Jump to content

Semi-displacement launches


Recommended Posts

I am getting to a stage in age and fitness where singlehanding a yacht is becoming a bit of a chore.

So I am thinking say a 9 m launch. Some short trips for a fish, but also Great Mercury and say up to Whangaroa in modest comfort.

I would prefer 10-12 knots rather than the 6-7 of a displacement hull and certainly don't need 20+.

As I understand it a true semi-displacement hull is designed so it produces some hydrodynamic lift, getting you out of the hole without full on planing.

This saves somewhat on fuel but more importantly I understand handling is improved particularly in rougher and following seas.

With some hulls and I think the Vindex may be one some have smaller motors eg 120 - 150 hp v 250. This gives the lower speed range but I suspect these are still planing hulls albeit underpowered for that.

Does anyone know of any kinds of launches specifically designed as semi-displacement. Thanks.

Link to post
Share on other sites

in the back of the boating mag there an ad for long ish moderate displacement launches, cant remember the details, can be bought as hull only or completed and anywhere in between.

 

more yacht type hull, easily pushed so you get reasonable speed (i think up to where you are looking) from relatively low power. Will have a look tonight for some names unless someone else can provide them first....

Link to post
Share on other sites

I'm a fan. Our family has a Woolley sedan launch. It is a 7-8 knotter, and was picked over a Vindex 32. Not your speed requirements but we've topped out at 14kn down a wave - not by choice!

Considerations are price, age, quality of build and equipment, maintenance - particularly glass over wood., interior volume, and interior layout. Then appearance, and emotion - functional or pretty to look at.

Expensive glass - Greenline 33, Franklin 925, Back Cove 30/33, then Logan 33. There were another couple of boats around too - a Pelham? Composite - Mike Pearce - Beale 10m. Then older - Vindex 30/32/3000. Plus sundry Pelins etc. Others in this size at the Oliver 2800/ Aztec / and Gulfstream etc. More a planing hull style but economic at 12-15kn

Then Classical - Williams, Wilson and Woolley. Beautiful, often retiring owners, but a range of layouts and beam. Others are Laing and Shipbuilders. Easily driven hulls but generally underpowered.

We have been lucky that we chose a boat that looks pretty on the marina and at anchor. It has a lot more beam than many of its style. We also have really good flow from cockpit to saloon to step down for the galley. And we have a smaller simple motor that uses the same amount of diesel as a yacht auxiliary.

Link to post
Share on other sites

A 9 m cat hull launch if kept light could do 9 /10 knots easily with 2x 20 hp . 2x 40hp 12 or more. Our 9.8m sailing cat will motor at up to 8 kts with 1 x10 hp and would do it easier without the mast. We have had a 46 ft cat doing over 8kts with 2 x 10 hp outboards. Unless you are averse to multihulls consider the merits of converting an existing sailing cat to motoring only. It may be as simple as removing and selling the mast,sails etc and have the benefits of stability , 2x motors/ rudders etc. However if you want to use a marina you would need to consider the extra beam. There is a NZ design of cat launch around that size which I think would fit a standard berth.

Link to post
Share on other sites

Yep the Sterling 33 was marketed as semi-displacement , but really over 10 knots they are dragging a lot of water, likewise the Coastal 32 unless you have 200hp - in which case a Vindex or Corsair hull is the go. A beale 12M version of the 10M with the corresponding increase in volume is a great concept, and powered with the Yanmar 180 will be amazingly economical. Am experimenting myself with a small launch --up to 12 knots out of 20hp this week!

Link to post
Share on other sites

Speed, Hp and distance traveled tend to be related in that, you can have a displacement Hull, easy to push through the water, needs little power, will take say 5hrs to get to Great Barrier which may take 100ltrs of Fuel to get there. Semi-displacement will need a slightly bigger engine and will get you there in say 3hrs and use about 100ltrs of fuel. A planning Hull will need a real big engine and will get you there in 1hr and use about 100ltrs of fuel. So the Fuel per hr usage may go up, but you get there faster, so tends to be about the same ( for same'ish Hull size of course). But the real cost is in the Engine and how many hrs you get before rebuild.

A big question unpc is what size are you thinking and what kind of "comfort" are you looking for?

Link to post
Share on other sites

Thanks for the helpful suggestions.

I am not asking for anyone to pick a boat fun though that might be.

That will depend on cash, my circumstances and preferences.

I would probably go towards the 70s and so an older boat. 28 ish though maybe a bit smaller if dry stacked. Boating costs but realistically it will only be for say 5-6 years.

The comfort factor – I mean aesthetically pleasing not spartan nor luxury or does it have to sleep 8 but it would be good to take the gc out though that might be mainly rare day trips every 2 years as they are overseas.

 

I understand a diesel is best working hard so is best at around 75% power and 90% of hull speed is economical, the last bit chewing up fuel.

For 24’ Lwl I get 2L per hour at say 6 knots, from a GM20 using say 14 hp.

To get 8 kts cruising I would need a LWL of about 42” though a 36 would peak at that on a displacement hull. Some hulls may better this.

 

So for an assumed trip of say 50 miles I estimate.

Displacement HP 25 speed 7 knots 7.2 hours fuel 14L.

Semi-displacement hp 120-150 knots 11-12 4.2 hours fuel 45L.+ Some claim 10Lph I suspect it might be closer to 14.

Planing hp 300 20 knots (economy) 2.5 hrs fuel 80L

 

In practice conditions would not allow planing at all times and a planning hull is less efficient than a displacement hull at displacement speeds.

 

A saving in time means less fatigue and opens up more alternatives on shorter trips eg fishing. Sure there is a cost and that has to be weighed, as fuel is not likely to get cheaper.

 

The handling in rough and downwind conditions also is important. I gather the s-d hull is better than the planing hull then.

 

I haven’t owned a launch so in the real world things might be a bit different from what I have gleaned from various opinions.

Link to post
Share on other sites

I did a short stint as a Broker and still have my finger on the pulse just for interest sake of what's around and prices, but nothing else.

In practice conditions would not allow planing at all times and a planning hull is less efficient than a displacement hull at displacement speeds.

That really is the defining factor. A Planning Hull may get you somewhere quicker, but in ruff conditions, you are back to Displacement speeds, so Planning Hulls are really for the ones that watch the weather for the best days.

Leaving aside the big go fast I have a small Penis type Planning Hulls(which fuel used is part of the Glory of ownership) the typical Fuel use per hr and speed of getting their, is very close to being the same, just as I said earlier. However, with a Displacement Hull, you can have several modes of economy. Push it hard and it will burn fuel too. A Planning Hull needs to be Planning to be most economical. Some can come down to displacement and be economical, but at the expense of not working the motor hard, so that is not good for the motorlng term.

In Hulls of 7 to 9M, you are going to find nothing much around in Displacement except the very old Timber Hulls and many are in need of expensive on going work. I imagine that is the last thing you want to spend retirement on. That is not to say that one or two don't come up.

9 to 11M Hulls gets you into some good boats for the dollars these days. However, there are some very good Planning Hulls in that range that I feel would very much fit in with your needs. I would suggest you take a good look at the slightly more modern Vindex's. The 32ft range can get you a lot of boat for the money. They have a modest engine size and are often fitted with Ford or Perkins engines (both Solid performers) or the Volvo engines and are quite economical and don't go like stuck cats. Depending on power, 14-16kts is a good middle of the range cruising speed. There are some faster. They are Comfortable and have everything you want. The flybridge version ios one of my favorite boats and still look very modern for their age.

Hartley and Pelin have several Hulls that are Glass over ply/timber. Also Steel, but at the age they are at, I suggest stay away from steel unless you can find an immaculate vessel.

Marklines are a very solid glass hull, but I find things like the Alloy window frames etc are getting tired on many now, so take special interest in inspecting those areas. They are also a very heavy Hull. That can be both a good and bad thing.

Link to post
Share on other sites

At the risk of being banned here...I have a 35' (incl duckboard) Pelin; 225hp Ford Lehman Turbo on a single shaft, glass over dd Kauri. Was built to survey 1980 (oversized shaft, prop, structure) so feels pretty heavy although I don't know actual weight. Was told it didn't plane until the trim tabs were added. Will plane at 11knts/2000rpm, but I'm happier tootling at 9knts/1800rpm (non-planing). Uses about 20l per hour. Yaw in a following sea is a bit of an issue I need to figure out how to deal with, but otherwise very comfortable. Won't really turn in reverse either which is a bitch for berthing.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...