Planing Dinghy: Hp vs Dinghy type vs #Passengers

[Jammer]

Originally Posted by Adelie

Update.
Keep the info coming. If anybody knows people that take hard dinghies out, I'd like a little info from them.

Yacht designer Evan Gatehouse cruised for several years in a catamaran using a GV10 hard dinghy. The GV10 is a boat of his own design and was built by his own hand. Plans for the GV10 are available from Bateau. Unlike most other hard dinghies, it is specifically designed for planing operation with higher horsepower outboards. Gatehouse also designed the GV11 which is a fresh design, not a stretch of the GV10.

These are probably the smallest boats that are actually designed for planing and for which information is available.

In my notes I have this quote attributed to a 2014 post by Gatehouse:
"Our GV10, with a well used 10 HP motor would plane at less than full throttle with about 425 lbs of passengers and an 85 lb motor." Gatehouse posted extensively at https://www.community.boatbuildercentral.com under his own name and a search there may turn up more details. He also used a 15 HP motor and reported in this 2008 post that:

• top speed with one person is 25.5 knots (!)
• does not plane with six people
• "just flies with a 15 HP engine and 3 people"

[MicHughV]

the standing up thing was more an attempt to keep the wood floorboards from rising up than a cowboy thing...I'm standing on one of the joints to keep it from buckling.....though it did become a habit in later years...

[lancelot9898]

I had a mid 1980's Archilles (10ft+) plywood floor and inflatable keel with a 5hp 2 stroke Nissan. I replaced the standard prop with a higher pitch one and was able to plane with 2 people aboard under mild conditions. We had to move forward to get up on plane at full throttle, but once on plane we could return to our original positions and back off the throttle. The weights involved were boat (120lbs) engine (45) me and wife (310)

I now have a Highfield high pressure floor 9.2 ft weighing in at 74 pounds and in the process of buying an outboard. Even though the dinghy is lighter it is over a foot shorter which will make it harder to plane. I'll probably get a 3.5 hp Tahatsu and replace the plastic prop with a metal one. I think I can probably plane it with me aboard but doubtfull with both of us. I just don't want to rig up a crane and prefer the ease of just lifting the outboard directly from the sailboat to dinghy. Also I could replace the carb with a larger one. I did put an epropotion electric on it and while leaving the marina with the two of us aboard I looked forward to opening it up once clear of the marina. Total disappointment and it did not even attempt to make plane. That may be the same result with the gas outboard.

[noreen]

* Aerotec 380 + 15hp plans with 320kg payload. * AB 8 UL + 6hp plans with 100kg payload. 130kg not any more. * AB 9 UL + 8hp plans with less than 200 kg. * Highfield 310 UL + 15 hp plans with 220kg payload. Still testing with more gear/load.

[chrisr]

Originally Posted by rslifkin

I'd call this the most surprising result in this thread. I was always under the impression that the Walker Bay dinghies just didn't plane at all.

esp 2 up with a 2hp outboard

perhaps we need to discuss the definition of 'planning' ??

cheers,

[Adelie]

Naval architecture-wise planing starts about 2.7-3.0 x S/L and you are definitely planing at 4x S/L

https://www.boatdesign.net/threads/p...6/#post-665352

S/L is Speed/Length ratio which equals Sqrt (LWL).

So planing may start as early as 2x hull speed.

See graphic for the hull and planing speeds for various length boats. For tenders, 10kt is a good estimate for planing.

[MicHughV]

A planing dink needs to have a planing hull...generally flat bottom with a V shape. You will also note most RIB's these days have " lifting strakes". These are essential planing hull items. You will also require sufficient speed which will require sufficient power. Like an airplane, you need a certain speed to generate lift to take off.

A "displacement" hull like my old fiberglass dink will never get on plane...ever..I've tried it, put two engines on it even, all I did was dig a deep hole in the water.

I rather doubt a 2 hp outboard will get a dink on plane.

I have seen twin hulled fishing boats...but they are basically a standard planing fishing boat hull cut in half...obviously requiring twin engines.

I think most dink manufactures have sorted this out. as there will be a plaque on the back with maximum weight carrying capacity, engine horsepower, etc.

As in all things, results will vary.

Sea state is an effective decider on how fast you will be able to go in your dink.

[flightlead404]

Apex AL310 aluminium hull RIB 18" tubes, Yamaha 9.9 outboard. Planes with my gf and myself ~280lbs total. Will not plane with 2 guys ~350lbs in it.

[Jammer]

Originally Posted by MicHughV

I think most dink manufactures have sorted this out. as there will be a plaque on the back with maximum weight carrying capacity, engine horsepower, etc.

The plaque is a compliance item that has nothing to do with planing ability.


Engine horsepower on the plaque is based on transom width multiplied by overall length then looked up in a table. Flat bottomed boats (jonboats) get derated. Manufacturers can choose to derate if they wish. Inflatables are exempt from the requirement but most manufacturers specify a limit anyway. Sailboats, canoes, kayaks, boats more than 20' long, and multihulls are also exempt.



Maximum weight carrying capacity is 20% of the weight required to sink the boat in most cases, 30% if the boat is certified for 2 hp or less. Some manufacturers derate to make it easier to pass the stability and level flotation requirements. Exemptions are the same as for engine horsepower.

[chrisr]

Originally Posted by Adelie

Naval architecture-wise planing starts about 2.7-3.0 x S/L and you are definitely planing at 4x S/L

https://www.boatdesign.net/threads/p...6/#post-665352

S/L is Speed/Length ratio which equals Sqrt (LWL).

So planing may start as early as 2x hull speed.

See graphic for the hull and planing speeds for various length boats. For tenders, 10kt is a good estimate for planing.

i think this is right up to a point, but because it neglects hull form it's fairly simplistic

as MicHughV says, some hull shapes wouldn't plane if thrown off a cliff...just dig a deeper hole in the water

others will get up and plane very easily...still others fall inbetween

thus this study could be misleading if only looking at hp v planning. hull form needs to be considered as well

btw, for me "planning" is taking place when the hull lifts up out of the water, out of displacement mode and skims. i'm sure we all know what it feels / looks like

incidentally, it takes considerably more power to get 'on the plane' than to stay there...

cheers,

[Adelie]

Originally Posted by chrisr

i think this is right up to a point, but because it neglects hull form it's fairly simplistic
If the dinghy doesn’t have a planing shape it won’t get near planing speeds regardless of motor size and data from those dinghies won’t make it into this chart

as MicHughV says, some hull shapes wouldn't plane if thrown off a cliff...just dig a deeper hole in the water

others will get up and plane very easily...still others fall inbetween

thus this study could be misleading if only looking at hp v planning. hull form needs to be considered as well

btw, for me "planning" is taking place when the hull lifts up out of the water, out of displacement mode and skims. i'm sure we all know what it feels / looks like

incidentally, it takes considerably more power to get 'on the plane' than to stay there...
I know and I believe I made that point earlier.

cheers,

Hull shape is not very important to this question. All the transom inflatables, RIBs and folding boats you can buy have at least an adequate planing shape or they wouldn’t have a market. The soft transom inflatable obviously can’t mount a big enough engine. The hard dinghies are the only ones where it’s up for debate but there don’t seem to be many people with those boats who put a big enough motor on to plane.

I’ll try directly PMing some of the members who have built hard dinghies if they’ve tried to plane with a big motor.

[Adelie]

Originally Posted by chrisr

…
incidentally, it takes considerably more power to get 'on the plane' than to stay there...

cheers,

I was wrong, I did not make this point earlier, I was going to but Panope had made the point and I elected not to repeat what he’d just posted.

[wingssail]

Originally Posted by Adelie

...The hard dinghies are the only ones where it’s up for debate but there don’t seem to be many people with those boats who put a big enough motor on to plane...

Let's don't forget that a large number of cruisers in Australia use "tinny's" which are hard dingys, usually made of aluminium, which plane so easily they don't need large motors.

[MicHughV]

I don't think this a clear cut thing...

Besides sailing....I also run a twin engined fishing boat. It's a 24' boat powered by two 150's. To get that boat up on plane quickly requires judicious use of the trim tabs out back, the engine trim and full on throttle. Trying to get the boat on plane slowly, just burns gas. It's an all or nothing thing.
It will also get on plane without all that "trim" effort, but struggles a bit. Bigger engines will help, 200 hp/engine will probably be sweet.

Once on plane, it has a "happy place"...which is around 3,500 engine rpm. It will be on plane at at around 3,000 rpm, but it tends to be squirrelly, it's on a plane, but just. Less than 3,000 rpm it will just wallow. I've experimented with 3,100 pm....3,200 rpm, etc...I actually get better fuel consumption at 3,500 rpm, than I do at 3,000 rpm.
You can hear it in the engines. At 3,000 rpm, engines are not "happy"...they feel like they are lugging. At 3,500 and above, the engines just sound better. I can generally tell from the the location of the spray alongside the boat, when I've got everything dialed in correctly.
You'd have to be there.

Sea state is another issue. To keep the boat on plane in rough seas, the bow needs to come down, so as to keep the hull as flat as possible, so the trim tabs must be down. How much down, varies a bit.

A single engine can get the boat on plane, but it's not easy, especially if the ocean is a tad rough. If the waves are behind me, it helps, if they are coming from ahead, not so easy.

I could also use the engine trim to cause the bow to come down, but then the engine thrust is not optimal.

On top of all that, those engines suck a prodigious amount of gas. I have a gas flow meter, so tend to keep an eye on this. Optimum gas mileage occurs only when both engines are synchronized and the boat is in it's "happy place". The amount of fuel burn jumps up considerably with each 100 rpm, once over 3,500 rpm. It requires constant attention to keep boat trim, engine trim and fuel consumption in harmony. At cruise, about 3,500 rpm I'm burning about 15 gal/hr....with the hammer down, about 5,000 rpm, that jumps up to around 45 gal/hr.

Switching back to to dinghies, I think much the same thing applies. I've never seen a dink with trim tabs, dink trim must be done by the occupants.
Same goes for engine trim, most dink motors stay in one state of trim all the time, occupants and weight placement is the " trim".

From my point of view, 15 hp is a good engine size for most dinks, 8-10 hp can do it, with some caveats. Smaller engines must work harder, but burn less gas. Bigger engines, while nice to have, weigh more, burn more fuel, etc...

Placement of the outboard fuel tank also needs consideration. Typically this is 6 gallons, close to 40 lbs when full. May not sound like much, but on an 11' RIB, that 15 hp is coming in at around 120-140 lbs or so...add 40 lbs of fuel in the rear, you've got close on 200 lbs on the transom....plus two adults, anchors, etc....

Just my point of view....nothing else..what you do with your dink is the deciding thing. If you plan to run out to the reefs, go big. If you're just putzing around the anchorage, no need to go big.

[chrisr]

Originally Posted by wingssail

Let's don't forget that a large number of cruisers in Australia use "tinny's" which are hard dingys, usually made of aluminium, which plane so easily they don't need large motors.

the tinnys are mainly 'cause the crocs find them harder to bite (RIBs are also called chew toys up north...)

the easier planning is just a fringe benefit

cheers,