Hull form selection

Hi all,

I would like to know the hull form selection guide line.

Thanks,

Donald

 

I'm afraid you're going ot have to be a liiiiitttttle more specific here.

 

Sorry,

I have a project of Fibre glass DIVER boat design about 55ft long with 10knots.
When I check the previous hull designs, I found that some are planning hull and some are semi displacement hull.

Which one should I select ? Why ?
What is the advantages and disadvantages of these hull forms.
Thanks for your replay

donald

 

Semi-Displacement

You're going to want to use a semi-displacement hullform. At that length and speed, you're not in the realm of planing yet. Semi-displacement will provide you with the best compromise in speed and seakeeping, particularly for a dive boat that needs to run offshore fast and often in waves.

 

'Fraid I'd have to disagree with you there bremerton-john. If max cruise speed is indeed 10 knots, and assuming a waterline lenth of 50ft, then the displacement speed of this hull would be around 9.5 knots. Sure - if you occaisionally want to push faster than this then semi-displacement is the way to go. Otherwise a true displacement hullform will give the greatest economy and probably more comfort as well.

 

Ahh, indeed. I had a momentary lapse of calculating ability. Somehow I was coming up with hull speed of 6 knots. 9.47 is correct, and thus a displacement hull would of course make more sense.

 

I would go with a semi-displacement. As Will points out 10 knots would be slightly beyond the limit of a full displacement hull, where as the semi-displacement should provide 10 to 15 knots. If you need to go faster than 15 knots then you would be better off with a planning hull, but it will pound when it gets rough. In addition, the semi-displacement hull will have a wider stern than a full displacement hull which will provide more room for the divers.

In my area, dive boats leave the night before and the divers sleep onboard. This allows the boat to make a slower, more fuel efficient transit, and the divers can start their first dive shortly after sunrise. The faster (day boats) only dive the front (sheltered) side of Catalina Island.

If your waters get rough (like the Pacific does around here), then I think roll stabilization would be a major benefit, especially at anchor. I have been on charter boats that rolled a lot and it made it very hard to get the dive gear on. I know of more than one dive boat that went out of business because the word of mouth said it rolled too much. Hard chines are better than round bilges. Passive roll stabilizers (paravanes) would probable be the easiest, cheapest and most effective at anchor, but you would have to slow to displacement speed to use them in route.

Regards;
Mike Schooley

 

Thought this topic might tickle your interest Mike! Leaving aside the displacement / semi-displacement hullform comparison for a moment (I think either would be good if it's designed well), have you considered the use of a catamaran? As long as you are operating in relatively sheltered waters, then there's a lot to be said for cats. Most of the dive operators on Oz's Barrier Reef run cats - they can be fast, are very stable, and provide a big, flat and wide deck to operate from.

 

Thanks everybody,

What is "paravanes" ? I only know bilge keel and skeg can control the roll stabilization.

How much effect on rolling for using the bilge keel ?
If we use skeg,we don't need to use bilge keel. Is it true ?

Regards,

Donald

 

Here is an informative article on roll attenuation http://www.kastenmarine.com/roll_attenuation.htm

And to give equal time to both sides here is the down side http://www.tc.gc.ca/MarineSafety/bulletins/2000/15_e.htm

Regards;
Mike Schooley

 

The NPL series (Bailey, from RINA, Monograph 4) provides hull form (and resistance) data for efficient semi-displacement hulls.

 

Dear CDBarry ,

I am affraid the NPL series will be out of range in L/B .
NPL series will be too narrow for yachts .

 

Paravanes are trailed from outriggers. They are somewhat flat devices that tend to submerge at speed. They attenuate rolling because they offer a flat surface to up and down movement.

 

NPL series L/B goes down to 3.33. I hope that's not too slender.

Note that loss of a paravane on one side can cause capsize if the other is not released or recovered.