why aren't there fast planing sailing dingies able to take outboard motors?

Reasons I've heard that I'm not really buying:
1)Planing when sailing "fast" is maybe 15 knots which is completely different than much faster motorboat planing.
Thats fine, 15 knots or less would be plenty fast, and lot faster than rowing.

2)Much of what allows these boats to plane under sail is their light weight, so if you add a motor and gas they will be too heavy to plane.
Makes some sense but the power required to push a sailing dingy to 15 knots seems like a very small and lightweight outboard. I've seen 2 fairly large people planing a dingy so subtract one of the 200+ lb crew and swap in an outboard should work.

3)Planing dingy hulls are completely different animal that motorboats and there is all sorts of Voodoo going on about interaction between sail bracing against dagger board that creates lift and a prop will throw a monkey wrench into all that because the prop creates SUCTION under the hull, etc.
Makes some sense but I'd think the gap could be bridged. Might have a more narrow window of speed VS load carried VS engine size/weight VS getting up on plane, but could still be a fairly large and useful window.

4)Planing in a planing dingy is not for the feint of heart and is an athletic event suitable for short time periods and results in getting wet, bruised and windblown, which isn't something motorboaters want. Its a rough ride and the dingy hulls aren't designed to mitigate slamming, only to sail fast, with no regard to comfort or even safety.
I'm thinking that is because the planing is done in fairly high winds and at least some chop, and involves hiking out into the bow spray. Shouldn't be too bad if motoring in less wind and waves at sailing dingy speeds.

 

Name one that doesn't

 

Hi Squidly. It depends on relative sizes and weights; and it depends on what you call fast. An Optimist probably won't like a 25 HP donk on the back, but maybe a light weight 2 stroke 2 HP might just work there, not sure about actual planing though, it's a tub, unlikely to go 'fast'. A small OB on its own shouldn't be too much weight, but a passenger and all the safety gear required to legally use a motor will add up on a small hull. 16 foot long sailing dinghys, eg Hartley TS 16, Cherry 16, and many others regularly carry 2 HP, 10 HP, even up to ~25 HP, and can sail, plane, motor sail, and just be powered, usually easily planing or semi-planing, depending on how much extra weight is being carried. Racing dinghies leave the OB off for max speed in races, but when cruising or camping, carrying everything including the kitchen sink, then an OB is very helpful, even though you might not quite get over the hump, or go 'fast'. Beach cats do go fast with a small OB on the back.

 

I was thinking more like these. RS Venture –stability and exciting features than almost any other dinghy https://www.rssailing.com/project/rs-venture/ I'm guessing they aren't built to take an outboard bracket bolted on the back.
I've never seen a Hartley/Cherry type with an engine that looks able to plane it, just 2-5hp range.

But its good to here there aren't any reasons why it wont work.

 

I'm sure that with a suitably reinforced transom, and maybe a slightly different mainsail sheeting arrangement, a short leg 5 - 10 HP OB on a RS - Venture, it would plane easily and be a versatile boat. Saying that having only seen the pictures, and not sailing one to see how it really is. With 8 people on board it may not be 'fast', but will get about nicely. The low freeboard at the transom may allow some stern wave over the back on quick de-acceleration, but it looks like it would work in general.
Sailing dinghies and small trailer sailers rarely carry a powerful motor to be able to plane fast, since they weigh down the stern, often have to be lifted on and off the transom for trailering, and are an easy target for theft.

 

The obvious answer is that most planing dinghies are built to race.It is well known that weight in the ends is detrimental to performance and consequently there is no real incentive to add strength and weight if it will harm the performance under sail.Additionally,the comparatively modest drive from the rig necessitates as little weight as possible,further loading the dice against carrying the weight or indeed having the hull shape as the probable increase in wetted surface would erode light weather performance.Some of the more traditional hull shapes also have a bit more rocker than is ideal for planing.What you may find is that a suitable boat,with a strong enough transom will plane quite well with very modest power,but it may not be an ideal powerboat.I once had a conversation with the manager of a local company that had built a good number of outboard weekenders and he despaired at the number of phone calls from people who wanted to fit an outboard that exceeded the power listed on the builder's plate.They always believed the hull shape would be fine,and it might well have been,but not one had ever considered the loading imposed by the mounting bracket on the transom or sought advice relating to the structural aspects of the alteration.Besides which planing under sail is more fun,we should all do it.Is there anybody on this thread who hasn't?

 

Or, from another perspective. A lightweight power skiff, on which sail can be added. I built in a daggerbox, and used the lug sail and rudder off my Michalak family skiff. It would plane downwind without problem, but was not set up for hiking out or ultimate windward performance. The idea was to sail with the favourable wind, and be able to motor at planing speed back to port. It sailed far better than expected for a boat with a modified flat run on the bottom panel.




I had used it also with the reverse Mirror rig, and outboards between 2.5 to 20hp. It would plane with 6hp. 22knots with 20hp. A very versatile boat, which did not require dressing up in a wet suit or oilies to have fun.

 

I have the plans for most of the Hartley TS range, including the 16, and the boat has been clocked apparently at 16 knots under sail. Given the bottom shape aft, doing the same under motor would give a major bow up attitude and require a hefty motor to do so. I looked at many sailing boats that could plane under sail, yet an engine being added for the same performance was always a detractor; hence my shift in looking at lightweight type power skiffs that could be adapted to sail instead. It got used a lot, just because it really didnt matter if the wind was going to die out later in the day, where getting home with the 2.5hp on the back of the better sailing Family Skiff could take hours, the planing skiff was 4x faster. It got the better half out on the water when she would otherwise be complaining it takes too long and "its boring".

 

Attached file shows an investigation for a planing motorsailer for a client.
I am convinced that the same basic principles can also be applied to a smaller boat with satisfactory or better results.
JS

 

Nice to know that it gave satisfaction,but did it plane under sail?

 

Downwind under the balanced lug in strong winds yes. Its a small sail, 91sqft. Broad reaching was a bit risky not being able to hike out, even though i had 2 25l water tanks about midships. The Mirror sails did not provide enough drive for planing in the winds i was happy to go out in, the lake picks up a nasty short chop in sustained wind. It weighed less than 300lb and floated in less than 4in, so it was very easily driven.

 

Swallow Boats seemed to have come up with an interesting concept with their Coast 250, but i just noticed that have put the molds up for sale due to "high production costs". Personally, I couldnt afford to run a 70hp outboard. I liked the design, it was very familiar with my box keel motorsailer project.



https://lh3.googleusercontent.com/g...UgnCSd8cD8yoFg2sCvq7-a6T061FHW58=w647-h485-no

 

I was thinking of super-sizing the Grumman Sport Boat to about 21' and 5' beam and calling it a SUV for Sports Utility Vessel able to sail, motor or row, which would look a lot like your boat. Your boat looks like a Sport Boat about that size but able to be stitch&glue. Few features I planned for SUV would be covered lockable cuddy/floation up front, barely able for uncomfortable sleeping but big enough to lock up the outboard. Another is a system that uses a single dimension plank for twin daggerboards, benches, table-tops, tiered cargo racks, outriggers and gang planks. If it required doubling up of the planks for a heavy crew member to hike out that would be OK. Also a baked in way to quickly and securely use sails as tent enclosure for camping or low profile cargo/passenger cover.

 

It can be done.
The aftship interceptor and effective spray strips are important.
What max-min payload?
JS

 

A little large to fit the dinghy description and if I was being harsh I might liken it to a Welsh MacGregor 26.I have a feeling that the investment in tooling to make it an efficient production process is hard to recoup in a market where each owner hopes to have his/her specific wishes catered for.If a company were to be building 30-40 a year for a decade or so,it would be different.