The effect of "rocker" on power choices for a skiff

[Roletter]

The summer before last I built Maynard Bray's Lumberyard Skiff for my son's 12th birthday. It turned out great, and he/we have gotten great enjoyment from it thus far with both oars and trolling motor. I have considered building a sailing rig for it, and still might, but in the interest of time, and the desire for extended range, am considering an outboard. What scant information I can find suggests an outboard of 8hp or less. So here is the question: Does anyone have any actual experience powering this hull? While it has a flat bottom for about 2/3 of the hull length, the back 1/3 has quite a bit of rocker. The hull in this section is still flat, but "runs" up above the waterline like the stern of a classic sailing hull. The intent of this design is both manuverability and dry feet when eased up on a shore-line, stern first.......so will she ever plane, or is she doomed to "hull speed" as all displacement hulls are? The answer affects the sizing of the outboard needed.... or wasted. Thanks

[Peerie Maa]

Hull speed. A stern wedge might help, but if the experiment fails you will curse me.

[neilm]

Use a 2-3 hp motor and you won't have to worry about it.

Neil

[Roletter]

Hmmm. I have seen the 'wedge' suggested somewhere. Think this is also known as a "whale tail" that bolts on the cavitation plate of the outboard. Am I with you?

[Roletter]

Neil,

This is sage advice, both for the wallet and unknows such as the strenth of the transom, stability under power, etc. The downside to this advice is it doesn't really satisfy one's natural curiosity of will she or won't she? If only God hadn't invented testosterone, decisions like this would be much easier..........

[Raka025]

She's a flat bottom boat and would most certainly plane with a 8 hp motor. Whether an 8 hp would hurt the transom, I don't know.

[TerryLL]

A flat bottom does not insure a boat will plane. A few other things are required for the hull to get on step and stay there. Typically, planing hulls require a flat planing surface straight back to the motor. Even a little rocker at the aft bottom usually dooms a hull to displacement speed, and the LYS has a huge amount of aft rocker.

[mmd]

Oh, I don't doubt that with sufficient horsepower & proper prop sizing that the boat would plane...

...you'd be looking at the sky while you vainly tried to control which direction it went in, but you would be planing.

[SaltyD from BC]

Oh, I don't doubt that with sufficient horsepower & proper prop sizing that the boat would plane...

...you'd be looking at the sky while you vainly tried to control which direction it went in, but you would be planing.

:DYup. A buddy has an aluminum seine skiff with a flat bottom and quite a bit of rocker. Originally these skiffs were made of wood they were meant to be rowed to provide just enough resistance at the net end to help peel the net off the drum. Then the seiner steered in a circle to make a loop and pick up the skiff man.

My buddy has a 9.9 merc on his skiff which is about 12 feet long and weighs 800 pounds. At 1/4 throttle it mashes along at about 4 knots. At half throttle it points its nose to the sky and is bloody dangerous! For a LY skiff with a similar profile to the bottom, I think I'd look into an electric trolling motor or if gas no more than 2hp.

[john welsford]

I'd endorse that , and add that the boat may become very unstable when being forced along in that attitude. Either stick with very low power or build something designed for the speed you want. No need to get any wetter than you have to.

JohnW



My buddy has a 9.9 merc on his skiff which is about 12 feet long and weighs 800 pounds. At 1/4 throttle it mashes along at about 4 knots. At half throttle it points its nose to the sky and is bloody dangerous! For a LY skiff with a similar profile to the bottom, I think I'd look into an electric trolling motor or if gas no more than 2hp.[/QUOTE]

[jimmy lee]

I built Maynard's Lumberyard Skiff also. I used my 2hp Johnson on Her just fine! I never needed to raise the throttle up no more than 2/3s, that was fast enough for Her. I presently sail Her. She is a pleasure to sail. Sincerely,Jimmy Lee

[Roletter]

Thanks to all who've replied thus far. The collective advice makes sense. I also suspect it would plane with enough hp given its flat bottom and sparse rocker forward, but she would probably ride bow high as has been suggested, and have strange tracking characteristics. Jimmy Lee, your input on the 2hp option is appreciated. She does fine with trolling motor in light air, its just that range is limited, and wind and current where we sometime use her makes that a very limiting method of propulsion. If anyone has used a larger outboard on this design, I would still be interested in hearing about your experience.

[hightop]

I built a Smith Island style crabbing skiff and used a 6.5HP Honda (inboard), and due to the rocker aft, anything over around 2000 rpm's just pulls the stern down, makes a lot of noise and increases hull speed marginally.

I would suspect that I (and you) could get along with about 2 HP.

[Cuyahoga Chuck]

As a rule of thumb getting a displacement hull 1 MPH over hull speed requires a doubling of HP and , supposedly, each additional MPH requires an additional doubling. The boat tries to climb it's bow wave but never gets on top. It is trying to climb a hill of water which requires a lot of urge from the motor and makes fuel usage skyrocket.
That said, the desire to mount the biggest possible motor on a rowboat will never be stamped out.

[James McMullen]

the correct shape for a good rowboat and the correct shape for a planing powerboat are opposite and antithetical to each other. You're really going to have to build another boat if you want an efficient, planing powerboat--and then it will be a bastard to row. This is an "either/or" situation.

[rbgarr]

My wife's uncle had two flat bottom skiffs, one (a local Brewer skiff) that has the rocker aft like the LYS (and which was influenced by the Brewer model) and the other flat to the transom. He had a five hp motor that he tried on the first for lobstering when he wanted to give up rowing his string of traps and it didn't work out. The weight aft even at rest left the boat unbalanced, bow high, and at speed it maneuvered really poorly. Reversing was dangerous. He had Brewer build the second, same length and beam, for use as the lobstering skiff and it worked fine, leveled out easily and would plane.

[mcdenny]

If you have enough speed but range is the issue, how about a larger battery bank? Doubling the battery bank more than doubles the range (google 'peukert effect'). You can also try an aftermarket prop (Kipawa, about $50) to improve performance. Trolling motors are propped to move a 2,000# bass boat, not a 200# skiff - it's kind of like driving in low gear all the time.

After getting used to that electric motor, a gas outboard is going be awfully noisy. Your hull speed is about 5 kts and 2 hp will push it that fast at an all up weight of 1000#. 6 or 7 kts will be the practical top speed no matter what. An 8 hp outboard will give you about 7.5 kts with lots of noise, waves, bad steering - and no joy!

Another nice but pricy option is a Torqeedo electric outboard.

BTW, 2 hp = 1500 watts = 62 amps at 24 volts.

[Spokaloo]

Im just feeling a little crazy today..

Why not put a pair of wedges right AT the turn of the rocker. Then cut a hole in the boat, making a well just aft of said turn. The wedges theoretically give the water a place to break away from the rest of the boat, effectively making a "transom" of sorts once the appropriate speed is reached. All weight will need to be forward of the motor to allow her to pick up that big section of hull aft of the wedge once you can achieve the breakaway of the water and the bow settling down. I would imagine its going to require something on the order of 10-15hp to accomplish, but since we are living in the theoretical world, why the hell not?

E

[TerryLL]

No rocker.


http://ford.physics.fsu.edu/tableboat.jpg

[wharf rat]

Since we're going all theoretical here...a properly designed power boat has the rocker in the first third of it's length, then a flat run to the stern. A sail boat on the other hand has it's flat run in the first two thirds of its length and it's rocker in the in the last third. Right?

So put the motor on one end and a rudder and tiller on the other. When you want to go power you (the hull) goes one direction, and when you want to sail it goes the opposite. Sorta' like a proa, but different. :D

OK, maybe not.

[ethanopia]

This is an excellent discussion and answers a burning question I've had in mind ever since my first trip in a Mackenzie River Drift boat as a fly angler. Would it be possible to have a boat that has enough rocker to row backward in the current like a Mac Drift boat does and have enough power in an outboard to motor upstream and drift back down to the launch site. The answer sounds like NO! A drit boat is built to row and row only.

This is why the ubiquitous aluminum jon boat has no rocker aft huh... great info, thanks!

[Wayne Jeffers]

This is an excellent discussion and answers a burning question I've had in mind . . .

Agreed!

What I've been puzzling about is not a drift boat, however, but an early-1930's outboard boat with only a little rocker aft. The rocker is compounded by lack of bearing aft (multi-chine design.) Looks like a modified row boat, probably intended for a lighter 3-HP or so outboard.

I had decided to build an old Phil Bolger outboard boat instead, with a dead-flat run aft, because I had concerns about meeting my objective of planing with an 8- to 10-HP outboard with the older design.

This pretty much confirms my choice. Pity, because the older design is so much prettier.

Wayne

[TerryLL]

This is an excellent discussion and answers a burning question I've had in mind ever since my first trip in a Mackenzie River Drift boat as a fly angler. Would it be possible to have a boat that has enough rocker to row backward in the current like a Mac Drift boat does and have enough power in an outboard to motor upstream and drift back down to the launch site. The answer sounds like NO! A drit boat is built to row and row only.

This is why the ubiquitous aluminum jon boat has no rocker aft huh... great info, thanks!

Well, it depends entirely on the speed of the current. You can easily power a MacDory to move at its displacement speed or a little beyond, say 5-6 knots. If the current is 4 kn, then you'll be able to get back upstream, though slowly. What you will not be able to do is plane upstream at 25 kn.

[Spokaloo]

Would it be possible to have a boat that has enough rocker to row backward in the current like a Mac Drift boat does and have enough power in an outboard to motor upstream and drift back down to the launch site. The answer sounds like NO! A drit boat is built to row and row only.

Look back a little in the history of the McKenzie river dory and you will find surprising answers.

The first MRD's were actually flat transomed jobs, if you can believe it. This was the initial shape:

http://www.raysriverdories.com/RRD_web_images/mightymac_snakervr.jpg

They were meant to absorb a wave with the flat, blunt transom and have minimal buoyancy aft, with the ability to be rowed into a chop on long flatwater stretches like the areas where the McKenzie meets the Willamette.

http://www.raysriverdories.com/misc/in_action/Picture%20023_lrg.jpg

As you can see in this shot, she keeps her chine up at oar:

http://www.raysriverdories.com/misc/in_action/Picture%20100_lrg.jpg

Afterwards, you hang your motor, stand in the transom, and she runs out on the prop on a plane. This shape resulted from some interaction with the Pacific City dory fleet, just across the Coast range from the river.

These boats run out well on a 9.9, though they are heavy for taking on and off the transom all the time.

E