 10 knots , it just hauls the back of the boat back straight. If quatering seas just set it to one side with bridle.

For long passages well worth the effort to deploy it, not so for a short run locally or in crowded waterways where someone might get caught up in the tow line, we deploy it out around 100 meters at sea so it is pulling through swells well behind you, its magic.

Still have it, still use it, it transforms any boat at or under 10 knots, we did use it on our Bayliner 4788 with great success up to 12 knots.

semi-planing said:

A photo of an Ed Monk designed Ocean Alexander hull. The boat cruises on the "inner" hull width/chine at slow speeds. When it begins to squat with application of higher power, the outer width provides additional lift.........

So, here's a hull with some genuine engineering applied to the efficiency aspect of design. When you read an OA ad that says engineering and performance are their overriding design objectives, they aren't blowing smoke. ]

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My OA 42 Altus is one of the best rides I've ever had on the water. (I believe, however, that the Offshore 48 has a better ride!)

Transitioning from 8 knots to 17 knots and on plane is a delight! Just an increase in speed with little to no bow rise. No tabs needed except for countering quarterly seas.

Sounds good,

What do you believe makes the difference with the ride, I am interested in your thoughts?

Cheers Chris D Liberty

Legend said:

My only solution to that other than a new course is to raise the bow as high as I can to try and avoid bow steer.

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I'd say those are your only options to minimise the yawing, along the the anticipated steering.

Every boat seems to have one type of water it doesn't like. With yours it's quartering seas.

With me, if the seas are quartering or more aft, the boat loves it. I leave it on AP with no yawing unless its very nasty. Anything more on the beam, is where I run into rolling problems.

Tough to find a boat that is steady in all conditions without some sort of stabilization.

Walt's good ride (and I'm not talking about a car) is probably because the boat's big and somewhat heavy but speed probably has more to do w it than anything else. The dynamics of boat motion favors speed ... up to a point that is probably higher than most trawlers cruise. Those big props probably have a stabilizing effect also.
Interestingly many boats have been built that are very similar but quite different in that the lower chine is soft .. Instead of hard. And for a different reason than the OA I think. The double chine lower being soft is to reduce the velocity of beam wise water velocity for the purpose of reducing spray and holding more water under the boat giving more lift. The hard chine low may be mostly for reducing wetted surface. Don't know if it works that way though.

AusCan,
I mirror your thoughts almost exactly. And for the obvious reason ... our boats are very similar.

manyboats said:

Walt's good ride (and I'm not talking about a car) is probably because the boat's big and somewhat heavy but speed probably has more to do w it than anything else.

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Couldn't agree more! The ability to alter one's speed to varying sea conditions is huge! When the sea is dead calm I run at trawler speeds and enjoy the noise reduction and the super smooth ride. But when things start to get a little bumpy, I put more coal on the fire to smooth things out. Both my wife and I really enjoy our boat's ability to do this.

With both engines running this is our speed range.

Walt actually that's not what I meant. However your point is well taken.

I'm talking about a dynamic form of stability that's present well above hull speed. A boat at rest or below hull speed rocks and pitches around sort of wallowing between waves but a big chunk of that wallowing motion disappears when a boat goes fast enough to leave the froth behind and leave her transom exposed to the air and sunlight. I don't have a name for it and don't recall reading about it either. Perhaps it's all just in my head wallowing around??? I like the motion of boats a little faster than FD excluding higher speeds w pounding and porpoising.

Walt maybe Larry could get rid of that rubber Duckie in the next slip in your avatar. Or you could take another picture. It dosn't flatter your boat.

manyboats said:

Walt maybe Larry could get rid of that rubber Duckie in the next slip in your avatar. Or you could take another picture. It dosn't flatter your boat.

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Will do! How's this?

Now take a pic w a little bit more .........

Naw just kidd'in.

Did you do that Walt? You're ahead of me.