What sort of efficiency can a 35'-40' planing hull make.

[Bigmarv]

If you want the Great Barrier Reef to last for long, best not to use any big engine for long.

[Galumay]

Originally Posted by Bigmarv

If you want the Great Barrier Reef to last for long, best not to use any big engine for long.

LOL! Do you know how much commercial shipping operates in those waters?

[Dave_S]

Thanks all.

I'll spend some time looking up the boats you have mentioned and maybe go and have a look at a couple.

[Pizzazz]

Originally Posted by boatpoker

My previous boat (below) with a single engine TAMD60B travelled at 10kts using 1.6gph and at 21kts using 8gph.

There are models that estimate required power for planing hulls, such as the Crouch formula V = C / (Disp/HP)^0.5, where V is in knots, C is a hull parameter between 150 and 200, displacement is in pounds and HP is shaft horsepower. We can get nmpg using gph = 22 / HP. Rearranging, we get

nmpg = 22 x C^2 / Disp / Speed. For example, for my boat C = 180, Disp = 35k lb, V = 20 knots, nmpg = 1 nmpg @ 20 knots, which is about right.

You can assume C=180 for most hulls, which simplifies the formula to nmpg = 325 / Disp (tons) / Speed (knots). I switched to tons to make it neater. Keep in mind, the Crouch formula typically underestimated the required power.

If you were doing 21 knots @ 8 gph, this means 2.6 nmpg, solving for displacement using the above formula, we get displacement < 6 tons or 13,000 lb. So, it was only possible you did 21 knots if your vessel displaced less than 13,000 lb. Not impossible but does not look like it from the picture.

[boatpoker]

Originally Posted by Pizzazz

There are models that estimate required power for planing hulls, such as the Crouch formula V = C / (Disp/HP)^0.5, where V is in knots, C is a hull parameter between 150 and 200, displacement is in pounds and HP is shaft horsepower. We can get nmpg using gph = 22 / HP. Rearranging, we get

nmpg = 22 x C^2 / Disp / Speed. For example, for my boat C = 180, Disp = 35k lb, V = 20 knots, nmpg = 1 nmpg @ 20 knots, which is about right.

You can assume C=180 for most hulls, which simplifies the formula to nmpg = 325 / Disp (tons) / Speed (knots). I switched to tons to make it neater. Keep in mind, the Crouch formula typically underestimated the required power.

If you were doing 21 knots @ 8 gph, this means 2.6 nmpg, solving for displacement using the above formula, we get displacement < 6 tons or 13,000 lb. So, it was only possible you did 21 knots if your vessel displaced less than 13,000 lb. Not impossible but does not look like it from the picture.

I don't know how you could say "does not look like it" without seeing the bottom.

It's a very unusual boat, forward 1/3 is extreme deep-V, Middle 1/3 quite a bit flatter with hard chines, aft 1/3 round bottom.

You can read about it in this magazine review, The fuel usage in this article is very close to my experience ... https://www.canadianyachting.ca/boat...trendsetter-40

[rslifkin]

Narrow beam significantly helps efficiency and can somewhat change the game between fast semi displacement and planing (think of the old commuter style hulls and the speed they could run with the low powered engines available). Looks like the test boat in the article topped out around 20 kts with a single 235hp engine. I'd expect a burn of ~13 gph for that engine at WOT. So that's still a bit off the 21 kts / 8 gph number, but certainly very efficient for a 40 footer.

[boatpoker]

Originally Posted by rslifkin

Narrow beam significantly helps efficiency and can somewhat change the game between fast semi displacement and planing (think of the old commuter style hulls and the speed they could run with the low powered engines available). Looks like the test boat in the article topped out around 20 kts with a single 235hp engine. I'd expect a burn of ~13 gph for that engine at WOT. So that's still a bit off the 21 kts / 8 gph number, but certainly very efficient for a 40 footer.

All I can say is I track my fuel usage meticulously. The photo in that article is a bit misleading, it must have been a choppy day. In flat water when we gave it full throttle there was almost zero bow rise and very little difference in wake betweem 10 and 21 knots. I asked a long time friend to watch the wake and tell me when we got on plane ... he couldn't tell the difference and he (some 50yrs ago) was a world hydroplane champ.

Here she is at 21 ....

[Pizzazz]

Originally Posted by boatpoker

I don't know how you could say "does not look like it" without seeing the bottom.

You can read about it in this magazine review, The fuel usage in this article is very close to my experience ... https://www.canadianyachting.ca/boat...trendsetter-40

It is definitely an impressive boat. I believe part of the confusion comes from imperial vs. US gallons and mph vs. knots, converting to standard US units I get estimated fuel burn of 1.8 nmpg (US gallons) @ 20 knots. A little below your numbers but impressive any way you look at it. Congratulations. It is the first time I come across a boat that can verifiably do close to 2 nmpg at 20 knots.

[rslifkin]

Originally Posted by Pizzazz

It is definitely an impressive boat. I believe part of the confusion comes from imperial vs. US gallons and mph vs. knots, converting to standard US units I get estimated fuel burn of 1.8 nmpg (US gallons) @ 20 knots. A little below your numbers but impressive any way you look at it. Congratulations. It is the first time I come across a boat that can verifiably do close to 2 nmpg at 20 knots.

There are plenty of boats that can do that, but usually they're a good bit smaller than 40 feet. For planing efficiency, hull form matters, but weight matters as much if not more. So as boats get bigger and heavier, it gets harder to get good fuel economy on plane.

[Hasbeen]

I ran a few boats in the Whitsundays on day trip & outer reef day trip & extended fishing charter.


45Ft heavy round bilge timber, Gardiner, 5 liters/hour at 8/9 knots.
53Ft heavy round bilge timber, Caterpillar, 13 liters/hour at 10/11 knots.
100Ft International alloy catamaran, twin 16V92TA GM diesels, 1220HP each, 50 gallons/hour each at 21 knots, 250 gallons/hour each at 29 knots.


When I wanted to spend a week out the reef it was the 45 footer I wanted.


I also ran a couple of light hard chine planing boats,

Plywood 48Ft twin Perkins 18 knots 35 L/H each.
Alloy 42Ft twin 8V92 GMs 23 knots 45 L/h each.


Both were not comfortable boats to travel in, were very uncomfortable on a bumpy night out the reef, but were less likely to make tourists seasick while traveling.

[rslifkin]

Originally Posted by Hasbeen

I ran a few boats in the Whitsundays on day trip & outer reef day trip & extended fishing charter.


45Ft heavy round bilge timber, Gardiner, 5 liters/hour at 8/9 knots.
53Ft heavy round bilge timber, Caterpillar, 13 liters/hour at 10/11 knots.
100Ft International alloy catamaran, twin 16V92TA GM diesels, 1220HP each, 50 gallons/hour each at 21 knots, 250 gallons/hour each at 29 knots.


When I wanted to spend a week out the reef it was the 45 footer I wanted.


I also ran a couple of light hard chine planing boats,

Plywood 48Ft twin Perkins 18 knots 35 L/H each.
Alloy 42Ft twin 8V92 GMs 23 knots 45 L/h each.


Both were not comfortable boats to travel in, were very uncomfortable on a bumpy night out the reef, but were less likely to make tourists seasick while traveling.

The 16V92TA fuel consumption numbers seem way too high. A single 1220hp 16V92TA at full power cannot burn anywhere near 250 gal/hr. Is that meant to be 250 liters / hour? That would seem about right.

[John Williams]

Originally Posted by rocks

We ran a 10m and about 4700kg fibreglass, LeisureCat with twin 250 hp outboard. The boat was best at about 16 to 22 knots in a 2m to 2.5 swell.
kts lt/hr
brace yourself 6 10
brace yourself 14 37
brace yourself 22 67
brace yourself 24 72
brace yourself 31 104

Then you have the yachties. We have a 10 tonne 41 foor cruising yacht. 20 year old Volvo 50 HP (based on Perkins design). When motoring we sit on 6 kts or a bit more and average 0.5 ltrs/nm. 3 litres an hour. Cost of fuel and maintenance of engine are not costs we are concerned with. Replacing rig every 10 years just cost us $15,000.

[NPCampbell]

One big efficiency feature to look for on gasoline engines is direct injection. Direct injection is around 10% more efficient than port fuel injection at planing speeds. Many modern gas engines have both types, direct at higher speeds and port at low speeds to keep the coke off the intake valves.

[Simi 60]

Originally Posted by John Williams

Then you have the yachties. We have a 10 tonne 41 foor cruising yacht. 20 year old Volvo 50 HP (based on Perkins design). When motoring we sit on 6 kts or a bit more and average 0.5 ltrs/nm. 3 litres an hour. Cost of fuel and maintenance of engine are not costs we are concerned with. Replacing rig every 10 years just cost us $15,000.

Free sails eh?

[Rightbrainer]

Get your hands on a copy of this book. All sorts of discussions about the various trade-offs in boat designs. Included many tables and graphs for you to address your questions. Very readable as well.