• UPDATED INFORMATION & ADVICE - PLEASE READ NOW

    'I didn't know/I wasn't told' will not be a valid defence if you fail to comply and lose your access to the off-topic area, core topic areas, or the entire YBW forum as a result. Full details can be found here, please read before you proceed.

Planing boat at D speeds

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
Well, inspired this weekend by the superb conditions in the Solent, and my current thoughts about semi-displacement alternatives, I thought I'd experiment with running my planing boat at D speeds. Max D speed was about 8 knots, then there's an sort of awkward zone between 8 and 16 knots when planing starts, then after about 22 she's planing properly.

Two hours later I have a couple of observations.

1. It's much more relaxing although of course it does take longer to get anywhere...
2. It uses a whole lot less fuel (no surprises there).
3. It's much more comfortable, except when encountering some swell in which case it's bloody awful.

OK that's three.

1 and 2 - well, really I quite like these. My musings are about point 3. The hull shape of my boat (which is not Itama-like in any way) won't help of course but it was very uncomfortable encountering the swell from (say) a 50' Princess doing 30 knots. Beam on - rolling, head on, slamming. Two sides of the same coin really. Would any Semi D boat in that position be a lot better? Dunno.

Stabiliser newbie here; maybe active stabilisation (fins) is an answer but I'm probably talking about a small ship. And they won't stop pitching.

<discuss>

PS anyone with a stabilised 45-50' semi-D boat out there?
 
Last edited:

BruceK

Well-known member
Joined
8 Feb 2015
Messages
7,219
Location
Conwy
Yachts have great big keels and huge flappy bits that stabalise them very well indeed but you know when that 50 foot Princess comes tootling along at 30 knts for a close pass, say half mile off, you still see the indolent and disgruntled peasants go into uproar and start throwing the teapot in their general direction. What kind of miracle are you hoping for? The walk on water kind or the Halleluja! I only lost half my gin kind?
 

ST840

Active member
Joined
22 Jun 2014
Messages
524
My boat is semi D and rolls like anything when hit by the wake of a larger boat. Especially when I’m at anchor and the ignorant to others big boy boaters come thrashing past at 20 knotts only 50 yards from us. Even if they are further away the rocking and rolling is pretty uncomfortable. To the point where we are forced to pull the anchor and stop fishing. TBH I don’t think any hull would be comfortable when subjected to that kind of selfish manoeuvres.
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
My boat is semi D and rolls like anything when hit by the wake of a larger boat. Especially when I’m at anchor and the ignorant to others big boy boaters come thrashing past at 20 knotts only 50 yards from us. Even if they are further away the rocking and rolling is pretty uncomfortable. To the point where we are forced to pull the anchor and stop fishing. TBH I don’t think any hull would be comfortable when subjected to that kind of selfish manoeuvres.
I don't think my experience with the P50 was deliberate on their part. Sunday in the Solent was very busy.
 
Last edited:

ST840

Active member
Joined
22 Jun 2014
Messages
524
I don't my experience with the P50 was deliberate on their part. Sunday in the Solent was very busy.
The solent is normally very busy, it’s where I boat too. Doesn’t mean it’s excusable manoeuvres though. People could have some consideration for others.
 

BruceK

Well-known member
Joined
8 Feb 2015
Messages
7,219
Location
Conwy
Well despite what you may have been told the flatter the bottom the less the roll so your barge is about as good as it gets without adding gyros / fins etc and they will absolutely help in general and anchorage conditions if you dont mind the power consumption but a 50 foot Princess is asking for too much if you're at displacement speeds. Not enough inertia.
 

James L

Well-known member
Joined
3 Jul 2012
Messages
2,052
Location
London / West Cork
I like to spend a lot of time at displacement speeds as well, weather and company allowing.
But if I see a big wash approaching I'll just add a few knots to stabilise things until it passes. I don't go to planing speeds, just up to 11 or 12 knots and I find it's enough to keep things relatively under control.
 

BruceK

Well-known member
Joined
8 Feb 2015
Messages
7,219
Location
Conwy
I've also discovered the yottie thrill of doing 10 knts when the motors are rev-ed for 7 working the currents. However I am dismayed I still dont get to save much fuel. They, them yotties seem to run on the smell of an oil rag and yet even when plodding along my fuel consumption for a given trip is not so great as to make the hours of boredom worth it. Maybe if I stuffed it into autopilot, hit the G&T and bribed a kid with a packet of Haribo's to keep watch I may find it more appealing. Maybe it's because I'm still driving all 12 cylinders. Dont buses have something that reduces the number of cylinders firing when they are at a stop? Something like that, firing on 3 instead of six per engine at speeds below 800rpm would be a boon.
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
I like to spend a lot of time at displacement speeds as well, weather and company allowing.
But if I see a big wash approaching I'll just add a few knots to stabilise things until it passes. I don't go to planing speeds, just up to 11 or 12 knots and I find it's enough to keep things relatively under control.
That's an idea, I'll try that next time.
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
I've also discovered the yottie thrill of doing 10 knts when the motors are rev-ed for 7 working the currents. However I am dismayed I still dont get to save much fuel. They, them yotties seem to run on the smell of an oil rag and yet even when plodding along my fuel consumption for a given trip is not so great as to make the hours of boredom worth it. ...
My experimenting yesterday produced the following, Tickover, 4.5kts, 7.5mpg or at 1100rpm, 8.5kts, 3.3mpg, although since fuel is about the cheapest thing in the annual budget after my Seastart subscription it's probably irrelevent.
 
Last edited:

BruceK

Well-known member
Joined
8 Feb 2015
Messages
7,219
Location
Conwy
Maybe it's because I push for max dsp hull speed I'm not seeing huge gains. I set it so the wake looks like what my tender might push out heavily laden at the same speed. I tell myself that's about 4hp wasted and good enough. Yet if I go at this displacement speed and cover my usual haunts racking up about 50 mile, I turn 2 hours travel into about 6 or even 7 tide depending and seem to use about a third of the fuel (on my old boat it's not an exact science is guestimation) A third of the fuel is a saving not to be sneezed at but it takes 3 times longer. So for short 1 to 2 hr hops to our anchorage for the weekend, spending so much time fighting the wander can actually get quite tiring while being in dire danger of loosing the best spots to some raggie in the marina. Especially my knackered I/O legs which are so wobbly they can-can spread enough to impress any sailor. Where as once on the plane she tracks true as a die so long as the trim tabs are level and is a much more relaxing if not social affair. I think if I was to go to a displacement trawler style it would be the new, new and quite pleasant and relaxing with an engine to match because expectation is achieved. Until then I go at displacement either because of maritime restrictions (Menai Strait) or to kill time waiting for the tide to be high enough to cross the estuary
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
Makes sense I think. We have a 30-40 minute trundle down the river at 5kts before hitting so-called open water, which on Sunday had more boats per sq mile than I've ever seen before.
 
Last edited:

LBRodders

Well-known member
Joined
3 Oct 2018
Messages
1,835
For us, particularly single outdrive, we wander too much at DS. Gains are lost on extra travel! 🤣
Plus at 8m, DS is tortoise like.

Horses for courses, planing boats are for planing, even in rougher weather.
 

rafiki_

Well-known member
Joined
19 Jan 2009
Messages
11,559
Location
Stratford on Avon
Scala, we used our Azi at a mix of displacement and planing. Worked brilliantly for us. Planing if we needed to get somewhere at a certain time, otherwise a mix. Our boat would happily plane at 16 kn, with no difference in fuel consumption up to mid 20 kn. she would sip fuel at 8 kn. I didn’t do anything between. And you’re right, fuel was one of our minor costs.
By the way, great to hear you are now mobile.
 

Greg2

Well-known member
Joined
24 Jun 2002
Messages
3,384
We are on our second semi-displacement boat, both with a round bilge design so they do roll in certain conditions but deal with a head sea with consummate ease. The previous owner of our Hardy said that it never slammed and from our experience to date I have no reason to disbelieve him.

The real benefit is that we can select any speed as long as we can live with the increased fuel burn at higher revs. No question that semi-displacement is less fuel efficient at higher revs and is likely to be slower overall than a planing hull but they tend to have good seakeeping qualities.

On passage we now cruise at two thirds of the speed of our old Broom 41 and half the speed of our old Fairline Targa 35. It all depends on your mindset! 😁
.
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
For us, particularly single outdrive, we wander too much at DS. Gains are lost on extra travel! 🤣
Plus at 8m, DS is tortoise like.

Horses for courses, planing boats are for planing, even in rougher weather.
I’ve got the wandering under control. Took some practice! On longer legs I use the AP which steers better than I can. I don’t want a D boat for exactly that reason. Having the speed if I want it.
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
Scala, we used our Azi at a mix of displacement and planing. Worked brilliantly for us. Planing if we needed to get somewhere at a certain time, otherwise a mix. Our boat would happily plane at 16 kn, with no difference in fuel consumption up to mid 20 kn. she would sip fuel at 8 kn. I didn’t do anything between. And you’re right, fuel was one of our minor costs.
By the way, great to hear you are now mobile.
thank you 😊 pity I screwed up my timing and missed you lot yesterday!
 

Scala

Well-known member
Joined
21 Feb 2004
Messages
4,532
Location
Home: Saffron Walden. Boat: Swanwick
We are on our second semi-displacement boat, both with a round bilge design so they do roll in certain conditions but deal with a head sea with consummate ease. The previous owner of our Hardy said that it never slammed and from our experience to date I have no reason to disbelieve him.

The real benefit is that we can select any speed as long as we can live with the increased fuel burn at higher revs. No question that semi-displacement is less fuel efficient at higher revs and is likely to be slower overall than a planing hull but they tend to have good seakeeping qualities.

On passage we now cruise at two thirds of the speed of our old Broom 41 and half the speed of our old Fairline Targa 35. It all depends on your mindset! 😁
.
Yes that’s it I think. I was surprised to find that my boat was so affected by the swell.

I’m especially surprised to find I enjoyed going slower albeit in gorgeous weather on a short passage.
 

seastoke

Well-known member
Joined
16 Mar 2013
Messages
8,328
Location
boat conwy
I’ve got the wandering under control. Took some practice! On longer legs I use the AP which steers better than I can. I don’t want a D boat for exactly that reason. Having the speed if I want it.
Well at DP you could tow your fenders.
 
Top