Minimum Horsepower Boats (Flat Bottom)

[Huntsmanvb]

I was thinking today about the subject of minimum horsepower boats: what flat bottom mono-hull designs will provide the greatest speed, carrying capacity, and functionality with minimal power?

I think you have to break the discussion down into the categories of displacement and planing boats. I would define "minimal" as under 10 hp for a planing boat, and under 5 for a displacement boat.

As for planing boats, the Bolger Sneakeasy comes immediately to mind


"Ultra simple launch for the home builder. This shape can't be improved for fast and simple building. "Instant Boat" is no exaggeration. 25HP (short shaft) drives her 30-32 mph. With 7.5 HP minimum she will make 12 mph and cruise at 9 or 10."


For displacement craft, I would think that something like a big FAO canoe would fit the bill:

http://www.archive.org/stream/fishin...e/n17/mode/2up

A big dory would likely be more seaworthy, but the long waterline of the FAO canoe should get a bit more displacement speed out of that 5 hp.

[brucehallman]

for a planing boat...for a displacement boat.

The tricky thing is that for these boat types, there is a not much distinction between planing and displacement.

In essence, Sneakeasy is planing at rest.

This is achieved by being very light weight for the amount of bottom area.

[Huntsmanvb]

The tricky thing is that for these boat types, there is a not much distinction between planing and displacement.

In essence, Sneakeasy is planing at rest.

This is achieved by being very light weight for the amount of bottom area.

Interesting. I wonder how the Sneakeasy would perform with, say, 5hp? Would it move along at more than hull speed, despite the amount of power being too low to plane according to conventional concepts? Is there a power level at which a hull shape like the FAO canoe would out-perform the sneakeasy (perhaps 2.5 hp?) Food for thought.

[brucehallman]

hull speed

I am not sure that "hull speed" has much meaning when talking about light narrow boats. As I understand it, a boat must be heavy before the interaction of bow wave and stern wave cause the "hull speed" hump. The Sneakeasy is too light weight to do this.

[Huntsmanvb]

I am not sure that "hull speed" has much meaning when talking about light narrow boats. As I understand it, a boat must be heavy before the interaction of bow wave and stern wave cause the "hull speed" hump. The Sneakeasy is too light weight to do this.

So perhaps the Sneakeasy would travel faster than a traditional displacement hull form (dory or canoe) even with very low HP?

[Ben Fuller]

The low horsepower planing types that were any size had a step in the hull. Look at the small outboard raceboats of the 30s. World War I Motor Torpedo boats used engines in the 100 hp range which were awesomely heavy and had steps. Carried 4 crew, an automatic weapon, a torpedo across channel and back. Problem with stepped hulls is that they are very sensitive to weight and placement of same.

[SeaB]

You may want to check out some of the boats used when racing British Seagulls. Start at the post dated Feb.20,2011 by "Tinker" in this thread:

http://www.saving-old-seagulls.co.uk...hp?f=15&t=2315

Also check out some of the boats used for the Seagull "RISR" [Round Island Seagull Race] in Bermuda

[skaraborgcraft]

That "flea" boat is very interesting,i like the fine bows.better to cut thru a chop than a slab sided flat bottom,i understand the original sneakeasy was a bit of a slammer.

[Huntsmanvb]

It seems like the biggest question is whether you want a double-ended shape, or if you what a full width transom on an long narrow boat. (like sneakeasy or the seagull boat).

People often assume that the double-ender produces less resistance for low power, but perhaps the lack of flotation in the stern makes the double-ended boat sit lower in the water, creating a stern wave that limits hull speed.

[Huntsmanvb]

It seems like the biggest question is whether you want a double-ended shape, or if you what a full width transom on an long narrow boat. (like sneakeasy or the seagull boat).

People often assume that the double-ender produces less resistance for low power, but perhaps the lack of flotation in the stern makes the double-ended boat sit lower in the water, creating a stern wave that limits hull speed.

The seagull boat posted above was shown doing 9 mph in the link.

[wizbang 13]

I built a boat for that Round Bermuda race in about 1990. Long skinny bastid. 7 knots with a 102. Won our class.The handicap then was you had to hand off a beer to every boat you passed, a full beer.

[JimD]

About 60 years ago 'XLNC', a 19 footer, came off the Atkin drawing board. Rated for 6 hp

[SeaB]

Also from Atkin "Scandal" which promises 9 mph for 3 hp. It shows the outline of an Elto "Ruddertwin" outboard, which was one of the very best outboards available in 1924, the date on the design. It is interesting to me that this hull was built of plank and so would be much heavier than the 20+foot long NZ built Seagull hull [built of light ply], yet produces the same top speed in a 14' 7" hull with a motor of similar power.

[stevebla]

Take a look at this Grand banks style boat. http://www.spirainternational.com/hp_texa.html

[Huntsmanvb]

On the subject of boats specifically designed for seagulls, check out the Poole canoe:



"So I was pleased a few weeks ago to learn of the Poole canoe a few weeks ago, and I’m now grateful to John for capturing these slender flattie skiffs with his camera before the original wooden boats disappear. I have the impression that they range up to around 22ft by 4ft or a little over. Thanks for the informative shots John!

.... British Seagull proprieter Mr Weyhope spent years experimenting to get the best speed out of the boats driven by a Seagull 102 model, which I’d guess was a 2hp type. Looking at the boats in these pictures, they mostly have the small amount of rocker I would expect for a low powered boat, though one or two seem to have rather flatter runs, which would suggest they were intended for a bigger power plant."

http://intheboatshed.net/2009/12/26/...poole-harbour/

[Daniel Noyes]

John Gardner recorded the lines of a 28' Chasion power dory that did 9mph with a 6 hp through any type of weather.
At the time it was built 9mph was a real speed boat.

[DanSkorupka]

The most efficient motor flat bottom boat i have ever heard of would have to be a proper power sharpie like Russel R by John and William Atkin. It is a true semi displacement hull, with a wide transom and some, but not extreme, aft rocker.

While narrow by most modern standards, its length to beam ratio of 4 offers quite a roomier boat than the over 6 of Sneakeasy.

To quote exactly:
"Plans of Russell R. show a neatly turned skiff 21 feet 10 inches over all; 20 feet 6 inches on the water line; 5 feet 8 inches in breadth on deck and 8 inches draft to the bottom of the skeg. The propeller and protecting shoe of the outboard motor project 6 inches below the skeg; the total underway draft thus being 14 inches--little enough by any measure. The freeboard at the bow is 2 feet 10 1/2 inches, the least freeboard is 1 foot 7 inches; and the height at the stern, 1 foot 10 inches. Altogether, this latest baby of the family is a shapely little packet. She has ample topside flare and will be reasonably dry in rough water, more so than many of her larger sisters. And she will prove to have proper balance under good or bad weather conditions."

RussellR-3.gif

Plans can be had from http://www.atkinboatplans.com/Oar/RussellR.html

Claimed speed is "a good 17 miles per hour" with a mere 12 horsepower. This may even be for the shown waterline which is a very heavy load, with both the transom and forefoot well submerged. lightly loaded I would expect her to exceed hull speed with a 6 or 8 hp engine

[dredbob]

Just a quick comment about the engines used in these old boats. The 6 hp (and others) referred to in old articles were not little fast turning outboards as might spring to mind today when you see "6 hp". Thet were almost certainly old slow turning, high torque, big cylindered engines turning large diameter propellers by todays standards.

Bob

[Ross Lillistone]

British Seagull power is not the best way to get maximum speed from a given horsepower, mainly because of gearing and propeller issues. However, the people who are keen get amazing results. I don't understand the rules when it comes to modifying the props and the gear ratios, but here is a photo of Steve Dorrington doing 6.5 knots using a 2.5hp British Seagull 40 Plus with (to the best of my knowledge) a standard prop and gearbox. The speed was measured by GPS, and the boat is one of my Flint rowing boats. I've recorded 6.1 knots using a Yamaha 2hp at part throttle with two adults and two teenagers aboard - i.e. an overload. The motor could have gone faster, but all it did was make bigger waves.


Steve Dorrington at 6.5 knots with a 2.5hp British Seagull Forty-Plus


6.1 knots at part throttle from a 2hp Yamaha - the boat is overloaded. Note the location of the bow and stern wave if you doubt the GPS!

Ross Lillistone www.baysidewoodenboats.com.au http://rosslillistonewoodenboat.blogspot.com/

[DanSkorupka]

Some planing sailing dinghies perform surprisingly well under motor if trimmed bow heavy. what happens IIRC is that despite having heavy aft rake they will still plane because even though the stern is not providing much lift, the wide flat sections amidship act as waterplane area, giving lift and allowing the boat to substantially exceed hull speed.

An extreme example of this is in the Vaurien class dinghy. The entire front half is flat bottomed, but the rear half is a rockered round bilge.

lines can be seen at http://vaurien.ru/engl/engl1.10.jpg

The person in the video (not me) claims 12 knots with a 3.5 horse tohatsu, alone and with a tiller extension.

[Wavewacker]

I have read lately that the lighter boats really are not as concerned with hull speeds as mentioned above. The Sneakeasy seems to be duplicated in other designs, long and skinny, and something I'm really interested in. Does the electric powed launch fall into your low hp category? I know the question addressed monohulls, but what about Skoota, Woods small power cat with similar hulls? Yellow Banana, another electric is interesting as well and while it is a tri-hull, the amas are really, to me, appear to be more of verticle foil with little floatation to them.


And then, how do you think these monohulls would react with trim tabs holding Sneakeasy on plane?

[DanSkorupka]

Bumping/reanimating this old thread, the above post was my first ever on this forum.

have something interesting to show y'all:
White River, 8 generators, upstream, 4 big guys, 21 footer, over 5 inches aft rocker, 25 hp merc jet (aftermarket jet IIRC kit so more like 15 or 20)
http://www.youtube.com/watch?v=phafRL9jiRM

[Huntsmanvb]

Bumping/reanimating this old thread, the above post was my first ever on this forum.

have something interesting to show y'all:
White River, 8 generators, upstream, 4 big guys, 21 footer, over 5 inches aft rocker, 25 hp merc jet (aftermarket jet IIRC kit so more like 15 or 20)
http://www.youtube.com/watch?v=phafRL9jiRM

What boat is that? I can see that it is a very narrow 21 foot scow. I have repeatedly seen reports of long narrow scows making great performance. I think the term was "McKenzie River" scow, a 4' by 20' (or more) plywood river boat.

[DanSkorupka]

It is a supreme 2048, a gussied up version of the Shawnee river boat.
It is narrow by modern standards but not quite as narrow as it looks and a whole lot wider and more useful than the sneakeasy, 48" wide bottom, beam over 5 feet at the gunnels for the whole length.
It's secret is the profile of the rocker: the midships are almost flat for a long ways, there is a brief region with a smaller radius to create the rocker, then a the rest of the aft run is straight (not truly flat but has a large radius) but not flat.
The low radius of the final run has a lot less "water on the spoon" effect than most displacement hulls which lessens the pull down on the transom a lot, while the flat midsection (not the transom) provides substantial dynamic lift
It is a wonderful compromise hull that is far easier to move and maneuver at low speeds than any pure planing hull but will still go faster than hull speed with little fuss, at the expense of poor handling and economy at speeds past the low teens, where a dedicated planing hull is the only real option.

I have contacted members of ozarkanglers forum who say they do not plow at slightly above hull speed but just seem to "levitate" the transition is so flawless that an inexperienced driver would only realize he or she started to plane by looking at the wake. This is partly due to the midsection being the main source of lift and partially due to the hulls waterline being so long that hull speed is so high that they start planing before they reach hull speed if properly trimmed.

Kinda pissed that the most efficient non pure displacement boat there is is made in blooge and itch fibre.
If you absolutely must have one and aren't willing to build it in wood (like I may do in a year or two) they do have a website: http://www.shawneeboats.com/models.htm

[DanSkorupka]

Here is a funny picture of someone attempting to have his cake and eat it too, for some reason it is on the manufacturer's website.


an expert will immediately see what is going but for the many newcomers on the forum I will spell it out.
The factory jet 40 is actually a 60, and uses a 3 cylinder fuel injected fourstroke.
all that weight will keep the stern in the drink, negating low speed economy and ease of rowing, and the excess power will do interesting things to the handling, plus stick the bow up God's (censored) during the hole shot (remember this almost a displacement hull here)

in short he made something that is horrible at low speeds and very dangerous to attempt to plane

most actual working watermen, and the smart rental fleets run a 15 or 20 hp 4 stroke prop motor (tohatsus and mercs are said to be popular for their light weight and tolerable cost) or a 25 hp 2 stroke jet, usually a bog standard Yamaha looper with an aftermarket jet kit.

[potomac]

Certainly not the most anything (long, fast, light), but very useful. Thomas Hill's Pamet Blue- relatively beamy, long, calls for a 2 hp, and at 91 lbs., car-topable.

http://www.thomasjhillboatdesigns.co...blue_boat.html

[Hal Forsen]

Here's a thread by a fella who built Bolger's Seahawk power dory.
http://forum.woodenboat.com/showthre...k-last-weekend

He had this to say:

Planes easily with the 6hp and one person. With gear and two aboard it still planes. Bolgers plans call list 20hp as a maximum. I can't imagine what that would feel like.

My boat is probably heavier than Bolger intended. I ended up using yellow pine plywood and epoxy coated the whole boat. The outside is covered in 10oz cloth doubled on the bottom inside and out. The extra weight has it sitting below the designed waterline. Even with the extra weight i'm happy with the performance.

[Huntsmanvb]

Check this out: http://www.dngoodchild.com/divide_for_small_craft.htm #5333--TESLIN

This 23-foot John Boat is a stable and fast river craft capable of carrying great loads.
Anyone who has ridden a John Boat similar to Teslin on swift and shallow river waters will not deny its advantages. It has the stability of a flat-bottomed rowboat, handles with the ease of a canoe, beaches like a landing craft and carries loads like a barge--all with minimum power. It can be rowed, sculled or poled with remarkable ease and a great deal of control. Because of its design, it shines in rapids. The long and slender hull doesn't bob up and down in the troughs--it rides a series of waves instead. Teslin was built four years ago and has been in use on Yukon rivers since. With a Mercury Mark 6. it moves
at speeds judged up to 15 mph with light loads and still has enough push to freight heavier loads upstream in a 5- to 7-mph current. A speed demon is in for a real thrill if he uses up to 25 hp. He need only be sure to have enough weight in the bow.

It has a maximum bottom width of only 29". More details here: http://www.scribd.com/doc/2897271/Teslin

[DanSkorupka]

Teslin is an absolute dream: I looked up the mercury mark 6; it is 5.9 hp with a muffler in the "silent super 6" model and a little over 7 hp without the muffler. plans are only 8 bucks (obviously meaning us dollars, for anyone outside the states).

I have a 5 hp briggs and stratton with a high reduction gearcase and a wide, steep pitch prop that will move pretty much anything at 15 statute mph limited by pitch and rpm to this value.
I think the 5 hp is an understatement: the block is continuous rated 6.5 hp at 2800 rpm with a 3000 rpm 7.2 hp wot, and this thing with its bigger carb and not as tiny muffler redlines at the limiter at 4000 rpm and has pretty brutal torque even at that value. Easy on the fuel budget at a hair over 1/2 gal/hr at full throttle and I would compare its "oomph" with a yamaha 6 or a tohatsu 8.

If I want to go faster I have an Evinrude Fastwin 18 with a moderately slow propeller.