Small Powerboat Seakeeping

[intrepid71]

From my own boating experience, I have found that passenger comfort aboard small powerboats traveling at speed through moderate to rough coastal sea conditions often leaves much to be desired. Small powerboats that fall in the 16 to 25 ft range are possibly the most popular of all recreational boat categories. Since many of these boats are operated on bodies of water large enough for a stiff afternoon breeze to kick up sea conditions that make traveling at cruising speed uncomfortable, it seems to me that this is an area in the field of boat design where significant improvement would be welcome.

The first place to start would be to explore the current state of the art. With that in mind I pose the following query: For a modest sized powerboat, say 22 ft., traveling at 30 knots in 4ft wind chop sea conditions, what boat type or hull configuration would offer the smoothest, most comfortable ride?

The second question is an extension of the first. Are there any boat designs or configurations that are not currently on the mass market that may offer significant improvement over currently available types? For example hydrofoils come to mind as a possible way to effectively "decouple" a boat hull from a rough water surface. Is this an area that is ripe for innovation? Do you think boat buyers, both recreational and govt./commercial, would be willing to pay for extra for a boat design that can offer significantly improved seakeeping at speed in many sea conditions?

[tom28571]

The best boat to run 30kts into 4' wind chop?

To me, 4' ain't no stinkin chop. Too many overstate the height of sea states. Not saying that you are doing that but 4' is pretty big stuff unless on the open ocean with widely spaced waves. Also if average waves are 4', then some will be much bigger, like 6' or so and these are the ones that you need to be concerned about. In closely spaced waves like that on shallower inland waters, most sensible people are ashore or running at reduced speed if they have to be out.

It may not be too difficult to come up with a boat that will do what you ask but what about its atributes and behavior in other situations?

Multihull?
Swath?
Foils?
Ground effect?
Sea sled - IVB?
Wave piercer?

All of these will probably have trouble at the length you want.

[intrepid71]

Well, for one thing when I am saying 4 ft chop I am talking about about a wave with 4 ft height from trough to crest, some may consider that to be a 2 ft wave height. A true 4 ft wave with a steep 8 ft face is too much for any 22' powerboat to take at speed IMO.

My point of reference is from boating on the South Shore of Long Island. On a typical summer day, a brisk southwest breeze often sets up and by late afternoon, the Great South Bay get rather snotty, especially when boat wakes coming from every direction get combined with the wind sea. My parents have 40' wooden Chris Craft which cruises along at about 16 to 18 knots. In those conditions they will take spray over the flybridge and have a few good thuds along the way. My boat is a 22' Mako Semi Vee, and running above 20 knots in those same conditions I will get pummeled and completely soaked in the course of a 30 minute ride across the bay. If a passenger is on board it is an uncivilized ride to say the least. Now granted, a deep vee or cat would offer some improvement, but my experience with the deep-vee at least indicates that this is a moderate improvement only and the ride would still be uncomfortable.

Starting with this common experience on the Great South Bay I can then extrapolate for people who operate on larger and deeper bodies of water and I see a need for better seakeeping in small powerboats. Of course, I haven't sampled every type of small powerboat out there, but that is why I sent the original question out to the forum.

[randy kahn]

Intrepid71, My expreience has proved a power cat (of good design) to be the best all around water craft for passenger comfort in the size range (and larger) that you decribed. I believe a real open water comparison, cat & mono-hull on the same day will always provide that proof. Please try it and let me know your opinion. Also choose a sloppy day you would not normally take your mono-hull outside so there will be no doubt. Thanks, Randy

[intrepid71]

Randy,

Thanks for the reply. I suspected that the powercat hullform probably represents the current "state of the art" for seakeeping in a small powerboat. I imagine a displacement type such as Glacier Bay would likely offer the best ride although it would be a bit slower than the planing cats due to higher wetted surface. I have never taken a ride on one of these boats, but one day I would like to try it. I have read that while the ride is superior to a deep-vee, the cat form is not a cure-all and you will still get bounced pretty hard if the conditions are right. Of course this is true for any vessel, but I guess my point is that I am not sure that the power cat form represents the best possible solution.

When it comes to a truly smooth ride it seems to me that a fully submerged hydrofoil would have the most potential. Of course a traditional hydrofoil design has some serious drawbacks such as deep draft which would make it unsuitable for a small boat. What I am envisioning is a small hydrofoil boat with foils that would retract into the bottom, eliminating the draft problem. Furthermore, the retraction mechanism would be designed such that foils could be deployed or retracted while the boat was at speed, giving the operator full control over when and where he wanted to be in hydrofoil mode. For stabilty and trim while on foils, perhaps some sort of wand-style feedback control system could be used, similar to the Rave sailing hydrofoils. Finally, there is the problem of keeping the prop in the water as the hull rises up, which might be solved by putting an outboard on a vertically swinging pantogram bracket and have this synchronized with the foils being retracted or deployed. Of course, putting all of this mechanism into a small, lightweight package that doesn't ruin the aesthetics of the boat and cost a fortune will be quite a trick. I have wrestled with the idea for a while and have decided that it is beyond my design ability. That doesn't mean there isn't someone out there who couldn't pull it off. When you think of all the complicated mechanisms they fit into a car, it really isn't asking all that much.

[tom28571]

I agree with Randy that a multihull of some sort is probably the best compromise available right now.

As far as I know wave height has always universally been the vertical distance from trough to peak.

My main point was that a 22' boat that could excel in the conditions you described will surely be way less than optimum most of the time and next to useless some of the time. Of course, nothing is certain until you try, so go to it.

A small cat is great in waves some of the time but when the water hits the underbelly at speed, you better be hanging on.

[mattotoole]

Cats used to be very popular in Australia, and probably still are. I rarely see them in North America though.

Some boats in this size range are advertised as having "seakeeping" hulls. This probably just means more deadrise than a typical runabout, but not quite deep-V. Am I right?

[woodboat]

From a ride perspective on a small boat the best I have ever been in was my friends old 17' AMF crestliner. Closed bow, very deep V, no lifting strakes that I remember so it was a very smooth bottom. It would slice through waves like the edge of your hand in the bathtub It was reasonably fast, about 42 MPH with an inline 165 HP 6 cylinder. I raced him in some rough stuff. I had a 15 ft Mckee craft, solid boat, hull shaped similar to a whaler. I was flying out of the water, catching air, heck shaking my teeth loose. I finally had to give up. He just pushed the throttle down and pulled away. Turns out he didn't know I was racing and he simply was driving slower and waiting for me. I remember watching it slice through with everyone sitting comfortably as I slowed to a crawl. I think current designs go for more lift and efficiency from there high speed runabouts. Of course a retractable hydrofoil sounds cool

[gerard baladi]

I have been looking for that perfect hull as well for the past year, as I encounter the same kind of sea and wind conditions described.

I am now looking at a Tunnel hull option 32' beam 8'6"(trailerable/twin 225Hp) . I am told that tunnel hulls are the cross of the 2 both worlds of deep V and Cats. Any comments?

What about Stolkraft Hull, has anybody tried them? They are so rare that I wonder why?

[intrepid71]

My take on tunnel hull is that it is basically a cat with assymetrical hulls. From head on it looks like a deep-vee with a tunnel down the center. I believe the idea is to get the seakeeping of a cat with handling characteristics of a deep-vee. Specifically, the tendancy to bank into turns rather than away from them. I have never ridden in such a boat, but it seems like good idea. The following link shows a boat that may not be a true tunnel hull, but it does have assymetrical hulls:

http://www.corsairmarine.com/0FoilerIndex.htm

The other very interesting feature it has is a hydrofoil situated between the hulls for additional lift.

[intrepid71]

Originally Posted by gerard baladi

What about Stolkraft Hull, has anybody tried them? They are so rare that I wonder why?

The main advantage of the Stolkraft hull is increased efficiency, I believe. I think they claim it has good seakeeping as well, but from the pictures I have seen it seems there will be plenty of relatively flat bottom area exposed to the waves at speed and usually that means pounding. I haven't ridden one so I can't say for sure.

As far as why they are so rare? It appears to be a complex hull shape which might be somewhat expensive to build. It is also not the prettiest shape in the world, which might limit its mass market appeal. Also it is patented design, which may be limiting its use. Perhaps the inventor is asking too much money.

[intrepid71]

Originally Posted by mattotoole

Cats used to be very popular in Australia, and probably still are. I rarely see them in North America though.

Some boats in this size range are advertised as having "seakeeping" hulls. This probably just means more deadrise than a typical runabout, but not quite deep-V. Am I right?

The max deadrise that you typically seein a small powerboat is 24 degrees at the stern. Anything more and you get diminishing returns and a tippy boat at rest. The amount of deadrise that is needed to make it a "seakeeping" hull is really however much the marketing people decide.

[intrepid71]

[quote=woodboat]From a ride perspective on a small boat the best I have ever been in was my friends old 17' AMF crestliner. Closed bow, very deep V, no lifting strakes that I remember so it was a very smooth bottom. It would slice through waves like the edge of your hand in the bathtub It was reasonably fast, about 42 MPH with an inline 165 HP 6 cylinder.

Yeah, removing the lifting strakes might help soften the ride. A nice heavy inboard also helps in giving a solid feeling ride, by putting more weight lower and forward of where an outboard would be.

[intrepid71]

Originally Posted by tom28571

I agree with Randy that a multihull of some sort is probably the best compromise available right now.

As far as I know wave height has always universally been the vertical distance from trough to peak.

My main point was that a 22' boat that could excel in the conditions you described will surely be way less than optimum most of the time and next to useless some of the time. Of course, nothing is certain until you try, so go to it.

A small cat is great in waves some of the time but when the water hits the underbelly at speed, you better be hanging on.

Regarding wave height, surfers consider it to be the height of the crest above the mean water level. So a four foot wave to them will have an 8 ft face. When talking boats, I will stick to the trough to peak definition.

Regarding your second point, I would have to disagree. It seems to me that any boat that can handle rough seas at high speed is not going to have trouble when the water is glass flat or any condition in between. Perhaps it would not be as effeicient as a boat optimized for flat water, but it would hardly be "useless". Of course the opposite is not true, a boat optimized for flat water may in fact be useless (and dangerous) in rough water.

[tom28571]

Originally Posted by intrepid71

Regarding wave height, surfers consider it to be the height of the crest above the mean water level. So a four foot wave to them will have an 8 ft face. When talking boats, I will stick to the trough to peak definition.

Regarding your second point, I would have to disagree. It seems to me that any boat that can handle rough seas at high speed is not going to have trouble when the water is glass flat or any condition in between. Perhaps it would not be as effeicient as a boat optimized for flat water, but it would hardly be "useless". Of course the opposite is not true, a boat optimized for flat water may in fact be useless (and dangerous) in rough water.

I'm not surprised that surfers would have different terminology, but should that change hundreds of years of nautical usage?.

On the second part. I stick by what I said, not your interpretation of what I said. By "excel" in rough water, I meant "optimized", just as you apparently meant by the "optimized" flat water boat.