Displacement or Semi-Displacement Hull

Hello,

Need some help with hull selection for coastal cruising where winds are typically 20 to 30 knots, displacement hull or semi displacement hull? Boat would also be used for short ocean passages between mainlands. How do the hull designs differ - moderate rocker, no rocker, part motor boat? Can a 24 to 30 foot cruiser safely handle these conditions on a regular basis? Primary design consideration is crew comfort while underway and at anchor.

 

You should read this first:
http://www.boatdesign.net/forums/powerboats/semi-displacement-hull-30169.html

Like everything in design, all depends how much you're willing to compromise...and no two solutions are the same.

 

Like the word implies, "semi" is not the real thing.
On these boats from the helm you cannot see where you are going because all you see are clouds, seagulls and the moon. When I see a semi-planing craft, the word impotence comes to mind.

I own a semi-dry neoprene suit with very tight collars, but still get awfully wet in it.

 

If you shave that beard, the collars will work better and you'll be much drier.

 

Coastal cruising in these wind strengths carry other sea state conditions, such as chop, swells, boarding waves, minimum and no wake zones and contrary currents. Given this set of variables, your crew and guests will be more comfortable in a full displacement vessel then attempting to "punch through" in a semi displacement craft. Quite often a skipper doesn't do the reasonable thing when conditions warrant it, such as slow down. I've found most will just hammer on, looking for speed and this is when the boat starts to pound and take on water, while the guests and crew resist motion distress. A vessel that hasn't this ability can't be driven too hard, so the crew and guests are more comfortable. A displacement craft will also be more economical to operate, though as you're aware, at a reduced speed.

 

Where is the cruising area?

 

Displacement or Semi-Displacement

Coastal shores of the North Atlantic during the summer - "Strong Wind Warning In Effect" nearly every day.

 

Heavy displacement is what I hear recommended (almost) all of the time also. Speed is often the reason others will suggest semi-displacement - to run from the storm.

Boat length is my the other critical factor. Most Cruisers are in the 40 foot plus range, but these cruisers are also designed for long ocean passages; which is not what I am looking for. I am looking to cruise the Eastern Sea Shore, where the longest ocean passage would normally be from Yarmouth, NS to Bar Harbour, Maine. It is the strong wind effects that I would be encountering most of the time. Does anyone have any recommendations on hull length for coastal cruisers?

 

Maybe you could run away, or maybe you could not.
For a 40-ft vessel the speed difference between a D and S-D vessel is 8.5 kts vs. 12-13 kts (approximately) or 50%. I think you should first try to figure out whether you could realistically manage to escape a storm with a 13 kts vessel, considering the time margin from the storm allert, most probable sea state in your operative area, most probable distance from a nearest port or haven, most probable wind, currents etc.

Would a difference between 12-13 kts in S-D vs. 8.5 kts in D regime really give you enough time to run away from a storm, considering all of the above parameters? The validity of the quoted claim depends on the answer to this question.

 

Speed has an advantage for getting to port before dark but the "run and hide from weather" in a faster boat only works 2% of the time. I don't know where Yarmouth is! A motor sailer could be your best choice. I don't like them but they deliver a high degree of safety and comfort.

 

If your primary concern is crew comfort, and the boat is limited to 24-30' long, a stabilized full displacement hull will be the most comfortable in all conditions. Heavy displacement and low speed, coupled with good motion damping and low accelerations, will keep people happy.

But that means spending longer periods at sea. If you don't want to spend long periods at sea, then you want higher speed and the semi-displacement hull is the answer. But you pay for that higher speed....big time.....

One answer to the higher speed in a sea question was created by Peter Thornycroft in about 1959, obviously building on the prior work by Laurent Giles. The Espoir is 33'6" by 9' 5", max speed is 23 knots with 350 HP.



My own answer to the question incorporates a hard chine aft for additional dynamic stability and lift.

 

Heavy displacement may not be necessary, but "healthy" displacement would be desirable. As has been mentioned, you're not going to "out run" storms. Those that think this way haven't a clue or pay at least 10 times your expected fuel bill. You're going to get "caught" out and the weather is going to turn on you. These are facts of life when people put to sea. The wise skipper plans his route and doesn't permit a schedule, to force his hand over weather concerns. If you want trouble fast, ignore this tip. Most difficulties at sea will be related to being there at the wrong time, which is forced by attempting to stick to a schedule. Waiting a day or two so a low pressure system passes or a boat system is completed fixed, through may be inconvenient, you'll arrive (late) with no damage or injuries.

As Tad mentioned, you can kiss goodbye to the fuel economy issue if you elect to travel at speeds where you can think about staying ahead of a weather front. This isn't often you can do this as it seems, at least with most people's luck, the weather pattern is moving at you, from the direction you want to go. Though occasionally, you might find a squall or front angling in to get you and a quick course change and WOT for a few hours can keep you out from under it. These are full plane vessels and you'll literally be tossing hundred dollar bills in the tank every few hours (or less, depending on size, vessel, speed, etc.). You will literally jump from a few gallons per hour, to dozens of gallons per hour, possibly many dozens, depending on what you elect to use.

40' isn't necessary, as a 30' boat can offer the comforts you seek. The additional length of 40' can help in the efficiency department and with motion, but you have to pay for this in other ways, such as slip fees, haul outs, etc. There are many design in this range, have you any specific ones that tickle your fancy?

 

Hello,

Thanks to all for the information. I agree with the full displacement hull recommendations also. I also agree that trying to run on a schedule is unsafe. The boat is strictly leisure time. As far as design objectives go, I am looking for a motor boat, diesel or outboard(s). I will want the most fuel efficient and reliable engine I can get, and believe that a marine diesel will likely be the best way to go. I do not need a whole lot of horsepower, and have not looked into the diesel option yet. As for the design, I would like a highly stable boat (to 180 degrees) with maximum interior space. I would like as much of the vessel to be as watertight as possible. When myself and company step outside the boat, we will be on the deck and the boat will not be underway. Looking at the initial design, the largest obstacle I am coming across is headroom for the 24-30 foot range vessel. The cockpit looks like the only place where standing room will be possible, so the cockpit will need alot of usable space - lavatory, cooking, etc. This will be a live-aboard vessel up to 6 months out of the year. Construction: Multi-Chine Wood Construction (partly for ease of construction). Materials: Wood or Duracore type Plank, epoxy and fiberglass - a low maintenance type vessel. A solid long lasting boat.

 

Power boats which have a positive righting arm at 179 degrees are rare, typically CG rescue or pilot boats. They are very specialized and do not have "maximum interior space". In fact they have the minimum interior space, arranged in a tall narrow box (headroom is not a problem) with tiny windows and nothing movable in it. This is not practical or achievable for live-aboard cruising.

"much of the boat as watertight as possible" This would be minimum criteria for any boat.......

 

This cannot be repeated too often!.............:!:

First obligation: learn weather
Second obligation: avoid weather
Third obligation: forget the meaning of "schedule"

Not competent to add my mustard on the semi displacement issue, I leave:
Regards
Richard