Sailboat as a powered cruiser?

[mackid068]

I have an idea. Perhaps if you're worried about stability by removing the sail rig, then you could use a cat hull or a boat with a full keel. Oh! I just thought of it. Use a lug mizzen or something of that nature, perhaps something like the farthest back sail on a ketch (same thing as a mizzen I think)

[Ssor]

The problem that you would encounter with a full keel is the depth of water needed to float the vessel, If you want to go exploring in the thin spots around the edges then you need to be able to pull the board up. I have friends that own a twenty something Macgregor and if they run aground they just get out and push. Don't be too afraid to keep the sailing aspect. You don't need a very big sail to move the boat and it can bring you home if the engine dies. We have been discussing the use of aircooled inboard engines on small boats on the wooden boat forum. It is done frequently. Sailboats already have a rudder.

[mackid068]

I don't think he's interested in the sailing aspect of boating, but I am not sure.

[oforberg]

I am in the process of rebuilding a O'Day Daysailer as a electric launch for our smaller inland Minnesota lakes. She will be powered by a Minnkota 54 LB thrust trolling motor and two 12 volt batteries.

It should reach hull speed and cruise silently and efficiently for about 8 to 10 hours on a full charge. It is a thru hull installation with up front steering.

Good luck on your venture.

My inspiration came from the boat in the second photo....Zinger on yahoo groups electricboats.

[wave1235]

I converted a Skipper 20 sailboat to a motor boat. A description and a picture is in my post under "Troller vs Trawler" thread. Search posts by wave1235 to find it. It's a great way to enjoy safety and comfort on the ocean at 20 mpg.

[mattotoole]

Originally Posted by tschienque

There was an old adage back when engines on boats were true auxilliaries, that you needed 1hp per ton to achieve hull speeed (square root of waterline length) in flat water.
You won't get scintillating performance and will take time to get upto speed but sounds OK for your situation/requirements.

I looked at a Santana 20 for some basic figures. LWL displacement etc

The Santana has 16' of waterline length (easy square root - how nice) would have a square root of 4 so speed = 4 knots. (most sailboats can achieve 1.34 x sqrt of LWL)

The displacement is 1,360lbs or .6 of a ton

So theory tells us that a 1hp engine would be sufficient to push boat at hull speed with more than 50% in reserve (only .6 hp required to get upto hull speed).

Don't know of any 1hp motors, but plenty of 2s, 2.5 and 3s that'll give you plenty of poke.

Well that's the theory. However, small outboards have small props and are not at their best pushing displacement craft.

Designers, what (if anything) is wrong with this scenario.


In tune with your MO consider a 4 stroke, they're very quiet (+fuel efficient).
Even quieter are electric trolling motors used one on my Dhyer dink for 5 years, gives you a new feeling pottering around anchorages.

Note that another Schock 20-footer, the Harbor 20, does indeed use an electric trolling motor. I don't know what the specs are but it's probably 1 HP or less. It seems to push these boats just fine (at hull speed) in flat water.

Another advantage of electric motors is that they produce full torque at low RPM. So an electric motor with lower rated HP can be used, and it can turn a bigger prop which is more efficient.

[wave1235]

With the high thrust Yamaha 9.9 hp I can get seven miles per hour which is greater than a 1.34 s/l. Big seas take lots of power however, and the 9.9 does ok but is not too much by any means. The 8 hp high thrust might have been enough but I wanted the extra electrical generation of the 9.9 as well as the extra cubic inches. That extra accelleration if a wave tries to push you around is worth the slight extra weight. I would not run an outboard which didn't steer lock to lock as the response is much quicker than holding the outboard straight and steering with a rudder. My Skipper 20 has an internal cut-away shoal keel which let it spin around quickly with the outboard and no rudder. I added a stationary rudder to Bug last weekend. This weekend I ran it around in the ocean and it really improved everything. The boat is faster, the steering takes a lot less effort, and quartering waves don't push the boat around any where near like before. The low speed handling forward and reverse are every bit as good if not better. The feel of the boat, which is hard to quantify to others, is like a larger,steadier boat and a stationary rudder is very much worth adding to any similar converted sailboat.

[SteamFreak]

Originally Posted by tschienque

There was an old adage back when engines on boats were true auxilliaries, that you needed 1hp per ton to achieve hull speeed (square root of waterline length) in flat water.
You won't get scintillating performance and will take time to get upto speed but sounds OK for your situation/requirements.

I know this might be alittle late in the discussion but I had heard that it was 1/2 ton per hp from an old marine engineer but considering he was speaking of larger fishing/motoryacht style hull and that no doubt has a greater requirement... When using that formula for hull speed, is there a range of hull types it conforms to or will that work for about anything? I ask because some source say that there's differences for broader hull forms than for narrow sailboat lines..

[wave1235]

With a heavy enough hull without too much frontal area you can get by with less hp per ton. With a smaller boat, the inertia is not available to maintain a constant speed through swells or chop and more power is required. The real power requirement is for crossing the bar. If a following wave starts pushing the stern around, then enough reserve thrust is required to get back perpendicular to the wave immediately. For this effort, hp means less than having enough prop size and torque for immediate accelleration. Frontal area is pushing the water. A narrow boat pushes easier than a beamier one. The contour of the hull entry makes a lot of difference also. Any hull can be pushed over hull speed with enough power A narrow hull with a sharper bow just uses less power to do so. The bow and stern waves will be at the ends of the hull at 1.34 X sq root of water line length for any hull at displacement speed. More easily driven hulls just use less hp to exceed this speed, though they too become less efficient as thier speed rises.

[messabout]

I converted a derelict Thistle to a beach cruiser, much reduced rig and removed thwarts and other unnecessary stuff. The boat was so much fun with a 2.2 HP Tohatsu that I often left the rig at home. It would go well enough with a 34 pound thrust troller. It also accepted a 9.9 Johnson that would make the boat go well past hull speed but it would throw a hellish wake. At just off idle the Johnson would make one mile in ten minutes and was very nearly silent. The Thistle was not the least bit uncomfortable in any sea state that I encountered. For quiet running and pleasing fuel economy this was a great boat. The Day Sailor that Oforberg is building will also make a neat boat for potting about quietly and happily. I confess that I often took along a shotgun to display to the Jet Ski maniacs. A Mossberg Mariner is an intimidating piece of hardware. What is it about a slow boat that causes those guys to want to spray water on you?

[FAST FRED]

"I confess that I often took along a shotgun to display to the Jet Ski maniacs.. What is it about a slow boat that causes those guys to want to spray water on you?"

Wrong approach,

Simply get scrap polly rope (that floats) and cut it into 3 ft lengths .

Tear it apart to stands , and simply float it as you sail.

When the bored fools ingest some into the jet ski intake , it gets REALLY QUIET .

And the waves go away.Till the seals are replaced .

FF

[messabout]

Fred;
Damn, I wish I had thought of that poly rope thing. I'll file that bit of information for future reference.

Actually all I want those guys to do is leave me alone. My shotgun display is totally illegal in Florida as you probably know. FWC officers have, a couple of times, contracted temporary blindness on my behalf. My choice of defensive tools converts me from intimidatee to intimidator almost immediately and the issue is resolved, no harm done. So far so good.

I am not a violent or angry man but I do have a limit to patience. I am not particularly proud of a long past episode when a slalom skier broke his leg when it hit my outstretched ash oar. He and his youthful companions were taking great glee in deviating from the slalom course, trying to sink my little dinghy with me in it. The oar was a defense mechanism that followed the sixth slalom run and all the spray that they made. This happened on a city lake and the cop who investigated the incident saw fit to threaten the offending skiers with jail time. He called it ADW. Justice served.

Back on topic. I submit that a boat similar to a Lighning, M20 or C scow, Thistle, Highlander, Flying Scot, will make a very pleasing boat for slow motoring. Any of those are trailerable, they'll perform very well with 4 to 6 HP, and they can be hade cheaply with some astute shopping. Yacht clubs and sailing squadrons sell off some of their outdated boats from time to time. Last month I saw such an example. They were selling doggy but usable Lasers and Sunfish for $25 There are also some "please haul this thing away" small keelboats for free.

[alan white]

I used to doodle a concept boat--- a 28 ft x 3ftx 1ft lightly ballasted, enclosed (with fighter jet cockpit) hull (no sail). Looked like a submarine. Maybe a 10 hp diesel, two berths, articulated floats that come down to stabilize at rest when desired. Idea being, it was self-righting, any weather boat, would pierce through waves (wet, but cockpit would have a sliding see-through cover), survive anything, beach, reach 15 kts (as if a single cat hull), even take-apart in the middle for trailering.

A.

[MikeJohns]

The overall key to power requirement is just where you pick on the speed-resistance curve to operate your vessel. Importantly you must also consider the area you want to operate in. Sheltered Semi or Ocean. If you want to motor against 50 knots in a seaway with a smaller engine it can be done, the tradeoff is a larger slower turning prop and a slower top speed.

Usually we are obsessed with speed and put in huge power packs that push the boat at maximum , it is often a big surprise to people to find that the 50 footer uses less fuel motoring alongside the 40 footer at 6 knots simply becasue she is operating in the relatively linear part of the resistance curve.

Originally Posted by tschienque

The reason Seagulls were so easy for owners to work on, was that they needed to be.
Some owners "boasted" of being able to strip and rebuild a Seagull blinforded and in the pouring rain.
They needed to - considering our weather and how often they broke down

Fouled plugs mainly, the big problem with them is the high oil content required in the fuel but this also gives them diesel engine longevity. 40 year old motors are still being used daily around here.

Several times in a sudden tempest amongst anchored boats I have seen inflatables flipped and blown onto the beach inverted with outboard attached, it says something that a sand filled drowned outboard can be used to motor the dinghy back to the yacht after an hours work while the modern one is a nightmare often proving impossible to strip with the facilities on a cruising yacht .

As Fred says some of the seagulls with a big relatively slow turning prop are capable of unbelievable thrust for such a small power rating.