Design Issues for small, sail/human powered cruising boats

Scantlings aren't just to keep the water out in mill pond conditions. The wracking and slamming of a dynamic three-dimension sea, torquing etc. stresses from propulsion systems as well as internal and exterior impacts have to be taken into account as well.

 

A couple of thoughts:

Most ocean-crossing row boats are significantly more than 5-foot beam. I'd say 6 feet and greater is general. The only 5-foot I've found, a solo boat, is expected to gross well over 3000 lbs. fully loaded. Can you imagine solo rowing 3ooo lbs.? I can appreciate the idea, but NOT for me!!!

I can appreciate rowing across oceans, but the boats are big and heavy. They need to be because they are so slow, which is partly because they are so big and heavy, but also because the power of one or two human's is so low. And rowing a big, heavy boat takes a LOT of calories, supplied by lots of food. They often require 100 days or more to cross the Atlantic - so they need a LOT of gear and supplies. So, again, they tend to be very heavy, and therefore very slow. They tend to be blown across by prevailing winds, and are very limited in moving against wind and waves . . . and are therefore subject to whatever the weather brings, for months at a time.

Using both sails and rowing (for when there's little or no wind), one can move noticeably faster, with much greater control, and therefore need less gear and supplies. And being smaller and much lighter, the forces are much less, which means they don't have to be as heavily built . . . and using foam core with lots of carbon fiber makes significant strength possible with much lighter weight . . .

I wouldn't imagine crossing an ocean in an Angus row-cruiser, with or without amas and sails. I'm sure Colin Angus would agree. On the other hand, he's designed a solar-electric autonomous ocean crossing research vessel that is rather long, narrow and light weight, as well as autonomous sail powered ocean crossing boats. At some point I'll send my design to him and see what he thinks.

 

Hi Scott

Have been thinking about your boat, in between having to deal with immature teenagers working on high rise construction sites, and cleaning a set of concrete steps for 50 minutes because I spilt some coffee, and in-between more and more covid tests, and an ageing stubborn mother

Firstly weight, yes the large pure rowing boats carry a lot of gear because crew eat a lot and drink a lot of water, and use a lot of energy. I take your word on the 1300 to 1400kg, yes lighter would be better. Being way way lighter than 3000 pounds would be a good thing. Going with sails, thus travel faster, shorter duration trips, less calories used, less food eaten etc. Yes your boat can now be lighter. But even 50 percent lighter is still 650kg.. You want to go half that again, 25 percent weight.. I think you will find it difficult to make everything super light

You may be able to make a boat for less than 50kg. I have my doubts to its strength carrying 300kg to 400kg load in large breaking seas. My suggestion is make it heavier stronger, add an extra 30kg or 40kg of stringers, more bulkheads, thicker foam, extra carbon etc. Maybe 6mm divinycell is strong enough, I have not used foam, I would be tempted to go thicker for hull sides. What weights are you using for pounds per square foot for your 1/4 inch divinycell foam plus inner and outer... and also 5/8 divinycell foam plus inner and outer?

My guess was over 500kg total weight, your thinking 300kg to 350kg. Just say for argument sake, the fully laden boat comes out of 400kg. I would argue that a boat at 54 inches beam is going to be much nicer to row and sail than one at 44 inches. Waterline beam when light would still be modest, 32 inches perhaps. I found that the main issue for me rowing was not the resistance of the hullform, it was windage. I could easily maintain 6km an hour for hours and hours. I think I rowed 20 hours non stop once, total weight me, boat plus stores, 160kg. However a strong wind would make the boat uncontrollable.

As you progress up in weight from marsh duck, it makes sense to increase beam

Extra food and water can be used early in a long voyage, after a week your boat can now be lighter

A few years ago I wrote up a list of 512 rowing boats from the internet. I looked at that list just now. I came up with a few in the large size range. One might be John Gardners Chamberlain Gynning Dory at 18'4 x 4ft 8. Another Deblois street dory by Clinton Chase, 18ft 6 x 4ft 10. A third William mills light dory at 17ft 11 x 4ft 5. All three have very nice lines. I think each if decked over would still have a little freeboard but not excessive in the 400kg to 500kg range. Links to all three boats are on my web-page on 500 rowing boat designs

54 inches is not a wide boat. It is still a long long way narrower than the 72 inch beam you referred to in professionally built and designed ocean rowing boats of approx 3000lb. If the 400kg to 500kg laden boat is deemed an intermediate weight compared with the light marsh duck and the heavy ocean rowers, then the beam should be intermediate as well. Thus 54 inches thereabouts seems about right. I doubt you will notice any real difference in rowing resistance. However massive benefits in rolling resistance, and carrying capacity. At 400kg or more total, the extra 30kg of extra weight from a stronger hull is going to make very little difference overall

 

I just did another doodle, I think this is a sensible vessel. Lines come from the seaworthy swampscott dory, in this case a dory built in 1915. I have estimated weight at 120kg, you may disagree, but even a skin on frame dory comes out heavier than what you are proposing. Something with the maths does not make sense, or maybe 6mm foam is not thick enough, or maybe you need to add a keel or a stronger gunnel, etc etc. Total weight is still 33 to 40 percent of the 3000lb of other ocean rowers

 

Peter, thanks for continuing interest and thoughts.

For now, a couple of quick thoughts:

With foam core, strength comes from the skins, not the core (different from plywood cores which provide most of the strength). The foam core material needs to be strong enough that it is not pulled apart. It's thickness contributes to stiffness of the panels, but not to strength.

Your latest doodle has very little form stability. I think it would roll very easily, and it isn't clear whether it could be made to self right.

I want much greater form stability. A fairly wide, flat bottom helps with that and enables a comfortable sleeping platform on the bottom of the boat which also contributes to stability when sleeping. I'm not certain whether I want maximum initial stability (36 or even 38 inches-wide flat bottom for a 44-inch beam boat), or less initial, but increasing reserve (perhaps as little as 28 inches max flat bottom width for the 44-inch beam). The latter, with much less waterline beam and wetted surface, particularly when lightly loaded, would be easier and faster rowing.

Windage, while important, is not as critical as in rowing (only) as when it's windy I'd be sailing, or in extreme conditions riding on drogue or sea anchor.

In any case, self-righting potential is important in any boat I'd want to use crossing an ocean.

 

PS: the over 3000 lbs. gross displacement I mentioned was for the only 5-foot ocean rowing boat I found, a very unusual one for a solo rower. Most of the one or two person ocean rowing boats are lighter.

Some examples:

Calderdale, 2 rowers, originally built 2001, 23.4 feet long, 6.3 feet wide, 550 lbs./250Kg empty, 1650 lbs./750Kg fully laden, veteran of a number of ocean crossings

State of the art Rannoch Ocean Rowing boats:
- R10: solo, 20 feet LOA, height 4'3", 1400 lbs. loaded
- R25: pairs, 24 feet, height 4'5", no weights provided, but obviously a good bit more than the solo
- R45 Elite: 3, 4 or 5 people, 28.35 feet long, height 5.12 feet, 2072.34 lbs. loaded

Whaleboat C-23, open water adventure rowboat, 23' long with 5'5" beam, open pairs, weighs 155 lbs./70 Kg.

 

For perspective: Jean Jacques Savin drifted across the Atlantic in 128 days from the Canary Islands to St. Eustatius Is. in the Caribbean, in his home built barrel - no motive power, just ocean currents and wind.

Recent solo rowers seem to take between 49 and 95 days for the crossing.
Prior to 2008, solo rowers seem to have generally taken between 60 and 110 days, presumably mostly due to heavier, lower tech vessels. Extremes were from 46 to 133 days.

Hans Lindemann took 76 days in his 17-foot Klepper Arius II sailing kayak in 1956.

Relatively recently Max Campbell sailed his 22-foot sloop from the Canary Islands to the Cape Verde Islands in 7 days and on to Grenada in another 20 days (article in Yachting World, May 2019).

Most larger or faster sailboats seem to take from 2 weeks to a month.

I imagine that I would be able to make the crossing in 4 to 6 weeks in the boat I'm designing.

 

While Marsh Duck is a cool craft, someone's calculations seem to be out. With respect, it's not likely to be Julian Bethwaite, Paul Bieker and the other skiffies, 0r Andy McDougal and the other Mothies.

The R Class in NZ has been building 12'9" racing dinghies with no weight limit for almost 70 years. Their carbon/foam boats still come in at similar weights to your much bigger boat. The Development Canoe (modern version of the Int Canoe) has a minimum weight with sail and spars of 109lb, and it's a much smaller boat. That weight is what design and construction experts like Steve Clark thought was the lowest practical.

On Sailing Anarchy years ago there was discussion about a proposed lightweight development racer about the same size as your boat (around 22' x 4'6). Andy Paterson, pro builder of champion lightweight Moths, Cherubs etc, reckoned 99lb for the hull in carbon and foam would be about as light as anyone can get - and that's with just 5mm of foam. In E glass he estimated 132lb.

Are you really that much more expert than a professional who has been a world leader in Moths, etc? It seems that either your weights are optimistic, or just about everyone in fast dinghy construction over the past few decades has been an incompetent. The latter is extremely unlikely. I've done enough miles offshore and spent enough time learning from the top guys to not stake my life on the hope that they are wrong and I am right.

 

That looks quite traditional and would probably function quite well as a rowing boat.I do wonder if you may have the cuddy arrangement back to front though.A good number of boat I have seen for this kind of thing have the shelter at the stern ,apparently so that if the rower decides to move inside in extreme weather,the windage keeps the boat head on to the waves.

 

The 20' Rannoch Solo rowing boat has the larger cabin at the bow -
https://www.rannochadventure.com/boats-2/rannoch-r10-solo

I am wondering if this is perhaps to improve the directional stability when rowing down wind - a large cabin at the stern might be affected by the wind and try to slew the boat around?

 

A good bit of the thrust moving many ocean rowing boats is the prevailing winds. Large front cabin with small aft catches that wind.

 

I have been thinking of a sail-assisred rowboat for some time.

The idea is to have a hull of about 5 ft beam and about 20 ft length.

The sail plan would be tandem boomed Lateen sails of about 25 sf each (this is a sail-assisted rowboat, not an oar assisted sailboat).

The idea of the tandem sails is that they can be balanced off each other to help keep the boat on course, when there is wind. This rig will offer three different sail combinations:
1.) full sail,
2.) fore sail up and aft sail down, and,
3.) aft sail up and fore sail down.

The sails would be flat-cut, so that their lift can be "killed", without them fluttering themselves to pieces.

Boomed Lateen sails were chosen, as they can have the shortest masts. These masts will be fitted with lazy-jacks, so the sails will more or less flake when the halyard is relieved. There will also be a boom crutch on deck for each sail.

The center cockpit will have a center board case that is open at the top, so it can double as a cockpit drain.

The boat would have a kick-up rudder but no skeg.

All-up weight goal would be a little over a metric ton. This will include about 70 gallons of drinking water.

Some sort of an awning would be rigged over the cockpit to keep the sun off the rower.

Hopefully a speed of about 3 kts can be reached in a dead calm, and maybe 2.5 kts can be sustained in those conditions.

Usually, the sails will be up and adding some thrust.

 

Sounds interesting. Please continue to share as you go forward.

 

Will this drain also flood the cockpit effectively defeating bailing efforts when recovering from a capsize?

 

Some small twin mast boats, for instance Ross Lillistone's lovely Periwinkle, have a third mast step-partner for balance under one sail.
I am having a fiberglass sail and (sliding seat) oar vessel @ 17' by 5' 6" built. It will sport a 100 ft² balanced lug rig with lazy jacks.
Foils are a funny thing. The dagger board will have a foil shape but barn door rudders behind skegs do not seem to get a noticeable real-world benefit from such niceties. Or so one successful boat designer-racer has stated.