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Old 02-03-2014, 16:49   #1
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Semi Displacement vs Planing

We are thinking of buying a 32 motor boat. I don't enjoy speed and am happy to cruise at 17 knots. What is the advantage over both of these hulls?
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Old 02-03-2014, 17:46   #2
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Re: Semi displacement v planing

32 ft boat at 17 knots is planing, or said another way, 17 kts is speed. What do you consider fast?
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Old 02-03-2014, 18:34   #3
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Re: Semi Displacement vs Planing

semi-planing vs. planing (no such thing as semi-displacement (that'll start a fight).
17knots in a planing hull will be much more fuel efficient than a semi-planing.
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Old 02-03-2014, 22:24   #4
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Re: Semi Displacement vs Planing

32 feet and 17 knots. Buy the planing hull with no regrets, you'll be far happier.
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Old 02-03-2014, 22:39   #5
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Re: Semi Displacement vs Planing

If you don't enjoy speed, how about a six-knot displacement boat?

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Old 03-03-2014, 02:06   #6
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Re: Semi Displacement vs Planing

Thank you for your replies. Just to fill you in . We are thinking of crossing. The English Channel and then cruising down the French canals. The Channel can be quite choppy so I am interested in which type of hull you think would be more comfortable on a 32 ft boat - planing or displacement. The boat we are considering is an Aquador 32c.
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Old 03-03-2014, 02:26   #7
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Re: Semi Displacement vs Planing

If you are going to do the French or any other canals be sure to check the "air height" of whatever model you are looking at. That is the height above water level of the highest point on the boat that is not removable /moveable. Some of the bridges are quite low - I think there is one in Paris that has only 13 ft / 4m between water level and the bridge span.
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Old 03-03-2014, 03:17   #8
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Re: Semi Displacement vs Planing

Good point!
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Old 03-03-2014, 03:47   #9
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Re: Semi Displacement vs Planing

Quote:
Originally Posted by 6woody7 View Post
Thank you for your replies. Just to fill you in . We are thinking of crossing. The English Channel and then cruising down the French canals. The Channel can be quite choppy so I am interested in which type of hull you think would be more comfortable on a 32 ft boat - planing or displacement. The boat we are considering is an Aquador 32c.
The Aquador is a nice boat which should be good for your purpose.

Motor boats of this size are not (!) for doing the Channel in a gale, and will be uncomfortable in any kind of unsettled weather. But you have fast cruise of 20 knots, so you can be in Cherbourg in 3 hours or so from the Needles, and coast hop from there.

The way you cruise in a boat like that is wait for a perfect weather wind, point, and squirt!

If you want a motorboat which is capable of handling stronger weather, and which will give you the range to make longer passages, not to mention capacity to carry the gear for longer-term cruising, then consider a trawler-type boat with a displacement hull. But this is much slower -- at 32 feet or so, we're talking maybe 7 knots cruising speed! But I'm not sure trawler-type boats make all that much sense in that size. I think "point & squirt" is probably the right approach to cruising in power boats of that size.
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Old 03-03-2014, 05:44   #10
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Re: Semi Displacement vs Planing

Greetings and welcome aboard the CF, 6woody7.

The Aquador 32C appears to be a “deep-V” planning hull.

Ad’ ➥ 32 C

Test Report ➥ http://www.gbm.se/UserFiles/Interboa...2032C_test.pdf
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Old 03-03-2014, 09:06   #11
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Re: Semi Displacement vs Planing

Semi displacement was once considered more " weatherly " then planing. The " Nelson" range of motor boats properly being the epitome of such form.

However the arrival and dominance of deep vee planing hulls have tended to now dominate all fast boats. Even the new RNLI lifeboat is deep vee planing hull

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Old 03-03-2014, 11:36   #12
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Re: Semi Displacement vs Planing

The question of semi-displacement v planing is an interesting one, I raised it on crew.org.nz a couple of weeks ago.
It is also discussed a number of times on boatdesign.com.
The subject can get technical and there is some disagreement amongst professionals.
It seems to me that yes some design factors allow semi-displacement hulls to exceed hull speed. This seems to give 12 knots or so or a bit more cruising speed.
A planing hull might do 30 knots but as fuel consumption rises with the cube of speed fuel use is heavy even in terms of distance traveled ie miles per litre.
Therefore many limit it to around 20 knots/hr.
17 knots is a planing speed in the usual sizes ie not destroyers or cats. I would think it unlikely in SD and it would involve high consumption.
Handling in a sea is likely to be better in a displacement and worse in a planing boat. I also note that conditions may not allow for planing and the planing boat would be the worst in that scenario.
As far as design goes it seems there is some imprecision and one has to look at the actual performance achieved. Weight is also a factor here.
What concerned me was that sometimes the same hull say a 32 is fitted with engines from 120-150 or 250-350 odd and the extent to which the speed and mileage reflect this without much difference in handling for the same speed and conditions. That is the boat does not become SD merely because of a lower speed given the same hull.
Whether one would prefer 6.5 or 7 knots or 12 or 20+ depends somewhat on the use, conditions, and intended distances and the wallet, and personal preference.
Those with planing boats may well opt for lighter conditions.
As for crossing the channel. I guess it depends on the weather window you want to wait for and the time factor. I gather the distance is short so I suspect time is not so important.
On the canals though I imagine there is a speed limit. I don't know but 5 knots would not surprise. Whatever it is you would not be planing at 17 indeed that would seem to be contrary to the point of the exercise namely puttering along enjoying the relaxation, scenery food and wine.
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Old 03-03-2014, 11:44   #13
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Re: Semi Displacement vs Planing

Thank you gird May and to you to Dave. The Aquador 32c is looking like a real favourite!
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Old 03-03-2014, 13:14   #14
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Re: Semi Displacement vs Planing

If you're doing the canals, you can't go fast, so the only time you'd use the speed is crossing the channel? In that case, would it be prudent to check fuel consumption and purchase a displacement yacht?
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Old 03-03-2014, 15:44   #15
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Re: Semi Displacement vs Planing

Quote:
Originally Posted by Stu Jackson View Post
If you're doing the canals, you can't go fast, so the only time you'd use the speed is crossing the channel? In that case, would it be prudent to check fuel consumption and purchase a displacement yacht?

That seems a good point, but I wonder if an hour's (?) worth of fuel at planing speeds -- occasionally -- would be a big deal to the OP. I could assume other boat features may end up driving the decision more so than brief periods where fuel consumption is higher... assuming one can pick decent weather windows for the crossing, and so forth...

Just a thought...

Have to admit, I had a smile when I read "I don't enjoy speed and am happy to cruise at 17 knots." I'd say that's fairly respectable rate of travel !

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