Devlin 33' picnic S/D: comments please...

[rustybarge]

Hi All,

I'm considering building this pretty plywood design in alloy. I would much appreciate your opinions about the specs of this hull.

Looking at the dead rise and bow entry will this boat have a soft ride?
At only just under 11' beam how will it handle in a beam sea?
It's heavy at nearly 7 tons displacement, I'd have guessed at 5 tons in ply?

. Power on the Topknot is a diesel sterndrive of 250 HP giving a cruising speed of 18 knots burning just under 4 gallons of fuel per hour and a top speed of 24 knots when you need to get home in a hurry. We will be doing several lengths of these vessels with the first being 32' and the second in the series being 36' long.

http://store.devlinboat.com/topknot-10_meter.aspx
Any other observations please.....

Specifications
Length: 33ft 4in 10.15m
Beam: 10ft 8in 3.25m
Draft: 2ft 2in .66m
Displacement: 14,900lbs 6,758kg
Power: Inboard diesel w/ stern drive
Hull type: Semi-displacement

 

[Rurudyne]

She's a really pretty thing. I wish you well in this!

Though one thing kinda confounds me, not really about this boat but just in general, and maybe just because I'm "cheap": it's a matter of power. The 250 hp diesel gives a top speed just 6 knots faster than the speed for the stated fuel consumption, but that means one might expect 18 kt from 80hp (20hp per hour per gallon, 80% throttle on a 100hp engine) and, fuel pump drain on a smaller engine being less, maybe a bit more ... so I guess the extra cost and weight of the larger engine doesn't make a lot of sense to me unless someone wants to go skiing behind the boat. Of course, for all I know, the drive type in question may not be available for the smaller engine.

 

[rustybarge]

She's a really pretty thing. I wish you well in this!

Though one thing kinda confounds me, not really about this boat but just in general, and maybe just because I'm "cheap": it's a matter of power. The 250 hp diesel gives a top speed just 6 knots faster than the speed for the stated fuel consumption, but that means one might expect 18 kt from 80hp (20hp per hour per gallon, 80% throttle on a 100hp engine) and, fuel pump drain on a smaller engine being less, maybe a bit more ... so I guess the extra cost and weight of the larger engine doesn't make a lot of sense to me unless someone wants to go skiing behind the boat. Of course, for all I know, the drive type in question may not be available for the smaller engine.

That's a good point. I'm just wondering if it's because it's a semi-displ. Hull, which push a wall of water in front of them at higher speeds rather than plane on top of the surface.

Of course if it was a really light weight ply boat a outboard might be a better choice!

I've always thought a slow planing hull that can stay up on the plane at 10 or 11 knots is a much better idea than a fast S/D hull...........

 

[Rurudyne]

I was just wondering at the power, not the design. As I said, she's a beauty.

My own preference, should I ever get to actually build, would be a lighter, slimmer displacement boat ... which should possibly be obvious given me not getting the need for speed.

Still, 5 mpg (not kt) seems pretty good for a 33' boat going nearly 21 MPH ... just not sure how I feel about 2.2 mpg for 27 MPH (250/20 = 12.5 GPH). She should be even more efficient at a tamer pace (one advantage of the smaller engine would be using proportionally more of its power when moving leisurely) and I kinda wish more data were given by the designer (but then again, most people seem to get the need for speed...). I suspect being SD isn't hurting her.

 

[rustybarge]

I was just wondering at the power, not the design. As I said, she's a beauty.

My own preference, should I ever get to actually build, would be a lighter, slimmer displacement boat ... which should possibly be obvious given me not getting the need for speed.

Still, 5 mpg (not kt) seems pretty good for a 33' boat going nearly 21 MPH ... just not sure how I feel about 2.2 mpg for 27 MPH (250/20 = 12.5 GPH). She should be even more efficient at a tamer pace (one advantage of the smaller engine would be using proportionally more of its power when moving leisurely) and I kinda wish more data were given by the designer (but then again, most people seem to get the need for speed...). I suspect being SD isn't hurting her.

I've just had a look at an old motor boat monthly magazine where they tested 3 identical 23' planing boats with different engines of 250hp Volvo cummins and Yanmar, but only 2.3 tons displacement and they average about 18-20 kts at 2500-3000revs at 20 ltr/hr which is 5.2 us.gal/hr or 4mpg, so the figures don't add up for 33' 7tons boat!!!

 

[rustybarge]

Ok, so the fuel consumption for those engines at WOT is 47ltr/hr at 37 kts in the planing 23'.

So the Devlin quotes 24kts max, so that makes 12 us.gal/hr = 2 mpg......

 

[Rurudyne]

Ok, so the fuel consumption for those engines at WOT is 47ltr/hr at 37 kts in the planing 23'.

So the Devlin quotes 24kts max, so that makes 12 us.gal/hr = 2 mpg......

Fun idea: if you build her exactly to spec make that part of the throttle (19+ kt) coin OP access on a second stick....

(Keep guests from getting too throttle happy on your dime...)

 

[rustybarge]

Fun idea: if you build her exactly to spec make that part of the throttle (19+ kt) coin OP access on a second stick....

(Keep guests from getting too throttle happy on your dime...)

...I was thinking if it comes out at 5 tons in alloy, the extra 2 tons could be extra fuel which is available tax free in the Channel Islands close to the Uk, gibraltar in the Med, and north Africa if you're brave enough; it's about half price at €0.70c a litre.
2 tons is a lot of diesel!

 

[Rurudyne]

... or beer....

 

[rustybarge]

Ordering 2 tons of alcohol in Muslim North Africa could be an interesting exercise!

Looking at the displacement of the Devlin of 7 tons, here's a comparison with a GRP 10 meter single engine flybridge of the same size, but it must be pretty heavy with the fly bridge in comparison to the Devlin and yet it weights 2 tons less. hundreds of this design has been sold, so it works extremely well: the speed predictions are well know to be optimistic, most owners report about 20 kts WOT.

Going on the basis that an alloy hull will be slightly lighter or approx the same weight as a GRP hull.........

JEANNEAU MERRY FISHER 925 built in 2005
Popular flybridge cruiser from Jeanneau

CONSTRUCTION:
GRP hull, deck and superstructure
Non-skid deck
Self draining cockpit
Hydraulic steering.

LOA: 9.76m (32'1")
LWL: 8.70m (28'5")
BEAM: 3.16m (10'3")
DRAFT: 0.85m (2'8")
WEIGHT: 4670kg (10295lbs)
Volvo D4 260 h.p. diesel engine
EVC controls
Fuel gauge
QL trim tabs
Bow thruster
Maximum speed approximately 24 knots
Cruising speed approximately 18 knots
TANKAGE
Fuel capacity 400 litres
http://www.ancasta.com/boats-for-sale/jeanneau-merry-fisher-925-28120/

 

[Rurudyne]

While their rough dimensions may be similar, it looks like the Devlin is closer to being plumb bowed which has got to affect things. Then there was the Merry Fisher's greater draft to consider. But 2 tons difference?

On a lark I looked up other images of the Merry Fisher and discovered that she has an almost faux-tri underbody, save for being fully submerged wings and all, with a substantial carved out area aft that wasn't quite a tunnel stern.

Btw, since you're considering alloy construction you may need to be aware that, at least in the States though comparable regulations likely exist elsewhere, environmental regulations may have made treated plywood and aluminum a potentially bad combo. I just came across this blog article, http://www.fishin.com/forums2/showthread.php/86885-Treated-plywood-deck-and-alum-boats-Don-t-do-it! , that details how plywood makers have started to substitute copper for arsenic in treated wood and this leads to corrosion issues with aluminum ... though one comment made claims it's been something of an issue even before that.

 

[rustybarge]

While their rough dimensions may be similar, it looks like the Devlin is closer to being plumb bowed which has got to affect things. Then there was the Merry Fisher's greater draft to consider. But 2 tons difference?

On a lark I looked up other images of the Merry Fisher and discovered that she has an almost faux-tri underbody, save for being fully submerged wings and all, with a substantial carved out area aft that wasn't quite a tunnel stern.

Btw, since you're considering alloy construction you may need to be aware that, at least in the States though comparable regulations likely exist elsewhere, environmental regulations may have made treated plywood and aluminum a potentially bad combo. I just came across this blog article, http://www.fishin.com/forums2/showthread.php/86885-Treated-plywood-deck-and-alum-boats-Don-t-do-it! , that details how plywood makers have started to substitute copper for arsenic in treated wood and this leads to corrosion issues with aluminum ... though one comment made claims it's been something of an issue even before that.

WOW!
Thanks very much for that link, I'm amazed, I've never heard of that before.
Aluminium is a very reactive metal, but I didn't realise it was that reactive.

Re. the Merry Fisher, I choose it as an example of a light layup semi-d boat as you can lots of heavier production boats than this; still a plywood boat should in theory be much much lighter than the Merry fisher. The hull on the Devlin has a double thickness of ply, this may be adding considerably to the weight.

The chines on the bottom edge of the MF hull could be to help steady the ride at slower speeds?

 

[Rurudyne]

I hadn't heard it before today either (I literally had just come across it).

It's maybe the kinda thing that needs to be heard more of though....

 

[Westfield 11]

If you want to sink an alloy boat, just toss a few copper pennies into a wet bilge.... It won't take that long to turn them into holes.

 

[rustybarge]

If you want to sink an alloy boat, just toss a few copper pennies into a wet bilge.... It won't take that long to turn them into holes.

I once went up in a glider and I had to empty out my pockets of change in case a coin fell out and jammed the controls........

.....I can see a new check list for alloy boats: Empty pockets....check.